Psychoacoustic tests for the study of human echolocation ability

ELSEVIER

Applied Acou.stics, Vol. 51, No. 4, pp. 399419, 1997 0 1997 Elsevier Science Ltd. All rights reserved

Printed in Great Britain PII: SOOOJ-682X(97)00010-8 0003-682X/97 $17.00+0.00

Psychoacoustic Tests for the Study of Human Echolocation Ability

Claudia Ariasa*b and Oscar A. RamoP

“Acoustical Research Center (CIAL), University of C6rdoba (UNC), Agencia Postal No. 4, Cuidad Universitaria, 5016 Ckdoba, Argentina

bResearch Center of the Faculty of Philosophy (CIFFyH), UNC, Argentina

(Received 10 December 1995; revised version received 28 December 1996; accepted 7 January 1997)

ABSTRACT

Four psychoacoustic tests were designed in order to study the human echolocation ability, i.e. the process of auditive perception of obstacles without the use of vision. The tests measure the subjects’ performance in repetition pitch detection and discrimination tasks - which are supposed to be involved in the short distance echolocation modality - using an echolocation paradigm as sound stimulus. The results, based on an experience, carried out with 30 sighted subjects with and without musical training and with one blind person, indicate that musical training did not seem to aflect the subjects’ performance in the tests. The subjects’ performance in the matching repetition pitch test seemed to indicate that they do indeed perceive a repetition pitch when they are stimulated with echolocating stimuli. Also, noise signals seemed to give better information than click signals in all the tests. The performance of the one blind subject in the matching repetition pitch test was, as we anticipated, the most relevant result. 0 1997 Elsevier Science Ltd.

Keywords: Human echolocation, obstacle perception, repetition pitch perception.

INTRODUCTION

The ability to perceive and avoid obstacles without the use of vision, as can be observed in some blind people, is a multivariate process that depends on

399

400 Claudia Arias, Oscar A. Ramos

both objective factors, related to the acoustical properties of the sound signal, the obstacle and the environment, and subjective ones, perceptual, cognitive, psychomotor abilities and personality variables. It is genuine but unexploited ability, called human echolocation, also known as ‘facial vision’, that can be observed in any person with normal audition in at least one ear.

Echolocation - that is, an active mode of perception - is defined as the ability to detect, discriminate and localize obstacles by processing the acoustic information contained in echoes produced by the reflexion of self- generated sounds on the surrounding obstacles. In a more general sense, this term is also used when electronics or environmental sounds are utilized. The self-generated sound is called the direct or emitted signal and the echo the reflected signal in an echolocation paradigm.

Visually handicapped people spontaneously and intuitively generate sounds as tongue clicks, snaps, hissings or vocalizations when they moved around in order to gather spatial information; thus they become active information processors. For reasons yet unknown, they have different capacities for using echolocation. Some of them are very proficient while, unfortunately, others seem not to be able to learn it.

Previous research

The research work in the human echolocation field is scarce and non-sys- tematic. The scientific experimentation in this field dates back to the early 1900s. However, it was not until the 40s that Dallenbach and co-workers at Cornell University - one of whom, Michel Supa,’ was blind - could make clear some important aspects: the audition is the seasonal basis of the echolocation and a pitch change in sounds is both necessary and sufficient condition.* The first link between human and animal echolocation, a research field where important advances have been made, also dates back to the ~OS.~,~

Subsequently, Kellogg, 5 Kohler6 and Rice7 studied the discriminatory power of this ability. They observed that both blind and sighted subjects could make precise judgements of distance, size and material and could even discriminate among obstacles of the same surface but different shapes. It could also be demonstrated that the sensitivity to detect small changes in a sound stimulus (in frequency and intensity) plays a role more important than do the absolute hearing thresholds in the echolocation ability.6

Practically no systemic research was reported until the 80~~3~ and, very recently, two related papers have been published. One is a very interesting theoretical paper that analyses the echolocation phenomenon from a psycho- echological perspective lo where the authors suggested that previous work on human echolocation needs to be replicated using perception-action paradigms.

Psychoacoustic tests for the study of human echolocation ability 401

The other paper” reported a relationship between specific auditory abilities and echolocation performance using nine blindfold sighted subjects. The authors developed an audiometric test battery in order to predict success in echolocation.

