Transcranial Magnetic Stimulation Affects Hand Choice

The choice of which hand to use to perform an action might seem simple enough, but a competitive decision-making process in the brain, most notably the posterior parietal cortex, is behind that choice, new research in the Proceedings of the National Academy of Sciences shows.

Several studies have shown that the posterior parietal cortex (PPC) has a critical role in planning reaching movements, but few have examined the underlying neural mechanisms behind the decision of which hand to use to perform an action.

Flavio T.P. Oliveira of the University of California, Berkeley, and colleagues investigated how the brain mediates the decision about which hand to use for a manual action (PNAS 2010 [Epub ahead of print] doi: 10.1073/pnas.1006223107]).

In the first of three experiments, the researchers instructed 13 right-handed study participants to reach for targets as quickly and accurately as possible. Researchers alternated between requiring participants to use their right hand only, left hand only, or either hand. The researchers estimated the point of subjective equality (PSE), the point at which subjects would be equally likely to use either hand; identified targets in which there was little or much uncertainty; and measured reaction time (RT), defined as the time from the onset of the target to the time they moved the cursor outside the starting circle.

Overall reaction times were faster when subjects were told which hand to use rather than allowed to choose. When allowed to choose, RTs were significantly longer for the targets around the PSE than for those targets in extreme locations. “The increase in RT suggests a cost associated with a competition between the action plans for each hand at locations where ambiguity in hand choice is maximal,” the researchers said.

In the second experiment, they used single-pulse transcranial magnetic stimulation (TMS) to temporarily disrupt brain activity in the left and right PPC in 10 right-handed subjects. Again, RTs were slower for targets around the PSE (400 ms) vs. those in extreme locations (388 ms). TMS led to marginally reliable increases in RT (392 ms for the left PPC, 394 ms for the right) vs. no TMS (386 ms).

Also, TMS to the left PPC alone led to an increase in the use of the hand ipsilateral to the stimulation site. Participants had a 4% increase in left-hand use after left-PPC stimulation, compared with the no-TMS condition, and a 5.2% increase relative to right-PPC stimulation.

In a follow-up control experiment involving 10 right-handed subjects, the researchers applied TMS to the left and right PPC and found no significant changes in hand preference.

These experiments suggest that hand choice involves a process that “resolves a competition arising from the parallel engagement of action plans for both hands,” the researchers said. “Serial models in which hand choice is made at a higher cognitive level without activation of action plans for both hands might have predicted an increase in RT with TMS but cannot account for the shift in hand use. Rather, the results indicate that motor planning is initiated before response selection is made.”

The study also shows that the PPC is proactively involved in deciding which hand an individual will use for a manual reach. “Although it is likely that a broad network of cortical and subcortical areas are involved in different aspects of decision-making, the present results highlight the critical role that the PPC has in transforming sensory information into free choices of action,” the researchers said.

Disclosures: The authors had no conflicts to disclose. The Natural Sciences and Engineering Research Council of Canada, the Canadian Institutes of Health Research, the National Institutes of Health, the National Science Foundation, and the Belgian American Educational Foundation provided support for this study.

The choice of which hand to use to perform an action might seem simple enough, but a competitive decision-making process in the brain, most notably the posterior parietal cortex, is behind that choice, new research in the Proceedings of the National Academy of Sciences shows.

Several studies have shown that the posterior parietal cortex (PPC) has a critical role in planning reaching movements, but few have examined the underlying neural mechanisms behind the decision of which hand to use to perform an action.

Flavio T.P. Oliveira of the University of California, Berkeley, and colleagues investigated how the brain mediates the decision about which hand to use for a manual action (PNAS 2010 [Epub ahead of print] doi: 10.1073/pnas.1006223107]).

In the first of three experiments, the researchers instructed 13 right-handed study participants to reach for targets as quickly and accurately as possible. Researchers alternated between requiring participants to use their right hand only, left hand only, or either hand. The researchers estimated the point of subjective equality (PSE), the point at which subjects would be equally likely to use either hand; identified targets in which there was little or much uncertainty; and measured reaction time (RT), defined as the time from the onset of the target to the time they moved the cursor outside the starting circle.

Overall reaction times were faster when subjects were told which hand to use rather than allowed to choose. When allowed to choose, RTs were significantly longer for the targets around the PSE than for those targets in extreme locations. “The increase in RT suggests a cost associated with a competition between the action plans for each hand at locations where ambiguity in hand choice is maximal,” the researchers said.

