Activation-inhibition coordination is considered a dynamic process that functions as a common mechanism in the synchronization and functioning of neurons, brain, behavior, and their sequencing/organization, including over these different scales. The concept has broad applicability, for example, in applications to maladaptivity/atypicality. Young developed the hypothesis to help explain the efficacy of right-hand reaching to grasp in 1-month-olds, a study that implicated that the left hemisphere is specialized for activation-inhibition coordination. This underlying left-hemisphere function, noted to characterize the left hemisphere right from birth, can explain equally its language and fine motor skills, for example. The right hemisphere appears specialized for less complex inhibitory skills, such as outright damping/inhibition. The hypotheses related to inhibition and hemispheric specialization that appear in the literature typically refer to right hemisphere skills in these regards.
activation-inhibition coordination;inhibition;laterality;hemispheric specialization;excitation/inhibition balance;development;brain networks
Inhibition is a widespread mechanism in all living matter and life processes. As shall be shown, Go/NoGo Task behaviors, approach-withdrawal mechanisms, and excitation-inhibition cellular process balances are some of the tasks and concepts related to the question. As shall be shown, in high-order organisms, inhibition is central to these functions, including at the level of the neuron, regional brain networks, wider connectomics, and behavior.
2. Activation-Inhibition Coordination Modeling
2.1. A Left-Hemisphere Activation-Inhibition Coordination Model
Young developed the concept to help interpret his findings (Young et al. 
; Young & Gagnon 
) that 1-month-olds exhibit better-coordinated arm and hand movements in reaching for a midline object (e.g., opening the hand and then contacting the object in the proper sequence and with the proper timing), even as the left hand moves about more in a nondirected fashion as if exploring the space in which the object is contextually situated. The findings of this advantage of the right hand and arm for this activity were deemed consistent with an early hemispheric specialization along adult lines, and with the left hemisphere being specialized for fine motor skills, aside from its language-related skills, and the right hemisphere for spatial and related skills.
Young attempted to find the commonalities in the language and fine motor skills of the left hemisphere relative to those of the right hemisphere, even at this early age. He was aware of standard approaches, for example, that considered it more of an analytic hemisphere compared to the synthetic right hemisphere, but considered that the refined movements in fine motor skills and language production involved a particular coordinated dynamic of precise activation with fine-tuned inhibition of interfering movements. In this regard, the activation-inhibition coordination model could accommodate the questions posed of the common nature of the function that underlies all left-hemisphere-related skills.
That is, the concept of activation-inhibition coordination enhances understanding of the central mechanism in the brain and behavior in which inhibition participates. Rather than considering inhibition in isolation, as in research on right hemisphere inhibition, or in terms of some sort of balance, as in the balance or ratio of excitation and inhibition in neuronal synaptic activity, the concept of activation-inhibition coordination is more comprehensive, subtle, and varied.
Note that the term activation-inhibition coordination is one unique to Young. Other than references to his research, the term is not found in data engine searches in psychology and related disciplines (PsychInfo, Web of Science, Scopus, Google Scholar; 28 April 2022)
As for the specifics of the concept (see Table 1
), Young posited that the left hemisphere is specialized for the sophisticated, longer term, and major alterations in activation-inhibition coordinations. The right hemisphere is specialized for, or can undertake less, complex inhibitions, such as outright damping or less sophisticated activation-inhibition coordinations (e.g., brief ones, or ones requiring minor adjustments).
Different types of activation-inhibition functions in the left and right cerebral hemisphere.
||Longer term synchrony
||Complex, sophisticated, interweaving (see next)
||Sophisticated, subtle interweaving of activation and inhibitory skills, with appropriate activations taking place because of the suppression of interference due to inappropriate alternative behavior, both when selecting adaptive goal-directed activity and during its (movement) transitions. Both subtle competing movements and gross interfering ones are countered and controlled
||Majorly modifying/disrupting sequential activation-inhibition coordinations
||Minorly adapting/refining sequential activation-inhibition coordinations [could be left hemisphere based, depending on context]
||Full suppression/damping activity over time
||Activation-inhibition synchrony instantaneously or for a short time period. In spatial processes, some information as figure highlighted and some as ground moderated