In the obstacle detection experiments reported in the scientific literature, in general, the subject had to perform a Yes/No task whereas, in the discrimination experiments, she/he had to decide which of two targets was present or which of two targets was further with respect to the subject or to another reference target. In the localization experiments, the researcher, for example, situated a target at some point in the horizontal plane within 90” to the left or the right of the subject. In some experiments, subjects could make any sound at will and scanning movements (moving the head from side to side) were allowed while, in others, subjects used artificial sounds and scanning movements were prevented. In all cases, the task was to point to the place that target was believed to be.S7

It is assumed that there are two basic modalities of human echolocation, based on different underlying psychoacoustic mechanisms. The first mechanism is involved in the detection of an obstacle, whereas the second one is involved in the detection, the localization and the discrimination of several of its physical characteristics.8-*2

At distances of more than 2-3 m and up to 5 m, the subject hears the direct or emitted signal clearly separated from the reflected signal. At distances less than 2 or 3 m between subject and obstacle, the person does not perceive two separate signals. Under certain conditions, the auditory system fuses both sounds onto a single stimulus and processes this combined signal which is perceived as a pitch shift. If the emitted signal is noise, the fused sound acquires a pitch-like quality.

Repetition pitch

Thurlow et ~1.‘~ were the first to describe the pitch perception phenomenon that occurs when a person listens to two identical pulse trains, one of which was delayed briefly with respect to the other. They observed that the frequency perceived was equal to the reciprocal of the smallest time separation, r, between leading edges of proximal pulses of the two trains. Thus, the pitch was equal to l/t and it was most easily noticed if the time separation between the pulses was continuously varied.

This phenomenon has been studied only in sighted subjects and is based on principles similar to those of periodicity pitch, residue pitch14 or virtual pitch. I5 The effect has been described in the literature under different names: reflexion tone, time separation pitch, time difference tone, rippled noise pitch and repetition pitch. Bilsen et al. described it in a more general sense, as the


phenomenon that occurs when a sound and the repetition (replica) of this sound are added after a brief delay and presented to a listener.i6 The former corresponds to the direct signal and the latter to the reflected one, in an ideal echolocation paradigm.

In the repetition pitch experiments, in general, the subject listened to pair of stimuli and judged which was higher in pitch or whether the pitch shifted up or down; some experiments used a pitch matching task.

Welchi was the first who pointed out the apparent paradox involved in the short echolocation mechanism due to the masking effect and the refrac- tory period of the neurons. He suggested that the ‘Thurlow pitch’ could explain this phenomenon, Furthermore, he designed a series of auditive tests to study several factors affecting echolocation ability in order to build an echolocating electronic aid for blind persons.

Bassett et a1.i8 described the pitch perception phenomenon that occurs when a person approaches a reflecting surface in the presence of certain sounds and related this to the ability to perceive and avoid obstacles that some blind people possess. They observed that the pitch varies inversely with distance and that the interference between the direct and the reflected signal produces a vibratory pattern in which some frequencies are cancelled and some others are augmented. They established the relation between the distance of the subject to the object d; the perceived frequency f and the sound velocity c, with the formula: f = c/2d.

McClellan et ~1.‘~ pointed out the similarity between both phenomena and established that f = c/2d = 117.

Purpose of the study

We are involved in a long-term project on the human echolocation process and its underlying mechanisms, whose main aims are to evaluate, predict and train the echolocation ability of the visually handicapped person. In order to achieve these purposes, we developed a PC based system named Rousettus, the only echolocating Megachiropteran; the other bat species of this order use vision for orientation.

Rousettus consists of three modules and one tool, the Obstacle Simulator that enables it to emulate obstacles, ‘phantom obstacles’, by controlling the echo structure, At present, we are planning to include principles of the so- called auditory virtual reality *‘, that will enable one to develop the spatial localization phase in the horizontal as well as in the vertical plane. The ECOTEST module allows one to analyse the psychoacoustic aspects of echolocation through the construction and administration of specially designed psychoacoustic tests, and the Evoked Potential module will enable one to obtain neural (objective) responses simultaneously with subjective


ones and so to study the neurological aspects of human echolocation ability. The tool enables one to create, to capture and to record the echolocation signals.

Once the Rousettus system is finished, it will be possible to develop an echolocation program, using virtual and real obstacles, to safely train the visually handicapped person to achieve independent mobility.

The present paper reports some exploratory investigations on the subject performance in psychoacoustic tests, specially designed to simulate acoustic conditions assumed to be involved in the short distance echolocation modality. The tests allow the analysis of blind and sighted subjects’ performance in repetition pitch detection and discrimination tasks with an echolocation paradigm as sound stimuli. Tests were constructed and administered with the ECOTEST,30 which is described in an accompanying Technical Note in this issue.

METHOD

Subjects

We tested two groups of 15 subjects each, with normal vision and audition, aged 18-25 years old, with (more than 12 years of formal training) and without musical training and with no training in the test tasks. One good independent traveller, without musical training, 50 years old, blind since his adolescence for a retinal injury caused by hereditary myophia, also comple- ted the test battery. His performance was not included in the statistical analysis and it is reported separately.

