Impact of walking speed and motion adaptation on optokinetic nystagmus-like head movements in the blowfly Calliphora

Abstract

The optokinetic nystagmus is a gaze-stabilizing mechanism reducing motion blur by rapid eye rotations against the direction of visual motion, followed by slower syndirectional eye movements minimizing retinal slip speed. Flies control their gaze through head turns controlled by neck motor neurons receiving input directly, or via descending neurons, from well-characterized directional-selective interneurons sensitive to visual wide-field motion. Locomotion increases the gain and speed sensitivity of these interneurons, while visual motion adaptation in walking animals has the opposite effects. To find out whether flies perform an optokinetic nystagmus, and how it may be affected by locomotion and visual motion adaptation, we recorded head movements of blowflies on a trackball stimulated by progressive and rotational visual motion. Flies flexibly responded to rotational stimuli with optokinetic nystagmus-like head movements, independent of their locomotor state. The temporal frequency tuning of these movements, though matching that of the upstream directional-selective interneurons, was only mildly modulated by walking speed or visual motion adaptation. Our results suggest flies flexibly control their gaze to compensate for rotational wide-field motion by a mechanism similar to an optokinetic nystagmus. Surprisingly, the mechanism is less state-dependent than the response properties of directional-selective interneurons providing input to the neck motor system.

Figure 1

The optokinetic nystagmus and experimental setup and design. (A) Left: Schematic diagram of human optokinetic nystagmus eye movements in response to a grating moving horizontally left to right (grey arrow). The black arrows indicate the point of fixation moving rightwards as the eyes move syndirectionally with the grating, compensating for its motion. As the yaw angle of the eye’s principal axis (θ) increases towards the limit of its angular range, the eyes saccade leftward, against the motion of the grating. Right: Schematic traces of the yaw eye angle (θ), indicating saccadic increases of the yaw angle against the direction of grating motion, and slow decreases in the yaw angle of the compensatory eye movements that follow the direction of grating motion. The net result is the characteristic sawtooth pattern of yaw eye angle. (B) Experimental design. Left: In every trial, a fly first viewed a pre-stimulus screen for 3 s (t = − 3 s to 0 s) of gratings that moved progressively in a front-to-back direction (‘progressive’) with either one of the following temporal frequencies, 0 Hz, 0.25 Hz, 1 Hz, 4 Hz, 10 Hz, or a grey screen. Center: We measured the yaw angle (θ) of the fly’s head. During the stimulus period (t = 0 to 0.5 s) the fly viewed gratings moving in the same direction simulating the image motion that would occur during a yaw rotation (‘rotation’) with one of the following temporal frequencies, 0.25 Hz, 1 Hz, 4 Hz, 7 Hz, 10 Hz, 13 Hz, 18 Hz, 25 Hz. Right: After the stimulus period (t = 0.5 to 1.0 s), the fly viewed a grey screen. The visual field of the display screens spanned 180° azimuth and 90° elevation.

Figure 2

Head movements in response to rotational image motion. (A) Example trials of head yaw movements of flies viewing 10 Hz progressive motion, followed by 4 Hz rotational image motion in the stimulus period. Top: After the onset of the yaw rotation stimulus (black dot), a saccade against the direction of motion (black line) is followed by a slow syndirectional turn (gray line; red dot marks end of stimulus period). Black arrow indicates small counter turn with the direction of motion at saccade start; white arrow indicates overshoot at saccade end where the head again moves with the direction of motion. Bottom: A saccade follows the onset of the yaw rotation stimulus (black line, arrows as before), then a slow syndirectional turn before a second saccade and further slow head turns. (B) Phase plots showing head yaw velocity and yaw head angle of examples in (A). Black and red dots indicate start and end of yaw rotation stimulus period; black and white arrows as in (A). (C) Head turns across all trials for fly #1 shown in (A–B) for the same pre-stimulus and stimulus conditions shown (n = 8 trials). Fast head turns (> 100°/s) are predominantly against the direction of motion. Their duration increases with peak velocity, but rarely last longer than 50 ms. Slow head turns (> 50 ms) are mostly in the direction of the motion stimulus. The magnitude of their peak velocity increases with the duration, but rarely exceeds 200°/s. (D–E). Peak velocity and duration of head turns of all flies (N = 18) for 10 Hz pre-stimulus progressive image motion and 4 Hz stimulus rotational image motion conditions of panels (A–C). (D). During pre-stimulus progressive image motion (‘progressive’), saccades and slow head turns occur in both directions. (E). During stimulus rotational image motion (‘rotation’), the distribution of saccades is skewed to positive head yaw velocities and slow turns are dominated by negative peak velocities. Red indicates head turns counting positively towards the OKI, and black indicates those counting negatively towards the OKI. Gray lines indicate thresholds used to classify saccades and slow head turns.

