Achromatic Gratings Research Articles

Contrast sensitivity of pattern transient VEP components: Contribution from M and P pathways.

The purpose of this study was to compare contrast sensitivity estimated from transient visual evoked potentials (VEPs) elicited by achromatic pattern-reversal and pattern-onset/offset modes. The stimuli were 2-cpd, achromatic horizontal gratings presented either as a 1 Hz pattern reversal or a 300 ms onset/700 ms offset stimulus. Contrast thresholds were estimated by linear regression to amplitudes of VEP components vs. the logarithm of the stimulus contrasts, and these regressions were extrapolated to the zero amplitude level. Contrast sensitivity was defined as the inverse of contrast threshold. For pattern reversal, the relation between the P100 amplitude and log of the stimulus contrast was best described by two separate linear regressions. For the N135 component, a single straight line was sufficient. In the case of pattern onset/offset for both the C1 and C2 components, single straight lines described their amplitude vs. log contrast relations in the medium-to-low contrast range. Some saturation was observed for C2 components. The contrast sensitivity estimated from the low-contrast limb of the P100, from the N135, and from the C2 were all similar but higher than those obtained from the high-contrast limb of the P100 and C1 data, which were also similar to each other. With 2 cpd stimuli, a mechanism possibly driven by the M pathway appeared to contribute to the P100 component at medium-to-low contrasts and to the N135 and C2 components at all contrast levels, whereas another mechanism, possibly driven by the P and M pathways, appeared to contribute to the P100 component at high contrast and C1 component at all contrast levels.

Objective assessment of chromatic and achromatic pattern adaptation reveals the temporal response properties of different visual pathways

The aim was to investigate the temporal response properties of magnocellular, parvocellular, and koniocellular visual pathways using increment/decrement changes in contrast to elicit visual evoked potentials (VEPs). Static achromatic and isoluminant chromatic gratings were generated on a monitor. Chromatic gratings were modulated along red/green (R/G) or subject-specific tritanopic confusion axes, established using a minimum distinct border criterion. Isoluminance was determined using minimum flicker photometry. Achromatic and chromatic VEPs were recorded to contrast increments and decrements of 0.1 or 0.2 superimposed on the static gratings (masking contrast 0-0.6). Achromatic increment/decrement changes in contrast evoked a percept of apparent motion when the spatial frequency was low; VEPs to such stimuli were positive in polarity and largely unaffected by high levels of static contrast, consistent with transient response mechanisms. VEPs to finer achromatic gratings showed marked attenuation as static contrast was increased. Chromatic VEPs to R/G or tritan chromatic contrast increments were of negative polarity and showed progressive attenuation as static contrast was increased, in keeping with increasing desensitization of the sustained responses of the color-opponent visual pathways. Chromatic contrast decrement VEPs were of positive polarity and less sensitive to pattern adaptation. The relative contribution of sustained/transient mechanisms to achromatic processing is spatial frequency dependent. Chromatic contrast increment VEPs reflect the sustained temporal response properties of parvocellular and koniocellular pathways. Cortical VEPs can provide an objective measure of pattern adaptation and can be used to probe the temporal response characteristics of different visual pathways.

Effects of environmental contaminant exposure on visual brain development: A prospective electrophysiological study in school-aged children

The Inuit from Nunavik (Northern Québec) are one of the most highly exposed populations to environmental contaminants in North America mainly due to the bioaccumulation of contaminants in fish and marine mammals that constitute an important part of their diet. This follow-up study aimed to assess the impact of exposure to contaminants on visual brain development in school-age Inuit children (mean age=10.9 years). Concentrations of polychlorinated biphenyls (PCBs), mercury (Hg), and lead (Pb) were measured in umbilical cord blood and again in blood samples at the time of testing, reflecting pre- and current exposure, respectively. Pattern-reversal visual evoked potentials (VEPs) were scalp-recorded at the occipital cortex. Visual stimulation consisted of achromatic gratings presented at four visual contrast levels: 95%, 30%, 12% and 4%. The relation between environmental contaminant body burdens and VEPs was examined by regression analysis controlling for confounding variables, including fish nutrients and other toxicants. No significant association was found for PCB exposure after statistical adjustments. Cord blood mercury level was associated with a reduction of the N75 amplitude at the highest contrast level and with a delay of the N75 latency at the 12% contrast level. Prenatal exposure to lead was associated with a delay of the N150 latency at most contrast levels. This study suggests that heavy metal exposure, in particular during the gestational period, can impair the development of visual processing.

