ORIGINAL ARTICLE
EEG Gamma Band Is Asymmetrically Activated by Location and Shape Memory Tasks in Humans Kenji NUMATA1, Yoshio NAKAJIMA2, Tadahiko SHIBATA2 and Shinobu SHIMIZU3 1
Department of Physical Therapeutics, College of Medical Sciences, Showa University, 1865 Tokaichiba-machi, Midori-ku, Yokohama 226-8555, Japan 2 Department of Physiology, School of Medicine Chiba University, 1–8–1 Inohana, Chuo-ku, Chiba 260-0856, Japan 3 Faculty of Rehabilitation, School of Allied Health Sciences, Kitasato University, 1–15–1 Kitasato, Sagamihara, Kanagawa 228-8555, Japan
Abstract. From the viewpoint of psychology, it is thought that perception analysis of the visual world includes two information processes: global (whole) and local (part) processes. It is assumed that the global process is carried out in the right hemisphere, and the local process, in the left hemisphere. In the present study, gamma EEG band activities during location memory (LM) task, as a global form, and shape memory (SM) task, as a local form, were calculated from the temporal, parietal and occipital areas using stimuli consisting of categorical patterns of small shapes. Gamma band activity during the SM task was greater than that during the LM task. It was assumed that the SM task requires a higher memory load condition than the LM task. In terms of the laterality ratio obtained from the whole electrode array, the gamma band was significantly activated in the right hemisphere during the LM task, and in the left hemisphere during the SM task. The gamma activation in the occipital area was significantly high in the right hemisphere for both tasks. High gamma band activation was observed in the right parietal area during the LM task and in the left temporal area during the SM task. It was concluded that global and local information processes occur in the left temporal areas and in the right occipitoparietal areas, respectively. The results of this study are useful in the assessment of visual cognition deficits in patients with cerebral hemispheric lesions in the physical therapy. Key words: gamma band EEG activity, location memory, shape memory, visual information process, human (J Jpn Phys Ther Assoc 5: 1–5, 2002)
It is well known in primates that the visual information
process is carried out via two cortical pathways: the occipitoparietal pathway, related to visuospatial recognition, and the occipitotemporal pathway, related to object recognition1-3). The presence of the two pathways has also been suggested in humans in neuropsychological studies of patients4)5) and by positron emission tomography (PET) in normal subjects6-8). Furthermore, it is known that brain function is related to attention, cognition and memory of visual spaces and objects are asymmetrically located in the left and right hemispheres9-11). Received: June 29, 2001 Accepted: January 12, 2002 Correspondence to: Kenji Numata, Department of Physical Therapeutics, College of Medical Sciences, Showa University, 1865 Tokaichiba-machi, Midori-ku, Yokohama 226-8555, Japan
In the field of neuropsychology, Navon et al.12) and Shulman et al. 13) pointed out that cognition of visual subjects is accomplished by two information process systems: global and local processes. In other words, a visual scene consists of global and local forms, and humans analyze its characteristics by sorting them into to global and local information process systems. Based on behavioral studies, it is assumed that the global process is localized in the right hemisphere, and the local process, in the left hemisphere14-17). Fink et al.18) employed PET to examine brain activity during character cognitive task using hierarchical stimuli consisting of large characters (as a global form), and small characters (as a local form). They showed that the cognitive task of the global form activated the right prestriate cortex, while the cognitive task of the local form activated the left inferior occipital cortex. Their
Numata, et al.
2
results seem to support the general concept of functional asymmetry of the human brain. However, Fink et al.10) recently reported contradictory results using hierarchical stimuli consisting of figures rather than characters. Thus, the functional localization of the two information process systems is not yet neurophysiologically understood. Gamma band EEG activity (usually in the 30 to 60 Hz band range) is proposed to be the frequency band which reflects cortical activity related to cognitive processes19)20) as well as short-term memory in visual discrimination task21). Numerous reports have shown that gamma band activity is a useful tool to study functional activities of the hemispheres in visual information processes. More detailed information is needed for evaluation for visual recognition in patients with cerebral vascular accident on physical therapy. In the present study, therefore, gamma band activities during location memory (LM) task, as a global form, and shape memory (SM) task, as a local form, were calculated from EEGs recorded from temporal, parietal and occipital areas. For the LM and SM tasks, stimuli consisting of categorized patterns of small shapes were used to reveal functional localization of global and local memory processes in the hemispheres.
