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Fig. 3. Locomotor responses to light and dark flash stimuli. (A) Transient
increases in light elicit a sharp spike in turn initiations. Larvae were
pre-adapted at 20 µW cm–2 white light and at time zero,
tested with a 500 ms pulse of 200 µW cm–2 (open circles,
N=8 groups) or maintained in constant illumination (closed circles,
N=8 groups). Activity was measured in 400 ms windows at the indicated
time points. A significant spike in turns was noted for the time window
coinciding with the light flash (two-tailed t-test,
P=0.0064) but not at any other time points. Scoot initiations were
not significantly altered by the light-flash (data not shown). (B) Turn
initiations (black circles) increase with the intensity of the light flash.
Larvae were pre-adapted at 20 µW cm–2 before being tested
with a series of 10 bright flashes at the indicated intensity levels, at 30 s
intervals (N=5 sets of larvae for each intensity). A significant
increase in the frequency of turn initiations compared to baseline levels was
found for light flashes of >1 log unit above baseline illumination
(*P<0.05). Scoot initiations (grey circles) in the same
larvae were slightly depressed compared to baseline, but this only achieved
significance at one intensity tested. (C) Transient decreases in light provoke
an increase in turn initiations. Larvae were pre-adapted at 200 µW
cm–2 and challenged with a 500 ms-long dark flash to 20 µW
cm–2 at time zero (open circles, N=10 groups) or
left in constant illumination throughout the experiment (closed circles,
N=10 groups). Turns were significantly increased in the 500 ms window
starting at the beginning of the dark flash (two-tailed t-test,
P<10–10), but not at any other time point. Scoots
were significantly reduced only in the time window corresponding to the
dark-flash, most likely reflecting the huge increase in turns at that time.
Scoot initiations were otherwise not affected (data not shown). (D) Larger
reductions in illumination elicit more turn responses, without evoking scoots.
Larvae were pre-adapted at 130 µW cm–2, then tested with a
series of 10 dark flashes of the indicated magnitude (N=6 per
intensity). Turn initiations (black circles) were significantly increased
(*P<0.05) for dark flashes of around 1 log unit and
greater whereas scoot initiations (grey circles) were reduced under the same
conditions, likely as a result of the large number of larvae initiating
turns.
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