Processing speed and intelligence are known to be strongly correlated with memory (Conway et al., 2002; Conway et al., 2003; Salthouse, 1996), and so it was thought prudent to investigate these factors as possible moderators. It was predicted there would be some main effects for processing speed and IQ in this sample, but this was not a focus of the research as it has been thoroughly covered in the above studies. Despite the moderating impact of processing speed and IQ on memory (working memory in particular), being well established, very few studies investigating depression and memory have taken these moderators into account. Given the literature on the relationships between depression, memory, and these potential moderators is sparse, no predictions were made regarding their effects on memory in the current study.
Processing Speed
There was a Depression x Age x Visual Processing Speed interaction for the recall task. The majority of this interaction was located in the Depression x Visual Processing Speed interaction for the older adults. In this small
interaction, the older adults in the low depression group maintained a reasonably stable level of performance on the recall task. On the other hand, the older adults in the high depression high visual processing speed group dramatically outperformed those in the high depression and low visual processing speed group (refer to Figure 1).
Comparisons of these results with previous research is not possible, as no studies were found that investigated relationships between STM, depression, and processing speed. Ilsley et al. (1995) measured psychomotor speed along with memory in a sample of adults with major depression. Psychomotor speed and reaction time are often used as measures of processing speed (Salthouse, 2000). While Ilsley et al. found a significant difference between the depressed and non-depressed groups on the measure of psychomotor speed, they did not find any significant differences between the two groups on STM performance. A similar study also found no significant difference between depressed and control participants on measures of STM (Austin et al., 1999). They measured reaction time and psychomotor speed, and found that reaction time (but not psychomotor speed) was significantly affected by depression. Because neither of these studies found that STM was affected, they did not perform any further analyses to look at the relationships between STM, depression, and processing speed in their samples. Also, the above studies used adult (as opposed to older adult) samples. The majority of the Depression x Age x Visual Processing Speed interaction was located in the older adults. There is no literature that helps to explain why the combination of high visual processing speed and high depression had such a positive effect on the recall task in the older adults. It is fascinating that the two high depression and high visual processing speed groups have the highest level of recall. It is interesting to speculate that high visual processing speed may be protective in any recall difficulties associated with late-life depression. This is especially so, given the opposite trend of low visual processing speed being more beneficial in the reading span task of working memory. More research is needed to focus on the factors involved in
the better identified short-term memory performance, especially with regard to older adults and the influence of visual processing speed.
The reading span task was the only working memory measure that was affected by a Depression x Visual Processing Speed interaction. Regardless of age, performance on the reading span task was better in the high visual processing speed groups for both low depression and low stress, as compared to the low visual processing speed groups. However, the reverse was seen for the high depression and high stress groups, with performance in the high visual processing speed group being worse than that in the low visual processing speed group. Interestingly, the higher visual processing speed appears to be disadvantageous for those in the high depression and high stress groups. It is difficult to explain these small effects due partly to the lack of previous research on processing speed and memory in combination with depression. Nebes et al. (2000) found that processing resources (processing speed and working memory) mediated depression-related change on other cognitive tasks for older adults. They also found processing speed contributed to the considerable amount of variance in the depression and working memory relationship, but noted a significant amount continued to be explained by working memory once processing speed was controlled for. Under this relationship, processing speed and working memory both deteriorate with depression. Such a linear explanation does not support the results found in the present study in which poorer performance in working memory was found in those with a higher level of visual processing speed in the high depression and high stress groups. In contrast to Visual Processing Speed, Verbal Processing Speed had little effect, an unexpected outcome. All of the tasks had a verbal component, and verbal working memory is supposed to be maintained in the phonological loop. Thus, it was expected that Verbal Processing Speed would at least have some effect on the memory scores. However, there were no main effects for Verbal Processing Speed, and no interactions with depression.
Intelligence
In the IQ analysis, there were two noteworthy interactions between Depression and IQ. Both of these interactions were with memory types that were anticipated to be subject to the greatest impairment in depression, working memory and time-based prospective memory. Not all of the findings lie in the predicted directions, however. Firstly, there was a small Depression x IQ interaction for the working memory task of reading span. Overall, the scores across all groups improved with higher IQ. The greatest difference in performance was in the high depression group for both ages. Those in the high depression and high IQ groups outperformed those in the low depression high IQ groups (as well as all low IQ groups) when compared with their own age groups.
As with the Visual Processing Speed results, a couple of studies have compared depressed and non-depressed samples on aspects of intelligence, but have not taken the next logical step of investigating IQ as a moderator of working memory ability in depression. Christopher and MacDonald (2005) measured verbal IQ through the use of the Vocabulary subtest of the Revised Wechsler Adult Intelligence Scale (WAIS-R) and found no significant difference in scores between their depressed and non-depressed participants. They did find differences between the groups in working memory, but they did not investigate the effect IQ may have had on the working memory differences. Rose and Ebmeier (2006) used the n-back task to research depression-related working memory impairment. They used the National Adult Reading Test (NART) to estimate IQ. They found that, while IQ was a mediator for working memory across both the depressed and non-depressed groups, it did not contribute to the depression-related working memory impairment observed in their sample. These results are difficult to interpret given that Rose and Ebmeier should not have used ANCOVA to analyse their results, because of the inability to randomly assign depression or IQ. There is little in previous research to account for the combination of high IQ and high depression having better
working memory performance found in the present study. Even if this is related to a higher IQ being associated with greater processing and capacity resources that are better able to manage the cognitive symptoms of depression and still attend to memory tasks, this still does not explain the superior performance seen in those with the higher depression scores.
Secondly, there was a medium-sized interaction between Depression and IQ for the time-based prospective memory task. The pattern revealed in this analysis is that for time-based prospective memory, those in the lower IQ groups, irrespective of age, exhibited a decrease in task performance in the high depression group. Those in the high IQ groups showed a slight increase in performance with higher depression. The result is consistent with attentional resource and inhibitory control models of depression-related decline as intelligence relies on attention and cognitive capacity. Those with lower IQs are assumed to have fewer processing resources to rely on. These resources may become taxed by rumination in depression to the point of interfering with memory. Those in the higher IQ group may have had extra processing resources at their disposal and the memory task may not have reached their resource threshold.