Learning to imitate novel motion sequences

A cognitive nose? Evaluating working memory benchmarks in the olfactory domain

Working memory (WM) processes are assumed to operate on a wide variety of sensory materials, yet WM research rarely extends beyond sight and hearing. In this systematic review, we integrate research from studies that address WM in olfaction, the sense of smell, spanning the last 50 yr (N = 44). We assessed whether 21 proposed "benchmarks" for WM generalize to olfactory WM. Seven benchmarks generalized to olfaction, whereas 2 failed to generalize. Evidence was insufficient to address the remaining 12 benchmarks (4 had mixed support and 8 were yet unaddressed). We conclude that the available evidence indicates that the sense of smell has a short-term memory system that mostly resembles WM processes in "higher" senses, although there are exceptions related to how olfactory WM performance is associated with other functions. We argue that researchers studying WM should explicitly consider evidence outside of the audio-visual senses when establishing theoretical frameworks. Further, we point out avenues for future research that may help close the remaining gaps in knowledge on this neglected topic.

Dynamic resource allocation in spatial working memory during full and partial report tasks

Serial position effects are well-documented in working memory literature. Studies of spatial short-term memory that rely on binary response; full report tasks tend to report stronger primacy than recency effects. In contrast, studies that utilize a continuous response, partial report task report stronger recency than primacy effects (Gorgoraptis, Catalao, Bays, & Husain, 2011; Zokaei, Gorgoraptis, Bahrami, Bays, & Husain, 2011). The current study explored the idea that probing spatial working memory using full and partial continuous response tasks would produce different distributions of visuospatial working memory resources across spatial sequences and, therefore, explain the conflicting results in the literature. Experiment 1 demonstrated that primacy effects were observed when memory was probed with a full report task. Experiment 2 confirmed this finding while controlling eye movements. Critically, Experiment 3 demonstrated that switching from a full to a partial report task abolished the primacy effect and produced a recency effect, consistent with the idea that the distribution of resources in visuospatial working memory depends on the type of recall required. It is argued that the primacy effect in the whole report task arose from the accumulation of noise caused by the execution of multiple spatially directed actions during recall, whereas the recency effect in the partial report task reflects the redistribution of preallocated resources when an anticipated item is not presented. These data show that it is possible to reconcile apparently contradictory findings within the resource theory of spatial working memory and the importance of considering how memory is probed when interpreting behavioral data through the lens of resource theories of spatial working memory.

Differential impact of endogenous and exogenous attention on activity in human visual cortex

How do endogenous (voluntary) and exogenous (involuntary) attention modulate activity in visual cortex? Using ROI-based fMRI analysis, we measured fMRI activity for valid and invalid trials (target at cued/un-cued location, respectively), pre- or post-cueing endogenous or exogenous attention, while participants performed the same orientation discrimination task. We found stronger modulation in contralateral than ipsilateral visual regions, and higher activity in valid- than invalid-trials. For endogenous attention, modulation of stimulus-evoked activity due to a pre-cue increased along the visual hierarchy, but was constant due to a post-cue. For exogenous attention, modulation of stimulus-evoked activity due to a pre-cue was constant along the visual hierarchy, but was not modulated due to a post-cue. These findings reveal that endogenous and exogenous attention distinctly modulate activity in visuo-occipital areas during orienting and reorienting; endogenous attention facilitates both the encoding and the readout of visual information whereas exogenous attention only facilitates the encoding of information.

The Embodied Brain of SOVEREIGN2: From Space-Variant Conscious Percepts During Visual Search and Navigation to Learning Invariant Object Categories and Cognitive-Emotional Plans for Acquiring Valued Goals

