Michael Andres

Dissociating the role of ventral and dorsal premotor cortex in precision grasping.

Small-object manipulation is essential in numerous human activities, although its neural bases are still essentially unknown. Recent functional imaging studies have shown that precision grasping activates a large bilateral frontoparietal network, including ventral (PMv) and dorsal (PMd) premotor areas. To dissociate the role of PMv and PMd in the control of hand and finger movements, we produced, by means of transcranial magnetic stimulation (TMS), transient virtual lesions of these two areas in both hemispheres, in healthy subjects performing a grip–lift task with their right, dominant hand. We found that a virtual lesion of PMv specifically impaired the grasping component of these movements: a lesion of either the left or right PMv altered the correct positioning of fingers on the object, a prerequisite for an efficient grasping, whereas lesioning the left, contralateral PMv disturbed the sequential recruitment of intrinsic hand muscles, all other movement parameters being unaffected by PMv lesions. Conversely, we found that a virtual lesion of the left PMd impaired the proper coupling between the grasping and lifting phases, as evidenced by the TMS-induced delay in the recruitment of proximal muscles responsible for the lifting phase; lesioning the right PMd failed to affect dominant hand movements. Finally, an analysis of the time course of these effects allowed us to demonstrate the sequential involvement of PMv and PMd in movement preparation. These results provide the first compelling evidence for a neuronal dissociation between the different phases of precision grasping in human premotor cortex.

Dissociable roles of the human somatosensory and superior temporal cortices for processing social face signals

Faces are multi‐dimensional stimuli bearing important social signals, such as gaze direction and emotion expression. To test whether perception of these two facial attributes recruits distinct cortical areas within the right hemisphere, we used single‐pulse transcranial magnetic stimulation (TMS) in healthy volunteers while they performed two different tasks on the same face stimuli. In each task, two successive faces were presented with varying eye‐gaze directions and emotional expressions, separated by a short interval of random duration. TMS was applied over either the right somatosensory cortex or the right superior lateral temporal cortex, 100 or 200 ms after presentation of the second face stimulus. Participants performed a speeded matching task on the second face during one of two possible conditions, requiring judgements about either gaze direction or emotion expression (same/different as the first face). Our results reveal a significant task–stimulation site interaction, indicating a selective TMS‐related interference following stimulations of somatosensory cortex during the emotional expression task. Conversely, TMS of the superior lateral temporal cortex selectively interfered with the gaze direction task. We also found that the interference effect was specific to the stimulus content in each condition, affecting judgements of gaze shifts (not static eye positions) with TMS over the right superior temporal cortex, and judgements of fearful expressions (not happy expressions) with TMS over the right somatosensory cortex. These results provide for the first time a double dissociation in normal subjects during social face recognition, due to transient disruption of non‐overlapping brain regions. The present study supports a critical role of the somatosensory and superior lateral temporal regions in the perception of fear expression and gaze shift in seen faces, respectively.

Contribution of Hand Motor Circuits to Counting

The finding that number processing activates a cortical network partly overlapping that recruited for hand movements has renewed interest in the relationship between number and finger representations. Further evidence about a possible link between fingers and numbers comes from developmental studies showing that finger movements play a crucial role in learning counting. However, increased activity in hand motor circuits during counting may unveil unspecific processes, such as shifting attention, reciting number names, or matching items with a number name. To address this issue, we used transcranial magnetic stimulation to measure changes in corticospinal (CS) excitability during a counting task performed silently and using either numbers or letters of the alphabet to enumerate items. We found an increased CS excitability of hand muscles during the counting task, irrespective of the use of numbers or letters, whereas it was unchanged in arm and foot muscles. Control tasks allowed us to rule out a possible influence of attention allocation or covert speech on CS excitability increase of hand muscles during counting. The present results support a specific involvement of hand motor circuits in counting because no CS changes were found in arm and foot muscles during the same task. However, the contribution of hand motor areas is not exclusively related to number processing because an increase in CS excitability was also found when letters were used to enumerate items. This finding suggests that hand motor circuits are involved whenever items have to be put in correspondence with the elements of any ordered series.

Temporal Dissociation between Hand Shaping and Grip Force Scaling in the Anterior Intraparietal Area

In humans, both clinical and functional imaging studies have evidenced the critical role played by the posterior parietal cortex, and particularly by the anterior intraparietal area (AIP), in skilled hand movements. However, the exact contribution of AIP to precision grasping remains debated. Here we used transcranial magnetic stimulation (TMS) to induce virtual lesions of the left and/or right AIP in subjects performing a grip-lift task with either hand. We found that, during movement preparation, a virtual lesion of AIP had distinct consequences on precision grasping of either hand depending on its time of occurrence: TMS applied 270–220 ms before the fingers contacted the manipulandum altered specifically the hand shaping, whereas lesions induced 170–120 ms before contact time only affected the grip force scaling. The lateralization of these two processes in AIP is also strikingly different: whereas a bilateral lesion of AIP was necessary to impair hand shaping, only a unilateral lesion of the left AIP altered the grip force scaling in either hand. The present study shows that, during movement preparation, AIP is responsible for processing two distinct, temporally dissociated, precision grasping parameters, regardless of the hand in use. This indicates that the contribution of AIP to hand movements is “effector- independent,” a finding that may explain the invariance of grasping movements performed with either hand.

Contribution of hand circuits to counting.

