Biomedical subjects

M Fahle

Publications and source records attributed to M Fahle.

At least 73 records · Page 4Linked to original sources

PubMed1994

Dyslexic children learn a new visual strategy for reading: a controlled experiment.

Recent studies by Geiger, Lettvin and Zegarra-Moran have proposed a new non-reading test for the diagnosis of dyslexia, and a new method for remediation. The latter involves the learning of a "visual strategy". On adult dyslexics the test was reliable and the remediation apparently effective. The purpose of this study is to confirm the usefulness of the remediation and test with children. Dyslexic children (3rd-6th grade) were divided into two groups. The experimental group (9) was given a new remedial regimen of practise. The control group (6) continued the remedial process given in their school. After 3 months of practising their separate regimens all the dyslexic children who were in the two groups were retested and compared. The "experimental" dyslexics improved in reading by 1.22 grade level on average while the "control" dyslexics improved by 0.17 grade on average. The form-resolving field (FRF) plots narrowed significantly for the experimental dyslexics while they changed little for the control dyslexics. At the end of the second testing the control dyslexics were also given the new regimen of practise. Five months later all the dyslexic children were tested for the third time. The initial control dyslexics who later practised the regimen (2) improved in reading by 2-2.5 grades and their FRF plot narrowed. The experimental dyslexics continued to improve yet further. All the dyslexic who practised the new regimen started at an average of 2.5 grades behind their expected grade/age level and after 8 months were at an average 0.75 grades behind their expected grade/age level. This is on average 1.75 grade level improvement in reading within 8 months, a rate of improvement larger than that of ordinary reading subjects. The dyslexic children were compared with matched grade/age ordinary reading children for reference. The study confirms the usefulness of the test and the applicability of the remediation method for children. It also shows that improvement under that method is quite rapid.

Child↗

PubMed1994

Human pattern recognition: parallel processing and perceptual learning.

A new theory of visual object recognition by Poggio et al that is based on multidimensional interpolation between stored templates requires fast, stimulus-specific learning in the visual cortex. Indeed, performance in a number of perceptual tasks improves as a result of practice. We distinguish between two phases of learning a vernier-acuity task, a fast one that takes place within less than 20 min and a slow phase that continues over 10 h of training and probably beyond. The improvement is specific for relatively 'simple' features, such as the orientation of the stimulus presented during training, for the position in the visual field, and for the eye through which learning occurred. Some of these results are simulated by means of a computer model that relies on object recognition by multidimensional interpolation between stored templates. Orientation specificity of learning is also found in a jump-displacement task. In a manner parallel to the improvement in performance, cortical potentials evoked by the jump displacement tend to decrease in latency and to increase in amplitude as a result of training. The distribution of potentials over the brain changes significantly as a result of repeated exposure to the same stimulus. The results both of psychophysical and of electrophysiological experiments indicate that some form of perceptual learning might occur very early during cortical information processing. The hypothesis that vernier breaks are detected 'early' during pattern recognition is supported by the fact that reaction times for the detection of verniers depend hardly at all on the number of stimuli presented simultaneously. Hence, vernier breaks can be detected in parallel at different locations in the visual field, indicating that deviation from straightness is an elementary feature for visual pattern recognition in humans that is detected at an early stage of pattern recognition. Several results obtained during the last few years are reviewed, some new results are presented, and all these results are discussed with regard to their implications for models of pattern recognition.

Attention↗

PubMed1994

Definition of thresholds for stereoscopic depth.

In the laboratory, thresholds for stereoscopic depth perception are usually determined by asking observers to discriminate between a stimulus with a given depth offset and its mirror image. Threshold is most often defined as the disparity difference that yields 75% or 83% correct responses. Disparities used for clinical tests of stereopsis are much higher. Here it is argued that, among other factors, this is because of the fact that clinical tests usually require the detection of a depth difference (offset versus no offset), rather than the discrimination between two directions of depth difference (in front versus behind). From a formal comparison of the two tasks, the data show that discrimination, or classification is easier by at least a factor of 2 than detection. The contribution of variations of the threshold criterion and learning to the differences between stereoacuity as measured in laboratory and clinic is also discussed. These differences are relevant to the design of tests for clinical use.

