Middle Intraparietal Sulcus Research Articles

Automatic coding of environmental distance for walking-related locomotion in the foot-related sensory-motor system: A TMS study on macro-affordances

We have recently described a facilitation effect for the execution of a walking-related action in response to distant objects/locations in the extrapersonal space. Based on the parallelism with the well-known effect of “micro-affordance”, observed during the execution of functionally appropriate hand-related actions towards manipulable objects, we have referred to this effect in terms of “macro-affordance”. Here we used transcranical magnetic stimulation (TMS) to investigate whether a foot-related region located in the human dorsal precuneate cortex plays a causal role in the generation and maintenance of such behavioral effect. This question was addressed by comparing the magnitude of the facilitation effect during an incidental go/no-go task, i.e. advantage for walking-related actions to pictures framing an environment from a far vs. near distance, during three different TMS conditions. The three TMS conditions were collected in all subjects in a randomized order and included stimulation of: i. a foot-related region in the anterior precuneus, ii. a control region in the middle intraparietal sulcus (mIPS), and iii. a sham condition.Enrollment in the TMS protocol was based on analysis of individual performance during a preliminary session conducted using a sham stimulation.TMS was administered at a low frequency range before the beginning of each condition. The results showed that stimulation of the foot-related region in the anterior precuneus produced a significant reduction of the walking-related facilitation effect as compared to both stimulation of the active-control region and the non-active sham stimulation. These findings suggest that the foot-related sensory-motor system directly participates in the process of extraction of the spatial features (i.e. distance) from an environmental scene that are useful for locomotion. More in general, these findings support an automatic coding of environmental affordance or “macro-affordances” in the walking-related sensory-motor system.

Electrocorticography of Spatial Shifting and Attentional Selection in Human Superior Parietal Cortex.

Spatial-attentional reorienting and selection between competing stimuli are two distinct attentional processes of clinical and fundamental relevance. In the past, reorienting has been mainly associated with inferior parietal cortex. In a patient with a subdural grid covering the upper and lower bank of the left anterior and middle intraparietal sulcus (IPS) and the superior parietal lobule (SPL), we examined the involvement of superior parietal cortex using a hybrid spatial cueing paradigm identical to that previously applied in stroke and in healthy controls. In SPL, as early as 164 ms following target onset, an invalidly compared to a validly cued target elicited a positive event-related potential (ERP) and an increase in intertrial coherence (ITC) in the theta band, regardless of the direction of attention. From around 400–650 ms, functional connectivity [weighted phase lag index (wPLI) analysis] between SPL and IPS briefly inverted such that SPL activity was driving IPS activity. In contrast, the presence of a competing distracter elicited a robust change mainly in IPS from 300 to 600 ms. Within superior parietal cortex reorienting of attention is associated with a distinct and early electrophysiological response in SPL while attentional selection is indexed by a relatively late electrophysiological response in the IPS. The long latency suggests a role of IPS in working memory or cognitive control rather than early selection.

Memory accumulation mechanisms in human cortex are independent of motor intentions.

Previous studies on perceptual decision-making have often emphasized a tight link between decisions and motor intentions. Human decisions, however, also depend on memories or experiences that are not closely tied to specific motor responses. Recent neuroimaging findings have suggested that, during episodic retrieval, parietal activity reflects the accumulation of evidence for memory decisions. It is currently unknown, however, whether these evidence accumulation signals are functionally linked to signals for motor intentions coded in frontoparietal regions and whether activity in the putative memory accumulator tracks the amount of evidence for only previous experience, as reflected in "old" reports, or for both old and new decisions, as reflected in the accuracy of memory judgments. Here, human participants used saccadic-eye and hand-pointing movements to report recognition judgments on pictures defined by different degrees of evidence for old or new decisions. A set of cortical regions, including the middle intraparietal sulcus, showed a monotonic variation of the fMRI BOLD signal that scaled with perceived memory strength (older > newer), compatible with an asymmetrical memory accumulator. Another set, including the hippocampus and the angular gyrus, showed a nonmonotonic response profile tracking memory accuracy (higher > lower evidence), compatible with a symmetrical accumulator. In contrast, eye and hand effector-specific regions in frontoparietal cortex tracked motor intentions but were not modulated by the amount of evidence for the effector outcome. We conclude that item recognition decisions are supported by a combination of symmetrical and asymmetrical accumulation signals largely segregated from motor intentions.

