RESUMEN
Many studies infer the role of neurons by asking what information can be decoded from their activity or by observing the consequences of perturbing their activity. An alternative approach is to consider information flow between neurons. We applied this approach to the parietal reach region (PRR) and the lateral intraparietal area (LIP) in posterior parietal cortex. Two complementary methods imply that across a range of reaching tasks, information flows primarily from PRR to LIP. This indicates that during a coordinated reach task, LIP has minimal influence on PRR and rules out the idea that LIP forms a general purpose spatial processing hub for action and cognition. Instead, we conclude that PRR and LIP operate in parallel to plan arm and eye movements, respectively, with asymmetric interactions that likely support eye-hand coordination. Similar methods can be applied to other areas to infer their functional relationships based on inferred information flow.
Asunto(s)
Lóbulo Parietal , Lóbulo Parietal/fisiología , Animales , Macaca mulatta , Masculino , Neuronas/fisiología , Movimientos Oculares/fisiología , Desempeño Psicomotor/fisiología , Red Nerviosa/fisiologíaRESUMEN
The canonical view of motor control is that distal musculature is controlled primarily by the contralateral cerebral hemisphere; unilateral brain lesions typically affect contralateral but not ipsilateral musculature. Contralateral-only limb deficits following a unilateral lesion suggest but do not prove that control is strictly contralateral: the loss of a contribution of the lesioned hemisphere to the control of the ipsilesional limb could be masked by the intact contralateral drive from the nonlesioned hemisphere. To distinguish between these possibilities, we serially inactivated the parietal reach region, comprising the posterior portion of medial intraparietal area, the anterior portion of V6a, and portions of the lateral occipital parietal area, in each hemisphere of 2 monkeys (23 experimental sessions, 46 injections total) to evaluate parietal reach region's contribution to the contralateral reaching deficits observed following lateralized brain lesions. Following unilateral inactivation, reach reaction times with the contralesional limb were slowed compared with matched blocks of control behavioral data; there was no effect of unilateral inactivation on the reaction time of either ipsilesional limb reaches or saccadic eye movements. Following bilateral inactivation, reaching was slowed in both limbs, with an effect size in each no different from that produced by unilateral inactivation. These findings indicate contralateral organization of reach preparation in posterior parietal cortex.SIGNIFICANCE STATEMENT Unilateral brain lesions typically affect contralateral but not ipsilateral musculature. Contralateral-only limb deficits following a unilateral lesion suggest but do not prove that control is strictly contralateral: the loss of a contribution of the lesioned hemisphere to the control of the ipsilesional limb could be masked by the intact contralateral drive from the nonlesioned hemisphere. Unilateral lesions cannot distinguish between contralateral and bilateral control, but bilateral lesions can. Here we show similar movement initiation deficits after combined unilateral and bilateral inactivation of the parietal reach region, indicating contralateral organization of reach preparation.
Asunto(s)
Movimiento , Lóbulo Parietal , Lateralidad Funcional/fisiología , Movimiento/fisiología , Lóbulo Parietal/fisiología , Tiempo de Reacción , Movimientos SacádicosRESUMEN
Primates use their arms in complex ways that frequently require coordination between the two arms. Yet the planning of bimanual movements has not been well-studied. We recorded spikes and local field potentials (LFP) from the parietal reach region (PRR) in both hemispheres simultaneously while monkeys planned and executed unimanual and bimanual reaches. From analyses of interhemispheric LFP-LFP and spike-LFP coherence, we found that task-specific information is shared across hemispheres in a frequency-specific manner. This shared information could arise from common input or from direct communication. The population average unit activity in PRR, representing PRR output, encodes only planned contralateral arm movements while beta-band LFP power, a putative PRR input, reflects the pattern of planned bimanual movement. A parsimonious interpretation of these data is that PRR integrates information about the movement of the left and right limbs, perhaps in service of bimanual coordination.
