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A Thiele

Publications and source records attributed to A Thiele.

At least 37 records · Page 2Linked to original sources
PubMed2000

Neural correlates of contrast detection at threshold.

Human psychophysical studies have demonstrated that, for stimuli near the threshold of visibility, detection of motion in one direction is unaffected by the superimposition of motion in the opposite direction. To investigate the neural basis for this perceptual phenomenon, we recorded from directionally selective neurons in macaque visual area MT (middle temporal visual area). Contrast thresholds obtained for single gratings moving in a neuron's preferred direction were compared with those obtained for motion presented simultaneously in the neuron's preferred and antipreferred directions. A simple model based on probability summation between neurons tuned to opposite directions could sufficiently account for contrast thresholds revealed psychophysically, suggesting that area MT is likely to provide the neural basis for contrast detection of stimuli modulated in time.

Animals↗
PubMed2000

Fluorescence and photochemical characterization of phytochromes A and B in transgenic potato expressing Arabidopsis phytochrome B.

Phytochrome in etiolated sprouts of wild type (WT) potato and its transgenic strains (DARA5 and DARA12) expressing Arabidopsis thaliana phytochrome B (phyB) was investigated using low-temperature (85 K) fluorescence spectroscopy and photochemistry. Phytochrome content, [Ptot], position of the Pr emission and excitation spectra, lambda(max), and extent of the Pr-->lumi-R, gamma1, and Pr-->Pfr, gamma2, phototransformations (at 85 and 273 K, respectively) were shown to vary in the transgenic lines and WT depending on tissue used (upper vs. lower parts of etiolated sprouts) and light-induced phytochrome depletion. Differences in the parameters between the transgenic lines and WT were detected which were interpreted in terms of the two phenomenological Pr types: a labile Pr' with gamma1 approximately 0.5 consisting of a major phytochrome A (phyA) fraction (phyA') and a relatively conserved Pr" with gamma1 = 0 comprising a minor phyA fraction (phyA") and phyB. Both DARA lines had higher [Pr"] as compared with WT in the lower parts of etiolated stems, especially after light-induced phytochrome depletion (residual phytochrome in DARA5 and DARA12 made up to one-third of its initial level vs. <5% in WT). These differences were associated with the expression of Arabidopsis phyB in the DARA lines and its higher light stability than that of phyA. Arabidopsis phyB expressed in potato was characterised by lambda(max) = 683/669 nm in the emission/excitation (absorption) spectra and gamma1 = 0. PhyB also revealed a relatively low gamma2 (approx. 0.5) and its early red drop as compared with the gamma2 wavelength dependence for phyA. This is believed to contribute to the lower signalling ability of phyB and to confine the region (red) of its physiological activity.

Arabidopsis↗
PubMed2000

Comparison of red-green equiluminance points in humans and macaques: evidence for different L:M cone ratios between species.

The human spectral luminosity function (V(lambda)) can be modeled as the linear sum of signals from long-wavelength-selective (L) and middle-wavelength-selective (M) cones, with L cones being weighted by a factor of approximately 2. This factor of approximately 2 is thought to reflect an approximate 2:1 ratio of L:M cones in the human retina, which has been supported by studies that allow for more direct counting of different cone types in the retina. In contrast to humans, several lines of retinally based evidence in macaques suggest an L:M ratio closer to 1:1. To investigate the consequences of differences in L:M cone ratios between humans and macaques, red-green equiluminance matches obtained psychophysically in humans (n = 11) were compared with those obtained electrophysiologically from single neurons in the extrastriate middle temporal visual area of macaques (M. mulatta, n = 5). Neurons in the middle temporal visual area were tested with sinusoidal red-green moving gratings across a range of luminance contrasts, with equiluminance being defined as the red-green contrast yielding a response minimum. Human subjects were tested under analogous conditions, by a minimally distinct motion technique, to establish psychophysical equiluminance. Although red-green equiluminance points in both humans and macaques were found to vary across individuals, the means across species differed significantly; compared with humans, macaque equiluminance points reflected relatively greater sensitivity to green. By means of a simple model based on equating the weighted sum of L and M cone signals, the observed red-green equiluminance points were found to be consistent with L:M cone ratios of approximately 2:1 in humans and 1:1 in macaques. These data thus support retinally based estimates of L:M cone ratios and further demonstrate that the information carried in the cone mosaic has functional consequences for red-green spectral sensitivity revealed perceptually and in the dorsal stream of visual cortex.

