Figure 1.

HEPES attenuates parasol ganglion cell surround. A, Responses of an ON parasol ganglion cell to a 300-μm-diameter spot (stimulus trace below) square wave stimulus modulated at 2 Hz in the absence (left) and presence (right) of 20 mm HEPES. The responses are similar in both conditions. B, Responses of a parasol ganglion cell to a 2000-μm-diameter spot (stimulus trace below) square wave stimulus modulated at 2 Hz in the absence (left) and presence (right) of HEPES. The control response is small and transient because of surround antagonism. The response in HEPES is larger and more sustained, indicating diminished surround antagonism. C, Response amplitude as a function of spot diameter in the absence (open circles) and presence (filled circles) of 20 mm HEPES. Stimuli were spots sinusoidally modulated at 2 Hz. Solid lines are difference of Gaussian receptive field model fits to the data; insets are two-dimensional profiles of the model fits. Responses at small spot diameters are similar, but responses at larger spot diameters are larger in the presence of HEPES, and the ratio of center to surround (cntr/surr) response strength increases. The inset on the right shows the mean response normalized to the maximum amplitude for nine cells in the absence (solid line) and presence (dotted line) of 20 mm HEPES. In the presence of HEPES, the surround response is reduce by 61%. CTR, Control.