Asunto(s)
Potenciales de Acción/fisiología , Movimiento/fisiología , Lóbulo Parietal/fisiología , Desempeño Psicomotor/fisiología , Animales , Escala de Evaluación de la Conducta , Electrofisiología , Lateralidad Funcional/fisiología , Macaca mulatta , Masculino , Corteza Motora/fisiología , Neuronas/fisiología , Movimientos Sacádicos/fisiología , Transducción de Señal/fisiologíaRESUMEN
The integrity of white matter architecture in the human brain is related to cognitive processing abilities. The corpus callosum is the largest white matter bundle interconnecting the two cerebral hemispheres. "Split-brain" patients in whom all cortical commissures have been severed to alleviate intractable epilepsy demonstrate remarkably intact cognitive abilities despite the lack of this important interhemispheric pathway. While it has often been speculated that there are compensatory alterations in the remaining interhemispheric fibers in split-brain patients several years post-commissurotomy, this has never been directly shown. Here we examined extra-callosal pathways for interhemispheric communication in the brain of a patient who underwent complete cerebral commissurotomy using diffusion weighted imaging tractography. We found that compared with a healthy age-matched comparison group, the split-brain patient exhibited increased fractional anisotropy (FA) of the dorsal and ventral pontine decussations of the cortico-cerebellar interhemispheric pathways. Few differences were observed between the patient and the comparison group with respect to FA of other long-range intrahemispheric fibers. These results point to specific cerebellar anatomical substrates that may account for the spared interhemispheric coordination and intact cognitive abilities that have been extensively documented in this unique patient.
Asunto(s)
Cuerpo Calloso/patología , Imagen de Difusión por Resonancia Magnética , Fórnix/fisiopatología , Vías Nerviosas/patología , Anciano , Anisotropía , Mapeo Encefálico , Cerebro/patología , Cerebro/fisiopatología , Cuerpo Calloso/fisiopatología , Imagen de Difusión por Resonancia Magnética/métodos , Imagen de Difusión Tensora/métodos , Femenino , Fórnix/patología , Humanos , Vías Nerviosas/fisiopatología , Pruebas NeuropsicológicasRESUMEN
We often orient to where we are about to reach. Spatial and temporal correlations in eye and arm movements may depend on the posterior parietal cortex (PPC). Spatial representations of saccade and reach goals preferentially activate cells in the lateral intraparietal area (LIP) and the parietal reach region (PRR), respectively. With unimanual reaches, eye and arm movement patterns are highly stereotyped. This makes it difficult to study the neural circuits involved in coordination. Here, we employ bimanual reaching to two different targets. Animals naturally make a saccade first to one target and then the other, resulting in different patterns of limb-gaze coordination on different trials. Remarkably, neither LIP nor PRR cells code which target the eyes will move to first. These results suggest that the parietal cortex plays at best only a permissive role in some aspects of eye-hand coordination and makes the role of LIP in saccade generation unclear.
Asunto(s)
Brazo/fisiología , Lóbulo Parietal/fisiología , Animales , Mapeo Encefálico , Macaca mulatta , Masculino , Red Nerviosa , Desempeño Psicomotor , Movimientos SacádicosRESUMEN
Bimanual coordination is critical for a broad array of behaviors. Drummers, for example, must carefully coordinate movements of their 2 arms, sometimes beating on the same drum and sometimes on different ones. While coordinated behavior is well-studied, the early stages of planning are not well understood. In the parietal reach region (PRR) of the posterior parietal cortex (PPC), the presence of neurons that modulate when either arm moves by itself has been taken as evidence for a role in bimanual coordination. To test this notion, we recorded neurons during both unilateral and bimanual movements. We find that the activity that precedes an ipsilateral arm movement is primarily a sensory response to a target in the neuron's visual receptive field and not a plan to move the ipsilateral arm. In contrast, the activity that precedes a contralateral arm movement is the sum of a movement plan plus a sensory response. Despite not coding ipsilateral arm movements, about half of neurons discriminate between different patterns of bimanual movements. These results provide direct evidence that PRR neurons represent bimanual reach plans, and suggest that bimanual coordination originates in the sensory-to-motor processing stream prior to the motor cortex, within the PPC.
