Abstract: Since the mid-1990s, psychologists and cognitive neuroscientists have been investigating in the area of object working memory and have gained valuable insights into the field. However, many issues and problems still need research attention. For example, there are different views concerning the dynamic change pattern of brain activation and the brain mechanism in object working memory tasks, and how the pattern of brain activation in high-load object working memory tasks differs from that in low-load tasks. The present study aimed to make use of the spatial-temporal properties of ERP slow waves to examine the pattern of brain activation when object information with different memory load was recoded and retained in WM.
Twenty volunteers (13 female and 7 male, mean age = 20.75 years) participated in the present study. None of the subjects had prior experience with the experimental task and all of the subjects were right-handed with either normal or corrected-to-normal vision. A delayed matching task was adopted. In each trial, a cue was presented on the center of the screen for 400ms, followed by a 400ms blank screen, and finally the to-be-memorized objects were displayed for 1800ms. Two (low-load condition) or four (high-load condition) objects were presented. After an inter-stimulus interval of 3000ms, the probe stimulus was presented for 1600ms. Participants had to respond within 1600ms and were instructed to respond as quickly and accurately as possible. EEG was recorded using 123 Ag/AgCl electrodes (positioned according to the international 10-20 system). The resulting ERPs time locked from the onset of the target stimulus to the probe stimulus were computed for each participant at all recording sites. Each condition was computed separately. Repeated-measures ANOVA was performed.
The results showed that slow cortical potentials of the high-memory-load task were more negative than that of the low-memory-load task in the dorsolateral prefrontal cortex (the effect was found between 700 to 1400ms after the display of the stimulus), and that slow cortical potentials of the low-memory-load task were more negative than that of the high-memory-load task in the left frontal (500-1800ms), left fronto-central (700-1800ms), left centro-parietal (1000-1800ms and 1800-4800ms), right frontal (1800 - 4800ms), and right fronto-central (1400-4800ms) regions. These results suggest that (1) the dorsolateral prefrontal cortex (DLPFC) may be responsible for the maintenance and rehearsal of object information and (2) the effects of memory load may be different for the two hemispheres

Key words: object working memory, effects of memory load, slow cortical potentials, event related potentials