Neurosurgery

Audiovisual speech perception is likely based on the association between auditory and visual information into stable audiovisual maps. Conflicting audiovisual inputs generate perceptual illusions such as the McGurk effect. Audiovisual mismatch effects could be either driven by the detection of violations in the standard audiovisual statistics or via the sensorimotor reconstruction of the distal articulatory event that generated the audiovisual ambiguity. In order to disambiguate between the two hypotheses we exploit the fact that the tongue is hidden to vision. For this reason, tongue movement encoding can solely be learned via speech production but not via others' speech perception alone. Here we asked participants to identify speech sounds while matching or mismatching visual representations of tongue movements which were shown. Vision of congruent tongue movements facilitated auditory speech identification with respect to incongruent trials. This result suggests that direct visual experience of an articulator movement is not necessary for the generation of audiovisual mismatch effects. Furthermore, we suggest that audiovisual integration in speech may benefit from speech production learning.

The term affordance defines a property of objects, which relates to the possible interactions that an agent can carry out on that object. In monkeys, canonical neurons encode both the visual and the motor properties of objects with high specificity. However, it is not clear if in humans exists a similarly finegrained description of these visuomotor transformations. In particular, it has not yet been proven that the processing of visual features related to specific affordances induces both specific and early visuomotor transformations, given that complete specificity has been reported to emerge quite late (300-450 ms). In this study, we applied an adaptation-stimulation paradigm to investigate early corticospinal facilitation and hand movements' synergies evoked by the observation of tools. We adapted, through passive observation of finger movements, neuronal populations coding either for precision or power grip actions. We then presented the picture of one tool affording one of the two grasps types and applied single-pulse Transcranial Magnetic Stimulation (TMS) to the hand primary motor cortex, 150 ms after image onset. Cortico-spinal excitability of the Abductor Digiti Minimi and Abductor Pollicis Brevis showed a detailed pattern of modulations, matching tools' affordances. Similarly, TMS-induced hand movements showed a pattern of grip-specific whole hand synergies. These results offer a direct proof of the emergence of an early visuomotor transformation when tools are observed, that maintains the same amount of synergistic motor details as the actions we can perform on them.

The discovery of mirror neurons revived interest in motor theories of perception, fostering a number of new studies as well as controversies. In particular, the degree of motor specificity with which others' actions are simulated is highly debated. Human corticospinal excitability studies support the conjecture that a mirror mechanism encodes object-directed goals or low-level kinematic features of others' reaching and grasping actions. These interpretations lead to different experimental predictions and implications for the functional role of the simulation of others' actions. We propose that the representational granularity of the mirror mechanism cannot be any different from that of the motor system during action execution. Hence, drawing from motor control models, we propose that the building blocks of the mirror mechanism are the relatively few motor synergies explaining the variety of hand functions. The recognition of these synergies, from action observation, can be potentially very robust to visual noise and thus demonstrate a clear advantage of using motor knowledge for classifying others' action.

Human languages can express opposite propositions by means of a negative operator not 23 that turns affirmative sentences into negative ones. Psycholinguistic research has 24 indicated that negative meanings are formed by transiently reducing the access to mental 25 representations of negated conceptual information. Neuroimaging studies have 26 corroborated these findings, showing reduced activation of concept-specific embodied 27 neural systems by negative versus affirmative sentences. This "disembodiment effect" of 28 sentential negation should have two distinct consequences: first, the embodied systems 29 should be computationally more free to support concurrent tasks when processing 30 negative than affirmative sentences; second, the computational interference should only 31 be reduced when there is a strict semantic congruency between the negated concept and 32 the referent targeted by concurrent tasks. We tested these two predictions in two 33 complementary experiments involving the comprehension of action-related sentences and 34 kinematic measurements of its effects on concurrent, congruent actions. Sentences 35 referred to actions involving either proximal or distal arm musculature. In Experiment 1, 36 requiring a proximal arm movement, we found interference reduction for negative proximal 37 sentences. In Experiment 2, requiring a distal arm movement, we found interference 38 reduction for negative distal sentences. This dissociation provides the first conclusive 39 evidence in support of a disembodiment theory of negation. We conclude that the 40 computational cost resulting from the insertion of an additional lexical item (not) in negative 41 sentences is compensated by solely storing a concept in affirmative form in semantic 42 memory, since its negative counterpart can be produced by transiently reducing the 43 access to such stored semantic information. 44 45

We are grateful to all commentators for their insightful commentaries and observations that enrich our proposal. One of our aims was indeed to bridge the gap between fields of research that, progressing independently, are facing similar issues regarding the neural representation of motor knowledge. In this respect, we were pleased to receive feedback from eminent researchers on both the mirror neuron as well as the motor control fields. Their expertise covers animal and human neurophysiology, as well as the computational modeling of neural and behavioral processes. Given their heterogeneous cultural perspectives and research approaches, a number of important open questions were raised. For simplicity we separated these issues into four sections. In the first section we present methodological aspects regarding how synergies can be measured in paradigms investigating the human mirror system. The second section regards the fundamental definition of what exactly synergies might be. The third concerns how synergies can generate testable predictions in mirror neuron research. Finally, the fourth section deals with the ultimate question regarding the function of the mirror neuron system.

We are grateful to all commentators for their insightful commentaries and observations that enrich our proposal. One of our aims was indeed to bridge the gap between fields of research that, progressing independently, are facing similar issues regarding the neural representation of motor knowledge. In this respect, we were pleased to receive feedback from eminent researchers on both the mirror neuron as well as the motor control fields. Their expertise covers animal and human neurophysiology, as well as the computational modeling of neural and behavioral processes. Given their heterogeneous cultural perspectives and research approaches, a number of important open questions were raised. For simplicity we separated these issues into four sections. In the first section we present methodological aspects regarding how synergies can be measured in paradigms investigating the human mirror system. The second section regards the fundamental definition of what exactly synergies might be. The ...