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Biomedical subjects

M Fahle

Publications and source records attributed to M Fahle.

At least 91 records · Page 5Linked to original sources
PubMed1991

Perceived depth scales with disparity gradient.

Perceived difference in depth between two adjacent stimuli decreases with increasing disparity gradient even if the disparity stays constant, ie when the stimuli approach each other along paths within fronto-parallel planes. This depth scaling effect is more pronounced with line stimuli than with two isolated points or two small symbols and is insignificant for easily discriminable symbols. The decrease in perceived depth is more pronounced for horizontal orientation than for oblique or vertical orientation. The ratio of perceived depth difference to displayed disparity difference also decreases when the distance between the stimuli increases at a constant gradient in depth. This is to say that we are more correct in our depth estimates for steep gradients in depth when the euclidean distance between the stimuli is short.

Attention↗
PubMed1991

A new elementary feature of vision.

Humans can discriminate among elementary features such as orientation, color, and depth in parallel. The author showed that discontinuities in lines, such as a vernier offset, can be detected in a flash presentation with subsequent mask, and an offset, even below the diameter of foveal photoreceptors, is identified among distractors. This result suggests that the visual system simultaneously detects misalignments or gradients of orientation in the hyperacuity range at many positions in the visual field. The direction of the vernier offset, however, cannot be classified simultaneously in several stimuli, but requires serial search.

Humans↗
PubMed1990

Impairment of vertical motion detection and downgaze palsy due to rostral midbrain infarction.

We present two cases with acute onset of vertical gaze palsy, mainly consisting of impaired downgaze and apraxia of downward head movements, together with neuropsychological deficits (hypersomnia, impaired attention and disorders of memory and affective control). CT and MRI revealed bilateral post-ischaemic lesions in the dorsomedial thalamus and the mesodiencephalic junction, dorsomedial to the red nucleus, thus being restricted to the territory of the posterior thalamosubthalamic paramedian artery, which includes the region of the rostral interstitial nucleus of the medial longitudinal fascicle as the main premotor nucleus for the generation of vertical saccades. In our patients, oculographic examination with electro-oculography and magnetic search coil recording showed severe impairment of downward more than upward saccades and only minor deficits of vertical pursuit and the vestibulo-ocular reflex. Visual functions were normal, with one exception: a psychophysical test of motion perception revealed a significant deficit in the detection of vertical movements. This could be due to a central adaptive mechanism which, in order to minimize oscillopsia, might elevate thresholds for vertical motion perception in cases of vertical gaze palsy. As an alternative explanation, lesions within the midbrain tegmentum could have damaged subcortical visual pathways involved in motion perception.

Cerebral Infarction↗
PubMed1990

[Eyelid akinesia after various techniques of facial nerve block].

Lid akinesia was investigated after the application of one of three techniques to achieve facial nerve blockade in each of 32 patients undergoing cataract surgery. Modifications of the O'Brien, Atkinson and Lint block techniques were applied in twelve, ten, and ten patients, respectively. Before and 1, 3, 5 and 10 min after administration of 5 ml 1% lidocaine (Xylocain) with naphazolin nitrate 1:20,000 (Privin) voluntary muscle activity of the orbicularis oculi muscle was recorded, and the area under the EMG curve was calculated for quantitative comparison of muscular activity between the groups. In addition, the force of lid closure was determined with a Müller blepharometer, and lid movements were evaluated on a subjective assessment scale. In six additional patients, the topographic distribution of a mixture of metrizamide (Solutrast) and the anesthetic solution was evaluated radiographically to assess potential causes of differences in the effectiveness of the block techniques. Within 1 min after modified O'Brien block, the voluntary activity of the orbicularis oculi muscle and the force of lid closure were significantly, lower than those after both the modified van Lint and the Atkinson techniques: furthermore, a significant decrease in the voluntary lid movement was observed within 3 min. Whereas the modified O'Brien block nearly abolished voluntary muscle activity, force of lid closure and lid movement, there was only a minor decrease in the area under the EMG curve and in the force of lid closure after the modified van Lint and Atkinson blocks (about 20%).(ABSTRACT TRUNCATED AT 250 WORDS)

Aged↗
PubMed1990

[Comparison of the Aulhorn flicker test with visual evoked potentials in the diagnosis of optic neuritis].

