Encoding Of Sound Spatial Information By Neurons In The Rat Auditory Cortex In Noise Environment

Many previous studies have shown that auditory cortical neurons are sensitive to sound spatial information, however, the neural mechanism of sound spatial coding is still not fully understood. Until now, detail studies on sound spatial coding have not been reported in the rat primary auditory cortex. Moreover, only few studies have investigated the effect of forward noise on sound spatial coding of auditory cortical neurons in cat in virtual acoustical condition. By employing electrophysiological technique, we investigated, in free field condition, spatial response areas in quiet and forward noise acoustical environment in the rat primary auditory cortex. The findings are as follows:1 Encoding of sound spatial information by neurons in the rat primary auditory cortexThe present study investigated spatial response areas of 151 neurons in the rat primary auditory cortex. The relationships between spike counts and average first-spike latencies in the spatial response areas were analyzed. The results showed that, more than half of cortical neurons (52.32%) exhibited contralateral preference in the frontal auditory space whereas other neurons exhibited ipsilateral preference (18.54%) and midline preference (18.54%), and only a few neurons were included in the category of omnidirection (3.31%) and complex (7.28%). For the overwhelming majority (84.81%) of contralateral preference neurons, the geometrical centers of the preferred spatial area were distributed in the up and middle portion of the contralateral space relative to the recording side. Most (77.14%) neurons responded strongly to stimuli from their preferred space with shorter average first-spike latencies, and responded weakly to stimuli from non-preferred space with longer average latencies. In the spatial response area, the spike counts were negatively correlated with average first-spike latencies. The auditory cortex might use the information of both spike counts and average response time to code sound spatial information. 2 The effect of forward noise on sound spatial coding of neurons in the rat primary auditory cortexWe investigated spatial response areas of 58 neurons in the rat primary auditory cortex under the conditions of quiet and forward noise. The changes of spike counts and average first-spike latencies in the spatial response areas under different conditions were analyzed. The results showed that, compared with that in quiet environment, under the condition of forward noise, the size of spatial response areas of the most (91.38%) cortical neurons was reduced, accompanied by a reduction of spike counts and lengthening of first-spike latency. For the majority (77.36%) of cortical neurons, the difference of the geometrical center of the preferred spatial area between quiet and forward noise conditions was within 30°in azimuth and elevation, the best response areas of most neurons (74.14%) were relative stable and their spike counts of preferred areas were still the largest under the condition of forward noise. In quiet condition, within the spatial response area of most (87.72%) neurons, the spike counts were negatively correlated with average first-spike latencies, whereas under the condition of forward noise, the majority (66%) of neurons maintained this negative correlation relationship. Under forward noise condition, the auditory cortex might code sound spatial information by relative stably keeping the function gradients of spike counts and first-spike latencies in the auditory spatial response areas of auditory neurons.