Orientation Selectivity Sharpens Motion Detection in Drosophila

Abstract

Detecting the orientation and movement of edges in a scene is critical to visually guided behaviors of many animals. What are the circuit algorithms that allow the brain to extract such behaviorally vital visual cues? Using in vivo two-photon calcium imaging in Drosophila, we describe direction selective signals in the dendrites of T4 and T5 neurons, detectors of local motion. We demonstrate that this circuit performs selective amplification of local light inputs, an observation that constrains motion detection models and confirms a core prediction of the Hassenstein-Reichardt correlator (HRC). These neurons are also orientation selective, responding strongly to static features that are orthogonal to their preferred axis of motion, a tuning property not predicted by the HRC. This coincident extraction of orientation and direction sharpens directional tuning through surround inhibition and reveals a striking parallel between visual processing in flies and vertebrate cortex, suggesting a universal strategy for motion processing.

Figure 1. The dendrites of T4 and T5 are directionally tuned

(A–D) Schematic of the T4 and T5 projection area, corresponding to the maximum intensity projection shown below, including the proximal medulla (Me), the lobula (Lo) and the lobula plate (Lp) with its four layers A–D (Maisak et al., 2013). Scale bar is 15 µm. A single region of interest (ROI) analyzed to extract the trace below is shaded. Traces display in vivo calcium responses to moving ON and OFF edges to one presentation of the stimulus (A) or the mean trace to repeated stimulus presentations, with individual response traces in grey (B–D). (E–G) Histograms plotting the normalized ROI count observed at each DS index value. Sample size (N = number of flies (ROIs)) was N = 12 (117) for T4 terminals in lobula plate layer C and N = 13 (74) for T4 dendrites in the proximal medulla (E), N = 13 (84) sparse T5 dendrites and N = 6 (37) dense T5 dendrites in (F) and N = 13 (84) sparse T5 dendrites and N = 10 (56) T5 axon terminals in layer C of the lobula plate (G). See also Figure S1.

Figure 2. T4 responses selectively enhance preferred direction signals

(A) X-T plots of apparent motion stimuli. A single bar appears for 0.5 s (stim 1); followed by a temporal delay of variable length (Δt), then a pair of bars appears for 0.5 s (stim 2). Shown are bar pair stimuli that create apparent motion to the right (PD), apparent motion to the left (middle panel, ND) and a control in which two pairs of bars are separated by Δt (flash control). (B,C) HRC and B-L models that are tuned to rightward motion, and their predicted responses to the stimuli shown in (A). The dotted lines display the linear prediction based on the sum of individual stim 1 and stim 2 responses. (D–F) Calcium signals imaged in the axon terminals of T4 and T5 in lobula plate layer C in response to a light bar version of the stimuli described above, at three different temporal delays. The dotted line represents the linear prediction based on the separate responses (recorded at Δt = 3 s). N = 14 (67) for axon terminals in (D, E) and N = 4(16) in (F). (G–J) Quantification of the peak calcium response at various time delays, comparing PD and ND responses (G), PD responses and the linear prediction (H), ND responses and the linear prediction (I) and double flash control responses with the linear prediction (J), for axon terminals of T4 and T5 in layer C of the lobula plate. * p <0.05, *** p<0.001, unpaired two tailed Student’s t-test with Bonferroni correction for multiple comparisons. See also Figure S2.

Figure 3. T5 responses selectively enhance preferred direction signals

(A–C) Calcium signals imaged in the axon terminals of T4 and T5 in lobula plate layer C in response to a dark bar version of the stimuli described in Figure 2, at three different temporal delays. The dotted line represents the linear prediction based on the separate responses (recorded at Δt = 3 s). N = 21 (114) for axon terminals in (A,B) and N = 6 (35) in (C). (D–G) Quantification of the peak calcium response at various time delays, comparing PD and ND responses (D), PD responses and the linear prediction (E), ND responses and the linear prediction (F) and double flash control responses with the linear prediction (G), for axon terminals of T4 and T5 in layer C of the lobula plate. * p <0.05, **p<0.01, *** p<0.001, unpaired two tailed Student’s t-test with Bonferroni correction for multiple comparisons. See also Figure S3.

Figure 4. Individual T4 and T5 neurons are orientation tuned and exhibit surround antagonism

