The effect of cognitive load on the retrieval of long term memory: an fMRI study

Background and Aim : One of the less well-understood aspects of memory function is the mechanism by which the brain responds to an increasing load of memory, either during encoding or retrieval. Identifying the brain structures which manage this increasing cognitive demand would enhance our knowledge of human memory. Manipulating load in working memory has been studied previously, but there is only one fMRI study on manipulating cognitive load in long term memory.Methods : In a novel design, we asked 32 (18F) healthy young volunteers, with the mean age of 30.16± 6.4 (20-39 years old), to memorize all possible details of 24 images over a 12-day period ending two days before the fMRI scan; by this, each participant had two nights of sleep before the scan, as the memory consolidation phase. Using the Depression Anxiety Stress Scales (DASS-21), Rey Auditory Verbal Learning Test (RAVLT), and Forward and Backward Digit Span tasks, the participants were checked to have a healthy mind and memory function. The images were of 12 categories relevant to daily events, with each category including a high and a low load image. Behavioural assessments on a separate group of participants (#22) provided the average loads of the images and approved our selection of images. The participants had to retrieve each previously memorized image in 15 seconds after hearing the retrieval cues during the fMRI scan, with their eyes closed; the high and low load trials were randomly intermixed and presented. We used a 3T Prisma MRI scanner, and 355 fMRI data volumes were acquired which increased the statistical power of the study.Results : Using both categorical and parametric fMRI design and data analyses, we observed eight brain structures showing the highest activation with increasing load of the retrieved images, viz. parahippocampus, cerebellum, superior lateral occipital, fusiform and lingual gyri, hippocampus, angular gyrus, and precuneus. Some structures showed reduced activation when retrieving higher load images, such as the anterior cingulate, insula, and supramarginal and post central gyri. The parametric study also showed a linear activation increment in six similar brain areas with increasing load, including precuneus, posterior cingulate, lingual gyrus, parahippocampus, fusiform, and cerebellum.Conclusion : The findings of this study revealed that the same network of brain areas show an elevated activation under a higher LTM retrieval load, compared to acquiring additional brain regions, which is a help to better understand the LTM storage and retrieval processes.

کلیدواژه ها:

Long term memory ، cognitive load ، functional MRI