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Visual Perception, Awareness and Self-Control:

the Brentano-Müller-Lyer illusion.C. Giovannella

ISIM_lab - ScuolaIADUniversity of Rome Tor Vergata

via della ricerca scientifica , 1; I - 00133 Romeinfo@mifav.uniroma2.it

G. GalliISIM_lab

University of Rome Tor Vergatavia della ricerca scientifica , 1; I - 00133 Rome

ABSTRACTIn this paper we present a study on visual perception,awareness and self-control, based on the visual explorationof the classical Brentano-Müller Lyer illusion and of someof its variants. The study has been realized with the help ofself-made software designed to analyze eye-trackerrecordings. In the case of subjects that have a certainfamiliarity with visual illusions, although not with all itsvariants, it is possible to observe, after some hundreds ofmilliseconds, the occurrence of a transition toward an awareexploration of the images. Moreover, it has been possible toshow that the awareness might or might not help - itdepends on the subject - to get out from the "trap" of theillusion.

Author KeywordsPerception of illusions, visual awareness, Brentano-MüllerLyer illusion, human factors, eye-tracking, humancommunication, visual communication channel.

ACM Classification KeywordsH.1.2 [Human information processing]

INTRODUCTIONOne of the best ways to study visual perception, awarenessand self-control, which are crucial in the human-computerinteraction that uses the visual channel, is to investigate theeffects induced in the subject by visual illusions. Among allthe possible visual illusions we chose the Müller Lyerillusion [1] because it is one of the most famous and moststudied visual illusions. In its simplest and well knownvariant it reveals itself when a subject is asked to indicatethe center of an arrow and s/he place the perceived center ina position that is closer to the tail than to the tip. In avariant, due to Brentano [2,3], the subject is asked to

compare two straight parallel segments terminated,respectively, by two tails and two tips of an arrow. Due tothe different termination the two segments are perceived tohave different lengths, see Fig . 1. It is an illusion whoseexplanation is still open to discussion. According to some itmay partly depend on cultural factors (a more or less visualfamiliarity with edged structures and landscapes). For atime the most accepted explanation has been that ofGregory [4] that ascribed the Brentano Müller-Lyer (BML)illusion to the different perspective perception that one hasof the two lines. According to this explanation the lineterminated with the tips might be perceived as the outeredge of a box, and thus, in a location closer than the otherwhich is perceived as the inner edge of a solid positionedfarther away (in fact, two objects appear to have the sameprojection only if the farthest object is larger/higher). Thehypothesis of Gregory, however, was questioned by somevariants of the BML illusion (see Fig. 2) in which thetermination of the segments allow to exclude anyrelationship between the perception of the illusion and the3D nature of the shapes. The illusion, moreover, remainseven when the segments are elicited and only the terminalornaments remain visible [5].

In the past, have been proposed also other explanations thatthen, over time seem to have lost strength. These include: a)the "eye-movement theory" [6] claiming that the illusionarises from the fact that the extent of eye movements can beinfluenced by the graphic arrangement (the ornaments inthe case of the BML illusion); this hypothesis, however,was abandoned following the demonstration that theillusion is seen even in the absence of a large number of eyemovements, or when the image is shown for a fraction ofsecond [7]; b) the "assimilation theory" [8] according towhich the visual system is unable to distinguish thecomponents of a figure, so that, in the case of the BMLillusion, the segment would appear longer or shorterdepending on the termination used (tails/tips); thisinterpretation is contradicted by 'existence of illusionsbased on so-called "size contrast effect" in which oneelement merged in a frame composed of elements of largersize appears smaller than when it is merged in a framecomposed of elements of smaller size; c) the probabilistictheory of perception [9,10] based on statistical relationships

Presented at International IUI 2010 Workshop on Eye Gaze in IntelligentHuman Machine Interaction.

between retinal images and inferences about visual stimulifrom reality (in particular segments of which you want topredict the correct length).

