- Email: [email protected]
Normative data for onset VEPs to red-green and blue-yellow chromatic contrast
Clinical Neurophysiology 110 (1999) 772±781
Technical note
Normative data for onset VEPs to red-green and blue-yellow chromatic contrast Vittorio Porciatti a,*, Ferdinando Sartucci b a
b
Institute of Neurophysiology, CNR, Pisa, Italy Department of Neuroscience, Institute of Neurology, University of Pisa, Pisa, Italy Accepted 10 December 1998
Abstract Objective: To better characterize the properties of chromatic VEPs to onset-offset of red-green and blue-yellow equiluminant patterns, and establish normative values for a set of stimuli able to elicit robust and reliable responses, suitable for the clinical application. Methods: Chromatic VEPs have been recorded (Oz lead) from 28 normal subjects (age range 20±53 years) in response to monocular presentation of both red-green and blue-yellow equiluminant sinusoidal gratings. Stimuli were generated by a Cambridge VSG/2 card and displayed on a Barco CCID monitor (14 £ 14 deg ®eld size). Spatial frequency, chromaticity, contrast and onset-offset duration were varied. Results: For both red-green and blue-yellow equiluminant stimuli, robust responses have been obtained with gratings of 2 c/deg, presented in onset (300 ms) offset (700 ms) mode, at contrasts ranging from 90 to 6%. In all observers, the VEP waveform consisted mainly of a negative wave at stimulus onset, with a latency rapidly increasing with decreasing contrast. For both red-green and blue-yellow stimuli, the VEP contrast threshold coincided with the psychophysical threshold. Conclusions: The results complement previous studies aimed at characterizing the properties of chromatic VEPs. In addition, normative data are provided for a set of stimulus characteristics suitable for the clinical routine. q 1999 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Parvocellular; Koniocellular; Magnocellular; Chromatic contrast; Luminance contrast; VEPs
1. Introduction The main interest for studying VEPs in response to patterns with pure chromatic contrast, is that of recording speci®c responses from colour-opponent pathways, anatomically and physiologically distinct from the achromatic pathway at retinal, geniculate and cortical level (Lennie and D'Zmura, 1988; Lennie et al., 1990; Merigan and Maunsell, 1993; Dacey and Lee, 1994; Derrington et al., 1984; Engel et al., 1997). The achromatic stream originates from large (parasol) ganglion cells projecting to magnocellular layers of the lateral geniculate nucleus (LGN). The red-green pathway originates from smaller (midget) ganglion cells relaying to parvocellular layers of the LGN. The blue-yellow pathway stems from bistrati®ed ganglion cells (with dendritic arbours in both ON- and OFF laminae of the inner plexiform layer) and project to the interlaminar (Koniocellular) neurons of the LGN. * Corresponding author. Tel.: 1 39-050-540770; fax: 540080. E-mail address: [email protected] (V. Porciatti)
1 39-050-
Following the pioneering work of Regan, Spekreijse and colleagues (reviewed in Regan, 1989), chromatic VEPs have received increasing attention for studying basic mechanisms of vision and clinical applications. This has been favoured by the recent development of affordable graphic cards and linear TV displays, which allow presentation of a wide variety of chromatic stimuli. Although some controversy exists on a possible contribution of luminance contrast contamination of responses (Kulikowski et al., 1996; Switkes et al., 1996), the body of evidence shows that the properties of the chromatic contrast VEPs differ greatly from those of luminance contrast responses. Overall, independently of the response type (transient and steadystate) and mode of presentation (reversal and onset-offset), differences between chromatic and achromatic VEPs are in the same direction (Murray et al., 1987; Berninger et al., 1989; Fiorentini et al., 1991; Rabin et al., 1994; McKeefry et al., 1996; Porciatti and Sartucci, 1996; Regan and He, 1996; Tobimatsu et al., 1996; Crognale et al., 1997; Porciatti et al., 1997). Chromatic VEPs, as compared with luminance VEPs, are slower and attenuate more steeply as a function of increasing spatial frequency, temporal frequency and
1388-2457/99/$ - see front matter q 1999 Elsevier Science Ireland Ltd. All rights reserved. PII: S13 88-2457(99)0000 7-3
CLINPH 98696
V. Porciatti, F. Sartucci / Clinical Neurophysiology 110 (1999) 772±781
decreasing contrast. These differences are expected on the basis of the known physiology of different subpopulations of generators of the visual pathway, indicating that chromatic VEPs re¯ect to some extent the activity of colouropponent neurons. By contrast, both colour-opponent neurons and colour-unselective neurons of the magnocellular pathway may contribute to luminance contrast VEPs. Recently, it has been shown that chromatic and achromatic VEPs are differently altered in disease (Russel et al., 1991; Crognale et al., 1993; Buttner et al., 1996; Heywood et al., 1996; Porciatti and Sartucci, 1996; Spinelli et al., 1996; Porciatti et al., 1997; Schneck et al., 1997; Tobimatsu and Kato, 1998), suggesting speci®c vulnerability of visual pathway subpopulations. This indicates that the combined assessment of chromatic- and luminance VEPs may be useful for a better understanding of neuro-ophthalmological disorders. However, chromatic VEPs, as compared with ordinary luminance contrast VEPs, may be considered a more delicate technique due to both technical requirements for generating stimuli and necessary controls for minimizing luminance intrusion. Extended clinical application of chromatic VEPs requires robust and reliable responses, relatively resistant to departures from optimal conditions. This study further characterizes some properties of chromatic VEPs to red-green and blue-yellow patterns, proposing normative data for a set of stimuli and recording conditions which appear feasible in a clinical context.
2. Methods 2.1. Visual stimuli Stimuli were horizontal sinusoidal gratings of different spatial frequency (0.5±1±2±3±4±5 c/deg) and contrast (6± 12±25±50±90%) generated by a widely used board (VSG/2 Cambridge Research) and displayed on a colour monitor (Barco CCID 7751, Kortrijk, Belgium) with a 14 bit resolution at 120 Hz, 512 lines per frame. The software required for generating stimuli and evaluating psychophysical responses can be easily obtained by modifying source codes of routines provided by the factory. Chromatic modulation was performed along L-M (red-green) and S-(L 1 M) (blue-yellow) cardinal axes of colour vision (Krauskopf et al., 1982), in order to selectively in¯uence the activity of the corresponding opponent mechanism. Chromatic-contrast patterns (red-green or blue-yellow) were obtained by superimposing (out-of-phase by 180 deg) red-black and greenblack (or blue-black and yellow-black, respectively) gratings of identical contrast. Luminance-contrast patterns (yellow-black and white-black, respectively) were obtained by superimposing the same gratings in-phase. As illustrated in the left panels of the example reported in Fig. 1, the chromaticity of chromatic-contrast patterns was varied by changing the relative luminance of superimposed gratings, as is now standard (Mullen, 1985). The ratio r of red to total
773
Fig. 1. Method for producing chromatic-contrast gratings with different chromaticity. Green-black and red-black gratings are superimposed out-ofphase by 180 deg. The relative luminance of red and green is varied to obtain all combinations of luminance- and chromatic contrast, including pure luminance- (r 0, r 1) and pure colour (r 0:5) contrast. Note that chromaticity is varied at constant Michelson contrast and mean luminance (sum of red and green mean luminances). See Section 2 for further details.
