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Biomedical subjects

M Fahle

Publications and source records attributed to M Fahle.

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

Component perimetry: a fast method to detect visual field defects caused by brain lesions.

PURPOSE: Noise field campimetry, performed according to Aulhorn and Köst, confronts patients with a large field of irregularly flickering dots, and many patients immediately perceive their visual field defects. The original method had a somewhat low specificity and sensitivity, especially for patients with visual field defects caused by cortical lesions. METHODS: The method was improved in two ways. First, the grain of the visual noise was increased toward the periphery of the visual field to accommodate the peripheral decrease in visual acuity. Second, the type of stimulus pattern was varied to include separate investigations of different visual components or functions (color, motion, temporal resolution, line orientation, stereoscopic depth, acuity, and figure-ground segmentation). To evaluate the reliability of the method, the visual fields were compared, as assessed by the new method, with those of conventional perimetry in 41 patients with neurologic disorders and 22 normal control subjects. RESULTS: The results were encouraging. All patients with suprageniculate lesions subjectively experienced visual field defects in component perimetry. Sizes of visual field defects obtained with both methods corresponded qualitatively with each other, with a highly significant correlation. The specificity of component perimetry was higher than that of the original noise field campimetry. CONCLUSIONS: This pilot study indicates that component perimetry is a subjective but relatively reliable method for detecting disorders of visual perception caused by lesions at different stages along the visual pathway, permitting fast screening of the visual field. In addition, this method seems to allow examination of the visual field, not only for defects in contrast sensitivity, as does conventional light perimetry, but also for the status of other components of vision such as color or motion perception. Further evaluation with larger patient cohorts is needed to allow exact assessment of the clinical usefulness of the method.

Adolescent↗
PubMed1999

Alcohol and visually guided saccades: gap effect and predictability of target location.

RATIONALE: Alcohol is known to affect most parameters of visually guided saccades, but it is unclear whether intoxicated subjects are able to utilize temporal or spatial pre-cues to compensate for their alcohol-related slowing of saccades. OBJECTIVES: We examined the effects of both temporal and spatial predictability on gain, latency, and peak velocity in sober and intoxicated subjects, e.g. by employing a temporal gap condition. METHODS: Saccades were recorded with subjects once sober and once intoxicated (0.8 g ethanol per kg body weight). Unpredictable and predictable target locations alternated in both the classical (no gap) and temporal gap condition (extinction of fixation point 200 ms before target onset). RESULTS: The gain was only slightly affected by alcohol, but increased for predictable target locations. After alcohol consumption, latencies increased, even in the gap condition and for predictable targets. However, intoxicated subjects took relatively more benefit from the gap than sober subjects did. In addition, they showed a "pretrial effect", i.e. their latencies depended on the condition for the previous saccade. For predictable target locations, latencies decreased and peak velocities increased both in sober and intoxicated subjects. Thus, intoxicated subjects were able to utilize both the spatial predictability and the temporal gap to speed up their saccades. CONCLUSIONS: These results lead to suggest that as far as the saccadic system is concerned alcohol predominantly affects the function of the superior colliculus (SC) and/or the oculomotor regions in the brain stem, either directly or indirectly. On the other hand, cortical areas assumed to mediate prediction and the gap effect seem to be less affected by alcohol intoxication.

Adult↗
PubMed1999

Learning of interpolation in 2 and 3 dimensions.

We investigated learning of spatio-temporal interpolation in depth and its relation to spatio-temporal interpolation in two dimensions by means of a vernier discrimination task. Performance improved with training but improvement did not or only partially transfer between opposite directions of motion in depth. Improvement was also at least partly specific for the eye and for the direction of two-dimensional motion used during monocular training. This specificity might explain the apparent specificity of interpolation in three dimensions. Training with stimuli moving in two dimensions increased performance for a stimulus moving in depth. The results indicate that improvement in spatio-temporal interpolation occurs at least partly on a stage preceding stereoscopic vision, and are a rare example for transfer of improvement between different perceptual tasks.

Depth Perception↗
PubMed1999

Display symmetry affects positional specificity in same-different judgment of pairs of novel visual patterns.

