MTBI and Working Memory
It is common for researchers and practitioners to see mild traumatic brain injury (MTBI) patients with normal CT or MRI scans, but who have a number of post-concussive complaints and cognitive deficits. The authors of this study deal with the concepts of "working memory" and "processing load." They define working memory as, "the 'online' storage of information necessary for performing cognitive operation," and processing load as, "the amount of information that must be held online to solve a particular problem."
The authors hypothesize that there is a physiologic basis for MTBI complaints, and that if a functional MRI (fMRI) were performed within one month following injury, it would show changes in both the working memory and processing load.
12 MTBI patients and 11 healthy controls had their brains scanned while performing memory tasks. Eleven of the patients had a normal CT and regular MRI scan. Both groups took a symptom checklist, and the MTBI patients reported more symptoms. Included symptoms were poor memory of recent events, difficulty in doing their job, and trouble with concentration.
Controls showed no decline in performance when moving from the low processing demand tasks to the high processing demand tasks. Both the controls and the patients had similar areas of brain activation and similar task performance. But, there were differences in how the memory worked in the patients and in the controls- patients showed increased activity in the right lateral parietal regions and the right dorsolateral frontal regions. The authors claim that it is unlikely that the differences in activation are related attentional difference or to distress or depression since both groups scored similarly on the symptom checklist; it is also not possible that the patients suffered neural loss since they showed performance similar to the controls in memory tasks. They conclude, "The ability to activate, modulate, or allocate processing resources in response to gradations of processing load may be impaired in the postacute period after MTBI." Which means that MTBI patients do not have decreases in brain resources (the information is all there) but have difficulty in the circuitry—they have difficulty moving, timing, and regulating access to the information. To explain all these incongruities between the patient's experience of difficulties and what test results imply, the authors write:
"One possible explanation is that the MTBI patients perceive the change in their ability to engage working memory easily and efficiently, and experience this change as 'having to work harder' to maintain accurate task performance. Perhaps this is then labeled as 'problems with memory.' If true, this might account for the discrepancy between the severity of complaints voiced by many MTBI patients and the relatively minor performance deficits often found in these individuals."
The MTBI patient then, is not malingering or experiencing depression or anxiety—which are common conclusions researchers come to when grappling with the chasm between test results and patients. This study makes headway, and suggests that cognitive complaints of MTBI patients are related to differences in brain activation and the condition of brain circuitry. The authors also point out that, "patterns of activation differences are likely to be more important than the overall level of activation."
McAlister TW, Saykin AJ, LA Flashman, et al. Brain activation during working memory 1 month after mild traumatic brain injury. Neurology 1999;53:1300-1308.