Adolescent↗

PubMed1994

An electrophysiological correlate of learning in motion perception.

We investigated learning in a motion-detection task using both psychophysical and neurophysiological methods in normal humans. A total of 20 naive observers had to discriminate between a small motion to the left versus to the right (jump displacement) or between a motion upward versus downward. Their performance improved significantly within less than 30 min in discriminating between directions in the psychophysical jump-displacement task. The improvement of performance with practice was very specific and did not transfer to the same stimulus rotated by 90 degrees. After training for the same task, multichannel evoked-potential recordings changed significantly in component latency and in the distribution of field potentials. This indicates that neuronal ensembles rather than single cells are involved in perceptual learning. Significant differences between the potential distributions occur for potentials at latencies of less than 100 ms over the occipital pole, suggesting an involvement of and plasticity in the primary visual cortex of human adults.

Adult↗

PubMed1993

Figure-ground discrimination from temporal information.

The separation of figures from ground is achieved by the visual system based on differences in features such as luminance, colour, depth, orientation, texture or motion. Temporal information, namely phase differences between groups of spatially homogeneous points, can also lead to a clear discrimination of an object shape. The time difference needed to separate figure and ground is around 5 ms over a range of temporal frequencies between 1.3 Hz and 30 Hz, both for sharply focused and for blurred points. These short delays are clearly below the temporal integration time of the visual system. The results have implications for theories on temporal binding and object recognition.

Humans↗

PubMed1993

Long-term learning in vernier acuity: effects of stimulus orientation, range and of feedback.

In hyperacuity, as in many other tasks, performance improves with practice. To better understand the underlying mechanisms, we measured thresholds of 41 inexperienced observers for the discrimination of vernier displacements. In spite of considerable inter-individual differences, mean thresholds decreased monotonically over the 10,000 stimuli presented to each observer, if stimulus orientation was constant. Generalization of learning seemed to be possible across offset-ranges, but not across orientations. Learning was slightly faster with error feedback than without it in one experiment. These results effectively constrain the range of conceivable models for learning of hyperacuity.

Feedback↗

PubMed1993

Retrobulbar blockade of somatic, motor, and visual nerves by local anesthetics.

Somatosensory, motor, and visual sensory blockade were investigated after retrobulbar injection of 3 mL 2% lidocaine, prilocaine, or mepivacaine plus hyaluronidase (15 U/mL) and naphazoline nitrate (1:20,000) in 90 cataract patients (n = 30 per group). Before injection as well as 20 and 90 minutes after injection, and then every 30 minutes, the quality of the retrobulbar blockade was evaluated in terms of the following factors until full recovery of function: (1) corneal sensitivity at the three extraincisional quadrants as determined with an esthesiometer; (2) horizontal and vertical motility, and elevation of the lid; (3) visual acuity on an arbitrary score scale ranging from 0 (no light perception) to 6 (visual acuity > 0.05); and (4) the time required for recovery from retrobulbar anesthesia. The data were analyzed by one- (anesthetic) and two-factor (anesthetic and time) analysis of variance. Full somatic recovery of corneal sensitivity occurred within 247 +/- 10.2 minutes after lidocaine, within 221 +/- 9.2 minutes after prilocaine, and within 280 +/- 8.5 minutes after mepivacaine (F = 10.1; P < .0001). Full motor recovery (all muscles) occurred within 290 +/- 5.8 minutes after lidocaine, within 258 +/- 5.7 minutes after prilocaine, and within 295 +/- 4.8 minutes after mepivacaine (F = 13.3, P < .0001). On the average, visual acuity decreased most after mepivacaine and least after lidocaine administration, although the differences between the three anesthetics in this regard were not significant. One patient temporarily lost vision after mepivacaine administration. Overall, the somatosensory and motor blockade were most pronounced after mepivacaine.

Adult↗

PubMed1993

Visual learning in the hyperacuity range in adults.