Our decision to work with sighted and naive subjects was based on two reasons: first, echolocation is a genuine human ability that can be observed in any person with normal audition, as mentioned above. The detection of an obstacle and the discrimination of its distance from an observer, i.e. the processes intended to be simulated by the repetition pitch tests administered by us, are relatively easy tasks that require practically no learning;tO,’ 1,21 second, to complete methodological adjustments without the need to recruit blind subjects. A musical training (ability) variable was included in the experimental design in order to check the hypothesis that a ‘privileged ear’ is not a necessary condition for echolocation.22-24

Signal description

Two types of signals were used, click and white noise, in order to mimic sound signals which could be generated by blind people when moving (tongue clicks and hissing, for example).


An echolocation paradigm was used for each type of signal, in which the direct signal was the output of a loudspeaker fed with a rectangular pulse of 4Ops, acquired and stored by the PC. The reflected signal was obtained in two ways: the so-called ‘artificial replica’ was a copied version of the direct signal attenuated by 3.5 dB. The ‘real replica’ was obtained by recording the whole event in a real situation, i.e. the true reflection produced in the presence of a real obstacle (a circular aluminum disc 0.50 m in diameter) (Fig. 1).

Two different delays between the direct and the reflected signal were used, 2 ms and 5 ms. The 2 ms delay generates the strongest repetition pitch according to Yost and Hi1125 and corresponds to 0.34m separation between the subject and the obstacle. This condition was called ‘RP72’, i.e. the direct signal is added to its 2 ms delayed replica. The 5 ms delay corresponds to 0.85m separation between the subject and the obstacle and generates the strongest repetition pitch according to Bilsen et aZ.26 This condition was called ‘RP75’, i.e. the direct signal is added to its 5ms delayed replica. The presentation of the direct signal alone was called ‘RPtO’, though this condition does not generate a repetition pitch (Fig. 2).

Procedure

In all the tests the same psychophysical method was used. The subject listened to pairs of successive stimuli (Tests 1A and 2A) or to a sequence of three successive stimuli (Tests 1B and 2B), that were diotically presented through headphones at about 60dB SPL. After each presentation, the subject had to respond according to the test instructions (Fig. 3).

The stimulus constructed with click signals was a periodic sequence of pulse pairs with pulse delay 7 of 2 ms or 5 ms; pulse-pair interval T = l/33 pps

Measurement Amplifier B&K 2607

Power Amplifier

Rousettus

Fig. 1. Recording procedure of echolocation signals.


and 200ms of total duration (D). Stimulus built with noise signals lasted 300 ms (D) and, in both cases, a silent period (S) of 200 ms separated the stimuli. In Fig. 4, an example of one stimulus condition is given.

The variables SIGNAL (click or noise), REPLICA (artificial replica or real replica) and RP CONDITION (for example, in Test IA: RPtO-RPrO or

_, i!

0 4.84 9.68 14.5 19.3 Miliseconds

I I -I0 4.84 9.68 14.5 19.3

Miliseconds

0.8 J I 0.6 ~......__.. ._ __.... 1’ ..___.. i . . ..__.__..____. (

-0.6 J I 0 4.84 9.68 14.5 19.3

Mdiseconds

-LJ

0 1.5 3.1 4.1

(a) Frequency (Khz)

-20 J 0 1.5 3.1 4.1

(b)

Frequency (Khz)

-I

01 I

L -20 0 I

I.5 3.1 4.7

Frequency (Khz)

Fig. 2. Temporal function and power spectrum of some of the direct and reflected signals used: (a) click, (b) click added to its 2 ms delayed replica, (c) white noise addded to its 5 ms

delayed replica.


RPrO-RPt2 or RPtO-RPtS) defined the different stimulus conditions and there were four repetitions for each one of them.

Psychoacoustic tests

Two phases of testing were conducted, one involving the detection of obstacles, designated Test 1, and one involving the discrimination of distance to an object, designated Test 2 (Fig. 5).

GENERA+bR __________ I I w COUNTER

I

I I

:AnPclnNES

Fig. 3. Experimental setup.

