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SELECTION IN VISION

A study of visual search

A thesis submitted in partial

fulfilment of the requirements for

the degree of Doctor of Philosophy

by

Robert T. Solman

Australian National University

January 1977

This thesis describes original research

carried out by the author in the Department of

Psychology at the Australian National University;

from March 1973 to January 1976.

Robert T. Solman

r i t

PREFACE

am indebted to Dr. Michael Cook of the Australian National

University. Dr. Cook was always willing to discuss my work, his

criticisms were invaluable, and his encouragement during the latter

stages was greatly appreciated. would like to thank Barbara Grosser

who typed this thesis, and Neville Whitworth who bu i It and rna i nta i ned

much of the equipment. would also I i"ke to acknowledge the •

Australian Government for awarding me a post-graduate research

scholarship.

Nine empirical investigati?ns have been reported in this

thesis. The results of five of these nine have been published, and

the results of the remaining four are being considered for publication.

I have listed below the five published papers and two manuscripts which

incorporate the remaining four studies.

SOLMAN, R.T. Influence of similarity between target and i_rrelevant

items on visual information processing. Perceptual and Motor

Skills, 1975, ~. pp.43-48.

Effect of target separation on selective attention.

Peroeptual and Motor SkiUs, 1975, ~. pp. 755-760.

Relationship between selection accuracy and exposure in

visual search. Peraeption, 1975, ~. pp.411-4!8.

Influence of selection difficulty on the time required for

icon formation. Peraeption, 1976, ~. pp.225-232.

Evidence that focal processing involves a build-up of a

visual object. British Journal of Psyahology, in press.

lv

Processi~g nominal information during first-stage analysis.

Being considered for publication by the Canadian Journal of

Psyahology.

Relationship beb1een accuracy of search and extended

exposure-time.

Pereeption.

Being considered for publication by

R.T.S.

PREFACE

ABSTRACT

TABLE OF CONTENTS

1. INTRODUCTION

1.1

1.2

STIMULUS CONTROL DURING SELECTION

SOME PROPERTIES OF VISUAL SELECTION

VISUAL SEARCH

ATTRIBUTES GOVERNING SELECTION

SERIAL AND PARALLEL PROCESSING

1.3 • THE EMPIRICAL INVESTIGATION

2. EXPERIMENT 1

2.1

2.2

2.3

2.4

2.5

INTRODUCTION

METHOD

DESIGN

MATERIALS AND APPARATUS

PROCEDURE

RESULTS

DISCUSSION

PARALLEL-SERIAL PROCESSING

A TWD-STAGE MODEL

FURTHER INVESTIGATION

Iii

1

3

3

6

8

10

12

13

14

14

15

16

23

24

27

28

v

vi

3. EXPERIMENT 2

3.1 . INlRODUCTION 30

3.2 METHOD 31

DESIGN 31

MATERIALS AND APPARATUS 32

PROCEDURE 32

3.3 RESULTS 34

3.4 DISCUSSION 37

4. EXPERIMENT 3

4.1 INlRODUCTION 38

4.2 METHOD 40

DESIGN 40

MATERIALS AND APPARATUS '•1

PROCEOI.RE 41

4.3 RESULTS 43

. 4. 4 DISCUSSION 48

A "TWD-STAGE MODEL 48

PARALLEL-SERIAL PROCESSING 50

RESPONSE SPEEDING 50

MASKING 51

MEMORY SPAN LIMITATIONS 51

4.5 FURTHER INVESTIGATION 52

5. EXPERIMENT 4

5.1 INTRODUCTION 53

5.2 METHOD 54

DESIGN 54


PROCEDI.RE 55

vil

5.3 RESULTS 55

5.4 DISCUSSION 56

ACCURACY/TIME CURVES 59

6. EXPERIMENT 5

6.1 INTRODUCTION 61

6.2' MEn-taD 61

DESIGN 62


PROCEDURE 62

6.3 RESULTS 64

6.4 DISCUSSION 65

IN SUMMARY 67

7. EXPERIMENT 6

7.1 INTRODUCTION 68

7.2 MEJHOD 69

DESIGN 70


PROCEDURE 71

7.3 RESULTS 72

7.4 DISCUSSION 75

7.5 FURTHER INVESTIGATION 81

8. EXPERIMENT 7

8.1 INTRODUCTION 82

8.2 ME1HOD 84

'THE PEST PROCEDURE 85

DESIGN 87


PROCEDURE 88

a.::>

a.4

RESULTS

DISCUSSION

DEVELOPMENT OF STRUCTURE

NAMING

9. EXPERIMENT a

9.1 INTRODUCTION

9.2 METI-IOD

DESIGN


PROCEDURE

9.3 RESULTS

9.4 DISCUSSION

FURTHER I !'NEST I GAT ION

10. EXPERIMENT 9

10.1 INTRODUCTION

10.2 METHOD

DESIGN


PROCEDURE

10.3 RESULTS

10.4 DISCUSSION

FURTHER INVESTIGATION

11. EM"IRICAL FINDINGS AND FINAL DISCUSSION

11.1 THE TIME CoURSE OF VISUAL SEARCH

11 THE STUDIES

DISCUSSION.

11.2 NAME DIRECTED SEARCH

THE STUDIES

DISCUSSION

v ll !

a9

92

92

96

9a

99

99

100

100

102

103

104

105

105

106

106

107

107

109

112

114

114

114

11a

119

119

121

11.3 . GEI'ERAL DISCUSSION

NEISSER'S DESCRIPTION OF THE TWD-STAGE MODEL

REFERENCES

APPENDICES

1. DATA RELEVANT TO EXPERIMENT 1









121

122

127

137

138

143

145

154

157

160

164

166

166

ABSTRACT

This thesis is concerned with the selective

processing of information which arrives via the visual system. The

introduction discusses the problem of stimulus control during selection

and specifies the aim of the thesis. Nine experiments are reported.

X

The first six examine the relationship between selection accuracy and

stimulus exposure-time. It is suggested that a plausible explanation of

the obtained results can be found in a two-stage .model of information

processing (e.g., Neisser, 1967). The remaining three experiments address

the question of whether a letter-name can direct the selection process,

and the results, while somewhat indecisive, do not indicate that it can.

Specifically,

a) in Experiment 1 accuracy of search for a single target

declined as the number of irrelevant items, and their

physjcal (shape) similarity to the target, increased.

These results could be explained either by sophisticated

parallel-serial models of information processing, or by a

two-stage model; Experiment 2 suggested that selection

was not based on a limited retinal region, thereby

negating the possIbilIty of the dIstance separatIng I terns

influencing the results of Experiment 1; Experiment 3

varied stimulus exposure-time along with similarity and

number of items. The results replicated the effects

obtained in Experiment 1, and the negatively accelerated

accuracy/time curves demonstrated a plateau effect. The

level of the plateau differed for each curve and these

differences were attributed to errors during .initial

xi

selection; Experiment 4 found the long term course of the

accuracy/time curves (i.e., exposure-time varied from 100

to 500 msec). This gave subjects the opportunity to begin

a second processing run, but, possibly due to !mage-decay

or to the absence of eye movements, no evidence of a second

run was detected; Experiment 5 supplemented the time

course data obtained in Experiments 3 and 4 by sampling

behaviour over an exposure-time range of 15 to 400 msec.

The results showed that both image-decay and the

opportunity for eye movements had no effect; the results

of Experiment 6 replicated those obtained in Experiment 3,

and indicated that while the relationship between target

and irrelevant items affected accuracy of selection, it did

not influence the time required for this selection.

b) Using the PEST procedure Experiment 7 firstly, demonstrated

that the process of bui !ding up a structural

representation of an alphabetic character requires time,

and secondly, indicated that the prior development of

object structure may be necessary for naming; Experiment 8

showed that the name of a target Jetter need not interfere

with its selection; In Experiment 9 a confounding of shape

and name information made a straight-forward interpretation

of the results difficult. The data, however, did not

suggest that nominal information was used to direct the

selection of the target.

Finally, the discussion speculates further on the nature of the

information processing system underlying the findings.

