PubMed Health⌕ Search

Biomedical subjects

P Thier

Publications and source records attributed to P Thier.

At least 19 recordsLinked to original sources
PubMed2001

Cortical substrates of perceptual stability during eye movements.

We are usually unaware of retinal image motion resulting from our own movement. For instance, during slow-tracking eye movements, the world around us remains perceptually stable despite the retinal image slip induced by the eye movement. This example of perceptual invariance is achieved by subtracting an internal reference signal, reflecting the eye movement, from the retinal motion signal. If the two cancel each other, visual structures, which do not move, will also be perceived as nonmoving. If, however, the reference signal is too small or too large, a false eye-movement-induced motion of the external world will be perceived. We have exploited our ability to manipulate the size of the reference signal in an attempt to reveal the structures in visual cortex, encoding the perception of self-induced visual motion rather than the retinal motion signal. Using EEG and lately also MEG recordings in human subjects and single-unit recordings in monkeys, we have been able to show that our ability to perceive the world as stationary despite eye-movement-induced retinal image slip is based on "late" parts of the cortical hierarchy of motion processing, sparing the early stages up to cortical area MT and, among others, involving cortex at the junction between the parietal and temporal lobes close to the parieto-insular-vestibular cortex. Lesions of this network in humans render the visual system unable to compensate for the visual consequences of eye movements, giving rise to severe dizziness, whenever the eyes move smoothly.

Animals↗
PubMed2001

Two types of neurons in the rat cerebellar nuclei as distinguished by membrane potentials and intracellular fillings.

Classically, three classes of neurons in the cerebellar nuclei (CN), defined by different projection targets and content of transmitters, have been distinguished. However, evidence for different types of neurons based on different intrinsic properties is lacking. The present study reports two types of neurons defined mainly by their intrinsic properties, as determined by whole-cell patch recordings. The majority of cells (type I, n = 63) showed cyclic burst firing whereas a small subset (type II, n = 7) did not. Burst firing was used to distinguish the two types of neurons because, as it turned out, pharmacological interference could not be used to convert the non-bursting cells to bursting ones. Some of the membrane potentials exclusively present in type I neurons, such as sodium and calcium plateau potentials, low-threshold calcium spikes, and a slow calcium-dependent afterhyperpolarization, were found to contribute to the generation of burst firing. Other membrane potentials of type I neurons were not obviously related to the generation of bursts. These were 1) the lower amplitude and width of the action potential during spontaneous activity, 2) a sequence of afterhyperpolarization-afterdepolarization-afterhyperpolarization following each spike, and 3) the high spontaneous firing rate. In contrast, type II neurons lacked slow plateau potentials and low threshold spikes. Their action potentials showed higher amplitude and width and were followed by a single deep afterhyperpolarization. Furthermore, they showed a lower firing rate at rest. In both types of neurons, a delayed inward rectification was present. Neurons filled with neurobiotin revealed that the sizes of the somata and dendritic fields of type I neurons comprised the whole range known from Golgi studies, whereas those of the few type II neurons recovered were found to be in the lowest range. In view of their size and scarcity, we propose that type II neurons may correspond to CN interneurons.

Action Potentials↗
PubMed2000

Encoding of movement time by populations of cerebellar Purkinje cells.

One of the earliest computational principles attributed to the cerebellum was the measurement of time. This idea was originally suggested on anatomical grounds, and was taken up again to explain some of the deficits in cerebellar patients. The contribution of the cerebellum to eye movements, in contrast, has traditionally been discussed in the context of motor learning. This view has received support from the loss of saccade adaptation, one of the key examples of motor learning, following lesions of the posterior cerebellar vermis. However, the relationship between the properties of saccade-related vermal Purkinje cells and the behavioural deficits that follow lesions is unclear. Here we report results from single-unit recording experiments on monkeys that reconcile the seemingly unrelated concepts of timing and motor learning. We report that, unlike individual Purkinje cells, the population response of larger groups of Purkinje cells gives a precise temporal signature of saccade onset and offset. Thus a vermal population response may help to determine saccade duration. Modifying the time course of the population response by changing the weights of the contributing individual Purkinje cells, discharging at different times relative to the saccade, would directly translate into changes in saccade amplitude.

