Fig. 6. Spike trains and peristimulus time histograms for a cell recorded in area MST show a reversal of selectivity depending on the location of the singular point. A, Neuronal activities during expansion (first phase of stimulus) and contraction (second phase) stimulation were recorded for nine locations of the singular point of the flow field. The arrangement of the histograms reflects the location of the singular point during the individual stimulations. One location was in the center of the visual field, and eight locations were arranged equidistantly on a circle of radius 15° around the center. The neuron favors the contraction stimulus when the singular point is placed in the upper left visual hemifield. In contrast, if the singular point is placed (Figure legend continues) to the right of the visual field center, the neuron favors expansion. B, Activities during rotational stimulation were recorded for 17 locations of the singular point. In addition to the 9 inner locations, 8 more locations were arranged on a second circle of radius 40°. Within the 9 central rotation stimuli (15° eccentric), no reversal of selectivity is observed. Instead, the neuron favors clockwise rotations (first stimulus phase) at most positions within the central 30° of the visual field. However, when the singular point is located 40° eccentric, it becomes apparent that the neuron favors counterclockwise rotation when the singular point is located in the upper left periphery of the visual field, and clockwise rotation when the singular point is located in the right or in the lower visual hemifield. C, Directional tuning for full-field frontoparallel translation is toward the left. The polar plot of the directional tuning was obtained by moving a full-field random dot pattern on a circular path in a frontoparallel plane, thereby covering all 360° of motion direction in a single trial. The receptive field covered the left half of the tangent screen, but an area of increased responsibility comprised the lower left hemifield.