1. INTRODI.X:TION

1.1 Stimulus Control During Selection

One of the most striking and ubiquitous aspects of

visual selection is the way In which characteristics of the stimuli can

and do control the selection process. This control is demonstrated both

1

in time-limited selection tasks and in visual scanning tasks. In both

cases the stimuli are usually constructed from the set of alpha-numeric

characters, and only a limited number of the items presented to the

subject are to be reported. When the time allowed for selection to take

place is limited the usual procedure is to display briefly a number of

Items and to Indicate those to be reported either by the presence of an

adjacent marker (e.g., Averbach and Coriell, 1961), or by specifying some

attribute, such as colour, which distinguishes relevant from irrelevant

Items (e.g.,Eriksen and Collins, 1969; Von Wright, 1968}. The marker or

attribute then controls selection in that it "drives" or directs the

selection mechanism(s) in such a way that the relevant items appear to be

processed, and are selected, to the exclusion of the irrelevant items.

This driving of the mechanisms of selection is even more dramatically

manifested In the scanning task. That Is, subjects are usually required

to scan a list containing upwards of fifty rows of five or six characters

each, with the aim of finding a specific character (termed the target)

(e.g., Neisser, 1963 & 1964). In the course of the search some attnbute(s)

of the target direct selection in such a way that the mass of non-target

characters Is rejected or excluded, and the target is perceived.

2

The selection of items defined by attributes such as

location, shape, and colour, implies that the specified attribute needs

to be processed prior to the recognition of the relevant Item, and this,

in its turn, suggests that, in some way or at some level, all items must

be processed before selection takes place. This somewhat paradoxical

suggestion has had ramifications for theories intended to explain

selective phenomena. For example, it was primarily an argument based on

this suggestion, and supported by empirical data (cf., Cherry, 1953;

Cherry and Taylor, 1954; Gray and Wedderburn, 1960; Treisman, 1964a, 1964b,

& 1964c), which led to Treisman's (1964a, 1964b, & l964c) revision of

Broadbent's (1958) "Filter" theory of attention. 1 Alternatively, it gives

support to those psychologists (e.g., Deutsch and Deutsch, 1963), who see

no need for a notion of selection during processing and would therefore

argue, as do Hodge (1959 & 1973) and Lawrence and La Berge (1956), that

selection in vision follows the complete processing of all Items. But, it

is not the intention of this thesis to enter the controversy over the

relative merits of the various theories of attention, 2 nor is it intended

that the perception/response question be considered in any detai1. 3 ·The

general aim of the investigations reported here is to explore some of the

properties of selection during visual search, and to seek an explanation

of search behaviour.

1. Some confusion surrounds the use of this term (see Spearman, 1937 for a detailed account), but for the purposes of this thesis and in line with Broadbent's (1958) description, it will be treated as a p~ceas, which by selectively allocating a scarce resource (capacity, effort, energy) allows the human information processing system to deal with some inputs to the exclusion of others.

2. The reader interested in theories of attention should refer to Broadbent (1958), Broadbent and Gregory (1964), Deutsch and Deutsch (1963), Deutsch, Deutsch, lindsay and Treisman (1967), Trelsman (1964a, 196/.jb, · 1964c, & 1969), as well as to corrprehensive reviews found in Egeth (1967), Kahneman (1973), Moray (1969), Neisser (1967), and Norman (1976).

3. Good reviews of the debate over whether selection in vision can occur during perception or whether this selection must wait until after the perception of all items, can be found in Egeth (1967) and Haber (1966).

3

1.2 Some Properties of Visual Selection

As the errp i rica l research reported in the present thesis

is concerned with visual search behaviour and the properties exhibited

therein, the following discussion will be restricted to a description of

visual search behaviour, and to a brief review of studies which examine

directly the attributes governing visual selection, and to a mention of the

debate concerning the mode of processing of elements from brief visual

displays.

1.2.1 Visual Search

The most theoretically oriented treatment of search

behaviour can be found in Neisser (1967), and Is based (in general) on the

results of search studies carried out by him and his colleagues (Neisser,

1963 & 1964); Neisser and Lazar, 1964; Neisser, Novick and Lazar, 1963).

In these experiments subjects scanned from top to bottom through vertical

lists, each consisting of fifty lines (called "items"), to find the target

embedded in each list. In general, the target was a single alpha-numeric

character, and the five or six alpha-numeric characters present in each

line were in some meaningless combination. The consistent finding was a

linear relationship between response latency and the vertical position of

the target in the list. Assuming that the starting time and stopping time

(includes target recognition time and response time) are independent of

the position of the target, their contribution is to the intercept of the

search time function, not to Its slope. The slope is then a measure of

the time required to examine the context items, and it is relatively

uncontaminated by response factors.

An efficient asymptotic search rate was achieved after

several days practice, and the following aspects of search behaviour were

recorded:

(a) Subjects reported that they did not "see" the context i terns,

and there was no evidence from post-search tests that these

items were recognized during the search. This result was

supported by Schapiro (1970), using a post-search recognition

task. However, the results of a study specifically designed

to examine the effects of varying degrees of physical

similarity between the target and particular irrelevant items

(Solman, 1975), showed a significantly higher post-search

recognition level for the most "target-like" item. This

suggested that the depth to which field items are likely to

be processed may depend on their physical similarity to the

target;

(b) Search for a given target was faster than search for its

absence;

(c) The physical similarity between the target and field items

had a marked affect on rate of search, with the search rate

declining as the similarity increased. This similarity effect

has been confirmed in a number of studies (Estes, 1972; Gibson

and Yonas, 1966a; Kaplan, Yonas and Shurcliff, 1966; Mcintyre,

Fox and Neale, 1970);

(d) After training, the search rate appeared to be indpendent of

the number of alternative targets, i.e., subjects were as fast

searching for one of ten possible targets as when required to

search for a single target. This result has been the subject

of some controversy, i.e., a similar finding was obtained in

a study by Gibson and Yonas (l966b), where searching for one

of two targets was no harder than searching for one alone.

But, Krlstofferson (1972) and Wattenbarger (1968) have shown

that only when the error rate is high is search equally rapid

5

for one and many targets. 4

The experimental results suggested (to Nelsser) that

recognition may best be described as a hierarchical, two-stage system of

decision processes. When searching for a particular target letter, the

first processing stage carries out elementary or low level tests on target

features (it was not possible to specify the exact nature of these features,

but they probably consist of physical or structural attributes, such as

angularity, openness, straight or curved lines, etc.). These tests are

carried out in parallel, and items which fail the test are passed by

without being subjected to second-stage processing. This model implies

that the perception of field items differs from the perception of targets.

The former are rejected without being recognized, and the latter are fully

processed or constructed and thereby recognized.

The suggestion that first-stage processing is carried

out on physical pattern features is consistent with the well known

physiological work of Hubel and Wiesel (1962 & 1965), and although there

Is no guarantee that the! r findings apply to human pattern perception, the

notion of pattern features is now widely accepted (cf., feature lists for

alpha-numeric characters by Geyer and De Wald, 1973; Gibson, 1969;

Sutcliffe, 1971). However, the assumption that these attributes are of a

physical kind, has been challenged by experimental results which suggest

that the names of alpha-numeric characters may also be processed at this

level (see Brand, 1971; Henderson, 1973; Ingling, 1972).

4. Further investigation of this question by Yonas and Pittenger (1973) produced evidence of a decline in rate of search and an increase in the number of errors, when search was for one of four targets as compared to one target alone. However, rather than suggesting that the Neisser et al., (1963) results are difficult to rep! icate, the authors point out that their findings indicate that, when conditions permit, subjects can and do make subtle adjustments in their speed· accuracy trade offs. This point was later confirmed and elaborated by Neisser (1974).

6

Neisser's (1967) explanation of search behaviour has not

been universally accepted. But, the important aspect of this behaviour is

that, whatever the pattern attributes are which direct the search, it is

efficient and fast with some forty to sixty items (Neisser, 1967)

processed every second.

1.2.2 Attributes governing selection

Discussion of the attributes which direct or govern

selection is more frequently found in the literature dealing with time-

limited selection than in the literature which deals directly with visual

search. This is perhaps due to the versatility of the partial-report

technique introduced by Sperling (1960). Sperling presented subjects

with a tachistoscopically displayed array of letters, and after

termination of the display they received an auditory tone specifying which

one of the presented rows was to be reported. The superiority of

performance under these conditions when compared with "who 1 e report"

(subjects were asked to report as many items as possible) was taken as

evidence for the existence of a short-term visual memory. 5 However, the

importance of the study in the present context is that, a) location was

used as a basis for selective processing of part of the display, and b)

the technique offers a method of investigating the effectiveness of other

attributes as controllers of selection.

