Representation of Stereoscopic Depth Based on Relative Disparity in Macaque Area V4

Abstract

Stereoscopic vision is characterized by greater visual acuity when a background feature serves as a reference. When a reference is present, the perceived depth of an object is predominantly dependent on this reference. Neural representations of stereoscopic depth are expected to have a relative frame of reference. The conversion of absolute disparity encoded in area V1 to relative disparity begins in area V2, although the information encoded in this area appears to be insufficient for stereopsis. This study examines whether relative disparity is encoded in a higher cortical area. We recorded the responses of V4 neurons from macaque monkeys to various combinations of the absolute disparities of two features: the center patch and surrounding annulus of a dynamic random-dot stereogram. We analyzed the effects of the disparity of the surrounding annulus on the tuning for the disparity of the center patch; the tuning curves of relative-disparity-selective neurons for disparities of the center patch should shift with changes in the disparity of the surrounding annulus. Most V4 tuning curves exhibited significant shifts. The magnitudes of the shifts were larger than those reported for V2 neurons and smaller than that expected for an ideal relative-disparity-selective cell. No correlation was found between the shift magnitude and the degree of size suppression, suggesting that the two phenomena are not the result of a common mechanism. Our results suggest that the coding of relative disparity advances as information flows along the cortical pathway that includes areas V2 and V4.

Neural Activity in Cortical Area V4 Underlies Fine Disparity Discrimination

“…The representation of stereoscopic depth is in an imperfect relative frame of reference. The incompleteness matches the properties of a fraction of V4 neurons whose tuning curve for center disparity shifts with 90% of surround disparities (Umeda et al, 2007; for V2, see Thomas et al, 2002; for V3/V3A, see Anzai et al, 2011). The reference frame of fine stereoscopic depth perception might reflect the properties of the disparity representations in V4.…”

“…This is in contrast to area MT, in which microstimulation and inactivation had no effects on decisions about fine disparity (Uka and DeAngelis, 2006;Chowdhury and DeAngelis, 2008). Another contrast between V4 and MT is the degree to which neurons are selective for relative disparity between adjacent visual features; a substantial proportion of V4 neurons are selective for relative disparity between the center and surround of concentric stimuli whereas virtually all MT neurons are not (Neri et al, 2004;Uka and DeAngelis, 2006;Umeda et al, 2007; for the selectivity of MT neurons to another type of relative disparity, see Krug and Parker, 2011). The two seemingly independent contrasts may not just be a coincidence, but the result of learning.…”

“…This adjustment maximizes the information of individual cells for the ideal observer, as long as the cells are selective to absolute disparity. For V4 neurons, the same procedure cannot maximize the information because the disparity tuning shifts with the surround disparity (Umeda et al, 2007). Therefore, we used the same surround disparity (0°) for all neurons.…”

“…Because many V4 neurons encode relative disparity (Umeda et al, 2007), area V4 is a possible site for the representation of fine stereoscopic depth perception. We trained monkeys to discriminate the disparity of the center with respect to the surround of a random-dot stereogram.…”

Neurons in Dorsal Visual Area V5/MT Signal Relative Disparity

“…In macaque ventral visual areas, a subset of disparity-tuned neurons in V2 (Thomas et al, 2002) and a larger fraction in V4 (Umeda et al, 2007) signal the relative disparity between spatially adjacent surfaces; neurons in inferotemporal cortex (TEs) signal the depth profile of curved surfaces (Janssen et al, 1999). By contrast, such neuronal signals are absent in V1 and dorsal area V5/MT (Cumming and Parker, 1999;Uka and DeAngelis, 2006).…”