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Michael Bach

Publications and source records attributed to Michael Bach.

At least 37 records · Page 2Linked to original sources
PubMed2004

High-frequency oscillations in human visual cortex do not mirror retinal frequencies.

Flash stimulation elicits oscillatory responses above 100 Hz in human visual cortex. It has been proposed that these are the result of retinal oscillations being directly relayed through the visual pathway to area V1. Experimental evidence, however, is scarce and contradictory. To address this issue, we performed a time-frequency analysis of simultaneously recorded retinal and cortical potentials. Matching frequencies would support the assumption of a direct relationship between retinal and cortical activities. In 4 of 7 subjects the frequency was significantly lower in the cortex than in the retina and in one subject it was significantly higher. The differences were in the range of 10-34 Hz and suggest that the cortical oscillations are not a simple echo of their retinal counterparts.

Electroencephalography↗
PubMed2004

The influence of defocus on multifocal visual evoked potentials.

BACKGROUND: In order to assess the influence of optical factors on the multifocal visual evoked potential (mfVEP), we obtained mfVEPs with optimal refraction and compared them to recordings with various degrees of dioptrical defocus. METHODS: Monocular mfVEPs were recorded from the right eye in eight normal subjects. Dartboard stimuli with 60 sectors arranged in six concentric annuli spanning 60 degrees were generated with a VERIS system and presented on a computer monitor. Two pairs of electrodes were placed 3 cm above and below and 3 cm to the right and left of the inion. Two sets of mfVEP records per subject were obtained, one with best-corrected visual acuity and another when the stimulus was defocused by +1.0, +2.0 or +3.0 D. A signal-to-noise ratio (SNR) measure was calculated for every response from the two channels. RESULTS: The effect of defocus depended on eccentricity: when defocus was at +2.0 D and higher, reducing visual acuity to <0.3, the central mfVEP responses were reduced to approximately 60%, while defocus had no marked effect at eccentricities >7 degrees. CONCLUSIONS: The results suggest that, in contrast to the mfERG, the mfVEP requires optimal refraction to correctly assess the cortical responses.

Adult↗
PubMed2004

Electrophysiological evidence for independent speed channels in human motion processing.

A variety of psychophysical studies suggests that motion perception in humans is mediated by at least two speed-tuned channels. To study the neurophysiological underpinnings of these channels in the human visual cortex, we recorded visual evoked potentials (VEPs) to motion onset. We applied an adaptation paradigm that allowed us (a) to isolate and extract direction-specific cortical responses and (b) to assess cross-adaptation in the speed domain. VEPs resulting from the onset of left- or rightward motion at either low or high speeds were recorded from three occipital recording sites in 11 subjects. For each of these test stimuli, responses were collected after adaptation to one of five different conditions: a static adaptation pattern (baseline), adaptation to low-speed motion (3.5 degrees/s) either in the same or in the opposite direction as the test, or adaptation to high-speed motion (32 degrees/s) either in the same or in the opposite direction as the test. We report considerable direction-specific adaptation for same adaptation and test speeds (by 28-37% of baseline response; p <.002), whereas there was no direction-specific adaptation across speeds. We supplement these electrophysiological data with corresponding psychophysical results. The lack of direction-specific cross-adaptation in the speed domain demonstrated with physiological and psychophysical techniques supports models of at least two speed-tuned channels in the human motion system.

Adaptation, Ocular↗
PubMed2004

Simulated nystagmus suppresses pattern-reversal but not pattern-onset visual evoked potentials.

OBJECTIVE: The aim of this study is to quantify and compare the effects of simulated horizontal nystagmus on pattern-reversal and pattern-onset visual evoked potentials (VEPs). METHODS: In eight visually normal subjects with normal oculomotor behaviour, we monitored eye movements and recorded pattern-reversal and pattern-onset VEPs from occipital electrodes. Subjects viewed the stimulus monocularly via a mirror, which was placed close to the eye and driven by a scanner at four different amplitudes (0, 1, 2, and 3 degrees ) with a 4 Hz saw-tooth waveform to simulate horizontal jerk-nystagmus. RESULTS: Retinal image motion nearly abolished the pattern-reversal VEPs (maximal reduction by 85%; mean reduction by 72%, P<0.001), while there was a non-significant reduction (mean reduction by 15%) of the pattern-onset VEPs. CONCLUSIONS: The differential effect of simulated nystagmus on pattern-reversal and pattern-onset VEPs resembles that reported in studies on nystagmus patients. We conclude that the interaction of retinal image motion with the stimulus is sufficient to explain the reduction of pattern-reversal VEPs in patients with nystagmus and propose simulated nystagmus as a useful tool to test the influence of nystagmus on the efficiency of VEP stimuli. SIGNIFICANCE: This study demonstrates how horizontal jerk-nystagmus can be simulated and suggests possible mechanisms by which nystagmus reduces VEP responses.

Adult↗
PubMed2004

Texture segregation in traumatic brain injury--a VEP study.

Visual evoked potentials (VEPs) were recorded to textures segregated by gradients in orientation or motion. Recordings were obtained in traumatic brain-injured (TBI) subjects and in normal controls. We analyzed both the low-level VEPs (llVEPs) evoked by homogenous stimuli, as well as the components associated with texture segregation (tsVEP) obtained through an appropriate linear combination. Our results suggest that the tsVEP, presumably higher up in the visual processing chain than the llVEP, is sensitive to TBI and can reveal further information as to the nature of possible information processing deficits after TBI. It could also help quantify cortical damage that is not revealed with more standard clinical tools.

