Arousal facilitates collision avoidance mediated by a looming sensitive visual neuron in a flying locust

Abstract

Locusts have two large collision-detecting neurons, the descending contralateral movement detectors (DCMDs) that signal object approach and trigger evasive glides during flight. We sought to investigate whether vision for action, when the locust is in an aroused state rather than a passive viewer, significantly alters visual processing in this collision-detecting pathway. To do this we used two different approaches to determine how the arousal state of a locust affects the prolonged periods of high-frequency spikes typical of the DCMD response to approaching objects that trigger evasive glides. First, we manipulated arousal state in the locust by applying a brief mechanical stimulation to the hind leg; this type of change of state occurs when gregarious locusts accumulate in high-density swarms. Second, we examined DCMD responses during flight because flight produces a heightened physiological state of arousal in locusts. When arousal was induced by either method we found that the DCMD response recovered from a previously habituated state; that it followed object motion throughout approach; and--most important--that it was significantly more likely to generate the maintained spike frequencies capable of evoking gliding dives even with extremely short intervals (1.8 s) between approaches. Overall, tethered flying locusts responded to 41% of simulated approaching objects (sets of 6 with 1.8 s ISI). When we injected epinastine, the neuronal octopamine receptor antagonist, into the hemolymph responsiveness declined to 12%, suggesting that octopamine plays a significant role in maintaining responsiveness of the DCMD and the locust to visual stimuli during flight.

FIG. 1.

Descending contralateral movement detector (DCMD) responses depended on the state of arousal. A: without an increase in arousal the DCMD response declined following repeated stimulation. Over the range tested, the rate of decline in the number of DCMD spikes generated increased as the interstimulus interval (ISI) was decreased. B: with approach speeds from 0.50 to 5.00 ms⁻¹, this decline in total DCMD spike numbers could be reversed by increasing arousal by mechanical stimulation of the hind-leg (filled bars).

FIG. 2.

Without arousal, the shape of the DCMD response envelope to looming stimuli, as well as DCMD spike numbers, depended on the interval between stimuli. Each graph shows DCMD responses to object approach at a particular speed but with different ISIs (20, 40, or 300 s). Approach speeds were 0.25, 0.50, and 1.00 ms⁻¹ in A, B, and C, respectively. Bars show the mean DCMD spike frequency in 50-ms bins, for 5 stimulus repetitions with a single locust. Ten spikes per bin would generate a maintained spike frequency of ≥200 Hz. With shorter ISIs, the number of spikes in the DCMD decreased and the maximum spike frequency occurred earlier in the approach (marked by *). The shift in the timing of the maximum response was greater for the slower stimulus approach speeds. Data from this experiment were also used in Fig. 5B.

FIG. 3.

Arousal restored the peak firing frequency and the ability of the DCMD to follow the increase in image size throughout object approach. With arousal, responses reach mean spike rates per 20-ms bin of almost 200 Hz. A–C: bars represent the DCMD mean spike frequency, over 50 stimulus repetitions at 20-s intervals, with approach speeds of 0.25, 0.50, and 1.00 ms⁻¹. The first 25 stimuli (gray bars) were delivered without any increase in arousal; then a further 25 stimuli (colored bars) were each preceded by a stroke to the hind leg tibia, raising the state of arousal. At 60 ms before projected collision, arousal significantly increases the DCMD response with approach speeds of 0.50 and 1.00 ms⁻¹ (P < 0.01; Mann–Whitney rank-sum test). D: instantaneous DCMD spike frequencies during the last 6 approaches are shown for the aroused and nonaroused trials.

FIG. 4.