Animals↗
PubMed2000

Reply

Explore the source record for details and available documents.

Journal Article↗
PubMed2000

Contextual influence on orientation discrimination of humans and responses of neurons in V1 of alert monkeys.

We studied the effects of various patterns as contextual stimuli on human orientation discrimination, and on responses of neurons in V1 of alert monkeys. When a target line is presented along with various contextual stimuli (masks), human orientation discrimination is impaired. For most V1 neurons, responses elicited by a line in the receptive field (RF) center are suppressed by these contextual patterns. Orientation discrimination thresholds of human observers are elevated slightly when the target line is surrounded by orthogonal lines. For randomly oriented lines, thresholds are elevated further and even more so for lines parallel to the target. Correspondingly, responses of most V1 neurons to a line are suppressed. Although contextual lines inhibit the amplitude of orientation tuning functions of most V1 neurons, they do not systematically alter the tuning width. Elevation of human orientation discrimination thresholds decreases with increasing curvature of masking lines, so does the inhibition of V1 neuronal responses. A mask made of straight lines yields the strongest interference with human orientation discrimination and produces the strongest suppression of neuronal responses. Elevation of human orientation discrimination thresholds is highest when a mask covers only the immediate vicinity of the target line. Increasing the masking area results in less interference. On the contrary, suppression of neuronal responses in V1 increases with increasing mask size. Our data imply that contextual interference observed in human orientation discrimination is in part directly related to contextual inhibition of neuronal activity in V1. However, the finding that interference with orientation discrimination is weaker for larger masks suggests a figure-ground segregation process that is not located in V1.

Action Potentials↗
PubMed1999

Saccadic dysmetria and adaptation after lesions of the cerebellar cortex.

We studied the effects of small lesions of the oculomotor vermis of the cerebellar cortex on the ability of monkeys to execute and adapt saccadic eye movements. For saccades in one horizontal direction, the lesions led to an initial gross hypometria and a permanent abolition of the capacity for rapid adaptation. Mean saccade amplitude recovered from the initial hypometria, although variability remained high. A series of hundreds of repetitive saccades in the same direction resulted in gradual decrement of amplitude. Saccades in other directions were less strongly affected by the lesions. We suggest the following. (1) The cerebellar cortex is constantly recalibrating the saccadic system, thus compensating for rapid biomechanical changes such as might be caused by muscle fatigue. (2) A mechanism capable of slow recovery from dysmetria is revealed despite the permanent absence of rapid adaptation.

Adaptation, Physiological↗
PubMed1999

GABAergic inhibition in the rat pontine nuclei is exclusively extrinsic: evidence from an in situ hybridization study for GAD67 mRNA.

As clearly indicated by our electrophysiological work, GABAergic inhibition plays a powerful role in the pontine nuclei (PN), the major link between cerebral cortex and the cerebellum. Using the technique of in situ hybridization for the mRNA encoding for the gamma-aminobutyric acid (GABA)-synthesizing isoenzyme glutamic acid decarboxylase67 (GAD67), we demonstrate here the total absence of potentially GABAergic neurons from the rat PN. This negative finding supports the notion that GABAergic inhibition in the PN of rats, unlike that of higher mammals, is exclusively based on extrapontine GABAergic afferents.

Animals↗
PubMed1999

The role of cortical area MST in a model of combined smooth eye-head pursuit.

