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PubMed1993

[Ipsilateral gustatory disturbance by thalamic hemorrhage].

Animal studies have recently demonstrated that the gustatory pathway ascends in the brainstem ipsilaterally without crossing, and terminates in the ipsilateral lower caudal limit of the thalamus, i.e., the nucleus ventralis posteromedialis parvocellularis (VPMpc). However, little is yet known about the corresponding anatomical arrangement in man. Some clinical reports on cases of ipsilateral gustatory disturbance caused by tegmental pontine hemorrhage have suggested that the human gustatory pathway may also ascend ipsilaterally in the brainstem. Our case is a 35-year-old man, who was admitted to the hospital because of double vision. Computed tomography revealed a small high density area in the right paramedian tegmentum of the upper midbrain. On the 7th hospital day, the patient suddenly complained of dysesthesia on the right side of the oral cavity. Neurological examination demonstrated remarkable total hypogeusia on the right side of the tongue. MR imaging revealed upward extension of the hematoma toward the lower caudal portion of right thalamus. This is first report to demonstrate unilateral gustatory disturbance due to lesion in the ipsilateral thalamus. It is also worth of note that unilateral gustatory disturbance caused by thalamic lesion can be accompanied by sensory impairment in the ipsilateral oral cavity.

Adult↗
PubMed1998

Colour discrimination thresholds in Parkinson's disease: results obtained with a rapid computer-controlled colour vision test.

A dysfunction of dopaminergic retinal neurons is thought to occur in Parkinson's disease, manifesting itself in impaired performance on various visual discrimination tasks. We have investigated whether differences in colour discrimination could readily be detected between a normal group and a Parkinsonian group, using a computer-controlled test of colour vision. Although some individual Parkinsonian patients showed an abnormal elevation of colour discrimination thresholds, there was no significant difference between the normal group and the Parkinsonian group.

Age Factors↗
PubMed1988

Color vision testing with a computer graphics system: preliminary results.

We report a method for computer enhancement of color vision tests. In our graphics system 256 colors are selected from a much larger range and displayed on a screen divided into 768 x 288 pixels. Eight-bit digital-to-analogue converters drive a high quality monitor with separate inputs to the red, green, and blue amplifiers and calibrated gun chromaticities. The graphics are controlled by a PASCAL program written for a personal computer, which calculates the values of the red, green, and blue signals and specifies them in Commité Internationale d'Eclairage X, Y, and Z fundamentals, so changes in chrominance occur without changes in luminance. The system for measuring color contrast thresholds with gratings is more than adequate in normal observers. In patients with mild retinal damage in whom other tests of visual function are normal, this method of testing color vision shows specific increases in contrast thresholds along tritan color-confusion lines. By the time the Hardy-Rand-Rittler and Farnsworth-Munsell 100-hue tests disclose abnormalities, gross defects in color contrast threshold can be seen with our system.

Adult↗
PubMed1995

The computer-based patient record vision contrasted with HIS/MIS.

The computer-based patient record (CPR), or electronic patient record, is an outgrowth of early medical information system (MIS) and hospital information system (HIS) efforts. However, significant additional requirements must be met to achieve the CPR vision. Contrasts between CPR and HIS/MIS are described. Factors key to completing the transitions are identified.

Confidentiality↗
PubMed1993

[The Freiburg Vision Test. A computer-assisted procedure with sequential strategy].

Single Landolt Cs were presented in one of four positions on a monitor. Subjects responded by pressing one of four buttons (forced choice). A computer selected the size of the Landolt Cs on a logarithmic scale using the "Best PEST" algorithm (Best Parameter Estimation by Sequential Testing), a modern procedure to measure psychophysical thresholds. For comparison, conventional measurements according to DIN 58220 (Deutsche Industrie Norm) were performed with Landolt Cs projected in eight positions, using three out of five correct responses as the threshold criterion. Examination of 23 patients (most of them with senile cataract) did not show any significant difference between the two tests in either visual acuity or reproducibility on two consecutive days. The coefficients of variation between sessions were 22% for the Freiburg test and 26% for the DIN test and 18% between the two tests, pooled over two sessions. The Freiburg acuity test thus appears to be numerically equivalent to the DIN 58220 procedure. In addition, it has a number of advantages: (1) examiner-dependent variability is reduced; therefore, the test can be used by inexperienced examiners; (2) the forced choice technique is used rigorously; (3) mistakes in confounding oblique directions are largely avoided; (4) the procedure is about twice as fast.

