What covariance mechanisms underlie green/red equiluminance, luminance contrast sensitivity and chromatic (green/red) contrast sensitivity?

In order to investigate the mechanisms underlying green/red equiluminance matches in human observers and their relationship to mechanisms subserving luminance and/or chromatic (green/red) contrast sensitivity, we tested 21 human subjects along these dimensions at 16 different spatial and temporal frequencies (spatial frequency, 0.25-2 c/deg; temporal frequency, 2-16 Hz) and applied factor analysis to extract mechanisms underlying the data set. The results from our factor analysis revealed separate sources of variability for green/red equiluminance, luminance sensitivity and chromatic sensitivity, thus suggesting separate mechanisms underlying each of the three main conditions. When factor analysis was applied separately to green/red equiluminance data, two temporally-tuned factors were revealed (factor 1, 2-4 Hz; factor 2, 8-16 Hz), suggesting the existence of separate mechanisms underlying equiluminance settings at low versus high temporal frequencies. In addition, although the three main conditions remained separate in our factor analysis of the entire data set, our correlation matrix nonetheless revealed systematic correlations between equiluminance settings and luminance sensitivity at high temporal frequencies, and between equiluminance settings and chromatic sensitivity at low temporal frequencies. Taken together, these data suggest that the high temporal frequency factor underlying green/red equiluminance is governed predominantly by luminance mechanisms, while the low temporal frequency factor receives contribution from chromatic mechanisms.

Spatial frequency tuned covariance channels for red-green and luminance-modulated gratings: psychophysical data from human adults.

Both chromatic and luminance-modulated stimuli are served by multiple spatial-frequency-tuned channels. This experiment investigated the independence versus interdependence of spatial frequency channels that serve the detection of red-green chromatic versus yellow-black luminance-modulated stimuli at low spatial frequencies. Contrast thresholds for both chromatic and luminance-modulated gratings were measured within 12 individual subjects using a repeated-measures design. Spatial frequencies ranged from 0.27 to 2.16 c/deg. A covariance structure analysis of individual differences was applied to the data. We computed statistical sources of individual variability, used them to define covariance channels, and determined the number and frequency tuning of these channels. For luminance-modulated gratings, two covariance channels were found, including one above and one below 1 c/deg [cf. Peterzell, & Teller (1996). Individual differences in contrast sensitivity functions: the coarsest spatial pattern analyzer. Vision Research, 36, 3077-3085]. For chromatic gratings, correlations between thresholds for most spatial frequencies were uniformly high, yielding a single covariance channel covering all but the highest spatial frequency tested. A combined analysis of both data sets recovered the same three covariance channels, and showed that detection thresholds for low-frequency red-green chromatic and luminance-modulated stimuli are served by separate, statistically independent processes.

Spatial frequency tuned covariance channels for red-green and luminance-modulated gratings: psychophysical data from human infants.

This study concerns the spatial-frequency-tuned channels underlying infants' contrast sensitivity functions (CSFs) for red-green chromatic stimuli, and their relationship to the channels underlying infants' CSFs for luminance-modulated stimuli. Behavioral (forced-choice preferential-looking) techniques and stationary stimuli were used. In experiment 1. contrast thresholds were measured in 4- and 6-month-olds, using isoluminant red-green gratings with spatial frequencies ranging from 0.27 to 1.53 c deg. In experiment 2. contrast thresholds were measured in 4-month-olds. using both red-green and luminance-modulated gratings in the same low spatial frequency range. Covariance analyses of individual differences were performed. Experiment 1 revealed one dominant covariance channel for the detection of red-green gratings, with a second channel contributing to detection of the highest spatial frequencies used. Experiment 2 revealed two to three channels serving color and luminance: but surprisingly these channels were not statistically separable for luminance versus chromatic stimuli. Thus, covariance channels for color and luminance that are independent for adults [Peterzell & Teller (2000). Spatial frequency tuned covariance channels for red-green and luminance-modulated gratings: psychophysical data from human adults. Vision Research, 40, 417-430] are apparently interdependent in infants. These data suggest that for infants, detection thresholds for chromatic and luminance-modulated stimuli may be limited by common mechanisms.