Arousal increased the number of spikes in the DCMD in response to approaching objects during the final stages of object approach. When the arousal of the locust was increased by stroking the hind leg tibia (dark bars), the number of spikes increased significantly. The inset in the bottom right graph shows the increasing DCMD spike frequency throughout the response to a 5 ms⁻¹ object approach. Projected collision (solid line) would have occurred 14 ms after motion ceased (dotted line). The duration of the DCMD response and section of the response analyzed for each object approach velocity depended on the velocity and was the final 400 ms for approach speeds of 0.25 ms⁻¹ (−400 to 0 ms), the final 200 ms for approach speeds of 0.50 ms⁻¹ (−220 to 0 ms), and the final 100 ms for approach speeds of 1 ms⁻¹ (−100 to 0 ms). The final 25 ms were used for approaches at 5 ms⁻¹. These intervals allowed a comparison of the DCMD responses to the same change in angular subtense (from 25 to 59°) with different loom speeds.

FIG. 5.

Arousal increased the number of high-frequency DCMD spikes. A: during object approach in an aroused locust, instantaneous spike frequency in the DCMD reached >200 Hz, particularly when collision was imminent. In this example, spike frequencies exceeded 200 Hz and 6 consecutive spikes occurred at ≥200 Hz (circled). B: approaches were presented at intervals of 40 s to either nonaroused (open bars) or aroused (filled bars) locusts. For nonaroused locusts, the numbers of DCMD spikes occurring at frequencies >200 Hz was low. When the arousal state of the locust was increased, the number of spikes at frequencies >200 Hz increased significantly (*P < 0.002, paired Student's t-test). In particular, increased levels of arousal led to increases in the number of approaches generating >6 consecutive spikes at 200 Hz: 56 of 148 approaches with increased levels of arousal and only 2 of 122 without arousal. Trials were in blocks of 6, with a total of 18 to 42 approaches per block. The analysis shown in Fig. 6B excluded the first 3 responses of the experiment in nonaroused locusts.

FIG. 6.

Flying protects the DCMD from habituation, enabling the locust to react to impending collisions even with very short intervals between successive stimuli. DCMD responses to looming objects in tethered flying locusts were very robust. Stimuli consisted of a dark disc of diameter 80 mm approaching the eye at 3 ms⁻¹. The end of disc movement is shown by a dashed vertical line. Experiments were performed on 4 locusts with 144 stimuli delivered when the locust was not flying, 144 stimuli during wind stimulation, and 144 during flight, each separated by an ISI of either 1.8, 10, 20, or 40 s (data from 20 and 40 s separation between stimuli are not shown). A: comparison of habituation in the DCMD neuron during repeated presentation of a stimulus looming at a velocity of 3 ms⁻¹ in 2 different behavioral states: wind stimulated and flying. i: total spike number per stimulus response. ii: peak instantaneous spike frequency per response. In flying locusts the peak instantaneous spike frequency remained >200 Hz at all ISIs. Top right: DCMD activity is shown for 3 looming stimuli (numbers 1, 3, and 5 of a sequence of 6), separated by an ISI of 1.8 s. B: the time course of the mean DCMD response to looming stimuli in the flying locusts is shown, divided into 10 ms bins. The responses persisted beyond the end of object approach (dashed line), with the maximum response occurring at the projected time of collision (solid line). The inset then compares DCMD responses in flying and nonflying wind-stimulated locusts in the 90 ms immediately prior to collision. Intervals during which flight had a statistically significant effect are marked with an asterisk (P < 0.02, Mann–Whitney rank-sum test).

FIG. 7.

Injection of epinastine, a specific blocker of neuronal octopamine receptors, significantly reduces the protective effect of flight on the evasive gliding behavior. Stimuli consisted of a dark disc of diameter 80 mm approaching the eye at 3 ms⁻¹ and were given in groups of 6, each stimulus separated by an ISI of 1.8 s. Experiments were performed on 12 locusts; 45 min prior to flight 6 had been injected with epinastine and 6 with saline as controls. An infrared beam was used to monitor wing-tip position and responses by flying locusts to imminent collision are plotted as the percentage of stimuli resulting in either 1) a pause or glide during flight lasting <1.8 s (dark bars); or 2) no-glide (gray bars). Stimuli delivered to nonflying locusts have not been plotted.