Biomedical subjects

P Lachapelle

Publications and source records attributed to P Lachapelle.

At least 19 recordsLinked to original sources

PubMed1992

Comparative effects of luminance and scatter on the pattern visual evoked potential and eye-hand reaction time.

We investigated the effect of reduced luminance and increased scatter on the pattern visual evoked potential and eye-hand reaction time evoked to a check size of 0.5 degrees in 10 normal subjects. Data analysis indicated that a reduction in luminance as well as an increase in scatter caused a statistically significant increase in the peak time of the pattern visual evoked potential P100 wave. The reaction time, however, was not significantly affected by the initial 0.9-log unit attenuation of the stimulus luminance or the 0.3 scatter filter. Further attenuation of luminance or increase of scatter also yielded statistically significant increases. Our results suggest that the reaction time is less affected by a reduction in luminance or an increase in scatter of a 0.5 degrees stimulus than the pattern visual evoked potential is and therefore represents a more reliable test to assess visual function, especially in the presence of medial opacities, which are known to reduce luminance and produce scatter.

Adolescent↗

PubMed1992

Influence of remote targets on directionality of striate neurons in rabbits.

The described investigations study the influence of additional targets located well outside the classical receptive field on responses to motion of cortical cells in rabbits. Animals are anesthetized and prepared for acute single cells recordings in a conventional manner. The interactions between remote targets and central stimuli are abolished with microinjections of lidocaine hydrochloride or GABA at the site excited by remote stimuli. Results show that responses to motion of cortical cells are particularly sensitive to these manipulations. Although supplementary targets fail to influence spontaneous activity of all cells, they do influence responses to motion. Overall, the directionality indices (DI) declined. (53 to 45.) This decline may express itself either by a decrease of responses in the preferred direction or an enhancement of responses in the non-preferred direction or both. By contrast, responses to stationary stimuli are unaffected by additional targets in the visual field. Globally, cells whose directionality index was superior to 50% were significantly more affected then cells whose DI was less than 50%. This result suggests that similarly to cats, the directionality of cells in the striate cortex rests on a very fragile convergence of excitatory and inhibitory influences.

Animals↗

PubMed1992

Evoked potentials are modified by long term exposure to trichloroethylene.

Two groups of New Zealand albino rabbits were respectively exposed to 350 and 700 ppm of trichloroethylene (TRI) 4 hrs/day, 4 days/week for 12 weeks. Weekly, visual evoked potentials (VEP) recordings were obtained under mesopic condition. Blood samples were also collected weekly to determine the concentration of TRI and its metabolites. Recordings from the 350 ppm group showed a significant (p less than 0.001) decrease in the amplitude of VEPs, while a significant (p less than 0.001) increase was observed in the 700 ppm group. Both effects were reversed to baseline values within six weeks after the last exposure. The observed modifications in VEP amplitudes were related to blood level of trichloroethanol. These results thus confirm the neuro-ophthalmotoxicity of TRI and support the hypothesis that trichloroethanol is a reliable marker of the effective neurotoxic dose of this organic solvent.

Animals↗

PubMed1991

Evidence for an intensity-coding oscillatory potential in the human electroretinogram.

Electroretinograms (ERGs) were obtained from normal subjects to various combinations of stimulus and background intensities to examine if, as shown in rabbits, one of the oscillatory potentials (OP2) could be identified as reflecting the absolute intensity of the stimulus (i.e. the intensity irrespective of the state of retinal adaptation). The results presented clearly demonstrate that the peak time and the amplitude of OP2 are highly correlated with the absolute intensity of the stimulus. Linear regression analysis (for light- and dark-adapted data combined) yields a correlation coefficient of r = 0.98 for the peak time of OP2 and r = 0.88 for the amplitude of OP2. The latter equations were obtained from ERGs where the brightest flashes were delivered in light-adaptation and the dimmest in dark-adaptation. This is the first demonstration of a high correlation between the intensity of the stimulus and the amplitude and peak time of a human ERG component which is shown to be linear irrespective of the state of retinal adaptation.

Adolescent↗

PubMed1991

Temporal relationship between the ERG and geniculate unit activity in rabbit: influence of background luminance.