This article develops a model of how reactive and planned behaviors interact in real time. Controllers for both animals and animats need reactive mechanisms for exploration, and learned plans to efficiently reach goal objects once an environment becomes familiar. The SOVEREIGN model embodied these capabilities, and was tested in a 3D virtual reality environment. Neural models have characterized important adaptive and intelligent processes that were not included in SOVEREIGN. A major research program is summarized herein by which to consistently incorporate them into an enhanced model called SOVEREIGN2. Key new perceptual, cognitive, cognitive-emotional, and navigational processes require feedback networks which regulate resonant brain states that support conscious experiences of seeing, feeling, and knowing. Also included are computationally complementary processes of the mammalian neocortical What and Where processing streams, and homologous mechanisms for spatial navigation and arm movement control. These include: Unpredictably moving targets are tracked using coordinated smooth pursuit and saccadic movements. Estimates of target and present position are computed in the Where stream, and can activate approach movements. Motion cues can elicit orienting movements to bring new targets into view. Cumulative movement estimates are derived from visual and vestibular cues. Arbitrary navigational routes are incrementally learned as a labeled graph of angles turned and distances traveled between turns. Noisy and incomplete visual sensor data are transformed into representations of visual form and motion. Invariant recognition categories are learned in the What stream. Sequences of invariant object categories are stored in a cognitive working memory, whereas sequences of movement positions and directions are stored in a spatial working memory. Stored sequences trigger learning of cognitive and spatial/motor sequence categories or plans, also called list chunks, which control planned decisions and movements toward valued goal objects. Predictively successful list chunk combinations are selectively enhanced or suppressed via reinforcement learning and incentive motivational learning. Expected vs. unexpected event disconfirmations regulate these enhancement and suppressive processes. Adaptively timed learning enables attention and action to match task constraints. Social cognitive joint attention enables imitation learning of skills by learners who observe teachers from different spatial vantage points.

Visuomotor Behaviour in Amblyopia: Deficits and Compensatory Adaptations

Amblyopia is a neurodevelopmental visual disorder arising from decorrelated binocular experience during the critical periods of development. The hallmark of amblyopia is reduced visual acuity and impairment in binocular vision. The consequences of amblyopia on various sensory and perceptual functions have been studied extensively over the past 50 years. Historically, relatively fewer studies examined the impact of amblyopia on visuomotor behaviours; however, research in this area has flourished over the past 10 years. Therefore, the aim of this review paper is to provide a comprehensive review of current knowledge about the effects of amblyopia on eye movements, upper limb reaching and grasping movements, as well as balance and gait. Accumulating evidence indicates that amblyopia is associated with considerable deficits in visuomotor behaviour during amblyopic eye viewing, as well as adaptations in behaviour during binocular and fellow eye viewing in adults and children. Importantly, due to amblyopia heterogeneity, visuomotor development in children and motor skill performance in adults may be significantly influenced by the etiology and clinical features, such as visual acuity and stereoacuity. Studies with larger cohorts of children and adults are needed to disentangle the unique contribution of these clinical characteristics to the development and performance of visuomotor behaviours.

Specific conditions for a selective deficit in memory for order in children with dyslexia

Short-term memory (STM) models distinguish between item and order memorization. The present study aims to explore how item and order STM are affected by the nature of the stimuli, the sequential versus simultaneous mode of presentation, the visual versus auditory presentation modality, the possibility of verbal recoding. A total of 20 children with dyslexia were matched one-by-one with 20 typically reading children on sex, age (8-14 years), and grade. Computerized STM tasks were administered while manipulating type (item vs. order), stimuli (letters vs. colors), sequentiality, input and output modality, as well as the presence/absence of articulatory suppression and distractors. General Linear Model analyses were conducted on accuracy scores for item and order STM. Both item and order recall scores were lower for children with dyslexia. Although order STM in the visual input condition turned out to be more impaired than item STM in the dyslexic group, both item and order memory impairments become evident when verbal recoding is prevented through articulatory suppression. Moreover, dyslexic children, unlike typical readers, were not facilitated by the linguistic nature of the stimuli to be remembered. The present findings suggest that the often-reported selective impairment of serial memory in dyslexia is restricted to stimuli that are verbal in nature or can be verbally recoded, whereas both item and order memory impairments become evident when verbal recoding is prevented through articulatory suppression. The presence of distractors is particularly detrimental to STM in the dyslexic group. The sensitivity to distractors, suppression, and stimuli in STM is predictive of reading performance.

