Inhibitory control

Inhibitory control is the ability to focus on relevant cues by suppressing irrelevant information or stimuli while performing any day-to-day task which involves attentional control (Miyake, Friedman, Emerson, Witzki, Howerter & Wager, 2000). Hasher, Zacks, and May (1999) state that inhibitory control has three main functions: a) the controlled inhibition function, which allows the access of relevant information to working memory, preventing the entrance of irrelevant information; b) the delete function of inhibition, which deletes or suppresses inappropriate information from working memory, and c) restrain strong responses, which is the inhibitory function that allows the evaluation of the appropriateness of responses before responses are provided.

More recent work carried out by Butler and Zacks (2006), who investigated eye movement control of 32 younger adults and 32 older adults, reported that the three functions mentioned above diminish with age. According to these researchers, eye movement tasks, such as the antisaccade task, in which the ability of overcoming a strong response is measured, involve executive control processes. In the antisaccade task, subjects have to avoid looking at the location where the cue appears;

instead, subjects have to move their eyes to the opposite direction to where the stimulus is presented. Butler and Zacks (2006) reported that older adults performed slowlier than younger adults due to the reduced ability of inhibiting misleading cues. Such finding suggests that executive processing deficits are associated with advanced age and can be considered as a support for the inhibitory deficit hypotheses of aging (Hasher et al., 1999; Zacks, Hasher & Li, 2000). This hypothesis posits that, with normal aging, the ability to ignore and delete irrelevant information in working memory declines, and the ability to retain and control strong responses is reduced.

As already said, inhibition is one of the executive functions which deteriorates with normal aging (Nielson, Langenecker &

Garavan, 2002). Consequently, age-related deficits in inhibitory control functions diminish the ability of ignoring inappropriate items (McDowd

& Shaw, 2000). As the control over attention declines, it is assumed that irrelevant information enters working memory, which impairs its efficiency (Zacks, Hasher & Li, 2000; Salthouse & Meinz, 1995; Alain

& Woods, 1999; Zellner & Bäuml, 2006). Likewise, Hasher, Lustig, and Zacks (2008) state that as the ability to maintain attention focused on relevant information diminishes across the lifespan, the performance on

tasks that require speed of processing and working memory are also influenced.

A classical task that has been used in age-related declines in inhibitory processes is the Stroop Color task (Kane & Engle, 2003). In the Stroop Color task, participants have to read the word printed irrespective of the ink color it is presented. If the word displayed and the color of the ink match - for example, „blue‟ is written in „blue‟ ink - this is a congruent trial. In contrast, if the word printed and the color of the ink displayed do not match, this is an incongruent item - for example,

„blue‟ is written in „red‟ ink - the conflict between the relevant information and the information to be ignored has to be solved.

Researchers (West & Alain, 2000; Langenecker, Nielson & Rao, 2004;

Spieler, Balota & Faust, 1996) who have used the Stroop task in order to investigate inhibitory control decline across the lifespan have observed an impairment of inhibitory processing with aging. In Langenecker et al.

(2004), for instance, age-related differences were observed in the frontal cortex. These researchers compared the performance of 13 younger and 13 older adults and used functional magnetic resonance imaging (fMRI) with a Stroop task. The study showed that both younger and older adults had greater activation of the prefrontal area of brain while performing the Stroop task. However, older adults activated more areas in the frontal cortex than younger adults in order to accomplish the task. The recruitment of multiple areas of the frontal cortex was interpreted as a reduction of inhibitory functioning efficiency during aging.

Another task used in the investigation of the relationship between inhibitory control functions and cognitive aging is the Simon task, which was the task used in the present study. This task, like the Stroop task, involves congruent and incongruent trials besides requiring controlled inhibition; however, instead of words, the Simon task may incongruent stimuli (the response key and the position of the stimuli do

4The Simon task 2 Colors and the Simon Arrow task are fully detailed in chapter III, section 3.4.4)

not correspond) and congruent stimuli (the response key and stimuli are on the same side) was smaller for younger adults than for older adults.

Responses to congruent trials are usually faster and more accurate than to incongruent trials in which the stimuli and response locations do not match. Older adults are more disrupted from the incongruent trials than younger adults due to the decline in the efficiency of inhibitory processing, increasing the Simon effect. Furthermore, the researchers argue that motor processes in visual tasks, which are controlled by an inhibitory process, change with age. Such change in motor activation affects the ability to react fast to any stimulus presented.

Having presented some of the most used inhibitory control tasks – the Stroop task and the Simon task - in the field of selective attention (Bialystok, 2006), some methodological issues regarding the design of tasks which assess inhibitory control functions will now be considered.

As observed by Bialystok, Martin, and Viswanathan (2005a) tasks should be developed or selected according to the population being investigated. According to these researchers, in order to design a task that assesses inhibitory control functions, the task must involve a conflict to be solved. In addition to that, when a study involves a wide range of ages or different language groups, finding a suitable inhibitory task that can be performed by all participants is essential. Considering these two aspects, the Simon task was selected to be used in the present study.

Furthermore, according to Bialystok et al. (2005a), the Simon task is appropriate to all ages because it is content-free, that is, it does not involve linguistic material. Furthermore, like the Stroop task, the Simon task offers a conflict: participants have to press the button which corresponds to the color presented on the screen. Half of the trials are incongruent, that is, they appear on the opposite side of the corresponding button. As already mentioned, such conflict is expected to generate slower responses to incongruent stimuli compared to congruent ones. The difference of response time in reacting to congruent and incongruent stimuli is called the „Simon effect‟, which measures the efficiency of inhibitory control. A reduced Simon effect reflects inhibitory function efficiency, that is, the smaller the difference between incongruent and congruent items, the more efficient inhibitory processes are (Bialystok et al., 2004, Bialystok et al., 2007).

Considering age-related differences, older adults, compared to younger adults, perform poorly on tasks or situations that require inhibition. Park (2000) explains that as we age, we have much more trouble concentrating on only one item. That is, we easily fail in

inhibiting distractions or conflicts. The difficulty in maintaining attention on a goal and quickly inhibiting competing stimuli affects many everyday activities.

In the present study, as already mentioned, the Simon task (the Simon task 2 Colors and the Simon Arrow task) was the measure used to investigate inhibitory control function. The Simon task 2 Colors was applied to a population of early and late bilinguals with ages ranging from 18 to 84, while the Simon Arrow task was administered only to late bilinguals. The Simon Arrow task was included in this research as another measure of inhibitory control, in addition to the Simon task 2 Colors, in order to verify whether the two versions of the Simon task assess inhibitory control in a similar way.