Individual Variability in Brain Activity: A Nuisance or an Opportunity?

Functional imaging research has been heavily influenced by results based on population-level inference. However, group average results may belie the unique patterns of activity present in the individual that ordinarily are considered random noise. Recent advances in the evolution of MRI hardware have led to significant improvements in the stability and reproducibility of blood oxygen level dependent (BOLD) measurements. These enhancements provide a unique opportunity for closer examination of individual patterns of brain activity. Three objectives can be accomplished by considering brain scans at the individual level; (1) Mapping functional anatomy at a fine grained analysis; (2) Determining if an individual scan is normative with respect to a reference population; and (3) Understanding the sources of intersubject variability in brain activity. In this review, we detail these objectives, briefly discuss their histories and present recent trends in the analyses of individual variability. Finally, we emphasize the unique opportunities and challenges for understanding individual differences through international collaboration among Pacific Rim investigators.

The influence of feedback valence in associative learning

The neural systems engaged by intrinsic positive or negative feedback were defined in an associative learning task. Through trial and error, participants learned the arbitrary assignments of a set of stimuli to one of two response categories. Informative feedback was provided on less than 25% of the trials. During positive feedback blocks, half of the trials were eligible for informative feedback; of these, informative feedback was only provided when the response was correct. A similar procedure was used on negative feedback blocks, but here informative feedback was only provided when the response was incorrect. In this manner, we sought to identify regions that were differentially responsive to positive and negative feedback as well as areas that were responsive to both types of informative feedback. Several regions of interest, including the bilateral nucleus accumbens, caudate nucleus, anterior insula, right cerebellar lobule VI, and left putamen, were sensitive to informative feedback regardless of valence. In contrast, several regions were more selective to positive feedback compared to negative feedback. These included the insula, amygdala, putamen, and supplementary motor area. No regions were more strongly activated by negative feedback compared to positive feedback. These results indicate that the neural areas supporting associative learning vary as a function of how that information is learned. In addition, areas linked to intrinsic reinforcement showed considerable overlap with those identified in studies using extrinsic reinforcers.

Sensitivity of the action observation network to physical and observational learning

Human motor skills can be acquired by observation without the benefit of immediate physical practice. The current study tested if physical rehearsal and observational learning share common neural substrates within an action observation network (AON) including premotor and inferior parietal regions, that is, areas activated both for execution and observation of similar actions. Participants trained for 5 days on dance sequences set to music videos. Each day they physically rehearsed one set of dance sequences ("danced"), and passively watched a different set of sequences ("watched"). Functional magnetic resonance imaging was obtained prior to and immediately following the 5 days of training. After training, a subset of the AON showed a degree of common activity for observational and physical learning. Activity in these premotor and parietal regions was sustained during observation of sequences that were danced or watched, but declined for unfamiliar sequences relative to the pretraining scan session. These imaging data demonstrate the emergence of action resonance processes in the human brain based on observational learning without physical practice and identify commonalities in the neural substrates for physical and observational learning.

The Angular Gyrus Computes Action Awareness Representations

Involvement of the right inferior parietal area in action awareness was investigated while taking into account differences in the conscious experiences of one's own actions; especially, the awareness that an intended action is consistent with movement consequences and the awareness of the authorship of the action (i.e., the sense of agency). We hypothesized that these experiences are both associated with processes implemented in inferior parietal cortex, specifically, right angular gyrus (Ag). Two blood-oxygenation-level-dependent functional magnetic resonance imaging studies employed a novel delayed visual feedback technique to distinguish the neural correlates of these 2 forms of action awareness. We showed that right Ag is associated with both awareness of discrepancy between intended and movement consequences and awareness of action authorship. We propose that this region is involved in higher-order aspects of motor control that allows one to consciously access different aspects of one's own actions. Specifically, this region processes discrepancies between intended action and movement consequences in such a way that these will be consciously detected by the subject. This joint processing is at the core of the various experiences one uses to interpret an action.

Neural Substrates of Contextual Interference during Motor Learning Support a Model of Active Preparation

When individuals acquire new skills, initial performance is typically better and tasks are judged to be easier when the tasks are segregated and practiced by block, compared to when different tasks are randomly intermixed in practice. However, subsequent skill retention is better for a randomly practiced group, an effect known as contextual interference (CI). The present study examined the neural substrates of CI using functional magnetic resonance imaging (fMRI). Individuals learned a set of three 4-element sequences with the left hand according to a block or random practice schedule. Behavioral retest for skill retention confirmed the presence of a typical CI effect with the random group outperforming the block group. Using a go/no-go fMRI paradigm, sequence preparation during the premovement study period was separated from movement execution. Imaging data for the two groups were compared for the first 1/3 and final 1/3 of training trials. Toward the end of training, behavioral performance between the two groups was similar, although the random group would later display a performance advantage on retention testing. During study time, the random group showed greater activity in sensorimotor and premotor regions compared to the block group. These areas are associated with motor preparation, sequencing, and response selection. This pattern of recruitment is consistent with the hypothesis that CI benefits in a sequencing task are due to improved capacity to actively prepare motor responses.