Asunto(s)
Potenciales de Acción/fisiología , Lateralidad Funcional/fisiología , Mano/fisiología , Movimiento/fisiología , Neuronas/fisiología , Lóbulo Parietal/citología , Desempeño Psicomotor/fisiología , Animales , Movimientos Oculares , Macaca mulatta , Imagen por Resonancia Magnética , Masculino , Modelos Estadísticos , Lóbulo Parietal/diagnóstico por imagen , Tiempo de Reacción/fisiología , Máquina de Vectores de Soporte , Factores de TiempoRESUMEN
Feedback and monetary reward can enhance motor skill learning, suggesting reward system involvement. Continuous theta burst (cTBS) transcranial magnetic stimulation (TMS) of the primary motor area (M1) disrupts processing, reduces excitability and impairs motor learning. To see whether feedback and reward can overcome the learning impairment associated with M1 cTBS, we delivered real or sham stimulation to two groups of participants before they performed a motor sequence learning task with and without feedback. Participants were trained on two intermixed sequences, one occurring 85% of the time (the "probable" sequence) and the other 15% of the time (the "improbable" sequence). We measured sequence learning as the difference in reaction time (RT) and error rate between probable and improbable trials (RT and error difference scores). Participants were also tested for sequence recall with the same indices of learning 60 min after cTBS. Real stimulation impaired initial sequence learning and sequence knowledge recall as measured by error difference scores and impaired sequence knowledge recall as measured by RT difference score. Relative to non-feedback learning, the introduction of feedback during sequence learning improved subsequent sequence knowledge recall indexed by RT difference score, in both real and sham stimulation groups and feedback reversed the RT difference score based sequence knowledge recall impairment from real cTBS that we observed in the non-feedback learning condition. Only the real cTBS group in the non-feedback condition showed no evidence of explicit sequence knowledge when tested at the end of the study. Feedback improves recall of implicit and explicit motor sequence knowledge and can protect sequence knowledge against the effect of M1 inhibition. Adding feedback and monetary reward/punishment to motor skill learning may help overcome retention impairments or accelerate training in clinical and other settings.
Asunto(s)
Retroalimentación Psicológica , Trastornos de la Memoria/psicología , Trastornos de la Memoria/terapia , Recuerdo Mental , Corteza Motora , Aprendizaje Seriado , Estimulación Magnética Transcraneal , Adulto , Concienciación , Femenino , Humanos , Masculino , Trastornos de la Memoria/etiología , Persona de Mediana Edad , Destreza Motora , Sistemas en Línea , Desempeño Psicomotor , Tiempo de Reacción , Recompensa , Ritmo Teta , Adulto JovenRESUMEN
Findings from previous transcranial magnetic stimulation (TMS) experiments suggest that the primary motor cortex (M1) is sensitive to reward conditions in the environment. However, the nature of this influence on M1 activity is poorly understood. The dopamine neuron response to conditioned stimuli encodes reward probability and outcome uncertainty, or the extent to which the outcome of a situation is known. Reward uncertainty and probability are related: uncertainty is maximal when probability is 0.5 and minimal when probability is 0 or 1 (i.e., certain outcome). Previous TMS-reward studies did not examine these factors independently. Here, we used single-pulse TMS to measure corticospinal excitability in 40 individuals while they performed a simple computer task, making guesses to find or avoid a hidden target. The task stimuli implied three levels of reward probability and two levels of uncertainty. We found that reward probability level interacted with the trial search condition. That is, motor evoked potential (MEP) amplitude, a measure of corticospinal neuron excitability, increased with increasing reward probability when participants were instructed to "find" a target, but not when they were instructed to "avoid" a target. There was no effect of uncertainty on MEPs. Response times varied with the number of choices. A subset of participants also received paired-pulse stimulation to evaluate changes in short-intracortical inhibition (SICI). No effects of SICI were observed. Taken together, the results suggest that the reward-contingent modulation of M1 activity reflects reward probability or a related aspect of utility, not outcome uncertainty, and that this effect is sensitive to the conceptual framing of the task.
Asunto(s)
Potenciales Evocados Motores/fisiología , Corteza Motora/fisiología , Probabilidad , Tractos Piramidales/fisiología , Recompensa , Incertidumbre , Adulto , Femenino , Humanos , Masculino , Estimulación Magnética Transcraneal , Adulto JovenRESUMEN
Studies of visually guided unimanual reaching have established that a saccade usually precedes each reach and that the reaction times (RTs) for the saccade and reach are highly correlated. The correlation of eye and hand RT is commonly taken as a measure of eye-hand coordination and is thought to assist visuospatial guidance of the hand. We asked what happens during a bimanual reach task. As with a unimanual reach, a saccade was executed first. Although latencies were fastest on unimanual trials, eye and hand RT correlation was identical whether just one or both hands reached to a single target. The average correlation was significantly reduced, however, when each hand reached simultaneously to a different target. We considered three factors that might explain the drop. We found that correlation strength depended on which hand reached first and on which hand reached to the same target as the saccade. Surprisingly, these two factors were largely independent, and the identity of the hand, left or right, had little effect. Eye-hand correlation was similar to that seen with unimanual reaching only when the hand that moved to the same target as the saccade was also the first hand to move. Thus both timing as well as spatial pattern are important in determining eye-hand coordination.
