CORTICAL AFFERENTS OF VISUAL AREA MT IN THE CEBUS MONKEY - POSSIBLE HOMOLOGIES BETWEEN NEW AND OLD-WORLD MONKEYS

Cortical projections to the middle temporal (MT) visual area were stud ied by injecting the retrogradely transported fluorescent tracer Fast Blue into MT in adult New World monkeys (Cebus apella). Injection site s were selected based on electrophysiological recordings, and covered eccentricities from 2-70 deg, in both the upper and lower visual field s. The position and laminar distribution of labeled cell bodies were c orrelated with myeloarchitectonic boundaries and displayed in flat rec onstructions of the neocortex. Topographically organized projections w ere found to arise mainly from the primary, second, third, and fourth visual areas (V1, V2, V3, and V4). Coarsely topographic patterns were observed in transitional V4 (V4t), in the parieto-occipital and pariet o-occipital medial areas (PO and POm), and in the temporal ventral pos terior area (TVP). In addition, widespread or nontopographic label was found in visual areas of the superior temporal sulcus (medial superio r temporal, MST, and fundus of superior temporal, FST), annectent gyru s (dorsointermediate area, DI; and dorsomedial area, DM), intraparieta l sulcus (lateral intraparietal, LIP; posterior intraparietal, PIP; an d ventral intraparietal, VIP), and in the frontal eye field (FEF). Lab el in PO, POm, and PIP was found only after injections in the represen tation of the peripheral visual field (>10 deg), and label in V4 and F ST was more extensive after injections in the central representation. The projections from V1 and V2 originated predominantly from neurons i n supragranular layers, whereas those from V3, V4t, DM, DI, POm, and F EF consisted of intermixed patches with either supragranular or infrag ranular predominance. All of the other projections were predominantly infragranular. Invasion of area MST by the injection site led to the l abeling of further pathways, including substantial projections from th e dorsal prelunate area (DP) and from an ensemble of areas located alo ng the medial wall of the hemisphere. In addition, weaker projections were observed from the parieto-occipital dorsal area (POd), area 7a, a rea prostriata, the posterior bank of the arcuate sulcus, and areas in the anterior part of the lateral sulcus. Despite the different nomenc latures and areal boundaries recognized by different models of simian cortical organization, the pattern of projections to area MT is remark ably similar among primates. Our results provide evidence for the exis tence of many homologous areas in the extrastriate visual cortex of Ne w and Old World monkeys.