Anatomy of the auditory thalamocortical system of the guinea pig

Abstract

We investigated the projection from the medial geniculate body (MG) to the tonotopic fields (the anterior field A, the dorsocaudal field DC, the small field S) and to the nontonotopic ventrocaudal belt in the auditory cortex of the guinea pig. The auditory fields were first delimited in electrophysiological experiments with microelectrode mapping techniques. Then, small quantities of horseradish peroxidase (HRP) and/or fluorescent retrograde tracers were injected into the sites of interest, and the thalamus was checked for labeled cells.The anterior field A receives its main thalamic input from the ventral nucleus of the MG (MGv). The projection is topographically organized. Roughly, the caudal part of the MGv innervates the rostral part of field A and vice versa. After injection of tracer into low or medium best-frequency sites in A, we also found a topographic gradient along the isofrequency contours: the dorsal (ventral) part of a cortical isofrequency strip receives d e r e n t s from the rostral (caudal) portions of the corresponding thalamic isofrequency band. However, it is not so obvious whether such a gradient exists also in the highfrequency part of the projection. A second, weaker projection to field A originates in a magnocellular nucleus that is situated caudomedially in the MG and was therefore named the caudomedial nucleus.The dorsocaudal field DC receives input from the same nuclei as the anterior field, but the location of the labeled cells in the MGv is different. This was demonstrated by injection of different tracers into sites with like best frequencies in fields A and DC, respectively. After injection of HRP into the 1-2-kHz isofrequency strip in field A and injection of Nuclear Yellow (NY) into the 1-2-kHz site in field DC, the labeled cells in the MGv form one continuous array that runs from caudal to rostral over the whole extent of the MGv. The anterior part of this array consists of NY-labeled cells; i.e., it projects to field DC. The caudal part is formed by HRP-labeled cells; i.e., it innervates field A. These findings indicate that there is only one continuous tonotopic map in the MGv. This map is split when projected onto the cortex so that two adjacent tonotopic fields (A and DC) result. The cortical maps are rotated relative to the thalamic map in that rostral portions of the MGv project to caudal parts of the tonotopic cortex and vice versa.The small field S receives its main thalamic input from a region situated in the rostral half of the MG medial to the ventraI nucleus (the rostromedial MG). After injection of tracer into the ventrocaudal belt, labeled cells were found dorsal, lateral, and ventral to the MGv. These cells form a continuous band that surrounds the MGv like a shell. A second population of labeled neurons was found in the caudomedial nucleus of the MG.Similarities and differences between the auditory thalamocortical systems in the guinea pig and other mammalian species are discussed.

Multiparametric Auditory Receptive Field Organization Across Five Cortical Fields in the Albino Rat

“…We found that thalamic inputs into VAF originated from regions of the MGBv that were about 0.75 mm caudal from the AI projection zone. This result is not unexpected given the known topographic cortical projection pattern along the rostrocaudal extent of an isofrequency region of the MGv, but nonetheless confirms that frequency-matched regions of VAF and AI receive inputs from nonoverlapping thalamic neuron populations (Cetas et al 2001;Huang and Winer 2000;Redies et al 1989;Velenovsky et al 2003). A comparable study in the guinea pig found that rostrodorsal regions of a single isofrequency contour in AI receives projections from the rostral pole of an isofrequency zone of MGv, whereas the caudoventral region of the same isofrequency contour received inputs from the caudal pole of the MGv (see Redies et al 1989; Fig.…”

Antidromic Activation Reveals Tonotopically Organized Projections From Primary Auditory Cortex to the Central Nucleus of the Inferior Colliculus in Guinea Pig

“…There is evidence supporting the existence of spatially distinct, functional regions within the ICC (Brunso-Bechtold et al 1981;Cant and Benson 2006;Loftus et al 2004;Oliver et al 1997;Ramachandran and May 2002;Roth et al 1978;Semple and Aitkin 1979;Shneiderman and Henkel 1987). This functional segregation of information originates as parallel projections from the brain stem to the ICC (Cant and Benson 2003) and appears to maintain some segregation up to A1, at least in association with different binaural features of sound (Andersen et al 1980;Calford and Aitkin 1983;Middlebrooks and Zook 1983;Middlebrooks et al 1980;Redies et al 1989a;Rodrigues-Dagaeff et al 1989;Velenovsky et al 2003). Therefore it is possible that stimulation of certain regions within the ICC elicit activity only within certain regions of A1.…”

Projections from auditory cortex contact ascending pathways that originate in the superior olive and inferior colliculus

“…Adjacent sections stained with thionin were used to draw nuclear borders. The MG was identified as described by Redies et al (1989). The subdivisions of the IC -central nucleus, dorsal cortex (ICd), and external cortex (ICx) -were identified using criteria described in guinea pigs and rats (FayeLund and Osen, 1985;Malmierca et al, 1995).…”