Dynamics of absolute and relative disparity processing in human visual cortex

Abstract

Cortical processing of binocular disparity is believed to begin in V1 where cells are sensitive to absolute disparity, followed by the extraction of relative disparity in higher visual areas. While much is known about the cortical distribution and spatial tuning of disparity-selective neurons, the relationship between their spatial and temporal properties is less well understood. Here, we use steady-state Visual Evoked Potentials and dynamic random dot stereograms to characterize the temporal dynamics of spatial mechanisms in human visual cortex that are primarily sensitive to either absolute or relative disparity. Stereograms alternated between disparate and non-disparate states at 2 Hz. By varying the spatial frequency content of the disparate fields from a planar surface to corrugated ones, we biased responses towards absolute vs. relative disparities. Reliable Components Analysis was used to derive two dominant sources from the 128 channel EEG records. The first component (RC1) was maximal over the occipital pole while the second component (RC2) was maximal over right lateral occipital electrodes. In RC1, first harmonic responses were sustained, tuned for corrugation frequency, and sensitive to the presence of disparity references, consistent with prior psychophysical sensitivity measurements. By contrast, the second harmonic, associated with transient processing, was not spatially tuned and was indifferent to references, consistent with it being generated by an absolute disparity mechanism. In RC2, the sustained response component showed similar tuning and sensitivity to references. However, sensitivity for absolute disparity dropped off, and transient signals were mainly driven by the lowest corrugation frequencies. SIGNIFICANCE STATEMENTSustained and transient mechanisms have been demonstrated across sensory systems and may reflect a common coding strategy. This strategy may be useful for the analysis of the form and duration of complex events in the case of sustained signals, and the onset and location of events in the case of transient signals. Here, we provide direct neural correlates of sustained and transient disparity mechanisms in human visual cortex. Early visual cortex is sensitive to both relative and absolute disparity, with the former being processed in a sustained fashion and the latter in a more transient fashion. Outside of early visual cortex, sustained relative disparity responses are readily measurable, but transient responses and responses to absolute disparity are not.