Biomedical subjects

T Meigen

Publications and source records attributed to T Meigen.

14 recordsLinked to original sources

PubMed2002

[The reproducibility of multifocal ERG recordings].

PROBLEM: Multifocal electroretinogram recordings (mfERG) can be used to detect a local dysfunction of the retina. In this study we tested both the intrasessional and inter-sessional reproducibility of mfERG amplitudes. METHODS: MfERGs from 6 eyes of 6 normal subjects were recorded on two different days using DTL electrodes. The relative coefficient of variation ( RCV) was used to quantify the amplitude reproducibility. We tested the effect of (a) session (inter- vs. intrasessional), (b) recording duration (7.3 vs. 3.6 min), (c) trace type (hexagon traces vs. ring averages), and (d) amplitude definition (peak-trough analysis vs. scalar product) on RCV. RESULTS: RCV was 6.5+/-0.4% (Mean+/-SEM, n=96) when averaged across all recording conditions and all subjects. The ANOVA showed a significant difference ( p=0.018) between hexagon traces and ring averages. Another significant effect ( p=0.016) occurred for the interaction of (a) and (b). CONCLUSION: MfERGs can be recorded with a high degree of reproducibility even for short recording durations and single hexagon traces. As the factor (a) did not show a significant effect, the new placement of the DTL electrode in the second session does not necessarily increase the retest variability compared to a second recording within the same session.

Artifacts↗

PubMed2001

[Reducing the duration of pattern ERG recordings by "time varying filtering"].

PROBLEM: In 1981, de Weerd proposed the method of time varying filtering (TVF) for estimating the "true" signal of an electrophysiological response if a noise response was additionally recorded. We studied whether this method can be used to predict the average pattern ERG (PERG) of all trials from a smaller subset of trials. METHODS: A total of 10 subjects with normal acuity participated in the experiment. Transient PERGs to reversing checkerboard patterns were recorded from both eyes using DTL electrodes. We calculated the mean deviation for partial averages of 6, 12, 20, 30, and 60 trials from the average PERG of all 120 trials. For each partial average, an additional noise response was derived by averaging the trials with alternating signs. RESULTS: When using TVF, the number of trials could be reduced by a factor of 1.43 to yield a PERG of similar quality to classical averaging. CONCLUSION: TVF allows the estimation of a "true" PERG response. This method can be used to significantly reduce the duration of PERG recordings.

Adult↗

PubMed2000

Summation of texture segregation across orientation and spatial frequency: electrophysiological and psychophysical findings.

Objects are usually segregated from ground by several visual dimensions. We studied texture segregation in checkerboards defined by gradients in spatial frequency, orientation or both frequency and orientation, using Gabor-filtered noise patterns. Saliency was measured electrophysiologically using the visual evoked potential (VEP) associated with texture segregation ('tsVEP') (an associated component in the visual evoked potential), and psychophysically by a 2AFC task. Spatial frequency and orientation stimuli evoked percepts of texture segregation and tsVEPs in all 11 subjects. The tsVEPs to combined stimuli were larger than those to each dimension alone, but smaller (74%) than the algebraic sum of tsVEPs to both individual dimensions. Psychophysical detection rates differed significantly between all conditions (P < 0.001), with highest rates for the combined stimuli. The findings suggest that segregation based on a combination of 'orientation' and 'spatial frequency' is more salient than that based on either of these alone. The significant deviation from full additivity in the tsVEPs suggests that simultaneous contrasts in spatial frequency and orientation have a common processing stage.

Evoked Potentials, Visual↗

PubMed2000

Standard for pattern electroretinography. International Society for Clinical Electrophysiology of Vision.

The pattern electroretinogram (PERG) is a retinal response evoked by viewing an alternating checkerboard or grating. It receives clinical and research attention because it can provide information about inner retinal cells and the macula. However, clinicians may have trouble choosing between different techniques for recording the PERG that have been described in the literature. The International Society for Clinical Electrophysiology of Vision has prepared a standard for a basic PERG recording procedure to aid new users in obtaining reliable responses and to encourage more uniformity among existing users.

Clinical Protocols↗

PubMed1999

On the statistical significance of electrophysiological steady-state responses.

