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Activation in the MT-complex during visual perception of apparent motion and temporal succession.
Previous studies have shown that MT (i.e., the MT-complex) is activated during visual perception of apparent motion. To further explore the function of MT, we measured activation in MT by positron emission tomography (PET) using a broad range of stroboscopic stimulus events in which (a) the frame rate was so fast that observers perceived stimulus frames as simultaneous, (b) the frame rate was slower and generated compelling impressions of apparent motion, or (c) the frame rate was so slow that observers perceived temporal succession (successive views of the same objects at different locations) instead of motion. As expected, the simultaneity condition showed no activation (reliable increase in regional cerebral blood flow, rCBF) in MT whereas the motion condition showed activation in both left and right MT. However, the succession condition showed even stronger activation in left and right MT than did the motion condition. MT seems implicated in perception of retinal stimuli as successive views of the same object at different locations whether or not the views are connected by apparent motion.
Alexithymia and visual perception.
60 subjects were administered the Toronto Alexithymia Scale-20, a screening test for alexithymia, and categorized by the usual scoring procedure. Each subject was then given the six Performance subtests of the Wechsler Adult Intelligence Scale-Revised which address aspects of visual perception, organization, and construction. Analysis indicated that the alexithymic group scored significantly lower on two tests of visual perception, Picture Completion and Digit Symbol of the Wechsler Adult Intelligence Scale-Revised.
Experiments on sensory-tonic field theory of perception. II. Effect of supported and unsupported tilt of the body on the visual perception of verticality.
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Spatially mediated capacity limits in attentive visual perception.
Modern theories conceptualize visual selective attention as a competition between objects for the control of cortical receptive fields (RFs). Implicit in this framework is the suggestion that spatially proximal objects, which draw from overlapping pools of RFs, should be more difficult to represent in parallel and with excess capacity than spatially separated objects. The present experiments tested this prediction using analysis of response time distributions in a redundant-targets letter identification task. Data revealed that excess-capacity parallel processing is possible when redundant targets are widely separated within the visual field, but that capacity is near fixed when targets are adjacent. Even at the largest separations tested, however, processing capacity remained strongly limited.
Elevated bilirubin and visual perception in pilots.
BACKGROUND: A fatal aircraft accident of a Bo-105 helicopter on May 20, 1996 led to the suspicion that the pilot's raised serum bilirubin level could have contributed to the accident. It was suspected that an increase of bilirubin beyond 20.52 micromol x L(-1) could disturb visual perception. METHODS: In 1998 and 1999, 101 pilots with functional hyperbilirubinemia, diagnosed as Gilbert's syndrome, were subjected to perceptual-psychological testing (signal detection test) during their periodic medical examinations at the German Air Force Institute of Aviation Medicine. The results of the psychophysiological testing were compared with a control group. RESULTS: Reaction time and perception range increased with age and were independent of serum bilirubin level. CONCLUSION: There is no evidence that raised serum bilirubin levels can diminish visual perceptional performance.
Effects of lithium on visual perception in manic-depressive patients without acute symptomatology.
Within a psychological theory of depression it is postulated that lithium influences the rigid and irregular patterns of psychic processes characterizing the premorbid state of patients with affective psychoses. Visual perception as a basic psychic function was, therefore, investigated in 16 patients, under long-term lithium treatment (= A), after 6 weeks of placebo (= B), after 6 weeks of reinstituted lithium (= A'). Series of clear, masked, and mixed stimuli (4 digit numbers) were presented tachistoscopically. It was found that lithium raises the perceptual threshold for all kinds of stimuli. Under lithium total performance for masked, i.e. complex stimuli was significantly changed, though not unidirectionally, if these stimuli were presented mixed with clear stimuli in randomized order. This finding of a lithium-induced increased variance of total performance, implying recognition and resolution of complex stimuli, was confirmed by a replication study in 6 patients having participated in the main study and being reexamined under continuous lithium medication without a placebo period. It is concluded that lithium modifies visual perception, and that possibly it influences the individual's expectancy towards the next stimulus during repeated exposures of mixed simple and complex targets.
Dyslexia and visual perception: is there a relation?
