Underlying mechanisms of the response specificity of expansion/contraction and rotation cells in the dorsal part of the medial superior temporal area of the macaque monkey
1. The dorsal part of medial superior temporal area (MST) has two unique types of visually responsive cells: 1) expansion/contraction cells, which selectively respond to either an expansion or a contraction; and 2) rotation cells, which selectively respond to either a clockwise or a counterclockwise rotation. In addition to selectivity for the mode of motion, both types of cells respond preferentially to movements over a wide field rather than over a small field. With the aim of understanding the underlying mechanisms of these selectivities, we carried out experiments on immobilized monkeys anesthetized with N2O. 2. Expansion/contraction and rotation of a pattern extending over a wide field contain three stimulus factors: 1) the spatial arrangement of different directions of movement, 2) the gradient in the speed of regional movement from the center to the periphery of the stimulus, and 3) the size change of texture components of the pattern in the expansion/contraction and the acceleration of movement of texture components toward the center of the stimulus in the rotation. The contribution of each factor to the activation of the cells was evaluated by comparing the response before and after removing the factor from the stimulus. The moving stimuli that lacked one or two of the factors were produced by the use of a cinematographic animation technique. 3. Withdrawal of the first factor, the spatial arrangement of different directions of movement, reduced the response of both Expansion/contraction and Rotation cells much more severely than either of the other two factors. We concluded that the first factor is far more important for activation than the other two. 4. These results are consistent with the model that Expansion/contraction and Rotation cells receive converging inputs from many directional cells with relatively small receptive fields in different parts of the visual field. Because MST receives strong fiber projections from MT, MT cells are candidates for the input cells. According to the model, if the convergence is organized so that the preferred directions of the input cells are arranged radially, the target cell will be an Expansion/contraction cell; if the input cells are arranged circularly, a Rotation cell will result.