Asunto(s)
Ojo , Mano , Desempeño Psicomotor , Movimientos Sacádicos , Animales , Lateralidad Funcional , Macaca mulatta , Masculino , Tiempo de Reacción , Factores de TiempoRESUMEN
The ability to assess frontal lobe function in a rapid, objective, and standardized way, without the need for expertise in cognitive test administration might be particularly helpful in mild traumatic brain injury (TBI), where objective measures are needed. Functional near infrared spectroscopy (fNIRS) is a reliable technique to noninvasively measure local hemodynamic changes in brain areas near the head surface. In this paper, we are combining fNIRS and frameless stereotaxy which allowed us to co-register the functional images with previously acquired anatomical MRI volumes. In our experiment, the subjects were asked to perform a task, evaluating the complexity of daily life activities, previously shown with fMRI to activate areas of the anterior frontal cortex. We reconstructed averaged oxyhemoglobin and deoxyhemoglobin data from 20 healthy subjects in a spherical coordinate. The spherical coordinate is a natural representation of surface brain activation projection. Our results show surface activation projected from the medial frontopolar cortex which is consistent with previous fMRI results. With this original technique, we will construct a normative database for a simple cognitive test which can be useful in evaluating cognitive disability such as mild traumatic brain injury.
Asunto(s)
Lesiones Encefálicas/fisiopatología , Juicio/fisiología , Espectroscopía Infrarroja Corta , Adulto , Bases de Datos Factuales , Femenino , Humanos , MasculinoRESUMEN
High-Definition transcranial Direct Current Stimulation (HD-tDCS) using specialized small electrodes has been proposed as a focal, non-invasive neuromodulatory technique. Here we provide the first evidence of a change in cortical excitability after HD-tDCS of the motor cortex, using TMS motor evoked potential (MEP) as the measure of excitability. Stimulation for 20 minutes at 1 mA with an anode centered over the hand area of the motor cortex and four surrounding return electrodes (anodal 4×1 montage) produced a significant increase in MEP amplitude and variability after stimulation, compared to sham stimulation. Stimulation was well tolerated by all subjects with adverse effects limited to transient sensation under the electrodes. A high-resolution computational model confirmed predictions of increased focality using the 4×1 HD tDCS montage compared to conventional tDCS. Simulations also indicated that variability in placement of the center electrode relative to the location of the target (central sulcus) could account for increasing variability. These results provide support for the careful use of this technique where focal tDCS is desired.
Asunto(s)
Estimulación Encefálica Profunda/métodos , Corteza Motora/fisiología , Estimulación Magnética Transcraneal/métodos , Adulto , Simulación por Computador , Potenciales Evocados Motores/fisiología , Femenino , Humanos , Masculino , Modelos Neurológicos , Corteza Motora/anatomía & histología , Proyectos Piloto , Adulto JovenRESUMEN
The primary motor cortex is important for motor learning and response selection, functions that require information on the expected and actual outcomes of behavior. Therefore, it should receive signals related to reward. Pathways from reward centers to motor cortex exist in primates. Previously, we showed that gamma aminobutyric acid-A-mediated inhibition in the motor cortex, measured by paired transcranial magnetic stimulation, changes with expectation and uncertainty of money rewards generated by a slot machine simulation. We examined the role of dopamine in this phenomenon by testing 13 mildly affected patients with Parkinson's disease, off and on dopaminergic medications, and 13 healthy, age-matched controls. Consistent with a dopaminergic mechanism, reward expectation or predictability modulated the response to paired transcranial magnetic stimulation in controls, but not in unmedicated patients. A single dose of pramipexole restored this effect of reward, mainly by increasing the paired transcranial magnetic stimulation response amplitude during low expectation. Levodopa produced no such effect. Both pramipexole and levodopa increased risk-taking behavior on the Iowa Gambling Task. However, pramipexole increased risk-taking behavior more in patients showing lower paired transcranial magnetic stimulation response amplitude during low expectation. These results provide evidence that modulation of motor cortex inhibition by reward is mediated by dopamine signaling and that the physiological state of the motor cortex changes with risk-taking tendency in patients on pramipexole. The cortical response to reward expectation may represent an endophenotype for risk-taking behavior in patients on agonist treatment.