Methods Subjects Eight right-handed college students (6 males and 2 females, 19 to 27 years old) participated in the present study. All were free of neurological disorders.
Visual memory tasks Figure 1 shows the flow chart of stimulation used for LM and SM tasks. One stimulation epoch was initiated by displaying a fixation point (cross) at the center of CRT at random intervals between 3,000 and 5,000 ms. Five types of figures (circle, square, upward-and downward-triangles and cross) were displayed for 60 ms. Immediately after the onset of the fixation point display, three out of the five types of figures (as a memory display), and at 2,000 ms after the onset, one out of the five types of figures (as a recall display) were presented at the center of an imaginary matrix of 3 × 3 squares on the CRT display. The center of the innermost square contained the fixation point, while the figures for memory or recall displays were presented at the center of one of the 8 outer squares (visual angle of each square: 0.9° and 1.3°). The locations for memory display were randomly arranged by a computer software. During the experiments, the subjects were instructed to gaze at the fixation point. In the LM task, the subjects were asked to memorize the position of the displayed figure (S1) (disregarding its shape) and to discriminate whether the location of the recall figure (S2) matches that of S1. In the SM task, they were asked to memorize the shape of the display figure (S1) (disregarding its location) and to discriminate whether the shape of the recall figure (S2) matches that of S1. The above two tasks were arranged so that each task consisted of 2 blocks, and each block consisted of 30 epochs. Four blocks in the 2 tasks were pseudorandomly carried out so that the frequency of correct answers became 30% for each task. After showing a recall display, the subjects were instructed to quickly press a button with their right index finger if S2 matched S1 in either task.
Fig. 1. Schematic illustration of the events in the location memory (LM) and shape memory (SM) tasks. In both tasks, subjects were asked to fixate a cross in centre of the CRT, and then memory and recall displays appeared for 60 ms respectively.
ASYMMETRICAL ACTIVATION IN HUMAN BRAIN
EEG recording and quantification The EEG data was recorded from 21 electrodes affixed to the scalp according to the international 10–20 system with reference to linked ears. EEG data of 5,000 ms duration [500 ms before and 4,500 ms after onset of memory display (S1)] were amplified with a band pass filter of 0.05–100 Hz and digitized at a sampling rate of 200 Hz. The EOG was simultaneously recorded to facilitate rejection of EEG data contaminated by eye movements. Electrode resistance was kept at less than 5Ω. The Fast Fourier Transform (FFT) with a Humming window was used to calculate power density (µV2/Hz) of the gamma frequency band at P3 (left parietal), P4 (right parietal), T3 (left temporal), T4 (right temporal), O1 (left occipital), and O2 (right occipital) areas. The evolutionary spectrum in each epoch was calculated using a time window of 320 ms, which was shifted by steps of 125 ms from the onset of the memory display to 2,000 ms afterward. The evolutionary spectrum was also calculated from EEG of 500 ms duration before the onset of the memory display (S1) as a baseline power. Gamma band activity was defined as the mean power of 40.6, 43.7 and 46.9 Hz. The mean powers at 17 latency steps during the 2,000 ms period from the onset of the memory display were subtracted from the baseline power. Statistics The baseline-corrected values were analyzed using the three-way ANOVA (task × electrode × 17 latency steps). The laterality ratios of the gamma band activity were also calculated at the temporal (T3 -T 4 ), parietal (P3 -P4 ) and occipital areas (O 1 -O 2 ) using (R-L)/(R+L) [27] and analyzed by the two-way ANOVA (task × area). A positive laterality ratio indicates a shift from the left to the right hemisphere and a negative ratio, a shift from the right to the left hemisphere. In addition, inter-group comparison was performed by the paired t-test.
