Flicker-induced eye movements and the behavioural temporal cut-off frequency in a nocturnal spider.

We investigated changes in the eye muscle activity in the spider Cupiennius salei as a response to temporal intensity modulations. These spiders are known to enhance eye muscle activity in their principal eyes when moving stimuli are detected in the secondary eyes. We measured the activity of the dorsal eye muscle using a small telemetric unit attached to the spiders' prosoma and confronted the animals to flicker stimuli presented on a cathode ray tube monitor. We registered a significant increase in eye muscle activity as response to temporal light intensity modulations, which implies that no directed motion is required to trigger the spiders' response. This allowed the determination of the behavioural temporal cut-off frequency. None of the frequencies higher than 8.6 cycles s(-1) and all of the frequencies lower than 4.3 cycles s(-1) elicited a significant increase in eye muscle activity. A behavioural cut-off frequency of only a few cycles per second is well in line with the temporal properties of the photoreceptor cells determined using intracellular recordings. A relatively low temporal resolution and a relatively high spatial resolution suit well C. salei's lifestyle as a nocturnal sit-and-wait hunter.