Using the partial-report technique, Von Wright (1968)

compared the effectiveness of a number of stimulus attributes. His

results showed that selection was possible if the attribute specified was

location, chromatic colour, achromatic colour, or size, but when

5. The existence of a rapidly decaying, short-term visual memory (see Averbach and Co riel 1, 1961; Coltheart, Lea and Thompson, 1974; Dick, 1974; Nelsser, 1967) is now generally accepted (Holding, 1970, 1971, 1972 ,& 1973 is a notable exception).

7

orientation was the basis for selection performance deteriorated markedly.

'In a similar study, using a circular arrangement of display items, Eriksen

and Collins (1969) showed that efficient selection was possible when a line

appeared beside the i tern to be reported, when the "to-be- reported" i tern

was indicated by an arrow radiating from the centre of the circle, and

when an instruction specified the report of an Item diametrically opposed

to the one indicated by the line. However, subjects were unable to select

efficiently when the cirterion was a number specifying a particular item

position.

While more remains to be done before it wi 11 be possible

to completely specify the attributes which can effectively direct the

selection process, both the Von Wright (1968) study and the Eriksen and

Collins (1969) study suggest (in contrast to the Brand (1971), Ingling

(1972), and Henderson (1973) studies) that they may be of a formal or

physical kind.

The partial-report technique has also proved useful for

investigating the "tuning" limits of selective processing. Eriksen and

his colleagues (Colegate, Hoffman and Eriksen, 1973; Eriksen and Hoffman,

l972a & 1973; Eriksen and Rohrbaugh, 1970). in a series of studies in

which they varied the interval between the onset of an indicator (usually

a line beside the position where a to-be-reported item was to appear) and

the display, showed that even when given sufficient time to fully process

the indicator, a) subjects were incapable of restricting processing to

the indicated item if the separation between adjacent itemswas less than

approximately one degree of visual angle, and b) the number of errors

increased with an increase in the total number of display items. These

results suggest that there are limits to the precision with which

processing capacity can be allocated, and the lower limit may be

represented (with the stimuli used) in spatial terms as an item separation

8

of one degree of visual angle. Also, the preliminary processing, which

isolates the controlling attribute in order that selection may occur, is

of sufficient depth for an increase in the total number of items to cause

a decrease in the probability of the to-be-reported item being selected.

1.2.3 Serial and parallel processing

The mode of processing of elements from brief visual

displays has been examined in some detail in recent years. 6 The most

productive method of investigation has been the detection paradigm, where

the subject is asked to make a simple response as soon as he detects some

predefined aspect of the display. The simple response, and prior

specification of what is to be looked for, means that there is no need to

store any but the predefined aspects of the display in short-term memory,

thereby limiting the involvement of memory. The two major detection tasks

are search, and same-different discrimination. In the visual search task

used the subject may be asked to indicate whether or not the target is

present in the array or alternatively, he may be asked to indicate which

one of two (or more} specified targets is contained in ar. array. In the

same-different task subjects are asked to indicate whether the elements of

the array are all identical to one another or whether (at least} one

element differs from the others.

A serial model is one in which a decision is reached by

examining the elements of a display one after the other, and a parallel

model allows for the stimultaneous examination of stimulus elements. Some

of the strongest support for the serial model comes from yes-no search

tasks. Sternberg (1966 & 1967} studied visual search in a situation where

memory search was also investigated. Subjects were given a short list of

6. Although this issue is not germane to the empirical work reported later in the thesis, the nature of some of the data necessitates that serial and parallel interpretations be considered. Consequently, a brief preliminary discussion is in order.

9

digits to memorise, and then presented with a visual display of a varying

number of digits. Themsk required that the subject Indicate whether or

not any of the digits in memory matched any of the digits In the display.

The principal finding was that reaction time increased with increases in

the number of display items, and Sternberg concluded that both V'isual and

memory search progress In serial at about the same rate (37 msec/element),

but memory search is exhaustive while visual search is self terminating.

Other investigators have also obtained results showing fairly uniform

increases in reaction time with increases in the number of display items

(e.g., Atkinson, Holmgren and Juola, 1969; Estes and Wessel, 1966;

Nickerson, 1966). Apart from some debate over whether search (visual and

memory) is self terminating or exhaustive, they support the suggestion

that the items are examined one after the other.

There are two main parallel processing models to be

found In the literature. One assumes that processing capacity is limited,

and that as the number of items (or amount of material ) to be processed

is increased, the available capacity must be more thinly spread over the

parallel input. Consequently, errors and reaction time would be expected

to show an increase as a function of the n4mber of display Items (see

discussions by Atkinson, Holmgren and Juola, 1969; Corcoran, 1971;

Townsend, 1971c). The other assumes that the processing capacity is

independent of the number of items to be processed (an Independent-

parallel-channels model), and that each element is allocated to a

separate channel. This model would predict no effect due to increases

In the number of display Items, unless the total number of items exceeded

the available number of channels. Results obtained by Egeth, Jonides, and

Wall 0972), and Gardner (1973) gave some support for a model of

independent parallel channels.

An interesting aspect of this research is that rather

than demonstrating that items are processed one at a time in serial or

simultaneously. in parallel, the data have suggested to some

investigators that the mode of processing need not conform exclusively

to either class of model (e.g., Estes, 1972; Rosenbaum, 1974). This

complicated situation has been further exacerbated by Townsend (1972)

pointing out that some serial and parallel models are empirically

indistinguishable.

1.3 The Empirical Investigations

The reported empirical studies take a look at

10

accuracy of search in brief visual displays. They were designed to

examine variables affecting the accuracy with which target item(s) are

selected in a tachistoscopically displayed circular arrangement of

alphabetic characters. The majority of the studies examined the effects

of changes in the number of irrelevant stimulus items, and changes in the

degree of visual similarity between target and irrelevant items. In

particular, the first study manipulated these variables at a masked

stimulus exposure-time of 100 msec. The tachistoscopic exposure was

intended to maximize any influence that variations in stimulus

characteristics might have on the accuracy of selection, and the results

did in fact show an interaction between number and similarity. Subsequent

discussion of this finding (see Experiment 1 for details) specified the

need for a concurrent variation of exposure-time, and the empirical

investigations which followed were designed to explore the relationship

between selection accuracy and masked exposure-time.

Although inquiry into the form of the relationship

between selection accuracy and exposure-time constitutes the main thrust

of the empirical investigations, the final three studies (i.e., Experiments

7, 8, and 9) have as their aim the investigation of a more specific issue.

That is, they were designed to examine the possibility that the name of

letters of different case can direct the selection process.

11

12

2. EXPERIMENT 1

2.1 Introduction

As referred to in the introduction to this thesis, Neisser

and his associates (Neisser, 1963; Neisser, Novick and Lazar, 1963;

Neisser, 1967) noted that an increase in the physical similarity between

the target item(s), and the irrelevant items in a visual search task

caused the search rate to decline. More recently, similarity between the

target and irrelevant items has been varied in studies by Estes (1972),

and Mcintyre, Fox, and Neale (1970) using brief visual displays. In

accordance with the earlier observations they found that increases in

similarity caused reaction time to increase and accuracy to decrease, and

thus confirmed the importance of similarity as a variable influencing the

selection process. This suggests that manipulation of the physical

similarity between the target and the irrelevant items (if accompanied by

variation in the number of irrelevant items), could aid in explaining

selection during visual search.

In this study the physical similarity between the target and

the irrelevant items, was manipulated in a situation using a

presentation sufficiently brief to force subjects to respond before they

had processed sufficient information to be confident of being correct.

This procedure was intended to maximise the influence of information

processing carried out early in the process of selecting and recognising

the target. In many studies evidence of errors made early in the

selection process is difficult to detect, as subjects usually have

sufficient time to compensate for incorrectly selected items by further

13

processing. On the other hand, if the input is avai !able (in the form of

the physical stimulus and/or in the form of a short-term visual store)

for only a brief period, accuracy of response should depend directly on

the precision of the early processing. Therefore, the aim of this study

was to examine the effect of variations in the similarity between target

and irrelevant items, on the accuracy of early processing during

selection of the target in a visual search.