Asunto(s)
Trastornos del Conocimiento/etiología , Enfermedad de Parkinson/complicaciones , Recompensa , Anciano , Anciano de 80 o más Años , Antiparkinsonianos/uso terapéutico , Trastornos del Conocimiento/tratamiento farmacológico , Electromiografía , Potenciales Evocados Motores/efectos de los fármacos , Potenciales Evocados Motores/fisiología , Femenino , Juegos Experimentales , Humanos , Masculino , Persona de Mediana Edad , Enfermedad de Parkinson/tratamiento farmacológico , Asunción de Riesgos , Estimulación Magnética TranscranealRESUMEN
Previous evidence suggests that directional social cues (e.g., eye gaze) cause automatic shifts in attention toward gaze direction. It has been proposed that automatic attentional orienting driven by social cues (social orienting) involves a different neural network from automatic orienting driven by nonsocial cues. However, previous neuroimaging studies on social orienting have only compared gaze cues to symbolic cues, which typically engage top-down mechanisms. Therefore, we directly compared the neural activity involved in social orienting to that involved in purely automatic nonsocial orienting. Twenty participants performed a spatial cueing task consisting of social (gaze) cues and automatic nonsocial (peripheral squares) cues presented at short and long stimulus (cue-to-target) onset asynchronies (SOA), while undergoing fMRI. Behaviorally, a facilitation effect was found for both cue types at the short SOA, while an inhibitory effect (inhibition of return: IOR) was found only for nonsocial cues at the long SOA. Imaging results demonstrated that social and nonsocial cues recruited a largely overlapping fronto-parietal network. In addition, social cueing evoked greater activity in occipito-temporal regions at both SOAs, while nonsocial cueing recruited greater subcortical activity, but only for the long SOA (when IOR was found). A control experiment, including central arrow cues, confirmed that the occipito-temporal activity was at least in part due to the social nature of the cue and not simply to the location of presentation (central vs. peripheral). These results suggest an evolutionary trajectory for automatic orienting, from predominantly subcortical mechanisms for nonsocial orienting to predominantly cortical mechanisms for social orienting.
Asunto(s)
Atención/fisiología , Corteza Cerebral/fisiología , Señales (Psicología) , Procesamiento de Imagen Asistido por Computador , Imagen por Resonancia Magnética , Red Nerviosa/fisiología , Comunicación no Verbal/fisiología , Orientación/fisiología , Reflejo/fisiología , Movimientos Sacádicos/fisiología , Adolescente , Adulto , Evolución Biológica , Mapeo Encefálico , Femenino , Lóbulo Frontal/fisiología , Humanos , Inhibición Psicológica , Masculino , Lóbulo Occipital/fisiología , Lóbulo Parietal/fisiología , Reconocimiento Visual de Modelos/fisiología , Tiempo de Reacción/fisiología , Lóbulo Temporal/fisiología , Campos Visuales/fisiología , Adulto JovenRESUMEN
In the lateralized simple reaction time (SRT) task with unimanual responses (Poffenberger paradigm), reaction times (RTs) are faster with ipsilateral (uncrossed) than with contralateral (crossed) response hand-target hemifield combinations. The difference between crossed and uncrossed responses (CUD) has typically been interpreted to reflect callosal transfer time. Indeed, acallosal subjects and split-brain subjects have longer CUDs than control subjects. However, a few recent studies have demonstrated that, contrary to classical findings, the CUD is also affected by non-anatomical factors. Here we show that the CUD is also affected by non-anatomical factors in patients with agenesis of the corpus callosum and complete commissurotomy where interhemispheric transfer must be subcallosal. We tested acallosal subject M.M. and split brain patient A.A. on a lateralized SRT task with their arms alternately uncrossed (natural arms position) or crossed (unnatural arms position) across blocks of trials. The results revealed a significant effect of arms crossing on the size and direction of the CUD as previously found in normal subjects [Mooshagian, E., Iacoboni, M., & Zaidel, E. (2008). The role of task history in simple reaction time to lateralized light flashes. Neuropsychologia, 46(2), 659-664]. This suggests that non-anatomical factors that modulate interhemispheric visuomotor integration may occur in absence of the corpus callosum. Anterior commissure and interhemispheric cortico-subcortical pathways are likely implicated in these effects.