Memory for Serial Order in Alzheimer’s Disease and Vascular Dementia: A Competitive Queuing Analysis

Objective

Competitive Queuing (CQ) models of memory for serial order comprise two layers: parallel planning where target items are activated and competitive choice where serial order is specified. The application of CQ models regarding healthy and pathological aging has received little attention.

Method

Participants included patients with Alzheimer’s disease (AD; n = 26), vascular dementia (VaD; n = 29), and healthy controls (HC; n = 35). Memory for serial order in the visual domain was assessed using the Object Span Task, where participants briefly viewed then drew a sequence of four figures. Percent correct and total errors (omissions, intrusions, repetitions, transpositions) were computed for each serial position.

Results

Significant primacy effects were detected in each group. AD and VaD participants were less accurate and showed more omission and between-trial repetition errors than HC (HC < AD = VaD, p < .05). VaD participants produced more transposition and intrusion errors than the AD and HC groups (HC < AD < VaD, p < .05). A group × position interaction was significant for omissions (p < .05), with AD and VaD participants producing more omissions in later serial positions (SP1 < SP2 < SP3 < SP4, all p values < .05).

Conclusions

Analysis of accuracy and errors by serial position identified unique patterns of performance across groups that suggest involvement of distinct layers of response activation and selection. Serial order difficulties in AD may be due to weakened activation of task items affecting later serial positions, whereas poor performance in VaD may be due to weakened activation plus interference from extraneous stimuli at all serial positions.

Desirability, availability, credit assignment, category learning, and attention: Cognitive-emotional and working memory dynamics of orbitofrontal, ventrolateral, and dorsolateral prefrontal cortices

BACKGROUND

The prefrontal cortices play an essential role in cognitive-emotional and working memory processes through interactions with multiple brain regions.

METHODS

This article further develops a unified neural architecture that explains many recent and classical data about prefrontal function and makes testable predictions.

RESULTS

Prefrontal properties of desirability, availability, credit assignment, category learning, and feature-based attention are explained. These properties arise through interactions of orbitofrontal, ventrolateral prefrontal, and dorsolateral prefrontal cortices with the inferotemporal cortex, perirhinal cortex, parahippocampal cortices; ventral bank of the principal sulcus, ventral prearcuate gyrus, frontal eye fields, hippocampus, amygdala, basal ganglia, hypothalamus, and visual cortical areas V1, V2, V3A, V4, middle temporal cortex, medial superior temporal area, lateral intraparietal cortex, and posterior parietal cortex. Model explanations also include how the value of visual objects and events is computed, which objects and events cause desired consequences and which may be ignored as predictively irrelevant, and how to plan and act to realise these consequences, including how to selectively filter expected versus unexpected events, leading to movements towards, and conscious perception of, expected events. Modelled processes include reinforcement learning and incentive motivational learning; object and spatial working memory dynamics; and category learning, including the learning of object categories, value categories, object-value categories, and sequence categories, or list chunks.

CONCLUSION

This article hereby proposes a unified neural theory of prefrontal cortex and its functions.

Towards solving the hard problem of consciousness: The varieties of brain resonances and the conscious experiences that they support

The hard problem of consciousness is the problem of explaining how we experience qualia or phenomenal experiences, such as seeing, hearing, and feeling, and knowing what they are. To solve this problem, a theory of consciousness needs to link brain to mind by modeling how emergent properties of several brain mechanisms interacting together embody detailed properties of individual conscious psychological experiences. This article summarizes evidence that Adaptive Resonance Theory, or ART, accomplishes this goal. ART is a cognitive and neural theory of how advanced brains autonomously learn to attend, recognize, and predict objects and events in a changing world. ART has predicted that "all conscious states are resonant states" as part of its specification of mechanistic links between processes of consciousness, learning, expectation, attention, resonance, and synchrony. It hereby provides functional and mechanistic explanations of data ranging from individual spikes and their synchronization to the dynamics of conscious perceptual, cognitive, and cognitive-emotional experiences. ART has reached sufficient maturity to begin classifying the brain resonances that support conscious experiences of seeing, hearing, feeling, and knowing. Psychological and neurobiological data in both normal individuals and clinical patients are clarified by this classification. This analysis also explains why not all resonances become conscious, and why not all brain dynamics are resonant. The global organization of the brain into computationally complementary cortical processing streams (complementary computing), and the organization of the cerebral cortex into characteristic layers of cells (laminar computing), figure prominently in these explanations of conscious and unconscious processes. Alternative models of consciousness are also discussed.

