Level-dependent Encoding Of Sound Spatial Information Of Neurons In The Rat Auditory Cortex

Many studies regarding auditory cortical neurons that encode sound spatial information in bats, cats, ferrets, mice and monkeys have been reported, but the encoding mechanism is not fully understood. There are few similar studies reported in rat primary auditory cortex, and the level-dependent encoding of sound spatial information have not been reported. Therefore, the purpose of this study is to explore the neural mechanism of sound spatial information coding at different acoustic stimulus level in the rat auditory cortex.In this study, we used electrophysiological technique to explore the changes of spatial response areas, spike counts, the average first-spike latencies, and the relationship between them under different acoustic stimululs levels in 142 neurons in the rat primary auditory cortex (AI). The changes of spatial preferred characteristics, the azimuthal angular range(AR), and best azimuth(BAz) at three acoustic level conditions were also analysed. The results showed that the majority of cortical neurons (69.72%,99/142) exhibited contralateral preference in the frontal auditory space, where the neurons exhibited more spikes but shorter latencies. In the spatial response areas of 122 neurons which had preferred spatial areas, the spike counts of most neurons (86.89%,106/122) were negatively correlated with the average first-spike latencies at 5-10 dB above threshold level (level 1). With the increasing of sound stimulus level, the majority of neurons (90.16%,110/122, level2), (90.16%, 110/122, level3) maintained this negative correlation relationship. Also, with the increase of sound stimulus level, the spatial response areas of neurons were enlarged, and associated with the increase of spikes counts and the decrease of response latencies. However, the change of most neurons'geometrical centers of the preferred spatial areas were less than 30°in both azimuth and elevation. In addition, the best azimuths(BAz) of most neurons at three acoustic level conditions were not significantly changed. The results showed that the preferred locations of auditory cortical neurons were maintained relatively stable at diferent stimuls levels. However, azimuthal angular range (AR) became smaller with the decrease of stimulus level, which showed enhanced spatial selectivity for preferred spatial areas of AI neurons.These results demonstrated that the encoding of acoustical spatial information of rat auditory cortical neurons were dependent on sound stimulus level. The neurons in the primary auditory cortex might code sound spatial information by keeping the functional gradients of spike counts and response latencies in the auditory spatial response areas relatively stable.