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Fig. 13.5. Measuring local response as multi-unit activity. (a) An example contour integration input, consisting of nine contour elements with different positions and orientations on the retina. Each element is an oriented Gaussian of length &sigma_a = 1.9 and width &sigma_b = 1.2. The firing rates of the retinal receptors are set according to these Gaussian values, plotted in gray scale from white to black (low to high). (b) The resulting activations of the GMAP neurons, measured as a leaky average firing rate with a 0.92 decay rate. The circles indicate areas where separate MUA values are measured. Each area is centered on a neuron whose receptive field is centered on one of the contour elements and whose orientation preference is the same as the element's orientation; due to local distortions in the retinotopic mapping, the circle's center in V1 is sometimes slightly displaced from the element's center on the retina. A few neurons outside the circles are also activated, driven by simultaneous input from two different contour elements. The circle radius is chosen such that the spurious activation is not included in the MUA measurements.