Daily and seasonal variations inlimulusvisual behavior

Abstract

Limulus visual thresholds were measured using psychophysical techniques over a period of almost one year. A pronounced seasonal trend was found; threshold varied over a range of approximately two log units and was lowest in Summer and highest in Winter. This seasonal trend had several properties:(1) Thresholds measured over the whole year were not normally distributed: In Summer, thresholds clustered around one value. In Winter, thresholds clustered around another value two log units higher. And in Spring and Fall, thresholds were more widely distributed between the two values.(2) Latencies were also season dependent: In both Summer and Winter, latency was a declining logarithmic function of intensity with the Summer latency versus intensity function shifted toward lower intensities. However, in Spring and Fall, the latencies declined at low intensities, then rose at intermediate intensities, and then fell again at high intensities.(3) Individual animals exhibited day-to-day threshold alterations of a magnitude that was comparable to the seasonal variation in group thresholds.(4) Temporal acuity, as measured by the temporal summation function, was unaffected by the seasonal and daily threshold variation.