Abstract
Although temporal information processing is important in auditory perception, the mechanisms for coding tonal offsets are unknown. We investigated cortical responses elicited at the offset of tonal stimuli using flavoprotein fluorescence imaging in mice. Off-responses were clearly observed at the offset of tonal stimuli lasting for 7 s, but not after stimuli lasting for 1 s. Off-responses to the short stimuli appeared in a similar cortical region, when conditioning tonal stimuli lasting for 5–20 s preceded the stimuli. MK-801, an inhibitor of NMDA receptors, suppressed the two types of off-responses, suggesting that disinhibition produced by NMDA receptor-dependent synaptic depression might be involved in the off-responses. The peak off-responses were localized in a small region adjacent to the primary auditory cortex, and no frequency-dependent shift of the response peaks was found. Frequency matching of preceding tonal stimuli with short test stimuli was not required for inducing off-responses to short stimuli. Two-photon calcium imaging demonstrated significantly larger neuronal off-responses to stimuli lasting for 7 s in this field, compared with off-responses to stimuli lasting for 1 s. The present results indicate the presence of an auditory cortical field responding to long-lasting tonal offsets, possibly for temporal information processing.
Highlights
When tonal stimuli at 5 kHz or 20 kHz lasting for 1 s were administered to mice, flavoprotein fluorescence responses were observed in A1, AAF, and A2 (Fig. 1a), as previously reported[17,18,19,20,23]
The auditory cortex is divided into various fields of distinct response properties; identification of cortical fields is an important step for understanding the roles of the auditory cortex
Optical imaging revealed the presence of many fields in the mouse auditory cortex
Summary
In the present study marked off-responses were observed at the offsets of long tonal stimuli lasting for 7 s. The peak off-responses were localized in a small cortical field between A1 and DP, and the same field was activated by offsets of short tonal stimuli lasting for 1 s, when the short stimuli were preceded by conditioning tonal stimuli. Using two-photon calcium imaging of neuronal responses, we confirmed that neurons in the off-response field showed significantly larger off-responses to tonal stimuli lasting for 1 s compared with off-responses to stimuli lasting for 1 s. These findings suggest the presence of an auditory cortical field coding long-lasting tonal offsets or tonal duration in mice
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