The comparative analysis of the retinal and lateral geniculate nucleus (LGN) intensity-response function revealed that the timing of the LGN unit response was highly correlated to that of one oscillatory potential (OP2). To examine if this OP2-LGN intensity-response function was retained irrespective of the state of retinal adaptation, we performed simultaneous recordings to the ERG and single-cell unit activity at the geniculate level evoked to a flash of constant energy while the level of background luminence was varied. For a stimulus of 6.7 cd m-2 sec, the shifts in latency induced by increasing the background luminance from 0 to 125 cd m-2 are of 1.17 +/- 0.61 msec for OP2, a value almost identical (P greater than 0.10; n = 36) to the one obtained at the geniculate level (1.11 +/- 0.88 msec). However, when a dimmer flash is used, the latency shifts are not so well correlated. The latter could be partly explained by the threshold nature of the resulting stimulus (i.e. high photopic background combined with a dim flash).

Action Potentials↗

PubMed1991

The effect of a slow flicker on the human photopic oscillatory potentials.

Flicker-induced modifications of the human photopic oscillatory potentials (OPs) were investigated with the use of flash stimuli of 0.89 and 8.9 cd m-2 sec in strength. When the dimmest stimulus is used, increasing the rate of presentation from 0.5 to 20 Hz augments the amplitude and peak time of OP2. For a brighter stimulus, the 10 Hz flicker significantly reduces the amplitude of OP2 and OP3, increases the peak time of OP2 and reduces that of OP4. The 20 Hz flicker increases the peak time and reduces the amplitude of OP2 and completely abolishes OP3, while it has no significant impact on the amplitude and timing of OP4. The data presented support the claim that each OP making the photopic response represent independent electrophysiological entities.

Adolescent↗

PubMed1990

The effect of iodoacetic acid on the electroretinogram and oscillatory potentials in rabbits.

It has been shown that a single injection of iodoacetic acid selectively (but temporarily) abolishes the b-wave of the electroretinogram. We examined whether such use of this chemical further substantiate our claim that the b-wave of the electroretinogram is a composite potential resulting from the summation (or integration) of faster retinal potentials, usually referred to as the oscillatory potentials. Full-field electroretinograms were recorded from adult New Zealand rabbits before and after a single, bolus injection of 15 mg/kg of buffered iodoacetic acid. Both the 1-1000 Hz electroretinogram and the 100-1000 Hz oscillatory potentials were recorded simultaneously. The oscillatory potentials considered in this study were those normally seen on the rising phase of the b-wave. Following the intravenous injection of iodoacetic acid, there was a progressive decrease in the amplitude and peak time of the b-wave. This observation also was reflected in the oscillatory potential recordings, in which the long-latency oscillatory potentials (3 and 4) progressively disappeared while oscillatory potential 2 remained. We believe that these findings further support our contention that the oscillatory potentials are major components of the b-wave.

Animals↗

PubMed1990

Oscillatory potentials as predictors to amplitude and peak time of the photopic b-wave of the human electroretinogram.

The oscillatory potentials are viewed by many as small oscillations of a high-frequency domain that ride on the b-wave of the electroretinogram. A study of electroretinograms and oscillatory potentials performed in 25 normal subjects was undertaken to substantiate my claim that oscillatory potentials are fast retinal potentials that are integrated to form the b-wave. The prominence of the OPs on the ascending limb of the b-wave was found to be only weakly correlated (r = -0.37) to the amplitude of the oscillatory potentials (measured in the 100-1000 Hz recordings). There was, however, a high correlation (r = 0.78) between the prominence of the oscillatory potentials and their frequency domain as determined by the peak-to-peak timing. Furthermore, the peak-to-peak timing of the oscillatory potentials was highly correlated with the b-wave peak time (r = 0.86) as well as with the 'a-wave trough to b-wave peak' time (r = 0.90), while the amplitude of the oscillatory potentials was correlated to the amplitude of the b-wave (r = 0.78). Interestingly, when combining the amplitude of the oscillatory potentials with the time interval between oscillatory potentials 2 and 3 and 3 and 4, a higher correlation (r = 0.88) was found with the b-wave amplitude. The latter finding would support my claim that the b-wave represents an integration (amplitude as a function of time) of the oscillatory potentials.

Adolescent↗

PubMed1990

The effect of 2-amino-4-phosphonobutyric acid on the oscillatory potentials of the electroretinogram.