The Aulhorn flicker test and visual evoked cortical potentials (VEP) are of great value for the diagnosis of optic neuritis (ON). In the present study, the two methods were compared for the first time within the same group of patients. The study comprised 405 eyes (175 suffering from active or subsided ON). The results were evaluated with a double-blind procedure. With the flicker test, the subjective brightness of flickering light is determined as a function of the flicker frequency. This test gives pathological results only in active ON and normalizes when the active phase is over. The test can discriminate between active and subsided ON as well as between the recurrent and chronic courses of the disease. Differentiation is not possible with the VEP, since the VEP latencies are prolonged even after the end of the active period of the disease. The sensitivity of the flicker test was 84.4%. The sensitivity of the VEP was 72.7% for our group of patients suffering from ON if the criterion of increased latency was used alone. In the diagnosis of multiple sclerosis (MS), the proportion of correctly identified pathological VEP results is increased because of the detection of demyelination of the optic nerve that causes no clinical symptoms. The specificity of the flicker test was 97.8% and that of the VEP 86.5%. If both methods were combined, the sensitivity was 98.4% and specificity 99.6%. The two methods obviously have different characteristics and seem to rely upon different demyelination effects. Each method has its advantages and disadvantages as well as optimal indications.(ABSTRACT TRUNCATED AT 250 WORDS)

Evoked Potentials, Visual↗
PubMed1989

[Assessment of visual acuity in simulation and aggravation].

Following a review of various tricks and techniques described in the literature for measuring the visual acuity of less cooperative patients, three new methods are presented. The first is based on the so-called "preferential-looking" method, which has been in use for several years to determine the visual acuity of infants and small children. The second method uses polarizing filters to separate the beam paths of the two eyes and a flashlamp to project the test symbols, to prevent the patient from finding out by blinking which eye is being tested. In the third method the stimuli are presented on a computer monitor; the beam paths of the two eyes are separated by special spectacles which can briefly cover either eye while a test symbol is being shown. The results so far are encouraging.

Humans↗
PubMed1989

Assessment of visual function in suspected ocular malingering.

Three new techniques are presented for assessing visual acuity in the presence of suspected malingering. The first technique is based on the preferential looking method commonly used to test acuity in infants. The second uses polarising filters to present stimuli briefly to both eyes or to each eye alone, without the patient's being aware of which eye is tested. The third presents the stimuli on a computer monitor and separates the ray paths of the two eyes by means of special spectacles that obscure vision in one eye for fractions of a second while a stimulus is presented.

Humans↗
PubMed1988

Naso-temporal asymmetry of visual perception and of the visual cortex.

The decrease of visual performance from the fovea towards the periphery depends upon the task tested. The slope of the decrease is generally steeper for hyperacuity than for spatial resolution and is steeper in the nasal than in the temporal hemifield. The naso-temporal asymmetry in the periphery of the visual field, beyond 20 deg eccentricity, is much more pronounced for hyperacuity than for spatial resolution. The psychophysical results show a close correlation to the cortical organization as revealed by autoradiography.

Adult↗
PubMed1988

Why do isoluminant stimuli appear slower?