In this framework not yet fully defined, we decided toinvestigate in detail, using an eye-tracker, the visualstrategies used to explore the classical BML illusion and itsvariants, first with subjects unaware of the illusory contentof the images and than with aware subjects. From ourobservations we have drawn a framework that, as discussedbelow: i) highlights the relevance of subjective and contextcharacteristics; ii) shows the existence of a transition to aconscious exploration of the image within a few hundredmilliseconds.

EXPERIMENTAL "SETTING"The experimental sessions was planned and carried out intwo successive stages.

During the first stage we asked 5 subjects - 2 males and 3females aged between 16 and 40 years - not experts ofvisual communication or of visual illusions, to observefreely for a maximum of 30 seconds the image shown infig. 1. The image was presented to them in inverted colors(white on black), not to tire the eye, as a part of a sequenceof images containing some other types of illusions, shuffledin a way, not provide any suggestion about the imminentexposure to the BML illusion.

Fig. 1 - The classical illusion of Brentano Müller-Lyer

In a second moment, due also to the inconclusive resultsobtained during the first experimental session with the eye-tracker, we made a further experiment with 3 subjects - 2males and one female aged between 20 and 28 years -having a good knowledge of issues related to visualperception and, also, a certain familiarity with the classicformulation of BML illusion. To these subjects we havebeen shown, still in inverted colors, also modified versionsof the BML illusion (see Figure 2) that they did not known:the "rocker" variant where tips and tails of arrows werereplaced by circles and the "cluster" variant in which thelines were elicited.

These two variants, plus the classic illusion, were includedwithin a sequence of images containing other types of

illusions. The classic version was shown last, in order not toinduce cognitive biases in the subjects. The task proposedto the subjects was to explore freely all the pictures for amaximum of 30 s.

Fig. 2 - Variants of the Brentano Müller-Lyer's illusion

Fig. 3 - Experimental settings: the investigator occupies theposition in the foreground

The presentation of the sequence of images was precededby a 9-point eye-tracker calibration procedure and wasfollowed by a 6-point calibration check, which was alsorepeated at the end of the sequence. Before and after thepresentation of each image the subject was asked to observea calibration point presented in non-predictable locations,located on an imaginary image frame. Calibration checksshowed that the spatial resolution and stability of the eye-tracker are about 35 pixels on a screen with 1024 x 768 (17") placed approximately 40 cm from subject's eyes.

During all the eye-tracking session we used a headrest (seefig. 3).

The recordings were made with a "home-made" eye-tracker, outcome of a collaboration developed with theUniversity of Sao Paulo. The Brazilian partners made thehardware and software for gaze tracking, while at theISIM_garage of Tor Vergata we developed the software fordata analysis. This latter, among other features, allows usto: a) superimpose the observed saccades and fixations tothe image; b) adjust the threshold of the eye movement overwhich it must be considered a saccade; c) select the lower

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and upper limits of the time window of observation; d) playthe "animation" of the eye movements and manually, like aslow motion, advance forward and backward the movieframe by frame; e) see the statistics on the spatialdistribution of the gaze, either by overlapping the imagewith a translucent layer of increasing opacity as function ofthe time spent in the area, or by means of a black filter thatbecome more and more transparent as a function of the gazedensity; to calculate the spatial distribution of the gaze thescreen has been divided in square tiles whose size can befreely adjusted by the experimenter.

Fig. 4 - A screenshot of the software used to analyze the datarecorded by the eye-tracker. Bottom left: one of the control

panels.

RESULTS

Classical BML illusion: subjects unaware of the illusion

Fig. 5 - Examples of visual exploration of the classical illusionof Brentano Müller-Lyer performed by subjects (NE1, NE2,

NE3) unaware of the illusory image (respectively A, B, C) andby subject E2 (D) (see text)

The visual exploration strategies used by the five subjectsnot aware about of the illusory content of the image arevery similar, some examples are shown in fig.1 A,B and C.They all started with exploring the graphical structure on

the left, starting mostly from the top, and after a thoroughvertical exploration of the structure switched on thestructure on the left to carry out a similar exploration. Thephysical borders of these vertical explorations weredetermined by the centers of gravity of the closingornaments (arrows' tails or tips). There is no evidence ofany strategies for visual comparison, except for somelocalized fixations at the center of the two graphicstructures, which were found during visual explorationperformed by the subject NE3 (after 16 s, time window notshown in fig.1) and by the subject NE4 (after 5 s). The factthat such fixations may have been determined bycomparative intentions, however, is to be demonstrated.