The finding that number processing activates a cortical network partly overlapping that recruited for hand movements has renewed interest in the relationship between number and finger representations. Further evidence about a possible link between fingers and numbers comes from developmental studies showing that finger movements play a crucial role in learning counting. However, increased activity in hand motor circuits during counting may unveil unspecific processes, such as shifting attention, reciting number names, or matching items with a number name. To address this issue, we used transcranial magnetic stimulation to measure changes in corticospinal (CS) excitability during a counting task performed silently and using either numbers or letters of the alphabet to enumerate items. We found an increased CS excitability of hand muscles during the counting task, irrespective of the use of numbers or letters, whereas it was unchanged in arm and foot muscles. Control tasks allowed us to rule out a possible influence of attention allocation or covert speech on CS excitability increase of hand muscles during counting. The present results support a specific involvement of hand motor circuits in counting because no CS changes were found in arm and foot muscles during the same task. However, the contribution of hand motor areas is not exclusively related to number processing because an increase in CS excitability was also found when letters were used to enumerate items. This finding suggests that hand motor circuits are involved whenever items have to be put in correspondence with the elements of any ordered series.

Time course of number magnitude interference during grasping.

In the present study, we recorded the kinematics of grasping movements in order to measure the possible interference caused by digits printed on the visible face of the objects to grasp. The aim of this approach was to test the hypothesis that digit magnitude processing shares common mechanisms with object size estimate during grasping. In the first stages of reaching, grip aperture was found to be larger consequent to the presentation of digits with a high value rather than a low one. The effect of digit magnitude on grip aperture was more pronounced for large objects. As the hand got closer to the object, the influence of digit magnitude decreased and grip aperture progressively reflected the actual size of the object. We concluded that number magnitude may interact with grip aperture while programming the grasping movements.

Dissociation of numerosity and duration processing in the left intraparietal sulcus: a transcranial magnetic stimulation study.

A possible dissociation of duration and numerosity processing was tested in an off-line repetitive transcranial magnetic stimulation (rTMS) design. Participants had to compare the numerosity of flashed dot sequences or the duration of single dot displays before and after 15 min of 1 Hz rTMS over one of three sites (the left or right intraparietal sulcus (IPS), or the vertex chosen as a control site). Compared to the control site, performance was only slowed down for the numerosity comparison task after the left IPS stimulation, whereas it was not affected for the duration comparison task for any of the parietal sites. These results show that the parietal area critically involved in numerosity processing is not involved in duration processing, revealing at least one cerebral site where duration and numerosity comparison processes dissociate.

Role of distinct parietal areas in arithmetic: An fMRI-guided TMS study

Although several parietal areas are known to be involved in number processing, their possible role in arithmetic operations remains debated. It has been hypothesized that the horizontal segment of the intraparietal sulcus (hIPS) and the posterior superior parietal lobule (PSPL) contribute to operations solved by calculation procedures, such as subtraction, but whether these areas are also involved in operations solved by memory retrieval, such as multiplication, is controversial. In the present study, we first identified the parietal areas involved in subtraction and multiplication by means of functional magnetic resonance imaging (fMRI) and we found an increased activation, bilaterally, in the hIPS and PSPL during both arithmetic operations. In order to test whether these areas are causally involved in subtraction and multiplication, we used transcranial magnetic stimulation (TMS) to create, in each participant, a virtual lesion of either the hIPS or PSPL, over the sites corresponding to the peaks of activation gathered in fMRI. When compared to a control site, we found an increase in response latencies in both operations after a virtual lesion of either the left or right hIPS, but not of the PSPL. Moreover, TMS over the hIPS increased the error rate in the multiplication task. The present results indicate that even operations solved by memory retrieval, such as multiplication, rely on the hIPS. In contrast, the PSPL seems to underlie processes that are nonessential to solve basic subtraction and multiplication problems.

Actions, Words, and Numbers A Motor Contribution to Semantic Processing?

Recent findings in neuroscience challenge the view that the motor system is exclusively dedicated to the control of actions, and it has been suggested that it may contribute critically to conceptual processes such as those involved in language and number representation. The aim of this review is to address this issue by illustrating some interactions between the motor system and the processing of words and numbers. First, we detail functional brain imaging studies suggesting that motor circuits may be recruited to represent the meaning of action-related words. Second, we summarize a series of experiments demonstrating some interference between the size of grip used to grasp objects and the magnitude processing of words or numbers. Third, we report data suggestive of a common representation of numbers and finger movements in the adult brain, a possible trace of the finger-counting strategies used in childhood. Altogether, these studies indicate that the motor system interacts with several aspects of word and number representations. Future research should determine whether these findings reflect a causal role of the motor system in the organization of semantic knowledge.

Number magnitude potentiates action judgements.

Motor actions can be simulated and generated through the perception of objects and their characteristics. Such functional characteristics of objects with given action capabilities are called affordances. Here we report an interaction between the perception of affordances and the processing of numerical magnitude, and we show that the numerical information calibrates the judgement of action even when no actual action is required. In Experiment 1, participants had to judge whether they would be able to grasp a rod lengthways between their thumb and index finger. The presentation of the rod was preceded by a number or a non-numerical symbol. When a small number preceded the rod, participants overestimated their grasp; conversely, when a large number preceded the rods, they underestimated their grasp. In Experiment 2, participants were requested to judge if two successive rods had the same length, a judgement that did not involve any grasping. The numerical primes had no effect on this judgement, showing that the magnitude/affordance interaction was not due to a simple perceptual effect. Finally, Experiment 3 showed that the interaction was not present with a non-numerical ordered sequence, thereby eliminating sequence order as a potentially confounding variable.