Biomedical subjects

P Bobak

Publications and source records attributed to P Bobak.

13 recordsLinked to original sources

PubMed1991

Effects of light adaptation on the response characteristics of human oscillatory potentials.

We have examined the response characteristics of the oscillatory potentials (OPs) of the human electroretinogram (ERG) obtained to ganzfeld flash stimuli presented against adapting fields. First, we determined the extent to which the OPs obtained to high luminance flashes change during the course of light adaptation to a cone-isolating adapting field. Regardless of the number of OP wavelets, the last OP wavelet increased in amplitude and decreased in implicit time to a greater extent than did the earlier wavelet(s). In addition, we examined the role of both flash and adapting field luminance in determining the wave form of the OPs. For each adapting field luminance that was tested, the number of OP wavelets increased as flash luminance increased, primarily resulting from the splitting of the last OP into 2 distinct wavelets. While the number of OP wavelets generally decreased as adapting field luminance increased, the amplitude of the last OP became larger. These functional distinctions between the last and the earlier wavelets are consistent with their representing the activity of different retinal generators.

Adult↗

PubMed1989

The (+/-) reference in pattern electroretinogram recording.

In this communication, the value of using a simultaneously acquired estimate of background noise, the (+/-) reference, is demonstrated in pattern electroretinogram (PERG) recording. During the signal averaging process, the (+/-) reference inverts the polarity of every other sweep; cycle-locked (stimulus-related) activity is thus cancelled out, leaving a noise distribution similar in statistical properties to that of the signal average. Assessing the presence of a PERG response by comparing the spectrum analysis of the PERG trace with that of the (+/-) reference is discussed. The results of the normal controls and patient groups are summarized and the raw waveforms from a representative normal observer and a patient with optic nerve damage are analyzed.

Electroretinography↗

PubMed1988

Visual evoked potentials to multiple temporal frequencies. Use in the differential diagnosis of optic neuropathy.

The usefulness of the visual evoked potential (VEP) in differential diagnosis increases when stimulus parameters such as check size and grating orientation are varied. In this study we varied the stimulation frequency. Temporal frequency-specific abnormalities were compared in three patient categories, including retrobulbar optic neuritis (eight patients), pseudotumor cerebri (11 patients), and thyroid eye disease (seven patients). All patients had minimal clinical evidence of optic nerve damage when tested. A 2.3 cycle-per-degree sinusoidal grating of 55% contrast was phase reversed at either 1 or 4 Hz. The P1 latency of the 1-Hz data and the phase at 8 Hz, the second harmonic of the 4-Hz input frequency, were measured. In retrobulbar neuritis, latency (phase) was severely abnormal at both temporal frequencies. In thyroid eye disease, VEP phase was abnormal at 8 Hz while the P1 latency was normal at 1 Hz. The P1 latency and phase were normal in most cases of pseudotumor cerebri. The results suggest differing mechanisms for damage in compressive vs primary demyelinating neuropathies.

Adolescent↗

PubMed1988

Cortical contrast gain control in human spatial vision.

1. We evaluated human visual cortical contrast gain using visual evoked potential (VEP) measurements. The steady-state VEP was elicited by 7.5 Hz contrast modulation of a 6 cycles/deg sinusoidal grating. The stimulus may be regarded as the sum of a steady grating (C) and a counterphase grating of the same spatial frequency (delta C). The counterphase grating is modulated sinusoidally in time. 2. The VEP was measured to combinations of different modulation contrasts (delta C) and different mean levels of grating contrast (C) which produced stimuli with contrast modulation depths (delta C/C) ranging from 0.0625 to 1.0 ('on-off'). 3. The VEP signals were Fourier analysed and the amplitude and phase of the first (7.5 Hz) and second (15 Hz) harmonic frequency components were examined. The monocular VEP to a contrast-modulated grating contains significant first and second harmonic frequency components. 4. The amplitude and phase of the monocular VEP was plotted as a function of delta C for each mean level of contrast explored. The amplitudes of both the first and second harmonic frequency components grow with increasing delta C. However, the slope of each function depends on the mean contrast (C): with higher levels of C, the slope of the function is more shallow. Furthermore, at each level of C the amplitude of the first harmonic frequency saturates at a lower delta C than does the second harmonic frequency component. VEP amplitude is therefore not determined by the absolute contrast change (delta C) alone. The VEP phase of the first harmonic frequency shows less dependence on either modulation or on mean contrast; the phase of the second harmonic frequency component is strongly dependent on mean contrast (C) but not on delta C. 5. When the second harmonic amplitude component of the VEP response (R) is expressed as Ractual/Rmax, where Rmax is the response to C = delta C (i.e. 'on-off'), all second harmonic VEP functions can be well fitted with a power function. This is not the case for the function of the first harmonic amplitude data. 6. A dichoptic VEP was obtained by presenting the steady and counterphase gratings to opposite eyes. The dichoptic VEP, in distinction to the monocular VEP, contains only a second harmonic frequency component. The amplitude of the second harmonic frequency component grows with increasing delta C, similar to the function seen for the monocular VEP.(ABSTRACT TRUNCATED AT 400 WORDS)

