APA Dictionary of Psychology

a hypothesis proposing that some adult brains can tolerate pathological changes without overt signs of disturbance because of the capacity of remaining neurons in the central nervous system to compensate for damaged or destroyed tissue. Thus, a person with a high brain reserve can sustain a greater amount of brain injury or deterioration before manifesting symptoms than a person with low brain reserve can. Implicit to this concept is the notion of a critical threshold level of functioning neurons below which normal activities can no longer be maintained and symptoms of disorder appear.

The validity of this hypothesis has been difficult to establish empirically, but the concept has been influential within neurology and cognitive science since it was first proposed to explain the observation that many individuals with Alzheimer’s disease who had extensive amyloid plaques and neurofibrillary tangles in their brains nonetheless showed few decrements in their intellectual abilities. This same discrepancy has since been observed in other types of dementia, Parkinson’s disease, and other neurological disorders.

Indeed, the lack of a direct relationship between the degree of brain pathology and the clinical manifestation of that damage makes it difficult to diagnose these conditions in their early stages during which degenerative alterations of cerebral anatomy have begun accumulating and intervention would be most effective. Various operational definitions of brain reserve capacity are used in studies, including overall brain volume, component structure volumes, head circumference, cerebral glucose metabolism, cortical thickness, the number of brain neurons, the density of their interconnections, regional cerebral blood flow, neural transmission speed, and various parameters of the sensory evoked potential. The term brain reserve at times is used interchangeably with cognitive reserve, despite the differing theoretical emphases of the two concepts. Also called neural reserve. See also functional plasticity.