The cortical medial superior temporal area (MST) is essential for the normal execution of smooth pursuit eye movements. Many pursuit-related neurons (visual-tracking neurons = VT neurons) in the lateral part of area MST (MSTl) are responsive to retinal image slip (r) as well as to eye (e) and head velocity (h) with similar preferred directions (isodirectionality). We show, by running a connectionist network with VT neuron-like elements, that an assembly of MSTl-VT neurons is able to reconstruct target motion in world-centered coordinates (t'). When t' is fed into a subsequent model stage, converting t' into gaze velocity (g') with varying contributions of e and h, the overall model is able to account for many of the salient properties of visually guided pursuit including the consequences of MSTl lesions. However, the analysis of the MSTl network also clearly indicates that isodirectionality is not a prerequisite for its performance. The investigation of a second model suggests that isodirectionality indeed does not result from functional but from developmental constraints. This second model is a connectionist network with hidden units, which similar to MSTl-VT neurons receive input from modality specific units encoding retinal slip, eye and head velocity. After training this network to offer t' as output, two subsets of hidden units emerged, one exhibiting isodirectionality, but not the other. Since only isodirectional hidden units contributed to the flow of information, the preponderance of isodirectional MSTl-VT neurons might be the result of developmental pruning, eliminating the second group.

Adult↗
PubMed1999

Eye movements of rhesus monkeys directed towards imaginary targets.

Is the presence of foveal stimulation a necessary prerequisite for rhesus monkeys to perform visually guided eye movements? To answer this question, we trained two rhesus monkeys to direct their eyes towards imaginary targets defined by extrafoveal cues. Independent of the type of target, real or imaginary, the trajectory of target movement determined the type of eye movement produced: steps in target position resulted in saccades and ramps in target position resulted in smooth pursuit eye movements. There was a tendency for the latency of saccades as well as pursuit onset latency to be delayed in the case of an imaginary target in comparison to the real target. The initial eye acceleration during smooth pursuit initiation elicited by an imaginary target decreased in comparison to the acceleration elicited by a real target. The steady-state pursuit gain was quite similar during pursuit of an imaginary or a real target. Our results strengthen the notion that pursuit is not exclusively a foveal function.

Animals↗
PubMed1999

Binding of signals relevant for action: towards a hypothesis of the functional role of the pontine nuclei.

If numbers matter, the projection that connects the cerebral cortex to the cerebellum is probably one of the most-important pathways through the CNS. Its extensive development as one ascends the phylogenetic scale parallels that of the cerebral hemispheres and the cerebellum, and it accompanies improvements in motor skills, suggesting that this system might have a decisive role in the generation of skilled movement. This article focuses on the pontine nuclei (PN), which are intercalated in the cerebro-cerebellar pathway, a large nuclear complex in the ventral brainstem of mammals, whose raison d'être has as yet not been examined. By considering recent morphological and electrophysiological findings, this article argues that the PN are an interface that is needed to accommodate the grossly different computational principles governing the cerebral cortex and the cerebellum.

Animals↗
PubMed1999

Absence of a common functional denominator of visual disturbances in cerebellar disease.

Several studies have demonstrated disturbances of visual perception in patients suffering from cerebellar disease. In an attempt to determine the cause of these visual disturbances and thereby the cerebellar contribution to vision, we designed two sets of experiments in which we tested (i) the possibility of a general magnocellular deficit in cerebellar disease and (ii) the alternative possibility of impaired spatial attention underlying visual disturbances in cerebellar patients. The first set of experiments consisted of a test of position discrimination, a parvocellular function and tests tapping different aspects of motion perception including speed discrimination, direction discrimination and the ability to extract a coherent motion signal embedded in noise. The second set of experiments compared the performance on two different classes of texture discrimination. The first one required fast and precise shifts of focal spatial attention ('serial search'), the second one, testing preattentive texture discrimination ('pop-out'), did not. In the first set of experiments cerebellar patients were impaired on the position discrimination task as well as several, albeit not all, tests of motion perception. The pattern of disturbances obtained was neither compatible with the notion of a selective magnocellular deficit nor the idea, originally put forward by Ivry and Diener (J Cogn Neurosci 1991; 3: 355-66) that visual deficits are secondary to an impaired measurement of time. In the second set of experiments, cerebellar patients showed normal performance on pop-out tasks and normal performance on all variants of the serial search task except for the one requiring comparison of a single element presented with a sample of the target in short-term memory. In summary, our results support the existence of visual disturbances in cerebellar disease, but provide evidence against a common, simple denominator such as a timing deficit, deficient cerebellar modulation of magnocellular circuitry, deficits of spatial attention or visual working memory.