2.2 Method

The task was visual search. Throughout the experiment a

trial consisted of the following steps. After placing a stimulus card in

the tachistoscope E gave a verbal ready signal upon which S fixated the

cross, and pressed a hand switch •1hich removed the cross and initiated

the display. The display remained for 100 msec, and was replaced by a 50

msec mask. 7 At the completion of each trial S recorded his response by

marking one of 18 possible target positions on a prepared sheet, and by

placing a confidence rating (4 ~ sure, 3 = almost sure, 2 = might have been, and 1 = guessing) alongside. A response was recorded as correct if

the target position or one either side was marked.

7. It is assumed that the backward masking procedure prevent further processing of the stimulus input. Whether this is due to the perceptual integration of the contours of the test stimulus with those of the mask (Eriksen, 1966; Kinsbourne and Warrington, 1962), or to the interruption of information processing caused by the arrival of the mask (Dilello, Lowe and Scott, 1974; Kahneman, 1968; Kohlers, 1968; Liss, 1968; Turvey, 1973), is unimportant here.

14

2. 2. 1 Des i g n

A three-way design was used, with the similarity between

the target and the irrelevant items (high-similarity, and low-similarity)

and the number of display items (2, 6, 12, and 18) varied within Ss, and

the order of presenting the high-similarity and the low-similarity

displays varied between Ss.

Subjects were 6 undergraduates (4 males, and 2 females)

with normal 6/6 vision, and each received $1 per hour for taking part in

the experiment. Three Ss searched high-similarity displays before low-

similarity displays, and vice versa for the other 3. All Ss searched

for 7, 1-hour sessions (3 practice, and 4 experimental).

2.2.2 Materials and apparatus

The .upper case letters of the alphabet provided an over-

learned and well-defined population of nameable visual forms, and a

subset of 13 items was selected for use in the experiment. To assess

the inter-item similarity the confusion matrices derived by Townsend

(1971a, and 1971b), and the feature analyses specified by Geyer (1970),

Gibson, Osser, Schiff, and Smith (1963), Laughery (1969), Rochester,

Johnson, Amdahl, and Mutter (1959), and Sutcliffe (1971) were examined.

The letter F provided a reasonable range of inter-item similarity when

compared with all other letters, and was selected as the target item.

After selecting the target, the non-target items were ranked in ascending

order on the basis of their physical similarity to the target, and the

top and bottom six letters were selected to constitute the high-similarity

(E,T,H,J,P,L) and low-similarity (C,G,O,Q,U,W) sets respectively.

15

Stimulus displays were constructed by mounting black

Letraset letters (18 pt Grotesque 216) on white cards. The letters

could be positioned at any of the 18 equally spaced loci on the

circumference of an Imaginary circle. When positioned, the letters were

0.27° of visual angle high with a stroke width of 0.06°, and a minimum

distance of 0.43° separating adjacent Items. In total 128 display cards

were constructed, I.e., there were 12 present (with an F) and 4 absent

(without an F) cards constructed for each level of number combined with

the two levels of similarity (see Figures 1, 2, 3, and 4 for examples).

The fixation card had a black cross mounted at Its centre

and the mask consisted of a jumble of broken, distorted, and whole

Letraset letters. All cards were displayed In a Model GB Scientific

Prototype Three-Channel Tachistoscope with channel luminance for fixation,

stimulus, and mask of 10.8, 17.2 and 38.7 mlllllamberts respectively.

Ss recorded their response on sheets with the 18 loci clearly shown.

2.2.3 Procedure

The nature of the experimental task was explained to S

upon his arrival at the first practice session. He was told that it was

a visual search task, that when present the target appeared only once,

that the fixation cross should be In clear focus before the initiation

of a display, that the confidence rating given was to refer to the

presence of the target and not to certainty concerning Its location, and

he was discouraged from responding negatively. He was then given 64

trials with one similarity level followed by 64 trials with the other, at

a stimulus exposure of 400 msec. The order of presenting the high-

similarity and low-similarity displays, with two consecutive experimental

sessions devoted to each, was maintained for any particularS throughout

16

the study. The same procedure was followed for practice sessions 2 and

3 using stimulus exposures of 200 msec and 100 msec respectively.

During each experimental session Ss searched on 16

practice (12 present and 4 absent), and 132 experimental (100 present,

and 32 absent) trials, at an arbitrarily selected brief stimulus exposure

of 100 msec. This provided a total of 50 responses for each

experimental condition. The 2, 6, 12, and 18 item displays were

randomly presented with 4 displays 3 present and 1 absent) at each

level. The modification of a simple random presentation, was used to

ensure that an appropriate absent card appeared with each 3 present cards.

2.3 Results

The experimental procedure raised the question of what ~1as

to constitute a correct response. During the practice sessions it was

noted that even when confident that they had found the target, subjects

were frequently unable to place it without making position errors, and

since these errors were concentrated one position either side of the

target position, the target position and one either side was scored as

correct.

The mean number of correct responses, for the high-

simi 1 ari ty and low-simi 1 ari ty conditions in combination with the number

of display items, are shown in Table 1 (individual data is shown in

Appendix 1, Table 1), and are illustrated as the percentage of correct

responses in Figure 5. Subjects were able to search the 2-item, low-

similarity displays without error, and the 2-item, high-similarity

displays with an average error of only 1%, and as a consequence, data for

these displays were not included in the statistical analysis.

p l T H F J

T H J

J T l H E

E

p

F

p

p

l T

p

H T

E

H

F

17

l T F P

H H

p L

T J

T

FIG. 1: Examples of high-similarity, present displays containing 2, 6,

12, and 18 items.

H H p p

l L

H p

T L L

E T T

T

' p

L l

l

J

J

J

H J

l

E

p T J

TTT l

H

T

fiG. 2: Examples of high-similarity, absent displays containing 2, 6,

12, and 18 Items.

18

H H

p

19

u Q u u Q u Q w

w 0 c w w 0 0 F c Q G u F

OGo wo GQu w

1 ..

Q F Q

c u u

G

FIG. 3: Examples of low-similarity, present displays containing 2, 6,

12, and 18 items.

F

uuw G 0

0 0 w

u WGu uw

c

0

u Q

0 Q

u G

Q

w

G

0 Q

Q

20

c WQ

w

u 0 OGG

0

FIG. 4: Examples of 1()'(1-simi larity, absent displays, containing 2, 6, 12, and 18 items.

21

TABLE

Mean number of correct responses (maximum 50) for the

combination of high-similarity and low-similarity conditions with the

number of display items.

Number of Disp Jay I terns High-Similarity Low-Simi Jarity

2 49.50 50.00

6 35.50 47.33

12 15.67 41.50

18 11 . 17 34.00

Observations of Figure 5 showed that accuracy declined as

the number of items contained in the displays increased, and that while

this occurred in both high-similarity and low··similarity conditions, it

was substantially greater in the former. These observations were

supported by a three-way analysis of variance carried out on the data.

The analysis (see Appendix 1, Tab Je 2) showed that both the rna in effects

of similarity between the target and the irrelevant items 8 and number of

displ9y items were significant (F(1,20) = 266.9, P < 0.01; and F(2,20) =

81.02, P < 0.01 respectively), and that the two-way interaction between

similarity and number "'as significant (F(2,20) = 11.89, P < 0.01). The

three-way interaction between similarity, number, and the order of

B. The high-similarity set of irrelevant items contained the letter E, and since F and E share common features (Solman, 1975), it was possible that the significant effect of similarity was caused by confusion between these two letters. To examine this possibility the contribution of E was removed (this was accomplished by considering as correct all responses whereForE were recorded (Appendix 1, Table 3) and the data reanalysed (Appendix 1, Table 4). The results confirmed the effect of similarity on accuracy (F(1,4) = 16.09, P < ~.05), and its interaction with the number of display items (F(2,8) = 6.48, P < 0.05). Therefore, the similarity effects in the main data (Table 1, and Figure 5) cannot be attributed solely to the subjects confusing F and E.

23

presentation of the high-similarity and low-similarity conditions also

reached significance (F(2,20) = 4.63, P < 0.05).

2.4 Discussion

The significant decrease In accuracy caused by the Increase

in physical similarity between the target and the irrelevant items, is

consistent with the data obtained by Estes (1972), and Mcintyre, et aZ.

(1970). The decline in accuracy with increasing numbers of display items

is consistent with the results of a number of reaction time studies (e.g.,

Atkinson, Holmgren, and Juola, 1969: Estes and Wessel, 1966; Holmgren,

1970), but it contradicts the findings of Egeth, Jonides, and Viall (1972).

