Topographic and cytoarchitectonic organization of thalamic neurons related to their targets in low‐, middle‐, and high‐frequency representations in cat auditory cortex
Authors: Imig Thomas J. , Morel Anne 
Published: January 2005

We studied the topographic organization of thalamic projections upon different ranges of cortical frequency representation. Thalamic neurons were labeled by injecting horseradish peroxidase (HRP) or tritiated bovine serum albumin into auditory cortex. Injections in individual brains were confined to the same range of frequency representation, and distributed through three or four tonotopic cortical fields in order to label as much of the thalamic projection upon a limited range of frequency representation as practicable. Low, middle, and high ranges of the frequency representation were injected in different brains. The spatial organizations of arrays of labeled neurons are described, and each array is divided into a ventral division and lateral posterior complex (lateral part of the posterior thalamic group), both composed mainly of small cells; and a medial division, composed mainly of medium and large cells. The ventral and medial divisions (located laterally and medially within the medial geniculate body (MGB), respectively), both contact the lateral posterior complex which is located rostrally. The HRP cytoarchitecture of the three divisions is described, and the portions of the ventral division corresponding with the physiologically and cytoarchitectonically defined ventral nucleus are identified. Relatively few labeled neurons were found within other thalamic areas.

The topographic organizations of the ventral division (and its tonotopic subdivision, the ventral nucleus), the lateral posterior complex (also tonotopically organized), and the medial division are described. There are planar and concentric components of the topographic organization in the ventral nucleus. Within the planar component, the low‐frequency area is located laterally and the high‐frequency area is located rostromedially. Within the concentric component, the low‐frequency area is located centrally and the high‐frequency area is located peripherally. Low‐, middle‐, and high‐frequency areas course without interruption through the planar and concentric components. In the lateral posterior complex, the low‐frequency area is located rostrally, and the high‐frequency area is located caudally adjoining the high‐frequency area in the ventral nucleus. The topographic organizations of the ventral nucleus and lateral posterior complex are consistent with tonotopic maps of these regions. The medium‐ and large‐cell portion of the medial division is also topographically organized, although there may be more overlap among low‐, middle‐, and high‐frequency arrays than in the ventral nucleus. Caudally in the medial division, the low‐frequency area is located lateral to the high‐frequency area, and rostrally the low‐frequency area is located ventral to the high‐frequency area.

In addition to studying the topographic organizations of individual subdivisions, we attempted to interrelate the topographic organizations of the ventral and medial divisions. Observations of arrays of labeled neurons coursing without interruption through the ventral division, medial division, and the lateral posterior complex for each of the frequency ranges injected suggest that there are areas of contiguity between frequency‐component arrays in the medial division with the corresponding arrays of the ventral division and lateral posterior complex. A block model of the topographic organization of the combined ventral and medial divisions was constructed using nonoverlapping low‐, middle‐, and high‐frequency components. The model displays some aspects of the shapes of labeled arrays seen in transverse sections and the spatial relationships among them. On the other hand, cytoarchitectonic observations suggest that the medial division partially overlaps with both the ventral nucleus and the lateral posterior complex. Careful comparison of the arrays of labeling with tonotopic maps of the ventral nucleus suggests that areas of low‐frequency medial‐division labeling overlap with areas of the middle‐frequency representation in the ventral nucleus.