Classical BML illusion: subjects aware of the illusionThe second experiment involved 3 subjects - E1, E2 andE3 - with good knowledge of the problems related to thevisual perception. Let's start by discussing how they haveperformed the visual exploration of the classical BMLillusion, keeping in mind that, as anticipated in the previousparagraph, such version of the illusion was the last one thathas been shown to the subjects. The behaviors observedwere very different among them and also different from thatobserved in the case of the unaware subjects.

The gaze of subject E1 was originally positioned on theright side of the graphical structure on the left for less than200 ms, then dwelt briefly on top of such structure, about 1s, and then started to compare the two structures byjumping from tails to tips, alternately at the top and thebottom of the structures. Although the subject knew theimage, he performed the visual comparison using thegraphic centroid of tips and tails of the arrows (see fig 6).

Fig. 6 - Early stage of the visual exploration of the classicalBrentano Müller-Lyer illusion performed by the subject E1

(see text)

Subject E2 resulted completely indifferent to the illusorycontent of the image. S/he conducted alternately scans ofboth graphic structures, but without any apparentcorrelation; each local exploration lasted for between 1.8and 2.5 s (see fig. 5).

Fig. 7 - Visual exploration of the classical Brentano Müller-Lyer illusion performed by the subject E3: (A) initial stage,

(B) later stage, after 4s (see text)

Subject E3, after an initial shift to the right of the graphicalstructure on the left, carried out a detailed visualexploration of both graphic structures along the first 4 s,and then started a rapid sequence of comparisons betweenthe upper and lower extremes of the two segments. Unlikesubject E1, during her/his investigation subject E3 tried tomaintain the centroid of the saccadic movements on theextremes of the two vertical segments.

The above observations indicate that the visual perceptionof what is contained in the image, including recognition ofillusion, occurs in a very short time, at most of the order offew hundred of milliseconds. After such initial stage of thevisual exploration the subjects start to follow a "voluntary"and "informed" path of exploration the characteristics ofwhich depend on the individual: we go from a visualexploration that pay attention only to the graphics (subjectE2), to the immediate comparison of the elements detectedduring the visual exploration performed by subject E1, tothe conscious comparison that subject E3 startedimmediately after an equally careful observation of thegraphic structures. It is worthwhile to stress that while inthe case of the subject E3 - that we could identified as themost thoughtful and controlled one - we observed a mentaleffort to force comparison between the extremes of thesegments (illusion unveiled), in the case of subject E1 theattractiveness of tips and tails is much stronger that the willof the subject to indicate a persistence of the illusoryperception. This observation, if from one side does notimply the need to invoke the "eye-movement theory" toexplain the origin of the perception of the illusion, on theother side shows that the attraction of the graphic features isreal and can provide guidance on the perceptual state of thesubjects with respect to the illusion.

BML illusion: "rocker" variantThe visual exploration operated by the subject E2 did notshow any relationship to the illusory content of the image,s/he performed a visual scan of the graphic structures verysimilar to that observed during a text reading: from left to

right, than a wrap, followed by a restart of exploration fromthe left.

Subject E1 in the first 850 ms carried out a comparativeexploration of the two graphic structures, holding the gazeslightly below the upper structure or slightly above thelower structure, then start a comparison between theextremes of the two lines, first at left then at right of the twographic structures, each lasting about 1.5 s; from time totime positioned the gaze at the center of the two "rockers".The comparison was made between the centers of thecircles (i.e. between the centers of gravity of the graphicterminals, as in the classical BML illusion). Such method ofexploration seems to show that the subject has understoodthe meaning of the image, although s/he needs to obtainvisual confirmations (positioning at the center of therockers).