Opinions differ concerning the relation between dyslexia and visual perception. In this pilot study we addressed this question by analyzing the results of visual perceptual testing on 19 elementary grade subjects diagnosed as having dyseidesia, one type of dyslexia that manifests as problems with sight-word recognition. The subjects, who have different degrees of severity of dyseidesia, were compared with 11 subjects tested as nondyslexic, but with reading problems. The Dyslexia Determination Test (DDT) and the Test of Visual Perceptual Skills (TVPS) were given to all subjects. Both multivariate analysis of variance (MANOVA) and univariate one-way analysis of variance (ANOVA) to test for differences in performance among four test groups (ranging from nondyslexic to increasing levels of severity of dyseidesia) showed no trends of either statistical or clinical importance. An individual with reading problems may be nondyslexic but have poor visual perceptual skills, or may be dyslexic with good visual perceptual skills. These results are consistent with the contention that reading problems caused by dyseidesia (as defined by the DDT) and those caused by visual perceptual deficiencies (as defined by the TVPS) are different. Although dyseidetic dyslexia results in characteristic coding patterns producing specific reading problems, it is probable that visual perceptual deficiencies contribute to learning problems that include general reading problems. Each condition would, therefore, require different forms of therapy. Limitations of this study and recommendations for future research are discussed.
Visual brain and visual perception: how does the cortex do perceptual grouping?
How the brain generates visual percepts is a central problem in neuroscience. We propose a detailed neural model of how lateral geniculate nuclei and the interblob cortical stream through V1 and V2 generate context-sensitive perceptual groupings from visual inputs. The model suggests a functional role for cortical layers, columns, maps and networks, and proposes homologous circuits for V1 and V2 with larger-scale processing in V2. An integrated treatment of interlaminar, horizontal, orientational and endstopping cortical interactions and a role for corticogeniculate feedback in grouping are proposed. Modeled circuits simulate parametric psychophysical data about boundary grouping and illusory contour formation.
Mechanism of visual perception of movement.
We here present a logical mathematical model for the visual perception of movement in man; it is based on the treatment of signals in the retina in relation to time and distance and on the statistical theory of the separation of signal from noise in the visual system. It shows good agreement with experimental results and has made it possible to determine the comparative level of internal noise of the visual system, the mean rate of spread of excitation from the photoreceptors along the surface of the retina, and many other characteristics.
Estimation of the timing of human visual perception from magnetoencephalography.
To explore the timing and the underlying neural dynamics of visual perception, we analyzed the relationship between the manual reaction time (RT) to the onset of a visual stimulus and the time course of the evoked neural response simultaneously measured by magnetoencephalography (MEG). The visual stimuli were a transition from incoherent to coherent motion of random dots and an onset of a chromatic grating from a uniform field, which evoke neural responses in different cortical sites. For both stimuli, changes in median RT with changing stimulus strength (motion coherence or chromatic contrast) were accurately predicted, with a stimulus-independent postdetection delay, from the time that the temporally integrated MEG response crossed a threshold (integrator model). In comparison, the prediction of RT was less accurate from the peak MEG latency, or from the time that the nonintegrated MEG response crossed a threshold (level detector model). The integrator model could also account for, at least partially, intertrial changes in RT or in perception (hit/miss) to identical stimuli. Although we examined MEG-RT relationships mainly for data averaged over trials, the integrator model could show some correlations even for single-trial data. The model predictions deteriorated when only early visual responses presumably originating from the striate cortex were used as the input to the integrator model. Our results suggest that the perceptions for visual stimulus appearances are established in extrastriate areas [around MT (middle temporal visual area) for motion and around V4 (fourth visual area) for color] approximately 150-200 ms before subjects manually react to the stimulus.
The visual perception of 3D shape.
A fundamental problem for the visual perception of 3D shape is that patterns of optical stimulation are inherently ambiguous. Recent mathematical analyses have shown, however, that these ambiguities can be highly constrained, so that many aspects of 3D structure are uniquely specified even though others might be underdetermined. Empirical results with human observers reveal a similar pattern of performance. Judgments about 3D shape are often systematically distorted relative to the actual structure of an observed scene, but these distortions are typically constrained to a limited class of transformations. These findings suggest that the perceptual representation of 3D shape involves a relatively abstract data structure that is based primarily on qualitative properties that can be reliably determined from visual information.
[Method of measuring the late period of visual perception in humans].
An ergonomic method for measuring the late period (LP) of visual perception of human pupillary reaction (PR) is proposed. Variabilities in the similarity of LP of human PR show that the accuracy of measuring PR LP by recording the parameters of perception and pupillometry is similar. The method is recommended for controlling the status and training the visual system of operators who trace several independent moving objects on the display.
Reliability of the Frostig Test of visual perception in a South African sample.
