Age-related Coding Of Sound Temporal Gaps By Neurons In The Rat Auditory Cortex

Studies regarding sound temporal information processing have been reported in human, chinchilla, mouse and rat. Most of these behavioral studies were focused on measuring the gap detection thresholds (GDTs), and found age-related changes of gap detection ability. However, the underling neural mechanisms are poorly understood. Therefore, the purpose of this study is to investigate whether the GDTs of rat auditory cortical neurons are dependent on age and on sound stimulus level. Rats were divided into three groups:infant group (postnatal days20-30), young adult group (postnatal weeks8-10), and middle-aged group (postnatal weeks28-30). We used electrophysiological technique to record the responses of250neurons in the primary auditory cortex of rats to various levels of noise with a silent gap within it, and then compared the gap detection thresholds and response latencies of auditory cortex neurons in the three groups of rats. The results are as follows:1. Age-related changes of GDTs of neurons in rat auditory cortexThe gap detection thresholds of rat auditory cortical neurons were dependent on the age of rats when the sound level was constant. At80dB SPL, the percentage of neurons exhibiting the shortest GDTs (0-5ms) was15%(15/95) in infant group, however the percent of neurons in young adult group and the middle-aged group was39%(26/65), and36%(34/94), respectively. At70dB SPL, the percent of neurons capable of detecting short-duration (0-5ms) gaps was11.8%(10/89) in infant group,35.4%(20/65) in young adult group, and31.5%(29/92) in the middle-aged group. At60dB SPL, only10.6%(20/64) of neurons whose GDTs were less than5ms in infant group, in contrast, the percentage of neurons was31.25%(22/88) in young adult group and25%(20/64) in the middle-aged group. Statistic analysis showed that the GDTs of cortical neurons in infant group were significant higher than that in young adult group and in middle-aged group. However, no significant differences were found between the GDTs of neurons in young adult group and that of neurons in middle-aged group. The results demonstrated that auditory cortical neurons in young adult rats displayed a better gap-detection ability than the neurons of infant rats. Moreover, the gap detection ability of auditory cortical neurons of rats remained in a relative stable level during the postnatal period from8weeks to28weeks.2. Level-dependent changes of GDTs of neurons in the rat auditory cortexWhen the sound stimulus level was at60dB SPL,70dB SPL and80dB SPL, the medians of GDTs of auditory cortical neurons in infant group were36ms,35ms, and26ms, respectively; the medians of GDTs in young adult group were21ms,21ms, and14ms, respectively; the medians of GDTs in middle-aged group were22ms,14.5ms, and12.5ms, respectively. Statistic analysis showed that there was a significant difference in GDTs in every group across three sound levels. The GDTs at60dB SPL were significant higher than that at70dB SPL and80dB SPL. The results demonstrated that GDTs of rat auditory cortical neurons were dependent on the sound stimulus level.3. Comparison of neural response latency in the auditory cortex of different groups of ratsWe compared the changes of neural response latencies to noise2in the sound sequence presented in the gap detection experiment for neurons from different groups of rats. The results showed a general trend, i.e., the response latencies were lengthened at shorter gap duration and became shorter with increasing gap duration; the response latencies were then stabled as the gap duration reached to a certain value. This trend was evident disregarding different groups and various sound presentation level. When the sound presentation level and the gap duration were constant, the response latency of auditory cortical neurons was significant higher in infant group than that in young adult group and the middle-aged group; however, there was no significant difference in latency between young adult group and middle-aged group. Within each group, at a constant gap duration, auditory cortical neurons had shorter response latency at a higher sound level and longer latency at a lower sound level. The results indicated that age, acoustic stimulus level and inter-stimulus interval all contributed to the change of response latency of auditory cortex neurons.