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

Contextual response time adaptation in the countermanding performance of rats

Humans and non-human primates are known to lengthen their response time (RT) to a go signal when they occasionally must cancel their responses following a stop signal in a countermanding task as well as to adjust their RT adaptively on a trial-by-trial basis. Less is clear regarding the adaptive RT adjustments in the countermanding performance of rodents. To investigate this question, male Wistar rats (N=12) were trained with food reward to press a lever directly below an illuminated light (go signal), but to countermand the lever press subsequent to a tone (stop signal) presented infrequently (25% of trials) at variable delays. Rats were then tested in a standard responding task (0% stop trials) or a countermanding task with a 10-s or 1-s TO interval following errors. Rats exhibited significant RT lengthening in the countermanding task, compared with the standard responding task, and RT shortening following consecutive correct go trials. They also show RT lengthening following both error trials in the standard responding task and unrewarded, non-canceled stop trials in the countermanding task. RT lengthening following erroneous stop trials was observed in sessions with a 10-s TO interval, but not with a 1-s TO interval. Analyses of RT distributions suggest that RT lengthening results largely from reduced sensitivity to the go signal, but also from reduced readiness. These findings indicate that rats exert control in the countermanding task by lengthening RT in anticipation of stop trials to avoid long, unrewarded TO intervals.

Effects of probability bias in response readiness and response inhibition on reaching movements

It is solidly established that unequal stimulus frequencies lead to faster responses to the more likely stimulus; however, the effect of this probability bias on response inhibition is still debated. To tackle this issue, we administered two versions of the stop-signal task to 18 right-handed healthy subjects. In one version, we manipulated the frequency of right and left targets appearance when subjects were required to produce speeded responses (no-stop trials) with the right arm, whereas stop signals occurred with equal frequencies after right or left targets (no-stop signal bias). In the other version, we manipulated the frequency of appearance of stop signals after right or left targets, whereas no-stop trials toward right or left targets had the same frequency (stop-signal bias). Surprisingly, we found a very modest, if any, increase in response readiness toward the more frequent stimulus. However, the no-stop signal bias had an effect on the speed of inhibitory control, as subjects were always faster to suppress a movement toward the side where targets were less likely to occur. Differently, the stop-signal bias had a much more powerful effect. In fact, subjects were faster to withhold movements toward the side where targets were more frequent, while they exhibited longer reaction times for reaches toward the more likely targets. Overall, these results suggest that action preparation and action inhibition are independent competing processes, but subjects tend to place automatically greater importance on the stop task.

Laterality of seizure onset and the simple reaction time: revamping the Poffenberger's paradigm for seizure surgery

BACKGROUND

Crossed-uncrossed differentials (CUDs) are viewed as surrogates for interhemispheric transfer time (IHTT). Not uncommonly CUDs assume statistically significant negative values (inverted CUDs). This raises doubts of the accepted interpretation of CUDs, i.e. intra- and inter-hemispheric routings of signals in uncrossed and crossed responses, respectively.

METHOD

Based on the evidence supporting directionality in callosal traffic, data are provided indicating that callosal transfers exclusively involve non-dominant responses and such transfers are modality non-specific. The evidence also indicates that neural handedness corresponds to behavioral only in a statistical manner and the former remains unchanged regardless of the subject's life experience.

RESULTS

The neurally dominant side is the side that is directly connected to the major hemisphere (command center). The connection of the non-dominant side to the command center is via the corpus callosum; therefore, a delay occurs in the reaction time of all non-dominant effectors, corresponding to IHTT. Accordingly, negative CUDs indicate a mismatch of neural and behavioral (avowed) handedness of the subject. This group comprises a minority of 15-20% of the population.

