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'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

Visual impairment on dentists related to occupational mercury exposure. Environmental Toxicology and Pharmacology

Abstract A detailed assessment of visual function was obtained in subjects with low-level occupational mercury exposure by measuring hue saturation thresholds and contrast sensitivity functions for luminance and chromatic modulation. General practice dentists (n = 15) were compared to age-matched healthy controls (n = 13). Color discrimination estimated by the area of Mac Adam ellipses was impaired, showing diffuse discrimination loss. There was also reduction of contrast sensitivity for luminance and chromatic (red-green and blue-yellow) modulation, in all tested spatial frequencies. Low concentrations of urinary mercury (1.97 ± 1.61 g/g creatinine) were found in the dentists group. Color discrimination as well as contrast sensitivity function, assessed psychophysically, constitutes a sensitive indicator of subtle neurotoxic effect of elemental mercury exposure

<i>Alouatta</i> Trichromatic Color Vision: Cone Spectra and Physiological Responses Studied with Microspectrophotometry and Single Unit Retinal Electrophysiology

<div><p>The howler monkeys (<i>Alouatta</i> sp.) are the only New World primates to exhibit routine trichromacy. Both males and females have three cone photopigments. However, in contrast to Old World monkeys, <i>Alouatta</i> has a locus control region upstream of each opsin gene on the X-chromosome and this might influence the retinal organization underlying its color vision. Post-mortem microspectrophotometry (MSP) was performed on the retinae of two male <i>Alouatta</i> to obtain rod and cone spectral sensitivities. The MSP data were consistent with only a single opsin being expressed in each cone and electrophysiological data were consistent with this primate expressing full trichromacy. To study the physiological organization of the retina underlying <i>Alouatta</i> trichromacy, we recorded from retinal ganglion cells of the same animals used for MSP measurements with a variety of achromatic and chromatic stimulus protocols. We found MC cells and PC cells in the <i>Alouatta</i> retina with similar properties to those previously found in the retina of other trichromatic primates. MC cells showed strong phasic responses to luminance changes and little response to chromatic pulses. PC cells showed strong tonic response to chromatic changes and small tonic response to luminance changes. Responses to other stimulus protocols (flicker photometry; changing the relative phase of red and green modulated lights; temporal modulation transfer functions) were also similar to those recorded in other trichromatic primates. MC cells also showed a pronounced frequency double response to chromatic modulation, and with luminance modulation response saturation accompanied by a phase advance between 10–20 Hz, characteristic of a contrast gain mechanism. This indicates a very similar retinal organization to Old-World monkeys. Cone-specific opsin expression in the presence of a locus control region for each opsin may call into question the hypothesis that this region exclusively controls opsin expression.</p></div

Ganglion cell response to stimulus contrast.

<p>(<b>A–B</b>) Response amplitude (left panels) and phase (right panels) as a function of stimulus luminance contrast for <i>Alouatta</i> MC cell (<b>A</b>) and PC cell (<b>B</b>). The results obtained with three temporal frequencies are given: 1.2 Hz (filled squares), 9.8 Hz (filled triangles), and 39 Hz (filled diamonds) for the MC cell; 1.2 Hz (filled squares), 9.8 Hz (filled triangles), and 19.4 Hz (filled diamonds) for the PC cell. Data representing response amplitude as a function of contrast have been fitted with Naka-Rushton functions. MC cell vigorously responded to low levels of luminance contrast, but responses rapidly saturated accompanied by advancement in response phase, especially at intermediate and high temporal frequencies. PC cells were quite insensitive to low levels of luminance contrast, but the responses did not exhibit saturation or phase advancement. (<b>C</b>) Temporal modulation transfer functions (temporal MTFs) for <i>Alouatta</i> MC cells (average of 7 cells) stimulated with luminance sinusoids (filled squares) and a PC cell stimulated with luminance (empty diamonds) and averaged responses of 4 cells to red-green (filled diamonds) sinusoids. Contrast gain was defined as the initial slope of the Naka-Rushton functions fitted to the amplitude versus contrast data such as those illustrated in the left panels (<b>A–B</b>). Michelson contrast and cone contrast were used for the luminance and red-green chromatic temporal MTFs, respectively. MC cells were much more sensitive than PC cells to temporal luminance modulation at all the temporal frequencies range and vigorously responded to very high temporal frequencies. On the other hand, PC cells were very sensitive to red-green contrast, especially at low and intermediate temporal frequencies.</p

Ganglion cell response to stimulus phase changes.

<p>(<b>A</b>) MC and (<b>B</b>) PC cell responses to heterochromatic stimuli. Phase protocol. The relative phases of the 554 and 638 nm LEDs were modulated with fixed modulation depths. MC and PC cells were stimulated at 9.8 Hz and 1.22 Hz, respectively. Stimulus size was 4 deg, mean retinal illuminance was 2000 Td. Response amplitudes (top panels) and phases (middle panels) are shown as a function of the phase difference between the luminance modulation in the red and green LEDs. Filled diamonds and empty squares represent the parameters for response first and second harmonics, respectively, extracted by Fourier analysis. Bottom panels: the histograms illustrate actual responses to two cycles of modulation, the arrows indicate the red/green phase difference for the histograms, and the red and green curves represent how the phase of the red and green lights changed for each stimulus condition. Luminance modulation corresponds to a relative phase of 0 deg, chromatic modulation to a relative phase of ±180 deg. MC cells responded to the phase protocol with a maximal response first harmonic amplitude when the green and red lights were modulated in phase and their response phase changed continuously with the phase difference between the modulation in the two LEDs. PC cells displayed a minimal response when the green and red lights were modulated in phase and their response phases changed abruptly in the region of minimal response.</p

Morphology and physiology of primate M- and P-cells

Catarrhines and platyrrhines, the so-called Old- and New-World anthropoids, have different cone photopigments. Postreceptoral mechanisms must have coevolved with the receptors to provide trichromatic color vision, and so it is important to compare postreceptoral processes in these two primate groups, both from anatomical and physiological perspectives. The morphology of ganglion cells has been studied in the retina of catarrhines such as the diurnal and trichromatic Macaca, as well as platyrrhines such as the diurnal, di- or trichromatic Cebus, and the nocturnal, monochromatic Aotus. Diurnal platyrrhines, both di- and trichromats, have ganglion cell classes very similar to those found in catarrhines: M (parasol), P (midget), small-field bistratified, and several classes of wide-field ganglion cells. In the fovea of all diurnal anthropoids, P-cell dendritic trees contact single midget bipolars, which contact single cones. The Aotus retina has far fewer cones than diurnal species, but M- and P-cells are similar to those in diurnal primates although of larger size. As in diurnal anthropoids, in the Aotus, the majority of midget bipolar cells, found in the central 2 mm of eccentricity, receive input from a single cone and the sizes of their axon terminals match the sizes of P-cell dendritic fields in the same region. The visual responses of retinal ganglion cells of these species have been studied using single-unit electrophysiological recordings. Recordings from retinal ganglion cells in Cebus and Aotus showed that they have very similar properties as those in the macaque, except that P-cells of mono- and dichromatic animals lack cone opponency. Whatever the original role of the M- and P-cells was, they are likely to have evolved prior to the divergence of catarrhines and platyrrhines. M- and P-cell systems thus appear to be strongly conserved in the various primate species. The reasons for this may lie in the roles of these systems for both achromatic and chromatic vision.</p