Previous reports have shown that an intravitreal injection of 2-amino-4-phosphonobutyric acid, a glutamate analogue that selectively blocks the photoreceptors' input to the on-bipolar cells, produces a rapid decrease in the amplitude of the electroretinographic b-wave. To our knowledge, the effect of this glutamate analogue has not been examined on the oscillatory potentials. We therefore conducted such a study. A needle electrode was inserted into the anterior chamber of the eyes of seven anesthetized and paralyzed rabbits to record simultaneously the electroretinogram and the oscillatory potentials. These responses were evoked by flashes of white light delivered in mesopic conditions. As expected, an injection of 2-amino-4-phosphonobutyric acid, produced a rapid decrease in the amplitude of the b-wave, while the a-wave was relatively spared. The amplitude of the oscillatory potentials also decreased rapidly. At maximal effect, the electroretinogram essentially consisted of a normal a-wave followed by small oscillations and no evidence of a b-wave. The 100-1000 Hz recording confirmed that the oscillations seen on the postinjection electroretinograms were remnants of the original oscillatory potentials.

Aminobutyrates↗

PubMed1990

Temporal relationship between ERG components and geniculate unit activity in rabbit.

The purpose of this study was to examine if there is an ERG component (alpha-wave, b-wave, oscillatory potentials) which can be used to account for the intraretinal processing time. To address this issue, ERGs and LGN unit responses were recorded simultaneously from anesthetized and paralyzed rabbits. A gradual decrease in the intensity of the light stimulus yields a progressive increase in the latency of the LGN unit response. A similar, highly correlated (r = 0.91 +/- 013), latency shift was also noted for one oscillatory potential (OP2). In comparison, correlation coefficients of r = 0.63 +/- 0.27 and r = 0.70 +/- 0.29 were obtained for the alpha- and b-wave respectively. Furthermore, in 77.6% of the cells examined, OP2 preceded LGN unit activity while the b-wave preceded LGN activity in only 17.8% of the cases. Our results suggest that, of all the retinal potentials considered, the peak time of OP2 better reflects the primary visual processes and the intraretinal time taken for visual processing.

Animals↗

PubMed1990

The oscillatory potentials in response to stimuli of photopic intensities delivered in dark-adaptation: an explanation for the conditioning flash effect.

Previous studies reported that the oscillatory potentials (OPs) evoked to the first flash of a series were always smaller than those produced by the later flashes. This conditioning flash effect (CFE) was suggested to arise from rod inhibition of cone-mediated OPs. We investigated this CFE with the use of two stimulus intensities: 10 cd sec m-2 and 1 cd sec m-2. While the highest intensity did yield the previously reported CFE, the dimmest intensity did not. Our results further indicated that with the brightest stimulus, there is a significant increase in the interpeak interval of the OPs, while dimmest stimuli failed to reveal a similar marked increase. We also noted a significant correlation between the frequency domain of the OPs (as estimated with the interpeak interval) and the amplitude of the OPs (individual or collective: SOPs). Our results would also suggest that the observed CFE could result from a cone inhibition of rod-mediated OPs.

Adaptation, Ocular↗

PubMed1990

A new speculum electrode for electroretinography.

A Storz model E4110 infant speculum was modified to record electroretinograms (ERGs) from anesthetized rabbits. The electrode combines the advantage of a corneal contact lens electrode (with blepharostat) without its major disadvantage, namely: corneal abrasion. Addition of i.v. tubings glued to both arms of the speculum allows for constant wetting of the eyeball with 0.9% NaCl. The latter not only prevents the eye (and cornea) from drying, but also favors the recording of reproducible ERGs with a noise level comparable to ERGs recorded with a corneal contact lens electrode.

Animals↗

PubMed1990

Maturation of the photopic b-wave and oscillatory potentials of the electroretinogram in the neonatal rabbit.

We examined the ontogenesis of the b-wave of the rabbit electroretinogram (ERG) in light adaptation and looked at the contribution of the oscillatory potentials (OPs) to the waveform at each stage of development. Recordings were obtained weekly from 2 to 5 weeks of age in six rabbits and every other day during the second week of life in six others. Comparative analysis of the ERGs showed a changing peak time and amplitude of the photopic b-wave with increasing age, consistent with the development of new components corresponding to the OPs. The results illustrate the importance of the OPs in determining both the amplitude and peak time of the photopic b-wave in the maturing rabbit.

Animals↗

PubMed1989

The diagnostic use of the second oscillatory potential in clinical electroretinography.

