Fig. 4.

Clustering of EPSP responses based on amplitude and rise time. A, Drawing of a retinal eyecup (as in Fig. 3). Local stimuli were presented at five stimulation locations labeled S1–S5, and EPSPs were detected by the AC window to estimate the preferred direction of each retinal area. Theright inset shows the stimulus pattern of 18 1.1°white squares that moved together in 1 of 12 directions within a square stimulus window of 8.8°. B, Three superimposed direction-tuning curves plotted as in Figure 1 but rotated to match the orientation of the eyecup shown in A. Thefilled circles show the direction tuning of spikes recorded at −60 mV during full-field stimulation. The open circles show direction tuning of all EPSPs detected by the AC window at −90 mV during full-field stimulation. The open squares show direction tuning of cluster 2 (C–E). The scaling is not the same for the different polar plots but demonstrates that these three response measures of this BON cell preferred similar directions.C–E, Amplitude–rise time scatterplots showing the distribution of EPSP shapes recorded in a BON cell hyperpolarized to −90 mV with the local pattern centered on the S2stimulation site shown in A. Each datapoint represents the shape of an individual EPSP recorded during equivalent 40 sec periods (10 sec of local pattern motion in 4 preferred and 4 null direction presentations, 8 5 sec periods of no motion). From the preferred data in C, boundaries were objectively determined to divide the scatterplot into three regions using cluster analysis. Note that there were many more events within the boundaries of cluster 2 inC than were recorded during the null directions inE. The stationary pattern (D) evoked the fewest events in all clusters. The direction-tuning curve of cluster 2 is plotted in B.