The Smallest Time Difference the Eyes can Detect with Sweeping Stimulation

The Smallest Time Difference the Eyes can Detect with Sweeping StimulationAuthor(s): Georg Von BekesySource: Proceedings of the National Academy of Sciences of the United States of America,Vol. 64, No. 1 (Sep. 15, 1969), pp. 142-147Published by: National Academy of SciencesStable URL: http://www.jstor.org/stable/59594 .

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THE SMA. LLEST TIME DIFFERENCE THE EYES CAN DETECT WI TH SWEEPING STIMI ULA TION *

BY GEORG VON BEKE'SY

LABOltATORlY OF SENSORY SCIENCES, UNIVERSITY OF HAW'AII, HONOLULU

Communicated July 3, 1969

Abstract.--Experiments in hearing showed that a time difference of 0.1 msec between the stimulation of the left and right ear by a similar click was enough to be recognized as a spatial shift of the integrated sound perception. Similar phenomena were obtained for taste stimulation on both sides of the tongue or with an odorous substance introduced to both nostrils. No similar phenomenon was obtained in binocular vision, when a short time difference between the stimulation of both eyes was introduced and the location of the fused image was ob- served. It was found that this peculiarity for vision could be eliminated when the stimuli for vision were made more identical to the stimuli used for the other sense organs, practically most of which were sNweeping stimuli. Sweeping light stimuli with time differences of 0.1 msec between both eyes could be detected as spatial shifts with the same precision as for the other senses. This wideins the expected similarities between the different sense organs.

The Smallest Time Difference the Sense Organs Can. Detect.--This paper describes one of a series of experiments' 2I which attempt to show- that there are quantita- tive similarities in the different sense organs.

It is well known that the ear can localize the direction of the sound source and its displacement in a horizontal plane with surprising accuracy. This holds for pure tones but especially for noise and clicks. It was the work of von Hornbostel and W ertheimer' which show-ed that for medium and low frequencies it is the time difference between the impact of the sound on both ears which determines the direction of the incoming sound. For high frequencies above 2000 cps it is mainly the loudness difference between both ears which influences the direction. sensation. Since the velocity of the sound is high, and change of the sound direc- tionI of a few degrees from the medium plane to one or- the other side can be detected, it became clear that time differences of the order of 0.1 msec betweenl both ears could be recognized.

Later, Katz4 demonstrated that two separate stimuli presented to a surface of the skin might produce a change in. the localization of the integrated stimulus if a time difference is present. Since it is known that the sense organs with large sur-- face area, like the ear, the retina, and the vibratory and pressure-sensation organs of the skin, all develop ontogemnetically from the same ectodermal tissue, the question comes up: Is the detection of the time difference betweerl tAwo similar stimuli of the order of a tenth of a millisecond a specific property of the ears or is it the general property of all the sense organls with large surface area?

It has been shown that simultaneous presentation. of stimuli to the surface of sense organs with large areas show similar properties in the localization of the sensationi., from sense organ to sense organ. Therefore, it was not surprising to

142



VOL. 64, 1969 PSYCHOLOGY: G. VON BEKESY 143

find, for taste and for smell, that when a small time difference is introduced between stimuli, a change in localization could be obtained when two similar stimuli are presented to the left side and the right side of the head.2 The shift of the localization always occurred in the direction of the stimulus which was presented first. As in directional hearing, there was no difficulty in showing that an odorous substance presented to both nostrils shifted away from the medium plane with a time difference of 0.1 msec between the stimuli presented to each nostril.

The Smallest Time Difference Which Can Be Detected in Vision.-As far back as 1864, Hering' had already developed a theory according to which a time difference between the two eyes would produce a change in the depth perception. Schon6 developed this theory further in assuming that depth perception is connected with the time difference between the arrival of the visual stimuli at the cortex. Bower' reported that two visual stimuli presented separately to both eyes with the time delay of between 1 and 3 msec can produce a change in depth perception under binocular fusion of the two light sources. Wist and Gogel8 found a time difference of about 30 msec was needed to produce a shift in depth perception.

In preliminary experiments we produced visual stimuli with two gas discharge tubes. They were fused together in binocular vision as one image, the depth of which (relative to a fixed target) could easily be varied by changing the lateral separation between the two gas tubes. A square pulse time pattern was used for each light stimulus varying from 1 to 200 msec in length. The luminance varied from 0.1 to 10 footlambert. The time difference between the two equal light pulses with constant luminance was varied from 0.1 to 30 msec. We did not succeed in producing a change in depth perception by varying the time difference between the two light stimuli. This seemed to indicate that the time difference similarities found for the different sense organs do not include vision.

