Encoding Of Sound Stimulus Frequency Information Of Neurons In The Rat Auditory Cortex In A Complex Acoustic Environment

In the natural acoustic environment, sound seldom occurs in isolation. Signal sounds are usually detected and discriminated by human and animals under certain acoustic context. Being the highest hierarchy of the central auditory system, auditory cortex plays an important role in the acoustic information encoding. The acoustic information processing could be dynamically modulated by the changing acoustic context. The main purpose of our study is to investigate how the primary auditory cortex (Al) encodes sound frequencies under the dynamically changing acoustic context. Using a forward masking paradigm, we recorded the frequency level receptive fields (FLRFs) or the frequency level response areas (FLRAs) of Al neurons of rats under both quiet and forward-masked conditions.Our study was divided into three parts and the results are as follows:1. The effects of preceding noise on the FLRF of Al neuronsThe white noise was used as masker sounds, the frequency and the level of the probe sounds were systematically varied. The masking effects to the FLRFs were mainly inhibitory, and the degree of inhibitory influences increased monotonically with the masker level. For some of the recorded neurons, the characteristic frequencies (CFs) were not changed significantly even under the high level masker conditions. For the other recorded neurons, their CFs shifted under the masked conditions, the higher the masker level, the more the CF shift was likely to happen. The bandwidths (BWs) of the FLRFs could be either narrowed or widened under the masked conditions and the percentage of neurons with narrowed BW was greater than the percentage of neurons with widened BW. The tip part of the FLRFs is more sensitive to the acoustic context than the middle part. The inhibitory masking effect gradually decayed as the inter-stimulus interval (ISI) getting longer. The higher the masker level, the longer the inhibitory masking effect lasted.2. The effects of preceding tones on the FLRF of Al neurons Pure tones were used as maker sounds in the forward-masking paradigm. The degree of modulation by forward pure tone on the FLRF was dependent on the masker frequency, masker level and the ISI. When the masker level and ISI were fixed, the inhibitory masking effects were stronger if the masker frequency was at or near to the neuron's CF rather than off the CF. The influences on Al neurons' CFs were also dependent on the masker frequencies. The probability of CF shift was greater if the masker frequency was nearer to the neuron's CF under quiet condition. The BWs of FLRFs were narrowed, widened, and not significantly variied under masking conditions. However, the masking effects on the BWs were mainly narrowed. In most cases, a preceding tone would lead to a stronger inhibitory effect on FLRFs than a preceding noise with the same sound stimulus level. The higher the masker level, the stronger the forward inhibitory effect was. The effects of the masker frequency and level were interacted in the forward suppression. The forward suppression effect on the FLRF was strongest if the masker frequency was at the neuron's CF and if the masker level was at the highest level. Al neurons'FLRFs gradually recovered as the ISI getting longer. The forward suppression could last for hundreds of milliseconds. Compared to the masker with frequencies off CF, the masker with frequencies at or near to CF led to longer forward suppression.3. The effects of preceding tones on the FLRA of Al neuronsThe masker frequency, level and ISI all contributed to the contextual modulation of the FLRAs of Al neurons. The masking effects were mainly inhibitory. However, facilitation occasionally occurred when the masker frequencies were off a neuron's CF. The most effective masker frequencies were positively correlated with the neurons'CFs. The characteristic frequencies or the best frequencies of some of the recorded neurons didn't significantly shift under all the tested masker conditions. In most cases, the best frequency ranges (BFRs) of the FLRAs of Al neurons were narrowed, which led to an increment of the selectivity to the BFs of neurons. Higher masker level and shorter ISI had stronger impact upon Al neurons'FLRAs.In summary, the results of our study indicated that the FLRFs and FLRAs of Al neurons were plastic rather than static. The impacts of a preceeding sound on the FLRFs and FLRAs of an Al neuron were related to the frequency and level of the preceding sound, and were also related to the CF of the neuron, and the ISI. The results suggest that the encoding of sound frequency information by auditory cortex neurons in a complex acoustic environment is influenced by both the frequency tuning of the neurons and the sound stimulus context.