Abstract

Images with certain spatial or temporal periodicity, and strong colour contrast can be responsible for discomfort. Images with these characteristics are rare in the natural images to which the human visual system has adapted and, perhaps for this reason, natural images are generally more comfortable than those that are unnatural. Text has the characteristics of an uncomfortable image, and there are simple steps that can be taken to reduce the discomfort. Uncomfortable (and unnatural) visual stimulation gives rise to strong physiological excitation in the visual cortex of the brain and some people, such as those with migraine, are far more susceptible to such strong excitation than others. Functional magnetic resonance imaging and near infrared spectroscopy indicate that the excitation induced by strong visual stimulation can be reduced by coloured filters individually chosen to reduce discomfort, and filters of different colour are without effect. The benefits of coloured filters on accommodation, on reading speed, and on other higher perceptual function can be understood as due to a reduction of over-excitation, with implications for treatment in a variety of medical conditions in which the cortex is hyperexcitable, and with implications also for the idiosyncratic manner in which colour is coded within the higher levels of the visual system.