(A) Polar plots displaying mean calcium responses in T4 and T5 axon terminals to static gratings of different orientations. Error bars are ±SEM. Layer A: N = 9(75), layer B: N = 9 (69), layer C: N = 9 (96), layer D: N = 9(78). (B,C) Peak calcium responses to horizontally or vertically oriented dark bars of variable width, imaged in T4 and T5 axon terminals in lobula plate layer A (N = 5 (9)) and layer B (N = 4 (10)) or layer C (N = 7 (38)) and layer D (N = 7 (25)). (D,E) Quantification of the difference in peak responses to horizontally and vertically oriented dark bars that were 7 to 15° wide (D) or light bars that were 5 to 10° wide (E), using averaged peak responses of the same ROIs as in panels (B,C) and Figure S4 B,C. * p<0.05, *** p<0.001, two tailed Student’s t-test. (F,H) Polar plots showing the mean calcium responses of a single T4 (F) or single T5 (H) cell. These same ROIs displayed direction tuning to 180° (T4) and 270° (T5) and thus represent putative layer A and D terminals, respectively (see Figure S4). (G,I) Normalized histogram comparing orientation selectivity index by ROI for clones with strong DS tuning for ON edges (G) or OFF edges (I) with ROIs obtained using the full T4/T5 GAL4 driver line. Individual clones were selected based on having a DS index > 0.5 for either moving ON or OFF edges respectively. Sample sizes were N = 4(16) flies (ROIs) for ON clones, N = 4 (17) for OFF clones and N = 9 (458) for the full T4/T5 pattern. See also Figure S4.

Figure 5. Orientation selectivity and antagonist surround require GABAergic signaling

(A) Polar plots displaying peak calcium responses in T4 and T5 axon terminals to static gratings of different orientations after application of the GABA_AR antagonist picrotoxin (PTX). Error bars are ±SEM. Layer A: N= 4(33). Layer B: N = 4(35). Layer C: N = 4(39). Layer D: N = 4(34). (B,C) Peak calcium responses in T4 and T5 axon terminals to static dark bars of various widths, after application of PTX, comparing horizontally and vertically oriented bars. N = 4 (20) for lobula plate layer A. N = 4 (24) for layer B. N = 4 (24) for layer C. N = 4 (22) for layer D. (D,E) Quantification of the difference in response to horizontally and vertically oriented dark bars that were 7 to 15° wide (D) or light bars that were 5 to 10° wide (E) after application of PTX, using averaged peak responses of the same ROIs used in panels (B,C) or in Figure S5 B,C. ** p<0.01, N.S. = not significant p>0.05, two tailed Student’s t-test. See also Figure S5.

Figure 6. Directionally tuned responses require GABAergic signaling

(A) Polar plots of calcium responses in T4 and T5 axon terminals to a moving dark bar, before and after application of PTX. Plots display the mean integrated responses to the bar moving in one of eight directions. Layers are color-coded: A = blue, B = green, C =red and D = yellow. Error bars are ±SEM. (B) Traces display mean responses. Shaded areas are ±SEM. The bar plots quantify the DS indices of these responses before and after PTX application to a moving dark bar at 100% contrast (upper plots). N = 10 flies; 50% contrast (middle plots), N = 6 flies; 25% contrast (lower plots), N = 6 flies. *** p<0.001, two tailed paired Student’s t-test. See also Figure S6.

Figure 7. OS and DS tuning properties are orthogonal

(A) Schematic of the stimulus design. Following an initial gray period, a static grating with a 20° spatial period appears at a particular orientation for 1.5 s. Then the grating moves orthogonal to the static pattern at 20°/s in one of 12 directions for 2 s. (B–D) Traces of mean calcium responses of T4 and T5 axon terminals of layer A in response to PD (120°) in (B), ND (300°) in (C) and an intermediate direction (30°) in (D). Shaded area is ±SEM. E, Polar plots displaying peak calcium responses of axon terminals in layer A to static gratings (reflecting orientation tuning) and to moving gratings (reflecting direction tuning). N = 14(108). The stars and arrowheads illustrate which epochs in (B–D) correspond to the values in (E). (F) Schematic illustration of the relationship between direction selectivity and orientation tuning in each of the four layers of the lobula plate. Histogram plotting the normalized ROI count for the angle θ between the OS and DS vector, ROIs were included in the analysis if they had sufficiently strong and DS responses to the moving grating (see Supplemental Experimental Procedures for details) (N ROIs = 171). (G) Schematic illustrating that the speed of a moving edge that is detected by a correlator will vary with motion direction. Predicted tuning width for T4 and T5 axon terminals calculated using measurements of PD responses to a grating moving at speeds ranging from 20°/s to 150°/s. (H) Gaussian fits to the measured tuning width of T4 and T5 axon terminals, normalized to the maximum response direction. Layer A: N = 14(108). Layer B: N = 15(106). Layer C: N = 15(135). Layer D: N = 15(109). FWHM denotes the full width at half maximum value. 95% confidence intervals are denoted in parentheses. For speed tuning measurements used to build the tuning width prediction in (G), Layer A: N = 3–14(11–108); Layer B: N = 3–15(17–106); Layer C: N = 4–15(25–135), and Layer D: N = 4–15(20–109). See also Figure S7.

Figure 8. An interaction between OS and DS sharpens DS tuning

A schematic summarizing how OS and DS properties of T4 and T5 can combine to narrow directional tuning. A HRC model demonstrates how preferred direction amplification leads to broad directional responses. This tuning can be made narrower by adding OS circuitry that suppresses responses to stimuli that are oriented slightly off axis.