Using circular stimulus displays containing 1 to 6 items, Egeth, et al.

found that the number of items did not influence reaction time in a number

of search tasks, and they considered their resu 1 ts to be l nd i cat l ve of

l·nformation processing along independent parallel channels. Ho.vever, when

the number of display items··was varied beb1een subject groups, reaction

times were significantly shorter for l-item displays than for 6-item

displays. This latter finding is not consistent with the independent-

parallel-channels interpretation, and suggests that subjects may have

adopted different strategies9 in the two situations and/or reaction time

measures, by allowing optional and unnecessary extra processing, may have

been ureliable as estimates of essential processing time. The substantially

greater performance decrement caused by increasing numbers of items in the

present high-similarity condition when compared with the low-similarity

condition (supported by the significant two-way interaction between

similarity and number), is consistent "lith the results obtained by Estes,

and its implications for models of information processing will be discussed

later.

9. The results of investigations by Neisser (1974) and Yonas and Pittenger (1973), suggest that the selection mechanism is sufficiently flexible for this to be a likely explanation.

24

The significant three-way interaction between similarity,

number and the order of presentation of the two similarity conditions,

reflected an Inconsistency in the performance of one group of subjects,

i.e., one subject group performed at a consistently higher level than the

other when searching low-similarity displays, but this superiority was not

maintained when searching 12-item and 18-item, high-similarity displays.

Therefore, it is of no obvious theoretical importance.

2.4.1 Parallel-serial processing

Any study of selection in vision needs to include some

mention of the mode of processing issue. But, as is forcefully pointed

out by Townsend (1972), the parallel-serial question is a complicated

one, and the sophistication of many of the theoretical models has

temporarily outstripped the ability of investigators to distinguish

between them. (" •.• despite its theoretical importance ( .•.• ) the

parallel-serial question should be ruled as unresolved in most of the

contexts in which it has been studied." Townsend, 1972, p.198). Under

these circumstances, there can be no suggestion that the present data are

unable to be explained by some parallel or serial information processing

models. However, they do present problems for relat·ively simple,

strictly serial and strictly parallel types.

For example:

(a) A serial mode of processing can account for the high-

similarity curve (see Figure 5) if the consequences of a brief

(100 msec) masked exposure are considered. Under these

conditions the mater.ial in the display and its "image", is not

av.ailable for sufficient time for subjects to process all items

from any but the 2-item displays. That is, in general the

the number of display items will exceed the average number

~rocessed. The most obvious prediction from a serial model,

25

is that once the number of display items exceed the

average number which can be processed in the time

available, any further increase should either have no

influence on this average value or slightly depress it

(if larger numbers introduce some early processing

difficulty). Estimates of the average number of items

processed from displays containing 6, 12, and 18 items

(see Appendix 1, Table 5 for individual data), show that

the numbers of items processed are 4.26, 3.76 and 3.69

respectively, and the differences between them are not

significant. On the other hand, the corresponding

estimates for the low-similarity curve show that the

average number of items processed is dependent on the

number of items present in the display, and not on the

processing time available. That is, the estimated

numbers are 5.68, 9.96, and 12.24 respectively and the

differences between them are significant. This result

would not be predicted by a serial model where item

processing times are equal, independent, and not

influenced by display density.

(b) A limited capacity parallel processing model with a

roughly equal distribution of capacity per item, can

account for the low-similarity curve but not for the high-

similarity curve. If there is a limit to the amount of

processing capacity available, then an increase in the

number of items to be processed requires a reduction in

the capacity allotted to each item. Since there is only

limited processing time, a reduction in capacity per

item should cause a roughly proportional decrease in

performance. A decrease of this kind can be seen in the

26

low-similarity condition, where the addition of 6 items

from 6 to 12 gave rise to a 12% drop in performance and

the addition of a further 6 items from 12 to 18 dropped

performance by 15%. The similar decrease in the high-

similarity case is not proportional (40% and 9%

respectively). In a simi Jar manner a model of

independent parallel channels with approximately six

avai !able channels (Egeth, et al. 1972) can only account

for the ]o>;;-similarity data. These results are explained

by assuming that the processing time is sufficient to

partially process 18 items,i .e., the six channels are

twice "refi !led", and a progressive decay of information

causes an increase in the proportion of errors with each

refill. To account for the high-similarity data, it is

necessary to assume that the increase in the d iff i cu 1 ty

of the discriminations increases processing time

sufficiently to prevent the channels being refilled.

While this is possible, the necessary threefold increase

l s not consistent l•li th a notion of independent channe 1 s,

The present data allows us to reject these simplified

models. There are, however, a variety of more sophisticated models based

on this parallel-serial distinction which might account for the results.

For example, a combination of parallel and serial processes would account

for the data by assuming that the mode of processing depends on the

difficulty of the selection task (i.e., a parallel mode when the target

and the irrelevant items are of different shape, and a serial mode when

they are similar). We cannot reject this on the basis of the present data.

27

2 .IJ. 2 A two-stage mode 1

As discussed in the introduction to the experiments,

Neisser considers that human information is characterised by a two-stage

hierarchy. The first stage or the period of general diffuse analysis

processes the entire parallel input in a fast, crude, and wholistic

fashion, and, as might be expected, is prone to error. In contrast, the

second stage or the period of detailed concentrated analysis processes

input "items" in a sequential, deliberate, attentive, and detailed manner.

And, this two-stage model of analysis provides a plausible explanation of

the interaction between the variable of similarity and the variable of

number.

Thus, the increase in the similarity between the target

and the irrelevant items, and the Increase in the number of irrelevant

items can be seen as changing the complexity of the task. That Is, the

feature processing carried out during first-stage analysis is more likely

to result in an error (i.e., result in the selection of an irrelevant

item) when, in the high-similarity condition, the irrelevant Items share

many features with the target. Also, as the number of Items is increased '

(independent of their feature relationship with the target), the mass of

feature material to be processed increases, and this will increase the

chances of an error during selection simply because there are likely to

be more items identified as having target-like features. These errors,

made during the early stage of processing, result in the detailed analysis

of an irrelevant item, and under conditions of lengthy or non-masked

exposures, they can be compensated for by the processing of more items.

However, in the case of the 100 msec masked exposure-time repeated

detailed processing is unlikely, and an increase in errors during the

first stage will cause a decline in performance. Hence, the interaction

28

between similarity and number, is due to the addition of the same numbers

of items in the high-similarity and low-similarity conditions, causing a

much greater decline in accuracy in the former. This is not surprising

since the addition of physically similar items introduces many target

features, and makes the task of selecting the target item, on the basis of

its features, very difficult.

2.5 Further investigation

At least two aspects of this study suggest avenues for

further investigation. That is:

(a) When constructing the stimulus displays, the dlstaoce between

adjacent items increased (see Figures 1, 2, 3, & 4). Since there is

reason to suspect that "traversing" empty space may require time (see the

torch-beam analogy detailed by Eriksen and Hoffman 1972b), in

experimental situations where time is limited, performance may be

influenced by the distance separating Items. While it is unlikely that

an effect of distance would alter the obtained pattern of results (see

Table I and Figure 5) in a manner invalidating the interpretations

detailed above, an examination of the issue is appropriate.

(b) The data obtained were sampled at a constant, arbitrarily selected,

stimulus exposure-time of 100 msec. A more comprehensive sampling

technique, exploring relationships between accuracy and stimulus exposure-

time, might throw more light on the nature of that early processing which

enables the selection of the target. Specifically, the experimental

findings of this study constitute a cross-section through a number of

accuracy/time curves (i.e., variation in exposure-time generates a curve

for each stimulus type). The obtained differences are, therefore,

differences in height at 100 msec, and they could represent many different

things, e.g., they may represent differences in slope or differences in . .

29

intercept.

30

3. E:

items share few physical features with the targets, then it should be a

relatively sim~le task to separate the ta~gets from the irrelevant items.

Once the targets are isolated, and if we accept that further processing

is carried out in a serial fashion (Eriksen and Colegate, 1971; Neisser,

1967), one target Y~i 11 be processed befor-e the other. If we consider

processino capacity to be analagous to a beam of light from a torch

(Eriksen and Hoffwan, 1972b), then it must be "moved" separately from

one target to the other, and if a brief masked stimulus exposure limits

the availability of the material, the total number of targets processed

should decli~e with increasing distance between them. The s;:>ecific aim

of the study to be reported was to examine this prediction.

