Optical recording studies on segmentation and induction of sounds in the auditory cortex

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This study was aimed to investigate neural mechanisms of grouping, segregation and temporal induction of sounds. Using an optical imaging technique with a voltage-sensitive dye RH795, neural activity was recorded in the primary auditory cortex (AI) of anesthetized guinea pigs. Complex tones (CT, 200-ms long) consisting of two pure-tones (PT) with 0-2.0-octave frequency separations and 0-100-ms time delays were used to study grouping and segregation. Gap-inserted (PGP) and white-noise (WN)-inserted pure tones (PWP) with 0-120-ms gap and WN lengths were used to study induction. They were presented to the ear contralateral to the recording cortex.In response to CT, strength of AI activity and widths of responding areas increased at 0.2-0.7 octave but did not at more than 0.8 octave. If one component in the CT was delayed more than 5ms from the other, the response to the delayed component was inhibited by the preceding one. These results suggest that AI of guinea pigs has a mechanism to group sounds with near frequencies (less than 0.7 octave) and short delays (less than 10 ms) and to separate those with far frequencies and long delays.When PGP with a gap shorter than 5 ms was presented, the AI response appeared only at the onset of the pre-gap tone, while it appeared at both the onsets of pre- and post-gap tones if the gap was longer than 5 ms. If WN was inserted into the gap, the response appeared only at the onset of the pre-WN tone, even if the WN period was longer than 5ms. These results suggest that the insertion of WN increases the threshold of gap detection in AI and this phenomenon may relate to auditory induction mechanism in the brain.