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Biomedical subjects

Matthew F S Rushworth

Publications and source records attributed to Matthew F S Rushworth.

17 recordsLinked to original sources
PubMed2006

Separate neural pathways process different decision costs.

Behavioral ecologists and economists emphasize that potential costs, as well as rewards, influence decision making. Although neuroscientists assume that frontal areas are central to decision making, the evidence is contradictory and the critical region remains unclear. Here it is shown that frontal lobe contributions to cost-benefit decision making can be understood by positing the existence of two independent systems that make decisions about delay and effort costs. Anterior cingulate cortex lesions affected how much effort rats decided to invest for rewards. Orbitofrontal cortical lesions affected how long rats decided to wait for rewards. The pattern of disruption suggested the deficit could be related to impaired associative learning. Impairments of the two systems may underlie apathetic and impulsive choice patterns in neurological and psychiatric illnesses. Although the existence of two systems is not predicted by economic accounts of decision making, our results suggest that delay and effort may exert distinct influences on decision making.

Animals↗
PubMed2006

Optimal decision making and the anterior cingulate cortex.

Learning the value of options in an uncertain environment is central to optimal decision making. The anterior cingulate cortex (ACC) has been implicated in using reinforcement information to control behavior. Here we demonstrate that the ACC's critical role in reinforcement-guided behavior is neither in detecting nor in correcting errors, but in guiding voluntary choices based on the history of actions and outcomes. ACC lesions did not impair the performance of monkeys (Macaca mulatta) immediately after errors, but made them unable to sustain rewarded responses in a reinforcement-guided choice task and to integrate risk and payoff in a dynamic foraging task. These data suggest that the ACC is essential for learning the value of actions.

Animals↗
PubMed2006

FEF TMS affects visual cortical activity.

We tested whether the frontal eye field (FEF) is critical in controlling visual processing in posterior visual brain areas during the orienting of spatial attention. Short trains (5 pulses at 10 Hz) of transcranial magnetic stimulation (TMS) were applied to the right FEF during the cueing period of a covert attentional task while event-related potentials (ERPs) were simultaneously recorded from lateral posterior electrodes, where visual components are prominent. FEF TMS significantly affected the neural activity evoked by visual stimuli, as well as the ongoing neural activity recorded during earlier anticipation of the visual stimuli. The effects of FEF TMS started earlier and were greatest for brain activity recorded ipsilaterally to FEF TMS and contralaterally to the visual stimulus. The TMS-induced effect on visual ERPs occurred at the same time as ERPs were shown to be modulated by visual attention. Importantly, no similar effects were observed after TMS of a control site that was physically closer but not anatomically interconnected to the recording sites. The results show that the human FEF has a causal influence over the modulation of visual activity in posterior areas when attention is being allocated.

Adult↗
PubMed2005

Attentional selection and action selection in the ventral and orbital prefrontal cortex.

Different accounts of the ventral and orbital prefrontal cortex (PFv+o) have emphasized either its role in learning conditional rules for action selection or the attentional selection of behaviorally relevant stimuli. Although the accounts are not mutually exclusive, it is possible that the involvement of PFv+o in conditional action selection is a consequence of its role in selecting relevant stimuli or that its involvement in attentional selection is a consequence of the conditional rules present in many attentional paradigms. Five macaques learned a conditional action-selection task in which the difficulty of identifying the stimulus relevant for guiding action selection was varied in a simple manner by either altering its distance from the action or presenting additional distracting stimuli. Simply increasing the spatial separation between the instructing stimulus led to slower responses. Experiment 1 showed that bilateral PFv+o lesions impaired conditional action selection even when attentional demands were kept to a minimum, but there was evidence that the impairment was exacerbated by manipulating stimulus selection difficulty. Experiment 2 confirmed the importance of PFv+o for conditional action selection even when stimulus selection difficulty was minimal. Experiments 3 and 4 demonstrated that the action-selection impairment was significantly increased by making identification of the behaviorally relevant stimulus difficult. PFv+o is central to the use of conditional rules when selecting courses of action, but conditional rules are also represented in premotor and striatal regions. A special contribution of PFv+o may be initial selection of behaviorally relevant stimuli.

Animals↗
PubMed2005

Quantitative investigation of connections of the prefrontal cortex in the human and macaque using probabilistic diffusion tractography.

