Tilt illusion during binocular rivalry

Vision Rex Vol. 17. pp. 327 to 328. Pergamon

Press

1977. Printed

LETTER TILT

ILLUSION

m Great

Britain.

TO THE EDITORS DURING lRrceived

BINOCULAR 29 June

A vertical grating appears tilted anticlockwise when superimposed over a clockwise tilted grating. This phenomenon is now thought to be a consequence of inhibitory interaction between orientation detectors (Blakemore, Carpenter and Georgeson, 1970; Carpenter and Blakemore, 1973). It has been shown that broadly tuned excitatory input from the lateral geniculate on to the cortical neurones is sharpened by inhibition by intracortical neurones which are even more broadly tuned to orientation (Andrews, 1965; Benevento, Creutzfeldt and Kuhnt, 1972; Blakemore and Tobin, 1972; Carpenter and Blakemore, 1973). There is considerable evidence for existence of orientation analysers in the visual cortex of the cat and the monkey and there is reason to believe that such neurones are also present in the human visual cortex (Mackay, 1957; Gilinksy, 1968; Blakemore and Campbell, 1969; Campbell and Maffei, 1970). It is known that stereopsis can co-exist with binocular rivalry (Triesman, 1962; Ramachandran, Rao and Vidyasagar, 1973) and it has also been shown (Blake and Fox, 1974a) that the suppression of binocular rivalry does not affect the process of adaptation to grating contrast and frequency. On the other hand from other experiments we do know that a number of visual processes are affected by binocular rivalry suppression (Lansing, 1964; Mackay, 1968; Fox and Check, 1968; Lack, 1974; Blake and Fox, 1974b; Blake, Fox and Westendorf, 1974; Ramachandran, 1975). It would be of interest to determine the effect of binocular rivalry on the tilt illusion since both are thought to be mediated by neural mechanisms located in the visual cortex.

PROCEDURE AND RESULTS

The stimuli, shown in Fig. 1 were presented in an amblyoscope (Clement & Clarke). Half-image A consisting of the inner vertical test grating and outer tilted inducing grating 0,, was presented to one eye and half-image R consisting of the horizontal suppressing grating O2 was presented to the other eye. The patterns were adjusted so that when viewed binocularly the inner test grating would be seen against the patternless illumined area cut out in 0, while the two outer gratings would be seen in a rivalrous field. Circular black lines drawn around each halfimage facilitated locking of the two images. The OS were five undergraduate students all of whom were naive as to the purpose of the experiment. The stimuli were presented in three conditions. In the first condition the stimuli were presented as mentioned above. In the second condition the outer gratings O1 and O2 were interchanged and in the third condition the horizontal suppessor grating was dispensed with and test grating was presented to one eye and inducing grating 0, to the other eye. Gratings of sufficiently low contrast were used to avoid formation of after

RIVALRY

1976)

images and to prevent dominance of the inducing grating its contrast was adjusted to be lower than that of the sup pressor grating. To begin with half-image A (Fig. 1) was shown to one eye and as expected all OS saw the inner test grating tilted in a direction opposite to that of the outer grating. Halfimage B of Fig. 1 was added to the other eye and the OS were asked to note carefully the orientation of the inner test grating and report and change they saw during the course of binocular rivalry. All OS reported the inner test grating to be alternating between two orientations-the tilted orientation which they saw earlier and the vertical. They were then asked to observe at what periods during rivalry the grating appeared vertical or tilted. This procedure was followed since it lessened the burden of tasks the OS had to perform. Though the OS found it difficult to make the observation which they attributed to confusion caused by rivalling patterns all of them reported that the vertical orientation of the test grating corresponded to the appearance of the horizontal grating O2 and that it was tilted when the inducing grating appeared. When both 0, and 0, were seen simultaneously as a c&s-cross pattern, tilt was perceived. When the test and inducing gratings were presented dichoptically it was found that the curve plotting the magnitude of the tilt illusion for different orientations of the test and inducing gratings was similar to that obtained in the monocular condition (Blakemore et al., 1970). It was assumed therefore that the same mechanisms mediate the effects in the two cases. In the second condition with the outer gratings interchanged the results were the same as with the previous set up. In the third condition only the test grating and the inducing grating were used. To facilitate binocular rivalry the field surrounding the test grating was illuminated brightly and the contrast of the inducing grating was reduced. It was found that as before the tilt observed during dichoptic stimulation disappeared when the inducing grating was suppressed. When the test and inducing grating were presented monocularly (halfimage A in Fig. 1) or when they were presented dichoptitally care being taken to prevent rivalry, it was found that the test grating was constantly tilted. It is unlikely therefore, that the changes in orientation of the test grating were spontaneous and unconnected with rivalry because, as mentioned above, the test grating appears constantly tilted when viewed continuously with the inducing grating.