Development of spatial frequency tuned "covariance" channels: individual differences in the electrophysiological (VEP) contrast sensitivity function.

We investigated the spatial frequency tuned channels underlying the contrast sensitivity functions (CSFs) of adults and infants. CSFs were measured in adults and in 8-, 14-, 20-, and 32-week-old infants, using the swept-contrast visual evoked potential (sweep-VEP). At each age, 8 to 21 subjects provided complete data. Subjects viewed achromatic sine wave gratings (0.3 to 8 c/deg) on a 20 degrees field presented on a CRT. Gratings were counterphased at 12 reversals/s (6 Hz). The second harmonic response was used to interpolate thresholds. We computed statistical "sources" of individual variability (or factors) underlying CSFs, then calculated the number, nature (discrete vs. continuous) and frequency tuning of "covariance" channels. CSFs from adults each contained three spatial frequency tuned covariance channels, consistent with psychophysical results spanning a similar spatial frequency range. Covariance channels in infants shifted upward in spatial frequency with age, with rapid shifts occurring between 8 and 14 weeks. The change in scale coincided with, and was probably determined by, developmental cone migration into the fovea and growth in eye size.

Spatial frequency masking with the sweep-VEP.

A suprathreshold sinewave grating can change the amplitude of the steady-state visual evoked potential (VEP) in response to a test grating if the two are close in spatial frequency (SF). The change in amplitude provides clues to underlying pattern analyzers. Masking was measured in 12 observers using the steady-state VEP. As a test grating reversed at 7 Hz, a masker of similar temporal frequency (9 Hz) but of variable SF was superimposed on it. Test gratings were 1, 3 and 8 c/deg (20% contrast). Within a 10 sec trial, the mask (20 or 40% contrast) was fixed at one of nine SFs or was swept across 19 SFs (5 octaves). The amplitude of the test response (at 14 Hz) was measured as a function of the SF of the masker. Group masking functions were broad (2-3 octaves) and revealed multiple minima. Functions for 1 and 3 c/deg tests each revealed minima near 1 and 3 c/deg. Functions for 8 c/deg tests revealed minima at 3 and 8 c/deg. Doubling the contrast of the mask from 20 to 40% increased masking but in a nonlinear fashion that enlarged the off-peak minima. Swept masks caused slightly more masking than fixed masks, and caused masked amplitudes to exceed unmasked amplitudes (i.e., enhancement) in one condition (3 c/deg.test, 20% contrast mask). The data suggest that each VEP masking function reflects the outputs of multiple spatial analyzers, that a discrete set of analyzers may underlie the data, and that the efficient sweep-VEP can measure SF tuning.

Hemisphericsymmetries in the identification of band-pass filtered letters Reply to Christman et al. (1997).

Christman, Kitterle, and Niebauer (1997) have examined the hypothesis that the two cerebral hemispheres are specialized for processing different ranges of spatial frequency. Their two experiments partially replicated an experiment of Peterzell, Harvey, and Hardyck (1989), who used Sergent's (1982) letter identification paradigm with spatial-frequency band-pass filtered letters as stimuli. We acknowledge the unusual strengths of Christman et al.'s experiments, but argue that the results support the original conclusion of Peterzell et al.: The results are not attributable to hemispheric asymmetries in spatial frequency processing.

Individual differences in contrast sensitivity functions: the lowest spatial frequency channels.