Phoneme restoration and empirical coverage of Interactive Activation and Adaptive Resonance models of human speech processing

Magnuson [J. Acoust. Soc. Am. 137, 1481-1492 (2015)] makes claims for Interactive Activation (IA) models and against Adaptive Resonance Theory (ART) models of speech perception. Magnuson also presents simulations that claim to show that the TRACE model can simulate phonemic restoration, which was an explanatory target of the cARTWORD ART model. The theoretical analysis and review herein show that these claims are incorrect. More generally, the TRACE and cARTWORD models illustrate two diametrically opposed types of neural models of speech and language. The TRACE model embodies core assumptions with no analog in known brain processes. The cARTWORD model defines a hierarchy of cortical processing regions whose networks embody cells in laminar cortical circuits as part of the paradigm of laminar computing. cARTWORD further develops ART speech and language models that were introduced in the 1970s. It builds upon Item-Order-Rank working memories, which activate learned list chunks that unitize sequences to represent phonemes, syllables, and words. Psychophysical and neurophysiological data support Item-Order-Rank mechanisms and contradict TRACE representations of time, temporal order, silence, and top-down processing that exhibit many anomalous properties, including hallucinations of non-occurring future phonemes. Computer simulations of the TRACE model are presented that demonstrate these failures.

Motion trajectory information and agency influence motor learning during observational practice

Fundamental to performing actions is the acquisition of motor behaviours. We examined if motor learning, through observational practice, occurs by viewing an agent displaying naturalistic or constant velocity, and whether motion trajectory, as opposed to end-state, information is required. We also investigated if observational practice is sensitive to belief regarding the origin of an agent. Participants had to learn a novel movement sequence timing task, which required upper-limb movements to a series of targets within a pre-specified absolute and relative time goal. Experiment 1 showed learning after viewing naturalistic and constant velocity, but not end-state information. For Experiment 2, in addition to learning the movement sequence, participants observed a series of movement stimuli that were either the trained or new sequences and asked to rate their confidence on whether the observed sequence was the same or different to observational practice. The results indicated that agency belief modulates how naturalistic and constant velocity is coded. This indicated that the processes associated with belief are part of an interpretative predictive coding system where the association between belief and observed motion is determined. When motion is constant velocity, or believed to be computer-generated, coding occurs through top-down processes. When motion is naturalistic velocity, and believed to be human-generated, it is most likely coded by gaining access to bottom-up sensorimotor processes in the action-observation network.

Violations of newly-learned predictions elicit two distinct P3 components

Sensitivity to the environment's sequential regularities makes it possible to predict upcoming sensory events. To investigate the mechanisms that monitor such predictions, we recorded scalp EEG as subjects learned to reproduce sequences of motions. Each sequence was seen and reproduced four successive times, with occasional deviant directions of motion inserted into otherwise-familiar and predictable sequences. To dissociate the neural activity associated with encoding new items from that associated with detecting sequence deviants, we measured ERPs to new, familiar, and deviant sequence items. Both new and deviant sequence items evoked enhanced P3 responses, with the ERP to deviant items encompassing both P300-like and Novelty P3-like subcomponents with distinct timing and topographies. These results confirm that the neural response to deviant items differs from that to new items, and that unpredicted events in newly-learned sequences are identified by processes similar to those monitoring stable sequential regularities.