luminance {red/(red 1 green)} and the ratio b of blue to total luminance {blue/(blue 1 red 1 green} could be varied from 0 to 1, where r 0, r 0:5 and r 1 de®ne green-black, red-green and red-black patterns, respectively, and b 0, b 0:5, and b 1 de®ne yellow-black, blueyellow and blue-black patterns. As shown in the right panel of Fig. 1, green-black and red-black gratings (r 0 and 1) contain pure luminance-contrast (simply de®ned by the Michelson contrast (Lmax 2 Lmin /(Lmax 1 Lmin )), irrespective of colour. For intermediate values of r there are different amounts of luminance- and chromatic contrast. At r 0:5 the chromatic contrast is maximal and the luminance contrast is zero (equiluminant point). It is worth noting that the variation of chromaticity (colour ratio) occurs at constant Michelson contrast for both green and red gratings and at constant mean luminance (de®ned as the sum of the mean luminances of red and green gratings). For red-green patterns, the display was viewed through yellow ®lters (Kodak Wratten 16) to attenuate wavelengths lower than 500 nm. This approach is believed useful to minimize the variable absorption of shorter wavelengths by the eye lens in older subjects (Fiorentini et al., 1996). CIE co-ordinates could be obtained from factory measurements, but were further evaluated by a Minolta Chromameter CS 100. For red-green, the CIE were red: x 0:637, y 0:362; green: x 0:416, y 0:582; mean luminance: 14 cd/m 2. For blue-yellow, the CIE were blue: x 0:075, y 0:377; yellow: x 0:377, y 0:348; mean luminance 6 cd/m 2. The photometer was also used to cali-
774
V. Porciatti, F. Sartucci / Clinical Neurophysiology 110 (1999) 772±781
brate look-up tables of the software used for linearization of voltage/luminance characteristics of the display (gamma correction). Calibration was found to be very stable during the annual follow-up. Stimuli subtended an area of 14 £ 14 deg at the viewing distance of 100 cm. Subjects ®xated a black spot at the centre of the screen. Further details can be found elsewhere (Fiorentini et al., 1996; Porciatti et al., 1997). 2.2. Equiluminance Equiluminance was established psychophysically for every subject with a variant of the method of heterochromatic ¯icker photometry. Red-green and blue-yellow gratings of 60% contrast were sinusoidally reversed at 15 and 10 Hz, respectively. Subjects adjusted r or b ratios to nil or minimized the perception of ¯icker. In some cases, the grating contrast was varied to emphasize the effect. The equiluminance point was the average of at least 6 independent measurements. In all subjects, for both red-green and blueyellow patterns the equiluminant point was found at r and b values close to 0.5 (Vl equiluminant point: see Fiorentini et al. (1996) for discussion). 2.3. VEP recording The issue of frequency and time-domain analysis of human chromatic VEPs has been addressed in some detail elsewhere (McKeefry et al., 1996). Steady-state responses to sinusoidal counterphase appear a suitable technique to evaluate the spatio-temporal properties of the colour-opponent system. With this method it is possible to successfully predict spatio-temporal thresholds (Fiorentini et al., 1991; Girard and Morrone, 1995; Macaluso et al., 1996), and evaluate the development (Allen et al., 1993; Morrone et al., 1993; Morrone et al., 1996), ageing (Fiorentini et al., 1996) and dysfunction (Porciatti and Sartucci, 1996; Spinelli et al., 1996; Porciatti et al., 1997) of the chromatic system. By contrast, transient VEPs to abrupt reversal of equiluminant chromatic gratings are meagre responses, not radically different from transient VEPs to reversal of luminance contrast (Murray et al., 1987; Berninger et al., 1989; Rabin et al., 1994; Fiorentini et al., 1996). However, onset presentation of chromatic patterns evokes robust transient VEPs with a well-de®ned waveform, different from that in response to similar patterns modulated in luminance contrast (Murray et al., 1987; Berninger et al., 1989; Tobimatsu et al., 1995; Regan and He, 1996). Since time-domain analysis of transient responses is a widely accepted clinical standard (Harding et al., 1996), we have further explored the characteristics of transient VEPs to onset-offset of chromatic gratings, with the aim of establishing a set of conditions suitable for clinical application. Red-green and blueyellow gratings of different chromaticity, spatial frequency and contrast were presented for a variable amount of time and exchanged with an equiluminant uniform background (patterns of zero contrast: yellowish for red-green, bluish-
white for blue-yellow). VEPs have been typically recorded from scalp electrodes (resistance , 5 kV ) placed at Oz (active), Mastoid (reference) and Cz (ground). Signals were ®ltered (0.3±100 Hz, 26 dB/oct) ampli®ed (50 000 fold), digitized (2 kHz, 12 bit resolution) and averaged (at least 80 sums), with a rejection of signals exceeding a threshold voltage. Partial averages (blocks of 10 sums) of total average were used to evaluate response consistency (standard error of amplitude of partial averages). The SEM was typically about 7% of the average VEP amplitude. Additional blocks were recorded when necessary to keep the SEM below 15% of the average amplitude. Responses to patterns of zero contrast were frequently recorded to have a measure of residual noise. The average peak-to-trough amplitude of residual noise was 2.1 mV, SEM 0.24. 2.4. Subjects Twenty-eight observers (14 males, 14 females, age range 19±53 years, mean 32:2 ^ 10:7) volunteered for psychophysical and electrophysiological measurements after a full description of the aims and methods of this study. All subjects had normal (1.0 or better) visual acuity with no or minimal (0.5±2.5 spherical, 0.25±0.75 cylindrical, diopters) refraction, and were fully corrected for the viewing distance. They had normal colour vision (Ishihara) and did not report any previous ocular or systemic disease. Two male subjects were not included because of previously undiagnosed colour anomaly of the protan type, which resulted in abnormal Ishihara scores and r colour ratios (see above). 3. Results Fig. 2 reports representative examples of VEPs recorded from the same subject in response to red-green and blueyellow equiluminant gratings of 2 c/deg presented in onsetoffset (300±700 ms) mode. For both red-green (Fig. 2A) and blue-yellow (Fig. 2B) patterns of 90% contrast, VEPs consist of a sharp negative de¯ection at stimulus onset with a peak latency of 120±130 ms. Unless otherwise speci®ed, the latency of chromatic onset VEPs refers to the peak latency of the negative wave, whereas amplitude is measured from the trough of the negative wave to the peak of the following positive wave. The amplitude of chromatic onset VEPs progressively decreases, and latency dramatically increases, with decreasing contrast. The response at stimulus offset consists of a broad positivity. The waveform of chromatic onset VEPs differs remarkably from that in response to luminance-contrast patterns of the same mean luminance and spatial frequency (Fig. 2C). At 120±130 ms, onset VEPs to high contrast luminance gratings display a major positivity, whose latency changes little with decreasing contrast. In addition, stimulus offset elicits a distinct response. In all tested subjects, chromatic VEPs displayed a comparable morphology. Overall, chromatic
V. Porciatti, F. Sartucci / Clinical Neurophysiology 110 (1999) 772±781
775
Fig. 2. Examples of VEPs to onset (300 ms) ± offset (700 ms) of horizontal sinusoidal gratings with pure chromatic (A,B) and luminance (C) contrast. Spatial frequency 2 c/deg, ®eld size 14 £ 14 deg. Note the different waveform of chromatic- and luminance onset VEPs. Also note that the peak latency (dotted lines) of chromatic VEPs, differently from luminance VEPs, increases dramatically with decreasing contrast (numbers to the lefts of tracings).
onset VEPs, as compared with luminance contrast VEPs, were of larger amplitude (see below). Fig. 3 shows an example of how chromatic onset VEPs change as a function of the colour ratio r and b (proportion
of red in the red-green, and blue in the blue-yellow mixture, respectively). As discussed in the Section 2, r and b values yielding equiluminance are expected at values around 0.5. Departure from the equiluminant point introduces lumi-
Fig. 3. Variation of onset VEPs as a function of colour ratio r (proportion of red in the red-green mixture) and b (proportion of blue in the blue-yellow mixture). For both red-green (A) and blue-yellow (B), waveforms with largest amplitude occur at r and b ratios of 0.5, close to the psychophysical equiluminant point (open arrows in C,D). Note in (A,B) that response latency is rather independent of colour ratio in the 0.4±0.6 range. Same conditions as in Fig. 2, contrast 90%.
776
V. Porciatti, F. Sartucci / Clinical Neurophysiology 110 (1999) 772±781
Fig. 4. Amplitude variation of onset VEPs to equiluminant gratings of 90% contrast as a function of spatial frequency. For both red-green (A) and blue-yellow (B), maximal amplitude occurs at about 2 c/deg in 4 different subjects. Same conditions as in Fig. 2.
Fig. 5. Effect of onset duration (numbers to the right of tracings in A) of red-green equiluminant stimuli on VEP amplitude and contrast threshold. (A,B) Maximal amplitude occurs for 50 ms duration due to summation of onset- and offset-related components. Note that with 50 ms duration both the psychophysical (C) and VEP (D) contrast threshold are greatly elevated, as compared to 300 ms duration. For further explanation see Section 3.
V. Porciatti, F. Sartucci / Clinical Neurophysiology 110 (1999) 772±781
777
Fig. 6. Average amplitude and latencies of onset VEP to pure chromatic- and achromatic contrast as a function of Log10 contrast. Error bars represent the SEM. For chromatic gratings (A,B), the contrast dependence of amplitude is virtually linear, whereas that of luminance gratings (C) saturates at moderate contrast. The range of latency variation with contrast is much larger for chromatic- (D,E) than luminance (F) gratings.
nance contrast contribution in the stimulus (about 20% for a 0.1 change in r and b) which may unacceptably contaminate the chromatic response. For r and b values ranging from 0.4 to 0.6, both response waveform and peak latency of the negative wave are unchanged. However, the response amplitude does change as a function of r and b. As shown in Fig. 3C,D, the chromatic VEP amplitude has a local maximum of around 0.5, at a point that matches the psychophysical equiluminant point (arrows). This experiment has been replicated in 10 subjects. Typically, VEPs had a minimum amplitude at about 0.3 and 0.7 colour ratio due to interaction between the different waveforms of chromaticand luminance responses. For colour ratios lower than 0.3 and higher than 0.7, the VEP waveform assumes the morphology of luminance-contrast VEPs illustrated in Fig. 2C, whose amplitude is largest at 0 and 1 colour ratios. Fig. 4 shows that onset VEPs to high contrast chromatic gratings have their maximal amplitude at intermediate spatial frequencies (2±3 c/deg), with a steep roll off for both higher and lower spatial frequencies. The spatial tuning appears sharper for blue-yellow patterns, and is consistent among subjects. The spatial tuning effect is present also at 25% contrast (not shown in ®gures). The effect of stimulus duration on onset VEPs to redgreen gratings is summarized in Fig. 5. Virtually identical results have been obtained with blue-yellow stimuli. As shown in Fig. 5A, onset duration 150 ms or longer produce a distinct positive offset response. By reducing onset dura-
tion, the positive offset response progressively merges with the onset response, resulting in a sharp increase of amplitude (Fig. 5A,B). With further reduction of onset duration below 50 ms, VEPs decrease in amplitude. With onset duration longer than 150 ms, the VEPs also decrease in amplitude. Overall, this effect is qualitatively similar to that previously described for luminance-contrast VEPs (Jeffreys, 1977) (for review, see Regan, 1989). That is, with a short-duration presentation, offset-related components summate with onset-related components. For luminance contrast stimuli, responses of maximal amplitude are obtained with very brief onset duration (25 ms). By contrast, prolonged presentation produce response attenuation due to adaptation of pattern-speci®c components. While a brief pattern presentation appears appropriate for luminance contrast stimuli, this is possibly not the case for stimuli of pure chromatic contrast. With chromatic stimuli, mixing the onset response (comparatively more tonic) with the offset response (comparatively more phasic) may introduce contribution from the luminance system (phasic). In addition, brief pattern presentations do not appear suitable to fully stimulate the chromatic system, due to its longer temporal integration as compared with the achromatic system (Smith et al., 1984; Swanson et al., 1987). In reply to Fig. 5C, the psychophysical contrast sensitivity for red-green gratings (the same stimulus used for VEP recordings) has been measured as a function of onset time. The contrast sensitivity drops dramatically for presentation times shorter than
778
V. Porciatti, F. Sartucci / Clinical Neurophysiology 110 (1999) 772±781
Fig. 7. Con®dence limits (95%) of latency of onset VEPs to equiluminant red-green (A) and blue-yellow (B) gratings of different contrast (spatial frequency 2 c/deg, ®eld size 14 £ 14 deg, 300 ms onset, 700 ms offset) for a population of normal subjects (n 28, meanage 32:2, SD 10:7).
