The mechanisms underlying visual motion detection can be studied simultaneously in different cell compartments in vivo by using calcium as a reporter of the spatiotemporal activity distribution in single motion-sensitive cells of the fly. As predicted by the Reichardt model, local dendritic calcium signals are found to indicate the direction and velocity of pattern motion but are corrupted by spatial pattern properties. The latter are canceled out by spatial integration, thus leading to a purely directional selective output signal in the axon. These findings attribute a specific computational task to the dendrites of visual interneurons and imply a functional interpretation of dendritic morphology.
The extraction of motion information from time varying retinal images is a fundamental task of visual systems. Accordingly, neurons that selectively respond to visual motion are found in almost all species investigated so far. Despite its general importance, the cellular mechanisms underlying direction selectivity are not yet understood in most systems. Blocking inhibitory input to fly visual interneurons by picrotoxinin (PTX), we demonstrate that their direction selectivity arises largely from interactions between postsynaptic signals elicited by excitatory and inhibitory input elements, which are themselves only weakly tuned to opposite directions of motion. Their joint activation by preferred as well as null direction motion leads to a mixed reversal potential at which the postsynaptic response settles for large field stimuli. Assuming the activation ratio of these opponent inputs to be a function of pattern velocity can explain how the postsynaptic membrane potential saturates with increasing pattern size at different levels for different pattern velocities ("gain control"). Accordingly, we find that after blocking the inhibitory input by PTX, gain control is abolished.Key words: direction selectivity; motion detection; membrane parameters; compartmental model; synaptic conductance; neural computation; gain control; flyThe fly has for long been a model system to study the processing and extraction of motion information from the time varying retinal images. In the third visual neuropile of the fly optic lobes a group of individually identifiable, motion-sensitive interneurons has been found. They are called lobula plate tangential cells (LP TC s) and are involved in visual course control (Hausen, 1984). In the blowfly Calliphora er ythrocephala this group comprises about 60 different cells. Via large dendritic arbors, these neurons spatially pool the signals of thousands of retinotopically arranged columnar elements . Most LP TC s studied so far display a directionally selective response: When the pattern is moving in the preferred direction (PD) of the cell, the cells become excited; when the pattern is moving in the anti-preferred or null direction (N D) of the cell, they become inhibited. Many L P TC s are nonspiking neurons. Rather than producing regular action potentials, they respond to visual motion by a graded shift of their membrane potential. Their directional selective responses are driven by at least two kinds of input elements, one being excitatory and the other inhibitory Borst et al., 1995) (Fig. 1a). As was revealed by in vitro studies (Brotz et al., 1995), the underlying dendritic receptors exhibit a pharmacological profile typical for insect nicotinic ACh receptors and picrotoxinin (P TX)-sensitive GABA receptors, respectively (Brotz and Borst, 1996).The computational structure of the fly motion detection system can be well described by a correlation type of elementary motion detector (EMD) Egelhaaf and Borst, 1989). This model for motion detection assumes a delay-andcompare mechanism for ea...
From our experiments combining in vivo calcium imaging and electrophysiology on fly vertical motion-sensitive cells (VS-cells) during visual stimulation, we infer different mechanisms of calcium entry within different dendritic compartments; while in the main dendritic branches calcium influx from extracellular space takes place only via voltage-activated calcium channels (VACCs), calcium enters the dendritic tips through VACCs as well as nicotinic acetylcholine receptors (nAChRs). Consequently, neuronal nACHRs of insects have to be assumed to be permeable to some extent for calcium under in vivo conditions.
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