Decline of the McCollough effect by orientation-specific post-adaptation exposure to achromatic gratings

The McCollough effect is a contingent color after effect induced by adapting to colored gratings for several minutes. It has been demonstrated that a long-lasting adaptation effect such as the McCollough effect can be diminished by exposure to achromatic versions of the induction stimuli. However, the orientation specificity of this effect of post-adaptation exposure is not known. Here we report the findings from two experiments conducted to determine the influence of achromatic gratings and their orientation on the strength of the McCollough effect. After adaptation to the McCollough stimuli, participants were exposed to achromatic gratings or to one of two control conditions (either waiting in the dark or different-orientation achromatic gratings). Results suggest a significant decline of the McCollough effect after perceiving achromatic gratings in comparison to waiting in the dark or being exposed to different-orientation achromatic gratings. Thus, the effect of post-adaptation exposure to achromatic gratings is orientation specific.

The spatial tuning of achromatic and chromatic vision in budgerigars

Birds are assumed to use half of their cones (double cones) to detect fine spatial detail while their other half (single cones) is used for color vision. However, the spatial resolution of the color pathway in birds has never been studied. We determined the spatial contrast sensitivity to achromatic and isoluminant red-green and blue-green color gratings in budgerigars (Melopsittacus undulatus). Contrast sensitivity to achromatic gratings has band-pass characteristics while that for red-green and blue-green gratings has low-pass properties. Maximum sensitivity is lower to blue-green than to red-green gratings and the acuity for both color gratings is less than half (ca. 4.5 cycles/degree) of that for achromatic gratings (ca. 10 cycles/degree). This suggests that achromatic vision in birds, as in humans and bees, is tuned for detecting fine detail while chromatic vision is tuned for viewing larger fields. Similar to humans, blue-sensitive cones contribute little to spatial vision. Moreover, budgerigars detected gratings having both achromatic and chromatic contrasts more reliably at high spatial frequencies than gratings with either of these contrasts, suggesting that the single and double cone pathways are incompletely separated. The study demonstrates the importance of the spatial dimension of color vision; fine patterns remain unresolved even if they present large color contrasts.

Transmission of colour and acuity signals by parvocellular cells in marmoset monkeys

The red-green axis of colour vision evolved recently in primate evolutionary history. Signals serving red-green colour vision travel together with signals serving spatial vision, in the parvocellular (PC) division of the subcortical visual pathway. However, the question of whether receptive fields of PC pathway cells are specialized to transmit red-green colour signals remains unresolved. We addressed this question in single-cell recordings from the lateral geniculate nucleus of anaesthetized marmosets. Marmosets show a high proportion of dichromatic (red-green colour-blind) individuals, allowing spatial and colour tuning properties of PC cells to be directly compared in dichromatic and trichromatic visual systems. We measured spatial frequency tuning for sine gratings that provided selective stimulation of individual photoreceptor types. We found that in trichromatic marmosets, the foveal visual field representation is dominated by red-green colour-selective PC cells. Colour selectivity of PC cells is reduced at greater eccentricities, but cone inputs to centre and surround are biased to create more selectivity than predicted by a purely 'random wiring' model. Thus, one-to-one connections in the fovea are sufficient, but not necessary, to create colour-selective responses. The distribution of spatial tuning properties for achromatic stimuli shows almost complete overlap between PC cells recorded in dichromatic and trichromatic marmosets. These data indicate that transmission of red-green colour signals has been enabled by centre-surround receptive fields of PC cells, and has not altered the capacity of PC cells to serve high-acuity vision at high stimulus contrast.

Psychophysical measurements of luminance and chromatic spatial and temporal contrast sensitivity in Duchenne muscular dystrophy.