Stimulus A Stimulus B

L Direct Signal

- Reflected Signal y = Pulse Delay

T = Pulse pair interval: l/T = 33 pph,

D = Total duration of the stimulus

S = Silence period

Fig. 4. Schematic description of one stimulus condition of Test 2A


Test 1, the obstacle detection phase, included two blocks. Test 1A consisted of 48 trials (2 x 2 x 3 x 4: SIGNAL x REPLICA x RP CONDITION x Repeti- tion) presented at random with the subject judging whether two successively presented sound stimuli were equal or different in pitch (Fig. 5(a)). In this test, trials involving RPrO-RPt2 or RPrO-RPt5 conditions simulate the presence of an obstacle since the direct signal changes by the effect of its reflection on the obstacle. Trials involving the RPrO-RPrO condition simulate the absence of an obstacle since the direct signal reflect on any obstacle.

remains unchanged when it does not

a) < cliyk , ) ( , nois, 3 click b)< , ) c-?iti7-3 I I

artificial I_J replica

Fj IRP1STIPYxJ

repetlllons repetltlona repelllluns repellllona

Fig. 5. Tests structure: (a) Test 1A: 2x2~3~4 (SIGNALxREPLICAx RP CONDI- TIONSxRepetitions), (b)Test 1B: 2x1~2~4. 0.8ms<tX<6.5ms,(c)Test2A: 2x2~4~4, (d)

Test 2B: 2x2~4~4.


Test 1B was designed in order to acquire some knowledge about the effective pitch perceived by a subject when listened to echolocation stimuli. It consisted of 16 trials (2x1~2~4) presented at random (Fig. 5(b)). The subject was asked to match the repetition pitch of the test stimulus to the repetition pitch of the matching stimulus (RPt,) by adjusting the pulse interval t, of an adjustable delay pair pulse generator (Fig. 3). The subject could switch over between test and matching stimulus or he could simultaneously listen to both stimuli as long as he thought this was necessary for a good pitch matching. He could also adjust the sound intensity of the matching stimulus at will. After the setting was made, the experimenter read off the time t, on a digital counter.

Because of the repetition pitch, it is very difficult to hear when the pulse delay is constant; to accommodate this a ‘3 stimuli descending scale’ in third octave intervals, separated by 150ms of silence, was constructed. The first two stimuli were included to facilitate listening to the repetition pitch of the test stimulus (the third one of the sequence). For example, when the subject was presented with the test stimulus Click RPt2 (RP= 500 Hz) he heard pulse stimuli A, B and C, the test stimulus, each separated by 150 ms of silence. The pulse delay t of Stimulus A was chosen as equal to 1.25, thus the RP was equal to 800 Hz; t of Stimulus B = 1.6 ms, RP = 630 Hz and r of the test stimulus (C) = 2 ms, RP = 500 Hz. The subject listened to this ‘descending scale’, paid attention to the RP of the test stimulus and adjusted the time delay t, of the matching stimulus (RPt,) until both stimuli were matched in (RP) pitch.

Test 2 examined discrimination of the distance between the subject and the obstacle. Test 2A consisted of 64 trials (2x2~4~4) presented at random, in which the subject listened to two successive stimuli and judged whether the second stimulus had a lower or a higher pitch than the first stimulus (Fig. 5(c)).

As repetition pitch varies inversely with distance, the stimulus conditions simulate the presence of an obstacle closer to (a higher pitch) or farther from (a lower pitch) a reference position (the first stimulus).

Test 2B also contained 64 trials (2x2~4~4) presented at random (Fig. 5(d)). The subject listened to a sequence of three successive stimuli, and judged whether the third stimulus was equal to the first or the second stimulus of the series. This task was equivalent to judging the relative distance of an object in relation to another two (the first and the second stimuli), previously presented.

All the tests began with three examples and four practice trials (discarded from the statistical analysis). Each subject performed tests lA, lB, 2A and 2B in this same order, in a semianechoic chamber, during one session lasting, approximately, 90 minutes. All subjects were paid for their participation.

Test lA, 2A and 2B were evaluated assigning 1 point for each correct answer and 0 for each incorrect one. A partial score for each stimulus


condition was calculated and the total score for each test by each subject was obtained summing the partial scores. Since the data were analysed in per- cent-correct form, an arcsine transformation of the data was used to nor- malize the variance.27

Test 1B was evaluated by computing the consistent and the octave matchings made by each subject in each one of the 16 trials of this test. A consistent pitch match was defined whenever there were two or more pitch matches within a window defined by f 10% of l/t. 28 For example, if the delay time t was chosen as equal to 2ms, the RPt was equal to 500 Hz, then the subject matching values between 450 and 550Hz were evaluated as consistent matchings and those between 225 and 900Hz and 275 and 1100Hz were evaluated as octave matchings.

Statistical method

The variables under analysis were SIGNAL (click vs noise), REPLICA (artificial replica vs real replica), RP CONDITION (for example, in Test 1A: RPtO-RPtO vs RPtO-RPt2 vs RPrO-RPt5) and GROUP (with vs without musical training).