Figure 3

Rotational image motion selectively recruits OKN-like head movements. (A) Properties of saccadic head turns during progressive image motion of all the pre-stimulus conditions (Left) and during rotational image motion of the 4, 7 and 10 Hz stimulus conditions (Right). Top: the angular size of all positive (red—indicating optokinetic head movement) and negative (black—indicating anti-optokinetic head movement) head saccades, as a function of the initial yaw angle at the start of the turn. N = 18 flies. Bottom: the rate of head saccades per trial per second per degree, as a function of the initial yaw angle at the start of the turn. Asterisks and p values indicate significance of statistical comparisons of the rate of saccades per trial per second for all initial angles (paired t-tests, N = 18; ** p < 0.01, *** p < 0.001). (B) Properties of slow head movements, from the same data and using the same plotting conventions as in (A). Here, negative slow movements with the direction of motion are plotted in red—indicating optokinetic head movements—and positive slow movements are plotted in black—indicating anti-optokinetic head movements.

Figure 4

Flexible performance of OKN-like head movements, independent of the locomotor state. (A) The joint distribution of walking speed and the maximum change of the head yaw angle within an entire trial (4 s), for all flies. Flies were able to move their head or keep them still independently of whether they walked or were stationary. The largest head yaw ranges occurred for the slowest walking speeds, and fast walking flies (> 30 mm/s) either displayed large head movements or kept their heads still. Top: the distribution of walking speeds indicated a bimodal distribution of stationary and walking flies, which we classified by applying a threshold value of 3 mm/s, below which flies were defined to be stationary. Right: the distribution of maximum changes in head yaw angle also indicated a bimodal distribution of flies with heads still or heads moved which we classified by applying a threshold value of 2.5°/s, below which flies were defined to have stationary heads. (B) Percentage frequencies of trials in which, from left to right, the flies were: standing with head stationary, 35 ± 18%; walking with head stationary, 21 ± 18%; standing with head moving, 14 ± 7%; walking with head moving, 30 ± 17%; all values, mean ± std, N = 18. Boxplots indicate the median and quartile ranges, and the whiskers indicate the range of data points that are not outliers. Asterisks indicate statistical significance of comparisons of rates of walking versus being stationary for flies with stationary heads, and moving heads (Wilcoxon signed rank tests, with Holm-Bonferroni correction for 4 comparisons; N = 18; ** p < 0.01, *** p < 0.001).

Figure 5

Impact of visual motion adaptation on temporal frequency tuning of the optokinetic index, OKI. (A) OKI during stimulus period of rotational image motion (‘rotation’) of walking flies (≥ 3 mm/s). The pre-stimulus conditions are labelled in each panel, and indicated by color and plotting symbol. Values were calculated for flies that contributed ≥ 3 trials per stimulus condition. Mean ± S.E.M. shown, with the range of the number of flies contributing trials per stimulus temporal frequency indicated. (B) OKI during pre-stimulus progressive image motion (‘progressive’) of flies contributing to the data shown in A. Boxplots indicate the median and quartile ranges, whiskers indicate the range of data points that are not outliers, and white symbols indicate mean values. *p < 0.05, n.s. not significant, t-test with Holm-Bonferroni correction for multiple comparisons, N = 18. (C) Walking speed during the stimulus period of rotational image motion. Plotting conventions as in (B). ** p < 0.01, paired t-test with Holm-Bonferroni correction for multiple comparisons, N = 18. (D) OKI of responses to 25 Hz rotational image motion, for all pre-stimulus conditions. Plotting conventions as in B. **p < 0.01, Wilcoxon signed rank test, N = 18. (E) OKI as a function of the walking speed, for the rotational image motion between 10 and 25 Hz (four data points), for all pre-stimulus conditions indicated by color and symbols (legend). Grey line indicates the linear fit that minimizes the sum of squared errors.

Figure 6

Impact of walking speed on temporal frequency tuning of the OKI. (A) OKI during pre-stimulus progressive image motion (‘progressive’), for all flies that contributed trials to the OKI during the stimulus period of rotational image motion, shown in (B), grouped by walking speeds (left to right): stationary (< 3 mm/s); slow walking (3–20 mm/s); fast walking (20–100 mm/s). The walking speeds are indicated by the greyscale intensity and symbols. Mean ± S.E.M. shown, and the range of the number of flies contributing trials per stimulus temporal frequency were: N = 10–13 (stationary), N = 14–17 (slow walking), N = 8–11 (fast walking). (B) OKI during the stimulus rotational image motion (‘rotation’), for all flies grouped by walking speed as in A. Mean ± S.E.M. shown. The range of the number of flies contributing trials per stimulus temporal frequency were as in (A), and indicated. The temporal frequency tuning of the OKI of stationary flies is qualitatively maintained over a range of walking speeds.