Functional organization of the auditory cortex in a native Chilean rodent (Octodon degus)

The tonotopic organization of primary auditory cortex (AI) and surrounding secondary regions has been studied in the Octodon degus using standard microelectrode mapping techniques. The results confirm and extend previous observations made in other species. The tonotopic organization of the largest field (AI) apparently covered the hearing range of O. degus. Low tonal frequencies were represented rostroventrally and high frequencies caudally, with isofrequency contours orientated dorsoventrally in a ventrocaudal slant. There were additional tonotopic representations adjacent to AI. Rostral to AI, a small field with a tonotopic gradient reversed with respect to that in AI (mirror image representation) was mapped and termed rostral auditory field (R). Best frequencies (BF's) in a range from 0.1-30.0 kHz were found in AI and R, with higher spatial resolution for the representation of lower BF's up to 10.0 kHz. Responses obtained in AI as well as in R were strong, with narrow tuning and short latencies. Caudal to AI, two small additional, tonotopically organized fields, the dorsoposterior field (DP) and the ventroposterior field (VP), could be distinguished. In fields VP and DP, high BF's were situated rostrally, adjacent to the high frequency representation in AI. Low frequency representations were found in caudal part of DP and VP fields. Responses to tone burst within DP and VP were mostly weak, with longer latencies and broader tuning compared to those found in AI and R.