Adult↗
PubMed1999

[Coronary angiography and change in antioxidative status].

PATIENTS AND METHODS: In a prospective study in 53 patients with stable angina pectoris symptoms the antioxidant status (glutathione peroxidase, glutathione, superoxid dismutase, malondialdehyde and selenium in serum and whole blood) was determined before and 4 to 6 hours after coronary angiography. According to the results of the coronary angiography the patients were classified in a group with "severe" (n = 16) and another with "moderate" coronary alterations. RESULTS: In both groups there was a significant reduction of selenium in serum and whole blood. The enzymes glutathione peroxidase and superoxide dismutase as well as glutathione and malondialdehyde changed only slightly. CONCLUSION: These results can be the cause of an increase of the formation of free radicals during coronary reperfusion (PTCA, implantation of stents in the group with "severe" coronary alterations) but could also be seen as a sign of formation of radicals by the method itself (in patients with "moderate" coronary alterations). Further investigations are indicated. Furthermore the amelioration of the antioxidant status of the organism by scavenger substances (vitamins A, B, C and selenium) should be discussed.

Aged↗
PubMed1999

The contribution of color to motion processing in Macaque middle temporal area.

The chromatic properties of an image yield strong cues for object boundaries and thus hold the potential to facilitate the detection of object motion. The extent to which cortical motion detectors exploit chromatic information, however, remains a matter of debate. To address this further, we quantified the strength of chromatic input to directionally selective neurons in the middle temporal area (MT) of macaque cerebral cortex using an equivalent luminance contrast (EqLC) paradigm. This paradigm, in which two sinusoidal gratings, one heterochromatic and the other achromatic, are superimposed and moved in opposite directions, allows the sensitivity of motion detectors to heterochromatic stimuli to be quantified and expressed relative to the benchmark of sensitivity for a luminance-defined stimulus. The results of these experiments demonstrate that the chromatic contrast in a moving red-green heterochromatic grating strongly influences directional responses in MT when the luminance contrast in that same grating is relatively low; for such stimuli, EqLC is at least 5%. When luminance contrast is added to the heterochromatic grating, however, EqLC wanes sharply and becomes negative (-4%) when luminance contrast is sufficiently high (>17-23%). Thus, the chromatic properties of an object appear to confer little or no benefit to motion processing by MT neurons when sufficient luminance contrast concurrently exists. These data support a simple model in which chromatic motion processing in MT is almost exclusively determined by magnocellular input. Additionally, a comparison of neuronal and psychophysical data suggests that MT may not be the sole contributor to the perceptual experience elicited by motion of heterochromatic patterns, or that only a subset of MT neurons serve this function.

Animals↗
PubMed1999

Decision-related activity in the macaque dorsal visual pathway.

Brain areas at higher levels of cortical organization are thought to be more involved in decision processes than are earlier, i.e. lower, sensory areas. Hence, neuronal activity correlated with decisions should vary with an area's position in the cortical hierarchy. To test this proposal, we investigated whether a change in neuronal activity during error trials depends in a systematic way on cortical hierarchical position. While macaque monkeys discriminated the direction of moving visual stimuli, the activity of direction-selective neurons was recorded in four extrastriate visual areas: V3A, the middle temporal area, the middle superior temporal area and the posterior part of the superior temporal polysensory area. Neuronal activity was significantly reduced in all areas when the monkeys made errors in judging the direction of stimuli moving in the preferred direction with low and intermediate luminance contrast. The amount of activity reduction was approximately 50% in all of the visual areas. Thus, the activity on error trials is reduced in early visual processing, independent of the hierarchy in the dorsal visual pathway. The activity reduction depended on stimulus contrast and the direction of the decision relative to the stimulus motion. It was profound and significant in all areas at low stimulus contrast. However, it was nonsignificant at high stimulus contrast. Our data suggest that activity reduction on error trials is due to lack of attention in association with stimulus expectation.

Animals↗
PubMed1999

Heterologous expression of Arabidopsis phytochrome B in transgenic potato influences photosynthetic performance and tuber development.

