PURPOSE: Neurons in middle temporal area (MT) have spatial antagonistic center-surround receptive fields. Surround activation globally inhibits responses to center stimuli is selective to their relative directions. These physiological observations are used to explain the center-surround motion repulsion effect. Furthermore, increasing the surround size above some radius does not suppress further neuronal responses, thus indicating a surround-to-center limit of interactions. In parallel, previous researches indicated that orientation tuning in V1 was modulated by near- and far-surrounds differently based on different anatomical circuits (intra-V1 horizontal connections and feedback connection from extrastriate cortex). Here, we want to investigate influences of near- and far-surround on center-surround motion repulsion effects and we used a simple physiological model of the repulsion effect to extract changes in surround inhibition strength and tuning widths with spatial distance.
METHODS: Human subjects performed motion discrimination tasks under near- and far-surround conditions, then, the model was fit to the data in order to understand the putative neuronal changes in center-surround effects with surround distance. Three major parameters were extracted: motion direction tuning width of the central neuronal population responding to the target motion (σ_s), surround-to-center inhibition strength of the maximum response rate of the center neurons (A_inh), and surround tuning width change with distance to the center.
RESULTS: All 11 subjects showed clear motion repulsion effects in near-surround condition while only 10 for the far-surround condition. The model predicted human performance well. σ_s in near-surround was significantly smaller than far-surround and the surround inhibition A_inh was stronger for near-surround condition.
CONCLUSIONS: These results showed that the increase of distance between surround and center not only decline the inhibitory effect of surround onto center, but importantly also increases the surround to center tuning width of inhibition.