layout. First is the case of DaVinci stereopsis (10), where as a consequence of occlusion, left- and right-eye-only points are interpreted to be in back of adjacent surfaces (see also ref.16). Second, and a new observation reported in this paper, are more restricted conditions (requiring more stringent luminance requirements), which indicate that unpaired points can also trigger the perception of surfaces continuing in front [see also von Szily (17) for related a case concerning silhouettes]. All of these demonstrations with half-visible points share an important characteristic. Not only is it important that there be only one eye stimulated but more important is the identity of the eye receiving the visual input. Interestingly, we as human observers are generally unaware as to which of our eyes received a given visual stimulus. It is also true for the majority of neurons in the extrastriate cortical visual pathway. Neurons here, say in V3, V2, and V4 are essentially all binocular (18). Each receives more or less equal amounts of neuronal activation independent of which eye received stimulation. Each of these neurons, therefore, is indifferent as to which eye was stimulated. Required for our phenomenon are neurons that have very different properties. Cells need to respond only to input from one eye and not the other. Where in the nervous system might this information be available?

The only obvious candidates are neurons in the striate cortex (V1). Here, because of the well-known ocular dominance structure of V1 (1), it is clear that there exist neurons that respond differentially to which eye received visual stimulation. Thus, we are drawn to the conclusion that information directly available from cortical area V1 is needed for the higher order interpretation of surface relations. One additional requirement is also pertinent. Cells in this area also need to respond only to one eye but not to both. Tuned inhibitory cells described by Poggio (19), if selectively excited by right or left eye stimulation, might be useful for this purpose, particularly if the suppressive tuning for disparity is fairly broad.

1. Hubel, D. H. ( 1988) Eye, Brain, and Vision (Scientific American Library, New York).

2. Felleman, D. J. & Van Essen, D. C. ( 1991) Cereb. Cortex 1, 1–47.

3. Sekuler, R. & Ganz, L. ( 1963) Science 139, 419–420.

4. Raymond, J. ( 1993) Vision Res. 33, 1865–1870.

5. Zeki, S. ( 1978) Nature (London) 274, 423–428.

6. Treisman, A. ( 1982) J. Exp. Psychol. Hum. Percept. Perform. 8, 194–214.

7. Barlow, H. B., Blakemore, C. & Pettigrew, J. D. ( 1967) J. Physiol. (London) 193, 327–342.

8. Nakayama, K., He, Z. & Shimojo, S. ( 1995) in Invitation to Cognitive Science, eds. Kosslyn, S. M. & Osherson, D. N. (MIT Press, Cambridge, MA), pp. 1–70.

9. Nakayama, K., Shimojo, S. & Silverman, G. H. ( 1989) Perception 18, 55–68.

10. Nakayama, K. & Shimojo, S. ( 1990) Vision Res. 30, 1811–1825.

11. Shimojo, S. & Nakayama, K. ( 1990) Vision Res. 30, 69–80.

12. Anderson, B. L. & Nakayama, K. ( 1994) Psychol. Rev. 101, 414–445.

13. Nakayama, K., Shimojo, S. & Ramachandran, V. S. ( 1990) Perception 19, 497–513.

14. Nakayama, K. & Shimojo, S. ( 1992) Science 257, 1357–1363.

15. Metelli, F. ( 1974) Sci. Am. 230, 90–98.

16. Anderson, B. L. ( 1994) Nature (London) 367, 365–368.

17. von Szily, A. ( 1921) Graefes Arch. Ophthalmol. 105, 964–972.

18. Burkhalter, A. & Van Essen, D. C. ( 1986) J. Neurosci. 6, 2327– 2351.

19. Poggio, G. F., Gonzales, F. & Krause, F. ( 1988) J. Neurosci. 8, 4531–4550.