Transgenic potato (Solanum tuberosum) plants expressing Arabidopsis phytochrome B were characterized morphologically and physiologically under white light in a greenhouse to explore their potential for improved photosynthesis and higher tuber yields. As expected, overexpression of functional phytochrome B caused pleiotropic effects such as semidwarfism, decreased apical dominance, a higher number of smaller but thicker leaves, and increased pigmentation. Because of increased numbers of chloroplasts in elongated palisade cells, photosynthesis per leaf area and in each individual plant increased. In addition, photosynthesis was less sensitive to photoinactivation under prolonged light stress. The beginning of senescence was not delayed, but deceleration of chlorophyll degradation extended the lifetime of photosynthetically active plants. Both the higher photosynthetic performance and the longer lifespan of the transgenic plants allowed greater biomass production, resulting in extended underground organs with increased tuber yields.

Arabidopsis↗
PubMed1998

[Retroperitoneal fibrosis and arthritis--a manifestation of the same illness].

Retroperitoneal fibrosis is a disorder in which the retroperitoneal fat is the site of a subacute and chronic inflammatory reaction and is subsequently replaced by dense fibrotic tissue. Rheumatoid nodules are chronic granulomata occurring at sites of pressure and movement, both near the body surface and internally. A 55-year-old sales-manager was admitted to radiation synovectomy after a 5 year history of excessive right and left knee effusions. There were no other clinical or laboratory abnormalities. The patient did not respond to either radioisotope synoviorthesis using radioactive Yttrium (90 Y), or to open synovectomy and prostetic surgery of the right knee. One year later, surgery of left ureter was necessary. Histological findings revealed the diagnosis of Ormond's disease. Comparative histological studies of synovial membrane of knee and retroperitoneal tissues showed local necrosis, fibrin deposition, lining cell proliferation, and infiltration by lymphocytes. Diagnosis of arthritis complicated by retroperitoneal rheumatoid nodules and retroperitoneal fibrosis was made. Serum rheumatoid factor has been negative. For the last 3 years, the patient has been on successful therapy with azathioprine. Rheumatoid nodules of the retroperitoneum have vanished completely and frequency of knee effusions decreased.

Arthritis, Rheumatoid↗
PubMed1997

Synchronization of neuronal activity during stimulus expectation in a direction discrimination task.

The dorsal pathway of the primate brain, especially the middle temporal area (MT or V5) and the superior middle temporal area (MST or V5a), is strongly involved in motion detection. The relation between neural firing rates and psychophysical performance has led to the assumption that the neural code used by these areas consists of the relative discharge rates of neuronal populations. As an additional neural code, temporal correlation of neural activity has been suggested. Our study addresses the involvement of such a code in awake monkeys performing a motion discrimination task. We found significant temporal correlations between simultaneously recorded pairs of units in areas MT and MST and other extrastriate cortical areas. Units recorded from the same electrode were more frequently synchronized than units recorded from different electrodes placed within the same or different cortical areas. Activity synchronization was present in the expectation period before stimulus presentation and could not be induced de novo by the stimulus. Rather, we found a contrast-dependent reduction of correlation strength on stimulus onset. Correlation strength did not vary systematically with stimulus directions. We conclude that under the conditions of this study, temporal decorrelation of MT and MST neurons could be used to detect the stimulus, but synchronization does not convey specific information about its direction of motion and therefore is unlikely to contribute to performance in our direction discrimination task. Activity synchronization in the period before stimulus onset could be related to attentive expectation.

Animals↗
PubMed1997

Visual responses of neurons from areas V1 and MT in a monkey with late onset strabismus: a case study.

One adult monkey (Macaca fascicularis) was investigated psychophysically and electrophysiologically after at least 5 years of late onset esotropic macrostrabismus (squint angle 52 deg). Behavioural tests revealed normal monocular visual and visuomotor functions. No indications of deep amblyopia or oculomotor asymmetry were found. The monkey used the left or right eye alternately at about equal frequencies. Single unit recordings from area VI disclosed a normal ocular dominance distribution. Most VI neurons from both hemispheres received binocular input. Thus, discordant visual information from corresponding retinal locations of the two eyes converged onto the cortical neurons. No evidence for anomalous retinal correspondence was found. Diplopia and confusion must therefore be avoided by suppression of vision through one eye to allow stable, unambiguous perception. Possible suppression was investigated by stimulating a neuron through the same eye when it was actively used for fixation in one set of trials, and when it was not used for fixation in another set of trials. Significant differences in these two stimulus conditions were found in 20/39 neurons from area VI and in 11/34 motion sensitive neurons recorded in the middle superior temporal area (MT). The normalized population activity in VI and MT was higher if cells were stimulated through the fixating eye. The data are discussed with respect to possible suppressive mechanisms helping to prevent double vision in strabismus and in binocular rivalry.