150 ms. A comparable effect is seen with VEPs, in the representative example illustrated in Fig. 5D. At high contrast, VEPs to 50 and 100 ms onset time have an amplitude substantially larger than that of VEPs to 300 ms onset time due to summation of onset- and offset-related components. By decreasing contrast, however, brief-onset VEPs decrease in amplitude with a very steep slope, resulting in a considerably higher contrast threshold as compared with VEPs to 300 ms onset presentation. Overall, ®ndings of this set of experiments indicate that a presentation time of about 300 ms represents a reasonable compromise to obtain speci®c onset responses of sizeable amplitude, at both high and low contrast levels. Amplitude and latency values of chromatic onset VEPs (2 c/deg, 300 ms onset, 700 ms offset) for the whole group (n 28) of control subjects are illustrated in Fig. 6. For comparison, the same ®gure reports data of onset VEPs to luminance contrast stimuli, obtained from a subset (n 6) of subjects with a comparable mean age. The contrast dependence is qualitatively similar for red-green and blueyellow. Amplitude decreases rather monotonically with decreasing Log10 contrast, whereas latency increases with an accelerating slope. The difference in latency between VEPs to 6 and 90% contrast is remarkable (120 ms for red-green and 130 ms for blue-yellow). The contrast- dependence of chromatic onset VEPs differs substantially from that of luminance-contrast VEPs. The latter typically saturate in amplitude at moderate contrast and change little in latency with contrast (about 20 ms from 6 to 90%). On average, onset VEPs to red-green have a larger amplitude and a shorter latency than those to blue-yellow (two-way repeated measures ANOVA, amplitude: F 1; 279 6:4, P 0:018; latency: F 1; 279 9:1, P 0:006). As
shown in Fig. 6A,B, the extrapolated contrast threshold appears in the same order for red-green and blue-yellow stimuli (2.9 and 3.8%, respectively). One should take into account, however, that the spectral sensitivities of long wavelength (L) and medium wavelength (M) cones are largely overlapping. As a consequence, the effective cone excitation (cone contrast) produced by red-green equiluminant patterns is much lower than the physical stimulus contrast (Michelson). This does not apply for blue-yellow stimuli, since the spectral sensitivity of short wavelength (S) cones is virtually non overlapping with those of L and M cones. Cone contrast (root mean square) for red-green stimuli can be calculated by transforming the CIE co-ordinates into cone excitation using the Smith and Pokorny (1975) primaries. In practice, it was equivalent to dividing the Michelson contrast by 3.6. Thus, in terms of cone contrast, the VEP threshold for red-green stimuli is much lower than that for blue-yellow stimuli. It is interesting that the average psychophysical threshold for the same stimuli (method of adjustment), corresponds well to the VEP extrapolated threshold. Psychophysical thresholds, measured in a subset of subjects (n 10) were 2:2 ^ 0:4% for red-green and 2:7 ^ 0:6% for blue-yellow. Fig. 7 shows the 95% con®dence limits (mean ^ 2 SDs) of latency for our sample of normal subjects. Conditions are identical to those described in Fig. 6. For both red-green and blue-yellow, the variability is approximately constant in the contrast range 90±25%. The coef®cient of variation (percent SD/mean) is about 10%, which is somehow smaller than that reported for onset VEPs to checkerboard patterns (Riemslag et al., 1981; Harding and Wright, 1986). At lower contrasts, the variability is unacceptably high for clinical application.
V. Porciatti, F. Sartucci / Clinical Neurophysiology 110 (1999) 772±781
4. Discussion Chromatic stimuli can be generated and presented in many different ways, and responses may be analyzed in both time and frequency domains. After inspection of the available literature, we realized that some conditions appear more suitable than others for application in a clinical context, in terms of robustness of response, ease of recording and interpreting data. In addition, the way in which chromatic stimuli are generated and calibrated should be standardized enough for exchanging information among laboratories. The methods described in this study, represent a reasonable compromise among the above variables, and the rationale for a particular choice has been described in detail in the Section 2. It is important to emphasize here, that sinusoidal chromatic gratings can be easily generated and controlled by means of graphic cards of widespread use, such as those we used. As compared with checkerboards, sinusoidal gratings appear a more suitable stimulus for chromatic stimulation. For checkerboard stimuli, VEPs to luminance contrast and chromatic contrast have similar morphology (Regan and Spekreijse, 1974), whereas grating stimuli give quite different waveforms (for discussion see Regan and He, 1996). In addition, the complex spectrum of checkerboard stimuli contains relevant contributions of high-spatial frequencies which emphasize problems of chromatic aberrations (Flitcroft, 1989), resulting in spurious luminance contrast. Several studies (Murray et al., 1987; Berninger et al., 1989; Rabin et al., 1994; Regan and He, 1996; Tobimatsu et al., 1996) have shown that chromatic contrast stimuli, when presented with the clinically-popular transient-reversal mode, produce meagre responses, probably due to the adaptation to standing contrast. By contrast, transient-onset presentation produce VEPs characterized by a robust negative potential, clearly different from the waveform of onset VEPs to luminance contrast. Onset VEPs to red-green an blue-yellow display a comparable waveform, in agreement with previous reports (Berninger et al., 1989; Rabin et al., 1994; Regan and He, 1996). Our data extend them by evaluating the contrast-dependence of chromatic onset VEPs and their interaction with stimulus duration. We have used a relatively long onset duration (300 ms; 700 ms offset), in order to separate onset- from offset-related components and allow full temporal integration of the stimulus (see Section 3). In this way, chromatic onset VEPs may be reliably recorded at low (6%) contrast levels. The VEP amplitude is approximately linearly related to Log10 contrast, and the linear extrapolation to 0 V yields a contrast threshold of about 3% for red-green and 4% for blue-yellow, in the range of corresponding psychophysical thresholds. Both the psychophysical and VEP contrast threshold are considerably higher with shorter onset duration. Rabin et al. (1994) used a brief onset (106 ms) offset (394 ms) duration to record VEPs to red-green and blue-yellow gratings of 1 c/ deg of different contrast. For both red-green and blue-
779
yellow stimuli, the VEP amplitude saturated at moderate contrast, and the contrast thresholds (extrapolated by 2nd order polynomials to 0 V), were higher (6.2 and 9.4%, respectively) than those found in the present study. Their results are in keeping with ours, showing higher contrast thresholds with shorter stimulus duration. Amplitude saturation may be due to the presence of offset components in the response, which display a contrast dependence different from that of onset components. Onset VEPs to high-contrast chromatic stimuli are band-pass tuned, with sharp attenuation at both higher and lower spatial frequencies, in agreement with previous studies (Murray et al., 1987; Berninger et al., 1989; Rabin et al., 1994; Tobimatsu et al., 1996). With the stimulation ®eld of a relatively large size (14 deg) that we have used, the peak spatial frequency is about 2 c/deg for both redgreen and blue-yellow stimuli. The fact that the form of the spatial frequency tuning for high contrast stimuli shows a distinct low-frequency fall-off is somehow intriguing, given that the psychophysical contrast sensitivity function is low pass (Mullen, 1985). However, at threshold, steady-state VEPs to red-green gratings are reported to match the psychophysical contrast sensitivity function (Fiorentini et al., 1991; Morrone et al., 1993). One delicate aspect of chromatic VEPs is the choice of stimulus ®eld size. On the one hand, a relatively large ®eld size is advantageous to maximize response amplitude (Kulikowski et al., 1991; Rabin et al., 1994; Korth and Nguyen, 1997). However, a large ®eld size may introduce spurious luminance contamination due to both chromatic aberration and retinal inhomogeneity (Stabell and Stabell, 1980). The theoretical implications of this issue have been discussed extensively elsewhere (Regan, 1989; Kulikowski et al., 1996; Switkes et al., 1996). The point here, is whether the choice of particular conditions may represent a reasonable compromise for clinical applications. Under the present conditions, possible luminance intrusions do not appear to contaminate signi®cantly the chromatic response. As shown by our results, onset VEPs to both red-green and blueyellow have a maximal amplitude at colour ratios close to the Vl equiluminance point. In case of substantial chromatic aberration, one would have expected a departure from that value due to a change in the effective contrast between different colours. The effect should have been stronger for blue-yellow, as compared with red-green patterns (Le Grand, 1956). In addition, when r and b colour ratios are intentionally mismatched from the equiluminant point by a substantial amount (i.e. reducing chromatic contrast and introducing at least 20% of luminance contrast, see Fig. 3), both the VEP waveform and peak latency do not change appreciably. The range of mismatch in the equiluminant point we have intentionally introduced is about twice the size as that reported for normal observers (Fiorentini et al., 1996). These results indicate that the response is largely dominated by chromatic contrast and is relatively resistant to departures from optimal conditions. This
780
V. Porciatti, F. Sartucci / Clinical Neurophysiology 110 (1999) 772±781
appears an advantageous characteristic for clinical application. Nevertheless, we think that evaluating individual equiluminant r and b colour ratios is an important preliminary control for congenital/acquired dyscromatopsies. In addition, a proper setting of equiluminant r and b colour ratios maximizes response amplitude and reduces its variability. As summarized above, the spatio-temporal properties of VEPs to pure chromatic contrast stimuli resemble corresponding properties of colour-opponent neurons, suggesting that their activity is somehow re¯ected in the response. The present results are in keeping with this view. In particular, speci®c temporal properties of chromatic responses are stressed. As compared with luminance contrast, chromatic onset VEPs have a longer integration time. In addition, response latency increases with decreasing contrast with a much steeper slope. A comparable dependence of latency on contrast was reported by Rabin et al. (1994), although they recorded chromatic VEPs over a smaller range of contrasts due to brief onset stimulus presentation. Responses to redgreen and blue-yellow display a comparable waveform, however their properties differ from each other. As compared with blue-yellow, onset VEPs to red-green have a larger amplitude, shorter latency and lower contrast threshold. These differential properties are in keeping with physiological properties of colour-opponent neurons, suggesting that red-green and blue-yellow onset VEPs re¯ect, at least in part, the activity originating in the Parvocellular and Koniocellular visual pathways, respectively. In conclusion, VEPs to onset-offset (300±700 ms) of sinusoidal gratings (2 c/deg, 14 deg ®eld size) display characteristics of speci®city and robustness suitable for the clinical application. In combination with established VEP techniques for luminance contrast stimuli, chromatic VEPs may help to better understand pathophysiology involving the visual pathway. The present study provides a basic framework in that direction. References Allen D, Banks MS, Norcia AM. Does chromatic sensitivity develop more slowly than luminance sensitivity? Vision Res 1993;33:2553±2562. Berninger TA, Arden GB, Hogg CR, Frumkes T. Separable evoked retinal and cortical potentials from each major visual pathway: preliminary results. Br J Ophthalmol 1989;73:502±511. Buttner T, Kuhn W, Muller T, Heinze T, Puhl C, Przuntek H. Chromatic and achromatic visual evoked potentials in Parkinson's disease. Electroenceph clin Neurophysiol 1996;100:443±447. Crognale MA, Switkes E, Adams AJ. Temporal response characteristics of the spatiochromatic visual evoked potential: non-linearities and departures from psychophysics. J Opt Soc Am (A) 1997;14:2595±2607. Crognale MA, Switkes E, Rabin J, Schneck ME, Haegerstrom-Portnoy G, Adams AJ. Application of the spatiochromatic visual evoked potential to detection of congenital and acquired color-vision de®ciencies. J Opt Soc Am (A) 1993;10:1818±1825. Dacey DM, Lee BB. The `blue-on' opponent pathway in primate retina originates from a distinct bistrati®ed ganglion cell type. Nature 1994;367:731±735.