When we compare the ear with the retina we have to realize that there is a definitive difference between a click and a light pulse. A light pulse represents a simultaneous stimulation of an area of the retina, but a click produces a traveling wave on the basilar membrane of the inner ear. This traveling wave is a sweeping mechanical stimulus along the cochlear partition. Therefore, a click does not produce, even at high frequencies, a simultaneous stimulation of an area in the cochlea. Since the traveling wave in the cochlea cannot be modified, the question arises: Can we bring the analogy between hearing and vision closer if we

apply for vision not a simultaneous but a sweeping stimulus similar to the stimu-

lus in the basilar membrane? By using such sweeping light stimuli, could it be

possible that the eyes might also detect small time differences?

The small time differences we obtained on the skin2 were also produced by

sweeping stimuli, since any vibratory stimulus on the surface of the skin produces

traveling waves which can be detected easily under stroboscopic observation. Even in the experiment on the stimulation of the tongue by salt solutions2 a

sweeping stimulus was employed, since the salt solution flowed along a tube with

an open side wall in contact with the surface of the tongue. In the investigation of smell, it seems reasonable to assume that the eddies in the nasal cavity, produced by the breathing in of air and the odorous substatnce, flow along the surface



144 PSYCHOLOGY: G. VON BEKESY PRoc. N. A. S.

of the mucosa linling and thus produce also a sweeping stimulus along the surface of the sense organ.

All this indicates that, in general, a sweeping stimulus occurs but not in vision. It is because of technical reasons that we have been experimenting with simultaneous stimulations of large areas of the retina.

Physiologically, a sweeping stimulus is different from a simultaneous one. One of the main reasons for this is that it continuously involves fresh receptors and, therefore, it produces a continuously new set of "on" effects.

Apparatus for the Observation of Binocular Time Differences wcith Sweeping Stimuli.-The apparatus shown in Figure 1 consists essentially of two white discs. With a little training and a black partition between both eyes it was possible for a subject to fuse both discs into one stimulus.

The white disc on the left side was attached to a metal strip orn the top, the white disc on the right side to a similar metal strip on the bottom. Both of these metal strips glided very precisely and smoothly in a horizontal plane with the help of four guides. Each of the strips was eccentrically attached to a gear (A or B in Fig. 1). The gears A and B were each driven by a separate gear in a differential gear box, which had a micrometer for phase adjustment. When gears A and B were in the same position, both white discs were moved sinusoidally in a horizontal line. By using the fine phase adjustment of the gear box it was possible to introduce a specific phase between the sinusoidal movement of the two white discs which could be read on the microrneter screw.

The observer achieved fusion of the two discs by looking straight forward; the distance between the two white discs was adjusted so that whenl the observer's eye was about 1 m from the target he could fuse the discs. The horizonital sinlusoidal

micrometer for the movable stripe ,'lamp for the stripes

differential

black wall between

adjustment

FIG. I.-A gear box produces a certain time delay adjustable by a micrometer screw between two eccentif c gears A and B. Their novement is tranasmitted to two whit;e discs which are seeii separately by both eyes and fused together. By this method it is possible to introduce a time delay between two moving optical stiImuli.



VOL. 64, 1969 PSYCHOLOGY: G. VON BEKESY 145

displacement of the two white discs was, between the two extreme positionrs, about 1.5 cm.

Behind the discs were two sheets with black arid white stripes. The white in the stripes was translucent and it was illuminated by a lamp placed behind the stripes. The two white dots were illuminated by a front light. One of the

sheets was movable in a horizontal direction, and it was adjusted by a micrometer screw so that the stripes appeared exactly in the same plane as the two white discs

when fused. By this method it was possible to observe very precisely any change in the depth perception of the two white discs, since in phase they appeared to

swing in exactly the same plane as the stripes. The stripes were motionless and

produced a well-defined reference plane. It is important to note that the gear box had to be made very precisely so there

was no backlash between the gears. To test a wider range of luminance of the two white discs, fiber optics were used

as shown in Figure 2. The two white discs of Figure I were replaced by the end

micrometer for l the adjustment

of the stripes

fiber optics

lateral movement produced by the

excenters

.. black wall between both eyes

FaG. 2.-Instead of the two white discs used in Fig. 1, fiber optics permit increasing the luminance of two discs to any desired value.

of two fiber optic tubes. The ends of these tubes were moved again by the two

eccentric gears shown in Figure 1. The other end of the fiber tubes was brought close to a lamp, and care was taken that the whole cross section of the fiber optic tube was equally illuminated. The luminance of this system could be easily changed from the threshold of vision to luminance values which produced dis- turbing after-images. The luminous ends of the tube moved in a slit made in the two striped sheets. One of them was again adjustable so that the stripes appeared to be exactly in the same plane as that of the illuminated end of the fiber optic tube during binocular fusion.