Adult↗
PubMed1995

Edge computation in human vision: anisotropy in the combining of oriented filters.

Above threshold, two superimposed sinusoidal gratings of the same spatial frequency (eg 1 cycle deg-1) and equal contrasts, and with orientations balanced around vertical, usually look like a compound structure containing vertical and horizontal edges. However, at large plaid angles (ie large differences between component orientations) and low plaid contrasts there is a tendency for the stimulus to appear as two overlapping gratings (component structure) with obliquely oriented edges. These dependencies of perceived spatial structure in plaids are incompatible with an edge-coding scheme that uses only circular filters to compute zero-crossings, but instead support the idea that different oriented filters can (compound percept) or cannot (component percept) be combined before edges are represented. Here, further evidence is presented in support of this hypothesis. Two-component plaid stimuli had plaid angles of 45 degrees or 90 degrees, and a range of plaid orientations (ie a range of orientations around which the plaid components were balanced). Observers indicated whether each stimulus was perceived as a compound or component structure for a range of plaid contrasts. In addition to angle and contrast effects, perceived spatial structure was also found to depend on plaid orientation: compound structures were perceived more often when the plaid components were balanced around the cardinal axes of the retina. It is suggested that the principles governing the combination of oriented-filter outputs might be learnt during the development of the visual system by using a Hebb-type rule: coactivated filters are more likely to combine their outputs when activated on future occasions. Given the prominence of vertical and horizontal orientations in a carpentered environment, this simple rule promotes a network that combines filters balanced around cardinal axes more readily than oblique axes, in agreement with the results.

Anisotropy↗
PubMed1997

Nested reentrant and recurrent computation in early vision: a Bayesian neuromorphic model applied to hyperacuity.

Hyperacuity is demonstrated in a neuromorphic model of the early visual system. The model incorporates Bayesian principles which are embodied in the dynamics of reentrant and recurrent feedback processes. Each retinotopically mapped area in the model represents a transformation of data from the visual field. Sensory information propagates in a bottom-up direction from one area to the next, while information based on Bayesian priors propagates in a top-down direction through reentrant connections. The 'bottom-up' and 'top-down' information maintain a separate existence in distinct layers of the model, but they interact through local connections within each area. Transformations between one area and the next are defined by the reentrant synaptic connections between areas, while local prior probability maps are defined by local recurrent connections within layers. The representation of hyperacuity is accomplished using a model of functional multiplicity: the large ratio of neurons in striate cortex compared with the number of afferent fibers projecting from the lateral geniculate nucleus. High functional multiplicity, in conjunction with hierarchical reentrant processing, allows the model to represent a fine-grained restoration of the line structure of visual input.

Bayes Theorem↗
PubMed1994

Discrimination of position and contrast in amblyopic and peripheral vision.

Many computational models of normal vernier acuity make predictions based on the just-noticeable contrast difference. Recently, Hu, Klein and Carney [(1993) Vision Research, 33, 1241-1258] compared vernier acuity and contrast discrimination (jnd) in normal foveal viewing using cosine gratings. In the jnd stimulus the test grating was added in-phase to the (sinusoidal) pedestal, whereas in the vernier stimulus the same test grating was added with an approx. 90 deg phase shift to the pedestal. In the present experiments, we measured thresholds for discriminating changes in relative position and changes in relative contrast for abutting, horizontal cosine gratings in a group of amblyopes using the Hu et al., test-pedestal approach. The approach here is to ask whether the reduced vernier acuity of amblyopes can be understood on the basis of reduced contrast sensitivity or contrast discrimination. Our results show that (i) abutting cosine vernier acuity is strongly dependent on stimulus contrast. (ii) In both anisometropic and strabismic amblyopes, abutting cosine vernier discrimination thresholds are elevated at all contrast levels, even after accounting for reduced target visibility, or contrast discrimination. (iii) For both strabismic and anisometropic amblyopes, the vernier Weber fraction is markedly degraded, while the contrast Weber fraction is normal or nearly so. (iv) In anisometropic amblyopes the elevated vernier thresholds are consistent with the observers' reduced cutoff spatial frequency, i.e. the loss can be accounted for on the basis of a shift in spatial scale. (v) In strabismic amblyopes and in the normal periphery, there appears to be an extra loss, which can not be accounted for by either reduced contrast sensitivity and contrast discrimination or by a shift in spatial scale. (vi) This extra loss cannot be quantitatively mimicked by "undersampling" the stimulus. (vii) Surprisingly, in some strabismics, and in the periphery, at relatively high spatial frequencies, vernier thresholds appear to lose their contrast dependence, suggesting the possibility that there may be qualitative differences between the normal fovea and these degraded visual systems. (viii) This contrast saturation can be mimicked by "undersampling" the target, or by introducing strips of mean luminance between the two vernier gratings, thus mimicking a "scotoma". Taken together with the preceding paper, our results suggest that the extra loss in position acuity of strabismic amblyopes and the normal periphery may be a consequence of noise at a second stage of processing, which selectively degrades position but not contrast discrimination.