Neural substrates of visuomotor learning based on improved feedback control and prediction

Motor skills emerge from learning feedforward commands as well as improvements in feedback control. These two components of learning were investigated in a compensatory visuomotor tracking task on a trial-by-trial basis. Between-trial learning was characterized with a state-space model to provide smoothed estimates of feedforward and feedback learning, separable from random fluctuations in motor performance and error. The resultant parameters were correlated with brain activity using magnetic resonance imaging. Learning related to the generation of a feedforward command correlated with activity in dorsal premotor cortex, inferior parietal lobule, supplementary motor area and cingulate motor area, supporting a role of these areas in retrieving and executing a predictive motor command. Modulation of feedback control was associated with activity in bilateral posterior superior parietal lobule as well as right ventral premotor cortex. Performance error correlated with activity in a widespread cortical and subcortical network including bilateral parietal, premotor and rostral anterior cingulate cortex as well as the cerebellar cortex. Finally, trial-by-trial changes of kinematics, as measured by mean absolute hand acceleration, correlated with activity in motor cortex and anterior cerebellum. The results demonstrate that incremental, learning-dependent changes can be modeled on a trial-by-trial basis and neural substrates for feedforward control of novel motor programs are localized to secondary motor areas.

The neural basis of love as a subliminal prime: an event-related functional magnetic resonance imaging study

Throughout the ages, love has been defined as a motivated and goal-directed mechanism with explicit and implicit mechanisms. Recent evidence demonstrated that the explicit representation of love recruits subcorticocortical pathways mediating reward, emotion, and motivation systems. However, the neural basis of the implicit (unconscious) representation of love remains unknown. To assess this question, we combined event-related functional magnetic resonance imaging (fMRI) with a behavioral subliminal priming paradigm embedded in a lexical decision task. In this task, the name of either a beloved partner, a neutral friend, or a passionate hobby was subliminally presented before a target stimulus (word, nonword, or blank), and participants were required to decide if the target was a word or not. Behavioral results showed that subliminal presentation of either a beloved's name (love prime) or a passion descriptor (passion prime) enhanced reaction times in a similar fashion. Subliminal presentation of a friend's name (friend prime) did not show any beneficial effects. Functional results showed that subliminal priming with a beloved's name (as opposed to either a friend's name or a passion descriptor) specifically recruited brain areas involved in abstract representations of others and the self, in addition to motivation circuits shared with other sources of passion. More precisely, love primes recruited the fusiform and angular gyri. Our findings suggest that love, as a subliminal prime, involves a specific neural network that surpasses a dopaminergic-motivation system.

Action outcomes are represented in human inferior frontoparietal cortex

The simple action of pressing a switch has many possible interpretations--the actor could be turning on a light, deleting critical files from a computer, or even turning off a life-support system. In each of these cases, the motor parameters of the action are the same but the physical outcome differs. We report evidence of suppressed responses in right inferior parietal and right inferior frontal cortex when participants saw repeated movies showing the same action outcome, but these regions did not distinguish the kinematic parameters by which the action was accomplished. Thus, these brain areas encode the physical outcomes of human actions in the world. These results are compatible with a hierarchical model of human action understanding in which a cascade of specialized processes from occipital to parietal and frontal regions allow humans to understand the physical consequences of actions in the world and the intentions underlying those actions.

Evidence for a distributed hierarchy of action representation in the brain

Complex human behavior is organized around temporally distal outcomes. Behavioral studies based on tasks such as normal prehension, multi-step object use and imitation establish the existence of relative hierarchies of motor control. The retrieval errors in apraxia also support the notion of a hierarchical model for representing action in the brain. In this review, three functional brain imaging studies of action observation using the method of repetition suppression are used to identify a putative neural architecture that supports action understanding at the level of kinematics, object centered goals and ultimately, motor outcomes. These results, based on observation, may match a similar functional-anatomic hierarchy for action planning and execution. If this is true, then the findings support a functional-anatomic model that is distributed across a set of interconnected brain areas that are differentially recruited for different aspects of goal-oriented behavior, rather than a homogeneous mirror neuron system for organizing and understanding all behavior.

