Background-induced flicker enhancement in cat retinal horizontal cells. II. Spatial properties.

1. Intracellular recordings have been made from cat retinal horizontal cells stimulated with flickering test spots. Dim backgrounds increase flicker amplitudes in response to small but not large test stimuli. 2. This background-induced flicker enhancement has been measured for different slit- and square-test stimulus widths and the results compared with two spatial models for the enhancement effect. 3. In the "dark test-region" model it is argued that rods within the test region are unresponsive to background stimuli because of prior saturation by the test stimulus. Background-evoked rod signals decay passively from regions outside the test stimulus through a syncytial network into the recording site, where they act on the cone-to-horizontal-cell synapse, increasing its gain. 4. In the "changing length-constant" model rod signals reduce the length constant of a syncytial network by uncoupling the cells within it. This causes an increased response to small but not large test stimuli. 5. Both models are analytically evaluated with the use of a conductive-sheet approximation to the syncytial network. Expressions are derived for network polarization [(V(0, 0)] as a function of stimulus size. The specific stimulus shapes considered are disks, rectangles, slits, and squares in both bright and dark varieties. From these expressions predictions of response enhancement as a function of stimulus size are made for both models. 6. The dark test-region model provides for an exponential decay of flicker enhancement as a function of slit width but a steeper-than-exponential decay with the width of squares, in close agreement with experimental data. 7. The changing length-constant model makes qualitatively similar predictions. Flicker enhancement declines nearly exponentially with slit width. For square-shaped test stimuli the predicted decline of flicker enhancement with size is somewhat shallower than either the dark test-region-model curve or the experimentally determined curve. 8. As recorded in the same set of cells and under the same set of stimulus conditions (with the use of both slit- and square-test stimuli), the mean length constant of the peak-to-peak flicker component in the horizontal-cell response is 168 +/- 18 (SE) microns with the background and 232 +/- 45 microns in the dark. The mean length constant for the background-induced flicker enhancement, as fit by dark test-region-model curves, is 186 +/- 22 microns (n = 9).(ABSTRACT TRUNCATED AT 250 WORDS)