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1、 Spatial working memory and strategy formation in adults diagnosed with attention deficit hyperactivity disorder1. Introduction Working memory is defined as a cognitive system that provides temporary storage and manipulation of the information necessary for activities such as reasoning, language com
2、prehension and learning (Baddeley, 1992). This process requires the ability to hold information on-line during a brief time until a decision and subsequent response is made (Goldman-Rakic, 1990). It has been postulated that there is a dissociation between basic memorial requirements of tasks and the
3、ir executive components which control response selection, e.g. the adoption of an overall strategy or plan, or the utilization of specific attentional inhibitory mechanisms. Thus performance is determined by two factors, one related to short-term spatial memory and the other to strategic factors (Ro
4、bbins, 1996). Several studies have shown that spatial working memory deficits in patients with frontal lobe excisions may be related to the inefficient use of a particular search strategy (Miotto, Bullock, Polkey, & Morris, 1996; Owen, Downes, Sahakian, Polkey, & Robbins,1990; Owen, Morris, Sahakian
5、, Polkey, & Robbins, 1996). This suggests that part of the deficit may arise from executive failure and use of a strategy can reduce the load on memory caused by interference from previous choices. Spatial working memory deficits have been found in studies comparing ADHD and normal children (Karatek
6、in & Asarnow, 1998; Gorenstein, Mammato, & Sandy, 1989; Shue & Douglas,1992). However, most studies have not considered whether deficits in spatial working memory may be accounted for by problems with strategy formation. The importance of strategy has been considered in other neurological or psychia
7、tric disorders. For example, strategy formation impairment has been shown to be linked to spatial working memory impairment patients with frontal excisions (Miotto et al., 1996) but not for patients with Aspergers Syndrome (Morriset al., 1999). The current study explores spatial working memory defic
8、its and strategy impairment of ADHD adults using the computerised Executive Golf task (cf. Feigenbaum, Polkey, & Morris, 1996), a development of the spatial working memory task designed by Morris et al. (1988). Working memory is of relevance in the investigation of adult ADHD because it is thought t
9、o be particularly impaired in relation to other types of psychiatric disorders that have associated neuropsychological deficits (Gallagher & Blader, 2001). This has been explored in a previous study by Dowson et al. (2004) using a different task but a similar methodology. In the Dowson et al. (2004)
10、 study the CANTAB method of measuring spatial working memory involves an array of spatial locations on a screen in which the participant has to avoid going back to previously successful locations.They found impairments on return errors and associated poor strategy when conducting the tasks. The Exec
11、utive Golf task is more akin to a real life activity in that, as well measuring spatial working memory it simulates a game of golf, the participant searching for places in which a golferis predicted to putt a ball, avoiding returning to these places during subsequent searches. The golf holes are pre
12、sented using three dimensional computer graphics with the golfer presented in the distance. The task has a game-like format, with the golfer putting the balls into the golf holes when a correct golf hole has been selected by the golfer. Performance can be improved using the strategy of following a p
13、redetermined search sequence, beginning with a particular place and returning to the same place for every new search sequence. The aim of the study was to investigate whether patients with ADHD had a spatial working memory deficit in adulthood and to establish whether the deficit persisted if a meas
14、ure of strategy formation was covaried in the analysis. It was hypothesised that ADHD patients would make a greater number of errors in relation to returning to previously successful locations (i.e. between subject errors), and that this deficit would increase with task difficulty.2. Methodology2.1.
