ADHD and Low Working Memory: A comparison
I was recently directed to a paper titled ‘Children with low working memory and children with ADHD: same or different?’ From the title alone this paper piqued my curiosity. As scientists we often focus on collecting large amounts of data, and in some cases forget the importance of forming new theories and ideas. For me, this paper casts the profiles of an ADHD diagnosis and a deficit in working memory, a common childhood issue, in a new light. What does it mean (if anything) to have a clinical diagnosis? And what is the best way of characterising children with problems of attention and memory?
Whilst I will summarise the main findings below, for those that want to read the paper for themselves it can be accessed here:
This paper provides a novel comparison of cognitive skills, executive function, educational attainment and behaviour between those with a low WM and those with a diagnosis of ADHD. It utilised three groups of children (8-11yrs): one group identified by routine screening as having a low WM, one group with diagnoses of ADHD and one group of typically developing children.
Age was controlled for across the groups and for each group the following measures were taken: Working memory (Automated working memory assessment); Executive function (Delis-Kaplan Executive Function System, The K-test of the Continuous Performance Test and Walk/Don’t Walk from the Test of Everyday Attention for Children); Academic ability (Wechsler reading, spelling, reading comprehension, mathematical reasoning and number operations and the four WASI subtests); And classroom behaviour problems (Connors teacher rating and the Behaviour rating inventory of executive function). Those children taking medication for their ADHD stopped taking it at least 24hrs prior to the session to ensure the effects of the drugs were eliminated by the time of testing.
The theory that makes up the basis for the research, includes the fact that both ADHD and low WM in childhood are significantly associated with poor educational attainment. It has also been established in the literature that there are associations between poor WM and inattentive behaviour and poor WM and executive function deficits. These elements combined suggest a high degree of overlap between the profiles of those with a deficit in WM and those with an ADHD diagnosis. In ADHD, we see executive function problems (deficits in response inhibition, attentional switching, planning and sustained attention) and also excessively high levels of motor activity and impulsive behaviour.
Some have argued that the executive function deficits in ADHD are a result of an underlying low WM, whilst others have proposed that ADHD involves two distinct neurodevelopmental systems:
1) a ‘cool/cognitive system’ that affects executive functions and WM
2) a ‘hot/affective system’ that leads to ‘aversion to delay that manifests as impulsive behaviour’.
It is this impulsive behaviour that manifests as excessive motor activity and problems in impulse control that are core characteristics of ADHD and are not associated with low WM. Thus, this study hypothesised that the two groups share a common deficit in the ‘cool executive function system’ reflected in the fact that both profiles exhibit lack of attentional control, but only those with the ADHD diagnosis have impairments in the ‘hot executive function system’ that results in hyperactivity and impulsivity.
The ADHD and low WM group, as expected, performed more poorly than the typically developing children on the tests of working memory (necessarily so for the latter group, because they were selected on this basis). Performance was significantly decreased for the low WM group in the two WM subtests that they were initially screened on. This suggests a selection artefact as ‘the two groups did not differ on the verbal WM task that was not used at screening’. In the executive function switching task no significant difference was seen between the low WM and ADHD group when considering accuracy, however, the low WM group showed a significantly slower performance than the ADHD and typical group. The sustained attention executive function task showed that both the ADHD and low WM children were significantly less accurate than the typically developing group and that they both made a significantly greater number of omissions than the typically developing group. The ADHD group also further made a significantly greater number of commission errors than the other two groups. No other significant differences were seen between the low WM group and the ADHD group on any other test of executive function. It is also worth noting that although there are a number of different models proposed for the functional structure of WM, the findings of this paper remain consistent across models on the basis that the ‘storage-only capacities of short-term memory (STM) and the capacity-limited attentional control functions of WM can be distinguished’.
They concluded that the ADHD and low WM groups have very similar profiles of WM ability and executive function impairment. They did however find two important differences: the low WM group were ‘slower to respond on several tasks’ and the ADHD group were ‘more hyperactive and exhibited more difficulties in controlling impulsivity in sustained attention’. The processing speed impairments in the low WM group were an unexpected finding. They state that ‘it does not appear to be a part of a broader problem in fluid intelligence, as controlling statistically for performance IQ had little impact’. They present the idea that the impairment in response time may be a result of sluggish cognitive tempo (SCT). Sluggish cognitive tempo is a ‘set of symptoms strongly associated with the predominantly inattentive form of ADHD that includes high levels of daydreaming, slow response times, poor mental alertness and hypoactivity.’ It could perhaps be the case that those with a low WM may correspond to those with SCT, a predominantly inattentive subtype of ADHD. At present SCT is not a commonly utilised diagnosis in the UK. The more hyperactive behaviour in the ADHD children however was expected. ‘They violated rules more frequently during a planning task, and made more commission errors on the Continuous Performance Test of sustained attention’. This elevated hyperactivity was also shown in the Connor’s teacher rating as increased impulsivity and oppositional behaviour.
Interestingly they found the two groups to have equivalently high levels of inattentive behaviour. Whilst it has been shown before that children with low WM often exhibit inattentiveness, this paper is the first to show that this inattentive behaviour is of a very similar degree to that of children with ADHD. In terms of the deficits in working memory, both groups exhibited deficits in the same regions and of a similar magnitude. They had impairment in ‘visuospatial short term memory, verbal working memory and visuo-spatial working memory’; but ‘their verbal short term memory scores fell within the typical age range’. The novel finding here is that there is such a high degree of correspondence between the profile and severity of the WM impairments in the two groups.
Previous research has also suggested a close relationship between ‘controlled cognitive attention and working memory’. This paper takes this understanding one step further suggesting that it is now looking more likely that there is a ‘link between poor working memory and overt inattentive and distractible behaviours’ as seen through the inattentiveness in the behaviour of those children with a low WM. In addition to being comparable in inattentiveness and working memory impairment, the two groups both exhibited ‘high rates of problem behaviours across a wide range of executive function behaviours’. Previous studies have also indicated that problem behaviours associated with executive function are seen in children with a low WM. This paper supports and extends this idea by showing that both groups ‘performed poorly on direct measures of switching, inhibition, sorting, planning, sustained attention and response suppression’.
When considering again, the cool (cognitively based) and the hot (affective) model of executive function, the pattern found in the results from this study suggest that there is some shared general executive deficit between the ADHD and low WM groups. The cool deficits manifest as inattention and low WM in both groups whilst the hot deficits, only present in the ADHD group, manifests as the hyperactivity. The additional delay in response times in the low WM group is the only component that does not fit with this theory, but the authors have identified that it ‘may be symptomatic of a subgroup of children with the predominantly inattentive form of ADHD who are characterised by SCT’.
In conclusion, this paper shows some very interesting preliminary findings into two conditions that are generally considered distinct. Both groups showed equal levels of underachievement when IQ differences were taken into account (‘they are indistinguishable in terms of their poor learning progress in mathematics and reading’). This is of great significance as those with a diagnosis of ADHD will almost certainly have learning support provided to them whilst those with a low WM will not. The educational needs of the group of children with a low WM are being overlooked as they do not show the disruptive hyperactivity of ADHD, even though in many respects the groups are indistinguishable.
Holmes J, Hilton KA, Place M, Alloway TP, Elliott JG and Gathercole SE (2014) Children with low working memory and children with ADHD: same or different? Front. Hum. Neurosci. 8:976. doi: 10.3389/fnhum.2014.00976