Fast temporal adaptation of on-off units in the first optic chiasm of the blowfly.

1. We recorded from spiking units in the first optic chiasm between lamina and medulla in the brain of the blowfly (Calliphora vicina). Both previously characterized neuron types, on-off units and sustaining units, were encountered. On-off units had a temporal frequency response with a lower cut-off frequency than blowfly photoreceptors. This low cut-off frequency is related to a fast temporal adaptation of the on-off units to trains of short light pulses. Temporal adaptation occurred independently for short on- and off-pulses. 2. On-off units only responded to stimuli of relatively large contrast. Contrasts of less than 10% gave little or no response.