3.2 Method

The task vms to search for both of two instances of a

single target letter, and an experiment

32

and the distance separating the targets (0.67, 1.27, 1.80, 2.20, 2.43,

and 2.53 degrees of visual angle) were varied within subjects.

Subjects were 4 undergraduates (2 males, and 2 females)

with normal 6/6 vision, who received $1 per hour as payment for taking

part in the experiment. Two Ss were given the exposures in ascending

order and the other 2 were given them in descending order. All Ss

completed 2 practice and 9 experimental sessions each of duration

approximately 1 hour. At the completion of each trial they recorded the

positions of the targets, indicated which was the first to be located or

if they were both located at the same time, and rated each on the four

point confidence scale (4 = sure, 3; almost sure, 2 =might have been,

1 =guessing). A target was c::>rrectly located if the correct position or

one either side was marked.

3.2.2 Materials and appar~tus

The apparatus was that used in Experiment 1. The letter

F was selected as the target and G, 0, Q, U and W were selected as the

irrelevant item set. The display items appeared at 12 clock positions

on th~ imaginary circle used in Experiment 1, and there were 36, 12-item

stimulus displays constructed each containing two F's. That is, given

the restriction that over the 36 displays each of the 12 positions was

equally likely to contain an F, a random selection of 6 of the 12

possible target arrangemen~s was made for each separation (see Figure 6).

3.2.3 Procedure

Upon arrival at the first practice session Ss were told

that the task v1as a visual search, that the target \·lotJld appear twice in

all displays, that the cross ¥!as to be fixated before the initiation cf

33

G Q

"" Q 0 w

0 0 F Q

u u w u

G f G u w

Q f F G

0

w Q G 0 u F F Q w u

w f G

G u 0 Q u 0 0 f w Q

FIG. 6: Examples of stimulus displays containing tvlo icer.tical targets.

the display, that the confidence rat! ngs were to refer to target

presence, and that gu"'sses were to be spread over the avai !able

positions. They we,re also informed that a response, including a

confidence rating, should be made to all targets. They were then

familiarised with the stimulus displays and given 54 practice trL;ls (18

at each exposure). Stimulus exposures of 40, 80 and 160 msec were given

in ascending or descending order during practice, and the order presented

to each subject was maintained throughout the study. The same procedure

11as followed for the second practice session using the experimental

stimulus exposures (20, 40 and 80 msec).

After completion of practice Ss attended 9 experimental

sessions. During ti1e first 8 they completed 6 practice (2 at each

exposure) and 108 experimental trials. The experimental trials were

administered in 3 blocks of 36 tria is, i.e., une block of trials for

each of the 3 exposure-times. During sesolon 9 only 36 experimental

trials were completed (12 at each exposunc.), and this made up a total o"

50 responses for each experimental condition.

3.3 Results

The results in Table 2 and Fi'gure 7 (see Appendix 2,

Table 6 for individual data) showed that the distance separating the

targets did not influence the number ocrrectly reported. A three-Nay

analysis of variance (see Appenrlix 2, Table 7), with the order of

presenting the !itimulus exposures as a between subjects effect, and

stimulus exposure and distance separating the t1·10 identical targets as

within subjects effects, supported this observation. That is,the distance

separating the targets was not a significant variable (F(5, 10) = 2.35,

P > 0.05). On the other hand, the stimulus exposure-time and interaction

35

of this variable with the distance separating the targets, both reached

significance (F(2,4) = 40.72, P < 0.01; and.F(10,20) = 6.49, P < 0.001

resrect i ve ly).

TABLE 2

Hean·number of correctly located targets for the three

experinental stimulus exposure-times and the six distances separating

the two targets.

Stimulus Exposure in msec.

Separation (Visual 20 40 80 Angle Deg reEos)

0.67 62.00 70.25 76.50

1.27 54.75 75.00 80.50

1. 80 56.50 73.00 74.25

2.20 54.25 64.25 75.50

2.43 51.00 74.50 74.50

2.53 56.75 72.25 76.00

The significant improvement in performance with

increasing stimulus exposure-ti!Tle (F(2,4) = 40.72, P < 0.01) was of no

direct concern. That is, it was not surprising that increasing the time

12 avai ]able for processing the input improved performance. However, an

explanation of the significant interaction between target separation and

stimulus exposure (F(10,20) = 6.49, P < 0.001) is not obvious from the

data. Observation of Figure 7 shows that performance at a seraration of

0.67° for 20 msec and at 2.20° for 40 msec, does not conform to the

general pattern of re3ults. The data obtained by Eriksen and Hoffman

12. l t should be noted that later studies (in particular Experiment 3) show that increases in stimulus exposure-time are not necessarily related to improvements in performance.

100

80

L!J

~ 40 '~ .... z L!J 0 a:: w £).. 20

stimVJius exposure (rn sec:)

80 O.,,.,..D

1·00 2·00 3·00 TARGET SEPARATION (deg, of visual angle)

FIG. 7: The percentage of correctly located targets for the three exposL!re-times and the six distances separating the targets.

(1972b) suggest that it is not possible for the focal mechanism to

selectively allocate capacity, if items zre sepa1·ated by less than 1n of

visual

4. EXPERIMENT 3

4.1 Introduction

In Experiment 1 the a':curacy of target selection, under

varying conditions of context similarity and number of context items,

was examined at a single arbitrarily selected stimulus exposure-time of

100 msec. It was pointed out that these effects can only be fully

interpreted if we kno\1 the form of the accuracy/time functions under each

condition, and as e consequence a sampling of behaviour over a range of

exposure-time is needed. It was the aim of this study to carry out just

such a samp 1 i ng, but first the .use of accuracy measures rather tha~

reaction times requires some justification.

Previous investigators of selection in vision have

usual!)' manipulated the number of display iter;-,s, 1·1ithout considering

relationships such as the physical similarity betv1een the target and the

irrelevant items, and they have relied to a large extent upon reaction

time as a measure of processing time (e.g., Atkinson, Holmgren, and

Juola, 1969: Estes and Wessel, 1966; Sternberg, 1967). Reaction time

has proved a powerful tool fer investigating information processing

mechanisms, but it suffers from a major disadvantage. When reaction

time is used subjects are given unlimited time and ambisuous instructions

{Edwards, 1961), being requested to perform the task both as quickly and

as accurately as possible. As a consequence they have considerable

freedom to choose a processing strategy and in particular might be

expected to carry out optional processing (usually in the interest of

accuracy) beyond the minimum required for successful completion of the

task. It is obvious that these are not optimal conditions for

demonstrating the essential stages in processing. For example, the

model of human information processing 11hich suggests that early

39

processing is carried out on the entire input in parallel (independent

of the nature of the further processing undergone by selected portions

of this input), might imply a high probability of error during the

early parallel stage. In particular, as we have seen, Neisser (1967)

suggested that first-stage processing is prone to error. Yet observed

error rates in reaction time experiments are low. However, given

unlimited processing time the absence of errors is not fatal to the

theory since subjects are given the opportunity to make further

selections when later processing reveals that the Initial selection was

incorrect.

The considerations above suggest that in general,

reaction time studies should be supplemented by accuracy studies, and

that a specific comparison of the two measures would be of interest.

Such a comparison was made in the foll~•ing study. Accuracy and reaction

time measures were compared for a visual search task using stirrulus

displays containing twelve items or six items, and two levels of

physical similarity between the target and the irrelevant item;'.

Subjects were required to call out the position of the target as quickly

as possible, and by exposing the stimuli for pre-set, brief, masked

periods, it was possible to obtain relationships betv1een accurcy (at

positioning the target) and exposure, and reaction time and exposure.

It was prod! cted that at the short exposures subjects accuracy would be

limited by the accuracy of the early stages of processing, and, in

particular their performance would reflect the accuracy of target

selection by these stages.

4.2 Method

The task was visual search, and an experimental trial was

essentially the same as described for Experiment 1. When S pressed the

switch a display and a timer were initiated. The display remained

visible for a prespeclfied duration, and its termination was followed by

a 50 msec mask. S was asked to call out the position of the target as

quickly as possible, and his voice stopped the timer allm·ling for the

collection of both accuracy and reaction time measures. Confidence

ratings ware also collected, and a response was considered correct if

the target position or one either side was reported.

