• inhibiting the prepotent response to imitate; and
• performing an action guided by the rule ‘do the opposite of what the experimenter is doing’.

The measure of executive function that this task provides is the number of errors in the conflict condition. The task is based on work by Luria and has been used with preschool children. Other variants of this task include Luria’s Knock/Tap game (in which the child must knock when the experimenter taps the table and vice-versa), the Opposite Worlds task (in which school-aged children are asked to say ‘one’ when they see a ‘2’ and to say ‘two’ when they see a ‘1’), and the Day/Night Stroop task (Gerstadtet al., 1994) in which children are instructed to say the word ‘day’ whenshown a line drawing of the moon and stars, and ‘night’ when
shown a line drawing of the sun.

You should be able to see how these tasks relate conceptually to the Stroop task, and how they are better suited to minds that are not yet at the stage of having overlearned (automated) the ability to read words.

Measuring the development of inhibitory control in infancy is particularly challenging. One method that is used is the structured observation of infant performance on certain tasks. Inferences are then made from these observations about inhibitory control. An example of this is the work of Diamond (2002). They observed infants of 8–11 months performing problem-solving tasks such as retrieving an object from an open box. They concluded that 9–10 month-old infants show evidence of inhibitory control, and that even in this narrow age band there was already evidence of older infants maintaining their attention on-task longer than the younger infants.

Most work on inhibitory control in childhood has focused on children who are 3 years of age and over. A range of tasks has been developed that is suitable for use with children of different ages. One of these is the Go/NoGo task (Drewe,1975). In one version of the task different letters are displayed, one after the other, on a computer screen. The child is instructed to press the space bar as quickly as they can whenever a letter is flashed onto the screen (Go), except when that letter is an ‘X’ (NoGo). Errors of commission, when a child presses the space bar mistakenly in response to the letter ‘X’, indicate a failure to inhibit. A variant of this task uses pictures of planes with a cartoon bomb as the ‘NoGo’ stimulus (Rubia et al., 2001).Note that in the ‘Simon says’ game, ‘do this’ (without the preceding ‘Simon says’) is equivalent to a NoGo stimulus.

Using tests such as these researchers have found that there are significant improvements in task performance between the ages of 3 and 6 years. In a study by Mahone et al. (2001), 87 typically developing children completed a computerized Go/NoGo task. Even though the 3-year-old children managed the task with few omission or commission errors, the researchers noted a developmental trend across their sample: increasing age was associated with steady and significant improvements in performance.More complex inhibitory control functions are tapped by non-verbal Stroop tasks (for example, Luria’s Day/Night, Handgame, Knock/Tap). These require children not only to inhibit a response (as in the Go/NoGo task), but also to execute a rule-guided action. The majority of 3 year olds fail the Day/Night task (Gerstadt et al., 1994), the Handgame (Hughes, 1996; Hughes, 1998a and b) and the Knock/Tap game (Perner and Lang, 2002). However, the majority of 4 year olds pass these tasks. Thus significant improvements in both simple and complex inhibitory control are evident in the pre-school years.

Developmental improvements in inhibitory control also continue throughout
childhood, as demonstrated by findings from studies with school-aged children using the Go/NoGo tasks (for example, Manly et al., 2001). Interestingly, findings from a brain imaging study by Casey et al. (1997) that used functional magnetic resonance imaging suggest that children and adultsshow similar patterns of brain activation during the Go/NoGo task, and that this activation is in the prefrontal cortex (see Research summary below).