Adolescent↗

PubMed1988

Steady-state visual evoked potentials to asymmetrical contrast.

The temporal frequency components in the steady-state visual evoked response (VEP) depend on the method of stimulus presentation; a first harmonic is generated to "on-off" patterns while a second harmonic occurs to both "on-off" and counterphase patterns. This study examined the VEP response in humans to patterns between these two extremes. In the main experiment, a 1 c/d sinusoidal grating was phase reversed sinusoidally at 8 Hz. The DC offset, however, was adjusted such that two different levels of peak contrast occurred during a temporal cycle. Within this context, a counterphase pattern would represent complete contrast symmetry and an "on-off" pattern would represent maximum contrast asymmetry during a temporal cycle. With this manipulation, the a) integrated luminance change; b) local luminance change; and c) total contrast remained constant. Only mean contrast varied. The amplitude of the first harmonic strongly depended on mean contrast. The amplitude and phase of the second harmonic, however, changed little across experimental conditions suggesting a dependence on one or more of the above three listed stimulus attributes which remained constant. In a supplementary experiment, this phase constancy was confirmed using other temporal frequencies.

Adult↗

PubMed1987

The effect of blur and contrast on VEP latency: comparison between check and sinusoidal and grating patterns.

Pattern defocusing was used to evaluate the contribution of different spatial frequency components in checks to VEP latency. Latency shifts with increasing blur (-2.5 to +2.5 diopters) were determined for sinusoidal grating and check patterns equated in fundamental spatial frequency. With both check and grating patterns, the effect of blur was greater the higher the spatial frequency. Given an equal fundamental spatial frequency, however, the latency of checks was more effected. This latency difference between check and sine patterns was pronounced at low fundamental spatial frequencies (large pattern) and decreased with higher spatial frequencies (small pattern). Latencies were then compared for patterns which were defocused vs. simply reduced in contrast. Results show that the increase in latency with defocused large checks is due to both fundamental and higher harmonic spatial components but with small checks, to the fundamental spatial frequency alone.

Adolescent↗

PubMed1987

Visual dysfunction in Parkinson's disease. Loss in spatiotemporal contrast sensitivity.

Flicker sensitivity and spatial contrast sensitivity (CS) were examined in a total of 99 patients with Parkinson's disease (PD). All patients were undergoing treatment with dopaminergic agents. Specific losses in sensitivity observed in PD were (1) a loss in flicker sensitivity which was most pronounced around the peak of the function (8 Hz) and (2) a loss near the peak of the spatial CS curve, often with no noticeable low frequency attenuation. Several PD patients affected by the 'on-off' syndrome were tested in both 'on' and 'off' phases, and the results show that the CS function switches in parallel with motor symptoms of the disease. These data suggest that not only is the visual system affected in PD, but that dopamine may have an essential role in receptive field organization in human vision.

Adult↗

PubMed1987

The visual evoked potential contrast response function in recovering optic neuritis.

