Functional magnetic resonance imaging investigation of brain regions associated with astringency.

Previous studies have investigated mechanisms of the perception of the five basic tastes at the peripheral and neural levels. However, little is known regarding the specific mechanisms and brain activity associated with the perception of astringency. In the present study, we aimed to clarify these mechanisms using functional magnetic resonance imaging (fMRI) in conjunction with taste stimuli, and to investigate the association between subjective appraisal of taste and brain activity. Brain activation to astringency was observed in the insula, superior orbitofrontal cortex, cingulate cortex, and frontal inferior triangularis. In addition, the right ventral anterior insula, which is part of the primary gustatory cortex, showed the strongest blood oxygen level-dependent (BOLD) response to astringent stimuli. Brain activation to bitter and sweet taste was observed in the insula. Each of the three tastes activated a different region of the insula. Also, a subregion in the right anterior insula responded to both astringent and bitter stimuli. Moreover, we observed relationships between the BOLD responsivity during astringent, sweet, and bitter stimuli and the participant's drinking habits regarding representative beverages of each taste. These results indicate a potential correlation between lifestyle and brain activity with regard to taste perception.

fMRI measurement of the integrative effects of visual and chemical senses stimuli in humans.

The purpose of this study was to evaluate the integrative effects of visual stimuli with chemical senses (olfactory and gustatory) stimuli in humans. Noninvasive measurement tools such as magnetoencephalography (MEG) and functional magnetic resonance imaging (fMRI) are used to describe the mechanism of olfactory information processing in the human brain, and the neurophysiological properties of olfactory-related neurons are described. The first study analyzed the interaction between visual and olfactory stimuli. Two odors (lemon and iso-valeric acid) were selected as pleasant and unpleasant odors, respectively and pleasant and unpleasant images were also selected. These cross-modal stimulus combinations were presented to the subject at random, and responses were measured by fMRI using an event related task. These results revealed that active brain areas with pleasant/unpleasant stimuli and matched/mismatched stimuli were different for memory and cognition. The second study analyzed the interaction between visual and gustatory stimuli. Total four conditions (hunger-not hunger, and intake-not intake of monosodium glutamate (MSG)) were tested. Visual stimuli were food-related and nonfood-related photos. A visual analog scale (VAS) was also used to evaluate before and at regular time intervals after intake of MSG, and responses were measured using fMRI. Brain activity related to feeding desire after intake of MSG occurred near the insula cortex, and orbito-frontal cortex, among other areas. These results on the integrative effects of visual stimuli with olfactory and gustatory stimuli, cross-modal and complex effects on olfaction and gustation were suggested to be obtained as an emotional response such as "pleasantness/unpleasantness" and as cognitive and memory responses such as "matching/mismatching" or the responses such as "feeding desire" afterwards intake of foods.

Cortical representation of taste-modifying action of miracle fruit in humans.

Red berries of a tropical plant called miracle fruit, Richadella dulcifica, reduce the sour and aversive taste of acids and add sweet and palatable taste. To elucidate the brain mechanism of this unique action of miracle fruit, we recorded taste-elicited magnetic fields of the human cerebral cortex. The initial taste responses were localized in the fronto-parietal opercular/insular cortex reported as the primary taste area. The mean latency of the response to citric acid after chewing miracle fruit was essentially the same as that for sucrose and was 250-300 ms longer than that for citric acid. Since it is known that stimulation with acids after the action of miracle fruit induces both sweetness and sourness responses in the primate taste nerves, the present results suggest that the sourness component of citric acid is greatly diminished at the level of subcortical relays, and mostly sweetness information reaches the cortical primary taste area. We propose the idea that the qualitative aspect of taste is processed in the primary taste area and the affective aspect is represented by the pattern of activation among the different cortical areas.

Reactivation of physical motor information in the memory of action events.