Derrington AM, Krauskopf J, Lennie P. Chromatic mechanisms in lateral geniculate of macaque. J Physiol (Lond) 1984;357:241±265. Engel S, Zhang X, Wandell B. Colour tuning in human visual cortex measured with functional magnetic resonance imaging. Nature 1997;388:68±71. Fiorentini A, Burr DC, Morrone C. Temporal characteristics of colour vision: VEP and psychophysical measurements. In: Valberg A, Lee BB, editors. From pigments to perception: advances in understanding visual processes, New York: Plenum Press, 1991. pp. 139±150. Fiorentini A, Porciatti V, Morrone MC, Burr DC. Visual ageing: unspeci®c decline of the responses to luminance and colour. Vision Res 1996;36:3557±3566. Flitcroft DI. The interactions between chromatic aberration, defocus and stimulus chromaticity: Implications for visual physiology and colorimetry. Vision Res 1989;29:349±360. Girard P, Morrone MC. Spatial structure of chromatically opponent receptive ®elds in the human visual system. Vis Neurosci 1995;12:103±116. Harding GFA, Odom JV, Spileers W, Spekreijse H. Standard for visual evoked potentials . Vision Res 1996;36:3567±3572. Harding GFA, Wright CE. Visual evoked potentials in acute optic neuritis. In: Hess RF, Plant GT, editors. Optic neuritis, Cambridge: Cambridge University Press, 1986. pp. 230. Heywood CA, Nicholas JJ, Cowey A. Behavioural and electrophysiological chromatic and achromatic contrast sensitivity in an achromatopsic patient. J Neurol Neurosurg Psychiatry 1996;60:638±643. Jeffreys D. The physiological signi®cance of pattern visual evoked potentials. In: Desmedt JE, editor. Visual evoked potentials in man: new developments, Oxford: Clarendon Press, 1977. pp. 134. Korth M, Nguyen NX. The effect of stimulus size on human cortical potentials evoked by chromatic patterns. Vision Res 1997;37:649±657. Krauskopf J, Williams DR, Heeley DW. Cardinal directions of color space. Vision Res 1982;22:1123±1131. Kulikowski JJ, Muray IJ, Russel MHA. Effects of stimulus size on chromatic and achromatic VEPs. In: Drum B, Moreland J, Serra A, editors. Colour vision de®ciencies X: proceedings of the ninth symposium of the international group on colour de®ciencies, Dordrect: Kluwer, 1991. pp. 51. Kulikowski JJ, Robson AG, McKeefry DJ. Speci®city and selectivity of chromatic visual evoked potentials. Vision Res 1996;36:3397±3401. Le Grand Y. Optique Physiologique, Tome III, l'eÂspace visuel. Paris: Masson, 1956. Lennie P, D'Zmura M. Mechanisms of color vision. Crit Rev Neurobiol 1988;3:333±400. Lennie P, Krauskopf J, Sclar G. Chromatic mechanisms in striate cortex of macaque. J Neurosci 1990;10:649±669. Macaluso C, Lamedica A, Baratta G, Cordella M. 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. Electroenceph clin Neurophysiol 1996;100:12±17. McKeefry DJ, Russell MH, Murray IJ, Kulikowski JJ. Amplitude and phase variations of harmonic components in human achromatic and chromatic visual evoked potentials. Vis Neurosci 1996;13:639±653. Merigan WH, Maunsell JHR. How parallel are the primate visual pathways? Annu Rev Neurosci 1993;16:369±402. Morrone MC, Burr DC, Fiorentini A. Development of infant contrast sensitivity to chromatic stimuli. Vision Res 1993;33:2535±2552. Morrone MC, Fiorentini A, Burr DC. Development of the temporal properties of visual evoked potentials to luminance and colour contrast in infants. Vision Res 1996;36:3141±3155. Mullen KT. The contrast sensitivity of human colour vision to red-green and blue-yellow chromatic gratings. J Physiol (Lond) 1985;359:381± 400. Murray IJ, Parry NRA, Carden D, Kulikowski JJ. Human visual evoked potentials to chromatic and achromatic gratings. Clin Vision Sci 1987;3:231±244.
V. Porciatti, F. Sartucci / Clinical Neurophysiology 110 (1999) 772±781 Porciatti V, Di Bartolo E, Nardi N, Fiorentini A. Responses to chromatic and luminance contrast in glaucoma: a psychophysical and electrophysiological study. Vision Res 1997;37:1975±1987. Porciatti V, Sartucci F. Retinal and cortical evoked responses to chromatic contrast stimuli. Speci®c losses in both eyes of patients with multiple sclerosis and unilateral optic neuritis. Brain 1996;119:723±740. Rabin J, Switkes E, Crognale M, Schneck ME, Adams AJ. Visual evoked potentials in three-dimensional color space: correlates of spatio-chromatic processing. Vision Res 1994;34:2657±2671. Regan D. Human brain electrophysiology. Evoked potentials and evoked magnetic ®elds in science and medicine. New York: Elsevier, 1989. Regan D, He P. Magnetic and electrical brain responses to chromatic contrast in human. Vision Res 1996;36:1±18. Regan D, Spekreijse H. Evoked potential indications of colour blindness. Vision Res 1974;14:89±95. Riemslag FCC, Spekreijse H, Van Walbek H. Pattern-reversal and pattern appearance-disappearance responses in MS patients. In: Spekreijse H, Apkarian PA, editors. Visual pathways. Electrophysiology and pathology, 27. The Hague: Dr. W. Junk Publishers, 1981. pp. 215±221. Russel MHA, Murray IJ, Metcalfe RA, Kulikowski J. The visual defect in multiple sclerosis and optic neuritis. Brain 1991;114:2419±2435. Schneck ME, Fortune B, Switkes E, Crognale M, Adams AJ. Acute effects of blood glucose on chromatic visually evoked potentials in persons with diabetes and in normal persons. Invest Ophthalmol Vis Sci 1997;38:800±810.
781
Smith VC, Bowen RW, Pokorny J. Threshold temporal integration of chromatic stimuli. Vision Res 1984;24:653±659. Smith VC, Pokorny J. Spectral sensitivity of the foveal cone photopigments. Vision Res 1975;15:161±171. Spinelli D, Angelelli P, De Luca M, Burr DC. VEP in neglect patients have longer latencies for luminance but not for chromatic patterns. NeuroReport 1996;7:815±819. Stabell U, Stabell B. Variation in density of macular pigmentation and in short-wave cone sensitivity with eccentricity. J Opt Soc Am 1980;70:706±711. Swanson WH, Uneno TVCS, Pokorny J. Temporal modulation sensitivity and pulse-duration thresholds for chromatic and luminance perturbations. J Opt Soc Am (A) 1987;4:1992±2005. Switkes E, Crognale M, Rabin J, Schneck ME, Adams AJ. Reply to `speci®city and selectivity of chromatic visual evoked potentials. Vision Res 1996;36:3403±3405. Tobimatsu S, Kato M. Multimodality visual evoked potentials in evaluating visual dysfunction in optic neuritis. Neurology 1998;50:715±718. Tobimatsu S, Tomoda H, Kato M. Parvocellular and magnocellular contributions to visual evoked potentials in humans: stimulation with chromatic and achromatic gratings and apparent motion. J Neurol Sci 1995;134:73±82. Tobimatsu S, Tomoda H, Kato M. Human VEPs to isoluminant chromatic and achromatic sinusoidal gratings: separation of parvocellular components. Brain Topogr 1996;8:241±243.
