Biomedical subjects

Holger G Krapp

Publications and source records attributed to Holger G Krapp.

11 recordsLinked to original sources

PubMed2006

Influence of electrotonic structure and synaptic mapping on the receptive field properties of a collision-detecting neuron.

The lobula giant movement detector (LGMD) is a visual interneuron of Orthopteran insects involved in collision avoidance and escape behavior. The LGMD possesses a large dendritic field thought to receive excitatory, retinotopic projections from the entire compound eye. We investigated whether the LGMD's receptive field for local motion stimuli can be explained by its electrotonic structure and the eye's anisotropic sampling of visual space. Five locust (Schistocerca americana) LGMD neurons were stained and reconstructed. We show that the excitatory dendritic field and eye can be fitted by ellipsoids having similar geometries. A passive compartmental model fit to electrophysiological data was used to demonstrate that the LGMD is not electrotonically compact. We derived a spike rate to membrane potential transform using intracellular recordings under visual stimulation, allowing direct comparison between experimental and simulated receptive field properties. By assuming a retinotopic mapping giving equal weight to each ommatidium and equally spaced synapses, the model reproduced the experimental data along the eye equator, though it failed to reproduce the receptive field along the ventral-dorsal axis. Our results illustrate how interactions between the distribution of synaptic inputs and the electrotonic properties of neurons contribute to shaping their receptive fields.

Action Potentials↗

PubMed2006

Spike-frequency adaptation and intrinsic properties of an identified, looming-sensitive neuron.

We investigated in vivo the characteristics of spike-frequency adaptation and the intrinsic membrane properties of an identified, looming-sensitive interneuron of the locust optic lobe, the lobula giant movement detector (LGMD). The LGMD had an input resistance of 4-5 MOmega, a membrane time constant of about 8 ms, and exhibited inward rectification and rebound spiking after hyperpolarizing current pulses. Responses to depolarizing current pulses revealed the neuron's intrinsic bursting properties and pronounced spike-frequency adaptation. The characteristics of adaptation, including its time course, the attenuation of the firing rate, the mutual dependency of these two variables, and their dependency on injected current, closely followed the predictions of a model first proposed to describe the adaptation of cat visual cortex pyramidal neurons in vivo. Our results thus validate the model in an entirely different context and suggest that it might be applicable to a wide variety of neurons across species. Spike-frequency adaptation is likely to play an important role in tuning the LGMD and in shaping the variability of its responses to visual looming stimuli.

Action Potentials↗

PubMed2006

A motion-sensitive neurone responds to signals from the two visual systems of the blowfly, the compound eyes and ocelli.

In the blowfly Calliphora vicina, lobula plate tangential cells (LPTCs) estimate self-motion by integrating local motion information from the compound eyes. Each LPTC is sensitive to a particular (preferred) rotation of the fly's head. The fly can also sense rotation using its three ocelli (simple eyes), by comparing the light intensities measured at each ocellus. We report that an individually identified tangential cell, V1, responds in an apparently rotation-specific manner to stimulation of the ocelli. This effect was seen with or without additional stimulation of the compound eye. We delivered stimuli to the ocelli which mimicked rotation of the fly's head close to that of the preferred axis of rotation of V1. Alternating between preferred and anti-preferred rotation elicited a strongly phasic response, the amplitude of which increased with the rate of change of light intensity at the ocelli. With combined stimulation of one compound eye and the ocelli, V1 displayed a robust response to ocellar stimuli over its entire response range. These findings provide the opportunity to study quantitatively the interactions of two different visual mechanisms which both encode the same variable--the animal's rotation in space.

Animals↗

PubMed2005

Population coding of self-motion: applying bayesian analysis to a population of visual interneurons in the fly.

