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.

Adaptation of response transients in fly motion vision. I: Experiments.

Two types of transient responses have been investigated in fly motion-sensitive neurons in the past: the impulse and the step response. In response to a brief motion pulse, cells show a sudden rise in activity followed by an exponential decay ('impulse response'). In response to the onset of a constant velocity stimulus, cells exhibit transient oscillations before settling to a steady-state value ('step response').

Orientation tuning of motion-sensitive neurons shaped by vertical-horizontal network interactions.

We measured the orientation tuning of two neurons of the fly lobula plate (H1 and H2 cells) sensitive to horizontal image motion. Our results show that H1 and H2 cells are sensitive to vertical motion, too. Their response depended on the position of the vertically moving stimuli within their receptive field.

Use of amplified fragment length polymorphism analysis to identify medically important Candida spp., including C. dubliniensis.

Non-Candida albicans Candida species are increasingly being isolated. These species show differences in levels of resistance to antimycotic agents and mortality. Therefore, it is important to be able to correctly identify the causative organism to the species level.

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.

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.

Clinical evaluation of a NASBA-based assay for detection of Candida spp. in blood and blood cultures.

The number of life-threatening opportunistic fungal infections has shown a dramatic increase. However, the diagnosis of candidemia remains difficult. Nucleic acid amplification assays may improve the detection rate and decrease the time needed for detection and identification of Candida spp.

Neural networks in the cockpit of the fly.

Flies have been buzzing around on earth for over 300 million years. During this time they have radiated into more than 125,000 different species (Yeates and Wiegmann 1999), so that, by now, roughly every tenth described species is a fly. They thus represent one of the most successful animal groups on our planet.

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.

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.

Recurrent network interactions underlying flow-field selectivity of visual interneurons.

Motion-sensitive large-field neurons found at higher processing stages in many species often exhibit a remarkable selectivity for particular flow fields. However, the underlying neural mechanisms are not yet understood. We studied this problem in the so-called lobula plate tangential cells (LPTCs) of the fly.

Spatial distribution of low- and high-voltage-activated calcium currents in neurons of the deep cerebellar nuclei.

The spatial distribution of low-voltage-activated (LVA) and high-voltage-activated (HVA) barium currents was investigated in neurons of the deep cerebellar nuclei (DCN) by combining barium imaging with voltage clamp. The current-induced fluorescence signal (DeltaF/F) of the HVA current was five times higher then the LVA-induced signal at the soma, but both signals were approximately equal in size in distant dendrites. This position-dependent shift of DeltaF/F indicates a non-uniform distribution of the underlying calcium channels.

Effects of mean firing on neural information rate.

We investigated the effect of mean firing on the information rate of a spiking motion-sensitive neuron in the fly (H1-cell). In the control condition, the cell was stimulated repeatedly by identical zero-symmetrical white-noise motion. The mean firing rate was manipulated by adding a constant velocity offset either in the same area of the receptive field where the dynamic stimulus was displayed or in a separate one.

Detection of Candida spp. in blood cultures using nucleic acid sequence-based amplification (NASBA).

Candida spp. are the main causes of fungal infections in immunocompromised patients. It is known, that the routinely used automated blood culture systems may fail to detect yeasts.

Cholinergic and GABAergic pathways in fly motion vision.

Background: The fly visual system is a highly ordered brain structure with well-established physiological and behavioral functions. A large number of interneurons in the posterior part of the third visual neuropil, the lobula plate tangential cells (LPTCs), respond to visual motion stimuli. In these cells the mechanism of motion detection has been studied in great detail.

Direction selectivity in ganglion cells: pre or post?

Contrary to a recent finding in rabbit retina, Borg-Graham reports that inputs to retinal ganglion neurons in turtles are already directionally selective.

Mechanisms of dendritic calcium signaling in fly neurons.

We examined the mechanisms underlying dendritic calcium accumulation in lobula plate tangential cells of the fly visual system using an in vitro preparation of the fly brain. Local visual stimulation evokes a localized calcium signal in the dendrites of these cells in vivo. Here we show that a similar localized calcium accumulation can be elicited in vitro by focal iontophoretic application of the cholinergic agonist carbachol.

Local current spread in electrically compact neurons of the fly.

Analyses of active and passive membrane properties predict an asymmetry in the spread of electrical current through a neuron. Simulated current injection into a large-diameter compartment of a biophysically realistic model neuron causes a local potential shift that can spread throughout the cell. In contrast, causing the same local potential shift in the dendritic tip of the same neuron results in only minimal changes in electrical potential in the rest of the cell.

Spatial distribution and characteristics of voltage-gated calcium signals within visual interneurons.

Most of our knowledge about insect calcium currents is derived from studies on cultured or dissociated somata. So far, only little data on calcium currents are available for neurons including their dendritic and presynaptic structures. Here we combined the switched-electrode voltage-clamp technique with optical recording using calcium-sensitive dyes in identified fly visual interneurons in vivo to characterize the voltage dependence and dynamics of calcium currents quantitatively and in a spatially resolved way.

The intrinsic electrophysiological characteristics of fly lobula plate tangential cells: III. Visual response properties.

In this last paper in a series (Borst and Haag, 1996; Haag et al., 1997) about the lobula plate tangential cells of the fly visual system (CH, HS, and VS cells), the visual response properties were examined using intracellular recordings and computer simulations. In response to visual motion stimuli, all cells responded mainly by a graded shift of their axonal membrane potential.

Separation of voltage- and ligand-gated calcium influx in locust neurons by optical imaging.

Calcium ions can enter neurons through either ionotropic transmitter receptors or through voltage-gated calcium channels. Thus, an observed rise in intracellular calcium concentration upon synaptic stimulation can be due to either one of these mechanisms or to both of them. We analyzed the individual contribution of transmitter- and voltage-gated calcium entry in non-spiking somata, acutely dissociated from thoracic ganglia of the locust Locusta migratoria.

Information theory and neural coding.

Information theory quantifies how much information a neural response carries about the stimulus. This can be compared to the information transferred in particular models of the stimulus-response function and to maximum possible information transfer. Such comparisons are crucial because they validate assumptions present in any neurophysiological analysis.