The functions of prefrontal cortex (PFC) areas are constrained by their anatomical connections. There is little quantitative information about human PFC connections, and, instead, our knowledge of primate PFC connections is derived from tracing studies in macaques. The connections of subcortical areas, in which white matter penetration and hence diffusion anisotropy are greatest, can be studied with diffusion-weighted imaging (DWI) tractography. We therefore used DWI tractography in four macaque and 10 human hemispheres to compare the connections of PFC regions with nine subcortical regions, including several fascicles and several subcortical nuclei. A distinct connection pattern was identified for each PFC and each subcortical region. Because some of the fascicles contained connections with posterior cortical areas, it was also possible to draw inferences about PFC connection patterns with posterior cortical areas. Notably, it was possible to identify similar circuits centered on comparable PFC regions in both species; PFC regions probably engage in similar patterns of regionally specific functional interaction with other brain areas in both species. In the case of one area traditionally assigned to the human PFC, the pars opercularis, the distribution of connections was not reminiscent of any macaque PFC region but, instead, resembled the pattern for macaque ventral premotor area. Some limitations to the DWI approach were apparent; the high diffusion anisotropy in the corpus callosum made it difficult to compare connection probability values in the adjacent cingulate region.

Adult↗
PubMed2005

Effects of an orientation illusion on motor performance and motor imagery.

Although the effect of visual illusions on overt actions has been an area of keen interest in motor performance, no study has yet examined whether illusions have similar or different effects on overt and imagined movements. Two experiments were conducted that compared the effects of an orientation illusion on an overt posture selection task and an imagined posture selection task. In Experiment 1 subjects were given a choice of grasping a bar with the thumb on the left side or right side of the bar. In Experiment 2 subjects were instructed to only imagine grasping the bar while remaining motionless. Subjects then reported which side of the bar their thumb had been placed in imagined grasping. Both the overt selection and imagined selection tasks were found to be sensitive to the orientation illusion, suggesting that similar visual information is used for overt and imagined movements, with both being sensitive to an orientation illusion. The results are discussed in terms of the visual processing and representation of real and imagined actions.

Brain↗
PubMed2005

Category-related activation for written words in the posterior fusiform is task specific.

Category-related brain activations have been reported in the posterior fusiform gyri when people view pictures of tools and animals, but only a single study has observed this pattern when the stimuli were words, rather than pictures. Here we replicate these category effects with words and provide evidence that distinctive patterns of activation are task specific. The results suggest that category-related activation in the posterior fusiform gyri can be driven either "bottom-up" by visual processing of images or "top-down" by word processing.

Adult↗
PubMed2005

Parietal rTMS disrupts the initiation but not the execution of on-line adjustments to a perturbation of object size.

Previous studies have implicated the human parietal lobes in the on-line guidance of action. However, no study to date has examined at what stage in the on-line adjustment process do the parietal lobes play their most critical role. Repetitive transcranial magnetic stimulation (rTMS) was applied over the left intraparietal sulcus as participants reached to grasp a small or large illuminated cylinder. On some trials, the illumination could suddenly switch from the small to large cylinder, or vice-versa. Small-Large switches were associated with relatively early grip aperture adjustments, whereas Large-Small switches were associated with relatively late grip aperture adjustments. When rTMS was applied early in the movement, it disrupted on-line adjustments to Small-Large target switches, but not to Large-Small switches. Conversely, when rTMS was applied late in the movement, it disrupted adjustments to Large-Small target switches but not to Small-Large switches. The timing of the disruption by rTMS appeared linked to the initiation of the adjustment. It was concluded that the left parietal lobe plays a critical role in initiating an on-line adjustment to a change in target size, but not in executing that adjustment. The implications of these results for current views of on-line control are discussed.

Adaptation, Physiological↗
PubMed2004

Interactions between decision making and performance monitoring within prefrontal cortex.

Our ability to judge the consequences of our actions is central to rational decision making. A large body of evidence implicates primate prefrontal regions in the regulation of this ability. It has proven extremely difficult, however, to separate functional areas in the frontal lobes. Using functional magnetic resonance imaging, we demonstrate complementary and reciprocal roles for the human orbitofrontal (OFC) and dorsal anterior cingulate cortices (ACd) in monitoring the outcome of behavior. Activation levels in these regions were negatively correlated, with activation increasing in the ACd and decreasing in the OFC when the selected response was the result of the participant's own decision. The pattern was reversed when the selected response was guided by the experimenter rather than the participant. These results indicate that the neural mechanisms underlying the way we assess the consequences of choices differ depending on whether we are told what to do or are able to exercise our volition.