Coding of sensory information often involves the activity of neuronal populations. We demonstrate how the accuracy of a population code depends on integration time, the size of the population, and noise correlation between the participating neurons. The population we study consists of 10 identified visual interneurons in the blowfly Calliphora vicina involved in optic flow processing. These neurons are assumed to encode the animal's head or body rotations around horizontal axes by means of graded potential changes. From electrophysiological experiments we obtain parameters for modeling the neurons' responses. From applying a Bayesian analysis on the modeled population response we draw three major conclusions. First, integration of neuronal activities over a time period of only 5 ms after response onset is sufficient to decode accurately the rotation axis. Second, noise correlation between neurons has only little impact on the population's performance. And third, although a population of only two neurons would be sufficient to encode any horizontal rotation axis, the population of 10 vertical system neurons is advantageous if the available integration time is short. For the fly, short integration times to decode neuronal responses are important when controlling rapid flight maneuvers.

Animals↗

PubMed2004

Spatial distribution of inputs and local receptive field properties of a wide-field, looming sensitive neuron.

The lobula giant movement detector (LGMD) in the locust visual system and its target neuron, the descending contralateral movement detector (DCMD), respond to approaching objects looming on a collision course with the animal. They thus provide a good model to study the cellular and network mechanisms underlying the sensitivity to this specific class of behaviorally relevant stimuli. We determined over an entire locust eye the density distribution of optical axes describing the spatial organization of local inputs to the visual system and compared it with the sensitivity distribution of the LGMD/DCMD to local motion stimuli. The density of optical axes peaks in the equatorial region of the frontal eye. Local motion sensitivity, however, peaks in the equatorial region of the caudolateral visual field and only correlates positively with the dorso-ventral density of optical axes. On local stimulation, both the velocity tuning and the response latency of the LGMD/DCMD depend on stimulus position within the visual field. Spatial and temporal integration experiments in which several local motion stimuli were activated either simultaneously or at fixed delays reveal that the LGMD processes local motion in a strongly sublinear way. Thus the neuron's integration properties seem to depend on several factors including its dendritic morphology, the local characteristics of afferent fiber inputs, and inhibition mediated by different pathways or by voltage-gated conductances. Our study shows that the selectivity of this looming sensitive neuron to approaching objects relies on more complex biophysical mechanisms than previously thought.

Action Potentials↗

PubMed2004

Insect-inspired estimation of egomotion.

Tangential neurons in the fly brain are sensitive to the typical optic flow patterns generated during egomotion. In this study, we examine whether a simplified linear model based on the organization principles in tangential neurons can be used to estimate egomotion from the optic flow. We present a theory for the construction of an estimator consisting of a linear combination of optic flow vectors that incorporates prior knowledge about the distance distribution of the environment and about the noise and egomotion statistics of the sensor. The estimator is tested on a gantry carrying an omnidirectional vision sensor. The experiments show that the proposed approach leads to accurate and robust estimates of rotation rates, whereas translation estimates are of reasonable quality, albeit less reliable.

Algorithms↗

PubMed2003

Plasticity in the visual system is correlated with a change in lifestyle of solitarious and gregarious locusts.

We demonstrate pronounced differences in the visual system of a polyphenic locust species that can change reversibly between two forms (phases), which vary in morphology and behavior. At low population densities, individuals of Schistocerca gregaria develop into the solitarious phase, are cryptic, and tend to avoid other locusts. At high densities, individuals develop instead into the swarm-forming gregarious phase. We analyzed in both phases the responses of an identified visual interneuron, the descending contralateral movement detector (DCMD), which responds to approaching objects. We demonstrate that habituation of DCMD is fivefold stronger in solitarious locusts. In both phases, the mean time of peak firing relative to the time to collision nevertheless occurs with a similar characteristic delay after an approaching object reaches a particular angular extent on the retina. Variation in the time of peak firing is greater in solitarious locusts, which have lower firing rates. Threshold angle and delay are therefore conserved despite changes in habituation or behavioral phase state. The different rates of habituation should contribute to different predator escape strategies or flight control for locusts living either in a swarm or as isolated individuals. For example, increased variability in the habituated responses of solitarious locusts should render their escape behaviors less predictable. Relative resistance to habituation in gregarious locusts should permit the continued responsiveness required to avoid colliding with other locusts in a swarm. These results will permit us to analyze neuronal plasticity in a model system with a well-defined and controllable behavioral context.

