Attention-Deficit/Hyperactivity Disorder (ADHD) is one of the most frequently diagnosed childhood psychiatric disorders, with worldwide prevalence rates estimated at 5.3%.1 A longitudinal study shows that approximately 65% of children with ADHD continue to show symptoms in adulthood.2,3 While adults with a history of ADHD may not meet the full criteria of symptoms listed in the DSM-IV, they continue to exhibit clinically significant symptoms.4 These continued symptoms are costly to society and the individual.5 For example, ADHD individuals were found to engage in higher risky driving behaviors and have more frequent car crashes.6–10

Current theories posit that executive function deficits account for poor outcomes in ADHD including driving problems. By contrast, we investigate a novel hypothesis that poor visual function might contribute to some of the negative outcomes. A review of the literature revealed several reports of ophthalmological problems in children with ADHD (see Table 1). For example, numerous studies report a significantly higher incidence of ADHD among children with convergence insufficiency.11–13 The ADHD trait was also found to be common in children with intermittent exotropia.14 Various visual function and ocular features including visual acuity, strabismus and ocular motility, near point of convergence and near point of accommodation were also tested in children with ADHD.15 The authors report that children with ADHD had a higher frequency of ocular and visual abnormalities, suggesting an early alteration of the development of neural and vascular tissues in central nervous system. Although Grönlund et al.15 found no effect of stimulant medication on visual function, another study reported that stimulant treatment seemed to improved visual field and best corrected distance visual acuity in children with ADHD.16

Not only have ophthalmological abnormalities been reported among individuals with ADHD, but also so have color perception deficiencies (see Table 2). Specifically, children with ADHD score poorly on clinical tests of blue color perception, but not red-green.17,18 Furthermore, children with ADHD show poorer game performance in a virtual environment when important on-screen information is displayed predominantly in blue-yellow colors compared to red-green colors.19 Several studies show decreased speed in color processing in ADHD population.20,21 Notably, the “retinal dopaminergic” hypothesis of color vision22 speculates that a deficiency in central nervous system (CNS) dopamine in ADHD may induce a hypo-dopaminergic tone in the retina, which in turn would have deleterious effects on short-wavelength (“blue”) cones. Blue cones are very sensitive to dopamine (as well as other neurochemical agents) and relatively scarce in number, so that the purported low dopaminergic tone in ADHD23 may affect blue-yellow color perception.

Numerous studies have reported possible visual deficiency including color perception in children with ADHD. However, it is unknown whether adults with ADHD also manifest problems in color vision or whether any visual problems have any functional impact in their everyday life. Accordingly, the aims of this preliminary study were to investigate visual function in everyday activities and color vision in adults with ADHD. Based on the retinal dopaminergic hypothesis, we hypothesized that adults with ADHD would show a specific color discrimination problem with blue colors.

This study provides the first evidence of color vision problems and functional visual impairments in adults with ADHD. Specifically, young adults with ADHD exhibited significantly more visual difficulties, as indicated by self-report on four out of eight areas in everyday visual activities. Moreover, the ADHD group reported more visual difficulties when driving. Also adults with ADHD exhibited specific problems in discriminating colors along the blue spectrum, as indicated by their performance on the FMT, as well as less efficient color discrimination and overall visual processing, as indexed by the longer time taken to complete the FMT and AQT scores, respectively.

In our study, ADHD individuals reported that they perceived significantly more difficulties compared to the control group in the following areas on the VAQ: depth perception, peripheral vision, visual search, visual processing, and items related to driving. Also, in the ADHD group self-rating of ADHD symptoms on the ASRS correlated significantly with VAQ subscales such as peripheral vision, visual search and visual processing speed, suggesting that these visual difficulties may be related to behavioral symptoms of inattention. However, those VAQ items pertaining to visual difficulties associated with driving did not correlate with ASRS scores. Previous studies have reported driving problems including risky driving behavior and vehicle collisions in adults with ADHD.6,7 Our findings on the VAQ, which are based on self-reported problems in visual function in everyday life, suggest that visual problems reported in previous studies may have functional consequences in real world activities including driving. Considering that visual perception is known to be one of the most important factors in driving, our findings warrants further investigation to delineate underlying mechanisms of visual difficulties in ADHD as well as the link between vision, attention and driving.34,35