When attempting to memorize action sentences (e.g., open an umbrella), performing the action of the sentence (enacted encoding) results in better memory performance than simply memorizing the sentences (verbal encoding). This memory enhancement is called the enactment effect. Magnetoencephalography (MEG) was used to elucidate whether the enactment effect is due to physical motor information or whether movement representation is the critical factor in the enactment effect. Physical motor information, which is implicated in the primary motor cortex, represents the speed, form, and kinematic sense of a movement, while movement representation indicates semantic and conceptual information such as movement formulae, movement ideas, and movement imagery, which are especially associated with the parietal cortex. We measured activities within the motor region and parietal cortex during a recognition test and compared activities during recognition with enacted and verbal encoding condition. The results showed that recognition performance was better for enacted encoding. The MEG data indicated that the left primary motor cortex with enacted encoding condition was activated in all subjects, though with verbal encoding condition, this activation appeared in only one subject. These activities were observed between 150 and 250 ms after recognition stimuli onset and were transmitted into the left parietal cortex. Moreover, activities in the right parietal cortex following enacted encoding were greater than those following verbal encoding, and the activities appeared 600-700 ms after onset of the recognition stimuli. These results suggest that the enactment effect occurs by the reactivation of the physical motor information and that this information facilitates activities related to movement representation.

Intelligibility of bone-conducted ultrasonic speech.

Ultrasound can be perceived through bone conduction by the profoundly deaf as well as by normal-hearing subjects. Moreover, speech signals modulated onto ultrasound can be detected through bone conduction. This study explored how well listeners can understand ultrasonic speech and the confusion patterns to evaluate and improve bone-conducted ultrasonic hearing. The intelligibility of Japanese words classified by familiarity and Japanese monosyllables with bone-conducted ultrasound was investigated. Results showed that the intelligibility of familiar words was higher than that of unfamiliar words. Further, the results of a monosyllable intelligibility test with bone-conducted ultrasound and those of a test with air-conducted sound showed a similar pattern of speech recognition with regard to the errors made. The relationship between speech intelligibility and sound level showed that the increase in the intelligibility of bone-conducted ultrasonic speech did not exceed the increase in the intelligibility of air-conducted speech as the sound level rose.

Auditory evoked fields to variations of interaural time delay.

Auditory motion can be simulated by presenting binaural sounds with time-varying interaural time delays. Human cortical responses to the rate of auditory motion were studied by recording auditory evoked magnetic fields with a 122-channel whole-head magnetometer. Auditory motion from central to right and then to central was produced by varying interaural time differences between ears. The results showed that the N1m latencies and amplitudes were not affected by the fluctuation of interaural time delay; however, the peak amplitude of P2m significantly increased as a function of fluctuation of the interaural time delay.

Nonlinear explanation for bone-conducted ultrasonic hearing.

Human listeners can perceive speech from a voice-modulated ultrasonic carrier presented via a bone-conduction stimulator. This study explored the psychoacoustic characteristics and underlying mechanisms of ultrasonic hearing by measuring difference limens for frequency (DLF) for pure tones modulated onto ultrasonic carriers. Human subjects were presented with two pulsed tones and asked to judge whether the first or the second had the higher pitch. When amplitude modulation was based on a double side-band transmitted carrier, the DLFs were as small as those from the air-conducted pure tones at 0.25-4 kHz. Ultrasounds yielded larger DLFs for tones with low (0.125 kHz) and high (6-8 kHz) frequencies. Results were essentially identical between the two types of carriers, sine wave (30 kHz) and bandpass noise (30+/-4 kHz), despite the different bandwidths in the ultrasonic range. When amplitude modulation was based on a double side-band suppressed carrier, DLFs corresponded to those from tones with double frequencies. These results suggest nonlinear conduction that demodulates audible signals from ultrasounds and provides inputs to the cochlea.

Auditory evoked magnetic fields in relation to iterated rippled noise.

Auditory evoked magnetic fields in relation to iterated rippled noise (IRN) were examined by magnetoencephalography (MEG). IRN was used as the sound stimulus to control the peak amplitude of the autocorrelation function of the sound. The IRN was produced by a delay-and-add algorithm applied to bandpass noise that was filtered using fourth-order Butterworth filters between 400-2200 Hz. All sound signals had the same sound pressure level. The stimulus duration was 0.5 s, with rise and fall ramps of 10 ms. Ten normal-hearing subjects took part in the study. Auditory evoked fields were recorded using a 122 channel whole-head magnetometer in a magnetically shielded room. The results showed that the peak amplitude of N1m, which was found above the left and right temporal lobes around 100 ms after the stimulus onset, increased with increase in the number of iterations of the IRN. The latency and estimated equivalent current dipole (ECD) locations of N1m did not show any systematic variation as a function of the number of iterations.