A descriptive data analysis (means, standard deviations, normal tests, box and normal plots and test for homogeneity of variance and residuals analysis) was carried out.

A bifactorial ANOVA of repeated measures, where the grouping factor was GROUP (with vs without musical training) and the repeated factor was TESTS (1A vs 2A vs 2B) was performed, in order to evaluate performance between and within the subject groups for the total scores obtained in the tests.

Four factor ANOVAs of repeated measures were also performed in order to evaluate performance between and within the subject groups for the scores obtained in each test separately (lA, 2A and 2B). The grouping factor was GROUP (with vs without musical training) and the repeated factors were SIGNAL (click vs noise), REPLICA (artificial replica vs real replica) and RI? CONDITION (In test 1A: RPtO-RPrO vs RPr&RPt2 vs RPt&RPtS. In test 2A: RPrO-RPt2 vs RPsO-RPr5 vs RPt2-RPr5 vs RPtS-RPr2. In test 2B: RPt2-RPtS-RPt2 vs RPt2-RPtS-RPt5 vs RPtS-RPt2-RPt2 vs RPtS- RPt2-RPt5). The GROUP variable was excluded from the design when no significant influence of this factor could be observed. The significant F values (p 5 0.05) were analysed with the post hoc test of Duncan.

In order to analyse the results of the pitch matching Test lB, only the subjects that made consistent or octave matchings were considered. The mean of the matchings values obtained by the subject for each test stimulus (four repetitions) and the average results from the two groups were calculated.


TABLE 1. Means standard deviations and standard errors of the scores obtained by the subjects as a

function of main factors and significant interaction effects @ 5 0.05)

Mean SD SE n

Tests 1 A, 2A and 2B Group

Trained Untrained

Test Al A2 B2

Group x Test Trained x Al Trained x A2 Trained x B2 Untrained x Al Untrained x A2 Untrained x B2

Teat 1A Signal

Click Noise

Replica Artificial Real

RP conditions l-O-r0 s(r72 7+75

Group Trained Untrained

Signal x Replica Click x Artificial Click x Real

Replica x RP Noise x Artificial Noise x Real Artificial x tO-t-0 Artificial x 2%~2 Artificial x rU-55 Real x z-O-50 Real x z-Q-s2 Real x rO-55

1.03 0.07 0.01 45 1.00 0.08 0.01 45

1.11 0.13 0.02 30 0.84 0.10 0.02 30 1.13 0.15 0.03 30

1.13 0.13 0.04 15 0.82 0.11 0.03 15 1.19 0.14 0.04 15 1.09 0.13 0.04 15 0.85 0.09 0.02 15 1.08 0.13 0.04 15

1.302 0.375 0.028 180 1.118 0 308 0.023 180

1.238 0.396 0.030 180 1.182 0.433 0.032 180

1.319 0.340 0.031 120 1.284 0.382 0035 120 1.027 0.455 0.042 120

1.230 0393 0.029 180 1.190 0.438 0.033 180

1.267 0.424 0.045 90 1.337 0.317 0.034 90

1.209 0.366 0.039 90 1.027 0.477 0.05 1 90 1.400 0.287 0.037 60 1.250 0.412 0.054 60 1.120 0.415 0.054 60 1.280 0.379 0.049 60 1.350 0.346 0.045 60 0.951 0.470 0.061 60

con timed

Psychoacoustic tests for the study of human echolocation ability

Table 1-contd

411

Test 2A Signal

Click Noise


RP conditions t0-r2 T&s5 T2--75 55-52


Signal x RP Click x 50-s2 Click x r&s5 Click x 522~5 Click x t5-s2 Noise x 70-52 Noise x 1f1-r5 Noise x ~2x5 Noise x 55-52

Replica x RP Artificial x So-52 Artificial x rO-~5 Artificial x ~2-75 Artificial x r5-s2 Real x ~0-52 Real x #r5 Real x r2-r5 Real x 55-r2

Teat 2B Signal

Click Noise


RP Conditions r2-s5-r2 s2-~5-75 t5-r2-r2 s5-r2-s5

0.744 0.519 0.048 240 0.961 0.510 0.062 240

0.746 0.528 0.048 240 0.959 0.501 0.062 240

0.834 0.517 0.076 120 0.718 0.484 0.066 120 1.139 0.450 0.104 120 0.720 0.538 0.066 120

0.832 0.562 0.054 240 0.874 0.486 0.057 240

0.685 0.489 0.089 60 0.725 0.485 0.094 60 1.115 0.472 0.145 60 0.453 0.407 0.059 60 0.983 0.506 0.128 60 0.711 0.486 0.093 60 1,162 0.429 0.151 60 0.988 0.522 0.129 60