Audiovisual crossmodal correspondences and sound symbolism: a study using the implicit association test

A growing body of empirical research on the topic of multisensory perception now shows that even non-synaesthetic individuals experience crossmodal correspondences, that is, apparently arbitrary compatibility effects between stimuli in different sensory modalities. In the present study, we replicated a number of classic results from the literature on crossmodal correspondences and highlight the existence of two new crossmodal correspondences using a modified version of the implicit association test (IAT). Given that only a single stimulus was presented on each trial, these results rule out selective attention and multisensory integration as possible mechanisms underlying the reported compatibility effects on speeded performance. The crossmodal correspondences examined in the present study all gave rise to very similar effect sizes, and the compatibility effect had a very rapid onset, thus speaking to the automatic detection of crossmodal correspondences. These results are further discussed in terms of the advantages of the IAT over traditional techniques for assessing the strength and symmetry of various crossmodal correspondences.

Audiovisual Temporal Recalibration: Space-Based versus Context-Based

Recalibration of perceived simultaneity has been widely accepted to minimise delay between multisensory signals owing to different physical and neural conduct times. With concurrent exposure, temporal recalibration is either contextually or spatially based. Context-based recalibration was recently described in detail, but evidence for space-based recalibration is scarce. In addition, the competition between these two reference frames is unclear. Here, we examined participants who watched two distinct blob-and-tone couples that laterally alternated with one asynchronous and the other synchronous and then judged their perceived simultaneity and sequence when they swapped positions and varied in timing. For low-level stimuli with abundant auditory location cues space-based aftereffects were significantly more apparent (8.3%) than context-based aftereffects (4.2%), but without such auditory cues space-based aftereffects were less apparent (4.4%) and were numerically smaller than context-based aftereffects (6.0%). These results suggested that stimulus level and auditory location cues were both determinants of the recalibration frame. Through such joint judgments and the simple reaction time task, our results further revealed that criteria from perceived simultaneity to successiveness profoundly shifted without accompanying perceptual latency changes across adaptations, hence implying that criteria shifts, rather than perceptual latency changes, accounted for space-based and context-based temporal recalibration.

A new method to evaluate order and accuracy of inaccurately and incompletely reproduced movement sequences

Studying imitation learning of long sequences requires the evaluation of inaccurately and incompletely reproduced movement sequences. In order to evaluate the movement reproduction, it has to be assigned to the original stimulus. We developed an assignment algorithm that considers the Spatial Neighborhood and Order of reproduction (SNOA). To evaluate the features of this analysis it was applied to human performance during learning of long pointing sequences under two conditions: stimulus-guided reproduction with high spatial accuracy and imitation learning with low spatial accuracy. The results were compared with a simple assignment considering Spatial Neighborhood only (SNA) and with a Manual Assignment (MA). In the stimulus-guided reproduction the error measures did not differ between the algorithms. In contrast, with imitation learning, SNOA and MA generated higher estimates of order and omission errors than SNA. The results show that SNOA can be used to automatically quantify the similarity of both movement structure and metric information between long target sequences and inaccurate and incomplete movement reproductions.

Computing an optimal time window of audiovisual integration in focused attention tasks: illustrated by studies on effect of age and prior knowledge

The concept of a "time window of integration" holds that information from different sensory modalities must not be perceived too far apart in time in order to be integrated into a multisensory perceptual event. Empirical estimates of window width differ widely, however, ranging from 40 to 600 ms depending on context and experimental paradigm. Searching for theoretical derivation of window width, Colonius and Diederich (Front Integr Neurosci 2010) developed a decision-theoretic framework using a decision rule that is based on the prior probability of a common source, the likelihood of temporal disparities between the unimodal signals, and the payoff for making right or wrong decisions. Here, this framework is extended to the focused attention task where subjects are asked to respond to signals from a target modality only. Evoking the framework of the time-window-of-integration (TWIN) model, an explicit expression for optimal window width is obtained. The approach is probed on two published focused attention studies. The first is a saccadic reaction time study assessing the efficiency with which multisensory integration varies as a function of aging. Although the window widths for young and older adults differ by nearly 200 ms, presumably due to their different peripheral processing speeds, neither of them deviates significantly from the optimal values. In the second study, head saccadic reactions times to a perfectly aligned audiovisual stimulus pair had been shown to depend on the prior probability of spatial alignment. Intriguingly, they reflected the magnitude of the time-window widths predicted by our decision-theoretic framework, i.e., a larger time window is associated with a higher prior probability.