3
Results Three-way ANOVA (task × electrode × latency step) revealed two main effects. First, the effect of task [F(1,4692) = 4.10, p = 0.0428] was revealed due to the fact that the gamma activity of the SM task (2.08 × 10-4 µV2/Hz) was higher than that of the LM task (1.76 × 10-4 µV2/Hz). Secondly, the main effect was found due to the electrode [F (5,4692) = 18.77, p
EEG Gamma Band Is Asymmetrically Activated by Location and Shape Memory Tasks in Humans Kenji NUMATA1, Yoshio NAKAJIMA2, Tadahiko SHIBATA2 and Shinobu SHIMIZU3 1
Department of Physical Therapeutics, College of Medical Sciences, Showa University, 1865 Tokaichiba-machi, Midori-ku, Yokohama 226-8555, Japan 2 Department of Physiology, School of Medicine Chiba University, 1–8–1 Inohana, Chuo-ku, Chiba 260-0856, Japan 3 Faculty of Rehabilitation, School of Allied Health Sciences, Kitasato University, 1–15–1 Kitasato, Sagamihara, Kanagawa 228-8555, Japan
Abstract. From the viewpoint of psychology, it is thought that perception analysis of the visual world includes two information processes: global (whole) and local (part) processes. It is assumed that the global process is carried out in the right hemisphere, and the local process, in the left hemisphere. In the present study, gamma EEG band activities during location memory (LM) task, as a global form, and shape memory (SM) task, as a local form, were calculated from the temporal, parietal and occipital areas using stimuli consisting of categorical patterns of small shapes. Gamma band activity during the SM task was greater than that during the LM task. It was assumed that the SM task requires a higher memory load condition than the LM task. In terms of the laterality ratio obtained from the whole electrode array, the gamma band was significantly activated in the right hemisphere during the LM task, and in the left hemisphere during the SM task. The gamma activation in the occipital area was significantly high in the right hemisphere for both tasks. High gamma band activation was observed in the right parietal area during the LM task and in the left temporal area during the SM task. It was concluded that global and local information processes occur in the left temporal areas and in the right occipitoparietal areas, respectively. The results of this study are useful in the assessment of visual cognition deficits in patients with cerebral hemispheric lesions in the physical therapy. Key words: gamma band EEG activity, location memory, shape memory, visual information process, human (J Jpn Phys Ther Assoc 5: 1–5, 2002)
It is well known in primates that the visual information
process is carried out via two cortical pathways: the occipitoparietal pathway, related to visuospatial recognition, and the occipitotemporal pathway, related to object recognition1-3). The presence of the two pathways has also been suggested in humans in neuropsychological studies of patients4)5) and by positron emission tomography (PET) in normal subjects6-8). Furthermore, it is known that brain function is related to attention, cognition and memory of visual spaces and objects are asymmetrically located in the left and right hemispheres9-11). Received: June 29, 2001 Accepted: January 12, 2002 Correspondence to: Kenji Numata, Department of Physical Therapeutics, College of Medical Sciences, Showa University, 1865 Tokaichiba-machi, Midori-ku, Yokohama 226-8555, Japan
In the field of neuropsychology, Navon et al.12) and Shulman et al. 13) pointed out that cognition of visual subjects is accomplished by two information process systems: global and local processes. In other words, a visual scene consists of global and local forms, and humans analyze its characteristics by sorting them into to global and local information process systems. Based on behavioral studies, it is assumed that the global process is localized in the right hemisphere, and the local process, in the left hemisphere14-17). Fink et al.18) employed PET to examine brain activity during character cognitive task using hierarchical stimuli consisting of large characters (as a global form), and small characters (as a local form). They showed that the cognitive task of the global form activated the right prestriate cortex, while the cognitive task of the local form activated the left inferior occipital cortex. Their
Numata, et al.
2
results seem to support the general concept of functional asymmetry of the human brain. However, Fink et al.10) recently reported contradictory results using hierarchical stimuli consisting of figures rather than characters. Thus, the functional localization of the two information process systems is not yet neurophysiologically understood. Gamma band EEG activity (usually in the 30 to 60 Hz band range) is proposed to be the frequency band which reflects cortical activity related to cognitive processes19)20) as well as short-term memory in visual discrimination task21). Numerous reports have shown that gamma band activity is a useful tool to study functional activities of the hemispheres in visual information processes. More detailed information is needed for evaluation for visual recognition in patients with cerebral vascular accident on physical therapy. In the present study, therefore, gamma band activities during location memory (LM) task, as a global form, and shape memory (SM) task, as a local form, were calculated from EEGs recorded from temporal, parietal and occipital areas. For the LM and SM tasks, stimuli consisting of categorized patterns of small shapes were used to reveal functional localization of global and local memory processes in the hemispheres.
