Executive function and Inhibitory Control

2.3.4.1. Response Initiation

A process that is associated with inhibitory control is the ability to initiate appropriate responses (response initiation) (Burgess & Shallice, 1996a). Studies examining this ability have highlighted how patients with frontal lobe lesions have shown poorer performance on tests of verbal fluency that examine response initiation when compared to patients with lesions to other areas of the brain (Burgess & Shallice, 1996a; Miller, 1984). The Verbal Fluency task is considered to be a classic test of executive

functioning because of its reliability and good discriminatory power (Phillips, 1997) (see section 5.4.3.1.). It is thought to involve processes of selecting, generating or initiating stimuli from a larger set (animal names or words beginning with a particular letter). Both the maintenance and monitoring of word production, and the generation and control of strategies in retrieving items from longer term memory, are also thought to be involved (Fletcher & Henson, 2001).

2.3.4.2. Response Inhibition

Various studies have focused on inhibitory control, a process involving the purposeful and controlled inhibition or suppression of over learned, dominant, automatic or prepotent (previous) responses that allows for more goal directed behaviour (Miyake et al., 2000). Inhibition has been defined by Simpson and Schmitter-Edgecombe (2000) as ‘the process by which the irrelevant stimulus is ignored’ (p. 310).

There are several neuropsychological tests commonly used in neuropsychological studies to examine the ability to both initiate and suppress responses. Traditionally Go/No-Go tasks (Drewe, 1975) have been employed to examine inhibitory control and response selection. Smith and Jonides (1999) have described use of the Stroop task (Stroop, 1935) to consider the relationship between inhibition and attention. This task consists of names of colours which are presented in different coloured print in relation to two conditions. In the first condition colour names are presented in the same colour print. The second condition involves colour names presented in different colour print. Performance in the latter condition is usually poorer due to competition arising between two conflicting executive processes. Attention is automatically focused on naming the word that is in conflict with the process of naming the colour of the print (the weaker process of the two). Attention and inhibition processes are subsequently brought into play and attention is diverted to processes relevant to the task (that is, naming the print colour and to deliberately inhibiting the irrelevant but automatic and more dominant process of naming the colour word) to ensure that the task performance is successful (Smith & Jonides, 1999). See section 2.2 for discussion of the relationship between response inhibition and the frontal lobes.

There appears to be speculation as to the relationship between response suppression and initiation (Ferret, 1974), One well-established neuropsychological measure, the Hayling test, developed by Burgess and Shallice (1996a; 1996c), has been used in clinical and research capacities to examine the ability to both initiate and to inhibit verbal material (see section 5.4.3.2.). The advantage of this test, unlike those employed in earlier studies, has been the possibility of examining both of these executive processes within the same task and under the same demands and conditions, thus reducing the potential

effect of background factors that may interfere with the findings.

2.3.4.3. The Hayling Test

The Hayling test involves two sections, both of which involve the presentation of 15 sentences in which the last word is missing. The individual is required to complete each sentence with a missing word; the missing word is expected to be strongly cued by the sentence itself with a ‘high probability of a particular response occurring’ (p. 264). Section A involves completing the sentence as quickly as possible with a word that best completes the sentence, thus examining the ability to initiate automatic responses. Section B, however, requires the individual to complete the sentence with a word that does not make sense in the context of the sentence, for example, by using a nonsensical word. This condition examines the ability of the individual to initially suppress an automatic, strongly cued, well-learned response prior to generating a novel word that makes no sense in the context of the sentence. Responses in section B are categorised according to whether they were unrelated to the sentence (as required by the

instructions), were connected to the sentence (a sensible completion) thus breaking the test instructions, or were semantically related to the sentence in some way. Response latencies are also examined to provide a measure of response initiation speed (sum of the response latencies) for each section. In particular, the difference between section B latencies minus section A latencies is calculated. This is thought to represent the thinking time required to generate an inappropriate, novel response compared to a word that best fits the context of the sentence as in section A.

Burgess and Shallice (1996a) examined the relationship of semantic or verbal inhibitory processes with the frontal cortex in an attempt to evaluate the functioning of the SAS

(see section 2.3.1.2.). In comparing patients with damage to the frontal lobe (the 'anterior' group) with the patient control group (posterior group with no frontal

involvement) that were matched for age and IQ, the anterior group were more impaired in their ability to initiate verbal responses and to suppress habitual responses as expected (see section 2.2). The anterior group appeared to have used their ’thinking time’ less effectively by taking significantly longer in both sections than the patient control group. The importance of considering both errors rates and response latencies in understanding the nature of spared abilities in tasks of inhibition has been supported by Christ et al. (2003).

To date there appears to be only one (unpublished) study that has applied the Hayling test to an MS group (Nathaniel-James, 2000). The study found MS participants to perform significantly worse on Part B of the Hayling test in term of response

suppression (number of errors) and efficiency in suppressing inappropriate responses (time latency). No differences were observed between the groups in terms of response initiation (part A).

Two perspectives will now be outlined in providing a theoretical underpinning to an understanding of response initiation and suppression.

2.3.5. A Cognitive Perspective to the understanding of initiation and inhibitory