Auditory evoked magnetic fields in relation to bandwidth variations of bandpass noise.

Auditory evoked magnetic fields in relation to the bandwidth of bandpass noise were examined by magnetoencephalography (MEG). Pure tone and bandpass noises with center frequencies of 500, 1000 or 2000 Hz were used as the auditory signals. All source signals had the sound pressure level set at 74 dB. The stimulus duration was 0.5 s, with rise and fall ramps of 10 ms. Eight volunteers with normal hearing took part in the study. Auditory evoked fields were recorded using a neuromagnetometer in a magnetically-shielded room. The results showed that the peak amplitude of N1m, which was found above the left and right temporal lobes around 100 ms after the stimulus onset, decreased with increasing bandwidth of the bandpass noise. The latency and estimated equivalent current dipole (ECD) locations of N1m did not show any systematic variation as a function of the bandwidth for any of the center frequencies.

The effect of 1/f fluctuation in inter-stimulus intervals on auditory evoked mismatch field.

This study focused on the effect of regularity of environmental stimuli on the informational order extracting function of human brain. The regularity of environmental stimuli can be described with the exponent n of the fluctuation 1/f(n). We studied the effect of the exponent of the fluctuation in the inter-stimulus interval (ISI) on the elicitation of auditory evoked mismatch fields (MMF) with two sounds with alternating frequency. ISI times were given by three types of fluctuation, 1/f(0), 1/f(1), 1/f(2), and with a fixed interval (1/f(infinity)). The root mean square (RMS) value of the MMF increased significantly (F(3/9)=4.95, p=0.027) with increases in the exponent of the fluctuation. Increments in the regularity of the fluctuation provoked enhancement of the MMF, which reflected the production of a memory trace, based on the anticipation of the stimulus timing. The gradient of the curve, indicating the ratio of increments between the MMF and the exponent of fluctuation, can express a subject's capability to extract regularity from fluctuating stimuli.

Auditory evoked magnetic fields in relation to interaural cross-correlation of band-pass noise.

Auditory evoked magnetic fields of the human brain were analyzed in relation to the magnitude of the inter-aural cross-correlation (IACC). IACC of the stimuli was controlled by mixing diotic bandpass and dichotic independent bandpass noise in appropriate ratios. The auditory stimuli were binaurally delivered through plastic tubes and earpieces inserted into ear canals of the nine volunteers with normal hearing who took part in this study. All source signals had the same sound pressure level. Auditory evoked fields (AEFs) were recorded using a neuromagnetometer in a magnetically shielded room. Combinations of a reference stimulus (IACC=1.0) and test stimuli (IACC=0.2, 0.6, 0.85) were presented alternately at a constant interstimulus interval of 0.5 s and MEGs recorded. The results showed that the N1m latencies were not affected by IACC; however, the peak amplitude of N1m significantly decreased with increasing IACC.

Effect of masker frequency on N1m amplitude in forward masking.

The effect of frequency on N1m has been investigated by various methods. However, it has not yet been measured using forward masking. In this study, the frequency specificity of N1m was investigated using forward masking. Although the masker frequency had some influence on N1m amplitudes, the results suggested that the frequency specificity of N1m was worse than that of a single-neuron or psychological tuning curve. This is probably because N1m includes various components, both frequency-specific and non-specific, some of which may be less affected by masking. Thus, our results agree with those of previous studies using intervening tones that suggested widespread neural representation in the auditory cortex.

Hemispheric processing of duration changes in speech and non-speech sounds.

Sound duration conveys phonemic information in some languages. The present study, using magnetoencephalography (MEG), examined whether the hemispheric activation associated with the processing of duration is different between speech and non-speech sounds in subjects whose native language uses duration as a phonemic cue. The magnetic mismatch negativity (MMNm) response was recorded for equal-duration decrements in vowel, sinusoidal, and spectrally rich complex sounds. Although the MMNm responses to duration changes were predominant in the right hemisphere, the distribution of this response for the vowel stimuli was significantly displaced leftward compared with that for the other two types of stimuli. The results suggest that the hemispheric distribution of the MMNm response to duration change depends on the linguistic relevance of the change.

Effect of a forward masker on the N1m amplitude: varying the signal delay.