In children with Duchenne muscular dystrophy, color vision losses have been related to dystrophin deletions downstream of exon 30, which affect a dystrophin isoform, Dp260, present in the retina. To further evaluate visual function in DMD children, we measured spatial, temporal, and chromatic red-green and blue-yellow contrast sensitivity in two groups of DMD children with gene deletion downstream and upstream of exon 30. Psychophysical spatial contrast sensitivity was measured for low, middle, and high spatial frequencies with achromatic gratings and for low and middle frequencies with red-green and blue-yellow chromatic gratings. Temporal contrast sensitivity was also measured with achromatic stimuli. A reduction in sensitivity at all spatial luminance contrasts was found for the DMD patients with deletion downstream of exon 30. Similar results were found for temporal luminance contrast sensitivity. Red-green chromatic contrast sensitivity was reduced in DMD children with deletion downstream of exon 30, whereas blue-yellow chromatic contrast sensitivity showed no significant differences. We conclude that visual function is impaired in DMD children. Furthermore, we report a genotype-phenotype relationship because the visual impairment occurred in children with deletion downstream but not upstream of exon 30, affecting the retinal isoform of dystrophin Dp260.

Receptive field asymmetries produce color-dependent direction selectivity in primate lateral geniculate nucleus

Blue-on receptive fields recorded in primate retina and lateral geniculate nucleus are customarily described as showing overlapping blue-on and yellow-off receptive field components. However, the retinal pathways feeding the blue-on and yellow-off subfields arise from spatially discrete receptor populations, and recent studies have given contradictory accounts of receptive field structure of blue-on cells. Here we analyzed responses of blue-on cells to drifting gratings, in single-cell extracellular recordings from the dorsal lateral geniculate nucleus in marmosets. We show that most blue-on cells exhibit selectivity for the drift direction of achromatic gratings. The standard concentric difference-of-Gaussians (DOG) model thus cannot account for responses of these cells. We apply a simple, anatomically plausible, extension of the DOG model. The model incorporates temporally offset elliptical two-dimensional Gaussian subfields. The model can predict color-contingent direction and spatial tuning. Because direction tuning in blue-on cells depends on stimulus chromaticity, spatial frequency, and temporal frequency, this property is of little value as a general mechanism for image movement detection. It is possible that anatomical wiring for color selectivity has constrained the capacity of blue-on cells to contribute to spatial and motion vision.

Parvocellular pathway impairment in autism spectrum disorder: Evidence from visual evoked potentials

In humans, visual information is processed via parallel channels: the parvocellular (P) pathway analyzes color and form information, whereas the magnocellular (M) stream plays an important role in motion analysis. Individuals with autism spectrum disorder (ASD) often show superior performance in processing fine detail, but impaired performance in processing global structure and motion information. To date, no visual evoked potential (VEP) studies have examined the neural basis of atypical visual performance in ASD. VEPs were recorded using 128-channel high density EEG to investigate whether the P and M pathways are functionally altered in ASD. The functioning of the P and M pathways within primary visual cortex (V1) were evaluated using chromatic (equiluminant red–green sinusoidal gratings) and achromatic (low contrast black–white sinusoidal gratings) stimuli, respectively. Unexpectedly, the N1 component of VEPs to chromatic gratings was significantly prolonged in ASD patients compared to controls. However, VEP responses to achromatic gratings did not differ significantly between the two groups. Because chromatic stimuli preferentially stimulate the P-color but not the P-form pathway, our findings suggest that ASD is associated with impaired P-color pathway activity. Our study provides the first electrophysiological evidence for P-color pathway impairments with preserved M function at the V1 level in ASD.

Adaptation to Combinations of form, Colour, and Movement

Observers typically report illusory colour on achromatic gratings after being adapted to orthogonally oriented gratings presented in complementary colours, where the colour apparent on each grating is complementary to the one that had been presented with that grating during adaptation (McCollough, 1965 Science 149 1115-1116). We used this procedure, but presented homogenous fields at test instead of achromatic gratings. When adaptation stimuli moved in directions locally orthogonal to their orientation, we found that, for up to 7-8 min after adaptation, a flower-like illusory pattern was evident on both homogenous fields; after this time illusory radial lines and concentric circles were evident and were colour-contingent (eg for adaptation with green concentric circles and magenta radial lines, concentric circles were apparent on a magenta test field and radial lines were apparent on a green test field). When stimuli were stationary during induction, colour-contingent illusory forms were also apparent at test. The results demonstrate that an aftereffect, reciprocal to the McCollough effect, can be produced under appropriate induction conditions, and that this effect is not due to retinal afterimages.

