Visual system ofNotonecta glauca: A neuron sensitive to movement in the binocular visual field

Summary

• 1. Large-amplitude spikes were recorded from a movement-sensitive unit in the optic lobe ofNotonecta.

• 2. This neuron is sensitive to movement of a small dark spot within the receptive field, in any direction. Movement of a background — e.g., the movement of a large striped pattern — does not elicit discharges. The movement of the striped pattern can even suppress the response to an otherwise effective stimulus.

• 3. The observed region of high sensitivity to moving spots is large and is found within the binocular field. Excitation involves binocular interaction: a high firing rate can be induced only if both eyes see the moving object. Covering of either the contralateral or the ipsilateral eye reduces the spike frequency to less than one-half the binocular rate (Fig. 15).

• 4. In the velocity range tested, from 3.5 to 350°/s, the log of the firing frequency varies linearly with the log of the stimulus velocity (Fig. 7).

• 5. Varying the contrast, above its threshold value, had little effect on the spike rate. At a given weak contrast, however, the neuron responded to movement in the dorsal visual field, but not to movement in the ventral field (Fig. 9).

• 6. Repetitive stimulation of the same part of the retinal array elicits successively smaller responses. This habituation is partly alleviated if a different group of ommatidia is stimulated (Fig. 6).

• 7. When a spot of suitable size was moved through the ventral visual field along a path perpendicular to the median plane of the animal, there was a discharge-rate maximum ca. 20° ipsilateral to the median (Fig. 4). A spot moving in the median plane from ventral to dorsal elicited three maxima of excitation (Figs. 3, 5, 11, 15). WhenNotonecta is hanging in its normal posture, upside down below the water surface, its visual environment is divided into three parts: the region of the deep water, the zone of te totally reflecting portion of the surface, and the zone of the transparent water surface. Each of the peaks in the response corresponds to one of these zones of the visual field.

• 8. The peak corresponding to the region of total reflection changed its position with the angular size of the moving spot; a small spot elicited a peak near the horizon. The larger the spot, the farther the peak shifted ventralwards (i.e., upward; Fig. 11). Due to this effect, an object moving in the plane of the water surface at any distance would excite the neuron maximally if it were of the appropriate size, ca. 1/4 the length ofNotonecta (Fig. 13).