The Smallest Detectable Interocular Time Differences.-To observe the smallest

detectable difference, subjects were asked to fixate on the two white discs in Figure 1. The diameter of each white disc was

i

cm. For the fiber optic discs, the diameter of the illuminated circle was 3.5 mm. In both cases the distance of



146 PSYCHOLOGY: G. VON B.EKESY Pitoc. N. A. S.

observationi was 1 m. The amplitude of the oscillations betweenl the two maxi- mal excursions was 15 mm.

At the beginning of each. experiment the observers perceived an apparent ellipsoidal movemelnt of the fused image in depth. This movement is similar to the movements in the Pulfrich phenomenon. The elliptical movements change their depth if the phase between the two oscillations is changed.

The first step was to put the two discs in a position so that they appeared to move approximately in a straight line. After this, the distance between the two background-striped sheets was adjusted so that in depth the stripes appeared to be exactly irn the same plane in which the discs were moving. The position of these stripes gives a well-defined surface for the movement of the discs so that any displacement of the disc in its depth can be easily recognized. To obtain the smallest values for the phase difference during binocular stimulation, the adjustment of the plane of reference, set by the apparent distance of the two striped sheets, was very important.

A convenienrt luminance for the stripes was 10 footlambert. The luminance of the discs was set at 30 footlambert.

It was found that a sinusoidal movement of 1.6 cps gave reliable discrimination between an elliptic and a straight-line movement of the fused image of the two discs.

Given the length of one cycle, the phase difference between the two rotations was converted into a time difference which could be read on the micrometer screw.

The observations were made by starting with a small time difference between the discs and decreasing the time difference to zero, and then going over to a time difference of opposite value. The instant the observer had the feeling that the movement of the white discs was in a straight line, a reading was taken. Then. in the second step, the zero time difference was approached from the opposite direction. The deviations from the physical zero point for no time delay were recorded for several weeks with different subjects. The results show that the smallest time difference that can be recognized as such in binocular vision is 0.05 + 0.02 msec. This time difference varies a little with the lumimnance of the disc. In general, it is 10 to 20 per cent longer for a luminance of 5 footlambert compared to the values obtained for a luminance of 100 footlambert. But inside this luminance range, which is convenienlt for observatioln, the variations of the smallest time difference were small.

This surprisingly small time difference is in complete agreement with the time differences obtained for the other sense organs. As is well known in hearing, a time difference of 1 msec between the two sound waves reaching both ears can produce a complete lateralization to one side. The same numbers hold for vibratory sensations on the surface of the skin of the underarm; for olfactory stimulations, values of about 0.5 msec produce a complete lateralization to one side. The very smallest detectable time differences are on the order of 0.1 msec. All these time differences seem somehow to be connected with spatial perception. However, in vision the small time difference produces a change in depth, but in the other sense organs, a change in the lateral position.



VoL. 64,19.69 PSYCIOLOGY: G. VON. B dKESY 147

* The researclh repoirted here was supported by glrant GB-5768 from the Natiional Scienlce Foundation, grant NB-06890-01 from. the Institute of Neurological l)iseases and Blindness, USPHS, and grant M-14 from the Americanl Otological Society.

1 Bekesy, G. von, J. Gen. Physiol., 50, 519-532 (1967). 2Bekesy, G. von, Sensory Inhibition (Princeton, N. J.: Prilnceton JUniversity Press, 1967). 3 Hornbostel, E. M. von, and AM. Wertheimer, Akad. Wissen. Berlin, Sitzungsberichte, 15,

388-396 (1920). 4Katz, David, E. Abderhalden, ed., Handbuch der biologischen Arbeitsmethoden (Berlin:

Urban & Schwarzenberg, 1937), Abt. 5, Teil 7, pp. 879-918. 5 Hering, E., Zur Lehre vom Ortsinn der Netzhaut (Leipzig: Engelmann, 1864). 6 Schon, W., Graefe's Archiv fiur Ophthalmologie, 20--22 (1874-76) 24, (1878). 7 Bower, T. G. R., Nature, 210, 1081-1082 (1966). 8 Wist, E. :R., and W. C. Gogel, Vision Research, 6, 325-334 (1966).



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The Smallest Time Difference the Eyes can Detect with Sweeping Stimulation