Auditory sensation is affected by a forward masker, and this phenomenon has been demonstrated in a neural adaptation model and a temporal window (integration) model. To study forward masking in the central auditory system, the growth of the N1m amplitude was measured by varying the signal delay. In the adaptation model, the masking increases as the signal delay decreases. However, in our results, the minimum N1m amplitude was observed at a signal delay of 40 ms. As the signal delay decreased from 40 ms, the N1m amplitude increased although the masking increased. Our results suggest that the growth of the N1m amplitude largely depends on temporal integration at signal delays below 40 ms.

Magnetoencephalographic study of cortical activity evoked by electrogustatory stimuli.

Electrogustometry is a convenient method to examine taste acuity in clinical situations. Some basic properties of neural activity in the cerebral cortex in response to electrogustatory stimulation were revealed by measuring magnetoencephalography (MEG) signals with a whole-cortex-type system in response to varying intensities of anodal DC currents focally applied to the tongue surface in human subjects. Independent component analysis was used to eliminate stimulus artifacts in MEG signals. Electrogustatory stimulation with intensities of induced electric taste evoked responses bilaterally, mainly in the opercular-insular cortex with a mean onset latency of approximately 350 ms, while subthreshold electrogustatory stimulation induced modest responses in the cortex. Stronger stimulation induced a tingling sensation and elicited large transient responses in both the opercular-insular and somatic sensory cortices. This is the first description of the basic properties of human MEG responses to electrogustatory stimulation.

Magnetic cortical responses evoked by visual linear forward acceleration.

Cortical site processing the information of whole body linear acceleration has not yet been identified. In this study, neuromagnetic responses to visually induced linear forward acceleration were recorded in six healthy-right-handed adult subjects using a 122-channel whole cortex neuromagnetometer. Significant activation was estimated in the cortex around the posterior insula, which belongs to the vestibular cortex. Hence, it is suggested that the vestibular cortex not only receives vestibular input from the peripheral vestibular apparatus, but also processes the vestibular sensation from multi-modal information.

MEG responses during rhythmic finger tapping in humans to phasic stimulation and their interpretation based on neural mechanisms.

The phase-resetting experiment was applied to human periodic finger tapping to understand how its rhythm is controlled by the internal neural clock that is assumed to exist. In the experiment, the right periodic tapping movement was disturbed transiently by a series of left finger taps in response to impulsive auditory cues presented randomly at various phases within the tapping cycle. After each left finger tap, the original periodic tapping was reestablished within several tapping cycles. Influences of the disturbance on the periodic right finger tapping varied depending on the phase of the periodic right finger tapping at which each left finger tap was made. It was confirmed that the periodic tapping was disturbed not by the auditory cues but by the left finger taps. Based on this fact, in this paper each single left tap was considered as the stimulus, and the phase of the periodic tapping of the right index finger when the left tap was executed as the phase of the stimulus. Responses of the neural activities (magnetoencephalography, MEG), the tapping movement, and the corresponding muscle activities (electromyography) were simultaneously measured. Phase-resetting curves (PRCs) representing the degree of phase reset as a function of the phase of the stimulus were obtained both for the left sensorimotor cortex MEG response and for the right index finger tapping response. The shapes of both PRCs were similar, suggesting that the phase reset of the left sensorimotor cortex activities and that of the finger tapping rhythm were the same. Four out of eight subjects showed type-0 reset in Winfree's definition, and the others showed type-1 reset. For general limit-cycle oscillators, type-0 reset is obtained for relatively strong perturbations and type 1 for weak perturbations. It was shown that the transient response of MEG to the single left tap stimuli in type-0 subjects, where the phase was progressively reset, were different from those in type-1 subjects. Based on detailed analysis of the differences, a neural network model for the phase reset of the tapping rhythm is proposed.

Effect of stimulus duration for bone-conducted ultrasound on N1m in man.

Ultrasound can be heard by bone conduction in man. However, there has been no consensus about the perception mechanism of bone-conducted ultrasound (BCU). In the current study, to clarify the central auditory system of BCU, the effects of stimulus duration for 30 kHz BCU on N1m were compared with those for air-conducted 1 kHz tone bursts by magnetoencephalography. As a result, the growth of N1m amplitude for both stimuli saturated at the duration of 40 ms, which suggest that the temporal integration system of BCU is similar to that of audible sound. However, significant differences in the growth were observed below the saturation points. The results indicate a possibility that there are some differences in the central auditory system between BCU and audible sound.