Attentional priorities and access to short-term memory: Parietal interactions

The intraparietal sulcus (IPS) has been implicated in selective attention as well as visual short-term memory (VSTM). To contrast mechanisms of target selection, distracter filtering, and access to VSTM, we combined behavioral testing, computational modeling and functional magnetic resonance imaging. Sixteen healthy subjects participated in a change detection task in which we manipulated both target and distracter set sizes. We directly compared the IPS response as a function of the number of targets and distracters in the display and in VSTM. When distracters were not present, the posterior and middle segments of IPS showed the predicted asymptotic activity increase with an increasing target set size. When distracters were added to a single target, activity also increased as predicted. However, the addition of distracters to multiple targets suppressed both middle and posterior IPS activities, thereby displaying a significant interaction between the two factors. The interaction between target and distracter set size in IPS could not be accounted for by a simple explanation in terms of number of items accessing VSTM. Instead, it led us to a model where items accessing VSTM receive differential weights depending on their behavioral relevance, and secondly, a suppressive effect originates during the selection phase when multiple targets and multiple distracters are simultaneously present. The reverse interaction between target and distracter set size was significant in the right temporoparietal junction (TPJ), where activity was highest for a single target compared to any other condition. Our study reconciles the role of middle IPS in attentional selection and biased competition with its role in VSTM access.

Distinct cortical networks support the planning and online control of reaching‐to‐grasp in humans

A number of brain imaging studies have identified regions involved in the planning and control of complex actions. Here we attempt to contrast activity related to planning and online control in the human brain during simple reaching and grasping movements. In four conditions, participants did one of the following: passively observed a grasp target; planned a grasping movement without executing; planned and then executed a grasp; or immediately executed a grasp. Neural activity was measured using functional magnetic resonance imaging and activity in the various conditions compared. Two large, independent networks of brain activity were identified: (i) a planning network including the premotor cortex, basal ganglia, anterior cingulate, posterior medial parietal area, superior parietal occipital cortex and middle intraparietal sulcus; and (ii) a control network including sensorimotor cortex, the cerebellum, the supramarginal gyrus and the superior parietal lobule. These findings provide evidence that the planning and control of even simple reaching and grasping actions use different brain regions, including different parts of the frontal and parietal lobes.

How the brain integrates costs and benefits during decision making

When we make decisions, the benefits of an option often need to be weighed against accompanying costs. Little is known, however, about the neural systems underlying such cost-benefit computations. Using functional magnetic resonance imaging and choice modeling, we show that decision making based on cost-benefit comparison can be explained as a stochastic accumulation of cost-benefit difference. Model-driven functional MRI shows that ventromedial and left dorsolateral prefrontal cortex compare costs and benefits by computing the difference between neural signatures of anticipated benefits and costs from the ventral striatum and amygdala, respectively. Moreover, changes in blood oxygen level dependent (BOLD) signal in the bilateral middle intraparietal sulcus reflect the accumulation of the difference signal from ventromedial prefrontal cortex. In sum, we show that a neurophysiological mechanism previously established for perceptual decision making, that is, the difference-based accumulation of evidence, is fundamental also in value-based decisions. The brain, thus, weighs costs against benefits by combining neural benefit and cost signals into a single, difference-based neural representation of net value, which is accumulated over time until the individual decides to accept or reject an option.

