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Spatial strategy elaboration in egocentric and allocentric tasks following medial prefrontal cortex lesions in the rat
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Spatial Strategy Elaboration in Egocentric and AIIocentric Tasks Following Medial Prefrontal Cortex Lesions in the Rat Katia Ethier, Nathalie Le Marec, Pierre-Paul Romprr, and Roger Godbout Centre de recherche, H@ital Sacrd-Cceur and D@artement de psychiatrie, Universit~ de Montrdal, Montrdal, Canada
We evaluated the role of the medial prefrontal cortex (mPFC) in the elaboration of egocentric navigation strategies in a water maze (WM). Lesions of mPFC cell bodies was achieved in 21 rats using bilateral injections of ibotenic acid (IA); 13 control rats were injected with saline. After 17 days, rats had to learn an allocentric (using external cues: 10 lesioned, 7 saline rats) or an egocentric WM (using internal/kinetic cues: l0 lesioned, 6 saline rats) over six trials in a same session. The initial trajectory on the sixth trial was used as an index of the elaboration of a navigation strategy. In the egocentric test, lesioned rats were more rarely located in the target quadrant than control rats. No differences were found between lesioned and control rats in the allocentric test. These results show that lesions of the mPFC impairs the capacity to elaborate an egocentric navigation strategy. © 2001 AcademicPress
According to de Bruin et al. (1997) the medial prefrontal cortex (mPFC) plays a role in setting up egocentric but not allocentric cognitive maps in water maze (WM) tasks. Until now the role of the mPFC was based on gross nonspecific lesions techniques (cortical suction, radiofrequency lesions, etc.) severing fibers of passage, etc. The aim of the present study was to verify the role of mPFC in the elaboration of egocentric navigation strategies.
Me~od
Thirty-four young male Sprague-Dawley rats (350-400 g) were used. Selective excitotoxic lesions of mPFC cell bodies was performed in 21 rats using microinjections of ibotenic acid (IA) while 13 control rats were injected in the same region with saline. Ten days after surgery, rats were tested with either an allocentric (10 lesioned rats, 7 controls rats) or an egocentric W M task (10 lesioned rats, 6 controls rats). Lesions were confirmed by standard histological methods.
Procedure
On each trial of the allocentric W M test, rats started from a different quadrant of the pool relative to the previous trial while the hidden target platform always stayed at the same place. This procedure calls for the use of external cues in order to succeed. In the egocentric version the platform was always positioned in a straight line relatively to its starting position, which was located in a different quadrant from the previous trial. This procedure calls for the use of internal/kinetic cues in order to succeed. The rats had six trials of 60 s m a x i m u m to reach the hidden target platform where they remained for 30 s. Upon an unsuccessful trial, the experimenter placed the rat on the platform for 30 s. The dependent measure was based on an analysis of the animal initial trajectory. We know from a W M pilot study that naive rats need two quadrants to find a visible platform. Therefore, we analyzed the initial part of each rat's trajectory to verify if it was in the target area, e.g., in the quadrant containing the hidden platform, after two quadrant entries.
TENNET XI
135
Results Allocentric WM. There was no difference between lesioned and control rats; i.e., there was an equal proportion of rats from the two groups in the target quadrant (saline, 100%; lesioned, 80%, n.s.). Egocentric WM. Lesioned rats were less frequently found in the target quadrant compared to control rats (saline, 50%; lesioned, 9%, Z 2 (17.67, df = 1), p < .00003). Lesioned rats were neither found in the previous trial's target quadrant. Rather, 9/11 (88.1%) of the lesioned rats were found in the starting quadrant. Discussion We found no difference between control and lesioned rats in the allocentric WM while mPFC rats were impaired in the egocentric WM. In the egocentric WM test, behavioral analysis showed that lesioned rats were neither located in the previous trial's target quadrant, suggesting that they also did not use an allocentric navigation strategy as an alternative. The fact that they were located back in the starting quadrant could suggest that rats with a lesion of the mPFC are not capable of correcting a deficient navigational strategy as it is being elaborated and that they choose to home the starting position upon navigational strategy difficulties.
REFERENCES de Bruin, J. P. C., Swinkels, W. A. M., & de Brabander, J. M. (1997). Response learning of rats in a Morris water maze: Involvement of the medial prefrontal cortex. Behavioural Brain Research, 85, 47-55.
