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Michael Bach

Publications and source records attributed to Michael Bach.

14 recordsLinked to original sources
PubMed2003

Isolating motion responses in visual evoked potentials by preadapting flicker-sensitive mechanisms.

Onset of visual motion evokes a component in the EEG, the motion onset VEP. Exploring its motion specificity with a direction-specific adaptation paradigm, previous work demonstrated that less than 50% of the motion onset VEP represents actual motion detection. Here, we tested whether preadaptation of flicker-sensitive mechanisms can help to isolate motion-specific responses in the VEP. Flicker preadaptation was accomplished by limiting dot lifetime in the random-dot kinematograms that we used to study the direction specificity of motion adaptation. With unlimited dot lifetime, motion adaptation reduced the VEP amplitude to 35% (adapted direction) and 50% (opposite direction). With the shortest dot lifetime (40 ms), motion adaptation reduced the amplitude to 55% (adapted direction) and 70% (opposite direction). These findings suggest that random-dot kinematograms with short dot lifetimes could improve the investigation of human motion processing, be it in electrophysiology or other fields. While such stimuli successfully preadapt flicker-related components, they still evoke a sizable response, of which an estimated 70% is motion-specific.

Electrooculography↗
PubMed2003

Ocular prevalence versus ocular dominance.

UNLABELLED: Ocular dominance manifests itself in tests that contain stereo-objects with a disparity beyond Panum's area, e.g. in pointing a finger. These tests force subjects to decide in favour of one or the other eye. In contrast, ocular prevalence is determined using stereo-targets imaged within Panum's areas. These tests allow a graded quantification of the balance between the eyes. Here we present the computer-based Freiburg Ocular Prevalence Test in which stereo-disparate targets have to be aligned, and compare it with the Haase Stereo-balance Test that requires an estimation of the horizontal distance between stationary stereo-disparate objects. In addition, we compare ocular prevalence with ocular dominance. METHODS: (1) We measured the influence of a neutral-grey filter in front of one eye to assess the suitability of the Freiburg and the Haase Tests in revealing graded amounts of ocular prevalence. (2) About 20 subjects with equal vision of their two eyes underwent the Freiburg and the Haase Tests for ocular prevalence, and Parson's Monoptoscope Test for ocular dominance. RESULTS: (1) In both the Freiburg and the Haase Tests, the neutral-grey filter shifted ocular prevalence by about 50%. (2) An ocular prevalence of more than 10% occurred in 13 of the 20 subjects using the Freiburg, and in 14 using the Haase Test. On average, the ocular prevalence was 24.1+/-3.8% in the Freiburg and 32.0+/-8.2% in the Haase Test. The dominant eye coincided with the prevalent eye in 15 of the 20 subjects. DISCUSSION: The effect of the neutral-grey filter indicated that both the Freiburg and the Haase Tests can be used to measure fractions of ocular prevalence, although the Freiburg Test carries a higher reproducibility. Spontaneous ocular prevalence occurs frequently in persons with equal vision of their two eyes. This suggests that ocular prevalence does not represent a condition that requires treatment. Rather, partial suppression of one eye, the correlate of ocular prevalence, may play a physiological role in that it helps to disregard double images at stereo-disparities close to the limits of Panum's area.

Adult↗
PubMed2003

Adaptation characteristics of steady-state motion visual evoked potentials.

OBJECTIVE: Motion visual evoked potentials (motion VEPs) are used in clinical diagnosis and basic research. Employing steady-state rather than the usual transient motion VEPs simplifies statistical evaluation and might drastically reduce examination durations. Protocols for recording transient motion-onset VEPs usually involve fairly long recovery intervals between trials to avoid neural adaptation. This is not feasible for steady-state VEPs. We investigated how adaptation affects the steady-state motion VEP. METHODS: Oscillatory (13.3rev/s) and continuous uni-directional random-dot motion served as adaptation stimuli. Steady-state motion VEPs and, for comparison, transient motion VEPs were recorded. RESULTS: In the first experiment, we investigated how adaptation affects the recordings. Contrary to our expectation, we did not find any sizable effect. However, there was a large inter-individual variability in steady-state amplitude and no correlation across subjects between transient and steady-state amplitude. In the second experiment, we confirmed that the steady-state VEP reflects veridical motion processing by assessing its susceptibility to uni-directional pre-adaptation. CONCLUSIONS: Taken together, the results suggest that steady-state motion VEPs provide a fast method of recording motion responses without suffering from adaptation, but at the expense of inter-individual reproducibility.

Adaptation, Physiological↗
PubMed2003

Stereoacuity versus fixation disparity as indicators for vergence accuracy under prismatic stress.

