Magnocellular modulation of spatial vision

Perceived spatial separation between two points can be influenced by adaptation to a dynamic texture pattern, such as moving or flickering dots. We utilised this adaptation effect to probe magnocellular influence on spatial perception by manipulating the spatial and temporal frequency of the adapting stimulus and isolating the contribution of colour and luminance mechanisms. Neurones in the magnocellular layers of the lateral geniculate nucleus respond at higher temporal frequencies than neurones in primary visual cortex. Magno-cells also respond most at high temporal and low spatial frequencies, and are less sensitive to isoluminant chromatic patterns. After viewing an adapting pattern in one hemifield, participants reported which of two pairs of dots (presented in adapted and un-adapted hemifields) appeared to have greater separation. The separation of one of the dot pairs was varied to derive the point of subjective equality (PSE) and provide a measure of the compression effect in comparison to a baseline condition with no adaptor stimulus. In a sequence of experiments, we found that: (1) Adapting to luminance-defined dot patterns flickering at high temporal frequency (60Hz) induced significant spatial compression despite the adaptor being invisible; (2) After adaptation to a Gabor array, compression was strongest for arrays with lower (0.5 cpd) spatial frequency carriers; (3) Adapting to colour-defined isoluminant dots produced significant compression at low (3Hz) but not high (60Hz) flicker rates; (4) Compression induced by a pattern of moving coloured dots with varying luminance content produced compression after-effects that were weakest at isoluminance, and coincided with a loss of perceptual motion coherence. Across all experiments, adaptor properties preferentially targeting the magnocellular pathway produce stronger compression after-effects. Specifically, the compression effect is maximal within spatially low-pass, temporally band-pass transient luminance channels, indicating that metric properties of spatial vision are encoded by adaptable neural processes reflecting magnocellular pathway specialisation.