Cortical Coding Of Sound Source Azimuth And Sound Stimulus Level In Complex Acoustic Environments

Natural acoustic environment is often complex, the neural mechanism of how the auditory system efficiently extracts target signals in a complex acoustic environment is unclear. In the present study, we simulated the complex acoustic environment by using preceding noise or preceding tone stimuli, and investigated the mechanism of the cortical coding of sound source azimuth and sound stimulus level in a complex sound environment. The present study is divided into three parts.1. Preceding noise modulates the azimuth-level response areas of neurons in the rat primary auditory cortex.In the natural acoustic environment, sounds commonly occur with noise either simultaneously or in a temporal relation. Therefore, it is important to understand how a cortical neuron responses to sound signal in a noisy environment. Using forward masking paradigm, we investigated the effect of preceding noise on the azimuth-level response areas (ALRAs) of neurons in the rat auditory cortex. We found that a preceding superthreshold noise altered the shape and extent of the ALRAs, greatly reduced response magnitude and lengthened the first-spike latencies in the response areas. The effect was dependent on the level of noise:the higher the noise level, the greater the effect. Preceding noise evoked a non-uniform inhibition on firing rates and first-spike latencies across the azimuth-level response areas. Preceding noise affected least in firing rates and first-spike latencies of responses to the neurons'prefferred stimuli, and affected more on the firing rates and first-spike latencies of responses to the non-prefferred stimuli. This resulted in an increased selectivity of a cortical neuron for its preferred azimuth-level combinations. These obervations indicate that the response areas of AI neruons were not static but fluid; they can vary greatly in shape, extent, response magnitude and latency. The dynamic variation of the response areas, and the preference selectivity of AI neurons to their preferred stimuli might be related with sound information coding in a complex acoustic environment.2. Temporal effect of preceding noise on azimuth-level response areas of neurons in rat primary auditory cortex In the natural acoustic environment, sounds can arrive at the ears in quick succession. However, it is still unclear how neurons in the auditory cortex encode auditory signals with a temporal relation in a complex acoustic environment. In the present study, we focused on the effect of the interstimulus interval (ISI) between preceding noise and successive tone stimuli on the azimuth-level response area (ALRA) of neurons in the rat primary auditory cortex (AI). We found that after the offset of a preceding noise, the ability of AI neurons to respond to succeeding tone varied dynamically in time. At short ISI, the preceding noise could greatly reduce spike rates and lengthen the first-spike latencies across the ALRA. With increasing ISIs, the responses of AI neurons to stimuli from preferred azimuth-level combinations recovered faster than to other stimuli. The recovery was non-uniform in spike rates and first-spike latencies across the ALRAs. During the tens to hundreds of milliseconds after a preceding noise the selectivity of AI neurons to preferred stimuli increased. The degree of increasing in selectivity was decreased with increasing ISIs. At different ISIs, the spike counts and first-spike latencies in ALRAs of most AI neurons showed significant negative correlation. These results suggest that the non-uniform recovery from preceding stimulation might be one of the mechanisms for AI neurons to code their preferred stimuli in a noisy environment.3. The effect of a preceding sound on the level discrimination of a successive soundIn a complex acoustical environment, the ability to accurately discriminate the sound level in sequential sound condition is important for human and other species to process sound information. The present study focuses on the effect of a preceding sound upon the just noticeable difference (JND) in sound level discrimination of four subjects to a successive sound. The data were collected in the dichotic sound field. The direction and the level of both the preceding and the successive sounds were manipulated by the average binaural level and interaural level difference. An effective preceding sound can increase the JND when discriminating the level of a successive sound. When the preceding sound ABLs was lower than 40, it had no significant effect on the JND compared to the control, unmasked condition. With increasing ABLs of the preceding sound, the JND was increased when the level of the successive sound was at lower and moderate ABLs. The preceding sound had little effect on the JND when the successive sound was at higher ABLs. The effect of the preceding sound on the JND of the level discrimination to the successive sounds showed similar trend regardless of the difference of ILDs between the preceding and successive sounds. The results indicate that the influence of preceding sound on JND of level discrimination to the successive sound was related to both the preceding and the successive sound levels, but not to the sound source azimuth.