15、 Participants The study included 27 (21 male, 6 female) adult ADHD patients at the Maudsley Hospital,London UK. All patients met the DSM-IV criteria for ADHD following a full ADHD assessment (see Young & Toone, 2000). The assessment included a comprehensive psychiatric evaluation which included a se
16、mi-structured interview based on the DSM-IV criteria for ADHD. Each positive self-rating had to be endorsed by the assessor on the basis of supplementary questioning or other information (e.g.documentation). Additionally a parent was interviewed to establish whether there was a history of ADHD featu
17、res during early childhood (i.e. before the age of 7). This judgement was based upon an unstructured interview although in addition, and in order to obtain a final positive rating, a score of 15 or more was required on the Conners Global Index-Parent Scale CGI-P (Conners, 2000). For cases in which C
18、onners ratings were equivocal, a final positive rating was made if school reports and/or other childhood documentation provided evidence of hyperactivity and attentional difficulties. Of the 27 patients, 22 met criteria for ADHD,Combined Type; 3 for ADHD,Predominantly Inattentive Type and 2 for ADHD
19、, Predominantly Hyperactive-Impulsive Type. All participants were tested off-medication. They agreed to refrain from taking stimulant medication for 48 h prior to the assessment and confirmed this when seen. Exclusion criteria included a history of learning disability, a history of severe psychiatri
20、c disorder (this includes schizophrenia,bipolar and major depressive disorder), a primary diagnosis of substance abuse disorder,autistic disorders and/or a history of neuropsychological impairment or head injury. None of the participants in the sample fitted any of these exclusion criteria and hence
21、 none were exclude. Their mean age was 24.85 years (SD = 7.00, range 1642). Their mean predicted full scale IQ from the Intelligence Quotients (IQ) derived from the National Adult Reading Test Restandardised (NART-R) (Nelson, 1982; Nelson & Willison, 1991) was 101 (SD = 20) and their mean paternal s
22、ocial class score, using the HMSO classification of occupation, was 2.93 (SD = 2.32). Patients were compared with 27 healthy local community controls recruited through advertisements in GP Health Centres. Controls did not differ significantly from them on the following variables:sex (15 males, 12 fe
23、males: v2 = 2.08, df = 1, p = .15), age (mean = 25.78, SD = 4.89, Range1836: t(52) = .56, p = .58) predicted full scale IQ (mean = 105, SD = 16; t(52) = 1.69, p = .10)and paternal social class (mean = 2.59, SD = 1.97: t(52) = .57, p = .57).2.2. Tests and procedure The Executive Golf task is presente
24、d on a colour monitor with a touch sensitive screen. The participant is shown firstly an image of a golf course and a golfer. The course has a varying number of golf holes into which the golfer can putt a ball. Initially, the participant is instructed to guess into which hole the ball will be putted
25、 and respond by touching it. If correct,the computer emits a specific tone and the ball is shown going into the hole. For incorrect responses,a different tone is given, and the participant has to try again until the correct hole is found. Effectively they are instructed to search around the holes un
26、til they find the target one.After this, the target hole then moves to a new location. Only one ball can be putted into each hole in any one trial, such that, on subsequent trial searches the participant has to avoid choosing previously successful locations. The participant is specifically told to a
27、void going back to successful holes on subsequent searches. A series of searches are made until all the holes have had balls putted into them. After all the holes have had balls putted into them, the word finished is presentedat the top of the screen and the participant informed that a new trial is
28、about to begin with new balls and holes. The task contains five increasing levels of difficulty (i.e. levels 2, 3, 4, 6 and 8) represented by an increasing number of holes used in each trial. Each level of difficulty has a set number of trials as follows: Level 2 has two; Levels 3 and 4 have three;
29、Levels 6 and 8 have four trials; (Levels 5 and 7 were excluded from the design to keep the length of the test reasonable whilst maintaining the size range). The participant starts with the easier trials and proceeds to the more difficult ones. In order to prevent chance selections causing variabilit
30、y in the number of holes that a person needs to explore during each search, the number of such holes to be tried before finding the target one (discounting responses to previously used holes) is predetermined by the computer in a pseudo-random fashion (i.e. the same number of holes for each person).
31、 For each trial the computer records two types of errors. The first, Between Search Errors, involves selecting a hole into which a ball has already been putted during that trial. The second,Within Search Errors, involves selecting the same hole more than once in a single search while searching for t
32、he correct hole. In addition to these two types of errors, the computer records the sequence of touches made by the participant, which can later be analysed in terms of whether the participant has developed a specific Search Strategy.3. Results3.1. Within search errors The data are given in Table 1.
