411 research outputs found
Gearing up for action: attentive tracking dynamically tunes sensory and motor oscillations in the alpha and beta band
Allocation of attention during goal-directed behavior entails simultaneous processing of relevant and attenuation of irrelevant information. How the brain delegates such processes when confronted with dynamic (biological motion) stimuli and harnesses relevant sensory information for sculpting prospective responses remains unclear. We analyzed neuromagnetic signals that were recorded while participants attentively tracked an actor’s pointing movement that ended at the location where subsequently the response-cue indicated the required response. We found the observers’ spatial allocation of attention to be dynamically reflected in lateralized parieto-occipital alpha (8-12Hz) activity and to have a lasting influence on motor preparation. Specifically, beta (16-25Hz) power modulation reflected observers’ tendency to selectively prepare for a spatially compatible response even before knowing the required one. We discuss the observed frequency-specific and temporally evolving neural activity within a framework of integrated visuomotor processing and point towards possible implications about the mechanisms involved in action observation
Semantic radical consistency and character transparency effects in Chinese: an ERP study
BACKGROUND: This event-related potential (ERP) study aims to investigate the representation and temporal dynamics of Chinese orthography-to-semantics mappings by simultaneously manipulating character transparency and semantic radical consistency. Character components, referred to as radicals, make up the building blocks used dur...postprin
Linking sentence production and comprehension: The neural mechanisms underlying production and comprehension control processes
This thesis investigated the relationship between sentence production and comprehension. A combination of behavioural and neuroimaging techniques were used to examine the extent to which sentence production and comprehension engage common or distinct mechanisms, with specific focus on the processes engaged by semantic/syntactic competition. Behavioural studies in Chapter 2 indicated that high-competition cases were more difficult to understand and produce than low-competition cases, and that difficulty varied as a function of the number of alternative associations entertained during performance in both tasks. In Chapter 3, an fMRI study indicated that production and comprehension shared a common competition mechanism within left inferior frontal gyrus (LIFG). However, they engage distinctive networks that interact with LIFG, with production eliciting a larger network including areas involved in sentence planning and memory retrieval. Further asymmetries across tasks were revealed in Chapter 4, in which behavioural results and neural networks were compared across adults and adolescents. This study also demonstrated the occurrence of shifts in the neural networks involved in competition resolution throughout development, thereby providing a strong link between poor behavioural performance and the underdevelopment of pre-frontal inhibitory mechanisms in adolescents. Chapter 5 used an improved experimental paradigm from that in Chapters 3 and 4. The results showed that production elicits more activity than comprehension in the dorsal language route thus confirming the engagement of task-specific control processes. Interestingly, this study also revealed a common area of LIFG involved in both tasks, but also differences within LIFG, suggesting the possibility of task-specific circuitry. Together, the findings suggest that production and comprehension share fronto-temporal areas that store and manage abstract linguistic associations between words and structures. However, they differ in the manner in which linguistic information is used, as is evident by the recruitment of distinct networks. Implications for models of language processing are discussed
Postdiction: its implications on visual awareness, hindsight, and sense of agency
There are a few postdictive perceptual phenomena known, in which a stimulus presented later seems causally to affect the percept of another stimulus presented earlier. While backward masking provides a classical example, the flash lag effect stimulates theorists with a variety of intriguing findings. The TMS-triggered scotoma together with “backward filling-in” of it offer a unique neuroscientific case. Findings suggest that various visual attributes are reorganized in a postdictive fashion to be consistent with each other, or to be consistent in a causality framework. In terms of the underlying mechanisms, four prototypical models have been considered: the “catch up,” the “reentry,” the “different pathway” and the “memory revision” models. By extending the list of postdictive phenomena to memory, sensory-motor and higher-level cognition, one may note that such a postdictive reconstruction may be a general principle of neural computation, ranging from milliseconds to months in a time scale, from local neuronal interactions to long-range connectivity, in the complex brain. The operational definition of the “postdictive phenomenon” can be applicable to such a wide range of sensory/cognitive effects across a wide range of time scale, even though the underlying neural mechanisms may vary across them. This has significant implications in interpreting “free will” and “sense of agency” in functional, psychophysical and neuroscientific terms
Investigating the function of the ventral visual reading pathway and its involvement in acquired reading disorders
This thesis investigated the role of the left ventral occipitotemporal (vOT)
cortex and how damage to this area causes peripheral reading disorders.
Functional magnetic resonance imaging (fMRI) studies in volunteers
demonstrated that the left vOT is activated by written words over numbers
or perceptually-matched baselines, irrespective of the word’s location on the
visual field. Mixed results were observed for the comparison of words versus
false font stimuli. This response profile suggests that the left vOT is
preferentially activated by words or word-like stimuli, due to either: (1)
bottom-up specialisation for processing familiar word-forms; (2) top-down
task-dependent modulation, or (3) a combination of the two. Further studies
are proposed to discriminate between these possibilities.
Thirteen patients with left occipitotemporal damage participated in the
rehabilitation and fMRI studies. The patients were impaired on word, text and
letter reading. A structural analysis showed that damage to the left
occipitotemporal white matter, in the vicinity of the inferior longitudinal
fasciculus, was associated with slow word reading speed. The fMRI study
showed that the patients had reduced activation of the bilateral posterior
superior temporal sulci relative to controls. Activity in this area correlated
with reading speed.
The efficacy of intensive whole-word recognition training was tested.
Immediately after the training, trained words were read faster than
untrained words, but the effects did not persist until the follow-up
assessment. Hence, damage to the left vOT white matter impairs rapid
whole-word recognition and is resistant to rehabilitation.
The final study investigated the role of spatial frequency (SF) in the
lateralisation of vOT function. Lateralisation of high and low SF processing
was demonstrated, concordant with the lateralisation for words and faces to
the left and right vOT respectively. A perceptual basis for the organisation of
vOT cortex might explain why left vOT damage is resistant to treatment
Early cross-modal interactions and adult human visual cortical plasticity revealed by binocular rivalry
In this research binocular rivalry is used as a tool to investigate different aspects of visual and multisensory perception. Several experiments presented here demonstrated that touch specifically interacts with vision during binocular rivalry and that the interaction likely occurs at early stages of visual processing, probably V1 or V2. Another line of research also presented here demonstrated that human adult visual cortex retains an unexpected high degree of experience-dependent plasticity by showing that a brief period of monocular deprivation produced important perceptual consequences on the dynamics of binocular rivalry, reflecting a homeostatic plasticity. In summary, this work shows that binocular rivalry is a powerful tool to investigate different aspects of visual perception and can be used to reveal unexpected properties of early visual cortex
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