Visual-haptic cue integration with spatial and temporal disparity during pointing movements

Many perceptual cue combination studies have shown that humans can integrate sensory information across modalities as well as within a modality in a manner that is close to optimal. While the limits of sensory cue integration have been extensively studied in the context of perceptual decision tasks, the evidence obtained in the context of motor decisions provides a less consistent picture. Here, we studied the combination of visual and haptic information in the context of human arm movement control. We implemented a pointing task in which human subjects pointed at an invisible unknown target position whose vertical position varied randomly across trials. In each trial, we presented a haptic and a visual cue that provided noisy information about the target position half-way through the reach. We measured pointing accuracy as function of haptic and visual cue onset and compared pointing performance to the predictions of a multisensory decision model. Our model accounts for pointing performance by computing the maximum a posteriori estimate, assuming minimum variance combination of uncertain sensory cues. Synchronicity of cue onset has previously been demonstrated to facilitate the integration of sensory information. We tested this in trials in which visual and haptic information was presented with temporal disparity. We found that for our sensorimotor task temporal disparity between visual and haptic cue had no effect. Sensorimotor learning appears to use all available information and to apply the same near-optimal rules for cue combination that are used by perception.

Learning deferred imitation of long spatial sequences

Sequence learning has been the subject of research in various paradigms but has not been investigated for learning deferred imitation of long spatial sequences. In this task no guiding stimuli support the sequence reproduction and all sequence information has to be recalled from memory. We investigate this kind of imitation learning with a task in which a long sequence of spatial positions has to be reproduced without guiding stimuli, either by manual pointing or by ocular fixations. Sequences consisting of 20 positions were acquired after only 25 training trials. The persistence of learned sequences over several days showed that the sequence was retained in long-term memory. A transfer test revealed that the learned sequence was independent of the effector. A detailed analysis of the error distributions of pointing and ocular fixations was performed to characterize the guiding control signal. The independence of the variable position errors from the movement directions as well as the lack of error propagation between successive targets suggest that the reproduction in this learning task was guided by sequential positions rather than sequential displacements.

Eye movements and imitation learning: intentional disruption of expectation

Over repeated viewings of motion along a quasi-random path, ability to reproduce that path from memory improves. To assess the role of expectations and sequence context on such learning, subjects eye movements were measured while trajectories were viewed for subsequent reproduction. As a sequence of motions was repeated, subjects' eye movements became anticipatory, leading the stimulus' motions. To investigate how prediction errors affected eye movements and imitation learning, we injected an occasional deviant motion into a well-learned stimulus sequence, violating subjects' expectation about the motion that would be seen. This unexpected direction of motion in the stimulus sequence did not impair reproduction of the sequence. The externally induced prediction errors promoted one-shot learning: During the very next stimulus presentation, their eye movements showed that subjects now expected the new sequence item to reappear. A second experiment showed that an associative mismatch can facilitate accurate reproduction of an unexpected stimulus. After a deviant sequence item was presented, imitation accuracy for sequences that contained the deviant direction of motion was reduced relative to sequences that restored the original direction of motions. These findings demonstrate that in the context of a familiar sequence, unexpected events can play an important role in learning the sequence.

Chunking and compound cueing of movement sequences: learning, retention, and transfer

When teaching a complex sequence, the sequence is often chunked into components; however, this strategy may not always benefit learning, but may be detrimental. The hypothesis is that this occurs because chunking deprives learners of compound cues that could aid recall. To test this, participants learned 9-item movement sequences, either as three 3-item chunks or as one 9-item series. To undermine compound cueing, some sequences had several movements in common. Learning a sequence in chunks impaired motor skill acquisition only when participants could have exploited compound cues; it also led participants to adopt an alternative recall strategy, which transferred to novel sequences even though this was detrimental to recall.

