Society for Neuroscience 2005
Prog#583.1

DISSOCIATION OF STEREOSCOPIC DEPTH JUDGMENT FROM PERCEPTION OF A PLANE-IN-DEPTH: IMPLICATION FOR NEURAL MECHANISM OF STEREOPSIS
I.Fujita*; S.Yasuoka; S.Tanabe
Grad. Sch. Frontier Biosciences, Osaka Univ., Toyonaka, Japan

Observers perceive a plane-in-depth by viewing a binocularly correlated random-dot stereogram (RDS). An anticorrelated RDS evokes no such percept. This observation suggests that the visual system produces a representation of stereoscopic depth only in the presence of a globally matching solution to the correspondence problem. Neurons in areas V4 and IT have response characteristics consistent with this depth percept, whereas neurons in areas MT and MST do not. These findings impose a conundrum; how can MT neurons contribute to depth judgment in some stereoscopic tasks, as some studies have suggested, given that they do not solve the correspondence problem? Here we show that depth judgment can be made in a way consistent with disparity energy computation, but does not accompany perception of a plane. The subjects were asked to report whether the center portion of an RDS was seen nearer or farther than the annular portion. Dots in the center had a binocular disparity of 0.32 deg relative to those in the annulus, and were switched randomly from correlated to anticorrelated across trials. A delay between the two-eye images was also varied. The annulus was always binocularly correlated, providing a reference plane for the judgment of depth. The subjects reversed depth judgment for a correlated RDS with an interocular delay of ~100 ms; they reported "far" for crossed and "near" for uncrossed disparities. Perceived depth was also reversed for an anticorrelated RDS with no or small (~12 ms) interocular delays. The subjects showed this pattern of judgments despite their subjective report of seeing no clear surface. The psychophysical responses are predicted by a disparity energy model incorporating a spatiotemporal receptive field (Yasuoka et al., SFN2005). The results suggest that disparity energy signals in dorsal pathway areas can be used for depth discrimination without evoking surface perception.