In the second experiment, they used single-pulse transcranial magnetic stimulation (TMS) to temporarily disrupt brain activity in the left and right PPC in 10 right-handed subjects. Again, RTs were slower for targets around the PSE (400 ms) vs. those in extreme locations (388 ms). TMS led to marginally reliable increases in RT (392 ms for the left PPC, 394 ms for the right) vs. no TMS (386 ms).

Also, TMS to the left PPC alone led to an increase in the use of the hand ipsilateral to the stimulation site. Participants had a 4% increase in left-hand use after left-PPC stimulation, compared with the no-TMS condition, and a 5.2% increase relative to right-PPC stimulation.

In a follow-up control experiment involving 10 right-handed subjects, the researchers applied TMS to the left and right PPC and found no significant changes in hand preference.

These experiments suggest that hand choice involves a process that “resolves a competition arising from the parallel engagement of action plans for both hands,” the researchers said. “Serial models in which hand choice is made at a higher cognitive level without activation of action plans for both hands might have predicted an increase in RT with TMS but cannot account for the shift in hand use. Rather, the results indicate that motor planning is initiated before response selection is made.”

The study also shows that the PPC is proactively involved in deciding which hand an individual will use for a manual reach. “Although it is likely that a broad network of cortical and subcortical areas are involved in different aspects of decision-making, the present results highlight the critical role that the PPC has in transforming sensory information into free choices of action,” the researchers said.

Disclosures: The authors had no conflicts to disclose. The Natural Sciences and Engineering Research Council of Canada, the Canadian Institutes of Health Research, the National Institutes of Health, the National Science Foundation, and the Belgian American Educational Foundation provided support for this study.

The choice of which hand to use to perform an action might seem simple enough, but a competitive decision-making process in the brain, most notably the posterior parietal cortex, is behind that choice, new research in the Proceedings of the National Academy of Sciences shows.

Several studies have shown that the posterior parietal cortex (PPC) has a critical role in planning reaching movements, but few have examined the underlying neural mechanisms behind the decision of which hand to use to perform an action.

Flavio T.P. Oliveira of the University of California, Berkeley, and colleagues investigated how the brain mediates the decision about which hand to use for a manual action (PNAS 2010 [Epub ahead of print] doi: 10.1073/pnas.1006223107]).

In the first of three experiments, the researchers instructed 13 right-handed study participants to reach for targets as quickly and accurately as possible. Researchers alternated between requiring participants to use their right hand only, left hand only, or either hand. The researchers estimated the point of subjective equality (PSE), the point at which subjects would be equally likely to use either hand; identified targets in which there was little or much uncertainty; and measured reaction time (RT), defined as the time from the onset of the target to the time they moved the cursor outside the starting circle.

Overall reaction times were faster when subjects were told which hand to use rather than allowed to choose. When allowed to choose, RTs were significantly longer for the targets around the PSE than for those targets in extreme locations. “The increase in RT suggests a cost associated with a competition between the action plans for each hand at locations where ambiguity in hand choice is maximal,” the researchers said.

In the second experiment, they used single-pulse transcranial magnetic stimulation (TMS) to temporarily disrupt brain activity in the left and right PPC in 10 right-handed subjects. Again, RTs were slower for targets around the PSE (400 ms) vs. those in extreme locations (388 ms). TMS led to marginally reliable increases in RT (392 ms for the left PPC, 394 ms for the right) vs. no TMS (386 ms).

Also, TMS to the left PPC alone led to an increase in the use of the hand ipsilateral to the stimulation site. Participants had a 4% increase in left-hand use after left-PPC stimulation, compared with the no-TMS condition, and a 5.2% increase relative to right-PPC stimulation.

In a follow-up control experiment involving 10 right-handed subjects, the researchers applied TMS to the left and right PPC and found no significant changes in hand preference.

These experiments suggest that hand choice involves a process that “resolves a competition arising from the parallel engagement of action plans for both hands,” the researchers said. “Serial models in which hand choice is made at a higher cognitive level without activation of action plans for both hands might have predicted an increase in RT with TMS but cannot account for the shift in hand use. Rather, the results indicate that motor planning is initiated before response selection is made.”

The study also shows that the PPC is proactively involved in deciding which hand an individual will use for a manual reach. “Although it is likely that a broad network of cortical and subcortical areas are involved in different aspects of decision-making, the present results highlight the critical role that the PPC has in transforming sensory information into free choices of action,” the researchers said.

Disclosures: The authors had no conflicts to disclose. The Natural Sciences and Engineering Research Council of Canada, the Canadian Institutes of Health Research, the National Institutes of Health, the National Science Foundation, and the Belgian American Educational Foundation provided support for this study.