Technical note
Normative data for onset VEPs to red-green and blue-yellow chromatic contrast Vittorio Porciatti a,*, Ferdinando Sartucci b a
b
Institute of Neurophysiology, CNR, Pisa, Italy Department of Neuroscience, Institute of Neurology, University of Pisa, Pisa, Italy Accepted 10 December 1998
Abstract Objective: To better characterize the properties of chromatic VEPs to onset-offset of red-green and blue-yellow equiluminant patterns, and establish normative values for a set of stimuli able to elicit robust and reliable responses, suitable for the clinical application. Methods: Chromatic VEPs have been recorded (Oz lead) from 28 normal subjects (age range 20±53 years) in response to monocular presentation of both red-green and blue-yellow equiluminant sinusoidal gratings. Stimuli were generated by a Cambridge VSG/2 card and displayed on a Barco CCID monitor (14 £ 14 deg ®eld size). Spatial frequency, chromaticity, contrast and onset-offset duration were varied. Results: For both red-green and blue-yellow equiluminant stimuli, robust responses have been obtained with gratings of 2 c/deg, presented in onset (300 ms) offset (700 ms) mode, at contrasts ranging from 90 to 6%. In all observers, the VEP waveform consisted mainly of a negative wave at stimulus onset, with a latency rapidly increasing with decreasing contrast. For both red-green and blue-yellow stimuli, the VEP contrast threshold coincided with the psychophysical threshold. Conclusions: The results complement previous studies aimed at characterizing the properties of chromatic VEPs. In addition, normative data are provided for a set of stimulus characteristics suitable for the clinical routine. q 1999 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Parvocellular; Koniocellular; Magnocellular; Chromatic contrast; Luminance contrast; VEPs
1. Introduction The main interest for studying VEPs in response to patterns with pure chromatic contrast, is that of recording speci®c responses from colour-opponent pathways, anatomically and physiologically distinct from the achromatic pathway at retinal, geniculate and cortical level (Lennie and D'Zmura, 1988; Lennie et al., 1990; Merigan and Maunsell, 1993; Dacey and Lee, 1994; Derrington et al., 1984; Engel et al., 1997). The achromatic stream originates from large (parasol) ganglion cells projecting to magnocellular layers of the lateral geniculate nucleus (LGN). The red-green pathway originates from smaller (midget) ganglion cells relaying to parvocellular layers of the LGN. The blue-yellow pathway stems from bistrati®ed ganglion cells (with dendritic arbours in both ON- and OFF laminae of the inner plexiform layer) and project to the interlaminar (Koniocellular) neurons of the LGN. * Corresponding author. Tel.: 1 39-050-540770; fax: 540080. E-mail address: [email protected] (V. Porciatti)
1 39-050-
Following the pioneering work of Regan, Spekreijse and colleagues (reviewed in Regan, 1989), chromatic VEPs have received increasing attention for studying basic mechanisms of vision and clinical applications. This has been favoured by the recent development of affordable graphic cards and linear TV displays, which allow presentation of a wide variety of chromatic stimuli. Although some controversy exists on a possible contribution of luminance contrast contamination of responses (Kulikowski et al., 1996; Switkes et al., 1996), the body of evidence shows that the properties of the chromatic contrast VEPs differ greatly from those of luminance contrast responses. Overall, independently of the response type (transient and steadystate) and mode of presentation (reversal and onset-offset), differences between chromatic and achromatic VEPs are in the same direction (Murray et al., 1987; Berninger et al., 1989; Fiorentini et al., 1991; Rabin et al., 1994; McKeefry et al., 1996; Porciatti and Sartucci, 1996; Regan and He, 1996; Tobimatsu et al., 1996; Crognale et al., 1997; Porciatti et al., 1997). Chromatic VEPs, as compared with luminance VEPs, are slower and attenuate more steeply as a function of increasing spatial frequency, temporal frequency and
1388-2457/99/$ - see front matter q 1999 Elsevier Science Ireland Ltd. All rights reserved. PII: S13 88-2457(99)0000 7-3
CLINPH 98696
V. Porciatti, F. Sartucci / Clinical Neurophysiology 110 (1999) 772±781
decreasing contrast. These differences are expected on the basis of the known physiology of different subpopulations of generators of the visual pathway, indicating that chromatic VEPs re¯ect to some extent the activity of colouropponent neurons. By contrast, both colour-opponent neurons and colour-unselective neurons of the magnocellular pathway may contribute to luminance contrast VEPs. Recently, it has been shown that chromatic and achromatic VEPs are differently altered in disease (Russel et al., 1991; Crognale et al., 1993; Buttner et al., 1996; Heywood et al., 1996; Porciatti and Sartucci, 1996; Spinelli et al., 1996; Porciatti et al., 1997; Schneck et al., 1997; Tobimatsu and Kato, 1998), suggesting speci®c vulnerability of visual pathway subpopulations. This indicates that the combined assessment of chromatic- and luminance VEPs may be useful for a better understanding of neuro-ophthalmological disorders. However, chromatic VEPs, as compared with ordinary luminance contrast VEPs, may be considered a more delicate technique due to both technical requirements for generating stimuli and necessary controls for minimizing luminance intrusion. Extended clinical application of chromatic VEPs requires robust and reliable responses, relatively resistant to departures from optimal conditions. This study further characterizes some properties of chromatic VEPs to red-green and blue-yellow patterns, proposing normative data for a set of stimuli and recording conditions which appear feasible in a clinical context.
2. Methods 2.1. Visual stimuli Stimuli were horizontal sinusoidal gratings of different spatial frequency (0.5±1±2±3±4±5 c/deg) and contrast (6± 12±25±50±90%) generated by a widely used board (VSG/2 Cambridge Research) and displayed on a colour monitor (Barco CCID 7751, Kortrijk, Belgium) with a 14 bit resolution at 120 Hz, 512 lines per frame. The software required for generating stimuli and evaluating psychophysical responses can be easily obtained by modifying source codes of routines provided by the factory. Chromatic modulation was performed along L-M (red-green) and S-(L 1 M) (blue-yellow) cardinal axes of colour vision (Krauskopf et al., 1982), in order to selectively in¯uence the activity of the corresponding opponent mechanism. Chromatic-contrast patterns (red-green or blue-yellow) were obtained by superimposing (out-of-phase by 180 deg) red-black and greenblack (or blue-black and yellow-black, respectively) gratings of identical contrast. Luminance-contrast patterns (yellow-black and white-black, respectively) were obtained by superimposing the same gratings in-phase. As illustrated in the left panels of the example reported in Fig. 1, the chromaticity of chromatic-contrast patterns was varied by changing the relative luminance of superimposed gratings, as is now standard (Mullen, 1985). The ratio r of red to total
773
Fig. 1. Method for producing chromatic-contrast gratings with different chromaticity. Green-black and red-black gratings are superimposed out-ofphase by 180 deg. The relative luminance of red and green is varied to obtain all combinations of luminance- and chromatic contrast, including pure luminance- (r 0, r 1) and pure colour (r 0:5) contrast. Note that chromaticity is varied at constant Michelson contrast and mean luminance (sum of red and green mean luminances). See Section 2 for further details.