On-line grasp control is mediated by the contralateral hemisphere

Electrophysiological recordings from monkeys, as well as functional imaging and neuropsychological work with humans, have suggested that a region in the anterior portion of the intraparietal sulcus (aIPS) is involved in prehensile movements. With recent methodological advances using transcranial magnetic stimulation (TMS), we can now causally attribute anatomy with function to more precisely determine the specific involvement of aIPS in grasping. It has recently been demonstrated that aIPS is specifically involved in executing a grasp under conditions of both constant target requirements, as well as in correcting a movement under conditions in which a target perturbation occurs. In the present study, we extend these findings by determining the differential contribution of the left and right hemisphere to executing a grasping movement with the left and right hands. Transient disruption of left aIPS at movement onset impairs grasping with the right but not the left hand, and disruption of right aIPS impairs grasping with the left but not the right hand. We conclude that grasping is a lateralized process, relying exclusively on the contralateral hemisphere, and discuss the implications of these findings in relationship to models of hemispheric dominance for motor control.

Correlation between insula activation and self-reported quality of orgasm in women

Current multidimensional models of women's sexual function acknowledge the implicit impact of psychosocial factors on women's sexual function. Interaction between human sexual function and intensity of love has been also assumed, even if love is not an absolute condition. Yet, whereas great insights have been made in understanding the central mechanisms of the peripheral manifestations of women's sexual response, including orgasm, the cerebral correlates sustaining the interaction between women's sexual satisfaction and the unconscious role of the partner in this interpersonal experience remain unknown. Using functional imaging, we assessed brain activity elicited when 29 healthy female volunteers were unconsciously exposed to the subliminal presentation of their significant partner's name (a task known to elicit a partner-related neural network) and correlated it with individual scores obtained from different sexual dimensions: self-reported partnered orgasm quality (ease, satisfaction, frequency), love intensity and emotional closeness with that partner. Behavioral results identified a correlation between love and self-reported partnered orgasm quality. The more women were in love/emotionally close to their partner, the more they tended to report being satisfied with the quality of their partnered orgasm. However, no relationship was found between intensity of love and partnered orgasm frequency. Neuroimaging data expanded these behavioral results by demonstrating the involvement of a specific left-lateralized insula focus of neural activity correlating with orgasm scores, irrespective of dimension (frequency, ease, satisfaction). In contrast, intensity of being in love was correlated with a network involving the angular gyrus. These findings strongly suggest that intimate and sexual relationships are sustained by partly different mechanisms, even if they share some emotional-related mechanisms. The critical correlation between self-reports of orgasm quality and activation of the left anterior insula, a part of the partner-related neural network known to play a pivotal role in somatic processes, suggests the importance of somatic information in the integration of sexual experience. On the other hand, the correlation between activation of the angular gyrus and love intensity reinforces the assumption that the representation of love calls for higher order cognitive levels, such as those related to the generation of abstract concepts. By highlighting the specific role of the anterior insula in the way women integrate components of physical satisfaction in the context of an intimate relationship with a partner, the current findings take a step in the understanding of a woman's sexual pleasure.

Beyond grasping: representation of action in human anterior intraparietal sulcus

The fronto-parietal network has been implicated in the processing of multisensory information for motor control. Recent methodological advances with both fMRI and TMS provide the opportunity to dissect the functionality of this extensive network in humans and may identify distinct contributions of local neural populations within this circuit that are not only related to motor planning, but to goal oriented behavior as a whole. Herein, we review and make parallels between experiments in monkeys and humans on a broad array of motor as well as non-motor tasks in order to characterize the specific contribution of a region in the parietal lobe, the anterior intraparietal sulcus (aIPS). The intent of this article is to review: (1) the historical perspectives on the parietal lobe, particularly the aIPS; (2) extend and update these perspectives based on recent empirical data; and (3) discuss the potential implications of the revised functionality of the aIPS in relationship to complex goal oriented behavior and social interaction. Our contention is that aIPS is a critical node within a network involved in the higher order dynamic control of action, including representation of intended action goals. These findings may be important not only for guiding the design of future experiments investigating related issues but may also have valuable utility in other fields, such social neuroscience and biomedical engineering.

Wandering minds: the default network and stimulus-independent thought

Despite evidence pointing to a ubiquitous tendency of human minds to wander, little is known about the neural operations that support this core component of human cognition. Using both thought sampling and brain imaging, the current investigation demonstrated that mind-wandering is associated with activity in a default network of cortical regions that are active when the brain is "at rest." In addition, individuals' reports of the tendency of their minds to wander were correlated with activity in this network.