BACKGROUND: Fixation disparity has been widely used as an indicator for vergence accuracy under prismatic stress. However, the targets used for measuring fixation disparity contain artificial features in that the fusional contours are thinned out. We considered that stereoacuity might be a preferable indicator of vergence accuracy, as stereo targets represent natural viewing conditions. METHODS: We measured fixation disparity with a computer adaptation of Ogle's test and stereoacuity with the automatic Freiburg Stereoacuity Test. Eight subjects were examined under increasing base-in and base-out prisms. RESULTS: The response of fixation disparity to prismatic stress revealed the curve types described by Ogle and Crone. All eight subjects reached a stereoscopic threshold below 10 arcsec. In seven subjects the stereoscopic threshold increased before double vision occurred. CONCLUSION: Our data suggest that stereoacuity is suitable to assess the range of binocular vision under prismatic stress. As stereoacuity bears the advantage over fixation disparity in that it can be measured without introducing artificial viewing conditions, we suggest exploring whether stereoacuity under prismatic stress would be more meaningful in the work-up of asthenopic patients than is fixation disparity.

Adult↗
PubMed2003

Pattern-onset stimulation boosts central multifocal VEP responses.

Multifocal visual evoked potentials (VEP) allow one to assess whether stimulation at specific visual field locations elicits cortical activity; it might therefore enable us to conduct objective visual field perimetry. However, due to the cortical folding, which differs markedly between subjects, a particular electroencephalogram generator may fail to project signal on some recording electrodes. This may lead to false alarms for potential scotomata. Here we compare pattern-reversal and pattern-onset stimulation in their efficacy to activate the visual cortex and recorded mfVEPs to 60 locations comprising a visual field of 44 degrees diameter. We report three main findings: (1) Pattern-onset compared to pattern-reversal enhances the amplitude by 30% for stimulation of the central visual field (<10 degrees radius), while evoking 30% less response in the periphery (>15 degrees ). (2) Although pattern-onset and pattern-reversal responses differ markedly in their eccentricity dependence, they have a similar topographical distribution. (3) By combining both stimuli, the number of false positives was reduced to less than 1.5% of the visual field locations tested. We conclude that pattern-onset and pattern-reversal activate identical visual cortical areas but target different neural mechanisms within these areas. Furthermore, pattern-onset stimulation greatly increases the sensitivity of the mfVEP to assess the cortical representation of the central 10 degrees of the visual field.

Adult↗
PubMed2002

Interindividual variability of learning in stereoacuity.

BACKGROUND: In the evaluation of therapies aiming at binocular vision, for instance by the use of prisms or orthoptic training in the case of heterophoria, stereoacuity is often the primary outcome measure. To assess therapeutic effects it is necessary to separate them from perceptual learning with repeated testing. Learning stereoacuity has been investigated only in a few studies with up to six subjects. METHODS: To ascertain the interindividual variability of learning in stereoacuity we examined 24 subjects, 12 with and 12 without experience in psychophysical experiments. In a two-alternative forced-choice paradigm, subjects reported whether a vertical bar appeared in front of or behind a reference frame. Estimates of stereo threshold were obtained using an adaptive staircase procedure ("best PEST"). RESULTS: We found a highly significant learning effect ( P<0.0001) with a marked interindividual variability. In some subjects the stereoacuity improved by a factor of >30 and in others it did not improve at all. The median of the learning factor was 1.7. There was no significant difference between novices and experienced subjects. CONCLUSION: The great interindividual variability of learning in stereoacuity has important implications for therapeutic tests that use stereoacuity as an outcome measure: To distinguish therapeutic effects from improvements due to repeated testing, each subject's individual learning behaviour has to be taken into account, for example by starting out with an adequate training phase. The number of test repetitions required to reach a fairly constant level appears to be similar among individuals: in our paradigm most of the learning occurred within the first six blocks with 100 target presentations each.

Adult↗
PubMed2002

The distinction between eye and object motion is reflected by the motion-onset visual evoked potential.

Humans are able to distinguish eye movement-induced retinal image motion and physical object motion during smooth pursuit eye movements. We investigated the neurophysiological basis of this ability by comparing motion-onset visual evoked potentials (VEPs) to onset of: (1) physical object motion during fixation, (2) eye movement-induced retinal image motion, and (3) physical object motion during eye movements. Electro-oculographic (EOG) artifacts were removed and the influence of eye-movement quality was evaluated. Retinal image shift was of similar magnitude in all conditions (9 degrees /s) and elicited typical motion-onset VEPs, with N2 at occipital and P2 at central derivations. During smooth pursuit, physical object motion induced N2 and P2 of higher latencies than during fixation. In the absence of physical object motion, i.e., for exclusively eye movement-induced retinal image motion, the N2 amplitude was reduced. This is taken as evidence that the activity of detectors of physical object motion is reflected by a part of the N2 component. N2 also reflects eye movement-induced retinal image motion. It is concluded that headcentric motion detection and the detection of eye movement-induced retinal image motion is mediated by brain mechanisms with similar latencies and, within the resolution limits of VEPs, at similar locations.