Asunto(s)
Atención , Cuerpo Calloso/fisiopatología , Percepción de Movimiento , Vías Nerviosas , Percepción Espacial , Percepción Visual , Adulto , Agenesia del Cuerpo Calloso , Cuerpo Calloso/cirugía , Lateralidad Funcional , Humanos , Procesamiento de Imagen Asistido por Computador , Imagen por Resonancia Magnética , Masculino , Persona de Mediana Edad , Desempeño Psicomotor , Tiempo de ReacciónRESUMEN
Parallel processing of multiple sensory stimuli is critical for efficient, successful interaction with the environment. An experimental approach to studying parallel processing in sensorimotor integration is to examine reaction times to multiple copies of the same stimulus. Reaction times to bilateral copies of light flashes are faster than to single, unilateral light flashes. These faster responses may be due to 'statistical facilitation' between independent processing streams engaged by the two copies of the light flash. On some trials, however, reaction times are faster than predicted by statistical facilitation. This indicates that a neural 'coactivation' of the two processing streams must have occurred. Here we use fMRI to investigate the neural locus of this coactivation. Subjects responded manually to the detection of unilateral light flashes presented to the left or right visual hemifield, and to the detection of bilateral light flashes. We compared the bilateral trials where subjects' reaction times exceeded the limit predicted by statistical facilitation to bilateral trials that did not exceed the limit. Activity in the right temporo-parietal junction was higher in those bilateral trials that showed coactivation than in those that did not. These results suggest the neural coactivation observed in visuomotor integration occurs at a cognitive rather than sensory or motor stage of processing.
Asunto(s)
Actividad Motora , Lóbulo Parietal/fisiología , Tiempo de Reacción , Lóbulo Temporal/fisiología , Visión Ocular , Humanos , Imagen por Resonancia MagnéticaRESUMEN
Split-brain patients present a unique opportunity to address controversies regarding subcortical contributions to interhemispheric coordination. We characterized residual functional connectivity in a complete commissurotomy patient by examining patterns of low-frequency BOLD functional MRI signal. Using independent components analysis and region-of-interest-based functional connectivity analyses, we demonstrate bilateral resting state networks in a patient lacking all major cerebral commissures. Compared with a control group, the patient's interhemispheric correlation scores fell within the normal range for two out of three regions examined. Thus, we provide evidence for bilateral resting state networks in a patient with complete commissurotomy. Such continued interhemispheric interaction suggests that, at least in part, cortical networks in the brain can be coordinated by subcortical mechanisms.
Asunto(s)
Mapeo Encefálico , Encéfalo/fisiología , Lateralidad Funcional/fisiología , Vías Nerviosas/fisiología , Procedimiento de Escisión Encefálica , Adulto , Anciano , Epilepsia/cirugía , Femenino , Humanos , Procesamiento de Imagen Asistido por Computador , Imagen por Resonancia MagnéticaAsunto(s)
Mapeo Encefálico/métodos , Cuerpo Calloso/anatomía & histología , Cuerpo Calloso/fisiología , Potenciales Evocados Motores/fisiología , Adulto , Cuerpo Calloso/citología , Imagen de Difusión por Resonancia Magnética/métodos , Femenino , Humanos , Masculino , Red Nerviosa/citología , Red Nerviosa/fisiología , Estimulación Magnética Transcraneal/métodosRESUMEN
In lateralized simple reaction time (SRT) tasks with unimanual responses, reaction times (RTs) are faster with ipsilateral (uncrossed) than with contralateral (crossed) response hand-target hemifield combinations. The difference between crossed and uncrossed responses (CUD) is typically interpreted to reflect callosal transfer time. Indeed, split brain patients have much longer CUDs than control subjects. However, while many studies have supported the hypothesis that the CUD reflects callosal transmission time, a few studies have suggested that the CUD may be affected by non-anatomical factors. We investigated the nature of these inconsistent results in two experiments. In the first, we asked half of our subjects to cross their arms while performing the task. The CUD was not affected by arms crossing, supporting the anatomical model of the CUD. In the second experiment, however, all subjects were asked to cross their arms in half of the trials. In this experiment, arms crossing significantly affected the CUD, thus showing that spatial attention modulates the CUD. These latter results cannot be readily explained by a simple callosal relay interpretation of the CUD. Rather, the CUD seems to reflect a mix of anatomical and non-anatomical factors produced by task history. Thus, the seemingly inconsistent results of previous studies can be reconciled by taking into account differences in task history across studies.