The subject E3 made a very fast initial exploration of eachof the two structures, 700-800 ms each, starting from thecenters of the figure. Then, started a more detailedexploration lasting about 9-10 seconds. Only then initiateda tight visual comparison between the extremes of the twofigures. In the first phase of this stage of the visualexploration the centers of attraction seems to have been,like for subject E1, the centers of the circles, while towardsthe end the gaze tended more and more to focus on theborders of the segments and the saccadic jumps between thetwo structures taken a direction perpendicular to them. E3has proven once again to be a person who, having becomeaware of the illusory sense of the image, is able toovercome the illusion and effectively control the gazemovements.

In general, the experimental observations described abovesuggest that faced to an image with a not known illusorycontent, although one may know the generating schema, thetime required to transit from a "mechanical" perception to aconscious perception may be in the range of severalhundreds of milliseconds, while to get full control of thesituation may take a few seconds.

Fig. 6 - Early stage of the visual exploration of the rockervariant of the Brentano Müller-Lyer illusion performed by the

subject E1 (see text)

BML illusion with segment elicitation: "cluster" variantAs in previous cases, the subject E2 paid attention only tothe graphical structures and did not reveal any visualexploration strategy aimed at achieving any sort of

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comparison. Throughout the whole duration of the test s/hemoved back and forth from one structure to the next alongloops, first clockwise and then counterclockwise.

Fig. 7 - Comparison between the early stage of the visualexploration of the cluster variant of the Brentano Müller-Lyer

illusion performed by E1 (column A) and E3 (column O)

Subject E2 (see fig. 7 column A) in the first 700 msexplored the set of graphic elements placed in the top left ofthe image, in the following 1.5 s the group placed at thebottom left, then 850 ms were used to explore the groupplaced in the bottom right and, finally, for a couple ofseconds explored the last group placed at the top right. Theexploration, then, continued along horizontal and parallellines. In the case of this variant of the BML illusion, subjectE1 appeared disoriented and seemed not to have understoodthe illusory content of the image and its orientation (notethat in the previous cases s/he had almost instantly started avisual comparison between the two graphic components ofthe image).

The subject E3, faced with an image of which did notimmediately perceived the sense, positioned the gazeapproximately at the center of the four groups of elementsfor about 850 ms, after s/he made a rapid exploration of thetwo upper groups (first on the right and then on the left) andfinally after more than 2 s started to focus the gaze on theonly filled element of the clusters of circles. In thefollowing 3 seconds s/he tried to understand the relationshipbetween the two groups of elements placed at the top of theimage and only after 7.5 s s/he started to compare the twoelicited segments, (see the third frame of fig. 7, column O).

These observations bring into clear evidence, especiallywhen compared with the previous ones, the differencebetween the initial perception of an illusory image (which,as we have seen, is confined to the first few hundredmilliseconds), the conscious perception of the illusion and,

finally, the capacity to control and address the eyemovements.

DISCUSSION AND CONCLUSIONSThis work shows that by experimenting with subjectsfamiliar with perception and visual illusion issues it ispossible - by comparing visual reactions to known illusoryimages and to their unknown variants - to highlight theexistence of a transition from an "unconscious" visualexploration to a conscious exploration of the illusory image.The duration of the first exploratory phase does not exceeda few hundred milliseconds, and seems to vary according tothe familiarity one has with the image shown. Theoccurrence of an aware explorative phase clearly showshow it is in the very first moments that the subject has the"physiological" perception of the illusion and proceeds toits possible classification within already known visual andcognitive patterns. These results seem consistent with thehypothesis that identifies the reasons for the originalperception of the illusion in the probabilistic theory ofperception. Without such a perceptual stage, in fact, wouldnot be possible to make the transition to a fully consciousexploration of the image, which itself is characterized bythe development of highly subjective strategies. It is duringthe visual exploration of the already known images that thisbehavior is more emphasized. In the other cases, discussedin the previous paragraphs, it is well possible that, despitehaving full knowledge of the illusion, one is captured by theattraction of the graphic effects - as for unaware subjects.This is, probably, an indication of the difficulty to "watch"an image beyond its illusory content, so that, in practice, weremain prisoners of the latter.

