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.