<p>Earlier studies have shown impaired explicit test and normal implicit test performance in participants classified as depressed. A number of different models have been put forward to explain this 'typical' test dissociation including the memory systems, processing, and activation - elaboration models. Blaxton (1989, 1992) has pointed out that to date most test designs have confounded the memory systems and processing models. The aim of this series of experiments was to systematically compare the effects of depression on the processing and memory systems models and in so doing provide a more precise explanation for the effects of depression on human memory. Across Experiments 1 - 4 the performance of participants with depression or dysphoria were examined on implicit and explicit memory tests which were designed to tap either predominantly perceptual or conceptual processes. In Experiment 1 the conceptual tests of category association (implicit) and semantic cued recall (explicit) were compared with the perceptual tests of word fragment completion (implicit) and graphemic cued recall (explicit). In Experiment 2 the perceptual tests of perceptual identification (implicit) and the 'mixed' test of anagram solution (implicit) were compared with the conceptual free recall test (explicit). Both experiments used dysphoric university students and found no effects of dysphoria in comparison to normal controls matched for age, sex and education levels. Experiment 3 compared the conceptual category association (implicit) and free recall (explicit) tests with the perceptual word fragment completion test (implicit) using participants diagnosed with major depression disorder. This revealed significant impairments in both the conceptual tests while the perceptual test was intact. Experiment 4 compared the implicit word association test with the explicit word association test using dysphoric university students. Experiment 4 found that dysphoric participants were impaired in performing the explicit test while the implicit test remained intact. These findings suggest that dysphoria has no effect on implicit tests, but can effect conceptual explicit test measures. Clinical depression effects both conceptual implicit and conceptual explicit test measures. While these results support aspects of both the memory systems and processing models these findings may be best accommodated by a model which combines these models. The revised memory systems model is discussed as one means of achieving this.</p>
Medial Temporal Lobe Involvement in an Implicit Memory Task: Evidence of Collaborating Implicit and Explicit Memory Systems from fMRI and Alzheimer's Disease
