Toshimitsu Takahashi

Influence of skin blood flow on near-infrared spectroscopy signals measured on the forehead during a verbal fluency task.

Brain activity during a verbal fluency task (VFT) has been the target of many functional imaging studies. Most studies using near-infrared spectroscopy (NIRS) have reported major activation in the frontal pole, but those using PET or fMRI have not. This led us to hypothesize that changes in the NIRS signals measured in the forehead during VFT were due to changes in skin blood flow. To test this hypothesis, we measured NIRS signals and the Doppler tissue blood flow signals in the foreheads of 50 participants. The measurements were performed while each participant produced words during two 60-s periods with an interval of 100 s. In addition to a conventional optode separation distance of 30 mm (FAR channels), we used a short distance--5mm (NEAR channels)--to measure NIRS signals that originated exclusively from surface tissues. The oxygenated hemoglobin (oxyHb) concentration in the FAR and NEAR channels, as well as the Doppler blood flow signal, increased in a similar manner during the two periods of word production; the signal increase in the first period was twice as high as that in the second period. Accordingly, the mean changes in oxyHb concentration in the FAR channels were correlated closely with the changes in the NEAR channels (R(2) = 0.91) and with the integrated Doppler skin blood flow signal (R(2) = 0.94). Furthermore, task-related NIRS responses disappeared when we blocked skin blood flows by pressing a small area that covered a pair of optodes. Additionally, changes in the FAR channel signals were correlated closely with the magnitude of pulsatile waves in the Doppler signal (R(2) = 0.92), but these signals were not highly correlated with the pulse rate (R(2) = 0.43). These results suggest that a major part of the task-related changes in the oxyHb concentration in the forehead is due to task-related changes in the skin blood flow, which is under different autonomic control than heart rate.

Synchronization of spontaneous eyeblinks while viewing video stories

Blinks are generally suppressed during a task that requires visual attention and tend to occur immediately before or after the task when the timing of its onset and offset are explicitly given. During the viewing of video stories, blinks are expected to occur at explicit breaks such as scene changes. However, given that the scene length is unpredictable, there should also be appropriate timing for blinking within a scene to prevent temporal loss of critical visual information. Here, we show that spontaneous blinks were highly synchronized between and within subjects when they viewed the same short video stories, but were not explicitly tied to the scene breaks. Synchronized blinks occurred during scenes that required less attention such as at the conclusion of an action, during the absence of the main character, during a long shot and during repeated presentations of a similar scene. In contrast, blink synchronization was not observed when subjects viewed a background video or when they listened to a story read aloud. The results suggest that humans share a mechanism for controlling the timing of blinks that searches for an implicit timing that is appropriate to minimize the chance of losing critical information while viewing a stream of visual events.

Neuroanatomical discrimination between manipulating and maintaining processes involved in verbal working memory; a functional MRI study

We used functional magnetic resonance imaging (fMRI) to investigate neural correlates of processes concerning store and manipulation in verbal working memory. We prepared a revised lag 1 digit span, digit span and a simple number detection task. Specific activities in association with manipulating process were identified in the right middle (BA 9/46) and left precentral gyrus (BA 6). Activated areas specific to maintaining process were detected in the right middle (right BA 11/10) and medial (BA 6) frontal gyri, the right inferior parietal lobule (BA 40), and the left middle (BA 9) and inferior frontal gyri (BA 44). The process-nonspecific activated areas common to two processes were identified in the right inferior frontal gyrus (BA 47) and the left superior parietal lobule (BA 7). Using the signal percent change of each subject, we calculated the correlation coefficients among each activated area. The results of this analysis showed that two processes of verbal working memory were clearly discriminated. The two essential processes of manipulation and maintenance in working memory seem to activate process-specific and overlapping (process-nonspecific) areas, but the patterns of combination were definitely different.

Medial temporal lobe activation during context-dependent relational processes in episodic retrieval: An fMRI study

Previous studies have reported that the medial temporal lobe (MTL) structures contribute to the processing of relations among multiple stimuli in episodic encoding. There have been few studies, however, on the episodic retrieval requiring processing of relations among multiple components that was involved in our events. We used functional magnetic resonance imaging (fMRI) to investigate neural activities during the retrieval of relations within an organized episode and the recognition of an episodic component. Healthy, normal participants memorized 50 four‐scene comic strips before fMRI scanning. In the retrieval phase with fMRI scanning, participants were engaged in three tasks: a visual identification (VI) task, a story recall (SR) task, and a picture recognition (PRe) task. In the VI task, participants were asked to judge whether they could identify at least one female character in the two scenes presented vertically. In the SR task, participants were shown the first and last scenes from strips memorized previously and asked to judge whether or not the two scenes were from the same strip. In the PRe task, participants were shown two scenes and asked to judge whether they both belonged to the memorized scenes. The two contrasts of SR with VI and PRe with VI demonstrated some commonly activated areas, such as the bilateral middle frontal gyrus and cerebellum. More importantly, the SR task differentially activated the bilateral parahippocampal gyrus, whereas the PRe task differentially activated right prefrontal areas, including the inferior frontal and precentral gyri. The results suggest that the activity of the MTL structures may be strongly associated with episodic memory retrieval requiring context‐dependent relational processing. Hum. Brain Mapping 17:203–213, 2002.