Deciding whether a novel visual pattern is the same as or different from a previously seen reference is easier if both stimuli are presented to the same rather than to different locations in the field of view (Foster & Kahn (1985). Biological Cybernetics, 51, 305-312; Dill & Fahle (1998). Perception and Psychophysics, 60, 65-81). We investigated whether pattern symmetry interacts with the effect of translation. Patterns were small dot-clouds which could be mirror-symmetric or asymmetric. Translations were displacements of the visual pattern symmetrically across the fovea, either left-right or above-below. We found that same-different discriminations were worse (less accurate and slower) for translated patterns, to an extent which in general was not influenced by pattern symmetry, or pattern orientation, or direction of displacement. However, if the displaced pattern was a mirror image of the original one (along the trajectory of the displacement), then performance was largely invariant to translation. Both positional specificity and its reduction in symmetric displays may be explained by location-specific pre-processing of the visual input.

Adult↗
PubMed1999

Effects of biased feedback on learning and deciding in a vernier discrimination task.

We investigate the influence of biased feedback on decision and learning processes in a vernier discrimination task. Subjects adjust their decision criteria and hence their responses according to biased external feedback. However, they do not use learning processes to encode incorrectly classified stimuli. As soon as correct feedback is restored observers regain their original performance indicating an involvement of internal criteria. If the external feedback is switched off instead of being corrected, the rebound is less vigorous. The findings contradict predictions of supervised neural network models.

Decision Making↗
PubMed1999

Alcohol and visual performance.

1. The authors examined the effect of acute alcohol consumption on a set of visual tasks: visual short term memory, depth perception, and attention. 2. In a repeated measurement design, thirteen subjects performed the tasks once sober and once intoxicated with 0.8 g/kg body weight pure ethanol in orange juice (33% alcohol). Subjects underwent a neuropsychological (Benton test) and a psychophysical test (vernier discrimination) both assessing visual short term memory, the test d2 as a measure of attention and concentration, and a psychophysical depth perception task. 3. Subjects demonstrated significant alcohol-related impairments in depth perception and in visual short term memory as assessed by the vernier discrimination task. However, the neuropsychological Benton test and test d2 failed to reveal alcohol-related changes in performance-probably due to superimposed learning effects. Performance was neither correlated with blood alcohol levels (BAL) nor perceived intoxication. Even though the current BAL was known to the subjects, only half of them demonstrated a close correlation between BAL and perceived intoxication.

Adult↗
PubMed1998

Modeling perceptual learning: difficulties and how they can be overcome.

We investigated the roles of feedback and attention in training a vernier discrimination task as an example of perceptual learning. Human learning even of simple stimuli, such as verniers, relies on more complex mechanisms than previously expected--ruling out simple neural network models. These findings are not just an empirical oddity but are evidence that present models fail to reflect some important characteristics of the learning process. We will list some of the problems of neural networks and develop a new model that solves them by incorporating top-down mechanisms. Contrary to neural networks, in our model learning is not driven by the set of stimuli only. Internal estimations of performance and knowledge about the task are also incorporated. Our model implies that under certain conditions the detectability of only some of the stimuli is enhanced while the overall improvement of performance is attributed to a change of decision criteria. An experiment confirms this prediction.

Attention↗
PubMed1998

Deficits and recovery of first- and second-order motion perception in patients with unilateral cortical lesions.

Unilateral lesions in the posterior parietal cortex can degrade motion perception in the contralesional visual hemifield. Our aim was to investigate whether deficits caused by cortical lesions may be different for first- and second-order motion perception, and to study the time scale of any potential recovery. In nine patients with circumscribed lesions mainly in the parietal and fronto-parietal cortex, thresholds for direction discrimination were measured for stimuli presented peripherally in their ipsi- and contralesional hemifield. Subjects had to identify the direction of a vertically moving object embedded in a background of dynamic random dot noise. The object consisted of various proportions of signal and noise dots. Signal dots were either (a) coherently moving in the same direction as the object (first-order), (b) stationary (second-order: drift-balanced), or (c) coherently moving in the opposite direction (second-order: theta). Noise dots were flickering. Two patients showed significant threshold elevations for all three types of motion stimuli presented in their contralesional hemifield, while thresholds for ipsilesional targets were unaffected. Neither showed any selective deficit of first- versus second-order motion perception, but second-order motion was more impaired. Their lesions probably included the motion area V5-MT, which was spared in the other seven patients. One of the patients, who was retested several times during a 27-month postlesional period, showed complete recovery for first- and second-order motion direction discrimination, as well as for the detection of speed differences.

