Subjective difficulty in a verbal recognition-based memory task: Exploring brain-behaviour relationships at the individual level in healthy young adults.

The vast majority of fMRI studies of task-related brain activity utilize common levels of task demands and analyses that rely on the central tendencies of the data. This approach does not take into account perceived difficulty nor regional variations in brain activity between people. The results are findings of brain-behavior relationships that weaken as sample sizes increase. Participants of the current study included twenty-six healthy young adults evenly split between the sexes. The current work utilizes five parametrically modulated levels of memory load centered around each individual's predetermined working memory cognitive capacity. Principal components analyses (PCA) identified the group-level central tendency of the data. After removing the group effect from the data, PCA identified individual-level patterns of brain activity across the five levels of task demands. Expression of the group effect significantly differed between the sexes across all load levels. Expression of the individual level patterns demonstrated a significant load by sex interaction. Furthermore, expressions of the individual maps make better predictors of response time behavior than group-derived maps. We demonstrated that utilization of an individual's unique pattern of brain activity in response to increasing a task's perceived difficulty is a better predictor of brain-behavior relationships than study designs and analyses focused on identification of group effects. Furthermore, these methods facilitate exploration into how individual differences in patterns of brain activity relate to individual differences in behavior and cognition.

A ghost of retrieval past: a functional network of alpha EEG related to source memory in elderly humans.

The electrophysiological data from a study of source memory in young and older adults were analyzed in the frequency domain in order to find functional networks of alpha band activity related to source memory performance and cortical reorganization with age. Participants were instructed to remember noun pairs embedded in sentences, with sentences grouped into two temporally distinct lists. At test, in response to noun probes, participants made old/new, followed by source (i.e., list) judgments. Lower band alpha electroencephalography (EEG) recorded during recognition and source retrieval epochs was analyzed using spatial covariance analysis [Hum. Brain Mapp. 2 (1994) 79; J.R. Moeller, C. Ghez, A. Antonini, M.F. Ghilardi, V. Dhawan, K. Kazumata, D. Eidelberg, Brain networks of motor behavior assessed by principal component analysis, in: R.E. Carson, M.E. Daube-Witherspoon, P. Herscovitch (Eds.), Quantitative Functional Brain Imaging with Positron Emission Tomography, Academic, San Diego, 1998]: significant activation patterns were found in source retrieval epochs. For young subjects, the regional covariance pattern involved coactivation of right anterior and left posterior electrode sites. Source retrieval performance was predicted by the subject difference in pattern expression between epochs involving correct and incorrect source attribution. The older adults also exhibited significant coactivation of the right anterior and left posterior electrode sites. However, no compensatory activation sites were identified for these elders who performed near chance in source retrieval. In this regard, we replicated the main functional difference between good and poor performing elders reported in a recent O-15 PET study of source retrieval in young and older subjects [Neuroimage 17 (2002) 1394]. These alpha band and O-15 PET findings complement the age group differences found in the time-domain (ERP) analysis of our EEG data [Psychol. Aging 14 (1999) 390]. We suggest that spatial covariance analyses of spectral EEG and MEG data will reveal new information about functional brain activity relevant to normal aging.