An investigation of the effects of relative probability of old and new test items on the neural correlates of successful and unsuccessful source memory

The present event-related fMRI study addressed the question whether retrieval-related neural activity in lateral parietal cortex is affected by the relative probability of test items. We varied the proportion of old to new items across two test blocks, with 25% of the items being old in one block and 75% being old in the other. Prior to each block, participants (N=18) completed one of two types of study judgment on each of 108 object images. They then performed a source memory test with four response options: studied in task 1, studied in task 2, old but unsure of the study task, and new. Retrieval-related activity in regions previously identified as recollection-sensitive, including the left inferior lateral parietal cortex and bilateral medial temporal cortex, was unaffected by old/new ratio. Generic retrieval success effects – retrieval-related effects common to recognized items attracting either a correct or an incorrect source judgment – were identified in several regions of left superior parietal cortex. These effects dissociated between a middle region of the intraparietal sulcus (IPS), where activity did not interact with ratio, and regions anterior and posterior to the middle IPS where activity was sensitive to old/new ratio. The findings are inconsistent with prior proposals that retrieval-related activity in and around the left middle IPS reflects the relative salience of old and new test items. Rather, they suggest that, as in the case of more inferior left parietal regions, retrieval-related activity in this region reflects processes directly linked to retrieval.

Perceptual Fusion and Stimulus Coincidence in the Cross-Modal Integration of Speech

Human speech perception is profoundly influenced by vision. Watching a speaker's mouth movements significantly improves comprehension, both for normal listeners in noisy environments and especially for the hearing impaired. A number of brain regions have been implicated in audiovisual speech tasks, but little evidence distinguishes them functionally. In an event-related functional magnetic resonance imaging study, we differentiate neural systems that evaluate cross-modal coincidence of the physical stimuli from those that mediate perceptual binding. Regions consistently involved in perceptual fusion per se included Heschl's gyrus, superior temporal sulcus, middle intraparietal sulcus, and inferior frontal gyrus. Successful fusion elicited activity biased toward the left hemisphere, although failed cross-modal binding recruited regions in both hemispheres. A broad network of other areas, including the superior colliculus, anterior insula, and anterior intraparietal sulcus, were more involved with evaluating the spatiotemporal correspondence of speech stimuli, regardless of a subject's perception. All of these showed greater activity to temporally offset stimuli than to audiovisually synchronous stimuli. Our results demonstrate how elements of the cross-modal speech integration network differ in their sensitivity to physical reality versus perceptual experience.

Attentional responses to unattended stimuli in human parietal cortex

Right-sided parietal lesions lead to lateralized attentional deficits which are most prominent with bilateral stimulation. We determined how an irrelevant stimulus in the unattended hemifield alters attentional responses in parietal cortex during unilateral orienting. A trial consisted of a central spatial cue, a delay and a test phase during which a grating was presented at 9 degrees eccentricity. Subjects had to discriminate the orientation of the grating. The unattended hemifield was either empty or contained a second, irrelevant grating. We carried out a series of functional MRI (fMRI) studies in 35 healthy volunteers (13 men and 22 women, aged between 19 and 30 years) as well as a behavioural and structural lesion mapping study in 17 right-hemispheric lesion patients, 11 of whom had neglect. In the patients with but not in those without neglect, the addition of a distractor in the unattended hemifield significantly impaired performance if attention was directed contralesionally but not if it was directed ipsilesionally. In the healthy volunteers, we discerned two functionally distinct areas along the posterior-anterior axis of the intraparietal sulcus (IPS). The posterior, descending IPS segment in both hemispheres showed attentional enhancement of responses during contralateral attentional orienting and was unaffected by the presence of an irrelevant stimulus in the ignored hemifield. In contrast, the right-sided horizontal IPS segment showed a strong attentional response when subjects oriented to a stimulus in the relevant hemifield and an irrelevant stimulus was simultaneously present in the ignored hemifield, compared with unilateral stimulation. This effect was independent of the direction of attention. The symmetrical left-sided horizontal IPS segment showed the highest responses under the same circumstances, in combination with a contralateral bias during unilateral stimulation conditions. None of the six patients without neglect had a lesion of the horizontal IPS segment. In four of the 11 neglect patients, the lesion overlapped with the horizontal IPS activity cluster and lay in close proximity to it in another four. The remaining three patients had a lesion at a distance from the parietal cortex. Our findings reconcile the role of the IPS in endogenous attentional control with the clinically significant interaction between direction of attention and bilateral stimulation in right parietal lesion patients. Functional imaging in neglect patients will be necessary to assess IPS function in those cases where the structural lesion spares the middle IPS segment.