Electroencephalography↗
PubMed2002

The influence of ambient room lighting on the pattern electroretinogram (PERG).

It has been suggested that low ambient lighting conditions increase the amplitude of the PERG, but no data has been available on this issue. We recorded the transient PERG (0.8 degrees check size) and steady-state PERG (15 rev/s, 0.8 degrees and 16 degrees check size) under three lighting conditions: dark room, only illuminated by the stimulus (resulting in 30 lux), our standard room lighting (windows occluded, one lighted lamp, 200 lux) and fully lit room (full ceiling illumination with eight fluorescent tubes) resulting in rather bright 2300 lux. The stimulus luminance was 50 cd/m2. The sequence of lighting conditions varied for each subject and followed a balanced permutation of an ABCCBA scheme. Results showed a significant effect (P < 0.01) across lighting conditions, with no relevant difference between the 30 and 200 lux conditions, but a reduction down to 70% at the 2300 lux condition. This obtained across all check sizes and temporal conditions. As an example, the transient PERG P50-amplitudes were as follows: dark, 5.6 +/- 0.8 microV; medium, 5.3 +/- 0.6 microV and bright, 3.8 +/- microV (mean +/- SEM). Peak times decreased significantly with illumination (dark, medium or bright): 45.9 +/- 0.9, 43.1 +/- 0.6 or 40.8 +/- 0.8 ms. Contrast measurements quantitatively explained the noticeable reduction of PERG amplitude at the brightest illumination level simply by straylight, which reduced the display contrast. This suggests that bright sunlight should be excluded, and that lighting conditions should be moderately standardized at low or medium luminance levels for reproducible amplitudes and peak times.

Adult↗
PubMed2002

[Do prisms according to Hans-Joachim Haase improve stereoacuity?].

BACKGROUND: The "Measuring and Correcting Methodology" after H.-J. Haase (MKH) aims at converting "fixation disparity" into bicentral fixation, using prismatic spectacles. In the context of the MKH, fixation disparity is diagnosed by a series of subjective tests. According to H.-J. Haase, a long-standing fixation disparity can lead to "disparate correspondence" between the central areas of both retinae, which consolidates the fixation disparity and gradually converts a "young" into an "old fixation disparity". In "old fixation disparity" it is thought that bicentral fixation does not occur anymore, so that stereoacuity is impaired. However, prismatic spectacles can, according to H.-J. Haase, restitute bicentral fixation and consequently improve stereoacuity, even in some cases of "old fixation disparity". METHODS: Ten non-strabismic subjects with a visual acuity of >/= 1.0 in both eyes were examined. It turned out that all ten had, according to MKH, a "disparate correspondence", 5 subjects with a "young" and 5 with an "old fixation disparity". According to the MKH, a correcting prism was determined. All 10 subjects underwent the automatic Freiburg Stereoacuity Test, without and with the MKH-prism. RESULTS: Without the MKH-prism, the stereoscopic threshold ranged between 1.5 and 14.5 arcsec. With the MKH-prism, the values were not significantly different. CONCLUSION: Stereoacuity ranged between good and excellent in the 5 subjects with "young" as well as in the 5 subjects with "old fixation disparity". The MKH-prism did not improve the stereoacuity in any of the subjects. These results cast doubt on Haase's assertion that an "old fixation disparity" implies a reduced stereoacuity. Hence, the premise for a benefit of the MKH-prism with respect of stereoacuity is not substantiated. In the 5 subjects with a "young fixation disparity", the good stereoacuity is consistent with Haase's theory, so that a benefit of the MKH-prism for stereoacuity was not expected. In previous studies, stereoacuity was found to be better with the MKH-prism than without it. These studies are questionable since learning with repeated testing was not taken into account. We conclude that there is no sound evidence for the assumption that the MKH-prism can improve stereoacuity.

Adult↗
PubMed2002

[Do prisms according to Hans-Joachim Haase influence ocular prevalence?].