Vernier acuity and the magnocellular system revisited: Response to Skottun and Skoyles

Skottun and Skoyles (2009) recently presented a comment on Vernier acuity and magnocellular dysfunctions in fragile X premutation carriers ( Kéri & Benedek, 2009). The authors concluded that our finding that the magnocellular deficit, as revealed by luminance-contrast sensitivity measurements, is associated with impaired Vernier acuity for achromatic gratings does not mean that this procedure is suitable for the investigation of the magnocellular pathways, given that deficits outside the magnocellular system also lead to impaired Vernier acuity. However, Skottun and Skoyles (2009) did not consider the dissociation between achromatic and isoluminant color Vernier and the convergence of Vernier and luminance-contrast sensitivity measurements.

Color signals in the primary visual cortex of marmosets

This study concerns the input from short-wavelength sensitive (S) cone photoreceptors to the primary visual cortex (striate cortex, Brodmann area 17, area V1) in marmosets. Signals from S-cones are thought to reach V1 by way of the koniocellular layers of the dorsal lateral geniculate nucleus. However, it is not known whether the S-cone afferent signals cause selective activation of cytochrome oxidase-rich cortical "blob" domains. To address this question, intrinsic optical signals and extracellular responses of V1 neurons were recorded. Stimuli consisted of drifting achromatic gratings and gratings that stimulated selectively either the S-cones or the medium-long wavelength sensitive (ML) cones. All stimuli produced contrast-dependent activation throughout the imaged regions of V1. The S- and ML-cone-selective stimuli produced activation levels of respectively 30% and 80% of that to achromatic gratings. No spatial variation in the strength of S-cone activation was apparent, and the ratio of S to ML activation was constant across all imaged regions. Consistently, in all of the single neurons recorded from V1, the functional input from S-cones was weaker than the input from ML-cones. We conclude that in the primary visual cortex of marmosets, S-cone signals are uniformly distributed.

Objective assessment of chromatic and achromatic pattern adaptation reveals the temporal response properties of different visual pathways

Abstract Purpose To investigate the temporal response properties of magnocellular, parvocellular and koniocellular pathways using contrast increment/decrement visual evoked potentials (VEPs). Methods Static achromatic and isoluminant chromatic gratings were generated on a monitor (Michelson contrast 0.05 to 0.6). Chromatic gratings were modulated along R/G or subject‐specific tritanopic confusion axes, established using a minimum distinct border criterion. Isoluminance was determined using minimum flicker photometry for a 1‐degree field. Achromatic and chromatic VEPs were recorded to contrast increments and decrements of 0.1 superimposed on the static gratings. Field size for Tritan gratings was restricted to 3‐degrees to minimize the effects of luteal pigment. Results Achromatic increment/decrement VEPs were positive in polarity and largely unaffected by high levels of static contrast at low spatial frequencies. Responses to finer achromatic gratings showed marked attenuation as static contrast was increased. Chromatic VEPs to R/G or Tritan chromatic contrast increments were of negative polarity and showed progressive attenuation as static contrast was increased. Chromatic contrast decrement VEPs were of positive polarity and less sensitive to pattern adaptation. Conclusion The relative contribution of sustained and transient response mechanisms to achromatic processing is spatial‐frequency dependent. Chromatic contrast increment VEPs are highly sensitive to pattern adaptation consistent with high stimulus selectivity and the sustained temporal response properties of the parvocellular and koniocellular pathways.

Systematic misestimation in a vernier task arising from contrast mismatch

Luminance signals mediated by the magnocellular (MC) pathway play an important role in vernier tasks. MC ganglion cells show a phase advance in their responses to sinusoidal stimuli with increasing contrast due to contrast gain control mechanisms. If the phase information in MC ganglion cell responses were utilized by central mechanisms in vernier tasks, one might expect systematic errors caused by the phase advance. This systematic error may contribute to the contrast paradox phenomenon, where vernier performance deteriorates, rather than improves, when only one of the target pair increases in contrast. Vernier psychometric functions for a pair of gratings of mismatched contrast were measured to seek such misestimation. In associated electrophysiological experiments, MC and parvocellular (PC) ganglion cells' responses to similar stimuli were measured to provide a physiological reference. The psychophysical experiments show that a high-contrast grating is perceived as phase advanced in the drift direction compared to a low-contrast grating, especially at a high drift rate (8 Hz). The size of the phase advance was comparable to that seen in MC cells under similar stimulus conditions. These results are consistent with the MC pathway supporting vernier performance with achromatic gratings. The shifts in vernier psychometric functions were negligible for pairs of chromatic gratings under the conditions tested here, consistent with the lack of phase advance both in responses of PC ganglion cells and in frequency-doubled chromatic responses of MC ganglion cells.

