A re-evaluation of achromatic spatiotemporal vision: nonoriented filters are monocular, they adapt and can be used for decision-making at high flicker speeds

Masking, adaptation, and summation paradigms have been used to investigate the characteristics of early spatiotemporal vision. Each has been taken to provide evidence for (i) oriented and (ii) nonoriented spatial filtering mechanisms. However, subsequent findings suggest that the evidence for nonoriented mechanisms has been misinterpreted: possibly, those experiments revealed the characteristics of suppression (e.g., gain control) not excitation, or merely the isotropic subunits of the oriented detecting-mechanisms. To shed light on this, we used all three paradigms to focus on the “high-speed” corner of spatiotemporal vision (low spatial frequency, high temporal frequency) where cross-oriented achromatic effects are greatest. We used flickering Gabor patches as targets and a 2IFC procedure for monocular, binocular and dichoptic stimulus presentations. To account for our results we devised a simple model involving an isotropic monocular filter-stage feeding orientation-tuned binocular filters. Both filter stages are adaptable and their outputs are available to the decision-stage following nonlinear contrast transduction. However, the monocular isotropic filters adapt only to high-speed stimuli—consistent with a magnocellular sub-cortical substrate—and benefit decision making only for high-speed stimuli. According to this model, the visual processes revealed by masking, adaptation and summation are related but not identical.