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Daisy J Phillips, Thomas J McDougall, David R Badcock; Centre-surround Suppression of Contrast through the Form and Motion Pathways. Journal of Vision 2019;19(10):165c. doi: https://doi.org/10.1167/19.10.165c.
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Centre-surround suppression of contrast is the phenomenon where the perceived contrast of a central stimulus is reduced when presented with a surround. A number of stimulus features influence the amount of suppression the surround induces on the perceived contrast of the centre, with the focus of this study being on the orientation and direction of motion of the surround relative to the centre. Suppression declines as the similarity between centre and surround stimulus properties increases and this decline is thought to reflect multiple mechanisms of centre-surround suppression. However, this is yet to be established in the motion pathway in human participants. A two-mechanism model has been proposed involving a narrowly-tuned process, requiring very similar stimuli in the centre and surround, and a weaker, broadly-tuned process unselective for stimulus features. Five participants completed a two-interval forced choice contrast discrimination task relative to a 40% reference. Stimuli included both static and moving Gabor grating patterns whose high contrast (95%) surrounds had either parallel or orthogonal (90°) orientations relative to the centre. The motion manipulation included surrounds drifting in either the same or opposite direction to the centre. The results with static stimuli showed contrast suppression when surrounds were present and replicated the more substantial suppression when the centre and surround had the same orientation than when they were orthogonal. With motion stimuli all surround conditions were significantly more suppressed than the no-surround condition and the surround condition with the same orientation and direction of motion as the centre was significantly more suppressed than either the opposite direction of motion conditions or the orthogonal orientation conditions. There was no difference between the orthogonal orientation conditions and the opposite direction of motion conditions. These findings support the extension of the two mechanism model of contrast suppression to the motion pathways.
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