Toru Tsujimoto

Dynamic activities of the frontal association cortex in calculating and thinking

We found 5-7 Hz magnetic theta waves in the frontal association cortex of adult human subjects during calculation and musical imagination by using 37-channel SQUID gradiometers. Simultaneous recording from the left and right cerebral hemispheres with two sets of 37-channel gradiometers revealed that the theta activity appeared in a waxing and waning manner in the frontal cortices of both hemispheres during the mental exercises. Electrical current dipoles for the theta waves were estimated to occur repeatedly and scatteringly in various parts of the frontal lobes of both hemispheres during continuous and intense mental exercises for 2 min. The results suggest a dynamic mode of activities in the frontal association cortex during mental effort such as calculating and thinking.

Frontal mental theta wave recorded simultaneously with magnetoencephalography and electroencephalography

Frontal mental theta waves were recorded simultaneously with magnetoencephalography (MEG) and electroencephalography (EEG) in normal subjects. MEG analyses demonstrated that theta burst activities distribute widely in the frontal cortices of both the cerebral hemispheres during concentration on mental calculation. On the other hand, EEG showed the theta activities seemed maximal on the frontal midline part (Fz), as if they might have been generated mainly in the frontal midline cortical area.

Cortical field potentials associated with hand movements triggered by warning and imperative stimuli in the monkey

Monkeys were trained to move the hand in response to imperative visual stimulus (IS) given 1 s after warning visual stimulus (WS). With implanted electrodes in the cortices, surface-negative (s-N), depth-positive (d-P) sustained potentials between WS and IS were recorded in the prefrontal, premotor and supplementary motor areas in both hemispheres, and gradually increasing s-N, d-P potentials were seen in the forelimb areas of motor and somatosensory cortices contralateral to the hand. It is suggested that the sustained and gradually increasing potentials are related respectively to cortical activities associated with expectation and anticipation of the IS, and to those with a preparatory process for the movement. The latter appeared to be similar to the case of self-paced movements. These potentials may correspond respectively to the early and late components of CNV in the human.

Theta Oscillations in Primate Prefrontal and Anterior Cingulate Cortices in Forewarned Reaction Time Tasks

Previously, we introduced a monkey model for human frontal midline theta oscillations as a possible neural correlate of attention. It was based on homologous theta oscillations found in the monkeys prefrontal and anterior cingulate cortices (areas 9 and 32) in a self-initiated hand-movement task. However, it has not been confirmed whether theta activity in the monkey model consistently appears in other situations demanding attention. Here, we examined the detailed properties of theta oscillations in four variations of forewarned reaction time tasks with warning (S1) and imperative (S2) stimuli. We characterized the theta oscillations generated exclusively in areas 9 and 32, as follows: 1) in the S1-S2 interval where movement preparation and reward expectation were presumably involved, the theta power was higher than in the pre-S1 period; 2) in the no-go trials of go/no-go tasks instructed by S1, the theta power in the S1-S2 interval was lower than in the pre-S1 period in an asymmetrical reward condition, whereas it was moderately higher in a symmetrical condition; 3) the theta power after reward delivery was higher than in the unrewarded trials; 4) the theta power in the pre-S1 period was higher than in the resting condition; and 5) when the monkey had to guess the S1-S2 duration internally without seeing S2, the theta power in the pre-S1 period was higher than in the original S1-S2 experiment. These findings suggest that attentional loads associated with different causes can induce the same theta activity, thereby supporting the consistency of attention-dependent theta oscillations in our model.

Effect of cooling the dentate nucleus of the cerebellum on hand movement of the monkey

The functional role of the cerebellum in voluntary movement was investigated by local cooling of the cerebellar nuclei in three Japanese monkeys which performed hand movement tasks in response to visual stimuli. We implanted electrodes in various areas of the cerebral hemispheres to record field potentials in the cortex, and examined effects of the cooling upon the movement and field potentials. Cooling of the dentate nucleus ipsilateral to the moving hand reversibly increased the reaction time and reduced the size of surface-negative, depth-positive (s-N, d-P) field potential in the motor cortex contralateral to the hand. The potential preceded the movement by an almost constant time of about 100 ms in the normal condition. The cooling remarkably prolonged and deviated the time. By shifting the cooling probe to different distances from the nucleus, we noted various decreases of the cooling effect. We also found a close correlation between the size of the s-N, d-P potential and the reaction time, i.e., when the potential was small, the reaction time was long. These findings support the following ideas; the motor command for this task comes to the motor cortex through the cerebello-thalamo-cortical pathway which includes the dentate nucleus, and produces the s-N, d-P potential as EPSP currents in pyramidal neurons in the motor cortex. We also studied self-paced movement task. In some cases, the cooling reduced the size of readiness potential in the motor cortex.