BOLD coherence reveals segregated functional neural interactions when adapting to distinct torque perturbations

In the natural world, we experience and adapt to multiple extrinsic perturbations. This poses a challenge to neural circuits in discriminating between different context-appropriate responses. Using event-related fMRI, we characterized the neural dynamics involved in this process by randomly delivering a position- or velocity-dependent torque perturbation to subjects' arms during a target-capture task. Each perturbation was color-cued during movement preparation to provide contextual information. Although trajectories differed between perturbations, subjects significantly reduced error under both conditions. This was paralleled by reduced BOLD signal in the right dentate nucleus, the left sensorimotor cortex, and the left intraparietal sulcus. Trials included "NoGo" conditions to dissociate activity related to preparation from execution and adaptation. Subsequent analysis identified perturbation-specific neural processes underlying preparation ("NoGo") and adaptation ("Go") early and late into learning. Between-perturbation comparisons of BOLD magnitude revealed negligible differences for both preparation and adaptation trials. However, a network-level analysis of BOLD coherence revealed that by late learning, response preparation ("NoGo") was attributed to a relative focusing of coherence within cortical and basal ganglia networks in both perturbation conditions, demonstrating a common network interaction for establishing arbitrary visuomotor associations. Conversely, late-learning adaptation ("Go") was attributed to a focusing of BOLD coherence between a cortical-basal ganglia network in the viscous condition and between a cortical-cerebellar network in the positional condition. Our findings demonstrate that trial-to-trial acquisition of two distinct adaptive responses is attributed not to anatomically segregated regions, but to differential functional interactions within common sensorimotor circuits.

Anatomical substrates of visual and auditory miniature second-language learning

Longitudinal changes in brain activity during second language (L2) acquisition of a miniature finite-state grammar, named Wernickese, were identified with functional magnetic resonance imaging (fMRI). Participants learned either a visual sign language form or an auditory-verbal form to equivalent proficiency levels. Brain activity during sentence comprehension while hearing/viewing stimuli was assessed at low, medium, and high levels of proficiency in three separate fMRI sessions. Activation in the left inferior frontal gyrus (Broca's area) correlated positively with improving L2 proficiency, whereas activity in the right-hemisphere (RH) homologue was negatively correlated for both auditory and visual forms of the language. Activity in sequence learning areas including the premotor cortex and putamen also correlated with L2 proficiency. Modality-specific differences in the blood oxygenation level-dependent signal accompanying L2 acquisition were localized to the planum temporale (PT). Participants learning the auditory form exhibited decreasing reliance on bilateral PT sites across sessions. In the visual form, bilateral PT sites increased in activity between Session 1 and Session 2, then decreased in left PT activity from Session 2 to Session 3. Comparison of L2 laterality (as compared to L1 laterality) in auditory and visual groups failed to demonstrate greater RH lateralization for the visual versus auditory L2. These data establish a common role for Broca's area in language acquisition irrespective of the perceptual form of the language and suggest that L2s are processed similar to first languages even when learned after the "critical period." The right frontal cortex was not preferentially recruited by visual language after accounting for phonetic/structural complexity and performance.

The anterior intraparietal sulcus mediates grasp execution, independent of requirement to update: new insights from transcranial magnetic stimulation

Although a role of the intraparietal sulcus (IPS) in grasping is becoming evident, the specific contribution of regions within the IPS remains undefined. In this vein, transcranial magnetic stimulation (TMS) was delivered to the anterior (aIPS), middle (mIPS), and caudal (cIPS) IPS in two tasks designed to dissociate the potential roles of the IPS in either grasp planning or execution (task 1) and its involvement in error detection or error correction (task 2). Determining the involvement of specific regions of the IPS in perceptual (planning and error detection) versus motor (execution and correction) components of grasping allowed us to assess the ecological validity of competing computational models attempting to simulate reach-to-grasp movements. In task 1, we demonstrate that, when no on-line adjustment is necessary, TMS to aIPS (but not mIPS or cIPS) disrupts grasping; this disruption is only elicited when TMS is applied during the execution (but not the planning) phase of the movement. Task 2 reveals that TMS to aIPS (but not mIPS or cIPS) disrupts grasping in the presence of a perturbation; this disruption is only elicited when TMS is applied during the error correction (but not error detection) phase of the movement. We propose that the specific contribution of the aIPS in grasping is in the on-line computation of a difference vector based on motor goal, efference copy, and sensory inputs. This computation is performed for both stable and perturbed motor goals.

