Encyclopedia of Color Science and Technology

2016 Edition
| Editors: Ming Ronnier Luo

Visual Evoked Potentials

Reference work entry
DOI: https://doi.org/10.1007/978-1-4419-8071-7_107

Synonyms

Definition

The visual evoked potential (VEP) is a means of extracting from the spontaneous electrical activity in the brain, electrical changes that are directly related to a specific brain action. It is a means of analyzing the Electroencephalogram (EEG). The event-related information (“signal”) is extracted from the overall electrical activity (“noise”) by some form of correlation. Usually, repeated samples of EEG are triggered by a particular event (e.g., an abrupt visual change) and averaged together. Because this event causes a particular type of brain response, at a predictable time, the process of averaging these time locked responses accentuates the event-related signal and smoothes out any uncorrelated activity. By using a stimulus that only contains time-locked changes in color, the VEP can provide an indirect measure of color information processing in the brain.

Scalp-Recorded Electrical Signals from the Brain

The brain is constantly generating large amounts of noisy electrical signals, emanating from the vast number of neurones firing at any given time. This ever-changing activity can be recorded by attaching electrodes to the scalp. The spontaneous activity, known as the Electroencephalogram (EEG) (http://www.springerreference.com/docs/html/chapterdbid/116485.html), reflects all the current motor, sensory and cognitive processes. If we wish to study brain activity that is related to a particular function (e.g., hearing or seeing) we can isolate that part of the EEG which occurs in response to specific stimulation by sounds or visual stimuli. In order to elicit such stimulus-related visual response, known as the visual evoked potential (VEP), it is necessary to cause a change in the stimulus, since the evoked potential is actually a measure of a change in electrical activity.

The crudest form of visual stimulus would be an instantaneous flash of light. This is rather like taking a sledgehammer to crack a nut, and is not very well suited to probing specific types of brain function. However it is often used clinically to get some idea of whether there is a gross connection between the eye and the brain, for instance in cases of cortical visual impairment. Patterned stimuli are more frequently employed; typically these are carefully specified repetitive patterns such as checkerboards or gratings. These have to be temporally modulated to elicit a response; usually this takes the form of pattern reversal or pattern onset-offset. Taking the example of a black-white checkerboard, in pattern reversal mode the black and white squares exchange places at fixed intervals, every few hundred milliseconds. In on-off presentation, the checks are replaced periodically by a blank gray screen so that there is no change in space-averaged color or luminance. It is crucial that artifacts are avoided: an overall luminance change between the two phases would itself elicit an evoked response, but this would have no relationship to the spatial content of the stimulus. Figure 1 shows a typical response to a reversing black and white checkerboard; this has a widespread use in clinical ophthalmology and neurology.
Visual Evoked Potentials, Fig. 1

VEP elicited to a black and white checkerboard with each square measuring 1 degree of visual angle. The checks reversed every 250 ms. The key wavelets are negativities at about 75 and 135 ms, and a positivity at about 100 ms. The waveform was produced by averaging together fifty 250ms samples of EEG, each of which was triggered by an identical event (the instantaneous reversal of the checkerboard). Timings are relative to this trigger event

The electrogenesis of the evoked potential is not well understood, but the signals recorded from an electrode placed on the scalp above the occipital (visual) cortex are said to reflect mainly the activity of early visual areas V1 and possibly V2. The signals are recorded using a differential physiological amplifier, which takes the difference between the potential at the “active” site (e.g., the occiput) and a second electrode placed at a nonvisual or “reference” site (e.g., the vertex or an earlobe). In fact, the terms “reference” and “active” are misnomers as it is possible to record some level of visually related activity from anywhere on the scalp. An alternative name for this montage is unipolar. A bipolar electrode montage places both electrodes over the same cortical area, a few cm apart. This generates smaller signals but has the advantage that distant sources of electrical “noise” tend to be of the same magnitude at both electrodes and are therefore not as readily recorded using a differential amplifier.

Chromatic Evoked Potentials

A spatial stimulus is defined in terms of many parameters, including its size (how many degrees of visual angle it subtends on the retina), the pattern type (e.g., sinusoidal or square-wave gratings), its periodicity or spatial frequency (i.e., number of spatial cycles per degree of visual angle), the type of temporal modulation (e.g., sinusoidal or square-wave) and temporal frequency (number of temporal cycles per second). In order to elicit a chromatic response, the stimulus has to vary in color alone, in both spatial and temporal domains. It is important to achieve activation of color opponent mechanisms (see Fig. 2; [ Color Vision, Opponent Theory]).
Visual Evoked Potentials, Fig. 2

Spectral characteristics of the cone photoreceptors; subtractive and additive mechanisms giving rise to opponency; with thanks to Prof D. McKeefry

The simplest and most well-established method is to use a sinusoidal grating composed of two colors, say red and green (Fig. 3), which has been adjusted for each subject so that it is isoluminant. This means that, for an individual, there is no change in luminance across the stimulus, but only a change in color or chromaticity. This is usually achieved with a psychophysical procedure such as heterochromatic flicker photometry (HFP), in which the two colors are exchanged at a fairly high rate (about 16Hz), and their luminance reciprocally adjusted until the perception of flicker (which results from residual luminance contrast) is minimised. In on-off mode, the pattern would be replaced with a plain yellow field of the same space-averaged hue and luminance.
Visual Evoked Potentials, Fig. 3

Combining red and green gratings in phase to make an achromatic (yellow) stimulus and out of phase to make a chromatic (red-green) grating. Also shown is the offset phase when contrast is zero; note that this is the same for both stimuli

