Color discrimination is affected by modulation of luminance noise in pseudoisochromatic stimuli

Pseudoisochromatic stimuli have been widely used to evaluate color discrimination and to identify color vision deficits. Luminance noise is one of the stimulus parameters used to ensure that subject´s response is due to their ability to discriminate target stimulus from the background based solely on the hue between the colors that compose such stimuli. We studied the influence of contrast modulation of the stimulus luminance noise on threshold and reaction time color discrimination. We evaluated color discrimination thresholds using the Cambridge Color Test (CCT) at six different stimulus mean luminances. Each mean luminance condition was tested using two protocols: constant absolute difference between maximum and minimum luminance of the luminance noise (constant delta protocol, CDP), and constant contrast modulation of the luminance noise (constant contrast protocol, CCP). MacAdam ellipses were fitted to the color discrimination thresholds in the CIE 1976 color space to quantify the color discrimination ellipses at threshold level. The same CDP and CCP protocols were applied in the experiment measuring RTs at three levels of stimulus mean luminance. The color threshold measurements show that for the CDP, ellipse areas decreased as a function of the mean luminance and they were significantly larger at the two lowest mean luminances, 10 cd/m2 and 13 cd/m2, compared to the highest one, 25 cd/m2. For the CCP, the ellipses areas also decreased as a function of the mean luminance, but there was no significant difference between ellipses areas estimated at six stimulus mean luminances. The exponent of the decrease of ellipse areas as a function of stimulus mean luminance was steeper in the CDP than CCP. Further, reaction time increased linearly with the reciprocal of the length of the chromatic vectors varying along the four chromatic half-axes. It decreased as a function of stimulus mean luminance in the CDP but not in the CCP. The findings indicated that visual performance using pseudoisochromatic stimuli was dependent on the Weber´s contrast of the luminance noise. Low Weber´s contrast in the luminance noise is suggested to have a reduced effect on chromatic information and, hence, facilitate desegregation of the hue-defined target from the background.Fil: Cormenzana Méndez, Iñaki. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán; Argentina. Universidad Nacional de Tucumán. Facultad de Ciencias Exactas y Tecnología. Laboratorio de Luminotecnia; ArgentinaFil: Martín, Andrés. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán. Instituto de Investigación en Luz, Ambiente y Visión. Universidad Nacional de Tucumán. Facultad de Ciencias Exactas y Tecnología. Instituto de Investigación en Luz, Ambiente y Visión; Argentina. Universidad Nacional de Tucumán. Facultad de Ciencias Exactas y Tecnología. Laboratorio de Luminotecnia; ArgentinaFil: Charmichael, Teaire L.. Christian Brothers University; Estados UnidosFil: Jacob, Mellina M.. Universidade Federal do Pará; BrasilFil: Lacerda, Eliza M. C. B.. Universidade Federal do Pará; BrasilFil: Gomes, Bruno D.. Universidade Federal do Pará; BrasilFil: Fitzgerald, Malinda E. C.. Christian Brothers University; Estados Unidos. University of Tennessee; Estados UnidosFil: Ventura, Dora F.. Universidade de Sao Paulo; BrasilFil: Silveira, Luiz C. L.. Universidade do Ceuma; Brasil. Universidade Federal do Pará; BrasilFil: O´donell, Beatriz Maria. Universidad Nacional de Tucumán. Facultad de Ciencias Exactas y Tecnología. Laboratorio de Luminotecnia; ArgentinaFil: Souza, Givago S.. Universidade Federal do Pará; Brasi

Visual evoked cortical potential (VECP) elicited by sinusoidal gratings controlled by pseudo-random stimulation.

The contributions of contrast detection mechanisms to the visual cortical evoked potential (VECP) have been investigated studying the contrast-response and spatial frequency-response functions. Previously, the use of m-sequences for stimulus control has been almost restricted to multifocal electrophysiology stimulation and, in some aspects, it substantially differs from conventional VECPs. Single stimulation with spatial contrast temporally controlled by m-sequences has not been extensively tested or compared to multifocal techniques. Our purpose was to evaluate the influence of spatial frequency and contrast of sinusoidal gratings on the VECP elicited by pseudo-random stimulation. Nine normal subjects were stimulated by achromatic sinusoidal gratings driven by pseudo random binary m-sequence at seven spatial frequencies (0.4-10 cpd) and three stimulus sizes (4°, 8°, and 16° of visual angle). At 8° subtence, six contrast levels were used (3.12-99%). The first order kernel (K1) did not provide a consistent measurable signal across spatial frequencies and contrasts that were tested-signal was very small or absent-while the second order kernel first (K2.1) and second (K2.2) slices exhibited reliable responses for the stimulus range. The main differences between results obtained with the K2.1 and K2.2 were in the contrast gain as measured in the amplitude versus contrast and amplitude versus spatial frequency functions. The results indicated that K2.1 was dominated by M-pathway, but for some stimulus condition some P-pathway contribution could be found, while the second slice reflected the P-pathway contribution. The present work extended previous findings of the visual pathways contribution to VECP elicited by pseudorandom stimulation for a wider range of spatial frequencies

Contrast response functions for K2.1 first (filled circles) and second (empty circles) principal components at three spatial frequencies (0.4 cpd, 2 cpd, and 10 cpd) (A–C), and for K2.2 first (filled circles) and second (empty circles) principal components at the same spatial frequencies (D–F).