Action Potentials↗
PubMed1997

Eye position effects in monkey cortex. I. Visual and pursuit-related activity in extrastriate areas MT and MST.

We studied the effect of eye position on visual and pursuit-related activity in neurons in the superior temporal sulcus of the macaque monkey. Altogether, 109 neurons from the middle temporal area (area MT) and the medial superior temporal area (area MST) were tested for influence of eye position on their stimulus-driven response in a fixation paradigm. In this paradigm the monitored eye position signal was superimposed onto the stimulus control signal while the monkey fixated at different locations on a screen. This setup guaranteed that an optimized stimulus was moved across the receptive field at the same retinal location for all fixation locations. For 61% of the MT neurons and 82% of the MST neurons the stimulus-induced response was modulated by the position of the eyes in the orbit. Directional selectivity was not influenced by eye position. One hundred sixty-eight neurons exhibited direction-specific responses during smooth tracking eye movements and were tested in a pursuit paradigm. Here the monkey had to track a target that started to move in the preferred direction with constant speed from five different locations on the screen in random order. Pursuit-related activity was modulated by eye position in 78% of the MT neurons as well as in 80% of the MST neurons tested. Neuronal activity varied linearly as a function of both horizontal and vertical eye position for most of the neurons tested in both areas, i.e., two-dimensional regression planes could be approximated to the responses of most of the neurons. The directions of the gradients of these regression planes correlated neither with the preferred stimulus direction tested in the fixation paradigm nor with the preferred tracking direction in the pursuit paradigm. Eighty-six neurons were tested with both the fixation and the pursuit paradigms. The directions of the gradients of the regression planes fit to the responses in both paradigms tended to correlate with each other, i.e., for more than two thirds of the neurons the angular difference between both directions was less than +/-90 degrees. The modulatory effect of the position of the eyes in the orbit proved to balance out at the population level for neurons in areas MT and MST, tested with the fixation as well as the pursuit paradigm. Results are discussed in light of the hypothesis of an ongoing coordinate transformation of the incoming sensory signals into a nonretinocentric representation of the visual field.

Animals↗
PubMed1996

Optic flow processing in monkey STS: a theoretical and experimental approach.

How does the brain process visual information about self-motion? In monkey cortex, the analysis of visual motion is performed by successive areas specialized in different aspects of motion processing. Whereas neurons in the middle temporal (MT) area are direction-selective for local motion, neurons in the medial superior temporal (MST) area respond to motion patterns. A neural network model attempts to link these properties to the psychophysics of human heading detection from optic flow. It proposes that populations of neurons represent specific directions of heading. We quantitatively compared single-unit recordings in area MST with single-neuron simulations in this model. Predictions were derived from simulations and subsequently tested in recorded neurons. Neuronal activities depended on the position of the singular point in the optic flow. Best responses to opposing motions occurred for opposite locations of the singular point in the visual field. Excitation by one type of motion is paired with inhibition by the opposite motion. Activity maxima often occur for peripheral singular points. The averaged recorded shape of the response modulations is sigmoidal, which is in agreement with model predictions. We also tested whether the activity of the neuronal population in MST can represent the directions of heading in our stimuli. A simple least-mean-square minimization could retrieve the direction of heading from the neuronal activities with a precision of 4.3 degrees. Our results show good agreement between the proposed model and the neuronal responses in area MST and further support the hypothesis that area MST is involved in visual navigation.

Animals↗
PubMed1996

Neuronal activity in MST and STPp, but not MT changes systematically with stimulus-independent decisions.

Single units in the middle temporal area (MT), the middle superior temporal area (MST) and the upper part of the superior temporal polysensory area (STPp) of the monkey's brain were recorded during the discrimination of direction of visual motion near contrast threshold. In addition to stimulus-related decisions the monkeys often made stimulus-independent decisions while stimuli were well below detection threshold (contrast < 0.01%), or totally absent. During this condition MT neurones showed no systematic changes in their ongoing activity while in the MST and STPp stimulus-independent decisions in the preferred direction of a given neurone were accompanied by the highest ongoing activity. We take this as evidence for internally generated representations of direction of motion in higher visual areas of the superior temporal sulcus. Neurones involved in the initiation of a decision or action are described for the first time in area STPp.

Animals↗
PubMed1996

Neuronal responses in the motion pathway of the macaque monkey to natural optic flow stimuli.