Adult↗
PubMed2004

Different effect of dioptric defocus vs. light scatter on the pattern electroretinogram (PERG).

To assess the quantitative effect of optical deficiencies on the Pattern Electroretinogram (PERG), we degraded the optical imaging onto the retina by dioptric defocus (+1.75-5 D), and by scatter transparencies. With optimal and degraded optics we measured the visual acuity and the steady-state PERG, evoked by checkerboards (check size 0.2-16 degrees), and flash stimuli. When the two optical degradation methods were equated for acuity, scatter transparencies reduced the PERG amplitude more than defocus. For scatter transparencies, there was a significant (p < 0.001) effect at 16 degrees, escalating for smaller checks. For defocus, significant effects were only observed for check sizes of 1.6 degrees and smaller. We explain the differences between the two blur techniques by their differing angular scattering functions. Dioptrical defocus leads to an underestimation of the effect of media opacities on the PERG. For the clinically relevant check size of 0.8 degrees, halving acuity reduces the amplitude by 15% with defocus and by 30% with scatter. With an acuity of 0.8 or higher, no clinically relevant effect of suboptimal refraction is seen.

Electroretinography↗
PubMed2004

Early neural activity in Necker-cube reversal: evidence for low-level processing of a gestalt phenomenon.

Normally we experience the visual world as stable. Ambiguous figures provide a fascinating exception: On prolonged inspection, the "Necker cube" undergoes a sudden, unavoidable reversal of its perceived front-back orientation. What happens in the brain when spontaneously switching between these equally likely interpretations? Does neural processing differ between an endogenously perceived reversal of a physically unchanged ambiguous stimulus and an exogenously caused reversal of an unambiguous stimulus? A refined EEG paradigm to measure such endogenous events uncovered an early electrophysiological correlate of this spontaneous reversal, a negativity beginning at 160 ms. Comparing across nine electrode locations suggests that this component originates in early visual areas. An EEG component of similar shape and scalp distribution, but 50 ms earlier, was evoked by an external reversal of unambiguous figures. Perceptual disambiguation seems to be accomplished by the same structures that represent objects per se, and to occur early in the visual stream. This suggests that low-level mechanisms play a crucial role in resolving perceptual ambiguity.

Adult↗
PubMed2004

Direction tuning of human motion detection determined from a population model.

Mass responses that are obtained using electrophysiological or psychophysical techniques are inadequate to characterize motion detectors at the single-unit level. Therefore, we have modelled a population of motion detectors and fitted their mass response to motion-onset EEG data. By examining a single unit of the modelled population we could assess the range of directions and the level of adaptation an individual motion detector responded to. Stimuli were patterns of randomly distributed dots. After subjects had adapted to either a non-moving pattern or to motion in one fixed direction, the pattern moved in one of seven possible test directions. The population model assumed elementary motion detectors with a Gaussian angle-selectivity profile. The population response was calculated as the sum of all adaptation-weighted individual responses to the test direction. In all subjects, we observed sizeable effects of adaptation on test directions close to the adapted direction and small amounts of direction-independent adaptation for all test directions. The model fit explained 75% (Oz) and 94% (lateral derivation) of the total variance and revealed a motion detector bandwidth (full width at half maximum) of 72 degrees (Oz) and 62 degrees (lateral derivation). The maximal adaptation depth was estimated as 72% (Oz) and 66% (lateral derivation). Estimates inferred from the population model representing human motion detectors were found to be close to those obtained by single-cell data from non-human primates.

Electrooculography↗
PubMed2003

Sleep disturbances in somatoform pain disorder.

UNLABELLED: Patients with chronic somatoform pain often complain about sleep disorders. However, sleep disorder/disturbances are not an integrated part of the somatoform disorders in the DSM-IV and the ICD-10. Sleep is important for recreation. Deprivation of deep sleep stages is experimentally linked to muscle pain. Therefore, sleep disorder may play an important part in the persistence of somatoform pain disorder. The aim of the study was to evaluate the frequency of sleep disorder in patients with somatoform pain disorder and to correlate it with comorbid depression, pain parameters and psychosocial parameters. METHOD: In this study, 147 patients (mean age: 48.8 years; SD: 11.0) with the diagnosis of a somatoform pain disorder were studied with regard to affective comorbidity, pain duration (months), maximum pain within the last month, minimum pain within the last month and medium pain within the last month, psychosocial disability within the last month and the presence of a sleep disorder. RESULTS: Eighty-four percent of the patients had a sleep disorder. The patients with a sleep disorder had significantly higher maximum and medium pain, a significantly higher level of psychosocial disability and a significantly lower overall subjective well-being. The medium pain and psychosocial disability in leisure and social activities are significant predictors for sleep disorder. CONCLUSIONS: The presence of a sleep disorder may be a hint for higher pain intensity and a higher level of psychosocial disability. Sleep disorder may be a factor in the persistence and aggravation of pain as well as psychosocial disability. Therefore, sleep disorder should be integrated in the therapeutic targets. It is suggested that sleep disorder should be a diagnostic criterion in somatoform pain disorder.

Adult↗
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↗