4.2. 1 Design

Accuracy and reaction tlme measures were collected for

displays containing 12 items or 6 items, at exposures of 180, 100, 75,

55, 45, 35, and 20 msec when target and irrelevant items were simi Jar in

shape, and 100, 75, 50, 35, 25, 20, and 15 msec when they were

dissimi Jar. Taking the common exposures of 100, 75, .35, and 20 ~sec

enabled the treatment of the data as a three-way design, with similarity

varied between subjects, and number of items and stimulus exposure-time

varied vlithin subjects.

Subjects were 8 undergraduates (6 males and 2 females)

with normal 6/6 vision, and each received $1 per hour for taking part

in the study. All Ss searched for 9, 1-hour sessions (3 practice and 6

experimental), with 4 searching high-similarity and 4 searching 10\v-

similarity dispi

41


The materials and apparatus were as for Experiment 1

with the following exceptions: a) the target was again F, but E was

excluded from the high-similarity set of irrelevant items (leaving T, L,

J, P, and H), and C was excluded from the low-similarity set (leaving G,

D, Q, U, and W); b) vocal reaction times were collected by means of a

voice key, connected via a modular digital system to a Fluke Counter-

Timer; and c) stimulus items were placed at 12 equally spaced loci

corresponding to the 12 clock positions. When positioned the minimum

distance separating any two letters was 0.67° and 1.27° on 12-item and

6-item displays respectively.

When constructing stimulus displays the target was

positioned equally often at each clod< position, and 24 cards v1core

constructed for each combination of simililrity and number giving a total

of 96 cards (see Figure B).

4.2.3 Procedure


upon his arrival at the first practice session. During this explanation

it was emphasised that the position of the target should be called out

as quickly as possible, that the confidence ratings were to refer to

target presence not to the accuracy with which it could be positioned,

and that guessing responses should be evenly distributed. He was then

given 112 practice trials with the similarity level relevant to his

group, at exposures of 400, 300, 200, 150, 125. 100 and 50 msec. The

presentation order for the 12-item and 6-item displays was 12, 6, 12, 6,

or 6, 12, 6, 12 (2 Ss in each grcup received one of these orderings),

F p

L p

p H T

l T

p J H

J H T

J

Q u G u F G Q

w 0 G

F 0

w G w Q u

FIG. 8: Examples of high-similarity and ]a,,-sir,>ilarity stimulus displays containing 12 items or 6 items.

l

F

0

4J

and was held constant throughout the study. The cards were presented in

I; batches of 28 with 4 trials at each of the 7 stimulus exposure-times.

The second practice session followed the same procedure, and the third

'las used to obtain an estimate of the exposure required to achieve an

accuracy of 50%. These estimates served as guidelines for deciding on

the experimental exposure-times.

During the experimental sessions 1, 2, 4, and 5 Ss

completed 14 practice and 112 experimental trials, and during sessiors 3,

and 6 they completed 14 practice and 126 experimental trials. These

trials were administered in the manner of practice sessions a11d 2.

The experimental stimulus exposure-times were used and they provided a

total of 50 responses for each relevant experimental condition.

4. 3 Results

As mentioned in Experiment 1, the experimental procedure

raised the question of what was to constitute a correct response, and to

make a decision the distributions of responses around the target

position were examined. The response frequencies at distances of 2, 3,

13 4, s,· and 6 positions away from the target appeared to be constant (see Appendix 3 Table 8), and smaller than those at a distance of

position away. Consequently responses which indicated the target

position or one position either side were treated as correct, and the

13 To decide if the frequencies at distances not less than 2 posi t.ions away from the target were reflecting a fairly stable chance level of performance, linear regressions were calculated for each of the 28 conditions. Seventeen of the estimates of slope obtained from these regressions failed to reach significance (Appendix 3, Table 8). This supported the observation that errors in positioning the target were (in the maio) confined to one position either side of the correct one, and suggested that guessing responses had been evenly distributed.

scores obtained in this manner were corrected for chance 14 (uncorrected

data is shown in Appendix 3, Table 9).

The mean number of correct responses, and the mean vocal

reaction times for non-guessing correct responses, are shown in Table 3

(see Appendix 3, Tables 10, and 11 for individual data). In order to

assess the effects of the experimental variables, three-way analyses of

variance v1ere ccrried out on these data (the levels of exposure-time were

those common to both similarity levels, i.e., 100, 75, 35 and 20 msec).

The accuracy data revealed significant effects for similarity, number,

and exposure-time (F(1,6) = 11.71, P < 0.05; F(1,42) = 91.81, P < 0.01;

and F(3,ll2) = 176.99, P < 0.01 respectively), and significant interactions

of similarity by exposure-time,number by exposure-time, and similarity

by number by exposure-time (F(3,42) ~ 4.36, r < 0.01; F(3,42) = 4.32,

P < 0.01, and F(3,42) • 3.45, P < 0.05 respectively) 15 (see Appendix 3,

Table 12 for summary of analysis of variance). The reaction time data

revealed significant effects for number, and for exposure-time (F(1,6) =

7-57, P < 0.05; and F(,3,18 = 7.14, P < 0.01 respectively) (see Appendix

3, Table 13 for summary of analysis of variance).

14 Contributing to both correct and incorrect responses

45

TABLE 3

The average number of correct responses and the average

reaction time for correct non-guessing responses, for the high-similarity

and low-similarity conditions and the number of display items, as a

function of stimulus exposure-time.

12 ITEMS 6 I TEf1S Stimulus Reaction Reaction Exposure Number Time Number Time

(msec) Correct (msec) Correct (msec)

180 27.7 1039 38.3 1045 HIGH-

SIMILARITY 100 25.1 11 71 37.2 1010

75 22.0 991 34.0 866

55 20.3 12.46 30.7 963

45 21.3 112 7 26.3 1080

35 9.7 116 7 25.7 998

20 4.3 1456 3.7 1165

100 43.3 861 47.3 697

75 37.0 888 46.7 663 LOW-

SIMILARITY 50 39.7 942 45.0 663

35 27.9 960 112.0 791

25 21.0 1049 30.7 830

20 5.7 1012 16.0 937

15 0.0 1282 3.0 1043

Since the significant interactions revealed by the acct1racy

data merely reflected the general convergence of the four accuracy/

exposure-time curves (see Figure 9) as performance approached chance

level for brief exposures, they were of relatively minor theoretical

interest. Consequently, the main distinction between accuracy and

reaction ti~e measures revealed by the analyses, was the failure of

similarity to have a significant influence on the latter (F(1,6) = 2.43;

P > 0.10). The failure of this difference to reach significance was

particularly surprising in the light of previous findings (Estes, 1972;

Mcintyre, Fox, and Neale, 1970; tleisser, 1967), and hence to draw any

conclusions based on it would be hazardous. However, consideration of

the complete accuracy/exposure and reaction timefexposure relationships,

suggested that accuracy provided a better measure of the time required

for processing, particularly at very brief exposures. That is, fO!-

decreasing exposures of less than 50 msec accuracy decreased very

rapidly, whereas reaction time increased. There is nothing particularly

unusual about increasing reactior. times under the5e experimental

conditions (Teichner and Krebs, l97lJ), but it is difficult to interprzt

them as demonstrating a steady increase in processing time accompanying .a

steady decrease in stimulus exposure-time. More specifically, &nd

assuming that the mask terminated processing, the increasing reaction

times suggest that these measures can contain components thct do not

reflect processing time.

100

80

60

1-(.) w 0.:: 0.:: 0 40 (J

w C)

48

4.4 Discussion

The discussion will be concerned with interpreting the

experimental results demonstrated by the accuracy/exposure curves in

Figure 9. In particular, why does each of the curves display the marked

deceleration so that In each case improvement in performance beyond the

50 • • d • ff • 1 d 16 d I msec po1nt IS 1 -leU t to etect, an , now may differences in the

level of these "plateaus" be explained. However, it should first be

noted that the data of Experiment i sectioned these curves at the 100

msec point, and therefore, represent differences in the plateau levels.

