Abstract
In barbiturate-anesthetized animals, primary auditory cortex (A1) neurons usually show onset responses for tonal stimuli. It is generally believed that A1 neurons do not have information of offset. However, offset responses are usual in A1 of awake animals and ketamine- and halothan-anesthetizad animals. We compared tonal offset responses with the onset responses in A1 of the awake animals. A majority of the neurons with phasic responses showed offset responses to tonal onsets, whereas only a small percentage of the neurons with sustain responses showed offset responses. The phasic neurons with on-off responses had different combinations of offset and onset frequency-receptive-field (FRF): offset-FRF was similar to onset-FRF, wider, narrower, higher or lower than onset-FRF. The offset-FRF pattern of each neuron was not changed at shift of the stimulation intensity and duration. At shift of the stimulus intensity response amplitude changed but not time parameters. Neuron-distribution pattern plotted by FRF-edge frequencies was diffuse, and pattern-boundaries were not found. The onset and offset responses fused at 50ms stimulus-duration. The offset responses were evoked whether the preceding inhibition was present or absent. The offset responses had a similar peak-latency, longer half-decay time and lower peak-amplitude than those of onset responses. In conclusion, the offset-FRF patterns are diverse. The offset responses are precise and salient when the tonal duration is > 50ms. The offset responses may be evoked by active mechanisms, but not passive rebound facilitation of a preceding inhibition. [J Physiol Sci. 2007;57 Suppl:S105]
