Redundancy gain in the stop-signal paradigm: implications for the locus of coactivation in simple reaction time

The authors carried out 2 experiments designed to cast light on the locus of redundancy gain in simple visual reaction time by using a stop-signal paradigm. In Experiment 1, the authors found that single visual stimuli were more easily inhibited than double visual stimuli by an acoustic stop signal. This result is in keeping with the idea that redundancy gain occurs prior to the ballistic stage of the stop-signal task. In Experiment 2, the authors found that the response to an acoustic go signal was more easily inhibited by a double than by a single visual stop signal. This result provides conclusive evidence for a redundancy gain in the stop process--in a process that does not involve a motor response but rather its inhibition

At what stage of manual visual reaction time does interhemispheric transmission occur: controlled or ballistic?

Interhemispheric transfer (IT) time through the corpus callosum can be measured with a manual reaction time (RT) to lateralized visual stimuli (the so-called Poffenberger paradigm) by subtracting mean RT of faster uncrossed hemifield-hand combinations (not requiring an IT) from slower crossed combinations (requiring an IT). That the corpus callosum is involved in IT has been demonstrated by its dramatic lengthening in patients with a section of the corpus callosum. However, it is still unclear whether the signal transmitted by the corpus callosum concerns perceptual or motor stages of RT. To try and cast light on this question, in a first experiment we tested normal subjects on a partially modified Poffenberger paradigm with stop trials intermingled with go trials. In the former, subjects are supposed to refrain from responding following a stop signal (stop-signal paradigm). This paradigm can tease apart the contribution of the controlled and ballistic stages to overall RT and, used together with the Poffenberger task, enables one to assess the stage at which IT occurs. The controlled stage lies before the point of no return, i.e. the point beyond which the response cannot be inhibited, and concerns perceptual and pre-motor processes, while the ballistic stage occurs after the point of no return and concerns the motoric aspect of the response. We found that the slower responses typically obtained in the crossed conditions were more likely to be inhibited than the faster uncrossed responses and this suggests that IT occurs prior to the point of no return. Since the precise locus of the point of no return is uncertain, in a second experiment we used response force as a dependent variable reflecting the activation of the motor cortex. We found that none of the force parameters studied differed between crossed and uncrossed conditions while the temporal parameters confirmed the presence of an advantage of the uncrossed combinations. Altogether these results suggest that callosal IT of visuomotor information occurs at the stage of controlled (perceptual and pre-motor) processes and rule out the possibility of an IT at the motoric stage

Dissociating arbitrary stimulus-response mapping from movement planning during preparatory period: evidence from event-related functional magnetic resonance imaging

In the present study, we aimed to dissociate the neural correlates of two subprocesses involved in the preparatory period in the context of arbitrary, prelearned stimulus-response (S-R) associations, namely, S-R mapping and movement planning (MP). We teased apart these two subprocesses by comparing three tasks in which the complexity of both S-R mapping and MP were independently manipulated: simple reaction time (SRT) task, go/no-go reaction time (GNGRT) task, and choice reaction time (CRT) task. We found that a more complex S-R mapping, which is the common element differentiating CRT and GNGRT from SRT, was associated with higher brain activation in the left superior parietal lobe (SPL). Conversely, a greater number of planned finger movements, which is the common difference between CRT and both SRT and GNGRT, was associated with higher brain activation in a number of frontal areas, including the left supplementary motor area (SMA), left dorsal premotor cortex (dPM), and left anterior cingulate cortex (ACC). The left-hemisphere dominance for S-R mapping could be related to the fact that arbitrary S-R mapping is often verbally mediated in humans. Overall, these results suggest a clear dissociation in the preparatory-set period between the more abstract role of left SPL in activating the appropriate S-R associations and the more concrete role played by the SMA, dPM, and ACC in preparing the required motor programs