Of all the electroretinogram (ERG) components (a-wave, b-wave, and oscillatory potentials) only one oscillatory potential, OP2, was found to be significantly correlated with the absolute intensity of the flash stimulus (i.e., the intensity of the stimulus irrespective of the state of retinal adaptation). Our finding was further confirmed in single cell recordings of lateral geniculate unit activity in rabbits in which peak time of OP2 was found to correlate better with the geniculate activity. For these reasons we have identified OP2 as the "intensity coding" oscillatory potential of the ERG. In order to investigate if this new feature could have some clinical significance, we examined photopic ERGs recorded from patients affected with various retinopathies. In most instances the peak time of OP2 paralleled that of the b-wave, that is, in the ERG with delayed b-wave the peak time of OP2 was also delayed, while in ERGs with normal b-wave peak time the peak time of OP2 was also normal. However, in some conditions (especially in cone-rod diseases) a delayed OP2 was found in ERGs with normal b-wave peak times.

Adult↗

PubMed1989

The effect of diphenylhydantoin on the electroretinogram.

Acute administration of diphenylhydantoin (DPH) in rabbits produces a significant increase in the amplitude of the a-wave. A marked increase in the amplitude of the b-wave is also noted but the time course is slower than that for the a-wave. While in controls the oscillatory potential (OP) recordings essentially consist of three major types, recordings taken after DPH injection consist of one major OP (OP2), which appears to be a result of the fusion of the original OP2 with another OP produced by the DPH injection. A similar blend of OPs was also seen in ERGs recorded from three human subjects on DPH therapy.

Adolescent↗

PubMed1989

The electroretinogram in Stargardt's disease and fundus flavimaculatus.

A retrospective study was performed comparing the ERG results of 15 patients with Stargardt's disease and fundus flavimaculatus. Patients with fundus flavimaculatus had "fish-tail" lesions with or without macular changes, while the Stargardt's group had macular atrophy without fish-tail flecks. The mean visual acuity was 20/200 for the Stargardt's patients compared with a mean of 20/80 for the fundus flavimaculatus patients. The Stargardt's photopic and scotopic amplitudes were respectively 33% and 34% of normal, while the fundus flavimaculatus values were less impaired at 58% and 64% of normal.

Adolescent↗

PubMed1989

Optic nerve blockade influences the retinal responses to flash in rabbits.

The presence of retinopetal fibers in mammals has been debated many times in the past two decades. Do rabbits have a retinopetal system? This question is addressed with the present investigations. In anesthetized and paralysed rabbits the b-wave and the oscillatory potentials (OP) are recorded at the cornea. The optic nerve is isolated retrobulbarly and is gently hooked to a curved injecting capillary. Through the latter lidocaine hydrochloride is pressure injected. This drug interrupts the neuronal flow travelling along the nerve. A steel electrode is positioned in the optic chiasm allowing us to monitor the evoked field potentials from the tested and untested eyes. The optic nerve blockade produces the following observations: (1) the amplitude of the b-wave is not significantly altered; and (2) the amplitudes of the long latency OP are significantly increased. The retinal capacity to respond to a second flash after the application of an initial light pulse was evaluated by varying the interval between the two flashes. After optic nerve blockade the recovery of the retinal responsiveness is considerably slower. Fourier analysis indicated that the highest power increases occurred around 200 Hz. It is difficult to escape the suggestion that rabbits possess retinopetal fibers.

Animals↗

PubMed1988

Maturation of the electroretinogram of the neonatal rabbit.

The development of the electroretinogram in the rabbit has been said to proceed with first the a-wave, then the b-wave and last the oscillatory potentials. The aim of our study was to reexamine this claim with special attention to the oscillatory potentials. Albino rabbits from the same litter were studied at weekly intervals for five weeks from the first week of life. A Grass photostimulator was used in light and dark adaptation and 50 amplified responses were averaged. Both 1-1000 Hz (electroretinogram) and 100-1000 Hz (oscillatory potential) band-widths were recorded simultaneously. The a-wave was the earliest signal to appear, at the second week of life. A rapid growth of the b-wave and oscillatory potentials was then noted between the second and third weeks, followed by a slower change. They evolved at the same rate, each with an increase in amplitude and decrease in peak time. The change in form of the b-wave was consistent with the sequential formation of each potential as the rabbit matured. The finding of simultaneous development of the b-wave and oscillatory potentials in the aging neonatal rabbit is contrary to previous reports.

Animals↗