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Fig. 6. Locomotor activity is regulated by light. (A) The frequency of spontaneous turn (closed circles) and scoot (open circles) initiations are significantly greater in more intense light. Larvae (N=10 groups per light level) were adapted to each light level for 30 min before testing. For each group, a series of twenty 400 ms video recordings was made under constant conditions at the indicated light level and average activity computed. Regression lines for activity versus log(intensity) are shown (turns, r²=0.49; scoots, r²=0.51). (B) Both turn (Bi) and scoot (Bii) initiations show a gradual reduction during dark adaptation (DA). Larvae were pre-adapted to 400 µW cm^–2 white light for at least 3 h before being subjected to sudden darkness (open circles, N=10 groups) or were maintained under constant illumination (CI; closed circles, N=10 groups). At each time point, a series of twenty 400 ms video recordings was made and the average activity computed for each group. Two-way ANOVA for group and time after light extinction revealed significant group by time interaction for both scoots (F_(5,107)=3.3, P=0.009) and turns (F_(5,107)=5.4, P<0.001). (C) Both turn (Ci) and scoot (Cii) initiations rapidly increase after larvae maintained in constant darkness are suddenly switched to bright light. Within 15 min of illumination with 230 µW cm^–2 (light-adapted, LA; open circles, N=10 groups), both turn and scoot initiations reach levels similar to larvae maintained in bright light for several hours (for example, constant illumination groups in B). Control larvae maintained in constant darkness (CI, closed circles, N=10 groups) continue to show low levels of locomotor activity. Two-way ANOVA for group and time for time points after the onset of illumination revealed a significant main effect of group for both scoots (F_(1,107)=216, P<0.001) and turns (F_(1,107)=482, P<0.001), and a significant group by time interaction for turns (F_(5,107)=7.9, P<0.001). (D) Ultraradian light:dark cycles of 1 h each demonstrate that photic input directly modulates activity levels in larvae. Larvae were monitored over a 24 h period (consisting of 12 cycles), with a series of twenty 400 ms video recordings taken every 10 min (offset from the beginning of each transition by 5 min). During light cycles, larvae were exposed to constant 60 µW cm^–2, while during dark cycles (shaded brown) larvae were maintained in darkness. (Di) The initiation frequency of both scoots and turns closely follows the light:dark cycle periodicity. Orange broken curves show functions estimated by performing non-linear regression according to the model: activity=b1+b2^*sin(2^*time/b3+b4), such that b3 is the periodicity of the function (see text). (Dii) Mean initiation frequency for scoots (open circles) and turns (closed circles) for each time point during a 2 h period averaged over all 12 cycles. (E) Turn initiations show a transient increase for 5 s following the switch to sustained darkness (Ei, open circles, N=30 groups) compared to larvae maintained in constant illumination (200 µW cm^–2, closed circles, N=30 groups). No change in turn initiations occurs over the next 7 min. Immediately after light extinction, scoot initiations (Eii) are slightly reduced; however, after 60 s, scoots show a transient but highly significant increase above baseline levels. For each group of 400 ms recordings were collected at the indicated time points (^*P<0.05, t-test versus constant light). (F) Behavioral light adaptation begins 60 s after dark-adapted embryos are exposed to bright light (140 µW cm^–2, open circles, N=20 groups). After an initial spike in turns (Fi) elicited by the abrupt change in illumination, there is a lag of approximately 1 min in which turn initiations remain at similar levels to larvae maintained in constant darkness (closed circles, N=20 groups). Thereafter turn initiations rapidly climb to light-adapted levels. Scoot initiations (Fii) show a similar pattern, with an acute spike following light onset, a lag phase of 60 s, then a rapid increase to normal light-adapted levels (^*P<0.05, t-test versus constant dark).

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