The Role Of Inhibition In Cognition And Behavior

Cognition and the control of overt behavior rely on real-time orchestration of component cognitive processes or ''mental operations." Each operation achieves a step in the sequence of steps leading from stimulus to response, intention to action, or thought to thought. A major distinction is made between reflexive or ''automatic'' operations and voluntary or ''controlled'' operations. The more automatic an operation is, the more able it is to occur without intention, needing only the appropriate stimulus conditions or information inputs to trigger it; to occur outside of

Encyclopedia of the Human Brain Volume 2

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conscious awareness, without being noticed phenom-enologically; and to run in parallel with other mental operations.

Automatic operations gain these properties either from genetic hardwiring or, more frequently, from repetition under relatively unchanging conditions. Hence, they sometimes occur as reflexes, and they become quite prominent in familiar situations and well-practiced tasks. Controlled operations are the opposite. The more voluntary or controlled an operation is, the more its execution is intentional, conscious, and demanding of serial attention. Controlled processes become prominent when dealing with novel situations to which reflexes and habits are poorly adapted or when pursuing particular goals in situations that are likely to trigger reflexes and habits that would produce incorrect or inappropriate behavior if unrestrained.

To achieve flexibility in dealing with both the familiar, unchanging aspects of the world and with novel events, it is important to make automatic and controlled processes work in concert. Inhibition provides a tool for curbing or regulating automated responses in the service of controlled assessment and reaction.

Just how fundamental a role is played by inhibition ofautomated responses can be appreciated by thinking about the developmental trajectory of reflexes across the life span. One of the central principles of neurology is that disease processes affecting higher brain centers, especially the cerebral cortex, are revealed by reappearance of primitive reflexes. The knee jerk of a normal person is tonically inhibited, but it can become hyperactive after damage to the spinal cord that interrupts descending inhibitory pathways from the motor cortex. The sucking reflex of infants disappears after the nursing years but can reappear in a patient with Alzheimer's disease. Presumably, with the development of the nervous system, these primitive reflexes are inhibited throughout adulthood but may be disinhibited and reappear due to nervous system insult.

Other reflexes remain active throughout the life span. Among the most common are reflexive orienting reactions that involve the automatic deployment of attention to a suddenly appearing visual stimulus or a loud sound. Because these are very common occurrences, such attentional reactions are frequent and do not always occur at convenient times. When controlled deployment of attention is required by task performance, disruptive reflexive deployments may need to be inhibited.

Analogous problems can arise in controlling responses that are not reflexes but have become automated through practice. Attention deficit and hyperactivity disorder is a persistent individual difference characterized by an impulsive inability to resist engaging in prepotent or automated actions triggered by task-irrelevant stimuli in the environment or task-irrelevant thoughts in the mind. Intrusion of an unwanted or inappropriate automatic response occasionally occurs in normal children and adults as well. This can be seen in "slips of action'' in which distraction of attention while carrying out a low-frequency task can result in inadvertently executing a different task that is higher in frequency given the situation and stimulus environment.

Other inhibitory functions are aimed at suppressing unwanted or incorrect perceptions and thoughts rather than controlling attention or inhibiting overt actions. These extraneous mental representations may become activated through relatively automatic processes of generalization because they are similar to correct perceptions and thoughts, or they may become activated because the environmental situation is ambiguous and admits multiple interpretations, only one of which is relevant to the task at hand. It is obvious that the wrong interpretation can be reached in an ambiguous situation, and if so it will need to be replaced. Less obvious, perhaps, is the possibility that all the interpretations of an ambiguous stimulus might be computed relatively automatically at an early stage of processing on every occasion, with one chosen for further processing at a later stage. In either case, contextually inappropriate interpretations will be active at some point in task performance and will need to be put aside or eliminated. This may involve inhibition. In this regard, Sir John Eccles wrote the following in 1977:

I always think that inhibition is a sculpturing process. The inhibition, as it were, chisels away at the diffuse and rather amorphous mass of excitatory action and gives a more specific form to the neuronal performance at every stage of synaptic relay.

