The Observation and Execution of Actions Share Motor and Somatosensory Voxels in all Tested Subjects: Single-Subject Analyses of Unsmoothed fMRI Data

Many neuroimaging studies of the mirror neuron system (MNS) examine if certain voxels in the brain are shared between action observation and execution (shared voxels, sVx). Unfortunately, finding sVx in standard group analyses is not a guarantee that sVx exist in individual subjects. Using unsmoothed, single-subject analyses we show sVx can be reliably found in all 16 investigated participants. Beside the ventral premotor (BA6/44) and inferior parietal cortex (area PF) where mirror neurons (MNs) have been found in monkeys, sVx were reliably observed in dorsal premotor, supplementary motor, middle cingulate, somatosensory (BA3, BA2, and OP1), superior parietal, middle temporal cortex and cerebellum. For the premotor, somatosensory and parietal areas, sVx were more numerous in the left hemisphere. The hand representation of the primary motor cortex showed a reduced BOLD during hand action observation, possibly preventing undesired overt imitation. This study provides a more detailed description of the location and reliability of sVx and proposes a model that extends the original idea of the MNS to include forward and inverse internal models and motor and sensory simulation, distinguishing the MNS from a more general concept of sVx

On the Role of Object Information in Action Observation: An fMRI Study

Observing other people’s actions activates a network of brain regions that is also activated during the execution of these actions. Here, we used functional magnetic resonance imaging to test whether these “mirror” regions in frontal and parietal cortices primarily encode the spatiomotor aspects or the functional goal-related aspects of observed tool actions. Participants viewed static depictions of actions consisting of a tool object (e.g., key) and a target object (e.g., keyhole). They judged the actions either with regard to whether the objects were oriented correctly for the action to succeed (spatiomotor task) or whether an action goal could be achieved with the objects (function task). Compared with a control condition, both tasks activated regions in left frontoparietal cortex previously implicated in action observation and execution. Of these regions, the premotor cortex and supramarginal gyrus were primarily activated during the spatiomotor task, whereas the middle frontal gyrus was primarily activated during the function task. Regions along the intraparietal sulcus were more strongly activated during the spatiomotor task but only when the spatiomotor properties of the tool object were unknown in advance. These results suggest a division of labor within the action observation network that maps onto a similar division previously proposed for action execution

Priming of reach trajectory when observing actions: Hand-centred effects

When another person's actions are observed it appears that these actions are simulated, such that similar motor processes are triggered in the observer. Much evidence suggests that such simulation concerns the achievement of behavioural goals, such as grasping a particular object, and is less concerned with the specific nature of the action, such as the path the hand takes to reach the goal object. We demonstrate that when observing another person reach around an obstacle, an observer's subsequent reach has an increased curved trajectory, reflecting motor priming of reach path. This priming of reach trajectory via action observation can take place under a variety of circumstances: with or without a shared goal, and when the action is seen from a variety of perspectives. However, of most importance, the reach path priming effect is only evoked if the obstacle avoided by another person is within the action (peripersonal) space of the observer

Uncertainty and Invariance in the Human Visual Cortex

The way in which input noise perturbs the behavior of a system depends on the internal processing structure of the system. In visual psychophysics, there is a long tradition of using external noise methods (i.e., adding noise to visual stimuli) as tools for system identification. Here, we demonstrate that external noise affects processing of visual scenes at different cortical areas along the human ventral visual pathway, from retinotopic regions to higher occipitotemporal areas implicated in visual shape processing. We found that when the contrast of the stimulus was held constant, the further away from the retinal input a cortical area was the more its activity, as measured with functional magnetic resonance imaging (fMRI), depended on the signal-to-noise ratio (SNR) of the visual stimulus. A similar pattern of results was observed when trials with correct and incorrect responses were analyzed separately. We interpret these findings by extending signal detection theory to fMRI data analysis. This approach reveals the sequential ordering of decision stages in the cortex by exploiting the relation between fMRI response and stimulus SNR. In particular, our findings provide novel evidence that occipitotemporal areas in the ventral visual pathway form a cascade of decision stages with increasing degree of signal uncertainty and feature invariance

Differential involvement of prefrontal and parietal areas in human imitation revealed by fMRI adaptation

The neuronal system involved in action understanding and imitation, involves the ventral premotor cortex (Ba44), parietal areas and the Superior Temporal Sulcus (STS). Neuroimaging and neurophysiological studies implicate the ventral premotor cortex in the processing of action goals while the function of the remaining areas is largely unknown. The present study investigated whether the goal and the kinematics of the movement are differentially processed within these cortical areas. We used an event-related fMRI adaptation paradigm, in which fMRI responses to two sequentially repeated stimuli are lower than for different stimuli. Using kinematics morphs (Hill Pollick, 2000) we tested the hypothesis that the premotor cortex in humans processes information about the goal of an action while parietal regions code for the kinematics of the movement. Four different action types and their kinematics morphs were used for the experiments. We functionally localised the brain areas involved in action understanding and imitation. We then tested for fMRI responses in the different experimental conditions during the event related scans. In the first series of experiments we showed that the premotor cortex represents the goal of the movements independent of their kinematics. Adaptation effects were observed across changes in the movement kinematics in the premotor cortex but not in parietal regions. These results suggest that the premotor cortex represents the goal of movements independent of their kinematics, while parietal regions encode information about the movement kinematics. Surprisingly, adaptation effects in hMT+/V5 and STS were similar to these in the premotor cortex. No differences across conditions were observed in early visual areas (i.e. V1). Subsequent experiments tested fMRI responses when the kinematics of the action remained the same while the goal of the action changed

Neuropsychological evidence for a dissociation in counting and subitizing.

