Varieties of interhemispheric inhibition, or how to keep a good hemisphere down.

Recent views of cerebral laterality suggest that both hemispheres contribute to the performance of most cognitive tasks. This would appear to require regulatory mechanisms to coordinate, select, and integrate the processes subserved by each hemisphere. In this paper we consider whether interhemispheric inhibition is used to achieve unified performance from a bilateral system capable of producing simultaneous and potentially conflicting, outputs. We examine the theoretical bases and empirical support offered for three varieties of interhemispheric inhibition. Interhemispheric suppression refers to mechanisms that may permit one hemisphere to halt or prevent concurrent processing by the opposite hemisphere. Such inhibition may operate either in a reciprocal, or in a unidirectional, fashion. Interhemispheric isolation may require an inhibitory process that suspends information transfer to prevent potentially harmful interhemispheric intrusions. Finally, one hemisphere could impede processing within the other via interference, that is, by providing irrelevant or detrimental information. It is argued that, while such mechanisms are plausible, evidence supportive of each is not convincing. We consider why it has been difficult to obtain clear-cut support for interhemispheric inhibition and suggest some avenues for further research.