In patients who have sustained mild traumatic brain injury (MTBI), differences in working memory functional activity were observed with MRI even though patients performed as well as controls on working memory tests, according to a study published online July 24 in Radiology.
Study authors Chi-Jen Chen, MD, of Taipei Medical University/Shuang Ho Hospital in New Taipei City, Taiwan, and colleagues said the results indicated the inclusion of MR imaging evaluation may increase sensitivity to MTBI compared with neuropsychological evaluation alone.
“This study determined that, despite no apparent differences in neuropsychological tests, brain activation patterns differed between MTBI patients and control subjects in response to increasing working memory loads,” wrote the authors.
A total of 20 patients with MTBI and 18 healthy control subjects were included in the study. Subjects were given n-back continuous performance tests, and functional MRI was used to asssess brain activation patterns in response to the tests. In the MTBI group, imaging was conducted within one month after injury.
N-back working memory tasks are conducted by presenting subjects with a sequence of stimuli, and the subjects are tasked with indicating when the current stimulus matches one from a certain number of steps earlier in the sequence. In the current study, subjects were given one-back, two-back and three-back tests in which they monitored a series of numbers and indicated when the current number matched a previous number from one, two or three steps prior, respectively.
Chen et al reported that the study groups performed similarly well on the n-back working memory tasks. The groups were statistically similar for sex, years of education and performance on a digit span memory test in which subjects were asked to remember an entire sequence of numbers.
Brain activation patterns differed between subjects, with controls able to increase activation in working memory circuitry as the working memory load required for the tasks increased. “In contrast, MTBI patients were impaired in their ability to increase activation in working memory circuitry under both moderate and high working memory load conditions,” wrote the authors. “However, MTBI patients did show cerebral plasticity, as evidenced by more activation in some areas outside and inside the working memory circuitry as compared with control subjects.”
Nine MTBI patients underwent follow-up functional MRI studies six weeks after the initial tests, and these patients demonstrated an improvement of activation in response to increasing working memory loads. While the initial results were varied, and more research is required to understand the effect of MTBI on brain activity during working memory tasks, the authors said the improvement at follow-up supported the initial observation of decreased activation. They also said the improvements support the notion that patients’ brains are able to compensate for damage in response to memory loads.
“At present it is still not completely understood which mechanisms underlie functional restitution after brain damage,” wrote Chen et al. “However, functional MR imaging research may allow identification of specific areas of the brain that have experienced changes in activation patterns. This information may greatly increase our knowledge of the gross mechanisms of cerebral activity after brain damage.”