Neurones in higher visual motion areas in the superior temporal sulcus (STS) of the macaque monkey respond to abstract random dot optic flow stimuli. Higher motion areas may not only represent, but in a next computational stage also analyse the flow field to determine, for instance, the direction of heading for navigation purposes. Real world visual scenes differ in several aspects from these abstract optic flow stimuli. We tested the neuronal response to naturalistic optic flow stimuli which simulated egomotion in different virtual environments and contained different numbers of visual cues. Neuronal activity depended mainly on the position of the focus of expansion rather than on other visual cues. This finding supports the hypothesis that higher motion areas within the STS analyse optic flow in natural scenes and can thus signal the direction of heading.

Animals↗
PubMed1996

Monkey saccadic latency and pursuit velocity show a preference for upward directions of target motion.

Saccadic latency was studied as a function of the direction of sudden target displacements (steps) and of subsequent smooth target motion (ramps) in Macaca fascicularis. The monkey fixated a central spot that suddenly changed its position and then moved constantly at 10 deg s-1, thus eliciting initial saccades and subsequent pursuit eye movements (recorded by a magnetic search-coil technique). Latencies for initial saccades differed markedly in the vertical axis, being shorter in upward than downward directions for both step and ramp components of target motion. Saccadic latency was also related to the mean pursuit velocity, indicating that the oculomotor system accounts for the direction of step and ramp components of target motion in an integrative way.

Animals↗
PubMed1996

Anatomical and physiological investigation of auditory input to the superior colliculus of the echolocating megachiropteran bat Rousettus aegyptiacus.

The objective of this study was to investigate whether a representation of auditory space in the superior colliculus (SC) of the echolocating megachiropteran bat (Rousettus aegyptiacus) exists. Additionally the subcortical auditory connectivity of the SC was investigated. A total of 207 units were recorded in five awake animals while presenting acoustic stimuli (white noise, clicks, and pure tones) at different positions in space. Six units responded to acoustic stimulation. Three of these located within the superficial layers and one located in the intermediate layers were classified as omnidirectional units. Two units were located within the deep layers. One was classified as a hemifield unit, and the other as a frontal unit. All units responded phasically to acoustic stimulation with a latency of 4-150 ms. None of them could be activated by visual stimuli. We further examined the interaction of paired auditory and visual stimulation in 116 visually responsive units. Responses to visual stimulation were markedly altered by acoustic stimulation in 5 units. The influence of the acoustic stimuli was temporally and spatially restricted, and resulted either in a reduction or an elevation of unit responsiveness. Horseradish peroxidase was injected into the SC of eight animals to investigate the auditory subcortical connectivity of the SC. Retrograde labeling in auditory structures was rare compared with labeling found in nonauditory structures (e.g., retina, substantia nigra, parabigeminal nucleus). In auditory structures retrograde labeling was found mainly in the external nucleus of the inferior colliculus and in the nucleus of the brachium of the inferior colliculus. To a lesser extent it was found in the nucleus sagulum and in the area medial to the lemniscal nuclei. In one case the dorsal nucleus of the lateral lemniscus and the anterolateral periolivary nucleus were labeled. Our results reveal only a sparse auditory input into the SC of the flying fox, R. aegyptiacus. On the basis of single-unit recordings, we did not find an elaborate representation of auditory space as it is described for several other species. The existence of auditory and bimodal neurones, in combination with their response properties, nonetheless indicate that there might be a representation of auditory space in the SC of R. aegyptiacus.

Acoustic Stimulation↗
PubMed1994

Association of class II sequences encoding DR1 and DQ5 specificities with hypersensitivity to chironomid allergen Chi t I.

A panel of 188 unrelated Caucasian subjects who were exposed to the larvae of Chironomus thummi (Diptera, nonbiting midges) was HLA-typed by polymerase chain reaction amplification of the second exons of the DRB, DQA1, and DQB1 genes followed by dot-blot hybridization with sequence-specific oligonucleotide probes. Type I sensitization to the allergen Chi t I and a large number of other inhalant allergens was determined by RAST and skin testing. Sixty-one individuals were found to be sensitized to Chi t I, of whom 24 were sensitive to this allergen and to no other allergens tested. Statistical analyses showed that only in the latter group were the HLA-D genes DRB1*0101, DQA1*0101, and DQB1*0501 associated with IgE-responsiveness to Chi t I. These results suggest that HLA associations with responsiveness to certain allergens may be more striking in monosensitized subjects.

Adolescent↗