Biomedical subjects

Alexander Borst

Publications and source records attributed to Alexander Borst.

6 recordsLinked to original sources

PubMed2003

Neural image processing by dendritic networks.

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.

Animals↗

PubMed2003

Passive spatial and temporal integration of excitatory synaptic inputs in cerebellar Purkinje cells of young rats.

We have investigated the integration of excitatory (parallel fiber) synaptic inputs in cerebellar Purkinje cells of young rats in vitro and in a compartmental model of such a cell, based on 3D morphological reconstruction. Excitatory synaptic inputs at two independent dendritic sites were activated by electrical stimulation with various delays between the two stimuli. Population postsynaptic potentials summed linearly under current clamp condition when the two dendritic input sites were spatially separated (>200 microm) but sublinearly, in a delay dependent manner, when the input sites were close (<50 microm) to each other. Population postsynaptic currents measured under voltage clamp conditions summed linearly independent of the spatial and temporal separation of inputs. Summation of inputs in a passive compartmental model of a Purkinje cell was similar to that of Purkinje cells in vitro. We show that sublinear summation of neighboring inputs is independent of inhibitory mechanisms and suggest that sublinearity is mainly due to a locally reduced driving force.

Age Factors↗

PubMed2003

Adaptation of response transients in fly motion vision. II: Model studies.

The so-called 'Reichardt detector' can successfully account for many properties of fly motion vision. In its simplest form, the signals derived from neighboring image locations become multiplied after a low-pass filter has delayed one of them. This operation is done twice in a mirror-symmetrical form and the resulting output signals become finally subtracted. As predicted by this model, fly neurons respond to a brief motion pulse with a sudden rise in activity followed by an exponential decay. The time constant of this decay has been shown to shorten when tested after presentation of an adapting motion stimulus. In terms of the detector model this inevitably implies that the time constant of the low-pass filter is adapting. Given that, one would expect a concomitant shift of the steady-state response towards higher velocities, which, however, could not be experimentally verified. Here, we show that given a model with an additional temporal high-pass filter in the cross-arms of the detector, only the high-pass filter determines the time course of the impulse response. Assuming consequently that the time constant of the high-pass filter is the locus of adaptation resolves the conflicts mentioned above. Moreover, such an elaborated model with an adaptive time-constant faithfully mimics a particular contrast-dependency of transient response oscillations observed in fly motion sensitive neurons.

Adaptation, Ocular↗

PubMed2002

Dendro-dendritic interactions between motion-sensitive large-field neurons in the fly.

For visual course control, flies rely on a set of motion-sensitive neurons called lobula plate tangential cells (LPTCs). Among these cells, the so-called CH (centrifugal horizontal) cells shape by their inhibitory action the receptive field properties of other LPTCs called FD (figure detection) cells specialized for figure-ground discrimination based on relative motion. Studying the ipsilateral input circuitry of CH cells by means of dual-electrode and combined electrical-optical recordings, we find that CH cells receive graded input from HS (large-field horizontal system) cells via dendro-dendritic electrical synapses. This particular wiring scheme leads to a spatial blur of the motion image on the CH cell dendrite, and, after inhibiting FD cells, to an enhancement of motion contrast. This could be crucial for enabling FD cells to discriminate object from self motion.

Animals↗

PubMed2002

Different mechanisms of calcium entry within different dendritic compartments.

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.

Animals↗

PubMedDate not supplied

Noise, not stimulus entropy, determines neural information rate.

In the quest for deciphering the neural code, theoretical advances were made which allow for the determination of the information rate inherent in the spike trains of nerve cells. However, up to now, the dependence of the information rate on stimulus parameters has not been studied in any neuron in a systematic way. Here, I investigate the information carried by the spike trains of H1, a motion-sensitive visual interneuron of the blowfly (Calliphora vicina) using a moving grating as a stimulus. Stimulus parameters fall in two classes: those that have only a minor effect on the information rate like increasing the frequency bandwidth or the maximum amplitude of the stimulus velocity, and those which dramatically affect the neural information rate, like varying the spatial size or the contrast of the visual pattern being moved. It appears that, for a broad range of complex stimuli, the neuron covers the stimulus with its whole response repertoire regardless of the stimulus entropy, with the information rate being limited by the noise of the stimulus and the neural hardware.

Animals↗