Adaptation, Psychological↗
PubMed2004

Cognitive neuroscience: acting on numbers.

The parietal cortex is a central part of the brain's system for representing numbers and magnitudes. Activity in the parietal cortex might reflect number representation or actions made in response to the numbers.

Cognition↗
PubMed2004

Response-selection-related parietal activation during number comparison.

Neuroimaging studies of number comparison have consistently found activation in the intraparietal sulcus (IPS). Recently, it has been suggested that activations in the IPS vary with the distance between the numbers being compared. In number comparison, the smaller the distance between a number and the reference the longer the reaction time (RT ). Activations in the right or left IPS, however, have also been related to attentional and intentional selection. It is possible, therefore, that activity in this region is a reflection of the more basic stimulus and response-selection processes associated with changes in RT. This fMRI experiment investigated the effect of numerical distance independently from RT. In addition, activations during number comparison of single-digit and double-digit stimuli were compared. During number comparison blocks, subjects had to indicate whether digits were greater or smaller than a reference (5 or 65). In control blocks, they were asked to perform a perceptual task (vertical line present/absent) on either numerical or nonnumerical stimuli. Number comparison versus rest yielded a large bilateral parietal-posterior frontal network. However, no areas showed more activation during number comparison than during the control tasks. Furthermore, no areas were more active during comparison of numbers separated by a small distance than comparisons of those separated by a large distance or vice versa. A left-lateralized parietal-posterior frontal network varied significantly with RT. Our findings suggest that magnitude and numerical-distance-related IPS activations might be difficult to separate from fundamental stimulus and response-selection processes associated with RT changes. As is the case with other parameters, such as space, magnitude may be represented in the context of response selection in the parietal cortex. In this respect, the representation of magnitude in the human IPS may be similar to the representation of magnitude in other nonhuman primates.

Adult↗
PubMed2003

Functional specialization within medial frontal cortex of the anterior cingulate for evaluating effort-related decisions.

The rat medial frontal cortex (MFC) has been implicated in allowing animals to work harder to receive larger rewards. However, it is unknown what role the individual MFC regions [anterior cingulate cortex (ACC) and prelimbic-infralimbic cortex (PL-IL)] play in such decision making. To investigate this, we trained rats on a T-maze cost-benefit task with two possible courses of action, shown previously to be affected by complete MFC lesions. One response involved climbing a 30 cm barrier to obtain a large quantity of reward (high cost-high reward), whereas the other had a lower energetic demand but also a smaller reward gain (low cost-low reward). Before surgery, all animals preferred to select the high cost-high reward option. However, after excitotoxic ACC lesions, there was a complete reversal of behavior, with the ACC group selecting the low cost-low reward response on nearly every trial. In contrast, both control animals and rats with PL-IL lesions continued to choose to climb the barrier for the larger reward. When the same rats were tested on a delayed match-to-sample paradigm however, it was the PL-IL group that was significantly impaired at learning the response rule, with the performance of ACC rats being comparable with controls. This double dissociation indicates that the ACC is the important region within the MFC when evaluating how much effort to expand for a specific reward.

Analysis of Variance↗
PubMed2003

Semantic processing in the left inferior prefrontal cortex: a combined functional magnetic resonance imaging and transcranial magnetic stimulation study.

The involvement of the left inferior prefrontal cortex (LIPC) in phonological processing is well established from both lesion-deficit studies with neurological patients and functional neuroimaging studies of normals. Its involvement in semantic processing, on the other hand, is less clear. Although many imaging studies have demonstrated LIPC activation during semantic tasks, this may be due to implicit phonological processing. This article presents two experiments investigating semantic functions in the LIPC. Results from a functional magnetic resonance imaging experiment demonstrated that both semantic and phonological processing activated a common set of areas within this region. In addition, there was a reliable increase in activation for semantic relative to phonological decisions in the anterior LIPC while the opposite comparison (phonological vs. semantic decisions) revealed an area of enhanced activation within the posterior LIPC. A second experiment used transcranial magnetic stimulation (TMS) to temporarily interfere with neural information processing in the anterior portion of the LIPC to determine whether this region was essential for normal semantic performance. Both repetitive and single pulse TMS significantly slowed subjects' reactions for the semantic but not for the perceptual control task. Our results clarify the functional anatomy of the LIPC by demonstrating that anterior and posterior regions contribute to both semantic and phonological processing, albeit to different extents. In addition, the findings go beyond simply establishing a correlation between semantic processing and activation in the LIPC and demonstrate that a transient disruption of processing selectively interfered with semantic processing.