luminance {red/(red 1 green)} and the ratio b of blue to total luminance {blue/(blue 1 red 1 green} could be varied from 0 to 1, where r 0, r 0:5 and r 1 de®ne green-black, red-green and red-black patterns, respectively, and b 0, b 0:5, and b 1 de®ne yellow-black, blueyellow and blue-black patterns. As shown in the right panel of Fig. 1, green-black and red-black gratings (r 0 and 1) contain pure luminance-contrast (simply de®ned by the Michelson contrast (Lmax 2 Lmin /(Lmax 1 Lmin )), irrespective of colour. For intermediate values of r there are different amounts of luminance- and chromatic contrast. At r 0:5 the chromatic contrast is maximal and the luminance contrast is zero (equiluminant point). It is worth noting that the variation of chromaticity (colour ratio) occurs at constant Michelson contrast for both green and red gratings and at constant mean luminance (de®ned as the sum of the mean luminances of red and green gratings). For red-green patterns, the display was viewed through yellow ®lters (Kodak Wratten 16) to attenuate wavelengths lower than 500 nm. This approach is believed useful to minimize the variable absorption of shorter wavelengths by the eye lens in older subjects (Fiorentini et al., 1996). CIE co-ordinates could be obtained from factory measurements, but were further evaluated by a Minolta Chromameter CS 100. For red-green, the CIE were red: x 0:637, y 0:362; green: x 0:416, y 0:582; mean luminance: 14 cd/m 2. For blue-yellow, the CIE were blue: x 0:075, y 0:377; yellow: x 0:377, y 0:348; mean luminance 6 cd/m 2. The photometer was also used to cali-
774
V. Porciatti, F. Sartucci / Clinical Neurophysiology 110 (1999) 772±781
brate look-up tables of the software used for linearization of voltage/luminance characteristics of the display (gamma correction). Calibration was found to be very stable during the annual follow-up. Stimuli subtended an area of 14 £ 14 deg at the viewing distance of 100 cm. Subjects ®xated a black spot at the centre of the screen. Further details can be found elsewhere (Fiorentini et al., 1996; Porciatti et al., 1997). 2.2. Equiluminance Equiluminance was established psychophysically for every subject with a variant of the method of heterochromatic ¯icker photometry. Red-green and blue-yellow gratings of 60% contrast were sinusoidally reversed at 15 and 10 Hz, respectively. Subjects adjusted r or b ratios to nil or minimized the perception of ¯icker. In some cases, the grating contrast was varied to emphasize the effect. The equiluminance point was the average of at least 6 independent measurements. In all subjects, for both red-green and blueyellow patterns the equiluminant point was found at r and b values close to 0.5 (Vl equiluminant point: see Fiorentini et al. (1996) for discussion). 2.3. VEP recording The issue of frequency and time-domain analysis of human chromatic VEPs has been addressed in some detail elsewhere (McKeefry et al., 1996). Steady-state responses to sinusoidal counterphase appear a suitable technique to evaluate the spatio-temporal properties of the colour-opponent system. With this method it is possible to successfully predict spatio-temporal thresholds (Fiorentini et al., 1991; Girard and Morrone, 1995; Macaluso et al., 1996), and evaluate the development (Allen et al., 1993; Morrone et al., 1993; Morrone et al., 1996), ageing (Fiorentini et al., 1996) and dysfunction (Porciatti and Sartucci, 1996; Spinelli et al., 1996; Porciatti et al., 1997) of the chromatic system. By contrast, transient VEPs to abrupt reversal of equiluminant chromatic gratings are meagre responses, not radically different from transient VEPs to reversal of luminance contrast (Murray et al., 1987; Berninger et al., 1989; Rabin et al., 1994; Fiorentini et al., 1996). However, onset presentation of chromatic patterns evokes robust transient VEPs with a well-de®ned waveform, different from that in response to similar patterns modulated in luminance contrast (Murray et al., 1987; Berninger et al., 1989; Tobimatsu et al., 1995; Regan and He, 1996). Since time-domain analysis of transient responses is a widely accepted clinical standard (Harding et al., 1996), we have further explored the characteristics of transient VEPs to onset-offset of chromatic gratings, with the aim of establishing a set of conditions suitable for clinical application. Red-green and blueyellow gratings of different chromaticity, spatial frequency and contrast were presented for a variable amount of time and exchanged with an equiluminant uniform background (patterns of zero contrast: yellowish for red-green, bluish-
white for blue-yellow). VEPs have been typically recorded from scalp electrodes (resistance , 5 kV ) placed at Oz (active), Mastoid (reference) and Cz (ground). Signals were ®ltered (0.3±100 Hz, 26 dB/oct) ampli®ed (50 000 fold), digitized (2 kHz, 12 bit resolution) and averaged (at least 80 sums), with a rejection of signals exceeding a threshold voltage. Partial averages (blocks of 10 sums) of total average were used to evaluate response consistency (standard error of amplitude of partial averages). The SEM was typically about 7% of the average VEP amplitude. Additional blocks were recorded when necessary to keep the SEM below 15% of the average amplitude. Responses to patterns of zero contrast were frequently recorded to have a measure of residual noise. The average peak-to-trough amplitude of residual noise was 2.1 mV, SEM 0.24. 2.4. Subjects Twenty-eight observers (14 males, 14 females, age range 19±53 years, mean 32:2 ^ 10:7) volunteered for psychophysical and electrophysiological measurements after a full description of the aims and methods of this study. All subjects had normal (1.0 or better) visual acuity with no or minimal (0.5±2.5 spherical, 0.25±0.75 cylindrical, diopters) refraction, and were fully corrected for the viewing distance. They had normal colour vision (Ishihara) and did not report any previous ocular or systemic disease. Two male subjects were not included because of previously undiagnosed colour anomaly of the protan type, which resulted in abnormal Ishihara scores and r colour ratios (see above). 3. Results Fig. 2 reports representative examples of VEPs recorded from the same subject in response to red-green and blueyellow equiluminant gratings of 2 c/deg presented in onsetoffset (300±700 ms) mode. For both red-green (Fig. 2A) and blue-yellow (Fig. 2B) patterns of 90% contrast, VEPs consist of a sharp negative de¯ection at stimulus onset with a peak latency of 120±130 ms. Unless otherwise speci®ed, the latency of chromatic onset VEPs refers to the peak latency of the negative wave, whereas amplitude is measured from the trough of the negative wave to the peak of the following positive wave. The amplitude of chromatic onset VEPs progressively decreases, and latency dramatically increases, with decreasing contrast. The response at stimulus offset consists of a broad positivity. The waveform of chromatic onset VEPs differs remarkably from that in response to luminance-contrast patterns of the same mean luminance and spatial frequency (Fig. 2C). At 120±130 ms, onset VEPs to high contrast luminance gratings display a major positivity, whose latency changes little with decreasing contrast. In addition, stimulus offset elicits a distinct response. In all tested subjects, chromatic VEPs displayed a comparable morphology. Overall, chromatic
V. Porciatti, F. Sartucci / Clinical Neurophysiology 110 (1999) 772±781
775
Fig. 2. Examples of VEPs to onset (300 ms) ± offset (700 ms) of horizontal sinusoidal gratings with pure chromatic (A,B) and luminance (C) contrast. Spatial frequency 2 c/deg, ®eld size 14 £ 14 deg. Note the different waveform of chromatic- and luminance onset VEPs. Also note that the peak latency (dotted lines) of chromatic VEPs, differently from luminance VEPs, increases dramatically with decreasing contrast (numbers to the lefts of tracings).
onset VEPs, as compared with luminance contrast VEPs, were of larger amplitude (see below). Fig. 3 shows an example of how chromatic onset VEPs change as a function of the colour ratio r and b (proportion
of red in the red-green, and blue in the blue-yellow mixture, respectively). As discussed in the Section 2, r and b values yielding equiluminance are expected at values around 0.5. Departure from the equiluminant point introduces lumi-
Fig. 3. Variation of onset VEPs as a function of colour ratio r (proportion of red in the red-green mixture) and b (proportion of blue in the blue-yellow mixture). For both red-green (A) and blue-yellow (B), waveforms with largest amplitude occur at r and b ratios of 0.5, close to the psychophysical equiluminant point (open arrows in C,D). Note in (A,B) that response latency is rather independent of colour ratio in the 0.4±0.6 range. Same conditions as in Fig. 2, contrast 90%.
776
V. Porciatti, F. Sartucci / Clinical Neurophysiology 110 (1999) 772±781
Fig. 4. Amplitude variation of onset VEPs to equiluminant gratings of 90% contrast as a function of spatial frequency. For both red-green (A) and blue-yellow (B), maximal amplitude occurs at about 2 c/deg in 4 different subjects. Same conditions as in Fig. 2.
Fig. 5. Effect of onset duration (numbers to the right of tracings in A) of red-green equiluminant stimuli on VEP amplitude and contrast threshold. (A,B) Maximal amplitude occurs for 50 ms duration due to summation of onset- and offset-related components. Note that with 50 ms duration both the psychophysical (C) and VEP (D) contrast threshold are greatly elevated, as compared to 300 ms duration. For further explanation see Section 3.
V. Porciatti, F. Sartucci / Clinical Neurophysiology 110 (1999) 772±781
777
Fig. 6. Average amplitude and latencies of onset VEP to pure chromatic- and achromatic contrast as a function of Log10 contrast. Error bars represent the SEM. For chromatic gratings (A,B), the contrast dependence of amplitude is virtually linear, whereas that of luminance gratings (C) saturates at moderate contrast. The range of latency variation with contrast is much larger for chromatic- (D,E) than luminance (F) gratings.
nance contrast contribution in the stimulus (about 20% for a 0.1 change in r and b) which may unacceptably contaminate the chromatic response. For r and b values ranging from 0.4 to 0.6, both response waveform and peak latency of the negative wave are unchanged. However, the response amplitude does change as a function of r and b. As shown in Fig. 3C,D, the chromatic VEP amplitude has a local maximum of around 0.5, at a point that matches the psychophysical equiluminant point (arrows). This experiment has been replicated in 10 subjects. Typically, VEPs had a minimum amplitude at about 0.3 and 0.7 colour ratio due to interaction between the different waveforms of chromaticand luminance responses. For colour ratios lower than 0.3 and higher than 0.7, the VEP waveform assumes the morphology of luminance-contrast VEPs illustrated in Fig. 2C, whose amplitude is largest at 0 and 1 colour ratios. Fig. 4 shows that onset VEPs to high contrast chromatic gratings have their maximal amplitude at intermediate spatial frequencies (2±3 c/deg), with a steep roll off for both higher and lower spatial frequencies. The spatial tuning appears sharper for blue-yellow patterns, and is consistent among subjects. The spatial tuning effect is present also at 25% contrast (not shown in ®gures). The effect of stimulus duration on onset VEPs to redgreen gratings is summarized in Fig. 5. Virtually identical results have been obtained with blue-yellow stimuli. As shown in Fig. 5A, onset duration 150 ms or longer produce a distinct positive offset response. By reducing onset dura-
tion, the positive offset response progressively merges with the onset response, resulting in a sharp increase of amplitude (Fig. 5A,B). With further reduction of onset duration below 50 ms, VEPs decrease in amplitude. With onset duration longer than 150 ms, the VEPs also decrease in amplitude. Overall, this effect is qualitatively similar to that previously described for luminance-contrast VEPs (Jeffreys, 1977) (for review, see Regan, 1989). That is, with a short-duration presentation, offset-related components summate with onset-related components. For luminance contrast stimuli, responses of maximal amplitude are obtained with very brief onset duration (25 ms). By contrast, prolonged presentation produce response attenuation due to adaptation of pattern-speci®c components. While a brief pattern presentation appears appropriate for luminance contrast stimuli, this is possibly not the case for stimuli of pure chromatic contrast. With chromatic stimuli, mixing the onset response (comparatively more tonic) with the offset response (comparatively more phasic) may introduce contribution from the luminance system (phasic). In addition, brief pattern presentations do not appear suitable to fully stimulate the chromatic system, due to its longer temporal integration as compared with the achromatic system (Smith et al., 1984; Swanson et al., 1987). In reply to Fig. 5C, the psychophysical contrast sensitivity for red-green gratings (the same stimulus used for VEP recordings) has been measured as a function of onset time. The contrast sensitivity drops dramatically for presentation times shorter than
778
V. Porciatti, F. Sartucci / Clinical Neurophysiology 110 (1999) 772±781
Fig. 7. Con®dence limits (95%) of latency of onset VEPs to equiluminant red-green (A) and blue-yellow (B) gratings of different contrast (spatial frequency 2 c/deg, ®eld size 14 £ 14 deg, 300 ms onset, 700 ms offset) for a population of normal subjects (n 28, meanage 32:2, SD 10:7).
