Kent E. Higgins

Spatial contrast sensitivity: effects of age, test–retest, and psychophysical method

Spatial contrast sensitivity was tested twice in normal subjects from five age groups by using each of two different psychophysical methods. Results obtained by the method of adjustment showed a decline in sensitivity with increasing age at all spatial frequencies. In contrast, the forced-choice procedure yielded results indicating an age-related decline predominantly at high spatial frequencies, with the magnitude of the decline being generally similar to that obtained earlier by a tracking method [Vision Res. 23, 689 (1983)]. The decline that we observed cannot be due to pupillary changes with age, since this factor was controlled. Moreover, we suggest that the high-spatial-frequency decline in contrast sensitivity, although it is comparatively small, is too large to be due to changes in lens density. We therefore suggest that a neural component is responsible. In addition, interactions among the age of the subject, the spatial frequency used, and the psychophysical method used suggest that the method of adjustment should be avoided for age-related studies of vision.

Detection and identification of mirror-image letter pairs in central and peripheral vision

Reading performance is poorer in the peripheral than in the central visual field, even after size-scaling to compensate for differences in visual acuity at the different eccentricities. Since several studies have indicated that the peripheral retina is deficient with respect to spatial phase discrimination, we compared the psychometric functions for detection (D) and identification (I) of size-scaled, mirror-symmetric letters (i.e. letters differing in the phase spectra of their odd symmetric components) at three inferior field eccentricities (0, 4, and 7.5 deg) using a two-alternative, temporal, forced-choice procedure and retinal image stabilization to control retinal locus. Each subjects data were fit with Weibull functions and tested for goodness-of-fit under several hypotheses. This analysis revealed that while the psychometric functions were of constant shape across eccentricity for the respective tasks, they showed statistically significant variations in the D/I threshold ratios. However, these variations were so small that poorer reading outside the fovea is unlikely to be due to reduced letter discriminability that might occur secondary to a loss of peripheral field phase sensitivity.

Long-term stability of the dark focus of accommodation.

ABSTRACT For 1 year we monitored the stability of the accommodative state assumed in darkness (the dark focus) of five subjects. Our results are similar to previously reported individual mean values. The magnitude and pattern of the variability in the dark‐focus also demonstrate individual differences. Despite this individual variability, all five subjects oscillated around a stable mean dark‐focus value throughout their respective measurement periods. This study indicates the feasibility of using the dark‐focus as a basis for correcting the anomalous myopias, in particular night and empty‐field myopia.

Suprathreshold intensity--area relationships: a spatial Broca-sulzer effect.

Three experiments are reported in which a brightness-matching procedure was used to determine the form of the foveal intensity-area relationship at varying suprathreshold levels. With two exceptions, the results of these experiments were consistent with previous threshold-level investigations of the intensity-area relationship. First, the nominal degree of spatial summation at the smallest stimulus sizes (less than 2′–3′)consistently exceeded that predicted by Riccos law at all visibility levels. However, further analysis indicated that this “supersummation” phenomenon is more than likely attributable to calibration errors introduced by diffraction. Second—and more important—at the higher suprathreshold levels, the coefficient ot spatial summation becomes negative between 3′–6′, indicating that brightness decreased with increasing stimulus size. This effect was termed the spatial Broca-Sulzer effect by analogy to its more familiar temporal counterpart. The third experiment indicated that the form of the suprathreshold intensity-area relationship is, to some extent, dependent upon the size of the standard stimulus used in the brightness-matching task.

Nonequivalence of backgrounds during photopic dark adaptation.

Using the equivalent-background transformation, Crawford has demonstrated the equivalence of adaptive states (dark and light adaptation) in scotopic vision. The critical feature of this transformation is that the threshold sensitivity of the eye at a particular time in the dark can be specified in terms of the steady-state, light-adapted condition necessary to produce the same threshold for all target diameters. This indicates that a single variable may control the spatial integration of light. Equivalence was investigated in the present study for long-term photopic dark adaptation using two target diameters (0.13° and 0.33°) and four chromatic combinations of red and green test and adapting stimuli (red on red, red on green, green on green, and green on red). Except under special conditions employing color-defective observers for which there was more complete isolation of cone mechanisms, equivalence between adaptive states was generally found not to exist. It is concluded that more than one process is needed to control the spatial integration of light in the photopic system, and these are hypothesized to be the wavelength-dependent excitatory and inhibitory processes of the visual receptive field.

THE YOUNG CHILD'S ABILITY TO SEE A DIFFERENCE BETWEEN MIRROR-IMAGE FORMS

Are children who fail on a discrimination learning task involving mirror-image (MI) forms unable to see a difference between them? When given the more purely perceptual task of judging form pairs as same (identically oriented) or different (non-identically oriented, MI or non-MI) a much larger proportion of children show evidence of discriminating MI pairs, even though there is still an age-related tendency to judge them “same.” Only 8 out of 75 Ss in grade levels K through 3 actually failed to discriminate MI forms using this paired-comparison task. Even such non-discriminators were apparently able to “see a difference,” however, since they very readily pointed in different directions when asked to show which way the forms faced.

The spatial Broca-Sulzer and sensitization effects for foveal viewing.

Abstract Under certain conditions a small, circular stimulus field will appear brighter (the spatial Broca-Sulzer effect) and produce a higher increment threshold (Westheimers sensitization effect) than a larger stimulus field of the same retinal illuminance. In this experiment, a two stage procedure was used to compare the two effects for foveal viewing by determining if the different stimulus area-retinal illuminance combinations that are perceived equally-bright produce equivalent states of increment-threshold sensitivity. The results indicate that, while both effects are observed at comparable levels of retinal illuminance, the two effects cannot be considered synonymous.

Saturation Discrimination and the Degree of Diabetic Retinopathy

A blue-yellow deficiency is generally observed when color vision is altered secondary to diabetes. What is not clear, however, is the relationship between the degree of retinopathy and the degree of color deficiency. We have measured spectral saturation discrimination (from 430–680 nm) in normal and diabetic subjects as the first perceptible step from a white reference field (x = 0.45, y = 0.43). We used an optical system which permitted the patient to vary colorimetric purity (at constant luminance) by adjusting a single control knob. Constancy of luminance was assured by preliminary measurement of each patient’s relative luminosity function. All diabetic subjects had normal visual acuity. Diabetic patients without retinopathy showed normal saturation discrimination functions. However, in patients with retinopathy, there was little correlation between losses in saturation discrimination and the degree of retinopathy. That is, a patient having minimal retinopathy, can manifest the same loss in saturation discrimination as a patient having severe background retinopathy.

Tritan pedigree without optic-nerve atrophy.

ABSTRACT Results of several previous reports have questioned the occurrence of the tritan color deficiency independently of dominantly inherited optic atrophy. This report describes the results of testing 34 members of a pedigree (including four tritans) for whom optic atrophy can be ruled out according to criteria previously described by Krill et al.

Clinical spatial contrast sensitivity measurement: implications of artificial scotoma experiments.

Measurement of spatial contrast sensitivity in normal subjects showed that the change produced by an artificial central scotoma depended on temporal factors associated with grating presentation. Minimizing temporal transients produced a predominantly high spatial frequency loss. Accentuating temporal transients produced an additional low spatial frequency change in sensitivity.