In Fig. 4, a series of VEPs are shown in which luminance contamination is present in varying quantities. At isoluminance (the middle trace, shown in red), the onset response takes on a different shape, the achromatic positivity (seen at top and bottom of the figure) being replaced by a chromatic-specific negativity [1, 2, 3, 4, 5]. This achromatic-chromatic difference is not seen when checkerboards or reversal presentation are employed, and this is possibly because these stimuli are not optimal for the chromatic visual system, which, at least for red-green stimuli, is handled by the parvocellular (P) visual pathway [6] The P pathway is dominated by sustained nerve fibers, whilst reversal favors transient responses, and perhaps therefore activates preferentially the magnocellular system [7].
Visual Evoked Potentials, Fig. 4

VEPs to a range of red-green ratios. The isoluminant response (when, for this subject, RGR = 0.5) is shown in red. The gray plot is the equivalent achromatic VEP, recorded to a yellow/black grating. Note that there is no significant difference between this response and those to the red/black or green/black gratings

Although red-green stimuli are relatively easy to generate on a graphics display using the red and green primary phosphors, they usually contain significant contrast in both red-green and blue-yellow color opponent pathways. Since these properties are handled by different retino-cortical pathways (respectively the parvocellular and koniocellular streams), it is desirable to optimize the stimulus so that it activates a particular pathway. Physical colors are often represented in a 3-dimensional space, an example of which is given in Fig. 5. See [ Psychological Color Space and Color Terms] for more on color spaces. The two component colors of the stimulus are selected from this space, forming a vector. This vector can be located at a particular orientation in order to restrict activation to one channel (say blue-yellow), whilst keeping the others (red-green and luminance) constant. In Fig. 6, a range of VEPs have been recorded to onset of chromatic stimuli which all lie on the isoluminant plane in MBDKL space [8, 9]; thus the luminance channel is held constant. The vector has been rotated for each successive recording so that at some point it passes through each color channel. When the chromatic axis (φ) is 0–180o, the stimulus is modulated along the L/M cone axis (“red-green”); when φ = 90–270°, it preferentially activates the S-cone axis (“yellow-blue”). 10 additional recordings have been made at intermediate angles where one would expect a mixture of red-green and blue-yellow signals. The blue-yellow VEP, which taps the sluggish koniocellular pathway, has a much longer latency (time from onset of the stimulus to peak of the component) than the M or L-cone mediated responses (see Fig. 6 inset).
Visual Evoked Potentials, Fig. 5

MBDKL color space; with thanks to Prof D. McKeefry

Visual Evoked Potentials, Fig. 6

(Left) Occipital VEPs recorded to a range of chromatic axes, where 0/180 is cardinal red-green and 90/270 is cardinal blue-yellow (see Fig. 5). (Below) Latency of the chromatic negativity as a function of chromatic axis

Limitations

There are a few factors which need to be taken into account when recording chromatic evoked potentials. Firstly, these stimuli may be contaminated by luminance information due to chromatic aberrations. The key to avoiding the many complications arising from chromatic aberrations is to use low spatial frequency (<5c/deg) sinusoidal gratings and to restrict the number of cycles visible. There is another good reason to use low spatial frequencies: the chromatic visual system is tuned to these, showing a low-pass tuning curve with a half-height of about 3 c/deg and an upper limit of about 10 c/deg [10]. The issue of stimulus size becomes particularly important when blue–yellow stimuli are used. This is because the central retina contains a higher concentration of yellow macular pigment, meaning that it is not possible to set a single value of isoluminance across the whole stimulus [11, 12]. Isoluminance also varies from subject to subject, even in those with normal color vision, largely because of the high inter-subject variability in the proportion of long-wavelength to medium–wavelength photoreceptors [ Cone Fundamentals]. Thus it is always advisable to establish isoluminance for each subject tested. The presence of color vision deficiencies is also important of course. In extremis, dichromatic individuals lack red-green opponency [ Color Vision, Opponent Theroy], and therefore do not generate red-green chromatic evoked potentials (see Uses, below).

Uses

What benefits do chromatic VEPs bring, aside from giving us a tool to help understand the visual system? VEPs are useful in assessing the vision of preverbal individuals, and several studies have used VEPs to study the development of color vision in infants [13, 14]. There are many potential clinical uses, from the diagnosis of color vision deficiencies to the assessment of drug toxicity [15, 16, 17, 18]. Studies have applied the technique to measure the effect of environmental toxins, which might be expected to preferentially affect the highly sensitive color visual system [ Color Perception and Environmentally Based Impairment]. The chromatic VEP has proven to be of value in neuro-ophthalmology [19, 20, 21, 22, 23, 24], again because diseases such as optic neuritis seem to produce defective color vision. This may not be because the disease specifically targets color vision (for instance via the parvocellular pathway), but because the chromatic system is simply more sensitive to insult or injury, showing their effects more readily than the more robust achromatic visual system.

Cross-References

Supplementary material

Video. 1 (wmv 5193 kb)

References

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Copyright information

© Springer Science+Business Media New York 2016

Authors and Affiliations

  1. 1.Vision Science CentreManchester Royal Eye Hospital, Central Manchester University Hospitals NHS Foundation Trust, Manchester Academic Health Science CentreManchesterUK
  2. 2.Centre for Ophthalmology and Vision Sciences, Institute of Human DevelopmentUniversity of ManchesterManchesterUK