Walking modulates speed sensitivity in Drosophila motion vision

Abstract

Changes in behavioral state modify neural activity in many systems. In some vertebrates such modulation has been observed and interpreted in the context of attention and sensorimotor coordinate transformations. Here we report state-dependent activity modulations during walking in a visual-motor pathway of Drosophila. We used two-photon imaging to monitor intracellular calcium activity in motion-sensitive lobula plate tangential cells (LPTCs) in head-fixed Drosophila walking on an air-supported ball. Cells of the horizontal system (HS)--a subgroup of LPTCs--showed stronger calcium transients in response to visual motion when flies were walking rather than resting. The amplified responses were also correlated with walking speed. Moreover, HS neurons showed a relatively higher gain in response strength at higher temporal frequencies, and their optimum temporal frequency was shifted toward higher motion speeds. Walking-dependent modulation of HS neurons in the Drosophila visual system may constitute a mechanism to facilitate processing of higher image speeds in behavioral contexts where these speeds of visual motion are relevant for course stabilization.

Figure 1. Two-Photon Calcium Imaging of the Motion-Sensitive HSN Neuron in Drosophila

(A) Schematic of the walking fly recording setup. (B) We recorded from the HSN’s dendrite branch, outlined in dotted red (from fly 6). HSE: HS-Equatorial. Scale bars represent 20 µm (i) and 5 µm (ii). (C) Canonical responses of the HSN neuron (from fly 6) to a horizontally rotating vertical grating pattern moving at a temporal frequency of 1 Hz in trials in which the fly was standing still. We always recorded from the left half of the brain; therefore, a front-to-back preferred direction (PD) for the neuron corresponds to counterclockwise rotations of the pattern. (D) Temporal frequency tuning of HSN dendrites. Shown are the mean normalized peak responses across eight different flies. Error bars indicate mean ± standard error of the mean (SEM). (E) Directional tuning of HSN dendrites. The figure shows the mean normalized peak response (error bars indicate mean ± SEM) across four different flies, during 10 s of grating motion of different orientations (indicated by red arrows) moving at 1 Hz. In all cases, the peak response was calculated as the mean value from a 0.5 s time window centered at the peak of the response and was normalized to the neuron’s maximum peak response.

Figure 2. HSN Responses Depend on the Walking State of the Fly

(A) Single trials (upper traces, calcium signals; lower traces, rotational velocities calculated over 0.5 s) from the HSN neuron and fly shown in Figure 1C during stationary (trial 7, blue) and walking (trial 1, red) conditions. For all examples in this figure, the fly was stimulated with a large-field, horizontal grating moving at a temporal frequency of 4 Hz. (B) Mean ± standard deviation (SD) response of the neuron during walking (red) and stationary (blue) trials (n = 5 trials per condition). (C) Mean normalized peak response during walking (black bar) and stationary (gray bar) trials across all flies (Mann-Whitney test, ***p < 0.001, n = 8). (D). Comparison of HSN responses in the two different nonwalking conditions. Shown are the mean ± SD of three different trials of HSN responses to a vertical grating moving horizontally at a temporal frequency of 3 Hz when the fly was standing still on the ball (”still-legs”) or when the same fly had its legs waxed (“fixedlegs”). (E) We repeated the same comparison across different temporal frequencies for four different flies to obtain temporal frequency tuning curves under the two nonwalking conditions (curves comparison: Mann-Whitney test; p values ranged from 0.2 to 0.9).

Figure 3. HSN Responses Are Correlated with Walking Speed

(A) Rotational walking velocities during periods of PD stimulation (gray box) for four different trials (indicated in different colors) for a single fly. (B) Trial-to-trial variability in accumulated rotation caused by the fly walking on the ball for the same trials. (C) Calcium transients (i.e., fluorescence transients, %DF/F) in response to PD stimulation in the same trials. (D) Correlation between mean calcium transients during the last 5 s of PD stimulus presentation (black box) and mean rotational walking speed over 100 trials for the same fly. Mean walking speed calculated over the 5 s periods shown in black boxes in (B): periods used to compute calcium transients are later in order to account for the delay in the transients.

Figure 4. HSN Temporal Frequency Tuning Curve Is Significantly Amplified and Its Peak Shifts toward Higher Frequencies during Walking

(A) Temporal frequency tuning curves for HSN dendrites in walking and stationary conditions for a single fly (fly 3; walking: n = 10 trials; stationary: n = 5 trials). (B) Mean temporal frequency tuning curve of HSN under walking and stationary conditions across all flies. The responses of HSN were normalized to the maximal responses for each fly under stationary conditions and averaged across flies (for 0.25 Hz and 10 Hz, n = 6 flies; other frequencies, n = 8 flies). *p < 0.05; **p < 0.01; ***p < 0.001. (C) Mean HSN response gain across temporal frequencies (number of flies per frequency same as in B). The gain is defined as the maximum calcium transient measured when the fly was walking divided by the maximum calcium transient obtained in stationary flies.