Figure 9.

Mechanisms of direction selectivity. A, A neuron (from a 24-d-old pup) with an early arriving band of LFI centered at 31 kHz. The HFI arrived 5 ms late. B, The response was suppressed to 50% of control even when the excitatory tone was presented 1 ms earlier than the inhibitory tone, showing that inhibition generated by the 32 kHz tone arrived early. C, This neuron was direction selective, with DSI of 0.69. The upward sweep that included the LFI (30–50 kHz) elicited weak responses. However, responses to an upward FM sweep that excluded the LFI (33–53 kHz) were within 30% of maximum response to the downward sweep. D, A neuron (from a 18-d-old pup) with delayed LFI. The LFI centered at 33 kHz was delayed by at least 4 ms after the excitatory input. E, A 50% suppression of response to the excitatory tone was produced only when the inhibitory tone was advanced by 3 ms. F, This neuron was not direction selective (DSI of −0.09) but was rate selective for the upward sweeps that included the LFI (30–50 kHz). Starting the upward sweep inside the excitatory tuning curve (36–56 kHz) caused rate selectivity for upward sweeps to be eliminated (F). The vertical gray bars in A and D illustrate the excitatory frequency tuning at the intensity at which the TTI test was conducted. The vertical lines in A and D indicate the BF of the neuron. Dashed lines in B and E indicate the arrival time of 50% inhibition.