Phase-dependent interactions in visual cortex to combinations of first- and second-order stimuli
Hutchinson, Claire V. and Ledgeway, Timothy and Baker, Curtis L.,Jr. (2016) Phase-dependent interactions in visual cortex to combinations of first- and second-order stimuli. Journal of Neuroscience . ISSN 1529-2401
A fundamental task of the visual system is to extract figure-ground boundaries between objects, which are often defined not only by differences in luminance but also by "second order" contrast or texture differences. Responses of cortical neurons to both first- and second order patterns have been previously studied extensively, but only for responses to either type of stimulus in isolation. Here we examined responses of visual cortex neurons to the spatial relationship between superimposed periodic luminance modulation (LM) and contrast modulation (CM) stimuli, whose contrasts were adjusted to give equated responses when presented alone. Extracellular single unit recordings were made in area 18 of the cat, whose neurons show very similar responses to CM and LM stimuli as those in primate area V2 (Li et al, 2014). Most neurons showed a significant dependence on the relative phase of the combined LM and CM patterns, with a clear overall optimal response when they were approximately phase-aligned. The degree of this phase preference, and the contributions of suppressive and/or facilitatory interactions, varied considerably from one neuron to another. Such phase-dependent and phase-invariant responses were evident in both simple- and complex-type cells. These results place important constraints on any future model of the underlying neural circuitry for second-order responses. The diversity in the degree of phase dependence between LM and CM stimuli that we observe could help disambiguate different kinds of boundaries in natural scenes.
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