The mechanisms of collinear integration.
Low-contrast visual contour fragments are easier to detect when presented in the context of nearby collinear contour elements (U. Polat & D. Sagi, 1993). The spatial and temporal determinants of this collinear facilitation have been studied extensively (J. R. Cass & B. Spehar, 2005; Y. Tanaka & D. Sagi, 1998; C. B. Williams & R. F. Hess, 1998), although considerable debate surrounds the neural mechanisms underlying it. Our study examines this question using a novel stimulus, whereby the flanking "contour" elements are rotated around their own axis. By measuring contrast detection thresholds to a brief foveal target presented at various phases of flanker rotation, we find peak facilitation after flankers have rotated beyond their collinear phase. This optimal facilitative delay increases monotonically as a function of target-flanker separation, yielding estimates of cortical propagation of 0.1 m/s, a value highly consistent with the dynamics of long-range horizontal interactions observed within primary visual cortex (V1). A curious new finding is also observed: Facilitative peaks also occur when the target flash precedes flanker collinearity by 20-80 ms, a range consistent with contrast-dependent cortical onset latencies. Together, these data suggest that collinear facilitation involves two separate mechanisms, each possessing distinct dynamics: (i) slowly propagating horizontal interactions within V1 and (ii) a faster integrative mechanism, possibly driven by synchronous collinear cortical onset.
Cass
John
J
Department of Psychology, University of Sydney, Australia. jcass@physiol.usyd.edu.au
Alais
David
D
D015350:Contrast Sensitivity / Q000502:physiology*; D005556:Form Perception / Q000502:physiology*; D006801:Humans; D008297:Male; D008959:Models, Neurological; D010775:Photic Stimulation / Q000379:methods; D011601:Psychophysics; D011930:Reaction Time; D012684:Sensory Thresholds; D009435:Synaptic Transmission; D013997:Time Factors; D014793:Visual Cortex / Q000502:physiology; D014796:Visual Perception / Q000502:physiology*