Spatial frequency of the human short-wavelength-sensitive (blue) cone mechanism. Psychophysical studies and pattern-reversal visual evoked potentials

Chromatic visual evoked potentials: A review of physiology, methods and clinical applications

Objective assessment of the visual system can be performed electrophysiologically using the visual evoked potential (VEP). In many clinical circumstances, this is performed using high contrast achromatic patterns or diffuse flash stimuli. These methods are clinically valuable but they may only assess a subset of possible physiological circuitries within the visual system, particularly those involved in achromatic (luminance) processing. The use of chromatic VEPs (cVEPs) in addition to standard VEPs can inform us of the function or dysfunction of chromatic pathways. The chromatic VEP has been well studied in human health and disease. Yet, to date our knowledge of their underlying mechanisms and applications remains limited. This likely reflects a heterogeneity in the methodology, analysis and conclusions of different works, which leads to ambiguity in their clinical use. This review sought to identify the primary methodologies employed for recording cVEPs. Furthermore cVEP maturation and application in understanding the function of the chromatic system under healthy and diseased conditions are reviewed. We first briefly describe the physiology of normal colour vision, before describing the methodologies and historical developments which have led to our understanding of cVEPs. We thereafter describe the expected maturation of the cVEP, followed by reviewing their application in several disorders: congenital colour vision deficiencies, retinal disease, glaucoma, optic nerve and neurological disorders, diabetes, amblyopia and dyslexia. We finalise the review with recommendations for testing and future directions.

Color discrimination along the cardinal chromatic axes with VECPs as an index of function of the parvocellular pathway. Correspondence of intersubject and axis variations to psychophysics

Chromatic information is carried only by the parvocellular pathway, giving the neurophysiologist the opportunity for eliciting specific responses. Further subdivision of the parvo chromatic system in two opponent chromatic mechanisms is potentially of great interest, given that the anatomical correlate seems to reside in subclasses of parvo ganglion cells that show differences both in size and in susceptibility to disease. We separately recorded responses arising from each chromatic opponent mechanism using visual stimuli chosen to belong to one of the "cardinal" chromatic axes. A calibrated color monitor, driven by a high resolution (14 bits/gun) computer board, was used for visualization of 1 c/deg isoluminant color gratings, sinusoidally modulated in time at 4 Hz. VECPs were recorded at several color contrasts along both cardinal axes, allowing extrapolation of contrast thresholds. Psychophysical thresholds were derived in the same stimulus conditions for comparison and found to correlate very well with the electrophysiologically derived values, both as intersubject and axis differences. The S-(L+M) opponent mechanism consistently yielded higher thresholds, smaller amplitude, and higher phase lag than the L-M mechanism. This finding was largely explained by the perceptual non-uniformity of the CIE chromaticity diagram. Correcting the VECP data for the perceptual differences yielded comparable responses, supporting the view that the two mechanisms are similarly represented in the cortex. In conclusion, recording of cortical responses to color contrast stimuli belonging to the cardinal chromatic axes seems a reliable procedure and may prove to be useful in performing clinical evaluations that refine the assessment of the physiology of the visual system.

Comparison of the pattern reversal visual evoked potential mediated by separate cone systems

With the purpose of recording responses mediated by the 3 cone systems visual evoked potentials (VEPs) were elicited by the reversal of monochromatic checkerboards superimposed upon strong monochromatic backgrounds (yellow, purple and blue-green). The sensitivity to light of various wave lengths were measured as the reciprocal of the intensity necessary to elicit a VEP amplitude of 3 microV. The spectral sensitivity curves based on this VEP amplitude criterion in the presence of blue-green, purple and yellow adaptation showed peak sensitivities in the red, the green and the blue part of the spectrum, respectively. This indicates that the responses reflect separate modulation of the 3 different cone mechanisms. The potentials obtained with yellow adaptation differed from those obtained with purple and blue-green adaptation. The amplitude versus log intensity function was flatter and the latency of the major positive peak was increased by 20-25 msec. Repeated examinations of 4 subjects suggest that the method yields reliable latency measurements of responses mediated by separate cone mechanisms.

Visual evoked potentials in three-dimensional color space: correlates of spatio-chromatic processing

Visual evoked potentials (VEPs) were measured for sinusoidal gratings with spatio-chromatic modulation defined in a three-dimensional color space. The spatio-chromatic modulation of the gratings can be decomposed into contributions from an achromatic luminance varying component, an isoluminant component which modulates only the activities of L cones and M cones, and an isoluminant component corresponding to modulation of only S-cone activity. The emphasis of this report is the nature of VEPs resulting from isoluminant spatio-chromatic modulation. The VEP response was characterized along a number of spatial, temporal, and chromatic stimulus dimensions: contrast, spatial frequency, chromaticity in the isoluminant plane, chrominance/luminance ratio, orientation, and temporal frequency. Isoluminant VEPs resulting from stimuli modulating L and M cones are compared with those from S-cone modulation. When appropriate spatiotemporal conditions are employed, both types produce robust VEPs; however, the S-pathway VEPs show considerably longer latencies than do those from LM-pathway activation. The VEP results are compared to psychophysical and single unit electrophysiological observations. VEP latencies exhibit the lowpass character of psychophysical chromatic contrast sensitivity functions but VEP amplitudes show bandpass tuning along both the S and LM axes. An oblique effect, i.e. shorter latencies for horizontal and vertical gratings than for diagonal, is observed in the isoluminant VEP. S-pathway VEPs are used to demonstrate an electophysiological correlate of transient tritanopia. Normative amplitude and latency data for VEPs from selectivity stimulated chromatic mechanisms provide a baseline for clinical electrodiagnostic applications.

Interactions of spectral, spatial, and temporal mechanisms in the human pattern visual evoked potential

The human pattern onset-offset visual evoked potential (VEP) was studied with different colours and spatial frequencies presented on a steady homogeneous intensive yellow background. Under this condition a broad, late negative wave (N2) dominated by the blue-sensitive mechanism and a sharper earlier positive wave (P1) dominated by the red-green-sensitive mechanism can be observed. With a 460 nm pattern N2 shows a strong amplitude tuning at low-medium spatial frequencies. With 550 nm an early negative component (N1) is added showing an amplitude tuning at high spatial frequencies. Different spatial resolutions of the two colour mechanisms are thus indicated. With pattern-reversal stimuli the 550 nm stimulus shows a higher temporal resolution than the 460 nm stimulus.