0.608 0.446 0.079 60 0.522 0.448 0.068 60 1.234 0.386 0.161 60 0.620 0.503 0.081 60 1.060 0.487 0.138 60 0.914 0.439 0.119 60 1.043 0.490 0.136 60 0.821 0.566 0.107 60

1.149 0.367 0.074 240 1.296 0.338 0.084 240

1.239 0.348 0.080 240 1.206 0.372 0.078 240

1.208 0.364 0.111 120 1.301 0.307 0.199 120 1.242 0.330 0.114 120 1.137 0.415 0.104 120

continued


Table l-contd

Mean SD


Group x RP Trained x 52-s5-s2 Trained x s2-t5-s5 Trained x r5-r2-r2 Trained x s5-r2-r5 Untrained x r2-t5-r2 Untrained x r2-r.Sr5 Untrained x r5-t2-r2 Untrained x r5-t2-r5

Replica x RP Artificial x r2-r5-r2 Artificial x r2-r5-r5 Artificial x r5-r2-r2 Artificial x lr5-r2-r5 Real x r2-55-52 Real x r2-r5-r5 Real x r5-r2-r2 Real x rSr2-r5

1.286 0.330 1.159 0.378

1.327 0.309 1.320 0.283 1.248 0.333 1.247 0.387 1.089 0.379 1.282 0.330 1.236 0.330 1.028 0.416

1.159 0.405 1.324 0.288 1.231 0.344 1.240 0.332 1.257 0.314 1.278 0.324 1.253 0.319 1.035 0.464

SE n

0.083 240 0.075 240

0.173 60 0.172 60 0.182 60 0.162 60 0.142 60 0.167 60 0.161 60 0.134 60

0.151 60 0.172 60 0.160 60 0.161 60 0.164 60 0.166 60 0.163 60 0.135 60

RESULTS AND CONCLUSIONS

The data displayed acceptable agreement to normality, homogeneity of variances and independence of errors (see raw data in Table 1).

The analysis indicated a significant difference for the main factor TESTS, F(2.56) = 70.46, p =0 and for the interaction GROUP by TESTS, F(2.56)= 3.06, p=O.O55. The GROUP factor effect was manifested only in

T&S .-~3-- Trained .++Untrained

Fig. 6. Interaction effect for GROUPxTests, F(2,56) = 3.06, p = 0.055. Means of the total scores obtained as a function of GROUP and Tests; standard errors are displayed.


Test 2B (the subject had to decide whether the third stimulus was equal to the first or the second stimulus of the series) favouring the subjects with musical training (p 5 0.05). The 2A test (the subject had to decide if the second of two stimuli was higher or lower in pitch than the first) was the most difficult for both groups (p 5 0.05) (Fig. 6).

There was no significant difference between the performance of the two groups in Test 1A (“are two stimuli equal or different”). However, there were significant differences for the main factors SIGNAL, F(1.29)= 20.38, p=O; RP CONDlTION, F(2.58) = 17.34, p = 0 and for the interactions SIGNAL x REPLICA, F(1.29)= 12.96, p=O.OOl and REPLICAxRP CONDITION, F(2.58) = 6.12, p = 0.004. The REPLICA factor effect was only shown in the noise signal, favouring the noise added with its artificial replica (piO.01). Besides, the effect of the REPLICA variable was only manifested in the RPtO-RPt5 condition, favouring the artificial replica, too (p 1 0.05) (Fig. 7).

There was no significant difference, either, between the performance of the two groups in Test 2A (“is the second of two stimuli higher or lower in pitch than the first”). However, there were significant differences for the main factors SIGNAL, F(1.29) = 19.55, p = 0; REPLICA, F( 1.29) = 34.55, p = 0; RP CONDITION, F(3.87) = 18.77, p = 0, and for the interactions SIGNALx RP CONDITION, F(3.87) = 8.89, p = 0 and REPLICAxRP CONDITION, F(3.87) = 14.77, p = 0. Due to the first interaction, the effect of the SIGNAL factor was shown in the RPrS-RPt2 and RPr(rRPr2 conditions, favouring the noise signal (pLO.01, in both cases). The REPLICA effect factor, in the second interaction, was manifested in the RPtO-RPt2 and RPrO-RPr5 conditions, favouring the real replica (pLO.01 in both cases) (Fig. 8).

In the Test 2B (“is the third stimulus equal to the first or to the second stimulus of the series?“) there were significant differences for the main factors

Fig. 7. Test IA: (a) Interaction effect for SIGNALxREPLICA, F( 1,29) = 12.96, p = 0.001. (b) Interaction effect for REPLICAx RP CONDITIONS, F(2,58) = 6.12 p = 0.004; standard

errors are displayed.


