A statistical model in which the main effects explain variance of the dependent variable such that the total portion of explained variance is the sum of the portions explained by each individual effect. This is in contrast to an interaction effect, which is based on a multiplicative model. See Analyses of variance (ANOVA)
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Additive genetic effect
Genetic influences that operate in an additive fashion (i.e., in which two copies of a risk allele confers twice the risk of just one copy). It is the sum of mostly small effects of the alleles of the many genes influencing a particular trait. See Allele, Epistatic/epistasis, Gene
Adaptation
In evolutionary biology, the process by which those behavioral or other characteristics of individuals that promote survival in their particular environment evolve through the action of natural selection. Also refers to the outcome of the process. A troublesome concept in evolutionary biology, it refers to the process by which the structure and function of an …
Addison’s disease
Addison’s disease, also known as primary adrenal insufficiency, is relatively uncommon disorder affecting about 8,400 people in the UK. It occurs when the two adrenal glands situated atop of the kidneys are damaged giving rise to a deficiency in two essential steroid hormones (cortisol and aldosterone) due in most cases to an autoimmune response. Tending to present first …
Actomyosin
A protein complex synthesised from actin and myosin in skeletal (or striated) muscles that when stimulated shrinks and causes muscle contraction, with the energy being supplied by adenosine triphosphosphate. See Actin, Adenosine triphosphate (ATP), Myosin, Myofibrils and myofilaments, Proteins, Sarcomere, Striated (or striped or voluntary) muscle
Active sleep
A condition of the infant (also referred to as irregular or REM sleep and state II) when the eyes are closed, intermittent rapid eye movements can be observed through the eyelids, respirations are irregular and variable in frequency and amplitude, and motor activity varies from apparently random small limb movements to occasionally trunk movements and …
Activity-dependent organization
The property of the nervous system to use the pattern of its own activity to wire itself. The central concept is ‘fire together, wire together’, a shorthand term for the Hebbian synapse. See Cerebral cortex (development), Connectionist models, Experience-dependent processes, Hebbian synapse, Self-organization, Stimulus-driven development
Active intermodal matching (AIM)
This concept pertains to Meltzoff and Moore‘s proposed mechanism for facial imitation. Facial imitation involves intermodal matching because the infant sees the adult’s gesture, but does not see his own response as his own face remains unseen by him. Facial imitation thus requires matching across different sensory modalities (hence ‘intermodal’). It is ‘active’ because infants …
Action words
Words that describe, demand, or accompany action (e.g., brought, hired, investigated). See Action, Action syntax, Exception words, Symbols, Symbolic function
Activation (in a connectionist model)
A central aspectof information processing in connectionist models is the flow of activationbetween units through the weighted connections in the network. A unit’s activation is usually a value between0 and 1 (or between -1 and 1, depending on how the model is implemented). Inputunits receive information from the environment and their activation value isdetermined by …