Chromatic and Luminance Contrast Sensitivities in Asymptomatic Carriers from a Large Brazilian Pedigree of 11778 Leber Hereditary Optic Neuropathy

To determine whether asymptomatic 11778 LHON carriers demonstrated impairments in (1) chromatic red/green (R/G) and blue/yellow (B/Y) contrast sensitivity functions (CSF) and in (2) luminance contrast sensitivity functions in the spatial CSF (SCSF) and temporal CSF (TCSF) domains. Twenty-five carriers (8 male, 17 female; 34.1 +/- 15.1 years of age) of homoplasmic 11778 LHON from the same well-described family and 30 age-matched controls (17 male, 13 female; 29.2 +/- 7.1 years of age) were tested in one eye, randomly selected. Of the 25 eyes tested, 18 had normal fundus, 5 had swelling and microangiopathy, and 2 had temporal pallor. The R/G and B/Y CSFs were obtained after equiluminance correction with bichromatic horizontal sinusoidal gratings at 0.3, 0.7, and 2 cycles per degree (cpd); the SCSFs were obtained with achromatic gratings at 0.3, 2, 6, and 12 cpd; and the TCSFs were obtained at 2, 10, 20, and 33 Hz with sinusoidal modulation of a 2.7 degrees field with a superimposed spatial Gabor function. Differences between carriers and controls were statistically significant for all spatial frequencies of chromatic and luminance SCSFs, but not for the TCSFs. R/G equiluminance settings of carriers differed from those of controls (P < 0.001), requiring higher luminance in the green; B/Y equiluminance settings were not statistically different in carriers and controls. Fundus findings did not correlate with CS results. Luminance and chromatic spatial CS losses that affected all tested spatial frequencies, are reported in LHON asymptomatic carriers with the mtDNA 11778 mutation. No losses were found in the temporal CSF. An intriguing finding is that the blue system is substantially spared in this LHON family. These represent subclinical visual impairments in otherwise asymptomatic LHON carriers.

Chromatic and spatial properties of parvocellular cells in the lateral geniculate nucleus of the marmoset (Callithrix jacchus).

The parvocellular (PC) division of the afferent visual pathway is considered to carry neuronal signals which underlie the red-green dimension of colour vision as well as high-resolution spatial vision. In order to understand the origin of these signals, and the way in which they are combined, the responses of PC cells in dichromatic ('red-green colour-blind') and trichromatic marmosets were compared. Visual stimuli included coloured and achromatic gratings, and spatially uniform red and green lights presented at varying temporal phases and frequencies.The sensitivity of PC cells to red-green chromatic modulation was found to depend primarily on the spectral separation between the medium- and long-wavelength-sensitive cone pigments (20 or 7 nm) in the two trichromatic marmoset phenotypes studied. The temporal frequency dependence of chromatic sensitivity was consistent with centre-surround interactions. Some evidence for chromatic selectivity was seen in peripheral PC cells. The receptive field dimensions of parvocellular cells were similar in dichromatic and trichromatic animals, but the achromatic contrast sensitivity of cells was slightly higher (by about 30%) in dichromats than in trichromats. These data support the hypothesis that the primary role of the PC is to transmit high-acuity spatial signals, with red-green opponent signals appearing as an additional response dimension in trichromatic animals.

The impact of suppressive surrounds on chromatic properties of cortical neurons.

Stimulation of the suppressive surround of a cortical neuron affects the responsivity and tuning of the classical receptive field (CRF) on several stimulus dimensions. In V1 and V2 of macaques prepared for acute electrophysiological experiments, we explored the chromatic sensitivity of the surround and its influence on the chromatic tuning of the CRF. We studied receptive fields of single neurons with patches of drifting grating of optimal spatial frequency and orientation and variable size, modulated along achromatic or isoluminant color directions. The responses of most neurons declined as the patch was enlarged beyond the optimal size (surround suppression). In V1 the suppression evoked by isoluminant gratings was less than one-half that evoked by achromatic gratings. Consequently, many cells were most sensitive to achromatic modulation when patches just covered the CRF but were most sensitive to isoluminant modulation when patches were enlarged to cover the suppressive surround. Non-oriented neurons that were strongly chromatically opponent generally lacked suppressive surrounds. In V2 most neurons showed equal surround suppression from isoluminant gratings and achromatic gratings. This makes the relative sensitivity of V2 neurons to achromatic and isoluminant gratings mainly independent of the size of the grating. We also measured the chromatic properties of the CRF in the presence of differently colored surrounds. In neither V1 nor V2 did the surround alter the chromatic tuning of the CRF. Cortical mechanisms sensitive to chromatic contrast seem to provide little input to the suppressive surrounds of V1 neurons but substantial input to those of V2 neurons.

