Brightness alteration from Diphenhydramine

Brightness alteration refers to a substance-induced change in the perceived luminance of the visual environment that does not correspond to any actual change in lighting conditions. The world may appear as though someone has turned up (or down) a dimmer switch on reality, with surfaces, colors, and the overall visual scene appearing significantly brighter or darker than they would in the individual's normal perceptual state.

The visual system processes brightness through multiple mechanisms. Pupil dilation (mydriasis) is the most straightforward: many psychoactive substances cause the pupils to dilate, physically allowing more light to enter the eye and genuinely increasing the retinal illumination of the visual scene. However, brightness alteration goes beyond this mechanical effect -- even when pupil size is controlled for, subjects under the influence of psychedelics report enhanced brightness, suggesting central neural processing changes.

Brightening of the visual field is most commonly reported with psychedelics and stimulants. Psychedelics appear to enhance brightness perception through increased gain in the visual cortex -- the neural response to a given amount of light input is amplified, making the same objective illumination produce a stronger subjective brightness signal. This is consistent with the broader psychedelic effect of reducing the brain's normal filtering of sensory input, allowing more of the raw visual signal to reach conscious awareness. Stimulants enhance visual brightness primarily through increased arousal and attentional enhancement of visual processing, combined with the mydriasis that most sympathomimetic substances produce.

Darkening or dimming of vision is characteristic of opioids, sedatives, and heavy doses of cannabis. Opioids cause pupillary constriction (miosis), directly reducing retinal illumination. Beyond this mechanical effect, CNS depression reduces cortical arousal and visual processing intensity, making the world appear dimmer and less vivid. Heavy sedation with benzodiazepines or alcohol produces a similar general dimming of visual experience as part of broader sensory suppression.

Brightness alteration can have practical implications for comfort and safety. Psychedelic-enhanced brightness can make sunlight or bright screens uncomfortably intense, and sunglasses may be welcome even indoors. Conversely, the darkened vision produced by opioids and sedatives contributes to impaired visual function in low-light conditions, increasing the risk of trips, falls, and navigation errors.

The effect is generally dose-dependent and resolves completely as the substance is metabolized. In rare cases, altered brightness perception may persist as part of HPPD (Hallucinogen Persisting Perception Disorder), typically manifesting as enhanced brightness or halos around light sources.