Organization of the association cortical afferent connections of area 5: a retrograde tracer study in the cat

Abstract

The association (intrahemispheric) cortico-cortical afferent connections of area 5 were studied in the cat by means of retrograde tracing techniques involving horseradish peroxidase (HRP) free or wheat germ agglutinin-conjugated (WGA-HRP) or fluorochrome injections. Single or multiple injections were placed in different parts of areas 5a and 5b, the medial division of area 5 (5m), or in the anterior suprasylvian area (SsA). Labeled cells were plotted on projection drawings of the coronal sections and on two-dimensional "maps" of the cerebral cortex, which were produced according to an accurate and consistent procedure. The major findings of this study are: 1. All divisions of the anterior parietal cortex (areas 3a, 3b, 1, and 2) project to area 5 and to SsA. These projections, however, show marked differences in amount and topographical distribution, depending on the mediolateral and rostrocaudal location of the injections. 2. The motor cortex (areas 4 and 6) also projects heavily to area 5 and to SsA in a well-organized topographic fashion: Area 4 projects mainly upon areas 5a, 5m, SsA, and the medial part of 5b; area 6 projects mainly upon the lateral part of 5b and SsA. Moreover, the upper bank of the cruciate sulcus (areas 4 tau and 4 delta) projects to medial parts of area 5, and the lower bank (areas 4 tau, 6a alpha, and 6a beta) projects to lateral parts of area 5. 3. The somatosensory areas in the anterior ectosylvian gyrus and surrounding cortices (SIIm, SII, and SIV) are connected primarily with medial parts of area 5 (particularly 5a), and SsA. 4. Areas 7 and 7m and a number of visual areas (19, SVA, AmLS, PmLS, 21, 20, 18, ALS, and PLS) project in varying degrees to lateral parts of area 5b. Some of these areas also send weak to moderate projections to the medial part of 5b and the lateral part of 5a. 5. Sparse projections arising from the dorsolateral prefrontal, cingular, retrosplenial, granular insular, and suprasylvian fringe cortices were found to distribute in area 5 and SsA, particularly in lateral portions of 5b. 6. Quite abundant intrinsic connections also found, which were loosely organized according to a complex topographic pattern. On the basis of these and previous results (Avendaño et al., 1985), the identity of area 5 in the cat is discussed, and comparisons are made between this area and sectors of adjoining cortex of cat and primates.