Action Potentials↗

PubMed2003

Robustness of the tuning of fly visual interneurons to rotatory optic flow.

The sophisticated receptive field organization of motion-sensitive tangential cells in the visual system of the blowfly Calliphora vicina matches the structure of particular optic flow fields. Hypotheses on the tuning of particular tangential cells to rotatory self-motion are based on local motion measurements. So far, tangential cells have never been tested with global optic flow stimuli. Therefore we measured the responses of an identifiable neuron, the V1 tangential cell, to wide-field motion stimuli mimicking optic flow fields similar to those the fly encounters during particular self-motions. The stimuli were generated by a "planetarium-projector," casting a pattern of moving light dots on a large spherical projection screen. We determined the tuning curves of the V1-cell to optic flow fields as induced by the animal during 1) rotation about horizontally aligned body axes, 2) upward/downward translation, and 3) a combination of both components. We found that the V1-cell does not respond as specifically to self-rotations, as had been concluded from its receptive field organization. The neuron responds strongly to upward translation and its tuning to rotations is much coarser than expected. The discrepancies between the responses to global optic flow and the predictions based on the receptive field organization are likely due to nonlinear integration properties of tangential neurons. Response parameters like orientation, shape, and width of the tuning curve are largely unaffected by changes in rotation velocity or a superposition of rotational and translational optic flow.

Animals↗

PubMed2002

Multiplicative computation in a visual neuron sensitive to looming.

Multiplicative operations are important in sensory processing, but their biophysical implementation remains largely unknown. We investigated an identified neuron (the lobula giant movement detector, LGMD, of locusts) whose output firing rate in response to looming visual stimuli has been described by two models, one of which involves a multiplication. In this model, the LGMD multiplies postsynaptically two inputs (one excitatory, one inhibitory) that converge onto its dendritic tree; in the other model, inhibition is presynaptic to the LGMD. By using selective activation and inactivation of pre- and postsynaptic inhibition, we show that postsynaptic inhibition has a predominant role, suggesting that multiplication is implemented within the neuron itself. Our pharmacological experiments and measurements of firing rate versus membrane potential also reveal that sodium channels act both to advance the response of the LGMD in time and to map membrane potential to firing rate in a nearly exponential manner. These results are consistent with an implementation of multiplication based on dendritic subtraction of two converging inputs encoded logarithmically, followed by exponentiation through active membrane conductances.

Action Potentials↗

PubMed2002

Neural encoding of behaviourally relevant visual-motion information in the fly.

Information processing in visual systems is constrained by the spatial and temporal characteristics of the sensory input and by the biophysical properties of the neuronal circuits. Hence, to understand how visual systems encode behaviourally relevant information, we need to know about both the computational capabilities of the nervous system and the natural conditions under which animals normally operate. By combining behavioural, neurophysiological and computational approaches, it is now possible in the fly to assess adaptations that process visual-motion information under the constraints of its natural input. It is concluded that neuronal operating ranges and coding strategies appear to be closely matched to the inputs the animal encounters under behaviourally relevant conditions.

Adaptation, Physiological↗

PubMedDate not supplied

Multiplication and stimulus invariance in a looming-sensitive neuron.

Multiplicative operations and invariance of neuronal responses are thought to play important roles in the processing of neural information in many sensory systems. Yet the biophysical mechanisms that underlie both multiplication and invariance of neuronal responses in vivo, either at the single cell or at the network level, remain to a large extent unknown. Recent work on an identified neuron in the locust visual system (the LGMD neuron) that responds well to objects looming on a collision course towards the animal suggests that this cell represents a good model to investigate the biophysical basis of multiplication and invariance at the single neuron level. Experimental and theoretical results are consistent with multiplication being implemented by subtraction of two logarithmic terms followed by exponentiation via active membrane conductances, according to a x 1/b = exp(log(a) - log(b)). Invariance appears to be in part due to non-linear integration of synaptic inputs within the dendritic tree of this neuron.

Action Potentials↗