Abstract

Studies across a broad range of disciplines–from psychiatry to cognitive science to behavioral neuroscience–have reported on whether the magnitude of contrast sensitivity alterations in one group or condition varies with spatial frequency. Significant interactions have often gone unexplained or have been used to argue for impairments in specific processing streams. Here, we show that interactions with spatial frequency may need to be re-evaluated if the inherent skew/heteroscedasticity was not taken into account or if refractive error could plausibly differ across groups or conditions. By re-analyzing a publicly available data set, we show that–when using raw contrast sensitivity data–schizophrenia patients exhibit an apparent contrast sensitivity impairment at low, but not high, spatial frequencies, but that when using log-transformed data or when using generalized estimating equations, this interaction reversed. The reversed interaction, but not the overall contrast sensitivity deficit, would disappear if groups were matched on visual acuity. However, matching groups in this way is probably only defensible if acuity differences arise from optical blur. These analyses reconcile seemingly discrepant findings in the literature and demonstrate that properly reporting contrast sensitivity interactions with spatial frequency requires accounting for refraction error and skew/heteroscedasticity.
Keywords: contrast sensitivity, spatial frequency, visual acuity, heteroscedasticity, schizophrenia
Abbreviations : cs, contrast sensitivity; cpd, cycles per degree; SZ, schizophrenia; HC, healthy control; GEE, generalized estimating equations; PANSS, positive and negative syndrome scale;
When does contrast sensitivity depend on spatial frequency? Two ways to avoid spurious interactions
Contrast sensitivity–the inverse of contrast threshold–corresponds to how much contrast energy is needed to identify a target reliably. Deficits have been reported in glaucoma, glare, ocular hypertension, amblyopia, macular degeneration, dry eye, multiple sclerosis, Parkinson’s disease, and schizophrenia (Pelli & Bex, 2013). Contrast sensitivity enhancements have been reported in individuals with major depression, seasonal affective disorder, and among those at clinical high risk for psychosis (Keri & Benedek, 2007; Wesner & Tan, 2006). Contrast sensitivity is also adversely impacted by antipsychotic medication (Kelemen et al., 2013) and reductions in retinal dopamine synthesis (Jackson et al., 2012). Many of these studies report contrast sensitivity alterations that are non-uniform across the spatial frequency spectrum. Such interactions have either gone unexplained or have prompted authors to postulate specific biological differences. For example, in chronically ill schizophrenia patients, poor contrast sensitivity at the lower end of the spatial frequency spectrum has led some to hypothesize that cells in the magnocellular channel may be selectively impaired (Butler et al., 2005; Revheim et al., 2014; Martinez et al., 2012). Here, we leveraged a publicly available data set (Zemon et al., 2020) to consider whether illness-specific non-uniform reductions in contrast sensitivity could be explained by the heteroscedasticity or rightward skew inherent to contrast sensitivity data (i.e., with the variance being the highest for low-to-mid-range spatial frequencies and lowest for high spatial frequencies). We further considered whether potential group differences in visual acuity might generate more of a deficit at higher spatial frequencies. This second possibility was taken seriously since even small amounts of refractive error have been shown to worsen contrast sensitivity with high spatial frequency stimuli (Charman et al., 1979; Keane et al., 2014; Keane et al., 2022) and since visual acuity deficits are commonly observed in schizophrenia (Keane et al., 2016; Zemon et al., 2020). We show that both factors, taken together, strongly influence the direction and the statistical significance of reported interactions. Our results reconcile seemingly conflicting findings in the contrast sensitivity literature (Butler et al., 2005; Revheim et al., 2014; Zemon et al., 2020; Keri et al., 2002) and provide guidance on how to report such results.