Methods Subjects Eight right-handed college students (6 males and 2 females, 19 to 27 years old) participated in the present study. All were free of neurological disorders.
Visual memory tasks Figure 1 shows the flow chart of stimulation used for LM and SM tasks. One stimulation epoch was initiated by displaying a fixation point (cross) at the center of CRT at random intervals between 3,000 and 5,000 ms. Five types of figures (circle, square, upward-and downward-triangles and cross) were displayed for 60 ms. Immediately after the onset of the fixation point display, three out of the five types of figures (as a memory display), and at 2,000 ms after the onset, one out of the five types of figures (as a recall display) were presented at the center of an imaginary matrix of 3 × 3 squares on the CRT display. The center of the innermost square contained the fixation point, while the figures for memory or recall displays were presented at the center of one of the 8 outer squares (visual angle of each square: 0.9° and 1.3°). The locations for memory display were randomly arranged by a computer software. During the experiments, the subjects were instructed to gaze at the fixation point. In the LM task, the subjects were asked to memorize the position of the displayed figure (S1) (disregarding its shape) and to discriminate whether the location of the recall figure (S2) matches that of S1. In the SM task, they were asked to memorize the shape of the display figure (S1) (disregarding its location) and to discriminate whether the shape of the recall figure (S2) matches that of S1. The above two tasks were arranged so that each task consisted of 2 blocks, and each block consisted of 30 epochs. Four blocks in the 2 tasks were pseudorandomly carried out so that the frequency of correct answers became 30% for each task. After showing a recall display, the subjects were instructed to quickly press a button with their right index finger if S2 matched S1 in either task.
Fig. 1. Schematic illustration of the events in the location memory (LM) and shape memory (SM) tasks. In both tasks, subjects were asked to fixate a cross in centre of the CRT, and then memory and recall displays appeared for 60 ms respectively.
ASYMMETRICAL ACTIVATION IN HUMAN BRAIN
EEG recording and quantification The EEG data was recorded from 21 electrodes affixed to the scalp according to the international 10–20 system with reference to linked ears. EEG data of 5,000 ms duration [500 ms before and 4,500 ms after onset of memory display (S1)] were amplified with a band pass filter of 0.05–100 Hz and digitized at a sampling rate of 200 Hz. The EOG was simultaneously recorded to facilitate rejection of EEG data contaminated by eye movements. Electrode resistance was kept at less than 5Ω. The Fast Fourier Transform (FFT) with a Humming window was used to calculate power density (µV2/Hz) of the gamma frequency band at P3 (left parietal), P4 (right parietal), T3 (left temporal), T4 (right temporal), O1 (left occipital), and O2 (right occipital) areas. The evolutionary spectrum in each epoch was calculated using a time window of 320 ms, which was shifted by steps of 125 ms from the onset of the memory display to 2,000 ms afterward. The evolutionary spectrum was also calculated from EEG of 500 ms duration before the onset of the memory display (S1) as a baseline power. Gamma band activity was defined as the mean power of 40.6, 43.7 and 46.9 Hz. The mean powers at 17 latency steps during the 2,000 ms period from the onset of the memory display were subtracted from the baseline power. Statistics The baseline-corrected values were analyzed using the three-way ANOVA (task × electrode × 17 latency steps). The laterality ratios of the gamma band activity were also calculated at the temporal (T3 -T 4 ), parietal (P3 -P4 ) and occipital areas (O 1 -O 2 ) using (R-L)/(R+L) [27] and analyzed by the two-way ANOVA (task × area). A positive laterality ratio indicates a shift from the left to the right hemisphere and a negative ratio, a shift from the right to the left hemisphere. In addition, inter-group comparison was performed by the paired t-test.
3
Results Three-way ANOVA (task × electrode × latency step) revealed two main effects. First, the effect of task [F(1,4692) = 4.10, p = 0.0428] was revealed due to the fact that the gamma activity of the SM task (2.08 × 10-4 µV2/Hz) was higher than that of the LM task (1.76 × 10-4 µV2/Hz). Secondly, the main effect was found due to the electrode [F (5,4692) = 18.77, p
