Encoding Of Sound Spatial Information By Neurons In The Rat Primary Auditory Cortex In A Complex Acoustic Environment

Many previous studies have reported sound spatial information encoding of auditory cortical neurons in a complex acoustic environment, however, most of these studies were investigated under the condition of background noise. Few studies on sound spatial coding in the condition of forward masking noise have been reported in the rat primary auditory cortex, no reports were found regarding the level of forward masking noise on sound spatial coding. By employing electrophysiological technique, we investigated, in free field condition, the sound spatial information encoding of neurons in the rat primary auditory cortex under both the condition of signal along and different levels of forward masking noise. Moreover, we analyzed the influence of different levels of forward masking noise on spatial response areas. The findings are as follows:1 Sound spatial information encoding by neurons in the rat primary auditory cortexWe investigated spatial response areas of 90 neurons in the rat primary auditory cortex in signal alone condition, and analyzed the property of spatial response areas. The relationships between spike counts and average first-spike latencies in the spatial response areas were analyzed. The results showed that, under the condition of signal alone, the response areas of neurons can be divided into five categories:contralateral preference(63.3%), midline preference (20.0%), ipsilateral preference (3.3%), omnidirection (2.2%) and complex (11.1%). The majority (87.4%) of 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 the average first-spike latencies. These results are similar to the findings of our lab's previous studies. We analyzed the relationship between the characteristic frequency of the neuron and its preference category, and we also analyzed the relationship between the preference category and the correlation of spike counts and average first-spike latencies. No significant correlations were found regarding the above relationship. The functional gradients and the correlation of spike counts and first-spike latencies in the auditory spatial response areas might be properties of primary auditory cortical neurons in encoding sound spatial information. The auditory cortex might use the information of both spike counts and latencies to code sound spatial information.2 Sound spatial information encoding by neurons in the rat primary auditory cortex in forward masking noise environmentWe investigated spatial response areas of 72 neurons in the rat primary auditory cortex both in the condition of signal alone and forward masking noise at different levels. The changes of spike counts and average first-spike latencies and their relationships in the spatial response areas under different conditions were analyzed. We also analyzed the changes of preferred space under different conditions. The results are as follows: compared with signal alone condition, under the condition of forward masking noise, the size of spatial response areas of most (87.5%) cortical neurons was reduced, accompanied with a reduction of spike counts and the lengthening of first-spike latency. When the level of noise was increased, the size of spatial response areas was reduced further. Under the condition of signal alone, for the majority (84.7%) of neurons, the spike counts were negatively correlated with average first-spike latencies within the spatial response areas. Under the condition of forward masking noise, most (76.4%) neurons maintained this negative correlation relationship as well. When the level of forward masking noise was increased, more than half (65.3%) of the neurons maintained this correlation relationship. For great majority (94.8%) of cortical neurons, the changes of the responsive center under the condition of forward masking noise were within 30°both in azimuth and elevation, the best response areas were relatively stable and their spike counts of preferred areas were still the largest under the condition of forward masking noise. When the level of forward masking noise was increased, the best response areas of most neurons (79.3%) were still relatively stable. In a complex acoustic environment, the auditory cortex might code sound spatial information through adjusting the changes of spike counts and first-spike latencies to keep their function gradients in the auditory spatial response areas stable.