We investigated the amount, speed, and specificity of the learning of new visual tasks in adult humans using stereoscopic depth perception and vernier discrimination as the sensitive probes for learning. The results of psychophysical experiments in untrained adult observers indicate two phases of highly specific perceptual learning in humans: a fast phase and a slower one. Both types of learning might take place relatively "early" during pattern recognition in the visual cortex, since learning was very specific, without transfer between different stimulus orientations.

Depth Perception↗

PubMed1992

Effects of pattern element density upon displacement limits for motion detection in random binary luminance patterns.

The upper motion displacement threshold (Dmax) was determined with two-frame motion sequences of random binary luminance patterns, over a range of pattern element sizes and densities. Dmax was little affected by density at small element sizes (less than 5 arcmin), in agreement with previous reports. However, at larger element sizes (greater than 9 arcmin) Dmax increased as element density was reduced in the range 50-5%. We explain our findings by a model which takes into account spatial-frequency filtering prior to motion detection, and the effects of pattern density upon the statistics of random binary patterns. We also implicate the dependence of Dmax upon the contrast energy of the elements in broadband patterns, and provide a direct demonstration that Dmax is contrast limited over a wide range of pattern contrasts (72-2.5%). Previous reports that Dmax is independent of density should be modified to take into account the complex effects of density upon the statistics of random patterns, and the existence of physiological filtering prior to motion detection.

Humans↗

PubMed1992

Fast perceptual learning in visual hyperacuity.

In many different spatial discrimination tasks, such as in determining the sign of the offset in a vernier stimulus, the human visual system exhibits hyperacuity by evaluating spatial relations with the precision of a fraction of a photoreceptor's diameter. It is proposed that this impressive performance depends in part on a fast learning process that uses relatively few examples and that occurs at an early processing stage in the visual pathway. This hypothesis is given support by the demonstration that it is possible to synthesize, from a small number of examples of a given task, a simple network that attains the required performance level. Psychophysical experiments agree with some of the key predictions of the model. In particular, fast stimulus-specific learning is found to take place in the human visual system, and this learning does not transfer between two slightly different hyperacuity tasks.

Algorithms↗

PubMed1992

Visual memory for vernier offsets.

Human ability to perceive and remember precise spatial relationships was investigated in a vernier acuity task. An initial ("standard") vernier stimulus with a variable offset was presented for 100 msec. After a delay of 1, 4, or 8 sec, another vernier target (the "variable" stimulus) followed, also for 100 msec. Observers compared the offsets of the two stimuli with each other. For very small offsets, discrimination between smaller and larger offsets was very precise, in the hyperacuity range. Thresholds increased linearly with the spatial offset of the standard stimulus, suggesting that the precision of the mechanism solving this spatial task scales with offset size. Control experiments suggested that this effect was not due to variations in retinal eccentricity. Thresholds also increased with increasing delay between the presentations of the two stimuli. By varying the delay, we directly measured the fading of the spatial memory trace.

Humans↗

PubMed1992

The development of vernier acuity in human infants.

Vernier acuity, i.e. the detection of a small misalignment between lines, is about one order of magnitude finer than the resolution of periodic gratings in adult humans. This hyperacuity is generally attributed to cortical mechanisms, and the time-course of its development seems to differ from the development of grating resolution that probably is limited by retinal factors. We investigated 271 human infants and children between 2 months and 8 yr of age with essentially identical stimuli and experimental procedures. Vernier thresholds for Vernier targets were compared to grating resolution. The preferential looking experiments led to the following results: (i) Vernier acuity starts below grating resolution. (ii) Like grating resolution, Vernier acuity develops gradually, but more rapidly and longer; at the age of 5 yr performance becomes comparable to that of adults. (iii) Flanking borders without offset, added to the Vernier targets at various distances, did not affect thresholds consistently across distances and age groups.

Adult↗

PubMed1992

A colour-flicker analysis of visual function in patients with retinal detachment.

