| In natural environment, sound signals often occur with background noise. Therefore, in order to understand the neural mechanism of sound localization in a noisy environment, it is important for us to study the neural coding of sound spatial information by auditory cortex in background noise conditions. Many previous studies have reported the cortical coding of sound spatial information in cats under the conditions of background noise or forward masking noise. However, there are still many issues need to be studied. For example, how do background noise affect the spatial preference and the azimuth selectivity of population neurons in the auditory cortex? How do background noise influence the correlation between spikes and first-spike latencies in the spatial response areas of auditory cortical neurons? Therefore, the main purpose of this study is to investigate the impact of background noise on the neural coding of sound spatial information in the rat primary auditory cortex in free field conditions, and to further explore the neural mechanisms of cortical coding of sound spatial inforamtion in a noisy environment. The results are as follows:1. The property of sound spatial information coding of neurons in the rat primary auditory cortex in quiet conditionsUsing electrophysiological method, we studied the neural responses to sound stimuli from frontal auditory space and the first-spike latencies of118neurons in the primary auditory cortex of62rats in quiet conditions. The spatial response areas of each neuron were represented by either the number of spikes or first-spike latencies. The neurons were classified into five categories according to their spatial preference in azimuth:contralateral preference (68.64%,81/118), midline preference (16.95%,20/118), ipsilateral preference (3.39%,4/118), omnidirection (4.24%,5/118), and complex (6.78%,8/118). The data indicates that overwhelming majority of neurons preferred the sound from the contralateral space relative to position of the recorded neuron. According to the vertical preference in the spatial response area of neurons, we categorized the neurons into up preference (48.57%,51/105), midline preference (45.71%,48/105) and down preference (5.71%,6/105). The result indicates that most cortical neurons preferred acoustical stimuli from up and midline auditory space. For most neurons (85.59%,101/118), the spike counts and the first-spike latencies in the spatial response areas had a significant negative correlation. The neurons responded stronger and faster (shorter latency) to sound stimuli from their preferred space compared to sound stimuli from non-preferred space. These results suggest that the comprehensive information of spike counts and latencies in the spatial response areas of auditory cortical neurons might play an important role in sound spatial information encoding.2. The effect of background noise on sound spatial information coding of neurons in the rat primary cortexWe further explored the sound spatial encoding of the118cortical neurons in the presence of background noise. The result indicates that:1) Compared with that in quiet conditions, the spatial response areas of most (80.95%) neurons were monotonically reduced with increasing levels of background noise, accompanied by a reduction in spike counts and an increase in first-spike latency.2) Compared with that in quiet conditions, the deviation of the geometrical center of the preferred auditory space of most neurons were less than30°in both azimuth and elevation in the presence of background noise. The percentages of these neurons were95.19%,88.46%, and73.08%at three increasing levels of background noise conditions.3) The best azimuths of most neurons remained relatively stable in background noise conditions compared to that in quiet conditions. However, the preferred angular range of most neurons reduced as the noise levels increased, which indicates an increased selectively for sound azimuth.4) In quiet and in three increasing levels of background noise conditions, the percentages of neurons whose spikes counts and latencies had a significant negative correlation were decreased as the level of background noise increased, i.e.,85.59%,81.36%,71.19%, and50%, respectively. All of these results indicate that, under the conditions of background noise, the gradient of spike counts and latencies in the auditory response area, the geometrical center of the preferred auditory response, and the negative correlation between spike counts and response latencies remained relatively stable. Also, background noise resulted in an increased azimuth selectivity of most of the auditory cortex neurons. These properties might contribute to the underling neural mechanism of sound localization by animals and human in the background noise environment. |
