This special volume reflects the resurgence of interest within psychology in inhibitory processes, their related neural systems and their role in cognition.

Inhibition has been described as a factor that plays a role in ‘orchestrating’ cognitive performance in various domains like perception, selective attention, memory and motor processes. There are two main areas of research that have expressed interest in inhibitory processes and that are also reflected in the contributions to this special volume. One area is mainly theoretical and directed towards identifying the basic cognitive mechanisms underlying processes like interference and the focusing of attention. On the neural level others have attempted to identify the cortical and subcortical mechanisms and networks that regulate inhibition and facilitation processes.

An area of basic research in which investigators have shown a special interest in inhibitory mechanisms is selective attention. Although most studies of selective attention have focused on the facilitation of relevant target information against a background of non-relevant information, there is growing evidence that non-selected items are not passively ignored but may also be actively suppressed. These findings support the view expressed in the past that theories of attention must deal not only with facilitatory but also with inhibitory processes. Some important theoretical issues that were raised by investigators in this field are (a) are facilitation and inhibition separate processes with different neural substrates; (b) what is the time course of facilitation and inhibition; and (c) what is the temporal locus of these processes: do they operate at the level of early sensory, central, or response-related selection processes?

The second area of research on inhibitory processes has focused mainly on individual differences in the ability to inhibit processing of to-be-ignored objects. Decreased inhibition or increased distractibility during selective attention tasks has been reported in clinical populations like schizophrenics, patients with frontal lesions, parietal lesions and children with attentional disorders. Inhibitory functioning has also been related to individual differences in cognitive functioning in normal subjects and to differences in the age of subjects. For instance, in normal populations increased distractibility or decreased inhibition has been reported to occur in older as compared to young adults.

Our starting point that is also expressed in the title of this paper is that inhibitory systems are manifested in various different ways. To illustrate the diversity of inhibition we shall begin this article with presenting in Section 2 a global (although not exhaustive) overview of experimental paradigms and psychophysiological methods from which inhibitory processes have been inferred in the past. In the same section we report in greater detail a subset of experimental paradigms that have manipulated visual-spatial selective attention and discuss the question how in these paradigms facilitatory and inhibitory processes were separated at the levels of their behavioral and electrocortical manifestations. In Section 3 we shall briefly discuss some neural pathways and mechanisms that are involved in the regulation of facilitation and inhibition in tasks that involved visual selective attention. Our overview of paradigms together with the age-related literature leads us to propose in Section 4 a tentative framework for conceptualizing inhibitory processes in terms of three distinct but interacting neural systems at the level of anterior and posterior cortices and the brain stem.

A variety of experimental paradigms have used the concept of inhibition, sometimes explicitly but often also more implicitly, to account for operational outcomes like increases in reaction time or amount of errors in high interference conditions. Some of the paradigms have also implemented event-related potentials (ERPs) as electrocortical reflections of facilitatory and inhibitory processes.

It is still uncertain whether the different forms of ‘inhibition’ that were inferred from these paradigms are functionally related to another or reflect qualitatively different forms of inhibition.

The OR has been considered as an important element in many theories of involuntary attention. It is manifested as a complex pattern of skeletal, physiological and behavioral changes that are elicited by novel, unexpected or highly significant stimuli. The orienting response is a reflexive attentional mechanism that is thought to involve the frontal lobes and to facilitate information processing by enhancing perceptual sensitivity to environmental stimuli. In his influential monograph Kahneman (1973) has further extended this view by suggesting that one of the functions of the OR is ‘allocation of effort’ to further processing when a stimulus event has been evaluated as novel or significant. The allocation of effort was assumed to be accompanied by both a transient rise in arousal and inhibition. The facilitatory component becomes manifest in enhanced processing of the alerting or relevant event and a better orientation (‘priming’) towards future significant information. It is further assumed to be expressed in enhanced activity of the autonomic nervous system, in particular in the dilatation of the pupil. The inhibitory component is associated with a temporary inhibition of non-relevant events and is reflected in cardiac deceleration and decrease in somatic (muscular) activity.

The OR is also reflected in ERP components like the mismatch negativity (MMN: Näätänen, 1992) in the auditory stimulus modality, and N2b in the visual stimulus modality ( Rohrbaugh, 1984). Another ‘endogenous’ component that has been related to orienting is the P3 (P300) component. Two types of P3 may be distinguished namely a parietal maximal P3 to target stimuli (‘target P3′) and a fronto-central maximal P3 that is elicited by non-target deviant stimuli (‘novelty P3′).