BACKGROUND: Ocular prevalence is defined as an unequal weighting of the eyes in the directional perception of stereo objects. Opinions differ as to the cause and relevance of ocular prevalence. Hans-Joachim Haase suggested that ocular prevalence is due to fixation disparity, brought about by incomplete compensation of heterophoria. He further suggested that prismatic spectacles determined by his "measuring and correcting methodology" (MKH) could restore bicentral fixation and thus establish a perceptual balance between both eyes. METHODS: We examined 10 non-strabismic subjects with a visual acuity of > or = 1.0 in both eyes. It turned out that all 10 had a "fixation disparity type II", characterised according to Haase by a "disparate retinal correspondence". All subjects underwent the automatic Freiburg Ocular Prevalence Test, without and with MKH prisms. In addition we examined ocular prevalence under forced vergence and compared ocular prevalence with stereoacuity. RESULTS: Spontaneous ocular prevalence ranged between 1 and 69 %. Averaged over all 10 subjects, ocular prevalence without and with the MKH prisms were not significantly different. Statistical evaluation of single subjects revealed only in one of the 10 a significant difference (Bonferroni-corrected p = 0.001). In the subgroup of 5 subjects who underwent forced vergence, ocular prevalence remained unaltered between 0 and 18 Delta base out. The stereoscopic threshold of all 10 subjects ranged between 1.5 and 14.5 arcsec. There was no correlation between ocular prevalence and stereoscopic threshold (r = - 0.2, p = 0.5). CONCLUSION: Our results indicate that ocular prevalence is largely independent of phoria correction and vergence stress. The excellent stereoacuity of all subjects suggests that ocular prevalence is abandoned for the sake of optimal resolution when very small differences in depth have to be judged.

Adult↗
PubMedDate not supplied

Contrast adaptation: paradoxical effects when the temporal frequencies of adaptation and test differ.

Previous studies of human contrast adaptation employing visually evoked potentials (VEP) have revealed contradictory results, namely, either a reduction or an enhancement in VEP amplitude. In a cross-adaptation experiment, we explored the possibility that differences in the temporal frequency of adapting and test patterns played a role. Phase-reversing checkerboard stimuli [1-deg check size, temporal frequency 8.5 or 17 reversals per second (rps)] served as adaptation and test pattern with contrasts of 0 or 97%. In 13 subjects, we recorded both retinal (PERG) and cortical (VEP) steady-state responses simultaneously. In a balanced block design, all four combinations of the temporal adaptation and test frequencies were employed. Contrast adaptation reduced the PERG amplitude by about 20% in every temporal condition (P < 0.001). The VEP amplitude was strongly affected by adaptation, but the effect differed in magnitude and sign depending on condition: With identical adaptation and test frequency, amplitude was reduced by 15% (P = 0.07) at 8.5 rps and by 38% at 17 rps (P < 0.05). Adapting at 8.5 rps and testing at 17 rps had a tiny (14%) insignificant effect, whereas adapting at 17 rps and testing at 8.5 rps revealed an amplitude enhancement of 27% (P < 0.05). These strong temporal cross-adaptation effects (in the VEP, but not in the PERG) suggest that the adaptable cortical mechanisms (gain control) can be narrowly tuned in their temporal properties. A sizable adaptation effect can even change its sign when varying the temporal frequency by a factor of two. This finding resolves contradictions between previous VEP adaptation studies and reconciles them with psychophysical findings.

Adaptation, Physiological↗
PubMedDate not supplied

Contrast adaptation in retinal and cortical evoked potentials: no adaptation to low spatial frequencies.

Contrast adaptation occurs in both the retina and the cortex. Defining its spatial dependence is crucial for understanding its potential roles. We thus asked to what degree contrast adaptation depends on spatial frequency, including cross-adaptation. Measuring the pattern electroretinogram (PERG) and the visual evoked potential (VEP) allowed separating retinal and cortical contributions. In ten subjects we recorded simultaneous PERGs and VEPs. Test stimuli were sinusoidal gratings of 98% contrast with spatial frequencies of 0.5 or 5.0 cpd, phase reversing at 17 reversals/s. Adaptation was controlled by prolonged presentation of these test stimuli or homogenous gray fields of the same luminance. When adaptation and test frequency were identical, we observed significant contrast adaptation only at 5 cpd: an amplitude reduction in the PERG (-22%) and VEP (-58%), and an effective reduction of latency in the PERG (-0.95 ms). When adapting at 5 cpd and testing at 0.5 cpd, the opposite effect was observed: enhancement of VEP amplitude by +26% and increase in effective PERG latency by + 1.35 ms. When adapting at 0.5 cpd and testing at 5 cpd, there was no significant amplitude change in PERG and VEP, but a small effective PERG latency increase of +0.65 ms. The 0.5-cpd channel was not adapted by spatial frequencies of 0.5 cpd. The adaptability of the 5-cpd channel may mediate improved detail recognition after prolonged blur. The existence of both adaptable and nonadaptable mechanisms in the retina allows for the possibility that by comparing the adaptational state of spatial-frequency channels the retina can discern between overall low contrast and defocus in emmetropization control.

Adaptation, Physiological↗