Figure 2.

Doublet stimuli. (A,B) A combination of two square-wave, white-black luminance steps on mean luminance (gray) background, referred to as a doublet. The doublet is 14° wide and 75° high and has a fundamental row frequency of 0.053 cycles/°, near optimal for hoverfly HS neurons (Straw et al., 2006). We simulated doublet motion at 90°/s with the doublet at either full or 10% contrast in both the preferred (A) and anti-preferred direction (B). For display purposes the doublets are not shown at their true contrasts or size. (C,D). Normalized time-luminance graphs as seen at the first edge of the slit-window, i.e., the right hand edge for preferred, right-to-left motion, and the left hand edge for anti-preferred left-to-right motion. Solid black lines represent the full contrast condition, dashed gray lines show the 10% original contrast condition. (E) The doublet stimulus produces a characteristic triphasic response from the EMD model in the preferred direction (gray). The neuron's response (black) is also characterized by a triphasic response profile that closely resembles the model output. (F) The EMD output (gray) is similar, but inverted, in the anti-preferred direction. The neuron's response is shown in black. The star (^*) indicates a brief depolarization of the membrane potential. (G,H) The neural responses to the low contrast doublet. Arrowheads indicate the timing of the output peaks produced by the model in panels (E,F). Although responses are qualitatively indistinguishable from one recording to the next, absolute response magnitude can vary. To enable accurate comparison of the responses across the six stimulus conditions in Figures 2–4, we show the response to one neuron in which all six conditions were performed, n = 20.