0.2 1 : 0.2 0.5 2.5 5.2

RP CONDITIONS

0.6

0.4’ 0.2 0.5 2.5 5.2

RP CONDITIONS

‘m Arbl. .C>. Real

(b)

Fig. 8. Test 2A: (a) Interaction effect for SIGNALxREPLICA, F(3,87) = 12.96, p = 0. (b) Interaction effect for REPLICA x RP CONDITIONS, F( 3,87) = 6.12 p = 0; standard errors

are displayed.

I .4

1.35 .

1.3

g Y 1.25..

9 1.2..

f 1.15..

I.1

1.05’ Click Nmrc

SIGNAL

(4

I.6

I.5

I .4 2 8 I.3 m ; 1.2

z 1.1

0

0.9

0.8 2-i-2 2-i-5 5-1-2

RP CONDITIONS

~mTrained -GhUnh-ained

(b)

0.9’ : I i

2-5-2 2-s-5 5-2-2 s-2-5

RP CONDITIONS

-m Artificial ~Rral

(c)

s-2-5

Fig. 9. Test 2B: (a) Interaction effect for SIGNALx REPLICA, F( 1,28) = 25.19, p = 0. (b) Interaction effect for REPLICAxRP CONDITIONS, F(3,84) = 4.35 p = 0.007; standard

errors are displayed.


GROUP, F( 1.28) = 5.8 1, p = 0.023; SIGNAL, F( 1.28) = 25.19, p = 0, favouring noise signals; RP CONDITION, F(3.84)=4.63, p=O.O05 and for the interactions GROUPxRP CONDITION, F(3.84) = 3.74, ~~0.014 and REPLICAxRP CONDITION, F(3.84) = 4.35, p = 0.007. The post hoc tests indicated that only the scores obtained by subjects with musical training in the RPt2-RPtS-RPt2 condition were significantly superior to those obtained by subjects without musical training in the RPtS-RPt2-RPt5 condition (plO.05). Besides, only the scores of the RPt2-RPtS-RPr5 condition with artificial replica were significantly higher than that of the RPtS- RPt2-RPt5 condition with real replica (plO.05) (Fig. 9).

Figure 10 displays the average matching values for each test stimulus by the two groups as a function of the reciprocal of the delay (l/t). The data on the ordinate are the frequencies of the matching stimulus which the subjects matched in pitch to the test stimuli. The lines with the numbers above them indicate how many matches were obtained for that pitch and, in brackets, how many subjects did them. For instance, at l/t of 2OOHz in Fig. 10(b), 10 subjects with musical training made consistent matches 35 out of 40 times (mean = 201.4; SD = 4.9) and 6 subjects without musical training made consistent matches 14 out of 24 times (mean = 196.5; SD = 5.2). The matches made by the unique blind subject are also displayed.

The following can be observed: Stimulus 1, click RPt2 (l/t = 500Hz): No subject made consistent matches,

with the exception of one subject without musical training who made consistent matchings 2 out of 4 times. Eight subjects with musical training made inferior octave matches 27 out of 32 times. All the matches made by the blind subject were consistent matchings (mean = 533; SD = 19.07).

Click Noise 600> /

,’ -.

,,/ ,/ /

T xc ,/’ ,I

/’

h 300. B 5 200 2 ,M)_. ,,/”

,/

OY.. : 0 100 200 300 400 500 600 0 100 200 300 400 500 600

II T [Hz] I/r [Hz]

lf=J Trained .+- Untrained .-w Blind Subject ] [B Trained .6- Untrained .-~Blind Subject 1

(a) (b)

Fig. 10. Average matched pitch from the two groups as a function of the reciprocal of the delay (1 /T in hertz) for (a) RPr2 and RPr5 click stimuli and (b) RPr2 and RPT~ noise stimuli. The vertical bars represent the number of times the subjects matched that pitch to the test

stimulus; the number of subjects that made these matches is indicated in brackets.


Stimulus 2, click RP75 (l/t = 200Hz): No subjects made octave matchings. Six subjects with musical training made consistent matches 15 out of 32 times, whereas only 2 subjects without musical training made consistent matches 4 out of 8 times. The blind subject made consistent matchings 2 out of 4 times (mean = 192; SD = 1.41). The other two were failure matches.

Stimulus 3, noise RP72 (l/t = 500Hz): No subject made consistent matches. Six subjects with musical training made inferior octave matches 21 out of 24 times. Only one subject without musical training made inferior octave matches 3 out of 4 times. The blind subject made consistent matchings 2 out of 4 times (mean = 265; SD = 1.41). The other two were failure matches.

