中川 誠司

To investigate the characteristics of visual short-term memory in humans, brain magnetic fields evoked during a delayed paired comparison task were recorded using a whole-head neuromagnetometer. The visual stimulus consisted of a circle with different colors in each quadrant. In the memory condition, subjects reacted with the index finger, when the first stimulus (Sample) was identical in color configuration to the second stimulus (Test), and with the middle finger when they differed. For the control condition, the Subjects ignored the Sample, and moved the index or middle finger alternately in response to the Test. Extremely low frequency components of brain magnetic fields were observed 500 ms after the Sample onset in the temporal and/or the occipital region in the memory condition, but not in the control condition, Sources for the low frequency components were localized in the inferior part of the occipital lobe, in the vicinity of the supramarginal gyrus and the angular gyrus, and the inferior frontal gyrus, The results suggest that the activities in the inferior part of the occipital lobe controls the storage process of shortterm visual memory.

N1m is an auditory evoked brain magnetic field with a magnitude of 100 fT order observed over the auditory cortex, 100 ms after the onset of auditory stimuli. The N1m is often used as a landmark of functional localization in the cortex. However, the mechanism of the N1m has not yet been clarified. The N1m peak amplitude and latency are dependent on the specifics of the stimulus; duration, intensity, and sequence of stimuli. In this study, we examined the dependency of the N1m peak amplitude and latency on the stimulus duration and frequency. Trains of 0.2 ms clicks were used for auditory stimuli by changing the number of clicks and the click interval. Auditory brain magnetic responses evoked by the click trains were recorded from seven human adult subjects by a dc superconducting quantum interference device magnetometer. In the results of this study, the N1m amplitudes significantly increased as the stimulus duration increased and the amplitudes leveled when the stimulus duration reached 32 ms. The amplitudes produced by the trains with the same number of clicks showed greater values for 4-ms-interval trains. The N1m latencies significantly decreased as the stimulus duration increased and leveled at 32 ms. It is concluded that all clicks received within 32 ms were integrated and that this integration mechanism is dependent upon the click interval. Increased synchrony of neuronal cells at the cortical level can explain this integration mechanism. (C) 1999 American Institute of Physics. [S0021-8979(99)42908-1].

Auditory evoked magnetic fields were measured to investigate the reaction of the auditory cortex to a train of sounds. Auditory stimuli, 1000-Hz tone bursts (percentage of appearance: 90%), and 2000-Hz tone bursts (percentage of appearance: 10%) were presented sequentially. The stimuli were presented with the stilmulus onset asynchronies (SOAs) within one trial fixed at either 380 ms, 480 ms, 580 ms, 1080 ms, 1580 ms, or 2080 ms. Nlm amplitudes of 2000-Hz tone bursts, 1000-Hz tone bursts 1 position before, 1 position after, 2 positions after, and 3 positions after the 2000-Hz tone bursts, were investigated. As a result, (1) the amplitudes of Nlm of 2000-Hz tone bursts were significantly larger than those of 1000-Hz tone bursts except for SOA of 2080 ms, (2) the amplitudes of Nlm increased as the SOA increased except for 380 ms and 480 ms, (3) the Nlm amplitude ratios of 1000-Hz tone bursts to 2000-Hz tone bursts increased as the SOA increased, (4) no significant effects of serial position on Nlm amplitude of 1000-Hz tone bursts were observed.

To investigate the characteristics of short-term memory in the human brain, magnetoencephalograms (MEGs) evoked by a delayed paired comparison task were recorded. A visual stimulus, a circle with a different color in each quadrant, was presented. In the memory task, subjects were requested to move the index finger when the second stimulus (TEST) was identical to the first stimulus (SAMPLE), and the middle finger when it was not identical. In the control task, the subjects were instructed not to pay attention to the SAMPLE, and to move the index and middle fingers alternately. A slow magnetic activity was observed for between 900 and 1500 ms during the memory task in the occipital region of all subjects. Sources for this slow activity were localized in either the visual cortex and/or the posterior temporal region. The results suggest that these areas may be responsible for retaining the stimulus in visual short-term memory.

