Localized direction selective responses in the dendrites of visual interneurons of the fly

Abstract

Background: The various tasks of visual systems, including course control, collision avoidance and the detection of small objects, require at the neuronal level the dendritic integration and subsequent processing of many spatially distributed visual motion inputs. While much is known about the pooled output in these systems, as in the medial superior temporal cortex of monkeys or in the lobula plate of the insect visual system, the motion tuning of the elements that provide the input has yet received little attention. In order to visualize the motion tuning of these inputs we examined the dendritic activation patterns of neurons that are selective for the characteristic patterns of wide-field motion, the lobula-plate tangential cells (LPTCs) of the blowfly. These neurons are known to sample direction-selective motion information from large parts of the visual field and combine these signals into axonal and dendro-dendritic outputs.

Results: Fluorescence imaging of intracellular calcium concentration allowed us to take a direct look at the local dendritic activity and the resulting local preferred directions in LPTC dendrites during activation by wide-field motion in different directions. These 'calcium response fields' resembled a retinotopic dendritic map of local preferred directions in the receptive field, the layout of which is a distinguishing feature of different LPTCs.

Conclusions: Our study reveals how neurons acquire selectivity for distinct visual motion patterns by dendritic integration of the local inputs with different preferred directions. With their spatial layout of directional responses, the dendrites of the LPTCs we investigated thus served as matched filters for wide-field motion patterns.

Figure 1

Acquisition of voltage and calcium responses to visual motion. (a) Setup for simultaneous electrophysiology and calcium imaging by multifocal two-photon and conventional wide-field microscopy. Insets show the recording site (top left) and the multifocal beamsplitter for two-photon microscopy (bottom left). The motion stimulus consists of a drifting square wave grating generated by a light emitting diode board. (b) Response of a vertical system (VS)2/3 neuron to a grating moving in preferred (bottom left) and antipreferred (top left) direction and directional tuning (right). Arrow directions indicate the direction of motion; arrow lengths represent response amplitudes averaged over an interval of 4 s starting at motion onset minus the mean response during 2 s before motion onset. Black arrows signal increases (depolarization); red arrows signal decreases (hyperpolarization). (c) Calcium response to downward motion of the same VS cell stained with Oregon Green BAPTA-1. Time course of the calcium signal integrated over the whole dendrite (left) and series of colour-coded ΔF/F₀ images showing local differences in fluorescence intensity for various time points (right). Resting fluorescence F₀ was determined by averaging the last three frames before start of pattern motion. Images were taken at 10 Hz and 512 × 512 pixel resolution.

Figure 2

Local dendritic directional preferences of various lobula plate tangential cells (LPTCs). Calcium signals at the dendrites of three different LPTC classes: vertical system (VS)3 cells (a), neurons called Amacrine cell (b) and ventral centrifugal horizontal cells (c). (i) Local differences in fluorescence intensity recorded in a single cell during pattern motion in eight directions (α) in comparison to an average of the last three frames before onset of motion. Signals were averaged over the last 500 ms of stimulus motion. (ii) Integrated local calcium responses to pattern motion in eight directions - the same cell as in (i). The grid of region of interest (ROIs) is indicated in the upper left corner. White arrows show increases in calcium in the underlying ROI in response to motion in the direction of the arrow; red arrows show decreases. Arrow length (normalized to the arrow with maximum amplitude in the image) represents ΔF/F intensity. (iii): Resulting response vectors from vector summation of the individual response vectors to all stimulus directions (normalized as before) - the same cell as in (i). Arrow brightness represents overall response amplitude. (iv) Resulting response vectors of additional cells of each of the cell types recorded in different flies, calculated as in (iii). In (c), right column, the profile faintly visible in the dorsal area is a VS cell which was accidentally stained during tissue penetration. The cell did not noticeably contribute to the calcium signals. Images represent single recording traces and were taken at 10 Hz and 512 × 512 px resolution.

Figure 3

Influences of ipsi- and contralateral inputs on ventral centrifugal horizontal (vCH) calcium signals. (a and b) Image pairs showing local differences in fluorescence intensity in a single vCH cell each during pattern motion in eight directions (white numbers), with only the contralateral (left panels) or ipsilateral (right panels) half of the stimulus pattern visible. Signals were averaged over the last 500 ms of stimulus motion. Images represent single recording traces and were taken at 10 Hz and 512 × 512 pixel resolution. (c) Wiring diagram of connections between contralateral lobula plate tangential cells and vCH. Black arrows next to the cells indicate the PD of visual motion in the contralateral receptive field of the cells. +: Excitatory input, -: inhibitory input. Wiring of H1, H2 and V1 after [23].

Figure 4

Fine-scale direction tuning of vertical system (VS)1 cell dendrites. (a) VS1 cell stained with Oregon Green BAPTA-1 showing the positions of the recording areas B-D. (b) Calcium response field of the dorsomedial dendrite to a square grating moving in eight directions. Presentation as in Figure 3. (c) Response field of the dorsal and (d) of the lateral dendritic branch of the same VS cell. Images b-d were taken at 10 Hz and 256 × 256 pixel resolution with multifocal two-photon laser-scanning microscopy.