There is a long and ongoing debate about whether subitizing and counting are separable processes. In the present paper we report a single case, MH, who presents with a dissociation in subitizing and counting. MH was spared in his ability to enumerate small numbers accurately along with a marked inability to count larger numbers. We show that non-visual counting was intact and visual counting improved when a motor record of counting could be maintained. Moreover, when larger numbers of items were spatially grouped into 2 subitizable units, performance dramatically improved. However, color grouping did not aid MH's performance, despite his being sensitive to color segmentation. In addition, MH made more re-visits of inspected locations than controls, and he was less aware of a re-visitation being made. The data cannot be explained in terms of general working memory problems (verbal working memory was relatively spared), or general number comprehension problems (e.g., simple sums and counting of auditory items was intact); but they can parsimoniously be accounted for in terms of impaired visuo-spatial memory. The findings support the argument that at least some processes are specific to counting and are not required for subitization - in particular spatial coding and memory for previously inspected locations

The involvement of parietal and prefrontal areas in human imitation revealed by fMRI adaptation

The perception and imitation of human movement requires that the brain integrates information about the goal of the movement and the kinematics that define it. Neuroimaging and neurophysiological studies implicate the ventral premotor cortex (Ba44) in the processing of action goals while the role of the parietal cortex is not entirely clear. The aim of this series of experiments was to disentangle the role of both prefrontal and parietal areas in the imitation of human movement. To this end we used human arm movements presented as point light displays. The movements were manipulated parametrically to produce morphs that differed from each other in their kinematics. Three different action types -throwing, lifting and knocking movements- and their morphs were utilised for this study. We used a rapid event related fMRI adaptation paradigm, in which fMRI responses to two sequentially repeated stimuli are lower than for different stimuli. We begun by functionally localising the brain areas involved in the imitation of human movement. We then looked at the MR signal under the different experimental conditions during the event related scans. In a first experiment we investigated the basic adaptation effect; identical movements both in their action goals and kinematics were tested against movements that were different in both their goals and kinematics. Preliminary evidence suggests that prefrontal and parietal areas show adaptation under those different experimental conditions. Future experiments will test whether the parietal areas respond to different kinematics even when the goal of the movement is the same, by using the kinematics morphs

An fmri method for identifying the sequential stages of processing in the ventral visual pathway

Visual processing in the human ventral cortex entails extraction of features from retinal images that mediate perception. In the human ventral cortex, early and late visual areas have been implicated in the analysis of simple and complex features respectively. If we view this processing pathway as a sequence of decision stages, each extracting progressively more abstract and invariant features from the output of preceding stages, then by considering the relationship between signal uncertainty and the slope of a psychometric function, we can show that the extent to which the output of a decision stage is perturbed by noise added to the visual stimulus will be more threshold-like (steeper log-log slope) for a decision stage further down the decision cascade. To test this theory, we used images of scenes and added visual noise that matched the signal's spatial-frequency power spectrum. The resulting images were rescaled to maintain a constant mean luminance and rms contrast across all noise levels. We localized individually in each observer the retinotopic regions and the LOC, and measured event-related BOLD response in these regions during a scene discrimination task performed at 4 noise levels. Behavioral performance increased with increasing signal-to-noise ratio. We found that log BOLD signal change from fixation baseline vs. log SNR is well-described by a straight line for all visual areas. The regression slope increased monotonically from early to late areas along the ventral stream. A factor of 8 change in SNR produced little change to the BOLD response in V1/V2, but resulted in progressively larger changes in V4v, posterior (LO), and anterior (pFs) subregions of the LOC. In accordance with our theory on noise perturbation, the results suggest approximately ordered decision stages in the ventral pathway

Human fMRI Studies of Visual Processing in Noise

Processing of visual information entails the extraction of features from retinal images that mediate visual perception. In the human ventral cortex, retinotopic and higher visual areas (e.g. Lateral Occipital Complex-LOC) have been implicated in the analysis of simple and more complex features respectively. To test how processing of complex natural images progresses across the human ventral cortex, we used images of scenes and added visual noise that matched the signal in spatial-frequency power spectrum. The resulting images were rescaled to ensure constant mean luminance and r.m.s. contrast across all noise levels. We localized individually in each observer the retinotopic regions and the LOC and measured event-related BOLD response in these regions during a scene discrimination task performed at 4 noise levels. Behavioral accuracy increased with increasing signal-to-noise ratio (SNR). We found that log BOLD signal change from fixation baseline vs. log SNR is well-described by a straight line for all visual areas. The regression slope increased monotonically from lower to higher visual areas along the ventral stream. For example, changes by a factor of 8 in SNR produced little or no change to the BOLD response in V1/V2, but resulted in progressively larger increases in V4v, posterior, and anterior sub-regions of the LOC. These findings suggest that the use of visual noise can reveal the progression in complexity of the natural-image features that are processed across the human visual areas