Neural image processing by dendritic networks

Abstract

Convolution is one of the most common operations in image processing. Based on experimental findings on motion-sensitive visual interneurons of the fly, we show by realistic compartmental modeling that a dendritic network can implement this operation. In a first step, dendritic electrical coupling between two cells spatially blurs the original motion input. The blurred motion image is then passed onto a third cell via inhibitory dendritic synapses resulting in a sharpening of the signal. This enhancement of motion contrast may be the central element of figure-ground discrimination based on relative motion in the fly.

Fig. 1.

Realistic compartmental models of a vCH cell (green), connected to an HSS and an HSE cell (red). (Left Inset) Original fluorescent staining for comparison. (Scale bar is 100 μm in both pictures.) (Right Inset) Location of electrical synapses (white) used for these analyses.

Fig. 2.

Calibration and validation of the model. (A) I–V curve (current injection in HSE, voltage response in vCH). The model follows the rectification observed in real cells. (B) Action potentials in HSE model do not travel to the vCH model axon. (C) Current injection into HSS (Left) and in HSE (Right) results in a localized potential change only in the respectively superimposed dendrites of vCH. Potential change in vCH model is shown as a false-color image. The anatomy of HSS and HSE is outlined in black.

Fig. 3.

Example of the blurring effect of the HS–CH connection. (A) Potential spread in HSS model (Left) and vCH model (Right) after local current injection into HSS. (B) Same as A, but HSS and vCH models were not connected to each other. Current was injected in HSS (Left) and vCH (Right). (C) Quantification of model results: ranked membrane potential distribution in HSS and vCH model dendrites, connected and unconnected. For example, 6% of HS dendrites and 26% of vCH dendrites are >60% of the maximum potential (dotted line) when connected to each other. When current is injected into an unconnected vCH model, only 0.37% of its dendrites are >60% of the maximum potential.

Fig. 4.

Simplified model. (A) Two cylinders (HS and CH) are connected by five linear conductances surrounding the location of current injection. (B) In HS, the signal spreads after an exponential decay. The CH spread is broader. (C) The CH spread can be approximated by the sum of passive spread through each conductance.

Fig. 5.

Consequences of the CH cell dendritic image blurring for relative motion detection. An array of elementary motion detectors computes the image motion in a retinotopic way, feeding onto the dendrites of HS and FD cells. Via dendro-dendritic connections between HS and CH cells, this motion representation is blurred in the dendrites of the CH cell. By conveying inhibitory dendro-dendritic input to FD cells, being subtracted from the retinotopic input, an enhancement of the motion edges is achieved.