High-frequency SEP components generated in the somatosensory cortex of the monkey.

To investigate the origin of high-frequency somatosensory evoked potential (SEP) components, we recorded median nerve SEPs from the scalp and the depth in six monkeys. Laminar field potentials were analyzed in area 3b (N10; corresponding to human N20) and area 1 (P12; corresponding to human P25). After digital filtering (300–900 Hz), 4–6 components were identified, and the 1st to 4th peaks in area 3b (7–11 ms in latency) and the 3rd to 5th in area 1 (9–13 ms) showed clear polarity reversals between the surface and the depth of the cortex. These results provide direct evidence for intracortical origin of early high-frequency components in area 3b and of late ones in area 1.

Activation of the prefrontal, occipital and parietal cortices during go/no-go discrimination tasks in the monkey as revealed by positron emission tomography

The regions of the monkey brain involved in cognito-behavioral tasks were mapped using the H2(15)O positron emission tomography (PET) activation technique. Monkeys performed hand movements in an asymmetrically rewarded go/no-go reaction-time task with discrimination between green (go) and red (no-go) light signals. Regional cerebral blood flow (rCBF) was measured during the go/no-go task and a control task in which only go signals were presented. In the go/no-go task, when compared with the control, a significant increase in rCBF was noted in the following regions: (1) the principal sulci; (2) the anterodorsal frontal pole; (3) the anterior part of the inferior occipital sulcus which appeared to be the V4; and (4) the parieto-occipital region. The increase in the principal sulci may be related to the no-go decision and motor suppression in the area, as interpreted in our previous electrophysiological studies. The present results appear to support electrophysiological observations on visual perception and motor suppression and reflect a mode of their functional integration.

Directional organization of sensorimotor oscillatory activity related to the electromyogram in the monkey

OBJECTIVE To investigate cortical control of the electromyogram (EMG). METHODS We examined the directed transfer function (DTF) between the EMG and local field potential oscillations in the monkey sensorimotor area during both an isotonic muscle contraction task and the rest condition with EMG silence. DTF computation based on a multivariate model is suitable for analyzing the directional structure of a reciprocally interconnected system. RESULTS We found that DTF between the cortex and EMG in the beta band is predominantly centrifugal and largest at the anterior bank of the central sulcus. As for the cortico-cortical DTF within the sensorimotor area, the effective connectivity in the beta band was reciprocal across the central sulcus but was dominated by a posterior-to-anterior direction, especially during the muscle contraction task. The asymmetry of the DTF during the rest condition was inconsistent across the monkeys. CONCLUSIONS These findings indicate the functional relevance of field potential oscillations in the post- as well as pre-central gyri in generating EMG rhythmicity. SIGNIFICANCE This supports the idea that information flow from the post- to pre-central gyri is a key element in volitional muscle contraction.

Reactive and anticipatory control of posture and bipedal locomotion in a nonhuman primate.

Bipedal locomotion is a common daily activity. Despite its apparent simplicity, it is a complex set of movements that requires the integrated neural control of multiple body segments. We have recently shown that the juvenile Japanese monkey, M. fuscata, can be operant-trained to walk bipedally on moving treadmill. It can control the body axis and lower limb movements when confronted by a change in treadmill speed. M. fuscata can also walk bipedally on a slanted treadmill. Furthermore, it can learn to clear an obstacle attached to the treadmills belt. When failing to clear the obstacle, the monkey stumbles but quickly corrects its posture and the associated movements of multiple motor segments to again resume smooth bipedal walking. These results give indication that in learning to walk bipedally, M. fuscata transforms relevant visual, vestibular, proprioceptive, and exteroceptive sensory inputs into commands that engage both anticipatory and reactive motor mechanisms. Both mechanisms are essential for meeting external demands imposed upon posture and locomotion.

Prefrontal theta oscillations associated with hand movements triggered by warning and imperative stimuli in the monkey

In order to investigate the functional significance of theta oscillations in the brain, we recorded the cortical field potential in monkeys engaged in a visually-initiated hand movement task. In each trial a warning signal (S1) was followed 3 s later by an imperative signal (S2) to which the monkey had to respond to get a reward. The theta power in the prefrontal area 9 and the prelimbic area 32 was higher in the S1-S2 interval than in the pre-S1 period. This theta activity may be related to attentional processes and is probably a homologue of the human frontal midline theta (Fm theta) rhythms.