Listening to rhythms activates motor and premotor cortices.

We used functional magnetic resonance imaging (fMRI) to identify brain areas involved in auditory rhythm perception. Participants listened to three rhythm sequences that varied in temporal predictability. The most predictable sequence was an isochronous rhythm sequence of a single interval (ISO). The other two sequences had nine intervals with unequal durations. One of these had interval durations of integer ratios relative to the shortest interval (METRIC). The other had interval durations of non-integer ratios relative to the shortest interval (NON-METRIC), and was thus perceptually more complex than the other two. In addition, we presented unpredictable sequences with randomly distributed intervals (RAN). We tested two hypotheses. Firstly, that areas involved in motor timing control would also process the temporal predictability of sensory cues. Therefore, there was no active task included in the experiment that could influence the participant perception or induce motor preparation. We found that dorsal premotor cortex (PMD), SMA, preSMA, and lateral cerebellum were more active when participants listen to rhythm sequences compared to random sequences. The activity pattern in supplementary motor area (SMA) and preSMA suggested a modulation dependent on sequence predictability, strongly suggesting a role in temporal sensory prediction. Secondly, we hypothesized that the more complex the rhythm sequence, the more it would engage short-term memory processes of the prefrontal cortex. We found that the superior prefrontal cortex was more active when listening to METRIC and NON-METRIC compared to ISO. We argue that the complexity of rhythm sequences is an important factor in modulating activity in many of the rhythm areas. However, the difference in complexity of our stimuli should be regarded as continuous.

Human limb-specific and non-limb-specific brain representations during kinesthetic illusory movements of the upper and lower extremities.

Sensing movements of the upper and lower extremities is important in controlling whole-body movements. We have shown that kinesthetic illusory hand movements activate motor areas and right-sided fronto-parietal cortices. We investigated whether illusions for the upper and lower extremities, i.e. right or left hand or foot, activate the somatotopical sections of motor areas, and if an illusion for each limb engages the right-sided cortices. We scanned the brain activity of 19 blindfolded right-handed participants using functional magnetic resonance imaging (fMRI) while they experienced an illusion for each limb elicited by vibrating its tendon at 110 Hz (ILLUSION). As a control, we applied identical stimuli to the skin over a nearby bone, which does not elicit illusions (VIBRATION). The illusory movement (ILLUSION vs. VIBRATION) of each immobile limb activated limb-specific sections of the contralateral motor cortex (along with somatosensory area 3a), dorsal premotor cortex (PMD), supplementary motor area (SMA), cingulate motor area (CMA), and the ipsilateral cerebellum, which normally participate in execution of movements of the corresponding limb. We found complex non-limb-specific representations in rostral parts of the bilateral SMA and CMA, and illusions for all limbs consistently engaged concentrated regions in right-sided fronto-parietal cortices and basal ganglia. This study demonstrated complete sets of brain representations related to kinesthetic processing of single-joint movements of the four human extremities. The kinesthetic function of motor areas suggests their importance in somatic perception of limb movement, and the non-limb-specific representations indicate high-order kinesthetic processing related to human somatic perception of one's own body.

Sensory processing during kinesthetic aftereffect following illusory hand movement elicited by tendon vibration.

We investigated how the human sensory-motor system elicits a somatosensory aftereffect. Tendon vibration of a limb excites the muscle spindle afferents that contribute to eliciting illusory movements of the limb. After the cessation of vibration, a transient sensation in which the vibrated limb returns towards its original position (kinesthetic aftereffect) is often experienced, even in the absence of the afferent inputs recruited by the vibration. We vibrated the tendon of either the right wrist extensor or flexor muscle that elicited an illusory flexion or extension movement, which was followed by its corresponding extension or flexion aftereffect. First, we psychophysically investigated how the preceding illusory movement affects the aftereffect. Second, we examined the cortico-spinal excitability during the aftereffect to evaluate its changes from the time during the illusion. We measured the amplitude of the motor-evoked potential that is evoked by a single-pulse transcranial magnetic stimulation to the hand section of the contralateral motor cortex (M1). All 19 subjects experienced the aftereffect, and the amount of aftereffect was approximately 70% of the preceding illusion. During the illusion, the cortico-spinal excitability increased more in non-vibrated than in vibrated muscle, so as to reflect the illusory directions. During the aftereffect, the excitability was significantly reduced only in the non-vibrated muscle, with no change in the vibrated muscle, which, in turn, caused an opposite pattern in the unbalanced excitability between the two muscles, and the degree of unbalanced excitability was correlated with the sensation of aftereffect. The kinesthetic aftereffect seems to be elicited by a sensory process that is determined by the preceding illusory movements. Motor-cortical processing of the unbalanced sensory information from the stimulated and non-stimulated muscles may contribute to the elicitation of kinesthetic aftereffect.

