Abstract
Global processing involves pooling activity from early visual cortex in domain-specific ways. Examples include the perception of motion or form in glass patterns, and depth in random dot stereograms. Removal of high-frequency information via blur reduces sensitivity to global structure coded by sustained temporal channels (Burton et al., 2013). Here, we generalise this finding to the disparity domain by measuring both transient and sustained responses using the steady-state VEP. Dynamic random dot stereograms alternated between disparate and non-disparate states at 2 Hz. We applied six levels of Gaussian blur to monocular half-images, chosen as multiples of the separately measured blur detection threshold. When fused, stimuli were either a flat plane (absolute disparity only), or disparity gratings (0.5 and 1.2 cpd) containing both absolute and relative disparities. Responses were measured (N=25) at different disparities, and were decomposed into transient and sustained components via Fourier analysis. Reliable Component Analysis was used to identify cortical sources that responded in a consistent manner across trials, revealing a single component over midline occipital electrodes. Disparity thresholds to the plane were dominated by transient responses and were robust to blur, with only modest increases at the most extreme blur levels. By contrast, the response to the high spatial frequency grating was predominantly sustained and even a small amount of blur was detrimental to disparity sensitivity. For the 0.5 cpd grating, sustained response threshold increases were smaller but systematic, indicating that the effect of blur scales with the corrugation frequency. Our results suggest that multiple shape-from-X mechanisms utilise a duplex coding strategy that is not necessarily tied to the temporal dichotomies present in the retino-geniculate pathway. Our results also provide direct neural evidence for an association between absolute disparity mechanisms and transient coarse spatial processing. Relative disparity, by contrast, is processed by sustained, fine spatial mechanisms.