Steady-state stimulation is a useful paradigm in many physiologic and clinical situations, for ERG, Pattern-ERG and VEP. One of the advantages is the easy evaluation of the response via Fourier analysis. However, the question whether a given response is statistically significant or not has received little attention so far, although it is especially relevant in high noise, low amplitude recordings, as often occur in pathologic conditions. A given response is statistically significant if it is unlikely that its value is due to noise fluctuations. Thus appropriate estimates of noise and response are required. We have analytically derived formulas for the statistical significance of a given signal-to-noise-ratio s, based on two different estimates of noise: (1) Noise estimate by a 'no stimulus' recording, or by a '+/-average'. The former needs an additional recording, the latter can simultaneously be calculated as the standard average. (2) Noise is estimated as the average of the two neighboring spectral lines (one below, and one above the response frequency). Analytical solutions were obtained for both noise estimates that can easily be evaluated in all appropriate recordings. Noise estimate (1) performs much poorer than noise estimate (2), as can be seen from the following landmark values: Typical significance levels of 5%, 1%, and 0.1% require s values of 4.36, 9.95, and 31.6 (1), and 2.82, 4.55, and 8.40 (2). The noise estimate based on the neighboring frequencies can be easily applied after recording, provided that the noise spectrum is reasonably smooth around the response and frequency-overspill was avoided. It allows a quantitative assessment of low responses in physiological threshold analyses and pathological conditions, e.g., 'submicrovolt flicker-ERG'.

Electroretinography↗

PubMed1999

Do's and don'ts in Fourier analysis of steady-state potentials.

Fourier analysis is a powerful tool in signal analysis that can be very fruitfully applied to steady-state evoked potentials (flicker ERG, pattern ERG, VEP, etc.). However, there are some inherent assumptions in the underlying discrete Fourier transform (DFT) that are not necessarily fulfilled in typical electrophysiological recording and analysis conditions. Furthermore, engineering software-packages may be ill-suited and/or may not fully exploit the information of steady-state recordings. Specifically: * In the case of steady-state stimulation we know more about the stimulus than in standard textbook situations (exact frequency, phase stability), so 'windowing' and calculation of the 'periodogram' are not necessary. * It is mandatory to choose an integer relationship between sampling rate and frame rate when employing a raster-based CRT stimulator. * The analysis interval must comprise an exact integer number (e.g., 10) of stimulus periods. * The choice of the number of stimulus periods per analysis interval needs a wise compromise: A high number increases the frequency resolution, but makes artifact removal difficult; a low number 'spills' noise into the response frequency. * There is no need to feel tied to a power-of-two number of data points as required by standard FFT, 'resampling' is an easy and efficient alternative. * Proper estimates of noise-corrected Fourier magnitude and statistical significance can be calculated that take into account the non-linear superposition of signal and noise. These aspects are developed in an intuitive approach with examples using both simulations and recordings. Proper use of Fourier analysis of our electrophysiological records will reduce recording time and/or increase the reliability of physiologic or pathologic interpretations.

Artifacts↗

PubMed1997

Shift of equiluminance in congenital color vision deficiencies: pattern-ERG, VEP and psychophysical findings.

We compared electrophysiological responses [pattern-ERG (PERG) and VEP] and psychophysical measures to color stimuli to separate different forms of anomalous color vision. PERG and VEP were recorded from seven normals and 14 subjects with congenital color vision deficiencies. Stimuli were color checkerboards with 0.5 deg check size, phase reversing at 34 rev/sec. The luminances of the red and green parts were varied in opposite direction from 0 to 30 cd/m2, while the hue of individual squares and space-averaged luminance were held constant. This allowed for one equiluminance condition where flicker appeared fused. In the seven normals, the subjective equiluminance was reached at a luminance ratio red/(red + green) = 0.50-0.53. At that point, the PERG amplitude was moderately, and the VEP amplitude sharply reduced. In 14 color anomalous subjects both the PERG and VEP were sharply reduced at equiluminance. These dips were shifted compared to normals and the dip position corresponded to the predicted luminance ratios obtained by calculations from L- and M-cone activation using the Smith-Pokorny transformation. As we found a close correlation of the VEP-dip position and the anomalous quotient, these electrophysiological measures may allow objective assessment of color vision deficiencies.

Adult↗

PubMed1997

Similar electrophysiological correlates of texture segregation induced by luminance, orientation, motion and stereo.

Certain local features induce preattentive texture segregation. Recently, components in the visual evoked potential (VEP) associated with preattentive texture segregation (tsVEPs) have been demonstrated. To assess the similarity and dissimilarity of visual processing across visual dimensions, we compared VEPs and tsVEPs in texture segregation by luminance, orientation, motion and stereo disparity. We found tsVEPs across these four visual dimensions to be remarkably similar when compared to the "low-level" VEPs. The tsVEPs were always negative; their implicit time, peak latency and amplitude were (in msec/msec/microV): 91/234/-5.7, luminance; 84/257/-3.9, orientation; 80/295/-8.3, motion; and 95/310/-5.0 for stereo. The cross-correlation function, as a quantitative measure for similarity, on average was higher for the tsVEPs by a factor of 4.2 as compared to the low-level VEPs (P < 0.0001). The results suggest (1) that the tsVEPs represent activity of neural mechanisms that have generalised to some degree across visual dimensions; and (2) that these hypothetical generalisation mechanisms might exist already in the primary visual cortex.

Evoked Potentials, Visual↗

PubMed1997

Raster-scan cathode-ray tubes for vision research--limits of resolution in space, time and intensity, and some solutions.