<p>K2.1 first principal component is more sensitive to contrast than K2.1 second principal component and K2.2 first and second principal components. We fitted power functions to the mean values (not shown for clarity) and observed that the largest difference between K2.1 first and second principal components were seen at 0.4 cpd (z = 0.84 and 0.37, respectively) and 2 cpd (z = 0.57 and 0.38, respectively) and between K2.2. first and second principal components were seen at 2 cpd (z = 0.57 and 0.37, respectively). We fitted Michaelis-Menten functions to the mean values (not shown for clarity) and observed that the largest difference between K2.1 first and second principal components were seen at 0.4 cpd (g = 1.06 and 0.31, respectively). The difference in contrast gain are suggestive that K2.1 first principal component is dominated by M-pathway response, whereas the other components are dominated by the response of a less contrast sensitive pathway such as the P-pathway. Error bars are SEM.</p

Spatial luminance contrast sensitivity measured with transient VEP: comparison with psychophysics and evidence of multiple mechanisms. Investigative Ophthalmology and Visual

PURPOSE. To compare the spatial luminance contrast sensitivity function (CSF) obtained with transient visual evoked potentials (VEPs) with that obtained with psychophysical measurements. METHODS. The stimuli consisted of horizontal luminance gratings. In the VEP experiments, 0.4, 0.8, 2, 4, 8, and 10 cpd of spatial frequency were used, at 1 Hz square-wave contrastreversal mode. Eight to 10 Michelson contrasts were used at each spatial frequency. Contrast thresholds were estimated from extrapolation of contrast response functions. Psychophysical sensitivities were obtained with spatial gratings of 0.4, 0.8, 1, 2, 4, 6, 8, and 10 cpd and presented at 1 Hz square-wave contrast-reversal or stationary mode (dynamic and static presentation, respectively). CSF tuning was estimated by calculating the ratio between peak sensitivity and the sensitivity at 0.4 cpd. RESULTS. In all subjects tested (n ϭ 6), VEP contrast-response functions showed nonlinearities-namely, amplitude saturation and double-slope amplitude functions that occurred at low and medium-to-high spatial frequencies, respectively. Mean electrophysiological and psychophysical CSFs peaked at 2 cpd. CSF tuning for electrophysiology and dynamic and static psychophysics were, respectively, 1.08, 1.11, and 1.31. Correlation coefficients (r 2 ) between electrophysiological CSF and dynamic or static psychophysical CSF were, respectively, 0.81 and 0.45. CONCLUSIONS. Electrophysiological and psychophysical CSFs correlated more positively when temporal presentation was similar. Spatial frequencies higher than 2 cpd showed that at least two visual pathways sum their activities at high contrasts. At low contrast levels, the results suggest that the transient VEP is dominated by the magnocellular (M) pathway. (Invest Ophthalmol Vis Sci. 2007;48:3396 -3404) DOI:10.1167/iovs.07-0018 T he use of the visual evoked potential (VEP) to study the human spatial luminance contrast sensitivity started in the 1970s, when it was demonstrated that the steady state VEP amplitude decreases with stimulus log contrast after a straightline relationship. 1 Moreover, the steady state VEP was successfully used to derive the spatial luminance contrast sensitivity function (CSF) in humans. When compared with behavioral measurements, the electrophysiological CSF was similar to the psychophysical CSF. 1 It was also observed that at spatial frequencies between 1.5 and 3 cpd, the contrast response functions were describable by two regressions with different slopes. 1 Further studies in humans and nonhuman primates have reported such nonlinearity of the VEP amplitude as a function of the stimulus contrast 2-11 : either a straight-line relation for low contrasts followed by saturation at high contrasts or a double-slope straight-line relation such as those described earlier. Originally, it was proposed that these nonlinearities indicate the contribution of different retinal regions, 1 but more recent studies have suggested that the nonlinearities are related to different contrast sensitivity mechanisms from parallel visual pathways. 6,10 -12 In this study, we assessed the similarity of the spatial luminance CSF obtained with transient VEP, when compared with that obtained psychophysically with similar stimuli. We also analyzed whether transient VEP amplitudes can be related to visual parallel pathway sensitivities. The transient VEP can be used as an objective tool to study spatial luminance CSF. There was good agreement between the electrophysiological and psychophysical measurements obtained with the same temporal frequency. Our results suggest that the M pathway is the main source of the VEP amplitude, at least at low-luminance contrasts. Some of these results have been presented in abstract form (Souza et al. IOVS 2006;47:ARVO E-Abstract 5379). METHODS Subjects Six healthy adults (three men and three women; mean age, 21 Ϯ 2 years) were monocularly tested. For each subject, only the eye with smallest dioptric error was tested. All subjects had normal or corrected acuity to 20/20, which was assessed by measuring the eye&apos;s refractive state with an autorefractor/keratometer (Humphrey 599; Carl Zeiss Meditec, Dublin, CA). None of the participants reported previous ocular, neural, or systemic diseases that could affect the visual system. All procedures were performed according to the tenets of the Declaration of Helsinki and were approved by the Committe

Mean RMS amplitude of VECP kernels across the spatial frequency domain at high contrast stimulation.

<p>First order kernels showed very small or no signal at all stimulus conditions (K1, left). The highest response of the second order kernel first slice occurred at low spatial frequencies (K2.1, center). The highest response of the second order kernel second slice occurred at intermediate spatial frequencies (K2.2, right). Error bars are standard errors of the means (SEM).</p

Mean VECP kernel waveforms obtained from 9 subjects at three spatial frequencies.

<p>Left column: first order kernels (K1). Center column: second order kernel first slices (K2.1). Right column: second order kernel second slices (K2.2). Due to their amplitude versus spatial frequency (Fig. 2) and amplitude versus contrast (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0070207#pone-0070207-g003" target="_blank">Figure 3</a>) functions we have suggested that K2.1 is mainly dominated by the M-pathway response or a mixture of M- and P-pathway influence, whereas K2.2 is mainly dominated by the P-pathway response (see the text for more details).</p