Adult↗
PubMed1999

Impaired analysis of moving objects due to deficient smooth pursuit eye movements.

It is usually assumed that the raison d'être for smooth pursuit eye movements is an advantage in the visual analysis of moving objects due to the stabilization of the retinal image on the fovea. Although such benefits resulting from foveal pursuit are plausible, there have been few attempts to demonstrate them rigorously. Moreover, it is unknown whether and to what extent pursuit deficits due to neurological disease impair vision. In this study, therefore, we measured psychophysical thresholds for two different discrimination tasks assessing the visual analysis of moving objects as a function of smooth pursuit performance. Results from a group of healthy subjects were compared with those obtained from patients exhibiting catch-up saccades (n = 9) or saccadic intrusions in the form of square-wave jerks (n = 2). In a first set of experiments we measured acuity thresholds for Landolt optotypes moving horizontally at velocities of up to 14 degrees /s (dynamic visual acuity, DVA). In the control group (n = 20), DVA thresholds were indistinguishable from thresholds observed under stationary fixation due to efficient pursuit eye movements allowing continuous foveal stabilization of the retinal Landolt image. In contrast, all patients with catch-up saccades showed pursuit gains that decreased with increasing velocity, paralleled by a dramatic rise in DVA thresholds. Patients with square-wave jerks in turn revealed sufficient pursuit velocity but impaired foveation due to the involuntary saccades that occurred at similar frequencies independent of target velocity. In these patients, thresholds were more or less independent of the Landolt velocity but significantly raised compared with controls. Similar results were obtained in a test determining the sensitivity for vertical position steps of a given pursuit target. In summary, our results indicate that the lack of adequate pursuit eye movements is indeed deleterious for the visual analysis of moving objects.

Adult↗
PubMed1998

Electrical microstimulation distinguishes distinct saccade-related areas in the posterior parietal cortex.

Electrical microstimulation (0.1-ms bipolar pulses at 500 Hz, current strength usually between 100 and 200 microA) was used to delineate saccade-related areas in the posterior parietal cortex of monkeys. Stimulation-induced saccades were found to be restricted to the lateral intraparietal area (area LIP) in the intraparietal sulcus (IPS) and a region on the medial aspect of the parietal lobe (area MP, medial parietal area), close to the caudal end of the cingulate sulcus, whereas stimulation of area 7a did not evoke eye movements. Two different types of evoked saccades were observed. Modified vector saccades, whose amplitude was modified by the position of the eyes at stimulation onset were the hallmark of sites in area LIP and area MP. The same sites were characterized by a propensity of single units active in the memory and presaccadic response segments of the memory saccade paradigm. Goal-directed saccades driving the eyes toward a circumscribed region relative to the head were largely restricted to a small strip of cortex on the lateral bank and the floor of the IPS (the intercalated zone), separating the representation of upward and downward directed saccades in LIP. Unlike stimulation in LIP or MP, stimulation in the intercalated zone gave rise to head, pinnae, facial, and shoulder movements accompanying the evoked saccades. We propose that the amplitude modification of vector saccades characterizing LIP and MP may reflect a spatially distributed head-centered coding scheme for saccades. On the other hand, the goal-directed saccades found in the intercalated zone could indicate the use of a spatially much more localized representation of desired location in head-centered space.

Animals↗
PubMed1998

An electrophysiological correlate of visual motion awareness in man.