CONCLUSION

Comparing the response time of symmetrically located effector is a robust way of lateralizing a person's major hemisphere. The latter is also the site of initiation of seizures, as the minor hemisphere is bereft of independent motor activity. Sensory signals arising from the nondominant side of the body traverse the callosum before reaching the major hemisphere. Searching for ipsilateral somatosensory evoked potentials provides another approach in lateralizing the non-dominant side of the body (ipsilateral to the major hemisphere). Practical uses of a conceptually revamped Poffenberger paradigm in neurosurgery are briefly reviewed.

Voluntary Control of Multisaccade Gaze Shifts During Movement Preparation and Execution

Although the nature of gaze control regulating single saccades is relatively well documented, how such control is implemented to regulate multisaccade gaze shifts is not known. We used highly eccentric targets to elicit multisaccade gaze shifts and tested the ability of subjects to control the saccade sequence by presenting a second target on random trials. Their response allowed us to test the nature of control at many levels: before, during, and between saccades. Although the saccade sequence could be inhibited before it began, we observed clear signs of truncation of the first saccade, which confirmed that it could be inhibited in midflight as well. Using a race model that explains the control of single saccades, we estimated that it took about 100 ms to inhibit a planned saccade but took about 150 ms to inhibit a saccade during its execution. Although the time taken to inhibit was different, the high subject-wise correlation suggests a unitary inhibitory control acting at different levels in the oculomotor system. We also frequently observed responses that consisted of hypometric initial saccades, followed by secondary saccades to the initial target. Given the estimates of the inhibitory process provided by the model that also took into account the variances of the processes as well, the secondary saccades (average latency ∼215 ms) should have been inhibited. Failure to inhibit the secondary saccade suggests that the intersaccadic interval in a multisaccade response is a ballistic stage. Collectively, these data indicate that the oculomotor system can control a response until a very late stage in its execution. However, if the response consists of multiple movements then the preparation of the second movement becomes refractory to new visual input, either because it is part of a preprogrammed sequence or as a consequence of being a corrective response to a motor error.

Models of response inhibition in the stop-signal and stop-change paradigms

The stop-signal paradigm is very useful for the study of response inhibition. Stop-signal performance is typically described as a race between a go process, triggered by a go stimulus, and a stop process, triggered by the stop signal. Response inhibition depends on the relative finishing time of these two processes. Numerous studies have shown that the independent horse-race model of Logan and Cowan [Logan, G.D., Cowan, W.B., 1984. On the ability to inhibit thought and action: a theory of an act of control. Psychological Review 91, 295-327] accounts for the data very well. In the present article, we review the independent horse-race model and related models, such as the interactive horse-race model [Boucher, L., Palmeri, T.J., Logan, G.D., Schall, J.D., 2007. Inhibitory control in mind and brain: an interactive race model of countermanding saccades. Psychological Review 114, 376-397]. We present evidence that favors the independent horse-race model but also some evidence that challenges the model. We end with a discussion of recent models that elaborate the role of a stop process in inhibiting a response.

Interhemispheric transfer following callosotomy in humans: Role of the superior colliculus

It is now common knowledge that the total surgical section of the corpus callosum (CC) and of the other forebrain commissures prevents interhemispheric transfer (IT) of a host of mental functions. By contrast, IT of simple sensorimotor functions, although severely delayed, is not abolished, and an important question concerns the pathways subserving this residual IT. To answer this question we assessed visuomotor IT in split-brain patients using the Poffenberger paradigm (PP), that is, a behavioral paradigm in which simple reaction time (RT) to visual stimuli presented to the hemifield ipsilateral to the responding hand is compared to stimuli presented to the contralateral hemifield, a condition requiring an IT. We tested the possibility that the residual IT is mediated by the collicular commissure interconnecting the two sides of the superior colliculus (SC). To this purpose, we used short-wavelength visual stimuli, which in neurophysiological studies in non-human primates have been shown to be undetectable by collicular neurons. We found that, in both totally and partially callosotomised patients, IT was considerably longer with S-cone input than with L-cone input or with achromatic stimuli. This was not the case in healthy participants in whom IT was not affected by color. These data clearly show that the SC plays an important role in IT of sensorimotor information in the absence of the corpus callosum.