Multisensory activation of the intraparietal area when classifying grating orientation: a functional magnetic resonance imaging study.

Humans can judge grating orientation by touch. Previous studies indicate that the extrastriate cortex is involved in tactile orientation judgments, suggesting that this area is related to visual imagery. However, it has been unclear which neural mechanisms are crucial for the tactile processing of orientation, because visual imagery is not always required for tactile spatial tasks. We expect that such neural mechanisms involve multisensory areas, because our perception of space is highly integrated across modalities. The current study uses functional magnetic resonance imaging during the classification of grating orientations to evaluate the neural substrates responsible for the multisensory spatial processing of orientation. We hypothesized that a region within the intraparietal sulcus (IPS) would be engaged in orientation processing, regardless of the sensory modality. Sixteen human subjects classified the orientations of passively touched gratings and performed two control tasks with both the right and left hands. Tactile orientation classification activated regions around the right postcentral sulcus and IPS, regardless of the hand used, when contrasted with roughness classification of the same stimuli. Right-lateralized activation was confirmed in these regions by evaluating the hemispheric effects of tactile spatial processing with both hands. In contrast, visual orientation classification activated the left middle occipital gyrus when contrasted with color classification of the same stimuli. Furthermore, visual orientation classification activated a part of the right IPS that was also activated by the tactile orientation task. Thus, we suggest that a part of the right IPS is engaged in the multisensory spatial processing of grating orientation.

Dominance of gait cycle duration in casual walking.

In gait research, casual walking has been considered to be walking at a casual speed. However, it is unclear that walking speed is the most stable factor in casual walking compared to other factors such as cycle duration and stride length. Although walking speed can be calculated from cycle duration and stride length, it is not necessarily the case that these parameters are "stable" in the same manner. We therefore conducted an experiment to determine which of these three parameters is most stable, regarding walking speed as cycle speed and using the coefficient of variation across gait cycles as index of stability. Ten participants were invited to walk in their own casual manner, once a week for a period of four weeks. Cycle duration was measured by means of a foot switch attached to the right heel. To measure the moving distance, participants towed a distance meter. Stride length and cycle speed were measured using this device. Over the four-week period, cycle duration and stride length were stable, whereas cycle speed was the most variable parameter. Furthermore, in the results for each single day, the cycle duration was significantly more stable than the other parameters. These results suggest that, when we walk casually, cycle duration is the dominant factor, rather than stride length or walking speed.

Neural substrate of body size: illusory feeling of shrinking of the waist.

The perception of the size and shape of one's body (body image) is a fundamental aspect of how we experience ourselves. We studied the neural correlates underlying perceived changes in the relative size of body parts by using a perceptual illusion in which participants felt that their waist was shrinking. We scanned the brains of the participants using functional magnetic resonance imaging. We found that activity in the cortices lining the left postcentral sulcus and the anterior part of the intraparietal sulcus reflected the illusion of waist shrinking, and that this activity was correlated with the reported degree of shrinking. These results suggest that the perceived changes in the size and shape of body parts are mediated by hierarchically higher-order somatosensory areas in the parietal cortex. Based on this finding we suggest that relative size of body parts is computed by the integration of more elementary somatic signals from different body segments.

Tactile estimation of the roughness of gratings yields a graded response in the human brain: an fMRI study.

Human subjects can tactually estimate the magnitude of surface roughness. Although many psychophysical and neurophysiological experiments have elucidated the peripheral neural mechanisms that underlie tactile roughness estimation, the associated cortical mechanisms are not well understood. To identify the brain regions responsible for the tactile estimation of surface roughness, we used functional magnetic resonance imaging (fMRI). We utilized a combination of categorical (subtraction) and parametric factorial approaches wherein roughness was varied during both the task and its control. Fourteen human subjects performed a tactile roughness-estimation task and received the identical tactile stimulation without estimation (no-estimation task). The bilateral parietal operculum (PO), insula and right lateral prefrontal cortex showed roughness-related activation. The bilateral PO and insula showed activation during the no-estimation task, and hence might represent the sensory-based processing during roughness estimation. By contrast, the right prefrontal cortex is more related to the cognitive processing, as there was activation during the estimation task compared with the no-estimation task, but little activation was observed during the no-estimation task in comparison with rest. The lateral prefrontal area might play an important cognitive role in tactile estimation of surface roughness, whereas the PO and insula might be involved in the sensory processing that is important for estimating surface roughness.