Drosophila Spatiotemporally Integrates Visual Signals to Control Saccades

Abstract

Like many visually active animals, including humans, flies generate both smooth and rapid saccadic movements to stabilize their gaze. How rapid body saccades and smooth movement interact for simultaneous object pursuit and gaze stabilization is not understood. We directly observed these interactions in magnetically tethered Drosophila free to rotate about the yaw axis. A moving bar elicited sustained bouts of saccades following the bar, with surprisingly little smooth movement. By contrast, a moving panorama elicited robust smooth movement interspersed with occasional optomotor saccades. The amplitude, angular velocity, and torque transients of bar-fixation saccades were finely tuned to the speed of bar motion and were triggered by a threshold in the temporal integral of the bar error angle rather than its absolute retinal position error. Optomotor saccades were tuned to the dynamics of panoramic image motion and were triggered by a threshold in the integral of velocity over time. A hybrid control model based on integrated motion cues simulates saccade trigger and dynamics. We propose a novel algorithm for tuning fixation saccades in flies.

Keywords: feature detection; fixation; flies; flight; motion vision.

Figure 1. Flies generate bar-fixation saccades that are different than spontaneous saccades and dynamically tuned to bar motion

A) Arena to study visual fixation of bar with the fly free to rotate in yaw. The fly is suspended within a magnetic field and illuminated from below with infrared lights. A bar of random “on” and “off” pixel columns is superimposed over a similarly randomly-generated background. B) Top panel: fly heading (blue), relative bar position (black), and error angle (red) between the fly and the bar during bar fixation. Bottom panel: filtered velocity data. Horizontal dashed line: threshold for detecting saccades. Vertical lines: identified saccades. C) Body angle (blue) and bar reference angle (black) between saccades during bar fixation. The yellow line is the best-fit line through the body angle data. D) Top: space-time graph of bout shown in B. Each pixel row represents the azimuthal distribution of “on” and “off” pixels displayed in the arena at a point in time. Slanted lines indicate clockwise bar rotation over time. Vertical lines indicate the stationary background over time. The fly trajectory is shown in orange. Bottom: space-time graph of bout shown in B, but in fly’s visual reference frame (orange). The bar is highlighted in green shading for visual comparison. E) Histogram of bar position in fly reference frame for motion-defined and traditional dark bar stimuli, indicated with cartoons. Bar rotation velocity = ±75°s⁻¹. n = 12 animals with balanced experimental design. 112 trials (56 motion-defined bar, 56 dark bar). We analyzed CW and CCW data together and show the distribution of signed bar position with respect to the fly visual midline. F) Trajectory of body angle for instances in which the motion-defined bar passed across visual midline between saccades for clockwise (CW) and counter-clockwise (CCW) bar rotation. Thin lines are from individual flight trajectories, thick lines are means. To emphasize the contribution of smooth movement, fly trajectories are plotted within a narrow spatial window where saccades are less frequently generated (see Figure 2A). G) Box plot of saccade amplitude, duration, and angular velocity for bar fixation (black) and spontaneous (grey) saccades. The dynamics of bar-fixation saccades were statistically different from spontaneous saccades. Bar-fixation saccades: 2,968 saccades from 10 animals. Spontaneous saccades: 4,137 saccades from 23 animals. H) Body saccade dynamics are tuned to the bar velocity. Bar velocity had a significant effect on saccade duration, amplitude, and peak angular velocity (ANOVA, P < 0.001 for all). I) Flies generate an initial torque followed by a counter torque. The white line shows the mean torque. Shaded areas are ± 1 STD. J) Initial peak torques are tuned to the bar speed (p < 0.001). Peak (minimum) counter-torques scale with bar speed (p < 0.001). Except for E), n = 2,968 saccades from 10 animals. See also Figure S1 and Movie S1.

Figure 2. Bar-fixation saccades are triggered by a time integral of bar position error

A) The distribution centers of saccade-triggered error angles along the visual azimuth during bouts of visual fixation. Error angles between the flies’ heading and the bar position were offset by approximately ± 45°. Shaded bar indicates the number of saccades. Error is computed between visual midline and bar center. Bar width = 30°. B) The error angle between the bar and fly’s heading are baseline-subtracted and plotted in time prior to a saccade. The positive and negative pre-saccade errors correspond to CW and CCW stimuli, respectively. C) The magnitude of the saccade-triggering error is dependent on the speed of the bar (p < 0.001). D) The inter-saccade intervals (ISI) scale inversely with the speed of the bar (p < 0.001). E) The integrated error between saccades changed little as the speed increased (p=0.261). Insets for C–E (dotted lines) show how each quantity was evaluated. n = 2,698 saccades from 10 animals. See also Figure S2.

