Representation of the fovea in the superior temporal sulcus of the macaque monkey

Summary

The response properties of 633 neurons from striate and prestriate cortex were recorded in 3 hemispheres of two awake cynomolgus monkeys while they fixated or tracked a small spot of light. Of 254 penetrations located at 1 mm intervals, 39% were identifiable from visible electrolytic lesions or electrode tracks and were used to reconstruct the positions of all recording sites. A total of 226 cells were located in the superior temporal sulcus and 81 cells in area V1. The location and visuotopic organization of the foveal portion of the middle temporal (MT) visual area were determined in three hemispheres. MT was defined physiologically on the basis of direction-selectivity, receptive field size, and retinotopic organization. Of 170 MT neurons, most were motion sensitive, and 65% had a directionality index, (best — opposite)/best, of 0.6 or higher. MT was defined anatomically on the basis of myelin staining within the superior temporal sulcus (STS). On the posterior bank of the STS the physiologically defined border corresponded closely to a myelin border visible on our sections. Distinct myelin borders were not consistently identifiable on the anterior bank. The representation of the central fovea (eccentricities of 0–1 deg) was located partly on the floor, but mostly on the posterior bank of the STS at the extreme postero-lateral edge of MT. In all three hemispheres foveal MT extended onto the roof of a cleft formed between the posterior bank and a wide flattened area on the floor of the STS. This region lies 10–12 mm below the brain surface, measuring along a line normal to the surface at a point 2–3 mm antero-lateral to foveal V1. The area of MT was 6–9 mm² for the central fovea (0–1 deg), 15–24 mm² for the entire fovea (0–3 deg), and 28–40 mm² including the fovea and parafovea (0–10 deg). A visuotopic map of central foveal V1 (0–1 deg) was obtained in one animal. The measured area of this representation was 116 mm². Using published estimates of the total areas of cynomolgus MT and V1 (73 and 1200 mm² respectively) the ratio of central foveal to total area was calculated to be 0.10 for both MT (7.5/73) and V1 (116/1200), indicating that the relative magnification of the foveal versus the peripheral visual field is preserved in the mapping of V1 onto MT. A separate representation of the central visual field was found immediately adjacent to foveal MT. This region, the FST area (Ungerleider et al. 1982; Ungerleider and Desimone 1986a, b), was distinguishable from MT in three ways: 1) by the presence of occasional visually unresponsive cells, 2) by the presence of cells with very large receptive fields intermingled with cells whose receptive fields are comparable in size to those found in foveal MT, and 3) by an increased incidence of cells responding during tracking. Of 34 FST neurons, 53% had a directionality index of 0.6 or higher. An additional 22 cells recorded in the superior temporal sulcus were judged to be outside both MT and FST.