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Human Cortical Responses to Periodic Breaks in Collinearity
Dr. Russell Hamer
Monday, April 21, 2014, 12:00pm - 07:00pm
Professor Visitante, Instituto de Psicologia, Universidade de Sao Paulo, Sao Paulo, Brasil and the Smith-Kettlewell Eye
Humans and nonhuman primates are extraordinarily sensitive to breaks in the collinearity of lines and edges, an ability that is likely an important component of visual mechanisms that perceive and segregate objects. Adults with normal vision can detect breaks in collinearity on the order of 2 arc seconds of visual angle, as much 30-times more precise than the dimensions of the stroke elements of standard 20/20 eye chart letters (1 arcmin), yielding the reliable detection of line/edge offsets that are substantially smaller than the cones and their spacing in the foveal cone lattice. This ability, Vernier sensitivity, is one of several visual hyperacuities and has been studied extensively, starting in the late 19th century. The specific cortical and retinal processing mechanisms underlying Vernier are still being unraveled. This line of research is relevant to both theoretical issues regarding the mechanisms that permit hyperacuity performance, and to clinical visual assessment. Visual impairments, such as amblyopia, degrade Vernier sensitivity more than grating acuity, and thus Vernier measures are a sensitive additional tool for clinical evaluation, especially in the pediatric population at risk for such maladies. Since the early 1980s, we have known that we can record large visual evoked potentials (VEPs) from the scalp of adults, and more recently, from infants. Our approach to measuring cortical Vernier responses is to analyze steady-state VEPs in the frequency domain using the efficient technique developed by Norcia and colleagues, the Sweep VEP (sVEP). This approach permits a sensitive, reliable segregation of Vernier-specific responses from responses to motion or pattern per se. A typical recording from an infant or an adult using sVEP will often yield responses with extraordinarily high signal-to-noise ratio (as high as 10 – 30). In this study (normal adults) we examined the effects of grating contrast and gaps between grating elements on the sVEP responses to periodic breaks in collinearity of achromatic squarewave gratings. The results demonstrate that the even and odd-harmonic response components segregate Vernier (odd) from motion (even) responses. In addition, we show how suprathreshold sVEP parameters, such as peak amplitude, response gain and phase at peak amplitude, can reveal properties of the Vernier and motion mechanisms that are not accessible psychophysically. Finally, we examine our data in relation to a hypothesis in the literature that proposes that retinal signals emanating from magnocellular, not parvocellular, ganglion cells form the substrate for cortical calculation of Vernier signals.