Short-wavelength acuity: blue–yellow and achromatic resolution loss with age

Previous studies have indicated that peripheral resolution for achromatic gratings is sampling limited and directly related to the density of the underlying midget ganglion cell population. Previous studies by the authors have shown that peripheral resolution for blue-cone isolating gratings is also sampling limited, is robust to optical defocus and short-wavelength attenuation, and yields estimates of sampling density which correspond closely with the density of small bistratified ganglion cells. We measured peripheral resolution in a group of normal subjects ranging in age from 12 to 72 years, using both achromatic and blue-cone isolating gratings, to determine how performance (and hence ganglion cell density) changed with age for both systems. Resolution was higher for achromatic than blue–yellow gratings and performance was flat for both until the fifth decade. After this, performance declined for both at a rate of ∼14%/decade with no significant difference between the two rates of decline. Individual measurements of lens density were not correlated with short-wavelength sensitive resolution performance in the older subjects, further indicating that the decline in resolution was not attributable to pre-retinal absorption.

Changes in short‐wavelength resolution acuity with age and defocus in the periphery: implications for relative ganglion cell loss

Purpose: Resolution acuity is known to be sampling limited for achromatic gratings in peripheral vision. In addition, peripheral achromatic resolution is unaffected by defocus up to 3 D. We have also shown resolution for Short Wavelength Sensitive (SWS) system isolating gratings to be sampling limited in both the fovea and periphery. This means that measurements of peripheral resolution with either stimulus can potentially be used to estimate the density of the corresponding underlying retinal ganglion cell population. We wished to examine the effect of optical defocus on detection and resolution acuity for this stimulus and also determine how peripheral resolution declined with age for both the achromatic and SWS system.Methods: We measured detection and resolution for SWS‐isolating gratings in the fovea and at 20° eccentricity for optical defocus between 0 and 4 D. In addition we measured resolution acuity for both achromatic and SWS‐isolating gratings at 13° in four retinal locations in 50 normal subjects ranging in age from 12 to 70 years. Measurements of lens density (age‐related yellowing) were also made in subjects over 60 years of age.Results: Detection acuity was initially higher than resolution accompanied by observations of aliasing. Detection acuity declined steadily with defocus but SWS resolution acuity showed no decline until 1 D in the fovea and 3 D at 20°. Resolution acuity was higher for achromatic gratings than SWS‐isolating gratings at all ages and neither displayed any decline in performance until around 55 years. After this both declined in a parallel fashion at a rate of 10% per decade. The individual ratio of SWS/achromatic resolution (0.25) showed no correlation with increasing lens density.Conclusions: As resolution for both short‐wavelength and achromatic gratings is robust to optical attenuation and declines in a parallel fashion after 55 years there appears to be no selective loss of SWS‐driven ganglion cell density relative to achromatic density with age.

What limits detection and resolution of short-wavelength sinusoidal gratings across the retina?

Peripheral resolution acuity for achromatic gratings is known to be limited by the density of the underlying ganglion cell sampling array. After confirming isolation of the short-wavelength sensitive (SWS) system using chromatic adaptation methods, we wished to determine if resolution is limited by blue/yellow ganglion cell sampling (evidenced by a superiority of detection over resolution acuity and the perception of aliasing) and thus directly related to SWS-driven ganglion cell density. We measured detection and resolution acuity between 0° and 35°, using blue sinusoidal gratings superimposed on a yellow adapting background which ranged in intensity from 2.5 to 4.7 log Trolands. At all locations, a break could be observed in the acuity vs. illumination curves followed by a plateau. Detection and resolution acuity were the same for low background intensities, but resolution acuity was lower than detection at higher intensities, accompanied by observations of chromatic aliasing. SWS resolution is sampling-limited across the retina and agrees well with predicted performance based on anatomical estimates of small bistratified ganglion cell density.