Adult↗
PubMed2002

The role of rat medial frontal cortex in effort-based decision making.

We conducted a series of experiments to elucidate the role of rat medial frontal cortex (MFC) (including prelimbic, infralimbic, and cingulate cortices) in effort-based decision making. Rats were trained on a cost-benefit T-maze task in which they could either choose to climb a barrier to obtain a high reward in one arm (HR arm) or could obtain a small reward in the other with no barrier present (LR arm). Before surgery, all animals were selecting the HR arm on almost every trial. However, after excitotoxic lesions to MFC, the rats shifted to selecting the LR arm on almost every trial. This was not caused by a spatial memory or motor deficit because the same rats returned to selecting the HR arm when the amount of effort needed to be expended to obtain reward in either arm was equalized by putting an identical barrier in the LR arm. Additional experiments demonstrated that the change in effort-based decisions observed in the rats was not caused by a complete insensitivity to reward and effort because they returned to choosing the HR arm if either the cost was reduced (by making the barrier smaller) or the benefit was increased (increasing the food ratio differential). Instead, the MFC lesion shifted the animals' decision criterion, making them more likely to choose the LR arm than the sham-lesioned animals. These results imply that medial frontal cortex is important for allowing the animal to put in more work to obtain greater rewards.

Animals↗
PubMed2002

Components of switching intentional set.

Despite the intuition that we can shift cognitive set on instruction, some behavioral studies have suggested that set shifting might only be accomplished once we engage in performance of the new task. It is possible that set switching consists of more than one component cognitive process and that the component processes might segregated in time. We recorded event-related potentials (ERPs) during two set-switching tasks to test whether different component processes were responsible for (i) set initiation and reconfiguration when presented with the instruction to switch, and (ii) the implementation of the new set once subjects engaged in performing the new task. The response switching (RS) task required shifts of intentional set; subjects selected between responses according to one of two conflicting intentional sets. The results demonstrated the existence of more than one constituent process. Some of the processes were linked to the initiation and reconfiguration of the set prior to actual performance of the new task. Other processes were time locked to performance of new task items. Set initiation started with modulation of medial frontal ERPs and was followed by modulation over parietal electrodes. Implementation of intentional set was associated with modulation of response-related ERPs.

Adult↗
PubMed2002

The role of ipsilateral premotor cortex in hand movement after stroke.

Movement of an affected hand after stroke is associated with increased activation of ipsilateral motor cortical areas, suggesting that these motor areas in the undamaged hemisphere may adaptively compensate for damaged or disconnected regions. However, this adaptive compensation has not yet been demonstrated directly. Here we used transcranial magnetic stimulation (TMS) to interfere transiently with processing in the ipsilateral primary motor or dorsal premotor cortex (PMd) during finger movements. TMS had a greater effect on patients than controls in a manner that depended on the site, hemisphere, and time of stimulation. In patients with right hemiparesis (but not in healthy controls), TMS applied to PMd early (100 ms) after the cue to move slowed simple reaction-time finger movements by 12% compared with controls. The relative slowing of movements with ipsilateral PMd stimulation in patients correlated with the degree of motor impairment, suggesting that functional recruitment of ipsilateral motor areas was greatest in the more impaired patients. We also used functional magnetic resonance imaging to monitor brain activity in these subjects as they performed the same movements. Slowing of reaction time after premotor cortex TMS in the patients correlated inversely with the relative hemispheric lateralization of functional magnetic resonance imaging activation in PMd. This inverse correlation suggests that the increased activation in ipsilateral cortical motor areas during movements of a paretic hand, shown in this and previous functional imaging studies, represents a functionally relevant, adaptive response to the associated brain injury.

Adult↗