The results of the present study demonstrate that the majority of the variance in visual contrast sensitivity, at all spatial frequencies, was accounted for by nonshared environmental influences. There was a modest genetic component to contrast sensitivity during midlife across all tested spatial frequencies, with different heritability estimates for the various individual frequencies. The strongest effect based on the best-fitting model () was at the middle spatial frequency (6 cpd), for which the genetic contribution was 38%. Although the additive genetic effects were significantly different from zero, nonshared environmental influences explained the largest proportion of variation in contrast sensitivity at all frequencies, ranging from 62%–86%. We were unable to detect any influence of the environment shared by twins on contrast sensitivity during middle-age.
These findings may seem somewhat surprising; because contrast sensitivity is a basic biologically-based function, and one might accordingly expect more of the variance to be accounted for by genes. This is not necessarily the case, however; all things genetic must be biological, but all things biological do not have to be genetic. In this context, it is worth emphasizing the fact that heritability, which is the proportion of phenotypic variance that is attributable to genetic variance, is a statistical and a population construct that is not informative about specific genes or about particular individuals. The modest heritability of contrast sensitivity indicates little genetic variation, even though the basis of contrast sensitivity is likely to be genetic. Indeed, for some characteristics, little variation may be adaptive. For example, having two eyes is a characteristic that is determined by genes, but it must have extremely low heritability because—almost certainly for adaptive reasons—it manifests virtually no variation in the population.
It may be argued that poor reliability of the test, and hence measurement error, could have accounted for the results. The test estimated the contrast sensitivity threshold, with true threshold falling between the contrast level for which the participant accurately responded and the next contrast level for that same spatial frequency. Despite this design limitation, however, the reliability of the test is reasonable (.73 across spatial frequencies in patients with Alzheimer’s disease assessed with the Vistech, an earlier version of the FACT test; Cronin-Golomb et al., 1995
) with greater reliability at higher than lower frequencies. Others have likewise reported variable reliability with the Vistech, with the same pattern of better reliability at higher than lower frequencies (reviewed in Pesudovs, Hazel, Doran, & Elliott, 2004
). There is less information available regarding the FACT specifically, but Pesudovs and colleagues (2004)
compared the FACT and Vistech and found similar reliability for the two tests as well as reporting the usual pattern of better reliability at higher than lower frequencies for both tests. Bühren, Terzi, Bach, Wesemann, & Kohnen (2006)
reported that repeatability results with the FACT were consistent with those of earlier studies, but also acknowledged that in their study the luminance provided was lower than that recommended for use of the test. Further test-retest reliability assessment of the FACT is to be encouraged for future studies. In a separate study, we found that performance on the test correlated significantly with performance on a true threshold measure of contrast sensitivity in which participants (young adults, older adults, and individuals with Alzheimer’s disease) identified letters presented at varying levels of contrast (Cronin-Golomb, Gilmore, Neargarder, Morrison, & Laudate, in press
). This finding offers some support for the reliability and usefulness of the FACT, which has the further advantage over most other computerized or chart tests in that it assesses contrast sensitivity at multiple spatial frequencies. In light of these findings, the large amount of variance accounted for by nonshared (individual-specific) environmental influences in the results of the present study is probably not attributable primarily to measurement error, though such error may make some contribution especially at the lower spatial frequencies. The results can be taken to mean that individual-specific environmental events must play a role in modulating the genetic/biological phenomenon of contrast sensitivity. Determining what type of events—whether external or biological—have an impact on contrast sensitivity will be important for generating potential approaches toward improving it.
The age range of 52 to 60 at assessment describes individuals whose contrast sensitivity presumably has undergone a degree of normal age-related change, some of which could reflect incipient disease effects (Owsley et al., 1983
). As these individuals age, they are likely to develop age-related visual pathology (cataract, glaucoma, macular degeneration) as well as pathology associated with neurodegenerative disorders such as Alzheimer’s disease and Parkinson’s disease. Common age-related disorders that have a significant genetic component include glaucoma (Gottfredsdottir et al., 1999
), macular degeneration (Klein, Mauldin, & Stoumbos, 1994
), and general thinning of the retinal nerve fiber layer (Hougaard, Kessel, Sander, Kyvik, Sorensen, & Larsen, 2003
). In regard to healthy older adults, twin studies have revealed genetic contributions to aspects of vision that do not arise from disease processes, including the spherical equivalent of refractive error and axial length of the eyeball (Teikari & O’Donnell, 1989
; Valluri, Minkovitz, Budak, Essary, Walker, Chansue, Carbera, Koch, & Pepose, 1999
). As noted above, individual-specific environmental events—such as injuries, illness, or poor access to vision care, to name a few—may affect contrast sensitivity with age. That is, several possible genetic and environmental factors may alter the heritability of contrast sensitivity across the lifespan.
The all-male, relatively homogenous composition of our sample limits our ability to generalize these results to other populations. A further limitation of our study is that we used the FACT contrast sensitivity test as our sole measure of visual function besides corrected acuity. Because the contrast sensitivity threshold is not based on a continuous measurement, the threshold must be considered an estimate. An advantage of the FACT, however, is its ability to estimate thresholds at several spatial frequencies, which most tests of contrast sensitivity are not designed to do (Neargarder, Stone, Cronin-Golomb, & Oross, 2003
Contrast sensitivity is an important visual function because of its role in predicting cognitive and functional decline in normal aging and in common age-related disorders. The low heritability and the relatively strong influence of individual-specific environmental events that we observed for contrast sensitivity suggests that a focus of future research should be on identifying the types of individual-specific environmental experiences that influence this ability. Altering those experiences by mid-adulthood may conceivably result in the delay of decline in cognitive and functional abilities that is associated with visual impairment. Longitudinal examination of the respective influences of genes and environment on contrast sensitivity may further address the probable efficacy of vision-based interventions to improve cognition and daily function in adults from middle age to the later years of the lifespan.