The number and nature of spatial channels tuned to low spatial frequencies in photopic vision was examined by measuring individual differences in the contrast sensitivity functions (CSFs) of seven visually normal adults. Stationary, 51 cd/m2, low spatial frequency sinusoidal gratings between 0.27 and 2.16 c/deg were used as stimuli. Correlational and factor analyses revealed that the set of CSFs contained only one statistical source of individual variability at spatial frequencies below 1 c/deg (tuned to a peak of about 0.8 c/deg), and a second source above 1 c/deg (tuned to about 1.4 c/deg). The sources ("factor-channels") mapped well onto the two coarsest spatial frequency channels from some existing computational models. The analysis was applied also to earlier data from 4-, 6- and 8-month-old infants, in which two sources of variability have been found below 1 c/deg [Peterzell, D. H., Werner, J. S. & Kaplan, P. S. (1995). Vision Research, 35, 961-980]. The combined results are consistent with the hypothesis that in photopic vision of the neonate, there are two channels with peak sensitivities below 1 c/deg, and that these channels shift their tuning from lower to higher spatial frequencies by about a factor of four during development.

Individual differences in contrast sensitivity functions: longitudinal study of 4-, 6- and 8-month-old human infants.

Contrast sensitivity functions (CSFs) of 25 infants were measured longitudinally at 4, 6 and 8 months of age using a preferential-looking method and the method of constant stimuli. Sine-wave gratings varied from 0.27 to 4.32 c/deg, contained eight unattenuated cycles (with edges tapered to uniform gray), and rose to the desired contrast in 2 sec. (1) The average CSF was described on log-log coordinates by a band-pass function. With development it increased in overall sensitivity to contrast, shifted its peak toward slightly higher spatial frequencies, and increased its high frequency cutoff. (2) Log sensitivity at the CSF peak was slightly higher for female than male infants at 6 months, consistent with the hypothesis that vernier acuity (which also may differ between the sexes at this age) is partly mediated by analyzers tuned to low frequencies. (3) Within age groups the individual differences were such that log sensitivities for neighboring spatial frequencies generally correlated more highly than distant frequencies. With development the correlations among distant frequencies below 1.0 c/deg increased. Monte Carlo simulations of a model that shifts spatial analyzers to higher frequencies with age reproduced these results but simulations of adultlike, unshifting analyzers did not. (4) Measures taken 2 months apart tended to correlate more highly than those taken 4 months apart, though some individual differences in the CSF peak remained stable over 4 months.

Individual differences in the visual attention of human infants: further evidence for separate sensitization and habituation processes.

How many and what kinds of processes mediate infant visual attention? Are they influenced by stimulus "complexity?" Analyses were performed on visual fixation data from ninety 4-month-old infants; the time each infant spent looking at a black and white checkerboard pattern containing either 4 x 4, 12 x 12, or 20 x 20 checks per picture was measured for eight 10-s trials, with 10-s interstimulus intervals (Kaplan & Werner, 1986). Correlational and factor analyses revealed one significant source of individual variability (or factor) in the 4 x 4 data, and two significant sources in the 12 x 12 and 20 x 20 data. One factor gained strength over progressive trials and accounted for decreases in looking time. A second factor gained strength over the first few trials, then lost strength over the remaining trials, and accounted for initial increments in looking time. The two factors are consistent with a dual-process model of infant response dynamics; the first may represent habituation, a decremental process, and the second may represent sensitization, an incremental process that is activated by "complex" stimuli.

Individual differences in contrast sensitivity functions: the first four months of life in humans.

Contrast sensitivity functions of forty 4-month-old human infants were measured using a preferential-looking method and the method of constant stimuli. Circular sinewave gratings varied from 0.27 to 1.08 c/deg, contained eight unattenuated cycles (with edges tapered to uniform gray), and rose to the desired contrast in 2 sec. Log contrast sensitivities for variables close in spatial frequency correlated more highly than those that were farther apart in these data, and in data of 1-, 2-, and 3-month-olds from Banks and Salapatek [(1981) Journal of Experimental Child Psychology, 31, 1-45]. Factor analyses yielded at least two frequency-tuned factors per age group. Monte Carlo simulations of a quantitative model that shifts spatial mechanisms to higher frequencies with age reproduced the results for 4-month-olds, but simulations of adultlike, unshifting mechanisms did not. The data are consistent with the following conclusions: (a) individual differences in the sensitivity of spatial mechanisms may explain some individual differences in CSFs; (b) factor analysis may help to estimate mechanism tuning; and (c) spatial mechanisms may shift to higher frequencies during development.