It is usually held that perceptual spatial stability, despite smooth pursuit eye movements, is accomplished by comparing a signal reflecting retinal image slip with an internal reference signal, encoding the eye movement. The important consequence of this concept is that our subjective percept of visual motion reflects the outcome of this comparison rather than retinal image slip. In an attempt to localize the cortical networks underlying this comparison and therefore our subjective percept of visual motion, we exploited an imperfection inherent in it, which results in a movement illusion. If smooth pursuit is carried out across a stationary background, we perceive a tiny degree of illusionary background motion (Filehne illusion, or FI), rather than experiencing the ecologically optimal percept of stationarity. We have recently shown that this illusion can be modified substantially and predictably under laboratory conditions by visual motion unrelated to the eye movement. By making use of this finding, we were able to compare cortical potentials evoked by pursuit-induced retinal image slip under two conditions, which differed perceptually, while being identical physically. This approach allowed us to discern a pair of potentials, a parieto-occipital negativity (N300) followed by a frontal positivity (P300), whose amplitudes were solely determined by the subjective perception of visual motion irrespective of the physical attributes of the situation. This finding strongly suggests that subjective awareness of visual motion depends on neuronal activity in a parieto-occipito-frontal network, which excludes the early stages of visual processing.

Adult↗
PubMed1997

False perception of motion in a patient who cannot compensate for eye movements.

We are usually unaware of the motion of an image across our retina that results from our own movement. For instance, during slow-tracking eye movements we do not mistake the shift of the image projected onto the retina for motion of the world around us, but instead perceive a stable world. Following early suggestions by von Helmholtz, it is commonly believed that this spatial stability is achieved by subtracting the retinal motion signal from an internal reference signal, such as a copy of the movement command (efference copy). Object motion is perceived only if the two differ. Although this concept is widely accepted, its anatomical underpinning remains unknown. Here we describe the case of a patient with bilateral extrastriate cortex lesions, suffering from false perception of motion due to an inability to take eye movements into account when faced with self-induced retinal image slip. This is indicated by the fact that during smooth-pursuit eye movements, he perceives motion of the stationary world at a velocity that corresponds to the velocity of his eye movement; that is, he perceives the raw retinal image slip uncorrected for his own eye movements. We suspect that this deficiency reflects damage of a distinct parieto-occipital region that disentangles self-induced and externally induced visual motion by comparing retinal signals with a reference signal encoding eye movements and possibly ego-motion in general.

Adult↗
PubMed1997

[Cortical vertigo].

Dizziness of cortical origin is the subjective correlate of a disturbance of spatial orientation resulting from cerebrocortical dysfunction. Cortical dizziness in the form of vertigo is rare. If present, it most probably reflects a dysfunction of a vestibular representation in the insula. It may be accompanied by tinnitus, sensory disturbance and possibly also spontaneous nystagmus. The dysfunction of this region may result either from a focal seizure or from a lesion, for instance due to ischemia. Nondirectional, visual dizziness is most probably much more common than vertigo. This latter type of dizziness results from a functional disturbance of those parts of parietooccipital cortex, contributing to the discrimination of self-induced and externally-induced retinal image slip. It is not accompanied by additional symptoms and should immediately cease upon closure of the eyes or avoidance of ego motion.

Brain Mapping↗
PubMed1997

Comparison of psychophysical and evoked potential methods in the detection of visual deficits in multiple sclerosis.

We compared the diagnostic sensitivity of traditional visual tests such as the Snellen-test and pattern reversal VEPs with psychophysical and electrophysiological tests involving motion processing and psychophysical tests of contrast processing in detecting visual deficits in a group of MS patients. A total of 30 patients with a definite diagnosis of MS and 22 age-matched controls selected from a pool of healthy volunteers participated in this study. Visual evoked potentials elicited by reversing checkerboards and moving random dot patterns (motion-onset VEPs) were recorded. The recognition of motion-defined forms (motion-defined letter test, MDL-test) and of contrast reduced optotypes was measured psychophysically. Of 30 patients, 29 showed deficits in at least one of the tests applied. The highest detection rate was obtained for a simple psychophysical test, the MDL-test, which revealed abnormalities in 80% of the patients. This is about 12% more than pattern VEPs could detect. Conversely, abnormalities in motion-onset VEPs were found in only 16% of the patients. Our results show that by adding a simple psychophysical test of form-from-motion analysis our capability to demonstrate an involvement of the visual system in MS patients may be promoted considerably.

Adult↗