The central component of a mechanism described by Shallice to account for the selection of any one action from a range of possible actions (an action being identified as a goal-directed response). In this model, sensory information flows into the CENTRAL NERVOUS SYSTEM and undergoes CONTENTION SCHEDULING. This describes a process in which, through mutual inhibition, one RESPONSE is selected and all other responses are suppressed. This is a mechanism that allows for simple STIMULUS-response processing: all stimuli are assessed and the most potent and appropriate response selected. Contention scheduling mechanisms then activate effector mechanisms which generate appropriate behaviour. The proeess of contention scheduling has three possible outcomes: in the presence of stimulation from the environment (both the external environment [the world] and the internal environment [the body]) an appropriate response will be selected and inappropriate responses inhibited. In the absence of stimulation from the environment the system will do one of two things: it will persist with the last selected response (that is, show PERSEVERATION), or it will cease activity, producing no output. The supervisory attentional system is a mechanism that is proposed to intervene on the processes of contention scheduling, biasing it in particular directions. It may be, for example, that the response most likely to be activated by environmental stimuli is, for some superordinate reason, not desired and should, at that time, be suppressed.
The suppression of food intake by an individual on a diet is an example of this: environmental stimuli (both internal and external) can make eating the most highly selected response by a contention scheduling process, but the supervisory attentional system can suppress or divert this. As well as biasing contention scheduling, the supervisory attentional system has another important function: it can generate responses when none is being selected by environmental stimuli, thus avoiding the problems of perseveration or inactivation. This model of action selection has been influential in apparently diverse areas, including explanation of the effects on behaviour of AMPHETAMINE (see Robbins, 1982) and in explaining some of the behavioural difficulties expressed by patients With SCHIZOPHRENIA or FRONTAL LOBE damage (see Frith, 1992). Note also that the actions of the supervisory attentional system have an intimate relationship with the CENTRAL EXECUTIVE described by Alan Baddeley and his colleagues. (Indeed, Baddeley has made it explicit that the supervisory attentional system was a model for the central executive and has described how the functions of these equate). The actions of the supervisory attentional system are also clearly bound with the concept of EXECUTIVE FUNCTIONS and, as with executive functions, there is debate about the relationship between the functions of the supervisory attentional system and CONSCIOUSNESS. Some authors believe that the supervisory attentional system requires in some way conscious processing, while others argue that it does not. This is a particularly interesting point when considering whether animals other than humans operate a supervisory attentional system.
References
Frith C.D. (1992) The Cognitive Neuropsychology of Schizophrenia, Lawrence Erlbaum: Hove UK.
Robbins T.W. (1982) Stereotypies: addictions or fragmented actions? Bulletin of the British Psychological Society 35:297–300.
Shallice T. (1988) From Neuropsychology to Mental Structure, Cambridge University Press: Cambridge.
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