Visuo-motor integration and control in the human posterior parietal cortex: evidence from TMS and fMRI

The posterior parietal cortex is a fundamental structure for visuo-motor integration and control. Here I discuss recent transcranial magnetic stimulation (TMS) and functional magnetic resonance imaging (fMRI) studies that I interpret as suggesting four concepts. The evolutionary process has enlarged the human posterior parietal cortex while still preserving the internal structure of the posterior parietal cortex of other primates. Visuo-motor control in the posterior parietal cortex may be implemented by coding primarily action goals. The lateralization of visuo-motor functions in the posterior parietal cortex suggests that the left posterior parietal cortex is more concerned with tool use and the right posterior parietal cortex is more concerned with imitation of the actions of others. Finally, visuo-motor inter-hemispheric transfer through parietal callosal fibers occurs at the level of 'motor intention'.

Inhibitory control of reaching movements in humans

Behavioral flexibility provides a very large repertoire of actions and strategies, however, it carries a cost: a potential interference between different options. The voluntary control of behavior starts exactly with the ability of deciding between alternatives. Certainly inhibition plays a key role in this process. Here we examined the inhibitory control of reaching arm movements with the countermanding paradigm. Right-handed human subjects were asked to perform speeded reaching movements toward a visual target appearing either on the same or opposite side of the reaching arm (no-stop trials), but to withhold the commanded movement whenever an infrequent stop signal was presented (stop trials). As the delay between go and stop signals increased, subjects increasingly failed to inhibit the movement. From this inhibitory function and the reaction times of movements in no-stop trials, we estimated the otherwise unobservable duration of the stopping process, the stop signal reaction time (SSRT). We found that the SSRT for reaching movements was, on average, 206 ms and that it varied with the reaching arm and the target position even though the stop signal was a central stimulus. In fact, subjects were always faster to withhold reaching movements toward visual targets appearing on the same side of the reaching arm. This behavior strictly parallels the course of the reaction times of no-stop trials. These data show that the stop and go processes interacting in this countermanding task are independent, but most likely influenced by a common factor when under the control of the same hemisphere. In addition, we show that the point beyond which the response cannot be inhibited, the so-called point-of-no-return that divides controlled and ballistic phases of movement processing, lies after the inter-hemispheric transfer.

Visuo-motor pathways in humans revealed by event-related fMRI

Whether different brain networks are involved in generating unimanual responses to a simple visual stimulus presented in the ipsilateral versus contralateral hemifield remains a controversial issue. Visuo-motor routing was investigated with event-related functional magnetic resonance imaging (fMRI) using the Poffenberger reaction time task. A 2 hemifield x 2 response hand design generated the "crossed" and "uncrossed" conditions, describing the spatial relation between these factors. Both conditions, with responses executed by the left or right hand, showed a similar spatial pattern of activated areas, including striate and extrastriate areas bilaterally, SMA, and M1 contralateral to the responding hand. These results demonstrated that visual information is processed bilaterally in striate and extrastriate visual areas, even in the "uncrossed" condition. Additional analyses based on sorting data according to subjects' reaction times revealed differential crossed versus uncrossed activity only for the slowest trials, with response strength in infero-temporal cortices significantly correlating with crossed-uncrossed differences (CUD) in reaction times. Collectively, the data favor a parallel, distributed model of brain activation. The presence of interhemispheric interactions and its consequent bilateral activity is not determined by the crossed anatomic projections of the primary visual and motor pathways. Distinct visuo-motor networks need not be engaged to mediate behavioral responses for the crossed visual field/response hand condition. While anatomical connectivity heavily influences the spatial pattern of activated visuo-motor pathways, behavioral and functional parameters appear to also affect the strength and dynamics of responses within these pathways.