The transition from an "unconscious" visual exploration toa conscious one was observed also when the subjects wereexposed to known illusions. The time of this transition,appears to vary depending both on the degree of recognitionof the illusion and on the characteristics of the subject. Theacquisition of a full awareness of the illusory content of theimage changes depending on the image and subject, to thepoint that in some cases, in the time allowed forobservation, it is not achieved. The latter situation is similarto that observed in the case of subjects not familiar withvisual perception issues. With these subjects, in fact, doesnot seem possible to carry out studies on the transition froman "unconscious" visual exploration to a conscious one oron the acquisition of the full control on the illusion. All theunaware subjects that took part in the experiment not onlyseems to remain "prisoner" of the graphic geometry of theillusion, but it seems not unrealistic to conclude that they donot ask themselves questions about the meaning of theimages.

Although the study presented here has been performed on alimited number of subjects, the results seem to suggestinteresting lines of development concerning the humancommunication that uses the visual channel - such as, forexample, the prevalence of one effect among multiple

illusory effects, the perceptions induced by very fast orillusory advertising, the identification of the most effectivestrategy to educate the "gaze".

CONCLUSIONThis work shows that by experimenting with subjectsfamiliar with perception and visual illusion issues it ispossible - by comparing visual reactions to known illusoryimages and to their unknown variants - to highlight theexistence of a transition from an "unconscious" visualexploration to a conscious exploration of the illusory image.The duration of the first exploratory phase does not exceeda few hundred milliseconds, and seems to vary according tothe familiarity one has with the image shown. Theoccurrence of an aware explorative phase clearly showshow it is in the very first moments that the subject has the"physiological" perception of the illusion and proceeds toits possible classification within already known visual andcognitive patterns. These results seem consistent with thehypothesis that identifies the reasons for the originalperception of the illusion in the probabilistic theory ofperception. Without such a perceptual stage, in fact, wouldnot be possible to make the transition to a fully consciousexploration of the image, which itself is characterized bythe development of highly subjective strategies. It is duringthe visual exploration of the already known images that thisbehavior is more emphasized. In the other cases, discussedin the previous paragraphs, it is well possible that, despitehaving full knowledge of the illusion, one is captured by theattraction of the graphic effects - as for unaware subjects.This is, probably, an indication of the difficulty to "watch"an image beyond its illusory content, so that, in practice, weremain prisoners of the latter.

The transition from an "unconscious" visual exploration toa conscious one was observed also when the subjects wereexposed to known illusions. The time of this transition,appears to vary depending both on the degree of recognitionof the illusion and on the characteristics of the subject. Theacquisition of a full awareness of the illusory content of theimage changes depending on the image and subject, to thepoint that in some cases, in the time allowed forobservation, it is not achieved. The latter situation is similarto that observed in the case of subjects not familiar withvisual perception issues. With these subjects, in fact, doesnot seem possible to carry out studies on the transition froman "unconscious" visual exploration to a conscious one oron the acquisition of the full control on the illusion. All theunaware subjects that took part in the experiment not onlyseems to remain "prisoner" of the graphic geometry of theillusion, but it seems not unrealistic to conclude that they do

not ask themselves questions about the meaning of theimages.

Although the study presented here has been performed on alimited number of subjects, the results seem to suggestinteresting lines of development concerning the humancommunication that uses the visual channel - such as, forexample, the prevalence of one effect among multipleillusory effects, the perceptions induced by very fast orillusory advertising, the relationship between perception-and actuaction in very demanding environments (like flightsimulations, video games, etc..), the identification of themost effective strategy to educate the "gaze".

ACKNOWLEDGMENTSWe are particularly grateful to Prof. Carlos HitoshiMorimoto and Prof. Ronald Ranvaud for the fruitfulcollaboration that has allowed us to use the eye-trackerdeveloped at USP. We are also grateful to Ronald for theendless and interesting discussions

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