In terms of color perception, our finding of impaired FMT performance for the blue spectrum in adults with ADHD, is in line with findings from previous studies using the FMT with children with ADHD17,18 (see Table 1). Notably, no impairments were found for the red or green spectra, suggesting that the difficulty with blue cannot simply be attributable to problems with sustained attention: the latter would be expected to have an impact on FMT performance across all of the colors. But how are we to account for the finding that the hue discrimination problems were restricted to the blue spectrum and did not include problems with the yellow spectrum? According to opponent color theory,36 the blue and yellow visual system may be coupled and show similar outcome in visual functions. However, an alternate model of color vision, Dual-Process Theory37 proposed that the visual system is comprised of two stages. The first stage can be considered as the receptor stage, where the three retinal photoreceptors (S, M and L cones) traduce physical stimuli into neural signals. In the second stage, the neural signals from cones are combined to create two chromatic mechanisms: L–M and S–(L+M). The S–(L+M) mechanism combines positive inputs from S cones and negative inputs from the sum of L and M cone signals which lead to our perception of blue and yellow. Notably, deficiency in central nervous system (CNS) dopamine in ADHD may induce a hypo-dopaminergic tone in the retina, which in turn would have deleterious effects on short-wavelength cones. Therefore, we may speculate that this dopaminergic effect may have an impact on S-cones in the receptor stage, leaving yellow perception, which is a product of LM cones in the receptor level intact. Another possibility, albeit speculation, is that the observed ‘blue-color problem’ in ADHD reflects an alteration in an opsin/vitamin-A-based photopigment, called melanopsin, which is maximally sensitive to blue light and linked with regulation of circadian rhythms:38,39 circadian systems are impaired in ADHD.40

Furthermore, high correlations between CVSI score and several categories in VAQ imply possible impact of color vision in daily visual activities. Likewise, evidence of a color perception deficit is important given the extensive use of color in daily life and the use of color stimuli in many of the standard neuropsychological tests used in the assessment for ADHD (e.g. color–word Stroop test, Wisconsin Card Sorting Task). For example, one previous study reported an intriguing relationship between performance on the Wisconsin Card Sorting Task (number of trials to complete the first category, which is color) and color naming on the Stroop. Both tasks involve a high proportion of blue stimuli.21 Findings for color vision were not completely consistent in this study. For instance, self-reports such as the CVSI and the color discrimination variable on the VAQ did not indicate any group differences in perceived color vision while the FMT results showed significant group differences on the blue spectrum. This seemingly inconsistent finding with self-reported questionnaires suggests that color perception deficiency may be specific to blue colors, as suggested by the retinal dopaminergic hypothesis.

Finally, we found that the ADHD population were significantly slower in naming speed than the control group in all subscales of the AQT: color naming, form/number/letter naming and color–form/color–number/color–letter naming. Individuals with ADHD were also generally slower on the color discrimination task (FMT) than the control group. Furthermore, ADHD individuals committed more errors than controls except for the color naming subscale. This finding is somewhat inconsistent with previous studies that found differences in color naming speed.17,20 One important factor to consider when evaluating the findings from the AQT is that the AQT differs significantly from the Stroop test in that the AQT tasks measure general cognitive and perceptual processing speed41 whereas the Stroop color–word test is believed to be a classic executive function test. A study using regional cerebral blood flow has shown that during color–form naming task, brain activation in the parietal area is increased and activations in fronto-temporal regions are decreased, which indicates that the color–form test may require more perceptual processing than executive functions.42 Along with slower naming speed in FMT, this further reinforces the idea that individuals with ADHD experience difficulties in visual processing efficiency.

There are several limitations in this preliminary study. First of all, the sample size is relatively small and involves a large proportion of college students which makes it difficult to generalize the results to the entire ADHD adult population. We acknowledge that this preliminary study needs to be replicated with a larger, clinical sample for findings to be generalizable to the ADHD population. However, based on the effect sizes on the visual and color vision measures of this study that range from medium to large, the sample provided sufficient statistical power to detect group differences.43 Secondly, ADHD diagnosis in this study was determined by previous documentation of diagnosis and comorbidities were self-reported by the participants instead of a comprehensive diagnostic procedure. Another limitation is that visual function problems in daily life were mainly analyzed with a self-report measure which might entail information bias. Subsequent studies should use a more objective measure of visual function problems in everyday life. In addition, the limitation of using the FMT is that it requires the participant to arrange the caps in the best color order (i.e. from yellowish green to turquoise green). This process involves both accurate movement execution and sustained attention, which are known to be impaired in ADHD. Future studies might want to give greater consideration to the choice of color perception tests and to incorporate other approaches to disaggregate visual attention and color vision. Perhaps display based tests such as Color Assessment and Diagnosis (CADs) should be used in future studies to investigating RG and YB chromatic sensitivity more accurately.

Notwithstanding the study limitations, this study represents the first attempt to investigate the perceived difficulty in daily visual activities, and to test whether a previously reported problem in blue color perception in children with ADHD also manifests in adults with ADHD. Its interdisciplinary innovation lies in its application of a well-established measure of color vision (FMT) along with a less used measure of visual functioning to test a novel hypothesis (retinal dopaminergic hypothesis22) in a prevalent psychiatric condition (ADHD), which is not traditionally linked with visual problems. Our findings highlight the need for further research on the complex interactions between attention, color perception, and visual functioning and underlying explanatory mechanisms. Moreover, our findings indicate the need for optometrists, psychiatrists, and other clinicians to investigate visual function in the assessment of young adults with ADHD. The VAQ appears to be a psychometrically sound measure that may serve as a useful clinical tool for investigating self-reported visual function in daily life.

Dr Tannock has served as a consultant for Eli Lilly and Purdue Pharmaceuticals in the past 3 years, and Pearson-Cogmed provides software licences without cost for her research on working memory training in ADHD college students; neither Ms Kim nor Ms Chen have any conflicts of interest.