VEP measurement of the amplitude of accommodation

Modulation of cortical activity by spherical blur and its correlation with retinal defocus

Cortical activity, as recorded via electroencephalography, has been linked to the refractive error of an individual. It is however unclear which optical metric modulates this response. Here, we measured simultaneously the brain activity and the retinal defocus of a visual stimulus perceived through several values of spherical blur. We found that, contrary to the existing literature on the topic, the cortical response as a function of the overcorrections follows a sigmoidal shape rather than the classical bell shape, with the inflection point corresponding to the subjective refraction and to the stimulus being in focus on the retina. However, surprisingly, the amplitude of the cortical response does not seem to be a good indicator of how much the stimulus is in or out of focus on the retina. Nonetheless, the defocus is not equivalent to the retinal image quality, nor is an absolute predictor of the visual performance of an individual. Simulations of the retinal image quality seem to be a powerful tool to predict the modulation of the cortical response with the refractive error.

Empirical Evaluation of Visual Fatigue from Display Alignment Errors Using Cerebral Hemodynamic Responses

The purpose of this study was to investigate the effect of stereoscopic display alignment errors on visual fatigue and prefrontal cortical tissue hemodynamic responses. We collected hemodynamic data and perceptual ratings of visual fatigue while participants performed visual display tasks on 8 ft × 6 ft NEC LT silver screen with NEC LT 245 DLP projectors. There was statistical significant difference between subjective measures of visual fatigue before air traffic control task (BATC) and after air traffic control task (ATC 3), (P < 0.05). Statistical significance was observed between left dorsolateral prefrontal cortex oxygenated hemoglobin (l DLPFC-HbO₂), left dorsolateral prefrontal cortex deoxygenated hemoglobin (l DLPFC-Hbb), and right dorsolateral prefrontal cortex deoxygenated hemoglobin (r DLPFC-Hbb) on stereoscopic alignment errors (P < 0.05). Thus, cortical tissue oxygenation requirement in the left hemisphere indicates that the effect of visual fatigue is more pronounced in the left dorsolateral prefrontal cortex.

Accommodation power determined with visual evoked cortical potentials in psychogenic visual disturbances

Accommodation power was measured with pattern visual evoked potentials in 12 normal subjects and three patients suffering from psychogenic visual disturbances and accommodation failure. Steady-state visual evoked cortical potentials recorded by increasing a minus-power lens in front of the eye in 1 diopter steps. The regression line was established from the visual evoked cortical potential amplitudes vs accommodation stimulus plot, and the objective accommodation power was determined by extrapolating the line to zero amplitude. The accommodation power measured by visual evoked cortical potentials was larger by approximately 2 diopters than that obtained subjectively by near-point rule in normal subjects. The subjective accommodation power was remarkably low for their age in each patient but the objective power was normal. These results suggest that the decrease of accommodation in these patients was caused by psychogenic mechanisms, and visual evoked cortical potential measurements of accommodation could provide helpful information for diagnosis.

Electrodiagnosis in paediatric ophthalmogenetics

In the present overview, practical application of the visual evoked potential (VEP) in paediatric neuro-ophthalmology is described across a wide range of ophthalmogenetic disorders, including albinism, Pelizaeus-Merzbacher disease and spastic paraplegia. The VEP approach is based on a four parameter subdivision of the electrophysiological response which includes, (1) amplitude (microV), (2) latency (ms), (3) waveform (component specificity), and (4) topography (potential distribution across the electrode array). In the case studies presented, evoked potential measures provide clinically useful and even at times invaluable insights concerning the presence, extent and type of visual pathway compromise. The four parameter subdivision along with age matched normative standards provides a sensitive and reliable means to facilitate detection and diagnosis of sensory anomalies in the developing visual system. The non-invasive electrophysiological assessment of visual function and its maturational course also provides early identification of affected family members as well as an objective measure of various treatment regimes which, particularly for the pre-verbal and non-verbal child, is critical for effective patient care.

Accommodative response changes with age measured by pattern electroretinogram

To evaluate whether the transient pattern electroretinogram reflects accommodation, we studied the amplitudes of the P-N component of transient pattern electroretinograms that were elicited in normal volunteers by reversing the checkerboard pattern. The stimuli were presented at a rate of three-reversals per second at a viewing distance of 20 cm. Each subject wore a lens that corrected for distance vision. The ophthalmic lenses were placed in front of the eye. A +12-diopter lens was used first, followed by lenses in decreasing 1-diopter steps, including minus lenses, until no response was recordable. The P-N amplitude was then plotted against increased accommodative stimulus. The graph showed a rapid increase to around 4 diopters, and then a slow decrease with increasing minus lens power. The gradually decreasing part of the graph became steeper under cycloplegic conditions. The amplitude of accommodative response was defined as the difference in diopters between the lens powers for eliciting an electroretinogram amplitude after cycloplegia was achieved and in the untreated pupil. The amplitude of accommodative response attenuated significantly in those subjects older than 40 years.

Accommodation power determined with transient pattern visual evoked cortical potentials in diabetes

Changes of accommodation power by aging were measured with pattern reversal visual evoked cortical potentials (VECPs) in 45 normal subjects and 32 patients suffering from diabetes mellitus. Transient VECPs were recorded by increasing minus power lenses in front of the eye at one-diopter steps, up to the point where no VECP was recordable. It was found that the amplitude of the P100 component became attenuated linearly with increased accommodation stimulus. The regression line was calculated from the VECP amplitude vs. accommodation stimulus plots (in diopters) and the objective accommodation was determined by extrapolating the line to 0 microV amplitude. The measured accommodation power with pattern VECPs was attenuated significantly in the normal group over 40 years old and in diabetics, and also was found to be larger by approximately 2.5 diopters than that obtained subjectively by the near-point rule.

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.

Modification of pattern reversal VERs by ocular accommodation

The amplitudes and implicit times of the P-1 component of monocular transient pattern VERs elicited by reversing checkerboard targets were measured in 6 conditions: (1) Ocular accommodation (Ao) elicited by minus power ophthalmic lenses; (2) Ao stimulated by minus lenses with the natural pupil of the test eye dilated and replaced by an artificial pupil; (3) accommodative demand and pupil size fixed with retinal image size (Ir) changed by afocal minifying lenses, (4) accommodative demand and Ir fixed but pupil size changed; (5) Ao elicited by minus lenses for three sizes of reversing checkerboards, and (6) Ao elicited by minus lenses for a constant checkerboard size with three check sizes. The observed decrease in VER amplitude with Ao could not be attributed entirely to the effects of pupil size or Ir. The rate at which the amplitude of P-1 decreased with Ao was not affected by checkerboard check size, but was closely correlated with checkerboard field size. The implicit time of P-1 first decreased slightly below baseline values at low and moderate levels of Ao and then tended to increase significantly at higher levels. The increase in implicit time at high levels of Ao could be explained adequately by the effects of pupil size alone on the VER. However, where Ao occurred without pupillary miosis, the increase in implicit time could only be explained by focusing error.