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J P Ewert

Publications and source records attributed to J P Ewert.

At least 19 recordsLinked to original sources
PubMed2001

Neural modulation of visuomotor functions underlying prey-catching behaviour in anurans: perception, attention, motor performance, learning.

The present review points out that visuomotor functions in anurans are modifiable and provides neurophysiological data which suggest modulatory forebrain functions. The retino-tecto/tegmento-bulbar/spinal serial processing streams are sufficient for stimulus-response mediation in prey-catching behaviour. Without its modulatory connections to forebrain structures, however, these processing streams cannot manage perceptual tasks, directed attention, learning performances, and motor skills. (1) Visual prey/non-prey discrimination is based on the interaction of this processing stream with the pretectal thalamus involving the neurotransmitter neuropeptide-Y. (2) Experiments applying the dopamine agonist apomorphine in combination with 2DG mapping and single neurone recording suggest that prey-catching strategies in terms of hunting prey and waiting for prey depend on dose dependent dopaminergic adjustments in the neural macronetwork in which retinal, pretecto-tectal, basal ganglionic, limbic, and mesolimbic structures participate. (3) Visual response properties of striatal efferent neurones support the concept that ventral striatum is involved in directed attention. (4) Various modulatory loops involving the ventral medial pallium modify prey-recognition in the course of visual or visual-olfactory learning (associative learning) or are responsible for stimulus-specific habituation (non-associative learning). (5) The circuits suggested to underlie modulatory forebrain functions are accentuated in standard schemes of the neural macronetwork. These provide concepts suitable for future decisive experiments.

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PubMed1999

Forebrain and midbrain structures involved in prey-catching behaviour of toads: stimulus-response mediating circuits and their modulating loops.

In anurans, visual prey information is filtered in the retina and processed in interacting pretectal and tectal retinal projection fields. Neuropeptide Y is involved in pretecto-tectal inhibition. Information related to prey and its location in space is transmitted to the bulbar/spinal motor pattern generating systems by ensembles of efferent tectal and tegmental neurons. This basic stimulus-response (S-R) mediating circuit is influenced by forebrain loops. It is suggested that ventral striatum and lateral thalamic nucleus participate in a loop responsible for gating S-R. The hippocampal pallium modifies S-R via the anterior thalamus with regard to previous experience. Dopaminergic modulation influences prey-catching strategies.

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PubMed1999

Responses of single neurons in the toad's caudal ventral striatum to moving visual stimuli and test of their efferent projection by extracellular antidromic stimulation/recording techniques.

Previous work in anuran amphibians has shown that activity in the caudal ventral striatum correlates with visuomotor activity: orienting responses toward prey fail to occur after striatal lesions. Thus it has been suggested that the striatum influences visually guided behavior. Therefore, the present study investigates visual response properties from neurons recorded in the striatum. Extracellular recordings of 104 single neurons of the cane toad's (Bufo marinus) caudal ventral striatum (STR) reveal five different response properties: resting discharge activity uninfluenced by the visual test stimuli (group STR1, 24.0%); resting discharge activity increased by any moving visual object (STR2, 31.7%); preference to moving compact objects (STR3, 15.4%); preference to certain configurational moving objects (STR4a and b, 13.5%), and resting activity reduced by visual stimuli (STR5, 15.4%). The receptive fields of these neurons encompassed the contralateral (46%) or the entire field of vision (54%). Of the neurons recorded in the striatum, 34% responded to electrical stimuli applied in the rostral diencephalon to the ipsilateral lateral forebrain bundle (LFB) which connects the striatum with the optic tectum (e.g. either directly or via pretectum or tegmentum). Various electrically driven STR neurons (40%) have axons that project caudally through the LFB, which was suggested by their antidromic activation in response to electrical stimuli applied to the LFB in the rostral diencephalon. In the present study, the main striatal output is mediated by 'motion detectors' (STR2) and 'compact object perceivers' (STR3). It is suggested that the caudal ventral striatum is involved in visual attentional processes that allow the translation of perception into action.