Figure 3. Optomotor saccades are triggered by a threshold in the fixed time integral of velocity

A) Example 25-second trial of fly tracking the motion of a rotating visual landscape at 90°s⁻¹. Vertical lines represent individual saccades. For visual clarity, the panorama position is offset from 0 deg. B) Inset expanded from red box in panel A to show smooth optomotor movement. C) Mean retinal slip, in units of velocity, during the average 1.5 s interval of smooth movement between saccades across three different rotation speeds for CW (blue) and CCW (red) rotation across all animals. Gray area: SEM. D) Box plot of fly angular velocity with respect to motion of the visual landscape. Gray dashed line: slope of 1 where visual rotation velocity is equal to fly angular speed. E) Same data as panel D but for optomotor gain (fly speed divided by visual rotation speed) across visual rotation velocities. Gray dashed line = unity gain. F) Saccade dynamics across three speeds of panorama motion. Speed had a significant effect on dynamics (p < 0.001 for all). G) Top: Distribution of pre-saccade integrated error for all speeds pooled. Bottom: Pre-saccade integrated error is speed invariant. For all panels: 2,232 saccades during optomotor response for CW and CCW rotation, n = 10 flies, 451 trials. See also Figure S3 and and Movie S2.

Figure 4. Bar width does not influence saccade dynamics or pre-saccade integrated error

A) Switch between saccade and smooth movement as a function of bar width. Blue circles: median; error bars: 25th–75th percentile. Orange error bars: mean ± STD. n = 40 animals, 1380 trials, 32,802 saccades. Speed = ±75 ° s⁻¹. B) Saccade amplitude, duration and peak angular velocity is not influenced by bar width. C) Similarly, the pre-saccade integrated error varies little as the bar width increases. Bar velocity = ±75°s⁻¹. n = 10 animals, 2,575 bar-fixation saccades. D) Saccade amplitude across the full range of bar width tested. Bar velocity = ±75°s⁻¹. n = 10 animals, 10,067 saccades. See also Figure S4.

Figure 5. Flies use body saccades to track a bar on a counter-rotating background, quickly switching to smooth movement between saccades, and saccades dynamics, but not saccade-triggering error angle, are influenced by ground speed

A) The compound stimulus is depicted by a space-time diagram in shaded gray as in Figure 1D. Fly heading (blue) and error angle (red) between the fly and the bar. The fly switches between saccadic bar fixation and smooth movement. Note that the fly’s body angle lags behind the bar. B) Expanded view of the trajectory if a single saccade indicates rapid transitions between a bar pursuit saccade and smooth tracking of the ground. Median delay = 25 ms. C) Flies track the ground between saccades. Body angle (blue) is baseline-subtracted to show the time course of body angle between saccades during a bout of bar fixation. The yellow line denotes the best-fit line of the body angle data. n = 1,361 saccades from 10 animals. D) Top: space-time graph of task shown in A with bar and background revolving in opposite directions. Bottom: space-time graph in the fly’s reference frame showing rapid switching between saccades and smooth movement. The bar is highlighted in pink for visual comparison. E) Ground velocity has subtle but significant effect on saccade duration, amplitude, and peak angular velocity. F) Flies generate an initial torque followed by a counter torque. The black line shows the mean torque. Shaded areas are ± 1 STD. G) Initial peak torques and counter-torques are tuned to the ground speed. H) The error scales with ground speed and marginally for ISI whereas integrated error does not change. n = 1,361 saccades from 10 animals. See also Figure S5 and Movie S3.

Figure 6. A switched, integrate-and-fire model simulates saccade control

A) A parsimonious model of saccade control suggests parallel position and velocity controllers. Top: The velocity controller drives smooth movement and saccades whereas the position controller drives only saccades (bottom). Red lines: command (fire) signal when integrated error reaches a set threshold. The command signal triggers a change in switch state allowing the error to flow to generate appropriately scaled saccades. While not demonstrated explicitly, for the wide-field sensitive system we hypothesized that e_v scales with motion speed to provide the appropriate tuning of saccades. v_r: reference velocity; e_v: retinal slip error; v_a: actual fly velocity; x_r: reference position; e_p: position error; x_o: bar position. For the position controller, our data suggest a narrow, bilateral receptive field for integration of bar position. B) To determine whether a simple mechanism could simultaneously capture the constant-threshold saccade trigger and the tuning of control torque dependent on bar speed, we constructed a reduced-order, switched control model of bar fixation via a position controller. C) Error, integrated error and torque pulse amplitude generated for three simulated bar speeds (blue, green, red lines). The integrate-and-fire model fires when the integrated error reaches a constant threshold (C; Figure S6). The behavioral data is shown on the right for comparison. See also Figure S6.