Patients with a history of retinal detachment were examined by the following tests: (1) setting of an isoluminant match between yellow and green constituent squares of a checkerboard and (2) measurement of the flicker fusion frequency for a contrast-reversal of this yellow-green pattern. The results suggest that the test may provide useful data for assessing the severity of any damage to a given part of the retina. The fusion frequencies range from over 10 Hz (normal) to about 2 Hz for a severely impaired retina. The data for the relative amounts of red and green light needed to achieve a match rarely show any abnormality; thus, a standard anomaloscope test or pseudoisochromatic plates would not detect these deficiencies.

Adult↗

PubMed1991

Motion perception in the peripheral visual field.

Thresholds were determined for the perception of the motion of a single bar moving at different positions in the field of view. Performance in the temporal hemified was slightly superior to that in the nasal hemifield and depended on the orientation as well as on the direction of the motion. The perception of horizontal motion was better than that of vertical motion. In spite of large variations, centrifugal motion was significantly more readily perceived than centripetal motion.

Humans↗

PubMed1991

Correlations between electroretinography, morphology and function in retinitis pigmentosa.

In a retrospective study, data from 116 patients suffering from different forms of retinitis pigmentosa were analysed, and 15 categories comprising altogether 34 symptoms or clinical signs were tabulated from each patient's record. The 15 categories evaluated were: visual acuity, visual field diameter, ring or central scotoma, nyctalopia, susceptibility to glare, refraction, cataract, electroretinography, colour of the optic disc, bone-spicule pigmentation of the retina, retinal vessel diameters, tapetoretinal reflex, sex, heredity, and age. Correlations between the tabulated 34 subcategories or symptoms were calculated. The results of a factorial analysis of the data showed a high number of highly significant correlations between the different categories. It seemed possible to discriminate between two groups of categories, with the members of each group being closely correlated but correlations with members of the other group being much smaller, if not nonexistent. We tentatively associated the first group with the functional state of the central retina and the second group with the state of the peripheral retina.

Adolescent↗

PubMed1991

Perceptual rivalry between illusory and real contours.

The interactions between illusory and real contours have been investigated under monocular, binocular and dichoptic conditions. Results show that under all three presentation conditions, periodic alterations, generally called rivalry, occur during the perception of cognitive (or illusory) triangles, while earlier research had failed to find such rivalry (Bradley and Dumais 1975). With line triangles, rivalry is experienced only under dichoptic conditions. A model is proposed to account for the observed phenomena.

Computer Simulation↗

PubMed1991

Psychophysical measurement of eye drifts and tremor by dichoptic or monocular vernier acuity.

If the two segments of a vernier target are presented to different eyes, i.e. dichoptically, thresholds are three to four times higher than with presentation to the same eye. This increase in thresholds is mainly due to uncorrelated movements of both eyes, such as tremor and drifts, that occur even under steady fixation. The psychophysically measured thresholds allow one to calculate an upper estimate for the amplitudes of uncorrelated eye movements during fixation. This estimate matches the best results from direct eye position recording, with the calculated mean amplitude of eye tremor corresponding to roughly one photoreceptor diameter. The combined amplitude of both correlated and uncorrelated eye movements was also measured by delaying one segment of the vernier relative to its partner under monocular or dichoptic conditions. Fixation proved to be relatively stable, and trained observers could sustain eye position within a few min arc.

Cues↗

PubMed1991

Parallel perception of vernier offsets, curvature, and chevrons in humans.

A vernier offset is detected at once among straight lines, and reaction times are almost independent of the number of simultaneously presented stimuli (distractors), even if absolute orientation cues are masked by varied orientation of the verniers. This result implies that the human visual system processes vernier offsets in parallel. Reaction times for identifying one straight target among offset verniers, on the other hand, increase with the number of stimuli. The same is true for the identification of a vernier offset to one side among verniers offset to the opposite side, if absolute orientation cues are masked. These tasks require serial or semi-parallel processing. Chevrons and curved targets show the same pattern of results. Even deviations below a photoreceptor diameter can be detected at once. The visual system thus attains positional accuracy below the photoreceptor diameter simultaneously at different positions. I conclude that deviation from straightness, or change of orientation, is detected in parallel over the visual field. Discontinuities or gradients in orientation may represent an elementary feature of vision.

Discrimination, Psychological↗