[Effects of background noise on auditory response characteristics of primary auditory cortex neurons in awake mice]

To study the effects of different continuous background noises on auditory response characteristics of primary auditory cortex (A1) neurons in awake mice.

We performed in vivo cell-attached recordings in layer 4 neurons of the A1 of awake mice to investigate how continuous background noises of different levels affected the intensity tuning, frequency tuning and time characteristics of individual A1 neurons. According to the intensity tuning characteristics and types of stimulation, 44 neurons were devided into 4 groups: monotonic-intensity group (20 monotonic neurons), nonmonotonic-intensity group (6 nonmonotonic neurons), monotonic-frequency group (25 monotonic neurons) and monotonic-latency group (15 monotonic neurons).

The A1 neurons only had transient spike response within 10 to 40 ms after the onset of continuous wild-band noise stimulation. The noise intensity had no significant effects on the background firing rates of the A1 neurons (P>0.05). The increase of background noise resulted in a significant linear elevation of the intensity threshold of monotonic and nonmonotonic neurons for tone-evoked response (R²>0.90, P < 0.05). No significant difference was observed in the slopes of threshold changes between monotonic and nonmonotonic neurons (P>0.05). The best intensity of nonmonotonic neurons increased along with the intensity of the background noise, and the variation of the best intensity was positively correlated with the change of the threshold of the same neuron (r=0.944, P < 0.001). The frequency response bandwidth and the firing rate of the A1 neurons decreased as the noise intensity increased (P < 0.001), but the best frequency almost remained unchanged (P < 0.001). The increase of background noise intensity resulted in an increased first spike latency of the neurons to the same tone stimulus (P < 0.05) without affecting the time accuracy of the first action potential (P>0.05).

The acoustic response threshold of the A1 neurons increases linearly with the increase of background noise intensity. An increased background noise leads to compressed frequency band-width, a decreased firing rate and a prolonged spike latency, but the frequency selectivity and the time accuracy of auditory response to the same noise remain stable.