In order to investigate the characteristics of human visual short-term memory, evoked brain magnetic fields during a delayed paired comparison task were recorded with a whole-head neuromagnetometer. A visual stimulus was a circle with different colors (red, blue, green, and orange) in each quadrant. The interstimulus interval between the first stimulus (sample; duration 50 ms) and the second stimulus (test; duration 100 ms) was constant at 3.0 s. Two experiments were performed with different tasks; a memory task and a control task. In the memory task, subjects reacted with the index finner, when the test was identical to the sample in color configuration, and with the middle finger when they were not identical. In the control task, the subjects paid no attention to the sample, and moved the index or middle finger alternately in response to the test. Extremely low frequency components of the brain magnetic fields were observed between 900 and 1,500 ms in the occipital region in the memory task but not in the control task. Sources for the low-frequency components were localized in the visual cortex and/or the posterior temporal region. The results suggest that these areas may be engaged in retaining the stimulus in the visual shortterm memory.

In recent years, techniques have been developed for estimating internal electrical sources in the human brain from measurements of magnetic fields (MEG). In this study, we investigate how to estimate spatially distributed sources, since this information is important for analyzing the higher functions of the human brain. We used a 64-channel whole-head-type SQUID system to record MEG activities evoked by visually cognitive tasks. Three kinds of stimuli were presented: English words, nonsense words, and random dot patterns. Spatially distributed internal sources with latencies between 320 and 420 ms were estimated by using the sub-optimal least-squares subspace scanning technique. The estimated sources were located in the frontal region for random-dot stimuli, the left temporal region for nonsense-word stimuli, and the right postero-temporal region in addition to the other regions for English-word stimuli.

Visually evoked magnetoencephalograms (MEGs) associated with a delayed-match paradigm were measured to obtain functional information related to short-term memory processes. A delayed-match paradigm was introduced. The paradigm consists of a four-color-stimuli; a pair of circles with four differently-colored regions. The subjects retain the first image (Sample) until the second image (Test) appears. The MEGs were measured by a 122-channel whole-head-type SQUID system. A DC-like slow wave was observed at the period between latencies of 900 and 1500 msec during the short-term memory task. The source localization of the DC-like slow wave was estimated. The estimated sources were localized either in the occipital or postero-temporal region.

In this study, we investigate an estimation of internal sources with poor information about their source profile, which is important in order to analyze the higher functions of the human brain. We used a 64-channel whole-head-type SQUID system to record MEG activities evoked by visually cognitive tasks. Three kinds of stimuli were presented: English words, nonsense words, and random dot patterns. Spatially distributed internal sources with latencies between 320 and 420 msec were estimated by using a sub-optimal lease-squares subspace scanning technique. From three subjects data, the following results of distributed source estimations were shown: English words stimuli activated large area, the left temporal region, the parietal and right postero-temporal region, nonsense words activated the parertal region, random dot patterns activated the frontal region.

Visually evoked magnetoencephalograms (MEGs) associated with a delayed-match paradigm were measured to obtain functional information related with short-term memory processes. A delayed-match paradigm was introduced. The paradigm consists of four-color stimuli; a pair of circles with four differently colored regions. The MEGs were measured by a 122-channel whole-head-type SQUID system. A DC-like slow wave was observed at the period between latencies of 900 and 1,500 ms during the short-term memory task. The source localization of the DC-like slow wave was estimated. The estimated sources were localized either in the occipital or posterotemporal region.

Visually evoked magnetoencephalograms (MEGs) associated with a delayed-match paradigm were measured to obtain functional information related to shortterm memory processes. A delayed-match paradigm was introduced. The paradigm consists of a four-color-stimuli; a pair of circles with four differently-colored regions. The subjects retain the first image (Sample) until the second image (Test) appears. The MEGs were measured by a 122-channel whole-head-type SQUID system. A DC-like slow wave was observed at the period between latencies of 900 and 1500 msec during the short-term memory task. The source localization of the DC-like slow wave was estimated. The estimated sources were localized either in the occipital or postero-temporal region.

We observed the brain electrical activities associated with verbal cognitive processes by measuring both EEGs and MEGs evoked by visually cognitive tasks. English words, nonsense words and random dots were presented. We discussed the source estimation at a latencies of 150 msec and 360 msec using MEG data. At 150 msec, two equivalent current dipoles are localized in or near the primary visual cortex. The dipole sources for recognition of English words at 360 msec were estimated in both hemispheres, and the dipole moment in the left-hemisphere was larger than the dipole moment of the right hemisphere. However, we could not find sources common to all subjects.