Neuronal Activity Related to Reward Size and Rewarded Target Position in Primate Supplementary Eye Field

Several areas of the macaque brain are known to be related to the reward during the performance of saccadic eye-movement tasks. Neurons in the supplementary eye field (SEF) have been reported to be involved in the prediction and detection of a reward. We describe a group of neurons in the SEF that became active during the period of reward delivery after saccades toward a specific direction, but showed weaker activity in other directions, although the same amount of reward was given in each direction. Moreover, this directional reward activity was modulated by the reward size. Our results demonstrate that the SEF cells may reflect both reward amount and target positions toward which a movement was rewarded, and suggest that they may play an important role in providing information about the value of each saccade according to the spatial target location.

Subsecond reward-related dopamine release in the mouse dorsal striatum.

Reward presentation is known to induce transient bursts of midbrain dopamine neurons in monkeys and rats, and the reward-induced dopamine overflow has been detected in the rat ventral striatum. To detect reward-related dopamine release in the dorsal striatum of behaving mice (C57BL/6), we used voltammetry with carbon-fiber microelectrodes implanted into the dorsal striatum. Dopamine signals increased transiently after food delivery with a peak at 0.6 s after the delivery onset. The success in detecting transient reward-response of dopamine in behaving mice opens a wide range of application to studies in mutant mice.

Neuronal activity related to anticipated and elapsed time in macaque supplementary eye field

It is essential to sense anticipated and elapsed time in our daily life. Several areas of the brain including parietal cortex, prefrontal cortex, basal ganglia and olivo-cerebellar system are known to be related to this temporal processing. We now describe a number of cells in the supplementary eye field (SEF) with phasic, delay activity and postdelay activity modulation that varied with the length of the delay period. This variation occurred in two manners. First, cells became active with the shorter delay periods (GO signal presented earlier). We call these cells “short-delay cells”. Second, cells became active with the longer delay periods (GO signal presented later). We call these cells “long-delay cells”. However, such changed neuronal activity did not correlate with reaction time. These results suggest that the delay-dependent activity may reflect anticipated and elapsed time during performance of a delayed saccadic eye movement.

Different Distribution of the Activated Areas in the Dorsal Premotor Cortex during Visual and Auditory Reaction-Time Tasks

Sensorimotor association is an essential aspect of behavior. The dorsal part of the premotor cortex (PMd) is known to have an important role in sensorimotor association. Although it is suggested that the partially segregated groups of neurons are involved in sensorimotor association in different sensory modalities, it is not yet clear whether these groups occupy the PMd to the same or different extent. Therefore, we performed a functional magnetic resonance imaging study to compare activated regions in the PMd during simple reaction-time tasks with visual and auditory cues. Eight normal volunteers performed two simple reaction time tasks with a conventional on-off design; one is with a visual cue and the other is with an auditory cue. In both tasks, two regions in the left primary motor area (M1) (4a and 4p) and the bilateral PMd were activated. The two activated regions in the left M1 occupied the same areas in both the visual and the auditory tasks. However, in the PMd, the activated regions were situate medially during the visual task and laterally during the auditory task, along the precentral sulci. There was no overlap of significantly activated regions between two tasks, and areas specifically activated during the visual task were observed in the middle of the precentral sulci, bilaterally. The results suggest that the distribution of PMd subregions involved in sensorimotor association differ when the sensory cues are in different modalities.

Dominance of the left oblique view in activating the cortical network for face recognition

Faces in portraits are often depicted from the left 3/4 view (an oblique view of the face that is intermediate between the frontal view and left profile). Here, we used functional magnetic resonance imaging (fMRI) to show that, compared with photographs of right 3/4 views of familiar faces, photographs of left 3/4 views of the same faces elicited stronger neural responses in the right middle occipital/inferior parietal cortex, and right inferior frontal gyrus; which are known to be involved in face recognition. By contrast, there was no differential activation in the temporal cortex including the superior temporal sulcus and fusiform gyrus, which are thought to process face-related visual stimuli at a stage that precedes recognition. We suggest that the preference for the left 3/4 view of faces was produced at a later stage of facial information processing that involves attention or memory retrieval.

Differences in the Pulsatile Component of the Skin Hemodynamic Response to Verbal Fluency Tasks in the Forehead and the Fingertip.

Several studies have claimed that hemodynamic signals measured by near-infrared spectroscopy (NIRS) on the forehead exhibit different patterns during a verbal fluency task (VFT) in various psychiatric disorders, whereas many studies have noted that NIRS signals can reflect task-related changes in skin blood flow. If such a task-related skin hemodynamic response is also observed in the fingertip, a simpler biomarker may be developed. Furthermore, determining the difference in the response pattern may provide physiological insights into the condition. We found that the magnitude of the pulsatile component in skin hemodynamic signals increased on the forehead (p < 0.001 for N = 50, p = 0.073 for N = 8) but decreased on the fingertip (p < 0.001, N = 8) during the VFT, whereas the rate in both areas increased (p < 0.02, N = 8). We also did not find a repetition effect in both the rate and the magnitude on the fingertip, whereas the effect was present in the magnitude (p < 0.02, N = 8) but not in the rate on the forehead. These results suggest that the skin vasomotor system in the forehead could have a different vessel mechanism to psychological tasks compared to the fingertip.