In this article, we present several examples representing the types of inhibitory function we have just described. Some are examples of inhibition regulating the deployment of attention. Others are examples of inhibition enabling disengagement from ongoing or automatically triggered responding so that overt behavior can be under voluntary and strategic control.

Still others are examples of inhibition tuning and sharpening the representation of the stimulus produced by perception or the interpretation of the situation produced by higher cognitive processes.

ii. attending to a spatial location

Orienting of visual attention to a point of interest is commonly accompanied by overt movements of the head, eyes, or body. Attending may originate at will, such as when we decide to look at a particular location where something of interest is expected, or it may originate reflexively without intention when something captures our attention, such as when we orient to a flash of light in the dark or to a movement in the periphery of our vision. In everyday life there are constantly competing demands on attention by the outside world as well as from internally generated goals. The need for mechanisms to arbitrate between these competing demands is straightforward: This must be done so that they can be integrated, prioritized, or selected among to provide coherent and adaptive behavior.

Michael Posner developed a paradigm widely employed to study visual spatial attention. Figure 1 shows the basic features of this paradigm. In a typical experiment, the subject is first presented with three boxes on a computer screen. A trial begins with a fixation cross at the middle box. After a short interval, one of the boxes may flash briefly, and a target (an asterisk) appears in one of the peripheral boxes. The subject is asked to respond as quickly as possible, by pressing a key, to the appearance of the target. Reaction time (RT) is measured (in milliseconds) from target onset until the subject's response. It is possible to study covert attention by asking the participants not to move their eyes and by measuring keypress responses (rather than saccade latencies).

Figure 1 Basic paradigm for spatial attention, showing an exogenous cue and a valid trial.

Figure 2 Typical time course for effects of a peripheral non-predictive luminance cue (box 50% cue) and a central predictive arrow cue. The task is a simple RT key press response to the appearance of the target. Mean detection RTs are presented as a function of cue-target interval (SOA) for valid and invalid cue conditions.

Figure 1 Basic paradigm for spatial attention, showing an exogenous cue and a valid trial.

Figure 2 Typical time course for effects of a peripheral non-predictive luminance cue (box 50% cue) and a central predictive arrow cue. The task is a simple RT key press response to the appearance of the target. Mean detection RTs are presented as a function of cue-target interval (SOA) for valid and invalid cue conditions.

The cue may summon attention to the target location, in which case it is a valid cue, or it may summon attention to the wrong location, in which case it is an invalid cue. For volitional goal-directed shifts of attention, often called endogenous shifts of attention, a central arrow serves as a cue and predicts where the target is likely to occur in most trials. That is, in 80% of the trials the target will appear at the valid cue location and in 20% of the trials the target will occur at the invalid cue location. For reflexive stimulus-driven shifts of attention, often called exogenous shifts of attention, the peripheral luminance change that serves as a cue has no predictive value with respect to where the target will occur (e.g., in 50% of the trials the target will occur at the valid cue location and in 50% of the trials the target will appear at the invalid cue location). In order to measure the effectiveness of the cue in summoning attention, researchers have manipulated the time interval between cue onset and target onset [the stimulus onset asynchrony (SOA)]. The typical effects of the two types of cues are depicted in Fig. 2. The top panel of Fig. 2 shows the time course of a nonpredictive peripheral luminance cue on summoning reflexive exogenous attention, and the bottom of Fig. 2 shows the time course of a predictive central arrow. These results were achieved in a covert attention experiment, with no movement of the eyes. The two cues are similar in the sense that in both types of cues the facilitory effects begin at 50 msec. However, there are differences. With endogenous cueing, the facilitory effect appears to be more sustained. With peripheral cueing, the advantage at the cued location changes, after a few hundred milliseconds, into an inhibition resulting in longer RTs for the cued location. No such inhibition is seen with endogenous cueing. These features of Fig. 2 will be discussed next, each related to a different mechanism of inhibition generated by the deployment of spatial attention.

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