Comparative study of inter-and intrahemispheric cortico-cortical connections in gerbil auditory cortex

Topographic distributions and laminar pattern of cortico-cortical projections from the primary auditory field (AI), anterior auditory field (AAF), dorsoposterior field (DP), ventroposterior field (VP), dorsal field (D) and ventral field (V) were studied in relation to tonotopic maps in combined anatomical, electrophysiological and 2-deoxyfluoro-D-glucose (2DG) experiments. Distributions of axons were examined by means of retrogradely-transported fluorescent tracer Fast Blue (FB) injected in the primary (AI) and anterior (AAF) auditory field. Injections of fluorescent tracer were placed in electrophysiologically-identified locations of AI and AAF. Neurons in AAF, DP, VP and V project to AI in the ipsilateral hemisphere. This area also receives projections from AI, AAF and D from the contralateral hemisphere. In AI, DP and VP, neurons are connected with AAF in the ipsilateral hemisphere and AI and AAF in the opposite hemisphere. In all cases, patches of labeling are distributed along 2DG bands oriented parallel to the isofrequency line. Substantial numbers of retrogradedly labeled neurons with similar best frequencies (BFs) were observed in the ipsilateral and moderate to scant numbers in the contralateral hemisphere. In general, regions near the injection sites receive more densely-labeled projections than do more distant targets. In both hemispheres, the supragranular layer III contains the greatest concentration of cortico-cortical cells bodies; the granular and infragranular layer V contains a somewhat lower concentration

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

Retinotopic organization of striate and extrastriate visual cortex in the golden hamster (Mesocricetus auratus)

The visual topography within striate and lateral extrastriate visual cortices was studied in adult hamsters. The cortical areas 17 and 18a in the left hemisphere were electrophysiologically mapped upon stimulation of the right eye, correlating receptive field positions in the visual field with cortical recording sites. Reference lesions were placed at selected cortical sites. Like in rats and other mammals, the lateral extrastriate cortex contained multiple representations of the visual field. Rostral area 18a contained the rostrolateral maps, with medial and lateral divisions. More caudally and sharing a common border with V1, maps in lateromedial, posterolateral and posterior areas were found. More laterally and forming a ®third tier® of visual maps, anterolateral, laterolateral-anterior, laterolateral and laterolateral-posterior areas were found. There was also an indication of a possible pararhinal map. The plan so defined is virtually identical to that of rats. The results may be useful to understand a basic mammalian plan in the organization of the visual cortex