Adult↗
PubMed1998

Complex motion stimuli localize higher-order visual processing in normal observers and in patients with parietal lesions.

The present paper illustrates how modern techniques applied in neuroscience can help us to understand the processing of visual information in the human brain and, in turn, how they can be helpful to characterize functional deficits in patients with cortical lesions. Based on theoretical considerations, motion stimuli are developed that require very specific operations to be performed by the visual system. Computational models explaining the processing of these 'Fourier' and 'second-order' motion stimuli are characterized by increasing complexity. The same types of stimuli are used to map the distribution of cortical activity during motion perception by measuring the magnetic and electrical fields on the head surface. Clinical investigations of patients with lesions in the parietal cortex indicate specific deficits in the perception of such stimuli that can be related to the lesion sites.

Action Potentials↗
PubMed1998

The use of different orientation cues in vernier acuity.

The role of orientation cues in vernier acuity was investigated in five experiments that varied the salience of different possible reference systems: (1) retinal, (2) subjective (vestibular), and (3) stimulus-defined ("intrinsic"). We compared the eight possible combinations of presence and absence of the three types of information about vertical by changing the orientation of the vernier stimulus, by changing the angle between the two segments of the stimulus, by reducing the stimulus to two dots rather than two lines, and/or by tilting the subject's head or by having the subject lie in a supine position. The results show that vestibular vertical plays no important role in vernier discriminations, whereas retinal vertical and, to a lesser degree, the intrinsic reference do. A control experiment excluded the possible role of a fourth possible reference system, the frame of the monitor. Our results are compatible with a model of vernier acuity detection based on oriented receptive fields with inhibitory surrounds. They show further that observers can produce, in the case of two-dot stimuli, an internal standard for comparison and detect deviations from this standard with high precision.

Cues↗
PubMed1998

Limited translation invariance of human visual pattern recognition.

Visual object recognition is considered to be largely translation invariant. An earlier study (Foster & Kahn, 1985), however, has indicated that recognition of complex novel stimuli is partially specific to location in the visual field: It is significantly easier to determine the identity of two briefly displayed random patterns if both stimuli are presented at the same, rather than at different, locations. In a series of same/different discrimination tasks, we characterize the processes underlying this "displacement effect": Horizontal and vertical translations are equally effective in reducing performance. Making the task more difficult by increasing pattern similarity leads to even higher positional specificity. The displacement effect disappears after rotation or contrast reversal of the patterns, indicating that positional specificity depends on relatively low levels of processing. Control experiments rule out explanations that are independent of visual pattern memory, such as spatial attention, eye movements, or retinal afterimages. Positional specificity of recognition is found only for same trials. Our results demonstrate that position invariance, a widely acknowledged property of the human visual system, is limited to specific experimental conditions. Normalization models involving mental shifts of an early visual representation or of a window of attention cannot easily account for these findings.

Adult↗
PubMed1997

The role of visual field position in pattern-discrimination learning.

Invariance of object recognition to translation in the visual field is a fundamental property of human pattern vision. In three experiments we investigated this capability by training subjects to distinguish between random checkerboard stimuli. We show that the improvement of discrimination performance does not transfer across the visual field if learning is restricted to a particular location in the retinal image. Accuracy after retinal translation shows no sign of decay over time and remains at the same level it had at the beginning of the training. It is suggested that in two-dimensional translation invariance-as in three-dimensional rotation invariance-the human visual system is relying on memory-intensive rather than computation-intensive processes. Multiple position- and stimulus-specific learning events may be required before recognition is independent of retinal location.

Adult↗
PubMed1997

Visual learning and memory as functions of age.