Fig. 1. Transparencies of the patterns shown above were presented in an amblyoscope as mentioned in the text. T = Test grating. 0, = Inducing grating which was oriented 10” clockwise or anticlockwise. It was of the same frequency as the test grating. O2 = Suppressor grating. 327

Lsttcr to the tditors

The absence of tilt during binocular rivalry must mean that the suppressor pattern cuts off input for interaction of the two gratings to produce tilt. If it is assumed that information processing in the visual system occurs serially this in turn would mean that binocular rivalry “precedes” the physiologic locus mediating tilt illusion. Interpretation is. however, difficult because it is possible to build neuro-physiologica1 models where the “site” of binocular rivalry is “after” the orientation selective mechanism (Arden, personal communication). While it is known that the processes of stereopsis and adaptation to grating contrast and frequency are not affected by binocular rivalry, there is considerable evidence which indicates that the suppression of binocular rivalry is non-selective and that a number of visual processes are affected during binocular rivalry. The results of the present experiment would support this hypothesis and also indicate that the inhibition of binocular rivalry is different from the orientation specific inhibition of orientation selection. ilcknowledgemrnts~~l am grateful to Prof. A. S. Paintal, Prof. D. P. Agarwal and Dr. Prem Singh for their interest. My thanks are also due to Drs. V. S. Ramachandran and T. R. Vidyasagar for helpful discussions. This work was done under the tenure of a fellowship from Council of Scientific and Industrial Research at V.P. Chest Institute, Delhi. Unirersit!! Lnhoratory Physiology, Parks Street. Oxford, OX1 3PT, England

of

V. MADHUSUDHAN

RAO

REFERENCES Andrews

D. P. (1965) Perception

of contours

in the central

fovea. Nature, Land. 205. 1218~1220. Benevento L. A., Creutzfeldt 0. D. and Kuhnt U. (1972) Significance of intracortical inhibition. Nature, New Biol. 238, 124-l 26. Blake R. and Fox R. (1974a)Adaption to invisible gratings and the site of binocular rivalry. Nature, Land. 249,488.

Blake R. and Fox, R. (lY74b) Binocular rivalry suppressmn insensitive to spatial frequency and orientation change. him Rm. 14. 6X7 692. Blake R.. Fox R. and Wcstendorf L).. (1974) Visual size constancy occurs after binocular rivalry. l’isi~~rrRm. 14. 5x5. Blakemore c‘. and Campbell F. W.. (1969) On the existence of neurones in the human visual system selectively sensitive to the orientation and size of retinal images. .1. Phy.siol., Land. 203. 237m~260. Blakemore C.. Carpenter R. H. S. and Georgeson M. A. (1970) Lateral inhibition between orientation detectors in the human visual system. Nature, Lond. 228, 37--39. Blakemore C., Carpenter R. H. S. and Georgeson M. A. ( 197 I) Lateral thinking about inhibition. Natwc. Zmld. 234. 418 -419. Blakemore C. and Tobin E. A. (1972) Lateral inhibition between orientation detectors in the cat’s visual cortex. E.xpl Bruit1 Rcs. 15, 439440. Campbell F. W. and Maffei L. (1970) Electrophysiological evidence for existence of orientation and size detectors in the human visual system. J. PhpioL, Lond. 207. 635 652. Carpenter R. H. S. and Blakemore C. (1973) Interactions between orientations in human vision. Expl Brain Res. 18. 2X7--303. Fox R. and Check R. (1968) Detection of motion during binocular rivalry suppression. J. exp. Psychal. 78, 3% 395. Gilinsky A. S. (196X) Orientation-specific effects of patterns of adapting light of visual activity. J. opt. Sot. Am. 58. Ii 18. Lack L. (1973) Amplitude of visual suppression during the control of binocular rivalry. Pwcept. Psychophys. 13(3), 374-78. Lansing R. W. (1964) Electroenophalographic correlates of binocular rivalry in man. Science, 146, 1325-1327. MacKay D. M. (1957) Moving visual images by regular stationary patterns. Nature, Land. 180, 849. MacKay D. M. (1968) Evoked potentials reflecting interocular and monocular suppression. Nature, Lond. 217. XI 87 Ramachandran V. S., Madhusudhan Rao V. and Vidyasagar T. R. (1973) The role of contours in stereopsis. Ntrrurr, Land. 242, 412413.

Ramachandran V. S. (1975) Suppression of apparent motion during binocular rivalry. Nature. Lond. 256, 122-~1’3. Triesman A. (1962) Binocular rivalry and stereoscopic depth perception. Q. J/ r.up. Psycho/. 14(l), 23-27.