Functional Organization of Auditory Cortex in the Mongolian Gerbil (Meriones unguiculatus). I. Electrophysiological Mapping of Frequency Representation and Distinction of Fields

Abstract

The frequency representation within the auditory cortex of the anaesthetized Mongolian gerbil (Meriones unguiculatus) was studied using standard microelectrode (essentially multiunit) mapping techniques. A large tonotopically organized primary auditory field (AI) was identified. High best frequencies (BFs) were represented rostrally and low BFs caudally along roughly dorsoventrally oriented isofrequency contours. Additional tonotopic representations were found adjacent to AI. Rostral to AI was a smaller field with a complete tonotopic gradient reversed with respect to that in AI (mirror image representation) and was termed the anterior auditory field (AAF). BFs in the range from 0.1 to 43 kHz, apparently covering the hearing range of the Mongolian gerbil, were found in AI and AAF. Units in these two core fields responded to narrow frequency ranges with short latencies. Ventral to the common high-frequency border to AAF and AI, a rapid transition to very low BFs suggested the presence of a ventral field (V). Caudal to AI two small tonotopically organized fields were identified, a dorsoposterior field (DP) and a ventroposterior field (VP). The VP showed a tonotopic organization mirror imaged to that of AI, i.e. low frequencies were represented rostrally near the caudal border of AI, and high frequencies caudally. The DP showed a concentric frequency organization with high BFs located in the centre. Units in DP and VP fired less strongly, with considerably longer latencies, and responded to a broader range of frequencies than units in AI and AAF. Dorsocaudal to AI a dorsal field (D) was identified, harbouring units that responded to very broad ranges of frequencies. A tonotopic organization of field D could not be discerned. In the border region of AI and D, low-frequency responses were similar to those found in parts of AI and AAF, but without a clear-cut tonotopic organization. This region was termed Ald. The two core fields AI and AAF appeared to be located within the koniocortex, while the remaining fields lay outside. Our data show that the organization of the gerbil auditory cortex is highly elaborate, with parcellation into fields as complex as in cat or primates.

Centripetal and centrifugal reorganizations of frequency map of auditory cortex in gerbils

“…The amount of BF shifts we observed was related to differences in both distance and BF between recorded and stimulated cortical neurons. Iso-BF lines are not necessarily parallel to each other and show some variations among animals (22,29). Accordingly, in our data pooled from 11 animals, the area for the centripetal BF shift overlapped with the surround for the centrifugal BF shift, and neurons showing centrifugal BF shift were intermixed with those showing no BF shift.…”

Plasticity of the cochleotopic (frequency) map in specialized and nonspecialized auditory cortices

“…In the AI of the gerbil, frequencies between 0.1 and 43 kHz are systematically mapped, and frequencies from 0.1 to 10 kHz are somewhat over-represented compared to other frequencies (7). Most of our data were obtained by stimulating the neurons located at the approximate center of the AI (Fig.…”

Consequences of unilateral hearing loss: Cortical adjustment to unilateral deprivation

“…A rostral band marking the AAF, and a more caudal band marking AI were visible in each section (see Fig. 3; and Thomas et al, 1993;Scheich et al, 1993). Once AI and AAF were identified, a 50 × 50 μm window was used to sample the OD in cortical layer IV of each region (e.g., arrow in Fig 3).…”

“…Until recently, AI and AAF were considered to be, in essence, mirror images of one another, each possessing a strict and orderly tonotopic map (e.g., Merzenich et al, 1975;Knight 1977;Imig, 1980, Phillips andIrvine, 1982;Morel et al, 1993;Thomas et al, 1993;Stiebler et al, 1997;Rutkowski et al, 2003) and having similar anatomical connections (e.g., Anderson et al, 1980b;Imig and Reale, 1980;Imig and Morel, 1983), though AAF occupies a smaller area of cortex. This concept has been challenged in several species, based on anatomical, behavioral, and physiological evidence.…”