Comparison of visual evoked potential and psychophysical contrast sensitivity

Melanopsin-mediated amplification of cone signals in the human visual cortex

The ambient daylight variation is coded by melanopsin photoreceptors and their luxotonic activity increases towards midday when colour temperatures are cooler, and irradiances are higher. Although melanopsin and cone photoresponses can be mediated via separate pathways, the connectivity of melanopsin cells across all levels of the retina enables them to modify cone signals. The downstream effects of melanopsin-cone interactions on human vision are however, incompletely understood. Here, we determined how the change in daytime melanopsin activation affects the human cone pathway signals in the visual cortex. A 5-primary silent-substitution method was developed to evaluate the dependence of cone-mediated signals on melanopsin activation by spectrally tuning the lights and stabilizing the rhodopsin activation under a constant cone photometric luminance. The retinal (white noise electroretinogram) and cortical responses (visual evoked potential) were simultaneously recorded with the photoreceptor-directed lights in 10 observers. By increasing the melanopsin activation, a reverse response pattern was observed with cone signals being supressed in the retina by 27% (p = 0.03) and subsequently amplified by 16% (p = 0.01) as they reach the cortex. We infer that melanopsin activity can amplify cone signals at sites distal to retinal bipolar cells to cause a decrease in the psychophysical Weber fraction for cone vision.

Transient VEP and psychophysical chromatic contrast thresholds in children and adults

It has been found that humans are able to distinguish colours without luminance cues by about 2-4 months of age and that sensitivity to colour difference develops during childhood, reaching a peak around adolescence. This prolonged period of maturation is reflected by improvements in psychophysical threshold measures and by the VEP characteristics of morphology, latency and amplitude. An intra-individual comparison of VEP and psychophysical responses to isoluminant colour stimuli has not been made in children, however, and this was the aim of the present study. VEPs were recorded from 49 subjects, children (age range: 4.8-12.6 years) and adults (age range: 25.7-33.2 years). Psychophysical and VEP thresholds were both measured in 40 of those subjects. Nominally isoluminant chromatic (L-M) sinewave gratings were presented in onset-offset mode and identical stimuli were used for psychophysical and VEP recordings to allow comparison. In agreement with previous reports, morphology of the transient VEP in response to this stimulus differed considerably between children and adults. There was a significant difference between psychophysical and VEP thresholds in children, but not in adults. Our findings support and expand on previous work on maturation of the L-M chromatic pathway and indicate a larger discrepancy between VEP and psychophysical chromatic thresholds in children than in adults.

Decreased amplitudes in multiple sclerosis patients with normal visual acuity: a VEP study

Primary demyelination with relative preservation of axons is considered to be one pathological hallmark of multiple sclerosis (MS), a chronic inflammatory disease of the central nervous system. However, imaging and pathomorphological studies have stimulated a recent re-emergence of interest in the axonal, neurodegenerative aspect of MS pathology. Axonal injury appears to be a key factor of disability and permanent neurological deficit in MS patients. In the present electrophysiological study, visual potentials evoked by pattern reversal (VEPs) were recorded in 25 MS patients with normal visual acuity and unimpaired visual functions. Compared to a control population, VEP amplitudes for two different spatial frequencies were significantly decreased. From this observation, we conclude that an underlying pathological process threatening axonal integrity may not be reliably reflected by clinical parameters due to the distinct ability of the visual system to compensate for axonal loss. Pattern VEP may thus serve as an objective tool to diagnose and to monitor axonal pathology in MS. Focal conduction block due to demyelination as a cause for reduced VEP amplitudes can not be fully excluded, but would appear less likely since latency prolongation in the MS group was moderate compared to controls.