33、 For the analysis, difficulty levels two and three were excluded since these were considered practice trials, which enabled the participant to thoroughly learn the rules of the task. There were no significant main group effects or interaction. Since errors were minimal in both groups, no further ana
34、lyses were conducted.3.2. Between search errors The data are given in Fig. 1. An increase in the variance was observed as the mean values increased across the levels of difficulty. Therefore, in order to satisfy the assumptions of the ANOVA,the square root of the number of errors was used as the tra
35、nsformation procedure, producing approximate homogeneity of the variance across the cells of the group by level of difficulty variables.Repeated measures analyses were performed on the transformed data with Group (ADHD and normal controls) as a between-subject factor and Level of Difficulty (four, s
36、ix and eight holes)as a within-subject factor. To allow for the fact that when following Group Level of Difficulty interaction, this resulted in multiple tests, we reduced the significance level for these analyses to p 0.01. This is equivalent to a Bonferroni correction of halving the p = 0.05 value
37、 three times,this number corresponding to the number of comparison. The analysis indicated a significant main effect of group (F = 12.37, df = 1, 51, p = .001) suggesting the groups were significantly different on an average of levels. There was a Group Level of Difficulty interaction (F = 15.87, df
38、 = 2, 102, p .001) indicating that the degree of impairment increased differentially with difficulty. An independent samples t-test was performed on levels 4, 6 and 8 separately comparing the two groups in order to explore this interaction. The results presented in Fig. 1 show the patients were sign
39、ificantly impaired at Level 8 (t = 3.91, df = 51,p .001), with a trend at Level 6 (t = 2.47, df = 51, p .05) and no difference at Level 4(t = 1.0, df = 51, ns).3.3. Strategy formation Subsequent analyses were performed to identify if there were significant differences between the groups with respect
40、 to the use of strategy. As indicated in the introduction, an identified strategy is to follow a predetermined search sequence in which one begins each search at the same location (hole), skipping holes that have been putted into in a trial. To quantify the use of this strategy,the number of searche
41、s within a trial which begin with the same location were counted. A high score (many sequences beginning with different locations) indicates a low use of this strategy and a low score (many sequences beginning with the same location) represents a high use of this strategy. The data were collapsed ac
42、ross the levels to generate a total strategy score for each group;ADHD mean score 41.85 (SD = 4.61) and controls mean score 37.15 (SD = 7.57). An independent samples t-test comparing the two groups showed that the patients total strategy score was significantly higher than that obtained by the contr
43、ol group (t = 2.76, df = 51, p .01) suggesting they were more impaired in terms of their strategy formation ability.3.4. Effect of strategy on memory performance Covariate analyses with strategy using ANCOVA were performed in order to investigate if there is any change in the results of the Between
44、Search Errors analyses when the Total Strategy variable is taken into account. The results showed a significant Group effect (F = 5.81, df = 1, 51,p = -.02) and interaction (F = 8.64, df = 2, 102, p .001). Thus when taking into account group strategy, the difference in spatial memory performance bet
45、ween the ADHD patients and the normal controls remains.4. Discussion The study investigated whether ADHD adult patients were impaired on a test of spatial working memory, exploring the extent to which impairment could be accounted for by poor strategy formation. Spatial working memory was measured u
46、sing the computerised Executive Golf task,which is a development of the spatial working memory task designed by Morris et al. (1988).Two types of errors were recorded, a within search error in which a participant selects a spatial location that has been tried already during a specific search, and th
47、e between search error that involves going back to a location which has been successful on previous searches. The low number of within search errors for the ADHD group was comparable to the performance of the normal control group suggesting that the ability to keep track of locations within a search
48、 was well within their memory capacity. Thus they were not impaired on remembering simple sequences of spatial locations. Miotto et al. (1996) suggest that the memory system supporting this type of memory is likely to be the equivalent of the Visuo-Spatial Scratchpad (Baddeley & Lieberman, 1980; Log
49、ie & Marchietti, 1991), which is involved in retaining small amounts of visuo-spatial information in immediate memory. In contrast and as hypothesised, a substantial number of between search errors were recorded for the ADHD group compared with those obtained by the normal control group. Their greater impairment on this task became more evident with task difficulty as they made increasingly significant er