The Marianne Frostig Developmental Test of Visual Perception was administered to 31 preschool children (19 boys and 12 girls) whose mean age was 68 mo. An item analysis showed that 52% of the test items satisfied the requirements with regard to degree of difficulty and discrimination. KR-20 reliability coefficients varied between 0.31 and 0.58 for the different subtests, with r=0.72 for the full scale.
Activation of the prefrontal cortex in the human visual aesthetic perception.
Visual aesthetic perception ("aesthetics") or the capacity to visually perceive a particular attribute added to other features of objects, such as form, color, and movement, was fixed during human evolutionary lineage as a trait not shared with any great ape. Although prefrontal brain expansion is mentioned as responsible for the appearance of such human trait, no current knowledge exists on the role of prefrontal areas in the aesthetic perception. The visual brain consists of "several parallel multistage processing systems, each specialized in a given task such as, color or motion" [Bartels, A. & Zeki, S. (1999) Proc. R. Soc. London Ser. B 265, 2327-2332]. Here we report the results of an experiment carried out with magnetoencephalography which shows that the prefrontal area is selectively activated in humans during the perception of objects qualified as "beautiful" by the participants. Therefore, aesthetics can be hypothetically considered as an attribute perceived by means of a particular brain processing system, in which the prefrontal cortex seems to play a key role.
[Associative visual agnosia: role of the left hemisphere in visual perception (author's transl)].
Visual agnosia following ischaemic accidents of the left posterior cerebral artery is often associated with intellectual deficiencies, memory disorders, elementary perceptive disturbances and elements of visuoverbal disconnection, with the result that some authors reject the notion of visual agnosia. By using a relatively simple examination procedure it is, however, possible to clearly differentiate the various disturbances, as shown by the case of a right-handed 66-year-old man in whom this type of vascular accident occurred. Neither the reduced intellectual capacities nor the memory disorders can explain the differences observed in the treatment of visual stimuli, which was very disturbed, and the normal treatment of other types of stimulus. Elementary difficulties are not sufficient to prevent correct discrimination, as all tests of matching object, images, colours, and graphic signs were successfully accomplished. Identification was disturbed however: the patient could not show the use of objects presented visually, and this disorder is related to the visual characteristics of the stimuli, which excludes a simple visuogestural disconnection; results of tests of classifying types of object images, colours, and graphic signs were markedly abnormal. Naming of these stimuli was also affected, even when they were correctly identified; this results from the visuoverbal disconnection associated with the agnosia, but it is insufficient to account for it, as the patient could correctly use objects that had been wrongly named, but could not use those that had been poorly identified. This case, therefore, has enabled a visual identification disorder to be isolated independently from a discrimination problem, and visuoverbal or visuogestural disconnections. It is the significance of ths stimulus that is disturbed. In contrast, functional or categorical classification is respected in right temporo-occipital lesions. It is the individuality of a stimulus within a physical category which is disturbed.
Light from Maillard reaction: photon counting, emission spectrum, photography and visual perception.
Several authors have reported on high-sensitivity measurement of oxygen-dependent low-level chemiluminescence (CL) from Maillard reactions (MR), i.e. nonenzymatic amino-carbonyl reactions between reducing sugars and amino acids (also referred to as nonenzymatic browning). Here we report for the first time, that light from Maillard reactions can be seen by the human eye and also can be photographed. In parallel with visual perception and photography CL was monitored by means of a CL-detection programme of a liquid scintillation counter (LSC, single photon rate counting). CL emission spectrum was recorded by a monochromator-microchannel plate photomultiplier arrangement. CL intensity from reaction of 6-aminocaproic acid with D-ribose (200 mg each) in 5 mL H2O at pH 11 at 95 degrees C was high enough for visual perception after adaptation to absolute darkness. Reaction in dimethylsulphoxide (DMSO) exhibited strongly enhanced CL (10 mg each in 5 mL were sufficient for visual detection) and could be photographed (15 minutes' exposure, ASA 6400); all characteristics of Maillard specific CL (O2-dependence, no CL from nonreducing sugars, inhibition by sulphur compounds) remained. Visual detection of CL and measurement by LSC were in full concordance. The CL emission spectrum showed two broad peaks at around 500 nm and 695 nm. Fluorescence emission of the brown reaction mixture matched the blue-green part of the CL emission spectrum. Emission of visible light during Maillard reactions may partly originate from oxygen-dependent generation of excited states and energy transfer to simultaneously formed fluorescent products of the browning reaction.
[On certain mechanisms of visual perception].
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