On the nonrelation between spatial frequency and cerebral hemispheric competence.

The hypothesis that the two cerebral hemispheres are specialized for processing different visual spatial frequencies is discussed in reference to Christman's (1989) review. It is suggested that the relevant results from every study reviewed by Christman which support the hypothesis can also be explained in terms of the total amount of visible information or energy contained in a visual stimulus, and the right hemisphere's relative resistance to information degradation. A recent study provides evidence in support of this total visible information/energy hypothesis, and appropriate measures of information/energy are discussed. Furthermore, results from the studies reviewed may reflect response bias and not hemispheric specialization or competence. The few studies which examine response bias support such an interpretation.

Light, vision, and aging.

Recent research on aging of chromatic and spatial vision processes is reviewed. Changes in these visual processes with advancing age are largely continuous. Age-related declines in visual performance may be explained in terms of reductions in the illuminance of the visual stimulus due to changes in the ocular media and losses of efficiency at a neural level. Thus, some prominent characteristics of the senescent visual system are similar to those of the younger visual system operating at a lower ambient light level. One important determinant of retinal aging may be the light history of the individual, i.e., cumulative exposure to high-energy photons from solar radiation may accelerate retinal aging. If these abstractions from the literature are valid, then it will become more important to control the light environment throughout the life span.

Identification of letters in the predesignated target paradigm: a word superiority effect for the common word the.

The size of the perceptual unit used in reading was addressed using the predesignated target paradigm. Sixteen subjects viewed the following stimuli in random order: the words tee, the, tie, and toe; the nonwords eet, eht, eit, and eot; and the letters e, h, i, and o. Subjects fixated on the location of the center letter and identified the letter as e, h, i, or o, alternatives which were known to them at the onset. A word superiority effect was obtained for the common word the but not for the less common words tee, tie, and toe. The word superiority effect was attributable to bias rather than discriminability: Subjects exhibited a bias to perceive the words in this experiment as the (i.e., there was a bias to perceive h in the t e stimulus presentations). These results suggest that the common word the is processed in reading units that are larger than the letter, and that the system is biased to perceive common rather than uncommon words in data-limited conditions.

Spatial frequencies and the cerebral hemispheres: contrast sensitivity, visible persistence, and letter classification.

The hypothesis that the two cerebral hemispheres are specialized for processing different visual spatial frequencies was investigated in three experiments. No differences between the left and right visual fields were found for: (1) contrast-sensitivity functions measured binocularly with vertical gratings ranging from 0.5 to 12 cycles per degree (cpd); (2) visible persistence durations for 1- and 10-cpd gratings measured with a stimulus alternation method; and (3) accuracy (d') and reaction times to correctly identify digitally filtered letters as targets (L or H) or nontargets (T or F). One significant difference, however, was found: In Experiment 3, a higher decision criterion (beta) was used when filtered letters were identified in the right visual field than when they were identified in the left. The letters were filtered with annular, 1-octave band-pass filters with center spatial frequencies of 1, 2, 4, 8, and 16 cpd. Combining four center frequencies with three letter sizes (0.5 degrees, 1 degree, and 2 degrees high) made some stimuli equivalent in distal spatial frequency (cycles per object) and some equivalent in proximal spatial frequency (cycles per degree). The effective stimulus in the third experiment seemed to be proximal spatial frequency (cycles per degree) not distal (cycles per object). We conclude that each cerebral hemisphere processes visual spatial frequency information with equal accuracy but that different decision rules are used.