150 ms. A comparable effect is seen with VEPs, in the representative example illustrated in Fig. 5D. At high contrast, VEPs to 50 and 100 ms onset time have an amplitude substantially larger than that of VEPs to 300 ms onset time due to summation of onset- and offset-related components. By decreasing contrast, however, brief-onset VEPs decrease in amplitude with a very steep slope, resulting in a considerably higher contrast threshold as compared with VEPs to 300 ms onset presentation. Overall, ®ndings of this set of experiments indicate that a presentation time of about 300 ms represents a reasonable compromise to obtain speci®c onset responses of sizeable amplitude, at both high and low contrast levels. Amplitude and latency values of chromatic onset VEPs (2 c/deg, 300 ms onset, 700 ms offset) for the whole group (n 28) of control subjects are illustrated in Fig. 6. For comparison, the same ®gure reports data of onset VEPs to luminance contrast stimuli, obtained from a subset (n 6) of subjects with a comparable mean age. The contrast dependence is qualitatively similar for red-green and blueyellow. Amplitude decreases rather monotonically with decreasing Log10 contrast, whereas latency increases with an accelerating slope. The difference in latency between VEPs to 6 and 90% contrast is remarkable (120 ms for red-green and 130 ms for blue-yellow). The contrast- dependence of chromatic onset VEPs differs substantially from that of luminance-contrast VEPs. The latter typically saturate in amplitude at moderate contrast and change little in latency with contrast (about 20 ms from 6 to 90%). On average, onset VEPs to red-green have a larger amplitude and a shorter latency than those to blue-yellow (two-way repeated measures ANOVA, amplitude: F 1; 279 6:4, P 0:018; latency: F 1; 279 9:1, P 0:006). As
shown in Fig. 6A,B, the extrapolated contrast threshold appears in the same order for red-green and blue-yellow stimuli (2.9 and 3.8%, respectively). One should take into account, however, that the spectral sensitivities of long wavelength (L) and medium wavelength (M) cones are largely overlapping. As a consequence, the effective cone excitation (cone contrast) produced by red-green equiluminant patterns is much lower than the physical stimulus contrast (Michelson). This does not apply for blue-yellow stimuli, since the spectral sensitivity of short wavelength (S) cones is virtually non overlapping with those of L and M cones. Cone contrast (root mean square) for red-green stimuli can be calculated by transforming the CIE co-ordinates into cone excitation using the Smith and Pokorny (1975) primaries. In practice, it was equivalent to dividing the Michelson contrast by 3.6. Thus, in terms of cone contrast, the VEP threshold for red-green stimuli is much lower than that for blue-yellow stimuli. It is interesting that the average psychophysical threshold for the same stimuli (method of adjustment), corresponds well to the VEP extrapolated threshold. Psychophysical thresholds, measured in a subset of subjects (n 10) were 2:2 ^ 0:4% for red-green and 2:7 ^ 0:6% for blue-yellow. Fig. 7 shows the 95% con®dence limits (mean ^ 2 SDs) of latency for our sample of normal subjects. Conditions are identical to those described in Fig. 6. For both red-green and blue-yellow, the variability is approximately constant in the contrast range 90±25%. The coef®cient of variation (percent SD/mean) is about 10%, which is somehow smaller than that reported for onset VEPs to checkerboard patterns (Riemslag et al., 1981; Harding and Wright, 1986). At lower contrasts, the variability is unacceptably high for clinical application.
V. Porciatti, F. Sartucci / Clinical Neurophysiology 110 (1999) 772±781
4. Discussion Chromatic stimuli can be generated and presented in many different ways, and responses may be analyzed in both time and frequency domains. After inspection of the available literature, we realized that some conditions appear more suitable than others for application in a clinical context, in terms of robustness of response, ease of recording and interpreting data. In addition, the way in which chromatic stimuli are generated and calibrated should be standardized enough for exchanging information among laboratories. The methods described in this study, represent a reasonable compromise among the above variables, and the rationale for a particular choice has been described in detail in the Section 2. It is important to emphasize here, that sinusoidal chromatic gratings can be easily generated and controlled by means of graphic cards of widespread use, such as those we used. As compared with checkerboards, sinusoidal gratings appear a more suitable stimulus for chromatic stimulation. For checkerboard stimuli, VEPs to luminance contrast and chromatic contrast have similar morphology (Regan and Spekreijse, 1974), whereas grating stimuli give quite different waveforms (for discussion see Regan and He, 1996). In addition, the complex spectrum of checkerboard stimuli contains relevant contributions of high-spatial frequencies which emphasize problems of chromatic aberrations (Flitcroft, 1989), resulting in spurious luminance contrast. Several studies (Murray et al., 1987; Berninger et al., 1989; Rabin et al., 1994; Regan and He, 1996; Tobimatsu et al., 1996) have shown that chromatic contrast stimuli, when presented with the clinically-popular transient-reversal mode, produce meagre responses, probably due to the adaptation to standing contrast. By contrast, transient-onset presentation produce VEPs characterized by a robust negative potential, clearly different from the waveform of onset VEPs to luminance contrast. Onset VEPs to red-green an blue-yellow display a comparable waveform, in agreement with previous reports (Berninger et al., 1989; Rabin et al., 1994; Regan and He, 1996). Our data extend them by evaluating the contrast-dependence of chromatic onset VEPs and their interaction with stimulus duration. We have used a relatively long onset duration (300 ms; 700 ms offset), in order to separate onset- from offset-related components and allow full temporal integration of the stimulus (see Section 3). In this way, chromatic onset VEPs may be reliably recorded at low (6%) contrast levels. The VEP amplitude is approximately linearly related to Log10 contrast, and the linear extrapolation to 0 V yields a contrast threshold of about 3% for red-green and 4% for blue-yellow, in the range of corresponding psychophysical thresholds. Both the psychophysical and VEP contrast threshold are considerably higher with shorter onset duration. Rabin et al. (1994) used a brief onset (106 ms) offset (394 ms) duration to record VEPs to red-green and blue-yellow gratings of 1 c/ deg of different contrast. For both red-green and blue-
779
yellow stimuli, the VEP amplitude saturated at moderate contrast, and the contrast thresholds (extrapolated by 2nd order polynomials to 0 V), were higher (6.2 and 9.4%, respectively) than those found in the present study. Their results are in keeping with ours, showing higher contrast thresholds with shorter stimulus duration. Amplitude saturation may be due to the presence of offset components in the response, which display a contrast dependence different from that of onset components. Onset VEPs to high-contrast chromatic stimuli are band-pass tuned, with sharp attenuation at both higher and lower spatial frequencies, in agreement with previous studies (Murray et al., 1987; Berninger et al., 1989; Rabin et al., 1994; Tobimatsu et al., 1996). With the stimulation ®eld of a relatively large size (14 deg) that we have used, the peak spatial frequency is about 2 c/deg for both redgreen and blue-yellow stimuli. The fact that the form of the spatial frequency tuning for high contrast stimuli shows a distinct low-frequency fall-off is somehow intriguing, given that the psychophysical contrast sensitivity function is low pass (Mullen, 1985). However, at threshold, steady-state VEPs to red-green gratings are reported to match the psychophysical contrast sensitivity function (Fiorentini et al., 1991; Morrone et al., 1993). One delicate aspect of chromatic VEPs is the choice of stimulus ®eld size. On the one hand, a relatively large ®eld size is advantageous to maximize response amplitude (Kulikowski et al., 1991; Rabin et al., 1994; Korth and Nguyen, 1997). However, a large ®eld size may introduce spurious luminance contamination due to both chromatic aberration and retinal inhomogeneity (Stabell and Stabell, 1980). The theoretical implications of this issue have been discussed extensively elsewhere (Regan, 1989; Kulikowski et al., 1996; Switkes et al., 1996). The point here, is whether the choice of particular conditions may represent a reasonable compromise for clinical applications. Under the present conditions, possible luminance intrusions do not appear to contaminate signi®cantly the chromatic response. As shown by our results, onset VEPs to both red-green and blueyellow have a maximal amplitude at colour ratios close to the Vl equiluminance point. In case of substantial chromatic aberration, one would have expected a departure from that value due to a change in the effective contrast between different colours. The effect should have been stronger for blue-yellow, as compared with red-green patterns (Le Grand, 1956). In addition, when r and b colour ratios are intentionally mismatched from the equiluminant point by a substantial amount (i.e. reducing chromatic contrast and introducing at least 20% of luminance contrast, see Fig. 3), both the VEP waveform and peak latency do not change appreciably. The range of mismatch in the equiluminant point we have intentionally introduced is about twice the size as that reported for normal observers (Fiorentini et al., 1996). These results indicate that the response is largely dominated by chromatic contrast and is relatively resistant to departures from optimal conditions. This
780
V. Porciatti, F. Sartucci / Clinical Neurophysiology 110 (1999) 772±781
appears an advantageous characteristic for clinical application. Nevertheless, we think that evaluating individual equiluminant r and b colour ratios is an important preliminary control for congenital/acquired dyscromatopsies. In addition, a proper setting of equiluminant r and b colour ratios maximizes response amplitude and reduces its variability. As summarized above, the spatio-temporal properties of VEPs to pure chromatic contrast stimuli resemble corresponding properties of colour-opponent neurons, suggesting that their activity is somehow re¯ected in the response. The present results are in keeping with this view. In particular, speci®c temporal properties of chromatic responses are stressed. As compared with luminance contrast, chromatic onset VEPs have a longer integration time. In addition, response latency increases with decreasing contrast with a much steeper slope. A comparable dependence of latency on contrast was reported by Rabin et al. (1994), although they recorded chromatic VEPs over a smaller range of contrasts due to brief onset stimulus presentation. Responses to redgreen and blue-yellow display a comparable waveform, however their properties differ from each other. As compared with blue-yellow, onset VEPs to red-green have a larger amplitude, shorter latency and lower contrast threshold. These differential properties are in keeping with physiological properties of colour-opponent neurons, suggesting that red-green and blue-yellow onset VEPs re¯ect, at least in part, the activity originating in the Parvocellular and Koniocellular visual pathways, respectively. In conclusion, VEPs to onset-offset (300±700 ms) of sinusoidal gratings (2 c/deg, 14 deg ®eld size) display characteristics of speci®city and robustness suitable for the clinical application. In combination with established VEP techniques for luminance contrast stimuli, chromatic VEPs may help to better understand pathophysiology involving the visual pathway. The present study provides a basic framework in that direction. References Allen D, Banks MS, Norcia AM. Does chromatic sensitivity develop more slowly than luminance sensitivity? Vision Res 1993;33:2553±2562. Berninger TA, Arden GB, Hogg CR, Frumkes T. Separable evoked retinal and cortical potentials from each major visual pathway: preliminary results. Br J Ophthalmol 1989;73:502±511. Buttner T, Kuhn W, Muller T, Heinze T, Puhl C, Przuntek H. Chromatic and achromatic visual evoked potentials in Parkinson's disease. Electroenceph clin Neurophysiol 1996;100:443±447. Crognale MA, Switkes E, Adams AJ. Temporal response characteristics of the spatiochromatic visual evoked potential: non-linearities and departures from psychophysics. J Opt Soc Am (A) 1997;14:2595±2607. Crognale MA, Switkes E, Rabin J, Schneck ME, Haegerstrom-Portnoy G, Adams AJ. Application of the spatiochromatic visual evoked potential to detection of congenital and acquired color-vision de®ciencies. J Opt Soc Am (A) 1993;10:1818±1825. Dacey DM, Lee BB. The `blue-on' opponent pathway in primate retina originates from a distinct bistrati®ed ganglion cell type. Nature 1994;367:731±735.
