Hideyuki Kikyo

Hemispheric asymmetry in human lateral prefrontal cortex during cognitive set shifting

Functional organization of human cerebral hemispheres is asymmetrically specialized, most typically along a verbal/nonverbal axis. In this event-related functional MRI study, we report another example of the asymmetrical specialization. Set-shifting paradigms derived from the Wisconsin card sorting test were used, where subjects update one behavior to another on the basis of environmental feedback. The cognitive requirements constituting the paradigms were decomposed into two components according to temporal stages of task events. Double dissociation of the component brain activity was found in the three bilateral pairs of regions in the lateral frontal cortex, the right regions being activated during exposure to negative feedback and the corresponding left regions being activated during updating of behavior, to suggest that both hemispheres contribute to cognitive set shifting but in different ways. The asymmetrical hemispheric specialization within the same paradigms further implies an interhemispheric interaction of these task components that achieve a common goal.

Memory of music: Roles of right hippocampus and left inferior frontal gyrus

We investigated neural correlates of retrieval success for music memory using event-related functional magnetic resonance imaging. To minimize the interference from MRI scan noise, we used sparse temporal sampling technique. Newly composed music materials were employed as stimuli, which enabled us to detect regions in absence of effects of experience with the music stimuli in this study. Whole brain analyses demonstrated significant retrieval success activities in the right hippocampus, bilateral lateral temporal regions, left inferior frontal gyrus and left precuneus. Anatomically defined region-of-interests analyses showed that the activity of the right hippocampus was stronger than that of the left, while the activities of the inferior frontal gyri showed the reverse pattern. Furthermore, performance-based analyses demonstrated that the retrieval success activity of the right hippocampus was positively correlated with the corrected recognition rate, suggesting that the right hippocampus contributes to the accuracy of music retrieval outcome.

Temporal characterization of memory retrieval processes: an fMRI study of the ‘tip of the tongue’ phenomenon

‘Tip of the tongue’ (TOT) is a natural phenomenon in which people cannot retrieve a target word immediately, even though they feel confident that they know the target. This provides us an opportunity to understand the human memory system, because cognitive components of memory retrieval such as retrieval effort and successful retrieval are temporally dissociated from each other during the TOT states. The purpose of the present study was to reveal the neural correlates of the cognitive components of the retrieval process by separating cognitive phases of the TOT phenomenon using event‐related functional magnetic resonance imaging with multiple regression analysis. We demonstrated that the left dorsolateral prefrontal cortex (DLPFC) and anterior cingulate cortex were activated at the time of successful retrieval, and the left DLPFC also showed activation when the subjects successfully retrieved the target names as compared to when they gave up. This result suggests that the left DLPFC is specific to the successful retrieval process. During the TOT state, a number of regions were activated, and this suggests that widely distributed brain regions are engaged when people make a hard effort to retrieve a proper name in the TOT state. Our new approach employing temporal resolution of the TOT phenomenon may contribute to the understanding of the mechanisms of the human memory system.

2602 Mapping of response inhibition function of the prefrontal cortex using functional magnetic resonance imaging

ATSUSHI IRIKI’32. MICHIO TANAKA’, YOSHIAKI IWAMURA’ Monkeys were trained to retrieve food by watching their hand movement through a real-time video monitor instead of seeing it directly. Single unit activities of bimodal neurons, which integrate somatosensory and visual information to code the schema of the hand (Iriki et al., 1996), were recorded form the anterior bank of the intraparietal sulcus of the contralateral cerebral cortex. After training, additional visual receptive fields were formed around the image of the hand in the video screen. Their size and position were modified according to expansion, compression, or change of the position of the visual image in the video screen, although the posture or the position of the hand was not actually altered. This phenomenon was observed only after the monkeys learned to retrieve food through the video screen, suggesting that the self image, if exists in monkeys, was projected to the video screen as the result of learning.

A functional MRI study of orthographic processing of Kanji

Optical imaging of the intrinsic signal has been used to visualize functional structures. This signal induced by neural activation is related to concentration changes of deoxy-hemoglobin (deoxy-Hb), oxy-hemoglobin (oxy-Hb), blood volume (BV), and change in light scattering (LS). In this study we investigated spatial patterns of orientation columns derived from these different components in anesthetized and awake cat cortices. We recorded the intrinsic signal at 540, 570, and 620 nm simultaneously by using three CCD cameras and decomposed it into the above-mentioned components, A flashed grating or moving grating was used for visual stimulation. The average response evoked by two orthogonal gratings indicated the global change (GC) for each component and a difference image obtained from the two gratings indicated the local difference (LD) of each component. The LD of BV showed an identical columnar pattern with that of deoxy-Hb. These small laocal changes were superimposed on a larger global changes across the cortex, with increasing GC of BV, and deoxy-Hb. These results suggest that, as well as deoxy-Hb, neural activation induces a large increase in BV both in active and less active columns, but the amount is larger in active columns. We hypothesize that there is a fine control mechanism of blood flow at the columnar level. Analysis of oxy-Hb, LS and differences in awake and anesthetized condition will be also discussed.