Light units (LU) constituted 37% of the 181 units studied. They had monocular RFs, about 20° to 40° in diameter. These are ipsilateral units, insensitive to patterns or moving spots, except that moving a large white spot over an LU RF would cause an increase in firing rate. Similarly, moving a large black spot over the RF would suppress firing. LUs can be subdivided into two subclasses: tonic LUs and phasic or ON-type LUs. The firing rate of tonic LUs increased smoothly at ON; there was no burst. ON-type LUs gave a burst at ON then slowed to a steady, lower rate under LSS conditions.
A few of the LUs exhibited an ON-gated mechanoresponse. Such an LU would fire a burst in response to stroking a leg with a wire only immediately following its ON burst. Stroking the leg in the same manner in the dark, or several seconds following ON, produced no extra burst in this curious type of LU. In several instances, the microelectrode would pick up both a tonic and a phasic LU at the same time, indicating that their axons ran in intimate contact. Horridge et al. (1965) called Locusta tonic LUs type AD; phasic LUs were called type AC and BC (the BCs had small RFs, down to 7°).
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