Stimulus 4, noise RP75 (1/7=200Hz): No subject made octave matches. Ten subjects with musical training made consistent matches 35 out of 40 times. This stimulus also produced the greatest number of consistent matches by subjects without musical training: 6 subjects made consistent matches 14 out of 24 times. All the matches made by the blind subject were consistent matchings (mean = 202; SD = 5.62).

In summary, it was possible to draw the following conclusions: (1) Musical training did not affect subjects’ performance in the RP tests

using echolocation signals as test stimuli, although musically trained subjects made more consistent and octave matches than non-musically trained subjects.

Accordingly, it was pointed out that the ability to detect and discriminate an obstacle is a genuine ability, that it does not require a “privileged ear”; rather, an enhancement sound information processing produced by the training effect seemed to be involved.22-24

(2) Test 2A (“is the second of two stimuli higher or lower in pitch than the first?“) and Test 1B (match repetition pitch) were the most difficult for both groups. The last result was expected since matching this kind of stimulus is difficult, and requires a large amount of training.

(3) The analysis of the pitch matching data showed that the subjects do indeed perceive the repetition pitch when they are stimulated with echolo- eating stimuli.

The subjects made more consistent matches-including those in the inferior octave-with noise than with click signals. Stimulus 4 (noise RP75) produced the greatest number of consistent matches for subjects in both groups. In addition, all the consistent matches were made with stimuli formed by adding click/noise to its 5 ms delayed replica (RP = 200 Hz), with the exception of two identical consistent matches made with stimulus two by one subject without musical training. These last two results are in accordance with those of Bilsen et a/.,26 who concluded that noise with its 5ms delayed repetition produces the optimally perceived repetition pitch. All octave matches were made in the inferior octave, with stimuli formed by click/noise added to its 2 ms delayed replica (RP = 500 Hz).


(4) With respect to the SIGNAL variable, noise seemed to give better information than click in all the tests. However, the effect of the REPLICA variable was task-dependent: when the subject had to judge whether two stimuli were the same or different and when he/she had to judge whether the third stimulus was equal to the first or to the second stimulus of a series (Tests 1A and 2B, respectively), the artificial replica seemed to give better information. When the subject had to identify the direction of pitch change (Test 2A), the real replica was the most useful.

The performance of the unique blind subject can be summarized as fol- lows:

He obtained the second highest score (96% hits) in Test 1A where the task was to judge whether two stimuli were equal or different, and he obtained a high score (89% hits) in Test 2B where the task was to decide whether the third stimulus was equal to the first or to the second stimulus of the series. However, he performed at chance (52% hits) in Test 2A where the task was to decide whether the second stimulus was higher or lower in pitch than the first.

His performance in the matching pitch 1B test is, we believe, the most relevant result (see Fig. 10). Here, he was unique among the subjects without musical training in making only four failures over the total of 16 trials: 10 responses were consistent matchings and the remaining 2 were inferior octave ones. Only 3 musically trained subjects made 14 hits on 16 trials but with a different response pattern: 6 or 7 of their responses were consistent matchings and 7 or 8 responses were inferior octaves.

It is worth remarking that, being the first time that he performed this task just like the rest of the subjects, he never showed an erratic or confused behaviour. On the contrary, he always turned the knob of the device in the correct zone direction and never to other zones. These results might point towards Terhardt’s modeli which assumed that virtual pitch, generated from a previous learning stage, is a product of the gestalt auditive perception process.

Our interdisciplinary efforts, at present, are mainly aimed at designing critical experiments and completing the Rousettus system that will allow us an integral approach, subjectively as much as objectively, to the human echolocation ability in a behavioral and cognitive context.

We evaluate the results obtained at present29 as promising, and we accept the challenge that implies the study of this worthy but scarcely studied human ability, highlighting the multidisciplinary richness of this scientific field.

We hope, in the near future, to lay down the theoretical and practical basis of a training program for the assessment and development of the echolocation ability of the visually handicapped person.


ACKNOWLEDGEMENTS

The authors would like to thank Carlos Frassoni and Aldo Ortiz Skarp for their valuable work in the signals acquisition and creation phase and for the arrangement of the experimental set up, Professor R. Guy for his helpful comments and his encouragements and any anonymous reviewer who helped us improve an earlier version of this manuscript. Thanks also to all the sighted subjects, and especially to the blind person, who participated in the experiments. The research on which this article is based has been supported by Pid Conicor No. 2715/93. Proyecto de Investigation y Desarrollo de1 Consejo de Investigaciones de Cordoba, Argentina.

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Psychoacoustic tests for the study of human echolocation ability

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