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Word activation and aging: A PET study
ABSTRACTS
Word activation and Aging: A PET Study H. Chertkowl,2,3,, D. Bub 4, S. Murthal, 3, C. Hosein 1, D. Goldl,3,A. Evans 2. Lady Davis Institute for Medical Research 1, McConnell Brain Imaging Centre, McGill Universi~ 2, and Centre de Recherche, Centre Hospitalier Cotes-des-Neiges 3, Montreal, Canada. Dept of Psychology, UniversiU of Victoria, Victoria, British Columbia, Canada 4 When x,e encounter a visually presented word, cognitive studies suggest that we automatically activate representations in the brain for its orthographic form, its meaning, and even its corresponding sound. Previous investigations of word identification and reading have used results from human PET subtractions studies, along with cognitive investigations, to argue that reading involves brain areas in the occipital lobe (predoinantly on the left], the left temporal lobe, and the left inferior frontal lobe. Activation of these areas proceeds automatically, and occurs rapidly during viewing of single words. These mechanisms appear to be preserved with aging. At the same time, cognitive studies suggest that in the elderly, activation of words proceeds less automatically, and (unlike le~:ical access in younger individuals) occupies attentional resources (Nestor et al, 1989; Greenwood and Parasuraman, 1991). While the neural correlate of such decreased automaticity are unknown, we can hypothesize that "nonautomatic" activation would be accompanied by increased blood flow in brain areas associated with focussed attention. The network of areas found to be associated with attentional tasks generally involves (among other structures) the anterior cingulate and the thalamus (Laberge, 1990). We thus hypothesized that passive viewing of words'would be associated (in the elderly but not in younger individuals) with CBF activation of the anterior cingulate and possibly the thalamus. We have carried out a PET study focusing directly on this question. Method Subjects consisted of ten right handed male university students (18-29 years of age) and 10 elderly normal individuals (age 63-82). The stimuli used consisted of a baseline condition of upper case "XXX" stimuli, varying from three to eight letters in length, and an activation condition of Concrete Words (all of the words being basic level names of animals - e.g., FOX, LION, BEAR, etc,). The two conditions were presented in the center of the screen for 1000 ms duration with a 1000 ms interstimulus interval. PET scanning after bolus H2150 intravenous injection was carried out for the separate conditions. The scans were acquired on a Scanditronix PC-2048 tomograph over a 60 second period while the subjects were passively viewing the screen without making any response. Analysis was performed on a simple subtraction between the two stimulus conditions. An exploratory search strategy was employed for all peaks within the gray matter volume of 600 cm 3 or 200 resolution elements (resels). We accepted t-statistic values exceeding critena of t = +/- 3.55 (false positive rate 0.58 in the search space). Results In the young subjects, the only peak of activity noted was in the left precuneus area, Brodmann area 18, where a peak (t statistic level = 5.15) was noted at coordinates (Tailarach and Tourneux, 1988) of -1, -74, +20. In the elderly subjects, again only one area of significant activity was noted, this time occurnng in the anterior cingulate, Brodmann area 24, where a peak [ t-statistic level = 3.58] was noted at coordinates +3,+ 29, + 8. In addition, two negative peaks of decreased blood flow were noted, in the fight thalamus (t = -4.01, coordinates +5, -11, + 11) and deep in the left inferior parietal lobe (t = -3.72, coordinates 44, -47, +48). Conclusions The two conditions were chosen in order to remove activation due to basic visual perceptual processing of letters. Similar to other investigators, we fecund that there was little remaining in terms of significant differences in brain activation between these two conditions, implying that even a series of letters such as "XXX" produces activation of many brain areas associated with processing of words. The activity in the pre-cuneate region in young individuals may relate to engagement of brain areas for memory, encoding of the words (Fletcher et al, 1995). The elderly subjects demonstrated increased CBF flow in the anterior cingulate lobe, which we take to reflect involvement of attentional processes during the processing of words in the eldelry, but not the young subjects. The decreased CBF in the thalamus may in fact reflect involvement of the thalamus, another substrate for attentional processing. Why this thalamic CBF change is in a negative direction is unclear. Our results are consistent with the idea that lexical access is automatic in young individuals, but involves attentional resources in the elderly. References 1 Fletcher,P.C., Frith,C.D., Grasby, P.M. et al, brain,1995, 1 1 8 : 4 0 1 - 416 2 Laberge, D. J. of Cognitive Neuroscience, 1990, 2 : 358-372. 3. Greenwood,P. and Parasuraman, R., Developmental Neuropsychology, 1991, 7: 421-434. 4.Nestor,P., Parasuraman,R., and Haxby,J. Developmental Neuropsychology, 1989, 5: 141-158. $432
