http://www.dspace.cam.ac.uk:80/handle/1810/221815 Wed, 22 May 2013 04:47:07 GMT 2013-05-22T04:47:07Z http://www.dspace.cam.ac.uk:80/handle/1810/243944 Title: Fronto-parietal cortex in sequential behaviour Authors: Farooqui, Ausaf Ahmed Abstract: This dissertation investigates the fronto-parietal representation of the structure of organised mental episodes by studying its effect on the representation of cognitive events occurring at various positions within it. The experiments in chapter 2 look at the completion of hierarchically organized mental (task/subtask) episodes. Multiple identical target-detection events were organized into a sequential task episode, and the individual events were connected in a means-to-end relationship. It is shown that events that are conceptualized as completing defined task episodes elicit greater activity compared to identical events lying within the episode; the magnitude of the end of episode activity depended on the hierarchical abstraction of the episode. In chapter 3, the effect of ordinal position of the cognitive events, making up the task episode, on their representation is investigated in the context of a biphasic task episode. The design further manipulated the cognitive load of the two phases independently. This allowed for a direct comparison of the effect of phase vis-à-vis the effect of cognitive load. The results showed that fronto-parietal regions that increased their activity in response to cognitive load, also increased their activity for the later phases of the task episode, even though the cognitive load associated with the later phase was, arguably, lower than the previous phase. Chapter 4 investigates if the characteristics of the higher-level representations, like organization of task descriptions, have a causal role in determining the structure of the ensuing mental episode. Results show this to be true. They also confirm the results of earlier chapters in a different framework. Chapter 5 shows that the effect of episode structure is not limited to the elicited activity, but also affects the information content of the representation of the events composing the episode. Specifically, the information content in many regions of later steps is higher than that of earlier steps. Together, the results show widespread representation of the structure of organised mental episodes. Mon, 08 Oct 2012 23:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/243944 2012-10-08T23:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/243852 Title: Understanding language and attention: brain-based model and neurophysiological experiments Authors: Garagnani, Max Abstract: This work concerns the investigation of the neuronal mechanisms at the basis of language acquisition and processing, and the complex interactions of language and attention processes in the human brain. In particular, this research was motivated by two sets of existing neurophysiological data which cannot be reconciled on the basis of current psycholinguistic accounts: on the one hand, the N400, a robust index of lexico-semantic processing which emerges at around 400ms after stimulus onset in attention demanding tasks and is larger for senseless materials (meaningless pseudowords) than for matched meaningful stimuli (words); on the other, the more recent results on the Mismatch Negativity (MMN, latency 100-250ms), an early automatic brain response elicited under distraction which is larger to words than to pseudowords. We asked what the mechanisms underlying these differential neurophysiological responses may be, and whether attention and language processes could interact so as to produce the observed brain responses, having opposite magnitude and different latencies. We also asked questions about the functional nature and anatomical characteristics of the cortical representation of linguistic elements. These questions were addressed by combining neurocomputational techniques and neuroimaging (magneto-encephalography, MEG) experimental methods. Firstly, a neurobiologically realistic neural-network model composed of neuron-like elements (graded response units) was implemented, which closely replicates the neuroanatomical and connectivity features of the main areas of the left perisylvian cortex involved in spoken language processing (i.e., the areas controlling speech output – left inferior-prefrontal cortex, including Broca’s area – and the main sensory input – auditory – areas, located in the left superior-temporal lobe, including Wernicke’s area). Secondly, the model was used to simulate early word acquisition processes by means of a Hebbian correlation learning rule (which reflects known synaptic plasticity mechanisms of the neocortex). The network was “taught” to associate pairs of auditory and articulatory activation patterns, simulating activity due to perception and production of the same speech sound: as a result, neuronal word representations distributed over the different cortical areas of the model emerged. Thirdly, the network was stimulated, in its “auditory cortex”, with either one of the words it had learned, or new, unfamiliar pseudoword patterns, while the availability of attentional resources was modulated by changing the level of non-specific, global cortical inhibition. In this way, the model was able to replicate both the MMN and N400 brain responses by means of a single set of neuroscientifically grounded principles, providing the first mechanistic account, at the cortical-circuit level, for these data. Finally, in order to verify the neurophysiological validity of the model, its crucial predictions were tested in a novel MEG experiment investigating how attention processes modulate event-related brain responses to speech stimuli. Neurophysiological responses to the same words and pseudowords were recorded while the same subjects were asked to attend to the spoken input or ignore it. The experimental results confirmed the model’s predictions; in particular, profound variability of magnetic brain responses to pseudowords but relative stability of activation to words as a function of attention emerged. While the results of the simulations demonstrated that distributed cortical representations for words can spontaneously emerge in the cortex as a result of neuroanatomical structure and synaptic plasticity, the experimental results confirm the validity of the model and provide evidence in support of the existence of such memory circuits in the brain. This work is a first step towards a mechanistic account of cognition in which the basic atoms of cognitive processing (e.g., words, objects, faces) are represented in the brain as discrete and distributed action-perception networks that behave as closed, independent systems. Mon, 12 Oct 2009 23:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/243852 2009-10-12T23:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/243495 Title: Pattern codes for perceived gaze direction revealed by functional MRI Authors: Carlin, Johan D. Abstract: Perceiving the direction of another's attention is a critical component of normal social behaviour. Seminal electrophysiology studies demonstrated that single cells in macaque superior temporal sulcus (STS) are tuned to specific directions of social cues, including gaze direction, head view, and body posture. Furthermore, a subset of such neurons respond to a single direction across multiple cues, suggesting that the code is driven by the direction of another's social attention regardless of how this is conveyed. Attempts to reveal similar gaze representations in humans using fMRI have provided mixed results. This thesis describes research where multivariate pattern analysis (MVPA) methods are applied to fMRI data in order to better explain how the human brain and particularly STS codes perceived gaze direction. After describing the MVPA methods applied in this thesis, I first demonstrate that fMRI response patterns in anterior STS distinguish between the direction of dynamic head turns, but not between the direction of rotation in non-social ellipsoids. In subsequent work, anterior STS is found to code the direction of another's gaze in a head view-invariant manner, thus demonstrating a potential parallel to previous macaque evidence for single cells that code the direction of another's attention. However, comparisons that run both across species (macaque, human) and methods (electrophysiology, fMRI) are problematic. To overcome this limitation I next tested whether macaque STS distinguishes gaze direction and head view when responses are measured with fMRI. In conclusion, this thesis demonstrates the utility of applying MVPA to fMRI data to reveal socially-relevant representations of the direction of another's attention. The thesis particularly highlights anterior STS as a key region in supporting direction-specific representations of social cues. These results advance our understanding of how the brain codes socially-relevant information, and highlight possible similarities and dissimilarities between humans and macaques. Mon, 11 Jun 2012 23:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/243495 2012-06-11T23:00:00Z