Amblyopia↗
PubMed1976

Bicycle-pedal model for the first step in the vision process.

Computer simulation of the molecular dynamics of retinal during its photoisomerisation inside a restrictive active site gives a detailed model for the sequence of events in the first step of the vision process. It is proposed that the prelumirhodopsin intermediate contains a strained all-trans retinal molecule produced directly and rapidly from the 11-cis, 12-s-trans conformation in rhodopsin by a bicycle-pedal isomerisation. The model reproduces the main experimental observations and explains how the protein makes the photoisomerisation path unique.

Binding Sites↗
PubMed1985

Application of a computable model of human spatial vision to phase discrimination.

We have used a computable model of human spatial vision to make predictions for phase-discrimination experiments. This model is being developed to deal with a broad range of problems in vision and was not specifically formulated to deal with phase discrimination. In the model, cross correlation of the stimuli with an array of sensors produces feature vectors that are operated on by a position-uncertain ideal observer to simulate detection and discrimination experiments. In this report the stimuli are compound sinusoidal gratings composed of a fundamental and a higher-frequency component added in various phases. We compare model predictions with three key results from the literature: the effect of the contrast of the fundamental on phase discrimination, threshold phase difference as a function of the fundamental frequency, and the contrast required for phase discrimination as a function of the frequency ratio of the two grating components. In the first two cases, the predictions capture the main features of the data, although quantitative discrepancies remain. In the third case, the model fails, and this failure suggests additional restrictions on the combination of information across sensors.

Discrimination Learning↗
PubMed1993

Low vision rehabilitation of multiple sclerosis: a case report.

A 34-year-old white male presented with a previous diagnosis of optic atrophy secondary to multiple sclerosis, seeking low vision rehabilitation. This case report is presented to illustrate the importance of prescribing both conventional optical low vision devices, as well as computer low vision aids in order to provide comprehensive low vision rehabilitation for a young visually impaired adult.

Adult↗
PubMed1996

A model of temporal adaptation in fly motion vision.

A computational model is proposed to account for the adaptive properties of the fly motion system. The response properties of motion-sensitive neurons in the fly are modelled using an underdamped adaptive scheme to adjust the time constants of delay filters in an array of Reichardt detectors. It is shown that the increase in both temporal resolution and sensitivity to velocity change observed following adaptation to constant motion can be understood as a consequence of local adaptation of the filter time constants on the basis of the outputs of elementary motion detectors.

Adaptation, Ocular↗
PubMed1998

Anterior communicating artery aneurysm associated with tuberculum sellae meningioma--case report.

A 50-year-old male presented with a very unusual case of a calcified anterior communicating artery (AComA) aneurysm associated with a tuberculum sellae meningioma. Until 10 years previously, the patient had been a professional soccer player for 15 years. He noticed a slight decrease in visual acuity in the right eye 7 years before. The patient was in the care of an oculist throughout this period. Two months before admission, a significant and rapid decrease of vision in the right eye occurred. Computed tomography and magnetic resonance imaging showed a round-shaped, partially calcified tumorous lesion. Four-vessel angiography revealed a large AComA aneurysm. During surgery, a tuberculum sellae. meningioma was found in combination with an AComA aneurysm with a completely calcified wall. The meningioma was resected totally. The AComA aneurysm with a calcified wall could not be clipped or resected and was left alone. His visual deficit improved postoperatively.

Cerebral Angiography↗