Integrative stimulus-specific adaptation of the natural sounds in the auditory cortex of the awake rat.

Using oddball stimulus with pure tones, researchers have extensively investigated stimulus-specific adaptation (SSA), which has been regarded as a method of novelty detection, from the inferior colliculus (IC) to the auditory cortex (AC). However, until now, it is not clear whether SSA is preserved for natural sounds or whether it exists for spatial cues in the AC. Moreover, it is also unclear whether SSA integrates different types of cues within a single modality such as sound location and sound identity. Here, we addressed these issues using two natural sounds presented at two different locations while simultaneously performing extracellular recordings in the AC of awake rats. Our data showed that SSA was present in the AC for the natural sounds, the pure tones, and the spatial locations in the neuronal population. We also found that the AC response to a double deviant stimulus (a deviant sound at a deviant location) was stronger than that to a single (either a deviant sound or the same sound at a deviant location); this finding suggests that detecting unexpected events benefits from the integration of different cues within the same modality.

Frequency-Dependent Stimulus-Specific Adaptation and Regularity Sensitivity in the Rat Auditory Thalamus.

Recent electrophysiological studies in animals using oddball stimuli have demonstrated that neurons along the auditory pathway from the inferior colliculus to the auditory cortex (AC) have a strong response to rarely presented stimuli. This phenomenon is termed stimulus-specific adaptation (SSA), which is regarded as novelty detection. However, in the medial geniculate body (MGB), it is not clear whether SSA is frequency dependent or if neurons in the MGB are sensitive to the regularity of the stimuli. In this present study, we analyzed the relationship between stimulus frequency and SSA, as well as explored regularity sensitivity using extracellular recordings in the MGBs of rats with regular and irregular oddball stimuli. It was found MGB neurons exhibited strong SSA when the pure-tone stimulus of the oddball stimulus deviated far from the characteristic frequency, even in the ventral region of the MGB, suggesting that the MGB may contribute to SSA in the primary AC. Moreover, we found the neuronal population in the MGB was sensitive to high-order sound structure, where deviant responses were smaller and standard responses were stronger for irregular oddball stimuli. We conclude that regularity detection occurs in the MGB, but in a manner distinct from the AC.