Week in Review: Number 48 Supplemental

Natural Outdoor Lighting Improves Contrast Sensitivity
Contrast sensitivity is a measurement of luminance or color differences between the foreground and background of an object. In natural settings, it can affect visual function in situations of low lighting, glare, or fog, when contrast between the object and the environment is reduced. First author of the study explains, "Until now, vision research has operated under the assumption that luminance contrast does not change with light intensity. That is, a dark letter on a white page has the same contrast outdoors (under the brightest light) than indoors (under the dimmest light). Our study shows that this decades-old assumption is incorrect. We demonstrate that contrast sensitivity is strongly dependent on the amount of light and that natural, bright light stimulates the visual brain more effectively and improves eyesight.” The study, conducted in cat animal models and human subjects, showed  that "[as luminance range increases, contrast sensitivity increases more within cortical pathways signaling lights than those signaling darks. Conversely, when the luminance range is constant, light-dark differences in contrast sensitivity remain relatively constant even if background luminance changes." These differences in light-dark contrasts were largest for bright environments. They highlight that understanding that luminance settings affect visual contrast would reduce critical errors in research methodology and clinical care as well as improve cortical sensitivity that may help with better eyesight. Lead author of the study remarks, "The hope is that our study will serve as a catalyst for further investigation of contrast sensitivity and its role in current measures for the evaluation, diagnosis, and treatment of eye disease."

Link Between Dual Sensory Loss and Depression
Researchers in the U.K. investigated the link between combined vision and hearing sensory loss or impairment and its effects on mental health, such as anxiety and depression. Specifically, the researchers analyzed survey data of 23,089 adults from the the Spanish National Health Survey 2017. After taking into account demographic adjustments, the data showed that while people with either visual or hearing loss were more likely to report depression compared to the general population, the odds of having depression were 3.85 times higher for individuals who had dual sensory loss or impairment. Furthermore, individuals with dual sensory loss were 3.38 times more likely to suffer chronic anxiety compared to the general population. The authors report this being the first study to investigate the connection between combined visual and hearing loss and risk of both depression and anxiety. Lead author of the study states, “Difficulties with seeing and hearing affects many aspects of everyday life. It can affect ability to work, interaction with others and carry out physical activity, all of which are important for emotional wellbeing. Our study has found a significantly increased risk of mental health issues like depression and chronic anxiety if people suffer both vision and hearing loss." A co-author adds, "These findings show the importance of appropriate treatment for sensory loss as well as timely intervention for mental health issues. The strong link to mental health that we have found shows these issues cannot be ignored by health authorities and action must be taken to ensure the best possible care for those with sensory loss."

Aphantasia & Hyperphantasia: Neural Signatures of Visual Imagery Vividness Extremes
Some individuals can conjure up extremely vivid visual imagery in their mind's eye, while others lack this ability entirely. Researchers in the U.K. conducted a study, the first systematic neuropsychological and brain imaging study of people with aphantasia and hypephantasia, to examine the neurological basis of these phenomena. The team conducted functional magnetic resonance imaging (fMRI) on 24 people with aphantasia, 25 with hyperphantasia, and a control group of 20 people with mid-range imagery vividness. The scans showed that individuals with hyperphantasia have stronger connections between their visual cortex, which becomes active in both real vision and visual imagery, and the prefrontal cortex, involved notably in decision-making. These stronger connections were more apparent in resting state fMRI, that is, while the participants were relaxed (and possibly mind-wandering). In an active state fMRI paradigm, "there was greater anterior parietal activation among hyperphantasic and control than aphantasic participants when comparing visualization of famous faces and places with perception." For memory-related tasks, the authors report, "Despite equivalent performance on standard memory tests, marked group differences were measured in autobiographical memory and imagination, participants with hyperphantasia outperforming controls who outperformed participants with aphantasia." And finally, in personality tests based on the Revised NEO Personality Inventory, they saw more introversion in the aphantasia group and more extroversion in the hyperphantasia group. The researchers highlight that these neural signatures among extremes of visual imagery vividness "validate and illuminate this significant but neglected dimension of individual difference."

Smoking Cannabis Significantly Impairs Vision
Researchers in Spain conducted a study examining the effects of cannabis (marijuana) use on vision and the awareness of visual impairment among cannabis users. The authors stress that in Spain, cannabis use has increased within the past decade despite this drug of abuse being illegal. Thirty-one cannabis users participated in the study. In particular, the researchers evaluated visual function in assessments of static visual acuity, contrast sensitivity, stereoacuity, accommodative response, glare sensitivity, and pupil size at baseline and after smoking cannabis . The authors report, "The study found that smoking cannabis has significant adverse effects on all the visual parameters analyzed." Despite this, survey of the participants revealed that 30% of the cannabis users self-reported that their quality of vision did not suffer at all, while 65% responded that their vision worsened only slightly. Among those participants who did perceive a decrease in visual quality, only contrast sensitivity (specifically at the highest spatial frequency tested) was identified. The researchers state, "These results, together with the lack of awareness that the participants presented about the visual impairment caused by smoking cannabis, indicate the need to carry out awareness-raising campaigns, as this visual deterioration can pose a danger when performing everyday tasks."

In Other News
(1) Aphantasia explained: some people can’t form mental pictures
(2) Pseudo-hallucinations: why some people see more vivid mental images than others