Thalamic afferents to cytoarchitectonic subdivisions of area 6 on the anterior sigmoid gyrus of the dog: A retrograde and anterograde tracing study

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Abstract
The cytoarchitecture and thalamic afferents of cortical area 6 located on the anterior sigmoid gyrus were mapped and analyzed in the dog by means of cytoarchitectonic, horseradish peroxidase (HRP), and autoradiographic methods. Cytoarchitectonically, area 6 consists of medial and lateral subdivisions that correspond, respectively, to areas 6aα and 6aβ in the cat. In the dog, area 6aα is characterized by a wide layer III, the merging of borders between layers III and V, the presence of small‐to‐medium‐size pyramidal cells in layer V, and a pallisade arrangement of cells in layer VI. Area 6aβ appears more stratified, with a relatively acellular layer present between layers V and VI and the presence of large pyramidal cells in layer V. Neither area 6aα nor 6aβ contains a layer IV. Data obtained from injections of HRP into areas 6aα or 6aβ revealed that labeled thalamic neurons were distributed in a longitudinal band extending from the rostral part of the ventral anterior nucelus (VA) through the caudal part of the mediodorsal nucleus (MD). Labeled cells were observed in the ventral and ventral medial thalamic nuclei as well as in several of the intralaminar nuclei including the central lateral, central medial, parafascicular, and centrum medianum nuclei. A few labeled cells were also located in the suprageniculate nucleus. The densest thalamic labeling was present in VA and MD following injections into area 6aα. Equivalent or even larger injections into area 6aβ resulted in much less thalamic labeling. The band of labeled cells also extended into the hypothalamus, zona incerta, amygdala, claustrum, periaqueductal gray of the midbrain, and the nucleus of Darkschewitsch. Results from autoradiographic experiments showed that area 6 subdivisions receive a loosely organized topographic input from VA. Injections of tritiated amino acids were made into selected regions of VA and into the caudal part of MD, areas in which the largest numbers of HRP‐labeled cells were located. Area 6aα receives afferents primarily from the rostromedial part of VA and the caudal part of MD while area 6aβ receives its principal input from the caudal and lateral parts of VA with minimal input from MD. Axons originating from VA terminate in both layers I and III of area 6 while those originating from the caudal part of MD terminate only in layer III. The results of this study show that area 6 in the dog may be divided cytoarchitectonically into two distinct regions, each of which receives a characteristic pattern of thalamic inputs. The organization of these inputs, as well as those from other, nonthalamic subcortical areas, lends anatomical support for the functional division of area 6 into an electrophysiologically defined medial eye movement field and a lateral supplementary motor area.