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Material perception is an essential part of our cognitive function that enables us to properly interact with our complex daily environment. One important aspect of material perception is its multimodal nature. When we see an object, we generally recognize its haptic properties as well as its visual properties. Consequently, one must examine behavior using real objects that are perceived both visually and haptically to fully understand the characteristics of material perception. As a first step, we examined whether there is any difference in the behavioral responses to different materials in monkeys trained to perform an object grasping task in which they saw and grasped rod-shaped real objects made of various materials. We found that the monkeys’ behavior in the grasping task, which was measured based on the success rate and the pulling force, differed depending on the material category. Monkeys easily and correctly grasped objects of some materials, such as metal and glass, but failed to grasp objects of other materials. In particular, monkeys avoided grasping fur-covered objects. The differences in the behavioral responses to the material categories cannot be explained solely based on the degree of familiarity with the different materials. These results shed light on the organization of multimodal representation of materials, where their biological significance is an important factor. In addition, a monkey that avoided touching real fur-covered objects readily touched images of the same objects presented on a CRT display. This suggests that employing real objects is important when studying behaviors related to material perception.

Research has established that severe stress adversely affects hippocampal memory, and chewing has been suggested to restore impaired cognitive functions in the hippocampus. To address how chewing involves stress-attenuated hippocampal memory process, we measured the long-term potentiation (LTP) of hippocampal slices of adult male rats that had experienced restraint stress, including some rats that were allowed to chew a wooden stick during the stress period and other rats that were not. The three experimental conditions were: 1) restraint stress without chewing (ST), 2) restraint stress with chewing (SC), and 3) no treatment (CT). We prepared hippocampal slices and collected trunk blood from all experimental animals. For rats in the two stressed groups, we collected tissue and blood at one of three post-stress time points: immediately after, 24 h after, or 48 h after exposure to the stressor. We found that the magnitude of LTP in both group ST and SC was significantly attenuated immediately after stress exposure. However, within 24 h after the end of the stress period, LTP had returned to the control level in group SC whereas it remained low in group ST. At the same post-stress time point, we found that facilitation of N-methyl-d-aspartate (NMDA) receptors by bath-applied glycine had less effect on the magnitude of LTP in group SC than on group ST, suggesting that most NMDA receptors had already become functionally restored in group SC by that time. Plasma concentration of adrenocorticotropic hormone was significantly elevated only in group ST immediately after exposure to the stressor, reflecting the involvement of chewing in decreasing subsequent corticosterone secretion. Thus, the present study demonstrates that chewing ameliorates the stress-induced impairment of NMDA receptor-mediated LTP, suggesting chewing as a good strategy to cope with severe stress by suppressing excessive endocrine responses

The involvement of chewing in brain activity in humans has been studied. In our studies using functional magnetic resonance imaging (fMRI) and behavioral techniques, chewing resulted in a bilateral increase in blood oxygenation leveldependent (BOLD) signals in the sensorimotor cortex, supplementary motor area, insula, thalamus, and cerebellum. In addition, in the first three regions, chewing moderately hard gum produced stronger signals than chewing hard gum. However, in the aged group, the BOLD signal increases were smaller in the first three regions and higher in the cerebellum. Only the aged subjects showed significant increases in various association areas to which input activities in the primary sensorimotor cortex, supplementary area, or insula had positive path coefficients. Furthermore, chewing ameliorates the age-related decrease in hippocampal activities during encoding and that in retrieval memory. The findings suggest the involvement of chewing in memory processes.

A systemic effect of dysfunctional mastication has been suggested as a possible epidemiological risk factor for senile dementia. In recent years, we have evaluated the effects on cognitive function deficits in SAMP8 mice. Aged mice with dysfunctional mastication showed significantly reduced learning ability in a water maze test compared with age-matched control mice, whereas there was no difference between control and molarless young adult mice. Immunohistochemical analysis revealed that in the CA1 region of the hippocampus the molarless condition not only enhanced the age-dependent increase in the density and hypertrophy of GFAP-labeled astrocytes, it decreased the density of Fos-positive neurons and Nissl-stained neurons, or the amount of acetylcholine (ACh) release in the hippocampus, in the same manner. There was a similar age-dependent decrease in choline acetyltransferase in the medial septal nucleus. Furthermore, dysfunctional mastication induced an increase in plasma corticosterone levels. The findings suggest that dysfunctional mastication in aged SAMP8 mice causes abnormalities in the hippocampus through stress, leading to deficits in learning and memory.

It has been generally suggested that chewing produces an enhancing effect on ognitive performancerelated aspects of memory by the test battery. Furthermore, recent studies have shown that chewing is associated with activation of various brain regions, including the prefrontal cortex. However, little is known about the relation between cognitive performances affected by chewing and the neuronal ctivity in specified regions in the brain.We therefore examined the effects of chewing on neuronal activities in the brain during a working memory task using fMRI. The subjects chewed gum, without odor and taste components, between continuously performed two- or three-back (n-back) working memory tasks. Chewing increased the BOLD signals in the middle frontal gyrus (Brodmann's areas 9 and 46) in the dorsolateral prefrontal cortex during the n-back tasks. Furthermore, there were more prominent activations in the right premotor cortex, precuneus, thalamus, hippocampus and inferior parietal lobe during the n-back tasks after the chewing trial. These results suggest that chewing may accelerate or recover the process of working memory besides inducing improvement in the arousal level by the chewing motion.

It has been suggested that the BOLD effect contributes to heavily diffusion-weighted (DW) fMRI signal changes. The BOLD effect is usually interpreted as a change in transverse relaxation rate (deltaR2). In this study, deltaR2 during visual stimulation (VS) and hypercapnia (HC) DW fMRI experiments was estimated using a multiple spin-echo EPI acquisitions after motion-probing gradients. deltaR2 showed dependence on b-value during VS, but not during HC. The results suggest that deltaR2 at high b-value may demonstrate a higher sensitivity to neuronal activation than at lower b-values., 18th Scientific Meeting & Exhibition

The contribution of the BOLD effect in heavily diffusion weighted fMRI is still unclear. In this study, a multiple spin-echo echo-planar-imaging sequence after motion probing gradients was used to measure the transverse relaxation rate at different b-values. The results suggest that the contribution of BOLD to DW fMRI signal changes is independent of b-value. It is speculated that the main contribution to heavily diffusion-weighted functional MRI signal is not the BOLD effect., 17th Scientific Meeting & Exhibition

Society for Neuroscience (Washington, DC., USA)

The Kana Pick-out Test is proposed to activate the prefrontal cortex and widely used in Japan as a suitable test for screening persons with mild cognitive impairment. However, there is no evidence demonstrating that this test really activates the prefrontal cortex. In this study, we assessed brain regional activities during the computerized Kana Pick-out Test using an fMRI. This test resulted in significant increase of blood oxygenation level-dependent signals in the prefrontal cortex. Activation of prefrontal cortex by Kana Pick-out Test was greater than that when the story was only read. The results confirm that Kana Pick-out Test really activates the prefrontal cortex., 第３６回 日本磁気共鳴医学会大会

Society for Neuroscience 37th Annual Meeting