Raster-based cathode-ray tubes (CRTs) are increasingly used for stimulus presentation. While very flexible, their design based on consumer electronics can limit their value in vision research. Here their limitations of resolution in time, space, intensity and wavelength are systematically compiled. Often, ingenious ideas can circumvent such limitations for specific experiments. Some ad-hoc solutions, as well as the more general techniques of dithering and anti-aliasing, are presented.

Artifacts↗

PubMed1994

Asymmetries in preattentive line detection.

We tested the relative contribution of absolute orientation and collinearity to asymmetries in preattentive line detection. Subjects reported the location of a single line that was tilted by 16 deg against the orientation of the background lines. Perceptual pop-out strength was quantified as the threshold presentation time to correctly locate the salient element. We found that (1) threshold times were significantly shorter in vertical than in oblique background lines and that (2) threshold times were significantly shorter in collinear than in non-collinear background lines. Our data suggest that the sensitivity of preattentive processes for orientation gradients is modulated by both collinearity and absolute orientation.

Humans↗

PubMed1994

Asymmetries in texture perception.

Single elements in textures segregate preattentively from the background if they differ in special features from surrounding elements (pop-out). Oblique targets on a background of vertical line elements are more easily detected than vertical targets on a ground of oblique elements. We wanted to find out whether this perceptional asymmetry depends on the absolute orientation or on the alignment of the background lines. Using the method of constant stimuli, we measured the thresholds of detection times in five subjects using four different arrangements of line patterns. The subjects indicated the position of a salient element in a forced-choice procedure. For all subjects, detection times were shortest if background lines were either vertical or collinear. Otherwise, detection times were significantly longer by a factor of 3. Figure-ground segregation is facilitated if background lines are either vertical or collinear.

Form Perception↗

PubMed1993

Perceptual ranking versus visual evoked potentials for different local features in texture segregation.

PURPOSE: To validate recent reports of specific visual evoked potentials associated with preattentive texture segregation (tsVEPs) and to quantitatively compare perceptual segregation strength and tsVEP amplitudes across different local features. METHODS: Four local features were selected: crossed vs noncrossed, line segments differing in orientation by 90 degrees, U-shapes differing in orientation by 90 degrees, and U-shapes differing in orientation by 180 degrees. The two variants of each local feature were spatially arranged in a checkerboard pattern; for the first three features this led to pop-out of a "preattentive checkerboard." In seven subjects, perceptual segregation strength was assessed using ranking, and tsVEPs were recorded in these and three additional subjects. RESULTS: Statistically significant tsVEPs were obtained for the features crossed vs noncrossed and 90 degrees-line segments. Ranking results and tsVEP amplitudes were highly correlated (P < 0.001); the order of perceptual ranking and the order of tsVEP amplitudes were identical; 180 degrees-U-shapes had lowest ranking, mean tsVEP amplitude was 0.1 microV, close to noise. Line segments had maximal ranking, tsVEP amplitude was 1.5 microV. CONCLUSIONS: The results suggest that tsVEPs quantitatively reflect the activity of cortical mechanisms involved in texture segregation across various features.

Adult↗

PubMed1992

Electrophysiological correlates of texture segregation in the human visual evoked potential.

We investigated whether the visual evoked potential (VEP) reflects cortical processing associated with preattentive texture segregation. On a visual display unit we presented stimuli with various arrangements of oriented line segments that either led to the appearance of a "preattentive" checkerboard or did not. Two presentation modes were used (pattern onset at 1 Hz and rapid pattern change at 4.3 Hz), while luminance (57 cd/m2) and contrast (92%) of the line segments remained constant. VEPs were recorded in 7 human subjects. The VEP was analyzed as a linear combination of putative components, which are evoked by either local pattern, quasi-local orientation contrast or global preattentive structure. In the transient VEP, we found a negativity over the posterior pole at a latency between 161 and 225 msec (FWHM) in the linear combination designed to extract segregation-specific components. Peak amplitude reached 3.1 +/- 0.8 microV (mean +/- SEM) at 199 msec. This negative peak appeared only for textures containing orientation contrast. Steady-state analysis of the rapid presentation also revealed a significant component (P = 0.002) associated with texture segregation. These potentials either represent processing of orientation contrast or global processing of texture segregation. The results suggest that specific surface potentials, differing from cognitive potentials, can be derived which are associated with preattentive processing.

Adult↗

PubMedDate not supplied

Electrophysiological correlates of human texture segregation, an overview.

'Texture segregation' results from parallel processing in the visual cortex. It occurs when the stimulus contains spatial gradients within a visual dimension. We here present an introductory overview of the field, concentrating on electrophysiological correlates in the human EEG ('tsVEPs') of the neuronal processes underlying texture segregation. We describe the isolation of the tsVEP from the background EEG, give examples of the correlation between saliency and tsVEP amplitude and compare texture segregation between visual dimensions.

Animals↗