Executive Control Of Task Performance Stopping An Ongoing Thought Or Action

Sometimes we start thinking about something or start to perform an action, only to realize it does not fit with our primary goal or our general plan at the moment or that our goal has changed and it is no longer appropriate. Such situations call for a change in the course of thought or action. We must stop the current thought or action in order to be able to switch to another one. An experimental method for studying this common example of executive control is the stop signal paradigm developed by Gordon Logan.

In this paradigm subjects are engaged in a primary task. In a relatively small proportion of trials they are signaled to stop before executing the response to the primary task. For example, participants can be asked to respond in each trial by pressing a button to an X and another button to an O. In addition, they are asked to withhold their response if a tone is presented at any point during a trial. The tone is presented in 25% of the trials at various delays after onset of the primary stimulus. Performance in this task can be successfully modeled as a race between a "go process'' (perceiving and responding to the primary stimulus) and a "stop process.'' The stop process is conceived to be a separate sequence of mental operations that involves perceiving the stop signal and intervening in the ongoing sequence of operations involved in the go process or primary task. If participants finish the stop process before the go process, they inhibit their response to the primary stimulus. If they finish the go process before the stop process, they produce a response to the primary stimulus, failing to inhibit it. RT to the go signal can be measured directly, whereas stop-signal RT (the time needed to cancel the planned response) cannot be measured directly (since its behavioral signature is the absence of action and hence there is nothing overt to measure). However, Logan's race model provides quantitative methods for estimating the stop-signal RT.

Research applying this model shows that young adults can stop a wide variety of actions (key presses, hand movements, squeezes, and speech) very quickly, with an estimated latency from the stop signal of about 200 msec. Williams and colleagues reported that the ability to stop improves developmentally throughout childhood (i.e., stop-signal RT decreases) and then remains approximately constant across much of adulthood, falling off slightly but nonsignificantly in old age. Interestingly, speed of responding in most tasks that would be used as a primary task in this paradigm—the go process—also improves throughout childhood, but peak performance in young adulthood is followed by significant slowing beginning in middle adulthood. Thus, there appears to be a difference between the developmental trajectory of the execution of primary tasks and that of the type of inhibitory control represented by stopping. This supports Logan's hypothesis that processes governing inhibition are separate from those governing execution of speeded primary processing. Other research, however, shows that in old age the probability of stopping successfully does deteriorate, even if stop-signal RT remains fairly constant on those trials in which the stopping process succeeds. This suggests that old age may involve a loss of concentration in which inhibitory control processes are less likely to be implemented appropriately, even though they may still work effectively once deployed. Although this conclusion is consistent with the available data from the stopping paradigm, there is more to the relationship between age and inhibition. As will be discussed later, a major theory of cognitive aging proposes that most inhibitory functions decline in old age.

In addition to developmental changes, the ability to inhibit one's actions in the stopping paradigm is related to some personal characteristics and individual differences. Stop-signal RT varies with impulsivity, being longer for more impulsive individuals. Hyperactive children have trouble stopping. Their stop-signal RT is longer than that of normal controls and they fail to inhibit responding on many occasions. This pattern is not due to a failure to detect the stop signal itselfbut rather to a deficit in the inhibitory mechanism that implements stopping. Moreover, stimulant medication (methylphenidate) that improves behavioral symptoms of hyperactive children also improves their stopping performance.

Event-related potentials suggest two loci at which the stop process exerts its impact—a central locus in frontal cortex that acts on motor planning and execution operations and a more peripheral process that acts on descending motor commands after they have left cortex. Evidence on the frontal locus comes from application of the stop-signal paradigm to examine gaze control in the FEF of monkeys by Jeffrey Schall and colleagues. Recording from single neurons in FEF showed that movement-related activity, which began to increase when a signal to move the eyes was presented, decreased when a signal to cancel the saccade was presented. The activity associated with this inhibition began to decrease before the stop-signal RT was over. It seems that the preparation of a movement is a controlled process composed of both execution and inhibition processes. FEF is involved in the generation of saccades, as mentioned earlier; in addition, neurons in the FEF can specify saccade cancellation—a specific and well-documented example of inhibitory function.

Understanding And Treating Autism

Understanding And Treating Autism

Whenever a doctor informs the parents that their child is suffering with Autism, the first & foremost question that is thrown over him is - How did it happen? How did my child get this disease? Well, there is no definite answer to what are the exact causes of Autism.

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