Age-related changes have been reported for a range of perceptual and cognitive functions. We investigated visual acuity and vernier acuity, as well as visual memory and learning, in four age groups, each comprising 10 subjects and ranging from adolescence to an age group up to 66 years. The groups were matched on general abilities for visuospatial information processing. Visual acuity decreased slightly with increasing age. The reproduction of more complex geometrical material which exceeded short-term memory capacities was significantly impaired in the oldest group relative to young adults. However, vernier acuity thresholds and improvement of thresholds with training did not differ in the four groups. Short-term retention of visual stimuli across 1 or 4 sec was also comparable in the different age groups. Those visual abilities that are limited primarily by cortical, not retinal, factors remained relatively unchanged in our observers despite age-related changes in the eye. Visual memory functions which tap working and long-term memory abilities, on the other hand, showed a significant age-associated decline during adulthood.

Adult↗
PubMed1997

The spatial limit for motion detection in noise depends on element size, not on spatial frequency.

When a random spatial noise pattern is displaced for a short distance it seems to move coherently, but when the displacement exceeds a certain value, the direction of motion cannot be clearly perceived. We measured the displacement limit (Dmax) for a two-frame sequence and found that it depended on the size of the elements comprising the random pattern, even when low spatial frequencies were removed from the pattern by spatial band-pass filtering. Dmax depended strongly on contrast for the filtered patterns, but less so for the unaltered patterns. The data support a model for low level motion detection in which the maximum motion displacement that can be detected is determined by the mean separation of pattern elements, following a stage of low-pass spatial filtering, and in which the upper spatial displacement depends upon the pattern statistics, not upon the size of detectors in the visual system.

Filtration↗
PubMed1997

Specificity of learning curvature, orientation, and vernier discriminations.

Training significantly improves the performance of many perceptual tasks. Different visual tasks share some "front-end" neuronal mechanisms but rely (partly) on different neuronal mechanisms for further analysis. Perceptual learning might occur on the early common levels of visual information processing or else on the later, more specialized levels. Eighteen observers trained in three visual hyperacuity tasks, namely curvature, orientation, and vernier discrimination that probably share a common first stage of analysis based on detection of oriented line elements. Speed of improvement did not differ significantly between these tasks. There was no transfer of improvement from one task to another, indicating that the neuronal mechanisms underlying the three tasks are at least partly non-identical and that learning does not take place on the first common levels of analysis. This result constrains the possible localizations, in the human brain, of perceptual learning. The study also demonstrates that perceptual learning can be used as a tool to increase our knowledge on the sequence of operations during (visual) pattern recognition.

Discrimination, Psychological↗
PubMed1997

The role of feedback in learning a vernier discrimination task.

We compare improvement through training in vernier acuity under different feedback conditions in order to clarify the role of feedback during learning of a perceptual task and to test different (neural network) models of perceptual learning. Improvement of performance is measured in 49 observers under feedback, no feedback, uncorrelated feedback, partial feedback, and block feedback conditions. Correct feedback conditions yield a larger improvement of performance than manipulated and no feedback conditions. Providing feedback that is uncorrelated to the observers' responses prevents learning, while the effect of block feedback does not differ significantly from complete feedback. Our results cannot be explained by learning rules that depend exclusively on an external teacher or by models that propose learning in an exposure-dependent way with unsupervised learning rules but without top-down influences.

Discrimination, Psychological↗
PubMed1997

Missed targets are more frequent than false alarms: a model for error rates in visual search.

In many visual search tasks, reaction times (RTs) for target detection are measured as a function of display size. The corresponding error rates are usually low but increase with increasing display size. Missed-target errors are more common than false alarms. In recent models of visual search, the error rates were attributed to a premature search termination and error rates increasing with display size were interpreted as indicating a speed-accuracy trade-off and an underestimation of search times per item (obtained from RT slopes). A model is described in which errors occur as a result of imperfect rather than incomplete search (i.e., it is assumed that there are task-specific probabilities of categorizing a target or a distractor incorrectly). Signal-detection theory is used to show that the observed error rate properties can be attributed to an optimized decision strategy. "Corrections" of RT data are thus questionable.

Humans↗