Satoshi Tsujimoto

This study aimed at investigating the effect of distraction on working memory and its underlying neural mechanisms in children with attention-deficit/hyperactivity disorder (ADHD). To this end, we studied hemodynamic activity in the prefrontal cortex using near-infrared spectroscopy while 16 children with ADHD and 10 typically developing (TD) children performed a working memory task. This task had two conditions: one involved a distraction during the memory delay interval, whereas the other had no systematic distraction. The ADHD patients showed significantly poorer behavioral performance compared with the TD group, particularly under the distraction. The ADHD group exhibited significantly higher level of prefrontal activation than did TD children. The activity level was positively correlated with the severity of ADHD symptoms. These results suggest that the impairment in the inhibition of distraction is responsible for the working memory deficits observed in ADHD children. Inefficient processing in the prefrontal cortex appears to underlie such deficits.

The concept of the “mnemonic scotoma,” a spatially circumscribed region of working memory impairment produced by unilateral lesions of the PFC, is central to the view that PFC is critical for the short-term retention of information. Presented here, however, are previously unpublished data that offer an alternative, nonmnemonic interpretation of this pattern of deficit. In their study, Wajima and Sawaguchi [Wajima, K., & Sawaguchi, T. The role of GABAergic inhibiton in suppressing perseverative responses in the monkey prefrontal cortex. Neuroscience Research, 50(Suppl. 1), P3–P317, 2004] applied the GABAA antagonist bicuculline methiodide unilaterally to the PFC of two monkeys while they performed an oculomotor delayed-response task. Consistent with previous studies, errors for the initial memory-guided saccade were markedly higher when the cued location fell into the region of the visual field affected by the infusion. These erroneous saccades tended to select an alternative target location (out of a possible 16) that had not been cued on that trial. By extending the analysis window, however, it was observed that the second, “corrective” saccade often acquired the location that had been cued on that trial. Further analysis of the erroneous initial saccades indicated that they tended to be directed to a location that had been relevant on the previous trial. Thus, the deficit was not one of “forgetting” the cued location. Rather, it was one of selecting between currently and previously relevant locations. These findings suggest a need for a reconsideration of the concept of the mnemonic scotoma, which in turn invites a reconsideration of functional interpretations of sustained neuronal activity in PFC.

When we encode faces in memory, we often do so in association with biographical information regarding the person. To examine the neural dynamics underlying such encoding processes, we devised a face recognition task and recorded cortical activity using magnetoencephalography. The task included two conditions. In the experimental condition, face stimuli were preceded by biographical information regarding the person whose face was to be memorized, whereas in the control condition, nonsense syllables were presented before face stimuli. Behavioral results indicated that the biographical information about a person facilitated the recognition memory of their face. Magnetoencephalography signals showed clear visually evoked magnetic fields mainly in the occipitotemporal cortex, in response to the face stimuli that were to be encoded. The phasic peak was observed at 100–200 ms after onset of a face stimulus, which was followed by late latency deflections (200–400 ms). Comparison of the signal between conditions revealed that the preceding semantic information does modulate the neuromagnetic responses to the face stimuli. This modulation occurred primarily at the late latency component in the sensors over the occipitotemporal cortex. In addition, the effects of conditions were also observed in the signals from more anterior sensors, which occurred earlier than the effects in the occipitotemporal cortex. These results provide insights into the neural dynamics underlying the encoding of faces in association with their biographical information.

Behavior-guiding rules and strategies allow monkeys to avoid errors in rarely encountered situations. In the present study, we contrasted strategy-related neuronal activity in the dorsolateral prefrontal cortex (PFdl) and the orbital prefrontal cortex (PFo) of rhesus monkeys. On each trial of their behavioral task, the monkeys responded to a foveal visual cue by making a saccade to one of two spatial targets. One response required a leftward saccade, the other required a saccade of equal magnitude to the right. The cues instructed the monkeys to follow one of two response strategies: to stay with their most recent successful response or to shift to the alternative response. Neurons in both areas encoded the stay and shift strategies after the cue appeared, but there were three major differences between the PFo and the PFdl: (1) many strategy-encoding cells in PFdl also encoded the response (left or right), but few, if any, PFo cells did so; (2) strategy selectivity appeared earlier in PFo than in PFdl; and (3) on error trials, PFo neurons encoded the correct strategy—the one that had been cued but not implemented—whereas in PFdl the strategy signals were weak or absent on error trials. These findings indicate that PFo and PFdl both contribute to behaviors guided by abstract response strategies, but do so differently, with PFo encoding a strategy and PFdl encoding a response based on a strategy.

Recording from single neurons in awake macaque monkeys, the authors find that neurons of the frontal pole cortex encode decisions at the time of feedback, but do not carry signals typically seen in other prefrontal areas, such as information about sensory cues, strategies, working memory, future goals or movement plans. The frontal pole cortex (FPC) expanded markedly during human evolution, but its function remains uncertain in both monkeys and humans. Accordingly, we examined single-cell activity in this area. On every trial, monkeys decided between two response targets on the basis of a 'stay' or 'shift' cue. Feedback followed at a fixed delay. FPC cells did not encode the monkeys' decisions when they were made, but did so later on, as feedback approached. This finding indicates that the FPC is involved in monitoring or evaluating decisions. Using a control task and delayed feedback, we found that decision coding lasted until feedback only when the monkeys combined working memory with sensory cues to 'self-generate' decisions, as opposed to when they simply followed trial-by-trial instructions. A role in monitoring or evaluating self-generated decisions could account for FPC's expansion during human evolution.