4. 4.1 A two-stage model

The four accuracy/exposure curves (Figure 9) can be

explained by Neisser's (1967) tvJO-stage model. We need only suppose

that within the experimental range of exposure-time there is on the

average insufficient time for further selective processing If the first

item constructed Is not the target. Then If there is only sufficient

time for the construction of one item, the accuracy/time curves represent

fairly directly, the relationships between the.probablllty of a correct

selection on the first item processed and stimulus exposure-time. This

allows a straight-forward explanation (along the same 1 ines as for

Experiment 1) of the experimental findings. The significant effects of

increasing, a) the number of irrelevant items, and b) the similarity

between the target and these items, can be easily explained in terms of

16 Observations of accuracy/time curves in the present study (Figure 9) suggests a very sla• Improvement in performance for ex~osure-tlmes greater than 50 msec. However, separate analyses (see Appendix 3, Table 15) carried on the hlgh··simllarlty data at 55, 75, 100, and 180 msec, and on the la1-s lmi lari ty data at 50, 75, and 100 msec failed.to reveal any significant effect of exposure (F(3,6) = 4.07, P > 0.05 and F(2,4) = 4.72, P > 0.05 respectively).

increased difficulty at the feature processing stage .of selection. When

the number of stimulus Items Is irrcreased the task of selecting the

correct item (on the basis of its features) becomes more difficult, i.e.,

the probability of the selected item being the target decreases, or

conversely, the chances of an irrelevant item being incorrectly selected

are increased. This is because early processing is elementary and prone

to error, and as a consequence an incrense in the number of items

processed increases the amount of feature material and the 1 ikelihood that

an incorrect item will be selected. Changes in the similarity between the

target and the irrelevant items also influences the accuracy of the

feature processing stage of selection, with errors increasing as irrelevant

items become more like the target since it becomes more,difficult to select

the correct i tern on the basis of its features.

The shapes cf the curves, with the rapid improvement in

performance prior to 50 msec and post-50 msec stability, can be accounted

for by considering the processing tires required by the two stages of

analysis. The period up to ~nd including the rapid initial improvements

covers the times necessary to carry out an initial selection, i.e., ir-. the

main the processing required is coro;lleted in less than 50 msec. Selected

' items are constructed from the icon one after the other (Eriksen and

Co legate, 1971; Neisser, 1967), and since it might be expected that the

times necessary for this detal led analysis are at least as long as those

required for icon information, very few I terns can be constructed in less

than 200 rnsec. Therefore, the level of performance reached when

information is only available for a brief period, is largely dependent on

the probability of correct initial selection from the icon, and will shO'.v

very little improve.ment after the icon is complete •.

50

!;,4.2 Parallel-serial ~rocessing

A mode of processing interpretation of the accuracy/time

curves faces the problem discussed in Experiment 1. Namely, it was not

the intention of the study to distinguish between parallel and serial

models, and the data certainly are not capable of doing so. But, even

in the light of this difficulty strictly parallel and strictly serial

models do not account easily for the data. That is, as available

processing time increases, in the absence, of memory overload, the most

obvious prediction from these models would be an improvement in

performance, and yet observed performance fails to show significant

improvement after 50 msec. It may be possible to develop il parallel-

serial account of the data. In the present context, h01•ever, such an

account is relevant only if it ,can explain (along with an account of the

plateau effect) the differences between curves 1-1i thout asst•>ni ng that

they reflect different rates of error during selection. The author is

not familiar with a model of this kind, but it is possible th~t the

differences between levels may be due to mlsperceptions of irrelevant

items as targets or viee ver-sa. (The question of incorract selection or

mlsperception is discussed further in Experiment 6.)

4.4.3 Response speeding

When instructing the experimental subjects a speeded

response was requested. It is therefore possible that the fai iure of

performance to Improve sign! ficantly beyond 50 msec, IYas due to subjects

adopting a strategy which traded accuracy for· speed, i.e., as the

exposure-time Increased, instead of \'Jaitin~ for the additional lnform'!ltion

they hurried their response and accuracy suffered accordingly. However,

this does not affc:ct the previously detailed interpretation of the

differences in plateau level as reflecting the probability of correct

51

initial selection.

4.4.4 Masking

As previously mentio~ed (Footnote 7), whether a backward

masking procedure prevents further processing of the stimulus input due

to a perceptual integration of the contours of stimulus and mask, or

because the arrival of the mask interrupts the processing of the stimulus,

remains an issL•e. But, if interruption is accepted as the cause, then

the visual system must register stimulus and mask as separate events, and

in general this does not occur at very brief separations. Therefore,

masking in the present study probably occurred because it was impossible

to extricate the contours of the stimulus from the mask at very brief

separations, but was caused by the interruption of processing as the

separation increased (see Turvey, 1973). This could account for t'Je

first leg of the curves by Interpreting the period of rapid Improvement

as that required for the separation of stimulus and mask. But, it cioes

not explain the plateau effect which can still be interpreted In terms

of a tv10-s tage mode 1.

4.4.5 Memory span limitations

Superficially similar negatively accelerated curves were

obtained by Sperling (1963, Figure 5), and he attributed the failure of

performance to improve after an exposure of 50 msec to l imitations in an

immediate-visual-memory. Sperling used a recognition task which required

subjects to report as many items as possible, and vs a consequence, memory

span was exceeded after 4 or 5 items had been p1·ocessed. However,

performance in the present study would not be restricted by a limited

memory span since subjects are not required to remember irrelevant ite~s.

52

4.5 Further investigation

There are two aspects of the accuracy/time curves

(Figure 9) which allow for further investigation. Firstly, the failL1re

to detect a significant improvement in performance between exposure-times

of approximately. 50 and 180 msec, suggests than an investigation of the

forms of the curves beyond 200 msec may be of interest, and, secondly,

if the period during \vhich rapid improvement in performance occurs, is

in fact determined by the time necessary to complete selection from the

icon, then it v1ould be of some interest to know if these times are

independent of the difficulty involved in distinguishing the target from

the irrelevent items.

53

5. EXPERIMENT 4

5. 1 Introduction

FolJm,ling the suggestions for further investigation made

in the discussion of Experiment 3, the aim of this study \vas to extend the

accuracy/time curves (Figure 9) by sampling search oehaviour over a range

of 100 to 500 msec.

The previously obtained relationships between accuracy of

target selection and stimulus exposure-time (Experiment 3, Figure 9),

shmved that as exposure-time increased performance irP.proved rapidly at

first, but then stabilised. It was argued that Neisser's (1967) tHo-stage

model of analysis gave a plausible account of these results, i.e., rapid

improvement occ,urred when sufficient time VIaS provided to enable the

comp 1 et ion of firs t•·s tilge precessing, and performance stab i l i sed wh i l e a

selected item underwent detailed analysis. This model 1vas used by

Neisser to explain the data obtained when subjects searched for targets

embedded in lists (see the previous discussion of visual search on

pages 3 to 6 ), and it was assumed that first-stage" processing •1as

repeated until the target was found. The extension of exposure-time

beyond 200 msec was intended to bridge the gap ben,een the time-] imi ted

(brief exposures) and list conditions, and it was hoped that

commencement of a second processing cycle •1ould be demonstrated as a rapid

Improvement in performance after the previously obtained period of

stab i 1 i ty.

5.2 Method

The task was again visual search and an experimental trial

was the same as previously described in Experi~ent 3 apart from the

fo 1 lowing: a) no reaction time measures were co I lected and Ss responses

were not speeded, b) subjects .vere not required to give confidence

ratings, and c) for stimulus exposure-times greater than 180 rr1ay des lgn.

Subjects were 8 undergraduates (6 rna les and 2 females)

with normal 6!6 vision. They undertook the study partly on a voluntary

basis and partly for course credit. All Ss searched for 7, 1-hour

sessions (1 practice 18 and 6 experimental), with 4 searching high-similarity

displays and 4 searching low-similarity displays.

17 Maintaining the physical presence of the stimulus for a maximum of 180 msec, ensured that the input into the processing system .vas from a single eye fixation.

18 \-lhen carrying out this study (and .vhen running Experiment 5) funds were not avai ]able for the paynent of subjects. Consequently, they were somewhat reluctant volunteers and practice, unfortunately, .vas more limited than .vas considered desirable.

55


The materials and apparatus (minus a counter-timer) were

essentially the same as those used in Experiment 3.

5.2.3 Procedure


upon his arrival at the practice session. He was told to mark the

location of the target on the sheet provided and to distribute his guesses

over the avai ]able positions, and he was then given 112 practice trials

with the similarity level relevant to his group. The practice trials

were administered at the experimental exposure-times with 7 trials at each

(i.e., 56 trials for both 12-item and 6-item displays). The presentation

order for the 12-item and 6-item displays was 12, 6 or 6, 12 (2 Ss in

each group received one of these orderings), and was held constant

throughout the study.

During experimental sessions i, 2, 3, and 4 Ss

completed 16 practice and 128 experimental trials, and during sessions 5

and 6

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