Visual neurons are highly sensitive to changes in luminance around the mean level of light. The effects of changes in luminance contrast on the responses of retinal ganglion cells (RGCs) and V1 neurons have been extensively studied. Sharp transitions in luminance signify edges, which are key features in the visual environment. However, edges are rarely sharp and may be blurred. Edges are generally blurred because eyes have a restricted depth of field, sources of light are rarely point sources, edges are typically smooth, and rapid motion induces blur.

The purpose of this research was to understand how RGCs and V1 neurons represent blur in their responses. Extracellular recordings were made from rabbit RGCs and primate V1 cells, and we assessed the spike rate, latency, and linearity to sharp edges varying by contrast and high contrast edges varying by blur. We sought to determine if there is an equivalent contrast for blur based on the spike rate and latency. We also checked for linearity: whether the response of a neuron to a sharp edge was equal to the response to a blurred edge plus the response to the missing spatial components that were the difference between a sharp and blurred edge.

For the majority of RGCs, reducing the contrast of a sharp edge was similar to increasing the blur of a high contrast edge based on spike rate and latency. We hypothesized that this occurs because RGCs compute contrast by comparing the highest and lowest luminance values within their receptive field. Most brisk-sustained RGCs produced linear responses to blurred edges. Brisk-transient RGCs produced both linear and nonlinear responses.

Results obtained from primate V1 cells differed from the results obtained from most RGCs. For the levels of blur tested in V1 cells, contrast and blur are not interchangeable based on the response dynamics. In addition, all V1 cells examined produced nonlinear responses to blurred edges. We hypothesize that some cortical neurons actively filter out or separate blur perhaps to maintain constancy as objects move in and out of the plane of fixation.

Keywords: blur, contrast, retina, V1, rabbit, primate