The pupil, the dark center of the eye, functions like a camera’s aperture, regulating the amount of light entering the eye. Muscles within the iris, the colored part of the eye, control pupil size. In bright conditions, the pupil constricts to limit light intake, while in dim light, it dilates to allow more light to reach the retina. This natural adjustment also occurs in response to emotional states such as fear or excitement.
How LSD Affects Pupil Size
Lysergic acid diethylamide (LSD), commonly known as “acid,” causes significant pupil dilation. Individuals under the influence of LSD exhibit larger pupils, sometimes described as “dinner plate” pupils.
The Brain’s Role in Pupil Changes
LSD-induced pupil dilation primarily stems from the drug’s interaction with receptors in the brain, particularly the serotonin 2A (5-HT2A) receptors. LSD acts as an agonist, meaning it binds to and activates these receptors. This activation impacts the autonomic nervous system, which involuntarily controls many bodily functions, including pupil size. LSD’s influence on 5-HT2A receptors leads to increased activity in the sympathetic nervous system, often associated with the “fight or flight” response.
The iris contains two muscles that control pupil diameter: the sphincter pupillae and the dilator pupillae. The sphincter muscle, controlled by the parasympathetic nervous system, constricts the pupil, while the dilator muscle, controlled by the sympathetic nervous system, dilates it. Sympathetic activation, triggered by LSD’s interaction with serotonin receptors, causes the dilator muscle to contract. This contraction pulls the iris outward, increasing the pupil’s diameter and allowing more light to enter the eye.
Beyond Pupil Dilation
Beyond changes in pupil size, LSD can induce other visual and ocular effects. Users may report alterations in the perception of colors, often describing them as more saturated or vivid. Patterns might appear to shift or flow, and depth perception can be altered. Some individuals experience phenomena like “tracers,” where moving objects leave a visual trail, or halos around lights. These effects are primarily perceptual, resulting from the drug’s impact on brain activity and sensory processing, rather than direct physical changes to the eye itself.
Other Substances and Pupil Size
LSD is not the only substance that influences pupil size; many other compounds can cause similar or opposing effects. Stimulants such as amphetamines, cocaine, and MDMA commonly lead to pupil dilation. Hallucinogens like psilocybin (found in magic mushrooms) and mescaline also typically cause pupils to dilate. In contrast, substances like opioids, including heroin, morphine, and fentanyl, generally cause pupil constriction, often resulting in “pinpoint pupils.” These examples demonstrate that changes in pupil size are a common physiological response to a variety of substances affecting the nervous system.