VEP contrast response functions were obtained in 7 patients with recovered optic neuritis and in 6 age-matched controls. A 1.5 c/d sinusoidal grating was counterphase modulated at 4 Hz (8 reversals/sec) about a mean screen luminance of 141 cd/m2. Grating contrasts ranged from 2% to 85% and were randomly presented. In all control observers, VEP amplitude was a nonmonotonic function of contrast; an amplitude inflection occurred between contrasts of 33% to 47% followed by a sharp increase in amplitude at higher contrasts. The slope of the function relating VEP phase to contrast was variable between observers at low contrasts but consistently "flat" at high contrasts. In spite of normal acuity and contrast sensitivity, VEP amplitude was severely attenuated at high contrasts in all optic neuritis eyes but there were no contrast-dependent phase abnormalities. The results were directed toward the issues of a) suprathreshold contrast processing in optic neurtis, b) relating a nonmonotonic VEP amplitude function to the contrast-segregating properties of single cells, and c) the relationship between VEP abnormalities and visual performance.

Adult↗

PubMed1986

Temporal frequency-dependent VEP changes in Parkinson's disease.

We recorded steady-state (4.19 and 8.41 Hz) VEPs in 17 Parkinson's Disease (PD) patients and 18 control observers using on-off temporal modulation of a 2.3 c/deg sinusoidal grating. With 4.19 Hz, 20 out of 33 PD patient eyes had abnormal VEPs. However, only 8 of 33 eyes were abnormal with 8.41 Hz. "Routine" (counterphase) transient VEPs revealed delayed VEPs for only 7 out of 24 eyes of the patients. These findings suggest an "input" temporal frequency-dependent abnormality in the foveal pathway.

Adult↗

PubMed1984

VEPs in humans reveal high and low spatial contrast mechanisms.

The effect of contrast on visual evoked potential (VEP) amplitude was examined in nine observers. A 6.0 cycles/deg (cpd) grating was modulated in an "on-off" mode at 7.5 Hz. The VEP response contains significant first and second harmonic components: their growth with contrast is parallel, each function consisting of two limbs. The data are consistent with the hypothesis that the pattern VEP obtained with "on-off" presentation may reflect the contributions of "low" and "high" contrast neuronal populations demonstrated in physiological studies of the primate.

Evoked Potentials, Visual↗

PubMed1983

Pattern electroretinograms and visual-evoked potentials in glaucoma and multiple sclerosis.

Steady-state visual-evoked potentials and electroretinograms were simultaneously recorded in four patients with glaucoma and in five patients with multiple sclerosis. The stimuli included a homogenous field and a 2.3 cycles per degree sinusoidal grating that were counter-phase modulated at the rate of 7.5 Hz. We used narrow bandwidth spectral analysis to measure the response amplitudes and signal-to-noise ratios. Transient pattern visual-evoked potentials (1 Hz) were also measured for latency in each eye. We found abnormal pattern electroretinograms, based on the absence of a significant second harmonic component, in three of the four glaucomatous eyes although the homogenous field electroretinograms were normal. In the patients with multiple sclerosis, the pattern electroretinograms were abnormal in two eyes, but the transient visual-evoked potential latency had the highest diagnostic yield (seven of ten eyes).

Adult↗

PubMed1983

On the possible role of temporal delays of afferent processing in Parkinson's disease.

Visual evoked potential (VEP) latency is one measure of afferent delays in Parkinson's disease. New aspects of temporal processing were studied in the VEP of 7 patients and 10 normals. For this analysis the patterned stimulus was simultaneously modulated at two temporal frequencies. Phase shifts of non-linear VEP components in patients, compared to normals, revealed that an abnormality of temporal processing in this disease must be caused by altered neuronal interactions, not only by simple conduction defects.

Adult↗

PubMed1982

Pattern visual evoked potentials in the rat.

Pattern and flash visual evoked potentials (VEPs) were obtained in 25 adult male hooded rats. Different electrode placements were used to detect the origin of the VEPs. The spatial frequency and temporal modulation of the stimulating pattern and the refractive state of the eye were varied to investigate the specificity of these factors in pattern VEPs. Variations in latency, amplitude, and trace morphology were found which were consistent with the characteristics of the visual system of the rat.

Animals↗