There is ample evidence that the perception of movement, both real and apparent, is substantially impaired at isoluminance. Models of movement perception require spatial and temporal information about the stimulus. We ask whether changes at isoluminance result from a spatial or a temporal error or uncertainty. Reaction times to three kinds of stimulus were measured: (a) temporal stimuli, such as the onset of a square in a known location; (b) spatial stimuli, a vernier displacement of two squares; and (c) spatiotemporal stimuli, moving squares either starting or stopping. The results suggest that there is relatively little effect of isoluminance on purely temporal tasks (a). Longer reaction times were, however, obtained for detecting vernier offset (b). The reaction times to moving stimuli (c) were also slower at isoluminance to an extent that implies that perceived velocity at isoluminance is approximately 30% less than that seen at 8% contrast. The slowing of reaction times at isoluminance could be mimicked by adding random positional jitter to a nonisoluminant moving stimulus and also by presenting a low-contrast monochromatic stimulus. A simple explanation of the data is given in terms of a motion-detecting unit coupled to a temporal integrator. It is shown how such a unit can encode perceived velocity. The results of these experiments suggest that the neural coding of isoluminant stimuli is similar to that of low-contrast luminance stimuli and therefore that isoluminance may not be an effective method to find out whether specific visual mechanisms are color-blind.

Adult↗
PubMed1988

Local and global factors in disparity detection of rows of points.

The threshold for the detection of disparity between points 10, 20 and 60 min apart was shown to depend on the number of intervening points. Even a single point bisecting the imaginary line both in depth and in visual angle often causes an approximate doubling of threshold and performance continues to deteriorate up to a point spacing that makes the line appear continuous. The effect is less pronounced if the intervening points are removed from the imaginary line joining the endpoints. Disparity detection in this kind of task involves local depth comparison of adjoining points and a more global mechanism.

Depth Perception↗
PubMed1987

[Why two eyes?].

Having two eyes instead of just one is advantageous in at least six respects: the danger of blindness is decreased, the visual field is enlarged, stereoscopic depth perception is possible on the basis of binocular disparities, the position of the eyes relative to the head can be computed from the images of both eyes, visual obstacles in front of the horopter perceptually shrink, and the signal-to-noise ratio is increased.

Blindness↗
PubMed1987

The apparent size of objects situated in front of or behind the horopter.

Objects in the visual world that are located outside the horopter project upon non-corresponding parts of both retinae. Hence, most environments will generate--within a portion of the binocular field of view--retinal images that contain clearly dissimilar pattern elements at corresponding retinal points. Fusion is impossible there and the brain has to decide locally from which eye conflicting information is to be chosen. The binocular field of view thus consists of parts where information from both eyes contributes to perception and others where only one eye is 'switched on' at a given time. Two easily performed perception experiments demonstrate the advantage of having two eyes, even if fusion of parts of the images they receive is impossible: the apparent size of large objects outside the fixation plane (i.e., visual obstacles) is reduced. Quantitative measurements agree well with the predictions derived from a simple geometrical model. It is also shown that only a very limited number of objects can elicit 'true' (physiological) double images.

Humans↗
PubMed1987

Presenting stereoscopic stimuli with one monitor.

Two different stimuli can be presented to both eyes on the same monitor in rapid alternation (100 Hz), each stimulus being displayed for 4 msec-periods. Sector discs in front of both eyes, phase-locked to the alternations of the displays, assure that each stimulus is seen by only one eye.

Data Display↗
PubMed1987

Naso-temporal asymmetry of binocular inhibition.

Binocular rivalry in the periphery of the visual field shows a distinct naso-temporal asymmetry that closely resembles the naso-temporal asymmetry of retinocortical projections. In convergent squint, the fovea of the deviating eye has to compete with the strong temporal hemifield of the contralateral eye. In divergent squint, the fovea competes with the much weaker nasal hemifield. Convergent squinters become amblyopic, but divergent ones seldom do.

Adult↗
PubMed1986

Curvature detection in the visual field and a possible physiological correlate.

Discrimination of curvature in nearly straight stimuli was measured for twelve stimulus orientations, corresponding to the hours of the clock, at eleven positions in the visual field of human observers. Displacements below the diameter of a photoreceptor can be detected foveally. In the periphery of the visual field, stimuli oriented towards the fovea yield best results. This anisotropy of orientation discrimination seems not to have an optical cause. It has a close correlation both to the structure and functional properties of cat ganglion cells, to the organization of receptive fields in area 17 and to the geometry of ocular dominance stripes.

Animals↗