Alcohol-induced suppression of BOLD activity during goal-directed visuomotor performance

The neurophysiological influence of alcohol produces deficits of many cognitive functions, including executive and motor control processes. This study examined the acute effects of alcohol in the context of goal-directed visuomotor performance during functional magnetic resonance imaging (fMRI). Subjects consumed alcohol-laced gelatin during one scan session and non-alcoholic placebo gelatin in another. During each session, subjects performed a visuomotor target capture where they received continuous or terminal positional feedback information. Blood-oxygen level-dependent (BOLD) activity in the cerebellum was suppressed in the presence of alcohol, consistent with the known ethanol sensitivity of the cerebellum. A fronto-parietal network was identified as most affected by alcohol consumption, with differential patterns of BOLD contingent on visual feedback. Results indicate that alcohol selectively suppresses cognitive activity in frontal and posterior parietal brain regions that, in conjunction with cerebellar nuclei, are believed to contribute to the formation of internal cognitive models of motor representation and action.

Motor experience with graspable objects reduces their implicit analysis in visual- and motor-related cortex

Motor-related regions of parietal and prefrontal cortices have been shown to selectively activate when observers passively view objects that afford manual grasping. Yet, it remains unknown whether these cortical responses depend on prior motor-related experience with the object being observed. To address this question, we asked participants to undergo fMRI scanning while viewing exemplars of two different categories of graspable objects: one associated with extensive motor experience (door knobs) and one associated with no self-reported motor experience (artificial rock climbing holds). Despite participants' lack of experience grasping climbing holds, these objects were found to generate a systematic response in several visuomotor-related regions of cortex-including left PMv and left AIP. Interestingly, however, the response to door knobs did not include activity in any motor-related regions, being limited instead to a comparatively small bilateral area of lateral occipital cortex, relative to the more spatially extensive response in occipital and temporal cortex that was observed for climbing holds. This result suggested that object-specific responses in both visual- and motor-related cortex may in fact negatively correlate with object-specific motor experience. To test this possibility, we repeated the experiment using participants having extensive self-reported experience grasping climbing holds (i.e., veteran indoor rock climbers). Consistent with our hypothesis, both climbing holds and door knobs generated activity limited to lateral occipital cortex. Taken together, these data support the proposal that repeated real-world motor experience with an object category may lead to reduced implicit analysis in both motor- and visual-related regions of cortex.

Normalizing motor-related brain activity: subthalamic nucleus stimulation in Parkinson disease

OBJECTIVE

To test whether therapeutic unilateral deep brain stimulation (DBS) of the subthalamic nucleus (STN) in patients with Parkinson disease (PD) leads to normalization in the pattern of brain activation during movement execution and control of movement extent.

METHODS

Six patients with PD were imaged off medication by PET during performance of a visually guided tracking task with the DBS voltage programmed for therapeutic (effective) or subtherapeutic (ineffective) stimulation. Data from patients with PD during ineffective stimulation were compared with a group of 13 age-matched control subjects to identify sites with abnormal patterns of activation. Conjunction analysis was used to identify those areas in patients with PD where activity normalized when they were treated with effective stimulation.

RESULTS

For movement execution, effective DBS caused an increase of activation in the supplementary motor area (SMA), superior parietal cortex, and cerebellum toward a more normal pattern. At rest, effective stimulation reduced overactivity of SMA. Therapeutic stimulation also induced reductions of movement related "overactivity" compared with healthy subjects in prefrontal, temporal lobe, and basal ganglia circuits, consistent with the notion that many areas are recruited to compensate for ineffective motor initiation. Normalization of activity related to the control of movement extent was associated with reductions of activity in primary motor cortex, SMA, and basal ganglia.

CONCLUSIONS

Effective subthalamic nucleus stimulation leads to task-specific modifications with appropriate recruitment of motor areas as well as widespread, nonspecific reductions of compensatory or competing cortical activity.

Goal representation in human anterior intraparietal sulcus

When a child reaches toward a cookie, the watching parent knows immediately what the child wants. The neural basis of this ability to interpret other people's actions in terms of their goals has been the subject of much speculation. Research with infants has shown that 6 month olds respond when they see an adult reach to a novel goal but habituate when an adult reaches to the same goal repeatedly. We used a similar approach in an event-related functional magnetic resonance imaging experiment. Adult participants observed a series of movies depicting goal-directed actions, with the sequence controlled so that some goals were novel and others repeated relative to the previous movie. Repeated presentation of the same goal caused a suppression of the blood oxygen level-dependent response in two regions of the left intraparietal sulcus. These regions were not sensitive to the trajectory taken by the actor's hand. This result demonstrates that the anterior intraparietal sulcus represents the goal of an observed action.