Derrington AM, Krauskopf J, Lennie P. Chromatic mechanisms in lateral geniculate of macaque. J Physiol (Lond) 1984;357:241±265. Engel S, Zhang X, Wandell B. Colour tuning in human visual cortex measured with functional magnetic resonance imaging. Nature 1997;388:68±71. Fiorentini A, Burr DC, Morrone C. Temporal characteristics of colour vision: VEP and psychophysical measurements. In: Valberg A, Lee BB, editors. From pigments to perception: advances in understanding visual processes, New York: Plenum Press, 1991. pp. 139±150. Fiorentini A, Porciatti V, Morrone MC, Burr DC. Visual ageing: unspeci®c decline of the responses to luminance and colour. Vision Res 1996;36:3557±3566. Flitcroft DI. The interactions between chromatic aberration, defocus and stimulus chromaticity: Implications for visual physiology and colorimetry. Vision Res 1989;29:349±360. Girard P, Morrone MC. Spatial structure of chromatically opponent receptive ®elds in the human visual system. Vis Neurosci 1995;12:103±116. Harding GFA, Odom JV, Spileers W, Spekreijse H. Standard for visual evoked potentials . Vision Res 1996;36:3567±3572. Harding GFA, Wright CE. Visual evoked potentials in acute optic neuritis. In: Hess RF, Plant GT, editors. Optic neuritis, Cambridge: Cambridge University Press, 1986. pp. 230. Heywood CA, Nicholas JJ, Cowey A. Behavioural and electrophysiological chromatic and achromatic contrast sensitivity in an achromatopsic patient. J Neurol Neurosurg Psychiatry 1996;60:638±643. Jeffreys D. The physiological signi®cance of pattern visual evoked potentials. In: Desmedt JE, editor. Visual evoked potentials in man: new developments, Oxford: Clarendon Press, 1977. pp. 134. Korth M, Nguyen NX. The effect of stimulus size on human cortical potentials evoked by chromatic patterns. Vision Res 1997;37:649±657. Krauskopf J, Williams DR, Heeley DW. Cardinal directions of color space. Vision Res 1982;22:1123±1131. Kulikowski JJ, Muray IJ, Russel MHA. Effects of stimulus size on chromatic and achromatic VEPs. In: Drum B, Moreland J, Serra A, editors. Colour vision de®ciencies X: proceedings of the ninth symposium of the international group on colour de®ciencies, Dordrect: Kluwer, 1991. pp. 51. Kulikowski JJ, Robson AG, McKeefry DJ. Speci®city and selectivity of chromatic visual evoked potentials. Vision Res 1996;36:3397±3401. Le Grand Y. Optique Physiologique, Tome III, l'eÂspace visuel. Paris: Masson, 1956. Lennie P, D'Zmura M. Mechanisms of color vision. Crit Rev Neurobiol 1988;3:333±400. Lennie P, Krauskopf J, Sclar G. Chromatic mechanisms in striate cortex of macaque. J Neurosci 1990;10:649±669. Macaluso C, Lamedica A, Baratta G, Cordella M. 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. Electroenceph clin Neurophysiol 1996;100:12±17. McKeefry DJ, Russell MH, Murray IJ, Kulikowski JJ. Amplitude and phase variations of harmonic components in human achromatic and chromatic visual evoked potentials. Vis Neurosci 1996;13:639±653. Merigan WH, Maunsell JHR. How parallel are the primate visual pathways? Annu Rev Neurosci 1993;16:369±402. Morrone MC, Burr DC, Fiorentini A. Development of infant contrast sensitivity to chromatic stimuli. Vision Res 1993;33:2535±2552. Morrone MC, Fiorentini A, Burr DC. Development of the temporal properties of visual evoked potentials to luminance and colour contrast in infants. Vision Res 1996;36:3141±3155. Mullen KT. The contrast sensitivity of human colour vision to red-green and blue-yellow chromatic gratings. J Physiol (Lond) 1985;359:381± 400. Murray IJ, Parry NRA, Carden D, Kulikowski JJ. Human visual evoked potentials to chromatic and achromatic gratings. Clin Vision Sci 1987;3:231±244.
V. Porciatti, F. Sartucci / Clinical Neurophysiology 110 (1999) 772±781 Porciatti V, Di Bartolo E, Nardi N, Fiorentini A. Responses to chromatic and luminance contrast in glaucoma: a psychophysical and electrophysiological study. Vision Res 1997;37:1975±1987. Porciatti V, Sartucci F. Retinal and cortical evoked responses to chromatic contrast stimuli. Speci®c losses in both eyes of patients with multiple sclerosis and unilateral optic neuritis. Brain 1996;119:723±740. Rabin J, Switkes E, Crognale M, Schneck ME, Adams AJ. Visual evoked potentials in three-dimensional color space: correlates of spatio-chromatic processing. Vision Res 1994;34:2657±2671. Regan D. Human brain electrophysiology. Evoked potentials and evoked magnetic ®elds in science and medicine. New York: Elsevier, 1989. Regan D, He P. Magnetic and electrical brain responses to chromatic contrast in human. Vision Res 1996;36:1±18. Regan D, Spekreijse H. Evoked potential indications of colour blindness. Vision Res 1974;14:89±95. Riemslag FCC, Spekreijse H, Van Walbek H. Pattern-reversal and pattern appearance-disappearance responses in MS patients. In: Spekreijse H, Apkarian PA, editors. Visual pathways. Electrophysiology and pathology, 27. The Hague: Dr. W. Junk Publishers, 1981. pp. 215±221. Russel MHA, Murray IJ, Metcalfe RA, Kulikowski J. The visual defect in multiple sclerosis and optic neuritis. Brain 1991;114:2419±2435. Schneck ME, Fortune B, Switkes E, Crognale M, Adams AJ. Acute effects of blood glucose on chromatic visually evoked potentials in persons with diabetes and in normal persons. Invest Ophthalmol Vis Sci 1997;38:800±810.
781
Smith VC, Bowen RW, Pokorny J. Threshold temporal integration of chromatic stimuli. Vision Res 1984;24:653±659. Smith VC, Pokorny J. Spectral sensitivity of the foveal cone photopigments. Vision Res 1975;15:161±171. Spinelli D, Angelelli P, De Luca M, Burr DC. VEP in neglect patients have longer latencies for luminance but not for chromatic patterns. NeuroReport 1996;7:815±819. Stabell U, Stabell B. Variation in density of macular pigmentation and in short-wave cone sensitivity with eccentricity. J Opt Soc Am 1980;70:706±711. Swanson WH, Uneno TVCS, Pokorny J. Temporal modulation sensitivity and pulse-duration thresholds for chromatic and luminance perturbations. J Opt Soc Am (A) 1987;4:1992±2005. Switkes E, Crognale M, Rabin J, Schneck ME, Adams AJ. Reply to `speci®city and selectivity of chromatic visual evoked potentials. Vision Res 1996;36:3403±3405. Tobimatsu S, Kato M. Multimodality visual evoked potentials in evaluating visual dysfunction in optic neuritis. Neurology 1998;50:715±718. Tobimatsu S, Tomoda H, Kato M. Parvocellular and magnocellular contributions to visual evoked potentials in humans: stimulation with chromatic and achromatic gratings and apparent motion. J Neurol Sci 1995;134:73±82. Tobimatsu S, Tomoda H, Kato M. Human VEPs to isoluminant chromatic and achromatic sinusoidal gratings: separation of parvocellular components. Brain Topogr 1996;8:241±243.