Word activation and Aging: A PET Study H. Chertkowl,2,3,, D. Bub 4, S. Murthal, 3, C. Hosein 1, D. Goldl,3,A. Evans 2. Lady Davis Institute for Medical Research 1, McConnell Brain Imaging Centre, McGill Universi~ 2, and Centre de Recherche, Centre Hospitalier Cotes-des-Neiges 3, Montreal, Canada. Dept of Psychology, UniversiU of Victoria, Victoria, British Columbia, Canada 4 When x,e encounter a visually presented word, cognitive studies suggest that we automatically activate representations in the brain for its orthographic form, its meaning, and even its corresponding sound. Previous investigations of word identification and reading have used results from human PET subtractions studies, along with cognitive investigations, to argue that reading involves brain areas in the occipital lobe (predoinantly on the left], the left temporal lobe, and the left inferior frontal lobe. Activation of these areas proceeds automatically, and occurs rapidly during viewing of single words. These mechanisms appear to be preserved with aging. At the same time, cognitive studies suggest that in the elderly, activation of words proceeds less automatically, and (unlike le~:ical access in younger individuals) occupies attentional resources (Nestor et al, 1989; Greenwood and Parasuraman, 1991). While the neural correlate of such decreased automaticity are unknown, we can hypothesize that "nonautomatic" activation would be accompanied by increased blood flow in brain areas associated with focussed attention. The network of areas found to be associated with attentional tasks generally involves (among other structures) the anterior cingulate and the thalamus (Laberge, 1990). We thus hypothesized that passive viewing of words'would be associated (in the elderly but not in younger individuals) with CBF activation of the anterior cingulate and possibly the thalamus. We have carried out a PET study focusing directly on this question. Method Subjects consisted of ten right handed male university students (18-29 years of age) and 10 elderly normal individuals (age 63-82). The stimuli used consisted of a baseline condition of upper case "XXX" stimuli, varying from three to eight letters in length, and an activation condition of Concrete Words (all of the words being basic level names of animals - e.g., FOX, LION, BEAR, etc,). The two conditions were presented in the center of the screen for 1000 ms duration with a 1000 ms interstimulus interval. PET scanning after bolus H2150 intravenous injection was carried out for the separate conditions. The scans were acquired on a Scanditronix PC-2048 tomograph over a 60 second period while the subjects were passively viewing the screen without making any response. Analysis was performed on a simple subtraction between the two stimulus conditions. An exploratory search strategy was employed for all peaks within the gray matter volume of 600 cm 3 or 200 resolution elements (resels). We accepted t-statistic values exceeding critena of t = +/- 3.55 (false positive rate 0.58 in the search space). Results In the young subjects, the only peak of activity noted was in the left precuneus area, Brodmann area 18, where a peak (t statistic level = 5.15) was noted at coordinates (Tailarach and Tourneux, 1988) of -1, -74, +20. In the elderly subjects, again only one area of significant activity was noted, this time occurnng in the anterior cingulate, Brodmann area 24, where a peak [ t-statistic level = 3.58] was noted at coordinates +3,+ 29, + 8. In addition, two negative peaks of decreased blood flow were noted, in the fight thalamus (t = -4.01, coordinates +5, -11, + 11) and deep in the left inferior parietal lobe (t = -3.72, coordinates 44, -47, +48). Conclusions The two conditions were chosen in order to remove activation due to basic visual perceptual processing of letters. Similar to other investigators, we fecund that there was little remaining in terms of significant differences in brain activation between these two conditions, implying that even a series of letters such as "XXX" produces activation of many brain areas associated with processing of words. The activity in the pre-cuneate region in young individuals may relate to engagement of brain areas for memory, encoding of the words (Fletcher et al, 1995). The elderly subjects demonstrated increased CBF flow in the anterior cingulate lobe, which we take to reflect involvement of attentional processes during the processing of words in the eldelry, but not the young subjects. The decreased CBF in the thalamus may in fact reflect involvement of the thalamus, another substrate for attentional processing. Why this thalamic CBF change is in a negative direction is unclear. Our results are consistent with the idea that lexical access is automatic in young individuals, but involves attentional resources in the elderly. References 1 Fletcher,P.C., Frith,C.D., Grasby, P.M. et al, brain,1995, 1 1 8 : 4 0 1 - 416 2 Laberge, D. J. of Cognitive Neuroscience, 1990, 2 : 358-372. 3. Greenwood,P. and Parasuraman, R., Developmental Neuropsychology, 1991, 7: 421-434. 4.Nestor,P., Parasuraman,R., and Haxby,J. Developmental Neuropsychology, 1989, 5: 141-158. $432