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First published online August 3, 2006
Journal of Experimental Biology 209, 3170-3182 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02369

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Visual stimulation of saccades in magnetically tethered Drosophila

John A. Bender^* and Michael H. Dickinson

Division of Biology, California Institute of Technology, Pasadena, CA 91125, USA

View larger version (25K): [in a new window]   Fig. 1. Experimental design. (A) Fly orientation was determined at 101 Hz with a near-IR camera and realtime software. Visual stimulation was provided by a 32x64 matrix of LEDs wrapped in a cylinder around the tethered fly. N, magnetic north; S, south. (B) Flies were glued to a steel pin, which was set in a jewel bearing and held in a magnetic field so that the flies could rotate only about their functional yaw axis. (C) Flies were stimulated with virtual looming squares. The time course of the visual stimulus () was proportional to x, the distance between the stimulus and the eye, which was determined by the square's edge half-length (l=10 cm), approach velocity v, and acceleration. (D) Time course of visual stimulation for an approaching object with constant velocity (v=1.5 m s-1, solid line), acceleration (vinit=0, a=6.2 m s-2, dotted line), or deceleration (vinit=3.4 m s-1, a=-5.3 m s-2, broken line). The stimuli were discretized both spatially and temporally due to limitations of the LED arena (visual refresh rate: 800 Hz; pattern update rate: 50 Hz). The blue circles represent the approximate discretization of the constant-velocity stimulus, sampled every 10 ms.

View larger version (15K): [in a new window]   Fig. 2. Spontaneous behavior in the flight arena and methods used to quantify saccades. (A) Orientation data sampled every 10 ms (101 Hz) reveal periods of rapid turning (top). The small, slow (0.5 Hz) oscillations (arrow) were noticeable by eye, and do not represent tracking noise. The orientation data were low-pass filtered at 25 Hz before calculating the angular velocity (bottom). The saccade threshold (broken line) was set 4 s.d. away from 0, at about 350° s-1. (B) Magnification of the region within the shaded box from A, containing one saccade. Saccade duration (green lines) was the time during which the fly's angular velocity exceeded one-quarter of its peak angular velocity during that saccade (broken line). Saccade amplitude (blue lines) was the difference between the median of the 5 points before and after the saccade.

View larger version (31K): [in a new window]   Fig. 3. Saccade amplitude, duration and peak angular velocity are correlated. (A) Histogram of absolute saccade amplitude. The mean value of the data shown in this histogram is 35.2°. Saccades with amplitudes <15° or >150° were not analyzed, since they may represent tracking errors. (B,C) 2-dimensional histograms. (B) Saccade duration, sampled at 101 Hz, loses correlation with saccade amplitude as amplitude increases. (C) Peak angular velocity during a saccade is tightly coupled to saccade amplitude. The bottom of C is truncated at the angular velocity saccade threshold. N=35 flies, n=26535 saccades.

View larger version (26K): [in a new window]   Fig. 4. Stimulus expansion evokes saccades. (A) The time course of visual stimulation (stimulus half-angle, red trace), overlaid with the fly's orientation (black dots). The gray boxes during stimulus expansion and contraction represent the 500 ms time windows during which a saccade must occur to be considered `triggered' by the stimulus. The blue box is the time during which the spontaneous saccade rate was calculated. (B) Constant-velocity, accelerating and decelerating stimuli all elicit saccades (N=11 flies). (C) Stimuli expanding along only the horizontal, vertical or diagonal axes evoke saccades with the same probability as full-square expansion (N=9). (D) Stimuli approaching with different constant velocities have equal probability of triggering saccades. Stimuli with additional expanding edges (concentric squares) also evoke saccades at the same rate. Contraction of the low-velocity and concentric square stimuli inhibit the saccade behavior (N=15). *P<0.01; P<0.05, relative to the spontaneous rate. Total n=2933 saccades. The error bars represent the s.e.m.; the dotted line shows the spontaneous probability.

View larger version (30K): [in a new window]   Fig. 5. The time course of stimulation affects the time course of saccade probability. (A-I) Histograms of relative saccade probability as a function of time during the 500 ms stimulation window, binned by the camera's 10 ms frame rate. The window begins 30 ms after the first discrete change in the stimulus. The red traces are the time course of stimulus size, and the blue boxes represent the spontaneous saccade rate.

View larger version (39K): [in a new window]   Fig. 6. Saccade metrics [rows: (i) duration, (ii) amplitude and (iii) peak velocity] are affected by stimulus parameters (columns A-E). The red numbers equal the reduction in uncertainty about the value of each metric given knowledge of that stimulus parameter. The bottom row (iv) shows the relative probability of observing each value of the stimulus parameters. The broken line in Aiv shows the spontaneous saccade probability. Values are means ± s.e.m.; N=35 flies; n=2933 saccades.

View larger version (12K): [in a new window]   Fig. 7. The time course of torque production during a saccade by a magnetically tethered fly. Velocity and acceleration data were drawn from the saccades falling in a single bin of Fig. 3C (amplitude=17.5°, peak velocity=379° s-1, n=108 saccades), rectified and aligned to the time when the angular velocity exceeded one-quarter of its maximum value. The shaded area indicates the s.e.m.

View larger version (18K): [in a new window]   Fig. 8. Peak saccade probability occurs at a critical stimulus size regardless of the time course of stimulation. (A) The time of peak saccade probability (tpeak) for each stimulus was determined using a 5 Hz low-pass filter on the saccade probability histograms from Fig. 5 (not all shown here; black traces). The ratio l/v is a single metric determining the apparent size of a looming visual stimulus over time (red traces). (B) An iterative best fit to the constant-velocity (black circles), accelerating (dark red circle), and decelerating (dark blue circle) stimuli shows a strong linear relationship on this plot of tpeak versus l/v (regression coefficient r2=0.91). The lighter circles behind the accelerating and decelerating stimuli show how those points would move for different values of tpeak; the values for the other stimuli would shift only vertically because l/v is constant. The threshold stimulus size, crit, is derived from the slope of the black best-fit line, and equals 62° in this case. , the delay between the time of the critical stimulus (tcrit) and tpeak, is the y-intercept of this line, and evaluates to 49 ms. The orange circles are the partial (horizontal, vertical and diagonal) stimuli, and the green circle is the concentric square stimulus. The broken line was fit to the three full, constant-velocity stimuli (black circles) only (r2=1.00). From this broken line, crit=71° and =22 ms.