Experimental Measures

2.3.1 Wechsler Intelligence Scale for Children - Revised (WISC-R)

The WISC-R was employed to assess verbal intelligence. This is the most available and reliable measure of overall general intelligence (Wechsler, 1974) and provided an

opportunity to compare directly with previous studies using this scale.

Scores on the Verbal Scale have been shown to highly correlate with the Full Scale scores and was considered the best estimate of overall functioning (Wechsler, 1974) . The Verbal Scale consists of five main subtests including

Information, Similarities, Vocabulary, Comprehension and Arithmetic. These subtests are all administed verbally except for the first four items on the Arithmetic test. Since the Arithmetic task is the least predictive of Verbal and Full Scale intelligence (Weschsler, 1974), it was

omitted from the battery to reduce administration time. Administration and scoring of these tasks were in

2.3.2 A t t e n t i o n Deficit Disorder Rating Scale

The ADHD rating scale was d e v e l o p e d by DuPaul (1990)

b a s e d on the D S M -III-R diagnostic crit e r i a for ADHD. The

orig i n a l scale of 14 items was used to determine the p r e s e n c e and severity of A D H D symptoms w i t h age and sex a p p r opriate cut-offs for both parent and teacher ratings. The scale yields separate scores on i n a t tention and

hyperac t i v e / impulsivity factors to p r o v i d e D SM-IV

d i a g n o s t i c sub-types of A D H D groups including p r e d o m i n a n t l y inattentive, pr e d o m i n a n t l y hyperactive or combined types.

The scale has been used reliably to discrim i n a t e ADD with h y p e r a c t i v i t y and without h y p e r a c t i v i t y in a d d i t i o n to d i f f e r e n t i a t i n g them from LD children and normal controls

(Barkley, 1990).

To meet the DSM-IV criteria, four extra items were added (items 15, 16,17 and 18) to the scale to d e t e rmine the pres e n c e and severity of A DHD symptoms a d d ressed in c r i t e r i o n Al(f), Al(i), A2 (e) and c r i t e r i o n C (see Appe n d i x A) .

2.3.3 Chi l d Behaviour Checklist for Ages 4-18

The Child Behaviour Check-list (CBCL, A c h e n b a c k & Edelbrock, 1983; Achenbach, 1991) is a 113 item multi a x i a l q u e s t ionnaire yielding an 8 factor p r o f i l e and includes

Hyperactivity, Delinquent and A g g r e s s i o n which has b een

found to correlate highly with p s y c h i a t r i c diagnosis of A D H D and CD for six to 12 year olds (Edelbrock & Costello,

CBCL behaviour profiles on 60 ADHD children display elevated scores of the externalised factors (Hyperactivity, Aggression and Delinquency) of the scale (Edelbrock &

Achenbach, 1980). The high scores on the Aggressive and Delinquency factor is expected due to the oppositional behaviour generally reported with ADHD and the high level of co-morbidity of ADHD with conduct problems (Barkley, 1990) .

Interestingly, the CBCL behaviour profile also include elevated internalised scores on the obsessive and

compulsive factor for ADHD children. Barkley (1990) argues that rather than reflecting the level of neuroticism in this clinical group the latter factor is more indicative of inattentive and hyperactive behaviours.

The CBCL has been found to discriminate effectively between children with and without a range of clinical disorders including ADD, Depression and Anxiety (Barkley, 1990). Current reliability of ADHD behaviour profiles have been established with other measures of ADHD including the Conner's Behaviour Checklist (Barkley et al., 1990). The test-retest reliability of the scales for behavioural problems have been reported at .95 (one week) and .59 for the stability of mother's rating after 6 months. Inter­ rater agreement of child's behaviour between mothers and fathers was .98 for clinic referred children. Edelbrock and Achenbach (1980) report that there is a 95% agreement between clinicians and mother's rating of externalised behaviours in the 6 to 11 year old age range.

2.3.4 Vigilance Tasks

The vigilance tasks employed in this study were based on the Seidel Continuous Attention Test (SCAT; Seidel & Joschko, 1990) . This computerised task presents a series of numbers from one to nine for 200 msec at a rate of 1.5 sec per stimuli. Each task consists of 800 stimulus

presentations over a period of 20 min (40 events per minute). The target to non-target ratio is 6:40 (15%

target probability) and targets were randomised across sets of 50 stimuli.

The target in the simple simultaneous task was '9' which was similar to that employed by Gordon (1986). In the complex successive vigilance task the target was any two repeated digits for example '5-5' or '2-2' . This

variation on the SCAT reduced the priming effect of targets inherent in the A-X paradigm while maintaining similar

information processing demands and memory load of the task.