Neuronal and behavioral responses to visual form

To support perception and guide behavior, the visual system must extract form information from retinal images. Form cues vary in their complexity and spatial extent. Human and macaque observers can detect forms that are defined by differences in texture. These cues are useful for segmenting figures from their backgrounds, and may also contribute to object identification. In the first experiment, I measured the development of behavioral sensitivity to texture-defined form in macaques. Infants could discriminate texture orientation as early as 6 weeks. Peak sensitivity continued to improve up to 40 weeks, reaching half of adult levels by 10–17 weeks. Surprisingly, texture sensitivity matured earlier than basic spatial acuity, and much earlier than global form sensitivity. Thus, different aspects of form vision develop over different rates, with local form mechanisms such as those implicated in texture processing maturing earlier than global form mechanisms. In the second experiment, I measured neuronal responses to texture-defined form in macaque visual area V2. Most cells responded best to texture patterns containing orientations that matched their preferences for luminance gratings. In some, these responses were modulated by aspects of the texture-defined form, either by its orientation or motion direction. Only a few cells preferred texture patterns whose orientations could not be predicted from their grating tuning, and thus showed true selectivity for the texture-defined form. Consistent with human imaging studies, these results suggest that signals related to texture-defined form in primates are found mainly in areas downstream of V2. In the third experiment, I identified V1 neurons projecting to V2 by antidromic electrical stimulation, and characterized their visual response properties. Projection neurons included both simple and complex cells. Most were tuned for orientation but not for direction, and were suppressed by large stimuli. In addition, most showed significant binocular phase interactions, and were better driven by luminance-modulated than chromatically-modulated stimuli. Thus, the heterogeneity of V1 inputs to V2 contributes to the diverse neuronal response types found there. These results provide a foundation for future work on how V1 inputs contribute to V2 receptive fields, particularly in the context of form vision