Impulsive Response Initiation

2.7.2.1 Measurement of Impulsive Response Initiation

Impulsive response initiation has commonly been measured using a Continuous Performance Test (CPT) paradigm (Beck, Bransome, Mirsky, Rosvold, & Sarason, 1956), whereby participants make selective responses to a target stimuli (e.g., the single letter A; single-stimuli task), or a stimulus which is preceded by another stimulus (e.g., A followed by B; paired-stimuli sequence task); incorrect responses to stimuli other than the target, otherwise known as commission errors, are thought

16_{Behavioural impulsivity evident at the first stage of processing is also cognitive-}

based. This type of impulsivity is known as ‘reflection impulsivity’, whereby individuals have a tendency to make decisions under conditions of uncertainty, before they have obtained all the necessary information (Clark, Robbins, Ersche, & Sahakian, 2006; Kagan, 1965). This doctoral research will focus only on the aspects of behavioural impulsivity occurring in the second and third stage of processing.

Chapter 2: Literature Review

145

to index impulsive response initiation. The Immediate Memory/Delayed Memory Task (IMT/DMT; Dougherty, Marsh, & Mathias, 2002) was developed as an

extension of the CPT paradigm due to ceiling effects from insufficient task difficulty (Cornblatt & Keilp, 1994). In the IMT/DMT, participants respond selectively to a 5- digit target stimulus when it matches the preceding stimulus (e.g., ‘34589’ followed by ‘34589’). The IMT/DMT offers a more complex assessment than traditional CPT tasks, including two types of non-targets (‘catch’ stimuli which differ from the target by one digit and ‘non-target’ stimuli which are random) and an additional level of task difficulty by introducing a delayed memory component. In the delayed memory component, distractor stimuli are repeatedly presented in the interval between paired stimuli. The IMT/DMT also adopts a more restrictive measure of impulsive

inattention, focusing only on commission errors to similar non-targets, as these errors are theorised to result from an inability to withhold a response until stimulus processing is completed. This index of impulsivity has been validated in past

research with high trait impulsivity samples, including those diagnosed with alcohol dependence (Bjork, Hommer, Grant, & Danube, 2004), bipolar disorder (Swann, Anderson, Dougherty, & Moeller, 2001) and disruptive behaviour disorders (Dougherty et al., 2003; Dougherty, Bjork, Marsh, & Moeller, 2000a).

2.7.2.2 Alcohol and Impulsive Response Initiation

Only four studies have been conducted to date looking at the effects of alcohol on IMT/DMT performance (Dougherty et al., 2008; Dougherty, Marsh, Moeller, Chokshi, & Rosen, 2000b; Dougherty et al., 1999; S. C. Reed et al., 2012). These studies revealed a dose-dependent effect of alcohol on IMT performance, with no statistically significant change in impulsive response initiation detected when low

Chapter 2: Literature Review

146

alcohol doses were administered (mean peak BrAC .011% to .056%) but increased impulsive responding when high alcohol doses (mean peak BrAC .063% to .092%) were administered (Dougherty et al., 2008; Dougherty et al., 2000b; Dougherty et al., 1999; S. C. Reed et al., 2012). Closer examination of these studies also showed that detection of alcohol-induced increases in impulsive response initiation was

dependent on the specific index adopted. The ratio of commission errors to correct detections was identified as the most sensitive outcome indicative of impulsive response initiation (relative to the frequency of commission errors), accounting for systematic individual variation in discriminability (Dougherty et al., 2000b).

The results are more mixed when looking at impulsive response initiation under greater task difficulty, as indexed by the DMT. Dougherty et al. (1999) reported that ingestion of a low alcohol dose (peak mean BrAC .035%) increased commission error rate, and S. C. Reed et al. (2012) found that a moderate (mean peak BrAC .056%) and high (mean peak BrAC .092%) alcohol dose increased the ratio of commission errors to correct detections, relative to placebo administration. In contrast, Dougherty et al. (2000b) reported no effect of a low (mean peak BrAC .039%) or high (mean peak BrAC .091%) alcohol dose on commission error rate or ratio of commission errors to correct detections. The researchers theorise that the DMT may be less robust in detecting the effects of alcohol than the IMT, as it has fewer trials per block, increasing the variability of outcomes.

2.7.2.3 ED, Alcohol, and Impulsive Response Initiation

Overall, these studies suggest that alcohol administration increases impulsive response initiation in a dose-dependent manner, particularly when task difficulty is

Chapter 2: Literature Review

147

lower, with impairment typically evident following moderate doses (BrAC ≥ .063%). However, there have been no studies assessing whether ED co-ingestion increases, attenuates, or maintains alcohol-induced impairment of impulsive response initiation. In fact, there is a lack of research in this field investigating the effect of caffeine or ED in general, regardless of alcohol co-ingestion. Those few studies which have assessed impulsive response initiation following caffeine or ED administration have typically revealed no statistically significant effect of either substance. For example, Bernstein et al. (1994 ) found that children’s commission error rates on a single- stimuli CPT were not impacted by administration of caffeine (2.5mg/kg and

5.0mg/kg). Similarly, administration of 200mg caffeine or a standard 250ml ED has been shown to have no statistically significant impact on single-stimuli CPT

commission error rates relative to placebo administration (Gendle et al., 2009). A commonality across these studies is the use of a single-stimuli CPT paradigm, meaning that the absence of treatment effects may be a consequence of low task sensitivity. Thus, it cannot be inferred from this research whether ED consumption does have an effect on impulsive response initiation, independently or in

combination with alcohol.