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PubMed1998

Neuropeptide Y (NPY) or fragment NPY 13-36, but not NPY 18-36, inhibit retinotectal transfer in cane toads Bufo marinus.

Previous work suggests that retinotectal information processing is influenced by pretectotectal ipsilateral projections. Neuropeptide Y (NPY) participates in the pretectotectal transmission. The present investigation demonstrates that administration of porcine NPY to the tectal surface causes a profound and prolonged attenuation of the initial excitatory N1 wave of the summated tectal surface field potential (FP) evoked by diffuse light off stimulation. The FP's on response was affected as well, but was less sensitive to NPY. Administration of the fragment NPY 13-36, a Y2 receptor agonist, had a smaller effect than did NPY. Fragment NPY 18-36, however, showed no comparable influences. The data suggest that NPY in the cane toad's pretectotectal pathway controls retinotectal transmission in an inhibitory manner via a Y2 receptor mechanism.

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PubMed1997

Dopaminergic modulation of visual responses in toads. I. Apomorphine-induced effects on visually directed appetitive and consummatory prey-catching behavior.

This study confirms for a phylogenetically basal terrestrial vertebrate that dopaminergic modulations interfere with the visually directed appetitive and consummatory feeding behaviors orienting and snapping, respectively. (1) In common toads Bufo bufo, intralymphatic administration of the dopamine D2/D1-receptor agonist apomorphine led to a dose-dependent facilitation of prey-snapping in response to moving objects. The snapping activity reached a maximum 15-35 min after apomorphine injection. (2) To changes in configurational stimulus features, the basic pattern of discrimination was maintained; however, the acuity of discrimination was reduced due to the high snapping response level. (3) The apomorphine-induced facilitation of snapping was accompanied by a suppression of prey-oriented lunging and turning. Toads snapped only if prey occurred frontally in the visual field at a relatively short distance. The snapping behavior was fixed in its form and stereotyped regarding its immediate release. (4) About 90 min after apomorphine administration, prey-oriented turning behavior was restored and displayed a facilitatory rebound. (5) In comparative experiments with the species B. marinus, both prey-oriented turning and snapping responses were suppressed by apomorphine in a dose-dependent manner. (6) After pre-treatment with the dopamine antagonist haloperidol, apomorphine showed no measurable effect on the visual release of prey orienting or snapping. (7) The results contribute to the sensorimotor and the motivation hypothesis of dopamine function proposed for higher vertebrates and stimulate a comparative discussion of anatomic homologies and functional analogies.

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PubMed1997

Dopaminergic modulation of visual responses in toads. II. Influences of apomorphine on retinal ganglion cells and tectal cells.

The behavioral studies of Part I have shown in common toads that after systemic administration of the dopamine agonist apomorphine the prey-directed orienting turning movements are suppressed while prey snapping is facilitated. Part II focuses on retinal and tectal single cell responses to moving objects. (1) After systemic administration of apomorphine, the discharge rates of retinal class R2 and R3 ganglion cell fibres--recorded from the retino-tectal projection--speeded up in response to visual objects traversing their excitatory receptive fields. This enhancing effect was independent of the recording site in the retino-tectal map. (2) The diameters of the excitatory receptive fields of R2 and R3 neurons doubled their sizes. Probably, apomorphine enhances the center-dominated excitatory responses at the expense of the strength of the inhibitory surround. (3) The apomorphine-induced effects were fully developed 20-35 min after drug administration. (4) At the same time the discharge rates of T5.1 and T5.2 tectal neurons were reduced under apomorphine. The effect was independent of the recording site in the retino-tectal map. The diameters of the excitatory receptive fields of these tectal neurons were not influenced. (5) To changes in configurational stimulus features, the basic pattern of discrimination was maintained. (6) It is suggested that tectal output to the turn-generating motor network--mediated by T5.1 and T5.2 neurons--is modulated by a pretecto-tectal pathway which involves dopaminergic pretectal cells. (7) The enhanced snapping can be interpreted in terms of a modulation of reticular/hypoglossal structures by dopaminergic preoptic/hypothalamic/solitary systems.

