Adaptation in the Dorsal Belt and Core Regions of the Auditory Cortex in the Awake Rat.

The rat auditory cortex is divided anatomically into several areas, but little is known about the functional differences in information processing among these areas. Three tonotopically organized core fields, namely, the primary (A1), anterior (AAF), and ventral (VAF) auditory fields, as well as one non-tonotopically organized belt field, the dorsal belt (DB), were identified based on their response properties. Compared to neurons in A1, AAF and VAF, units in the DB exhibited little or no response to pure tones but strong responses to white noise. The few DB neurons responded to pure tones with thresholds greater than 60 dB SPL, which was significantly higher than the thresholds of neurons in the core regions. In response to white noise, units in DB showed significantly longer latency and lower peak response, as well as longer response duration, than those in the core regions. Responses to repeated white noise were also examined. In contrast to neurons in A1, AAF and VAF, DB neurons could not follow repeated stimulation at a 300 ms inter-stimulus interval (ISI) and showed a significant steeper ISI tuning curve slope when the ISI was increased from 300 ms to 4.8 s. These results indicate that the DB processes auditory information on broader spectral and longer temporal scales than the core regions, reflecting a distinct role in the hierarchical cortical pathway.

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.

Adaptation facilitates spatial discrimination for deviant locations in the thalamic reticular nucleus of the rat.

The capacity to identify unanticipated abnormal cues in a natural scene is vital for animal survival. Stimulus-specific adaptation (SSA) has been considered the neuronal correlate for deviance detection. There have been comprehensive assessments of SSA in the frequency domain along the ascending auditory pathway, but only little attention given to deviance detection in the spatial domain. We found that thalamic reticular nucleus (TRN) neurons exhibited stronger responses to a tone when it was presented rarely as opposed to frequently at a certain spatial location. Subsequently, we engaged signal detection theory to directly gauge neuronal spatial discriminability and found that discrimination of deviant locations was considerably higher than standard locations. The variability in neuronal spatial discriminability among the TRN population was directly related to response difference (RD) but not variance; meanwhile, further analyses attributed higher spatial sensitivity at deviant locations to larger RD. Astonishingly, a significant correlation was found between the amount of adaptation and deviant discriminability. Collectively, our results suggest that adaptation facilitates rare location discrimination by sharpening the response gap between two locations.