Figure 8.

Relative time course of activation of excitation and feed forward inhibition for approaching, receding and suddenly appearing objects as inferred from the experimental data. A. Excitation and inhibition are activated in parallel during approach, with excitation leading inhibition by approximately 50 ms. In control conditions, inhibition becomes only apparent during response termination as excitation decays earlier and faster than inhibition (shaded area). The relative strength of inhibition relative to excitation is expected to increase faster with angular size at high l/|v| values, to account for faster response termination (Figs. 3, 4). B. In response to receding stimuli, excitation is sharply activated, followed by delayed inhibition. This leads to a short summation time window (~10 ms) during which 1–2 spikes are reliably emitted. Thereafter, inhibition is expected to hover roughly at the same level as excitation, since additional spikes are sporadically emitted in response to the stimulus. A similar activation pattern is expected for suddenly appearing objects. However, excitation is likely to be activated less sharply, as the probability of spiking at the onset of the stimulus is decreased. The absolute activation level is expected to be smaller than for approaching objects.