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PubMed1997

Neural correlates of key stimulus and releasing mechanism: a case study and two concepts.

Major themes in neuroethology concern the specificity of key stimuli, neurones tuned to such stimuli, and the release of corresponding behaviour. Neurobiological data from the analysis of visuomotor functions of prey-catching and avoiding in amphibians support the view that retinal outflow in different combinations is pooled for further computation in interacting processing streams, rather than segregated into distinct retinal channels. The keys by which the visual system gets access to perceptual-motor categories are shown to derive from specific computational strategies that evaluate significant configurational features of objects. Rapid behavioural responses are assured by visuomotor pathways which, monitoring different aspects of visual objects, collectively select appropriate motor patterns. Responses can be adapted to varying environmental and internal conditions via modulating and modifying loops. This requires parallel distributed processing and integration at various levels in a macro-network.

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PubMed1996

Apomorphine-induced suppression of prey oriented turning in toads is correlated with activity changes in pretectum and tectum: [14C]2DG studies and single cell recordings.

In common toads, after systemic administration of the dopamine agonist apomorphine (APO), prey-oriented turning was suppressed. Searching for neural correlates, the present study shows APO-induced increases in glucose utilization in the retinorecipient pretectum and dorsal optic tectum and a decrease in the medial tectal output layers. This pattern of metabolic activity is resembled by neuronal discharge activities recorded in response to a prey stimulus; the discharge rates are increased in retinal ganglion cells and pretectal neurons and decreased in tectal output neurons. We suggest that an APO-induced enhancement of pretecto-tectal inhibitory influences contributes to the reduction of tectal output, thus suppressing prey-oriented turning behavior.

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PubMed1996

A key by which the toad's visual system gets access to the domain of prey.

Searching for principles that allow toads to distinguish between prey and nonprey, we wondered how the toad's prey-catching activity measured as R differs in response to changes in significant configurational stimulus features. Elongated shapes moving worm-like in the direction of their longer axes are preferred prey dummies; but a toad is not a worm detector, and a worm is not the unique prey-catching releaser. Considering the frequency distributions of R values, we show that the release of prey catching is in a specific manner sensitive to the relation between the extensions of an object parallel (xl1) and perpendicular (xl2) to its direction of movement. It is the xl1 and xl2 features-relating algorithm that provides the key (instruction) by which the toad's visual system gets access to the domain of potential prey in terms of configurational cues. This, within behaviorally relevant limits, largely invariant algorithm also holds for segmented stimuli. Further investigations show that this principle of object discrimination is not due to experimental procedures but emerges as a species-common property, of which different toad species take advantage in a species-specific manner. Neurobiological correlates are discussed.

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PubMed1996

Disproportionate distribution of field potentials across the toad's tectal visual map in response to diffuse light ON and OFF stimulations.

In toads Bufo marinus and Bufo bufo spinosus, field potentials (FPs) were recorded from the surface of the optic tectum at different sites of the visual map in response to a sudden diffuse darkening (OFF) and lightening (ON) of the visual field of the contralateral eye. The OFF and ON responses were differently pronounced or even failed to occur. The latency of the former was significantly less than the one of the latter. FP amplitudes of the OFF and ON responses were strongest in the representation of a horizonto-superior anterio-lateral portion of the visual field and weakest toward the posterior field of vision. This phenomenon suggests various interpretations for subsequent experiments.

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PubMed1996

Influence of pretectal lesions on tectal responses to visual stimulation in anurans: field potential, single neuron and behavior analyses.

