The Reaction Time Test: Processing Speed and Attention, Explained

Keel's reaction time test presents a visual target and asks you to respond as quickly as possible when it appears. Trials include both simple reaction time (respond to any target) and choice reaction time (respond to a specific target, withhold response to others). Response times are measured in milliseconds across multiple trials, and the task records both speed and error rate.

Reaction time tasks have been used in experimental psychology since Francis Galton's anthropometric laboratory in the 1880s. In modern cognitive neuroscience, they remain fundamental: the Human Connectome Project, the UK Biobank, and most large-scale aging studies include some form of reaction time measurement as a core cognitive marker.

Simple reaction time measures the baseline speed of the sensorimotor loop: visual cortex detects the target, prefrontal motor planning areas engage, and the motor cortex fires. The limiting factor is neural conduction speed — essentially, how fast signals travel along myelinated pathways. This makes simple reaction time a direct proxy for white matter integrity.

Choice reaction time adds a decision component. When you must distinguish target from non-target, the prefrontal cortex becomes heavily involved in response selection and inhibition. This engages executive function: specifically, the ability to suppress a prepotent response (respond) in favor of a contextually appropriate one (do not respond).

Together, these two variants capture processing speed, attentional vigilance, and inhibitory control — three domains that are both sensitive to daily state and, over time, sensitive to neurodegeneration.

A landmark 2014 meta-analysis by Deary and colleagues, drawing on multiple large longitudinal studies, found that reaction time measured in midlife is a significant predictor of mortality and neurological outcomes decades later. Faster reaction time at 45 predicted better cognitive outcomes at 70. This is not because reaction time causes good health — it is because reaction time reflects the integrity of the neural hardware underlying all cognitive function.

In Parkinson's disease, reaction time slowing often precedes motor symptoms by years. In Alzheimer's disease, choice reaction time — which requires working memory and executive control — degrades earlier than simple reaction time. In vascular cognitive impairment, white matter lesions slow neural conduction, producing reaction time changes that precede more dramatic cognitive symptoms.

Reaction time is one of the most acutely state-sensitive cognitive measures. A single night of less than six hours of sleep slows mean reaction time by 30-50ms — an effect comparable to a blood alcohol level of 0.05%. Caffeine improves simple reaction time for habitual users within 30 minutes of consumption. Body temperature, time of day (peak reaction speed typically occurs mid-to-late morning), ambient noise, and even hydration status all produce measurable effects.

This state sensitivity is a feature rather than a bug for daily tracking: it means Keel's reaction time measure is genuinely sampling your current cognitive state, not just your stable trait. The trend across many sessions — smoothed over the noise of daily life — is where the meaningful signal lives.