Hemispheric Processing Asymmetries: Implications for Memory Margaret G. Funnell, Paul M. Corballis, and Michael S. Gazzaniga Center.for Cognitive Neuroscience, Dartmouth College
Recent research has demonstrated that memory for words elicits left hemisphere activation, faces right hemisphere activation, and nameable objects bilateral activation. This pattern of results was attributed to dual coding of information, with the left hemisphere employing a verbal code and the right a nonverbal code. Nameable objects can be encoded either verbally or nonverbally and this accounts for their bilateral activation. We investigated this hypothesis in a callosotomy patient. Consistent with dual coding, the left hemisphere was superior to the right in memory for words, whereas the right was superior for faces. Contrary to prediction, perlormance on nameable pictures was not equivalent in the two hemispheres, but rather resulted in a right hemisphere superiority. In addition, memory lbr pictures was significantly better than for either words or faces. These findings suggest that the dual code hypothesis is an oversimplification of the processing capabilities of the two hemispheres. © 2001 AcademicPress
Observations of patients with unilateral brain lesions hint at a multitude of functional asymmetries between the two hemispheres of the brain. Paul Broca first noted the left hemisphere's dominance for language after an autopsy on his famous aphasic patient, Tan. Other syndromes resulting from unilateral brain damage also sparked interest in the specialized functions of the two cerebral hemispheres. Patients with
134
Spatial Strategy Elaboration in Egocentric and AIIocentric Tasks Following Medial Prefrontal Cortex Lesions in the Rat Katia Ethier, Nathalie Le Marec, Pierre-Paul Romprr, and Roger Godbout Centre de recherche, H@ital Sacrd-Cceur and D@artement de psychiatrie, Universit~ de Montrdal, Montrdal, Canada
We evaluated the role of the medial prefrontal cortex (mPFC) in the elaboration of egocentric navigation strategies in a water maze (WM). Lesions of mPFC cell bodies was achieved in 21 rats using bilateral injections of ibotenic acid (IA); 13 control rats were injected with saline. After 17 days, rats had to learn an allocentric (using external cues: 10 lesioned, 7 saline rats) or an egocentric WM (using internal/kinetic cues: l0 lesioned, 6 saline rats) over six trials in a same session. The initial trajectory on the sixth trial was used as an index of the elaboration of a navigation strategy. In the egocentric test, lesioned rats were more rarely located in the target quadrant than control rats. No differences were found between lesioned and control rats in the allocentric test. These results show that lesions of the mPFC impairs the capacity to elaborate an egocentric navigation strategy. © 2001 AcademicPress
According to de Bruin et al. (1997) the medial prefrontal cortex (mPFC) plays a role in setting up egocentric but not allocentric cognitive maps in water maze (WM) tasks. Until now the role of the mPFC was based on gross nonspecific lesions techniques (cortical suction, radiofrequency lesions, etc.) severing fibers of passage, etc. The aim of the present study was to verify the role of mPFC in the elaboration of egocentric navigation strategies.
Me~od
Thirty-four young male Sprague-Dawley rats (350-400 g) were used. Selective excitotoxic lesions of mPFC cell bodies was performed in 21 rats using microinjections of ibotenic acid (IA) while 13 control rats were injected in the same region with saline. Ten days after surgery, rats were tested with either an allocentric (10 lesioned rats, 7 controls rats) or an egocentric W M task (10 lesioned rats, 6 controls rats). Lesions were confirmed by standard histological methods.
Procedure
On each trial of the allocentric W M test, rats started from a different quadrant of the pool relative to the previous trial while the hidden target platform always stayed at the same place. This procedure calls for the use of external cues in order to succeed. In the egocentric version the platform was always positioned in a straight line relatively to its starting position, which was located in a different quadrant from the previous trial. This procedure calls for the use of internal/kinetic cues in order to succeed. The rats had six trials of 60 s m a x i m u m to reach the hidden target platform where they remained for 30 s. Upon an unsuccessful trial, the experimenter placed the rat on the platform for 30 s. The dependent measure was based on an analysis of the animal initial trajectory. We know from a W M pilot study that naive rats need two quadrants to find a visible platform. Therefore, we analyzed the initial part of each rat's trajectory to verify if it was in the target area, e.g., in the quadrant containing the hidden platform, after two quadrant entries.
TENNET XI
135
Results Allocentric WM. There was no difference between lesioned and control rats; i.e., there was an equal proportion of rats from the two groups in the target quadrant (saline, 100%; lesioned, 80%, n.s.). Egocentric WM. Lesioned rats were less frequently found in the target quadrant compared to control rats (saline, 50%; lesioned, 9%, Z 2 (17.67, df = 1), p < .00003). Lesioned rats were neither found in the previous trial's target quadrant. Rather, 9/11 (88.1%) of the lesioned rats were found in the starting quadrant. Discussion We found no difference between control and lesioned rats in the allocentric WM while mPFC rats were impaired in the egocentric WM. In the egocentric WM test, behavioral analysis showed that lesioned rats were neither located in the previous trial's target quadrant, suggesting that they also did not use an allocentric navigation strategy as an alternative. The fact that they were located back in the starting quadrant could suggest that rats with a lesion of the mPFC are not capable of correcting a deficient navigational strategy as it is being elaborated and that they choose to home the starting position upon navigational strategy difficulties.
REFERENCES de Bruin, J. P. C., Swinkels, W. A. M., & de Brabander, J. M. (1997). Response learning of rats in a Morris water maze: Involvement of the medial prefrontal cortex. Behavioural Brain Research, 85, 47-55.
Hemispheric Processing Asymmetries: Implications for Memory Margaret G. Funnell, Paul M. Corballis, and Michael S. Gazzaniga Center.for Cognitive Neuroscience, Dartmouth College
Recent research has demonstrated that memory for words elicits left hemisphere activation, faces right hemisphere activation, and nameable objects bilateral activation. This pattern of results was attributed to dual coding of information, with the left hemisphere employing a verbal code and the right a nonverbal code. Nameable objects can be encoded either verbally or nonverbally and this accounts for their bilateral activation. We investigated this hypothesis in a callosotomy patient. Consistent with dual coding, the left hemisphere was superior to the right in memory for words, whereas the right was superior for faces. Contrary to prediction, perlormance on nameable pictures was not equivalent in the two hemispheres, but rather resulted in a right hemisphere superiority. In addition, memory lbr pictures was significantly better than for either words or faces. These findings suggest that the dual code hypothesis is an oversimplification of the processing capabilities of the two hemispheres. © 2001 AcademicPress
Observations of patients with unilateral brain lesions hint at a multitude of functional asymmetries between the two hemispheres of the brain. Paul Broca first noted the left hemisphere's dominance for language after an autopsy on his famous aphasic patient, Tan. Other syndromes resulting from unilateral brain damage also sparked interest in the specialized functions of the two cerebral hemispheres. Patients with