Neural correlates of sequence encoding in visuomotor learning

To examine the neural basis of sequence learning, a fundamental but poorly understood human ability, we recorded event-related potentials (ERPs) while subjects viewed and memorized randomly directed sequences of motions for later imitation. Previously, we found that the amplitude of ERPs elicited by successive motion segments decreased as a function of each segment's serial position. This happened when subjects were required to remember the sequence, but not when they were performing a perceptual task. Here, to study the functional significance of this amplitude gradient in sequence learning, we presented each sequence several times in succession and examined changes in ERP amplitude as subjects learned the sequence through repeated observation and imitation. Behaviorally, with each repetition subjects grew more accurate in reproducing what they had seen. At the same time, ERPs grew smaller with each successive presentation, replicating and extending previous demonstrations of repetition suppression. Importantly, a comparison of ERPs to segments occupying different serial positions within a sequence revealed a decreasing amplitude gradient that grew steeper with sequence repetition. This sharpening of the amplitude gradient may reflect an explicit encoding process that relies on a magnitude code for serial order.

Eye movements are not a prerequisite for learning movement sequence timing through observation

The present experiment examined learning of a three-segment movement sequence using physical or observational practice, and whether permitting eye movements to be made during observation is a prerequisite for learning such a movement sequence. Specifically, participants were required to move a mouse cursor through a three-segment movement sequence in order to satisfy one of three movement time goals (800, 1000, 1200 ms). A yoked-participant design was used in which a physical practice group acted as a learning model, which was viewed simultaneously by two groups that carried out different observational practice procedures. An observation group was permitted to move their eyes whilst observing the model, whereas the fixation group was instructed to maintain fixation on a central target. The difference between pre-test and post-test data indicated that all the three experimental groups significantly altered their timing accuracy, variability and movement kinematics over practice, while the control group's behaviour was unchanged. These data indicate that movement time as well as the underlying movement control was learned following observation of a movement with or without an explicit contribution from eye movements, albeit to a lesser extent during the final segment of the sequence when compared to the physical practice group. The implication is that while similar processes might normally be involved in physical and observational practice, information afforded by eye movements during observation (e.g., efference copy and eye proprioception) is not necessary for movement sequence learning.

A new way to quantify the fidelity of imitation: preliminary results with gesture sequences

Imitation is a common and effective way for humans to learn new behaviors. Until now, the study of imitation has been hampered by the challenge of measuring how well an attempted imitation corresponds to its stimulus model. We describe a new method for quantifying the fidelity with which observers imitate complex series of gestures. Wearing a data glove that transduced movements of their digits, subjects viewed and then reproduced a sequence of gestures from memory. The velocity profile of each digit's flexion or extension was used to segment movements made during an imitation into gestures that can be compared against corresponding gestures in the stimulus model. The outcome is a multivariate description of each imitation, including its temporal characteristics, as well as spatial errors (in individual gestures and in the ordering of those gestures). As a demonstration, we applied this method to data from an imitation learning experiment with gesture sequences. With repetition, overall fidelity of imitation improved, with various aspects of the imitation improving at different rates. Confirming the approach's usefulness, when we varied the complexity associated with imitation, that variation was robustly reflected in our measures of imitation quality. Finally, we describe a simple way to extend our methods to make them useful not only in assessing imitation and imitation learning, but also in various settings in which the detection and characterization of subtle abnormalities in movement production is paramount.

Interactions between working memory and visual perception: an ERP/EEG study

How do working memory and perception interact with each other? Recent theories of working memory suggest that they are closely linked, and in fact share certain brain mechanisms. We used a sequential motion imitation task in combination with EEG and ERP techniques for a direct, online examination of memory load's influence on the processing of visual stimuli. Using a paradigm in which subjects tried to reproduce random motion sequences from memory, we found a systematic decrease in ERP amplitude with each additional motion segment that was viewed and memorized for later imitation. High-frequency (>20 Hz) oscillatory activity exhibited a similar position-dependent decrease. When trials were sorted according to the accuracy of subsequent imitation, the amplitude of the ERPs during stimulus presentation correlated with behavioral performance: the larger the amplitude, the more accurate the subsequent imitation. These findings imply that visual processing of sequential stimuli is not uniform. Rather, earlier information elicits stronger neural activity. We discuss possible explanations for this observation, among them competition for attention between memory and perception and encoding of serial order by means of differential activation strengths.