Investigating the fine structure of the anuran's optic tectum (OT), Székely and Lázár suggested that a focal tectal excitation would spread across the tectal network if there were no control by thalamic pretectal (TP) inhibitory influences. Disconnecting OT from TP by lancet lesions in toads, we show that visual prey-catching is hyperexcited and stimulus discrimination nearly abolished. Functional recovery exists. Micro-administration of the axon sparing excitotoxins kainic acid (KA) or ibotenic acid (IBO) confirm that the TP region is actually involved. Recordings from prey-selective tectal neurons in immobilized frogs reveal a KA-induced impairment of stimulus discrimination and an increase in spontaneous firing. Following TP-lesions, in freely moving toads a correlation is observed between any moving stimulus, enhanced neuron firing, and pray-catching. Tectal field potentials evoked by diffuse light on- and off-stimuli before and after administration of the conduction-blocking drug procaine (PRC) suggest that populations of tectal neurons are affected by pretectal inhibitory influences. Considering previous work on physiologically identified pretecto-tectal projection cells, three different kinds of pretecto-tectal influences are discussed. A working hypothesis suggests a loop by which striatal influences are controlling TP, thus gating and tuning the visual information processing in OT.

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PubMed1995

Responses of retinal and tectal neurons in non-paralyzed toads Bufo bufo and B. marinus to the real size versus angular size of objects moved at variable distance.

Toads judge the real size of moving visual objects during prey-catching. But neither ganglion cells of the retinotectal projection nor tectal neurons showed a comparable phenomenon in muscle paralyzed toads. In non-paralyzed toads, however, tectal neurons, unlike neurons, displayed a sensitivity to the real size of an object moving at variable distance to the animal. Interestingly, this property was obtained both in monocular T5 smallfield neurons and in most of the investigated T4 widefield neurons.

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PubMed1995

Effect of neuropeptide-Y on tectal field potentials in the toad.

Field potentials (FP) recorded from the surface of toad's optic tectum (OT) to electrical stimulation of the contralateral optic nerve (ON) show initial positive deflections P*, followed by negative wave N and positive wave P; the former result from axonal inputs, the latter two resemble excitatory and inhibitory postsynaptic processes, respectively. Electrostimulation of the pretectum ipsilaterally to the recorded OT--preceeding ON-stimulation--strongly attenuates the N wave, suggesting pretectotectal inhibitory influences. The N wave of the tectal FP evoked by ON-stimulation is reduced, too, after application of the neuropeptide-Y (NPY) to the tectal surface. Previous authors have shown that frog's pretectum contains NPY immunoreactive pretectotectal projecting cells.

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PubMed1994

Increases of excitatory receptive fields of retinal ganglion cells in common toads under apomorphine are not associated with size preference in prey-snapping.

After systemic administration of the dopamine agonist apomorphine (APO) in common toads Bufo bufo spinosus, the excitatory receptive fields of retinal R2 and R3 ganglion cells doubled their size and the response activities of these neurons to moving visual stimuli were generally increased. Comparable investigations of the stimulus-response relationships in the snapping behavior of toads pre- and post-APO showed a general increase in the snapping responses, but no shift in the preferred prey size. This is consistent with the current concept, suggesting that size evaluation in amphibians does not rely on retinal ganglion cell output but on interacting networks beyond the retina.

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PubMed1992

Configurational pattern discrimination responsible for dishabituation in common toads Bufo bufo (L.): behavioral tests of the predictions of a neural model.

Recently, a neural model of visual pattern discrimination for stimulus-specific habituation was developed, based on previous behavioral studies which demonstrated that toads exhibit a dishabituation hierarchy for different worm-like stimuli. The model suggests that visual objects are represented by temporal coding and predicts that the dishabituation hierarchy changes when the stimulus/background contrast direction is reversed or the stimulus size is varied. The behavioral experiments reported in this paper were designed to test these predictions. (1) For a pair of stimuli from the contrast reversal prediction, the experimental results validated the theory. (2) For a pair of stimuli from the size reduction prediction, the experimental results failed to validate the theory. Further experiments concerning size effects suggest that configural visual pattern discrimination in toads exhibits size invariance. (3) Inspired by the Groves-Thompson account of habituation, we found that dishabituation by a second stimulus has a separate process from habituation to a first stimulus. This paper serves as an example of a fruitful dialogue between experimentation and modeling, crucial for understanding brain functions.

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