Abstract
The ability to decide which of the two stimuli is presented first can be probed using a temporal order judgment (TOJ) task. When the stimuli are delivered to the fingers, TOJ decisions can be confounded by the fact that the hands can be moved to different locations in space. How and where this confounded information is processed in the brain is poorly understood. In the present set of experiments, we addressed this knowledge gap by using single-pulse transcranial magnetic stimulation (TMS) to disrupt processing in the right or left posterior parietal cortex (PPC) during a vibrotactile TOJ task with stimuli applied to the right and left index fingers. In the first experiment, participants held their hands in an uncrossed configuration, and we found that when the index finger contralateral to the site of TMS was stimulated first, there was a significant increase in TOJ errors. This increase did not occur when stimuli were delivered to the ipsilateral finger first. In the second experiment, participants held their hands in a crossed configuration and the pattern of errors was reversed relative to the first experiment. In both the first two experiments, significant increases in TOJ error were present with TMS over either hemisphere, regardless of arm configuration; however, they were larger overall following TMS over the right PPC. Control experiments using sham TMS indicated the systematic modulation in error was not due to nonspecific effects of the stimulation. Additionally, we showed that these TMS-induced changes in TOJ errors were not due to a reduced ability to detect the timing of the vibrotactile stimuli. Taken together, these results demonstrate that both the right and left PPC contribute to the processing underlying vibrotactile TOJs by integrating vibrotactile information and proprioceptive information related to arm position in space.
Highlights
A vital aspect of human behavior is the ability to keep track of time
Unlike with stimuli presented in the visual or auditory domain, when vibrotactile stimuli are used in a temporal order judgment (TOJ) task, it is possible to alter their spatial location by changing the configuration of the limbs, for example, by applying the stimuli to each index finger and varying the relative position of the hands with respect to the midline of the body
We were interested in the extent to which transcranial magnetic stimulation (TMS) over the posterior parietal cortex (PPC) could disrupt the processing associated with TOJ decisions and whether this would be modulated by the spatial configuration of the hands
Summary
A vital aspect of human behavior is the ability to keep track of time. Despite the clear need for temporal discrimination, the neural mechanisms underlying time processing are poorly understood. Yamamoto and Kitazawa (2001) demonstrated that when the hands are uncrossed, decisions regarding the temporal order of such stimuli are accurate even with ISIs as short as 70 ms; with the hands in a crossed configuration, ISIs of up to 600 ms are required to make accurate judgments. They suggested that this deficit is due to the additional time necessary to resolve the relative spatial location of the stimuli with the hands in the crossed configuration. Common to both of these accounts is the spatial remapping required to successfully perform the task
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