Individual differences in verbal short-term memory and reading aloud: Semantic compensation for weak phonological processing across tasks

According to contemporary accounts, linguistic behaviour reflects the interaction of distinct representations supporting word meaning and phonology. However, there is controversy about the extent to which this interaction occurs within task-specific systems, specialised for reading and short-term memory, as opposed to between components that support the full range of linguistic tasks. We examined whether individual differences in the efficiency of phonological processing would relate to the application of lexical-semantic knowledge to support verbal short-term memory, single word reading and repetition. In a sample of 83 participants we related nonword performance in each task (as a marker of phonological capacity in the absence of meaning) to the effects of word imageability (a lexical-semantic variable). We found stronger reliance on lexical-semantic knowledge in participants with weaker phonological processing. This relationship held across tasks, suggesting that lexical-semantic processing can compensate for phonological weakness which would otherwise give rise to poor performance. Our results are consistent with separable yet interacting primary systems for phonology and semantics, with lexical-semantic knowledge supporting pattern completion within the phonological system in a similar way across short-term memory and reading tasks

Impact of brain parcellation on prediction performance in models of cognition and demographics

Brain connectivity analysis begins with the selection of a parcellation scheme that will define brain regions as nodes of a network whose connections will be studied. Brain connectivity has already been used in predictive modelling of cognition, but it remains unclear if the resolution of the parcellation used can systematically impact the predictive model performance. In this work, structural, functional and combined connectivity were each defined with five different parcellation schemes. The resolution and modality of the parcellation schemes were varied. Each connectivity defined with each parcellation was used to predict individual differences in age, education, sex, executive function, self-regulation, language, encoding and sequence processing. It was found that low-resolution functional parcellation consistently performed above chance at producing generalisable models of both demographics and cognition. However, no single parcellation scheme showed a superior predictive performance across all cognitive domains and demographics. In addition, although parcellation schemes impacted the graph theory measures of each connectivity type (structural, functional and combined), these differences did not account for the out-of-sample predictive performance of the models. Taken together, these findings demonstrate that while high-resolution parcellations may be beneficial for modelling specific individual differences, partial voluming of signals produced by the higher resolution of the parcellation likely disrupts model generalisability

Opposing effects of semantic diversity in lexical and semantic relatedness decisions

Semantic ambiguity has often been divided into 2 forms: homonymy, referring to words with 2 unrelated interpretations (e.g., bark), and polysemy, referring to words associated with a number of varying but semantically linked uses (e.g., twist). Typically, polysemous words are thought of as having a fixed number of discrete definitions, or “senses,” with each use of the word corresponding to one of its senses. In this study, we investigated an alternative conception of polysemy, based on the idea that polysemous variation in meaning is a continuous, graded phenomenon that occurs as a function of contextual variation in word usage. We quantified this contextual variation using semantic diversity (SemD), a corpus-based measure of the degree to which a particular word is used in a diverse set of linguistic contexts. In line with other approaches to polysemy, we found a reaction time (RT) advantage for high SemD words in lexical decision, which occurred for words of both high and low imageability. When participants made semantic relatedness decisions to word pairs, however, responses were slower to high SemD pairs, irrespective of whether these were related or unrelated. Again, this result emerged irrespective of the imageability of the word. The latter result diverges from previous findings using homonyms, in which ambiguity effects have only been found for related word pairs. We argue that participants were slower to respond to high SemD words because their high contextual variability resulted in noisy, underspecified semantic representations that were more difficult to compare with one another. We demonstrated this principle in a connectionist computational model that was trained to activate distributed semantic representations from orthographic inputs. Greater variability in the orthography-to-semantic mappings of high SemD words resulted in a lower degree of similarity for related pairs of this type. At the same time, the representations of high SemD unrelated pairs were less distinct from one another. In addition, the model demonstrated more rapid semantic activation for high SemD words, thought to underpin the processing advantage in lexical decision. These results support the view that polysemous variation in word meaning can be conceptualized in terms of graded variation in distributed semantic representations

Semantics in speech production

The semantic system contributes to the process of speech production in two major ways. The basic information is contained within semantic representations, and the semantic control system manipulates that knowledge as required by task and context. This thesis explored the evidence for interactivity between semantic and phonological stages of speech production, and examined the role of semantic control within speech production. The data chapters focussed on patients with semantic aphasia or SA, who all have frontal and/or temporoparietal lesions and are thought to have a specific impairment of semantic control. In a novel development, grammatical class and cueing effects in this patient group were compared with healthy participants under tempo naming conditions, a paradigm which is thought to impair normal semantic control by imposing dual task conditions. A basic picture naming paradigm was used throughout, with the addition of different grammatical classes, correct and misleading phonemic cues, and repetition and semantic priming: all these manipulations could be expected to place differing loads on a semantic control system with either permanent or experimentally induced impairment. It was found that stimuli requiring less controlled processing such as high imageability objects, pictures with simultaneous correct cues or repetition primed pictures were named significantly more accurately than items which needed more controlled processing, such as low imageability actions, pictures with misleading phonemic cues and unprimed pictures. The cueing evidence offered support to interactive models of speech production where phonological activation is able to influence semantic selection. The impairment in tasks such as the inhibition of task-irrelevant material seen in SA patients and tempo participants, and the overlap between cortical areas cited in studies looking at both semantic and wider executive control mechanisms suggest that semantic control may be part of a more generalised executive system.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Global versus phonemic similarity : evidence in support of multi-level representation

There is long-standing debate about the extent to which children cognitively represent words in terms of global properties or phonological segments, yet few studies have investigated how children’s sensitivity to phonemic versus global similarity changes over time. The current study uses a mispronunciation-reconstruction task to measure both types of sensitivity within a cross-sectional (N=90, aged 3;2 to 5;7) and longitudinal sample (N=23, aged 3;2 to 5;1). The results show that children’s sensitivity to phonemes increases over the first two years of school but does not reach adult levels. The findings indicate that global similarity relations remain important throughout development and support the idea of multi-level representation

Efficient visual object and word recognition relies on high spatial frequency coding in the left posterior fusiform gyrus: Evidence from a case-series of patients with ventral occipito-temporal cortex damage

Recent visual neuroscience investigations suggest that ventral occipito-temporal cortex is retinotopically organized, with high acuity foveal input projecting primarily to the posterior fusiform gyrus (pFG), making this region crucial for coding high spatial frequency information. Because high spatial frequencies are critical for fine-grained visual discrimination, we hypothesized that damage to the left pFG should have an adverse effect not only on efficient reading, as observed in pure alexia, but also on the processing of complex non-orthographic visual stimuli. Consistent with this hypothesis, we obtained evidence that a large case series (n = 20) of patients with lesions centered on left pFG: 1) Exhibited reduced sensitivity to high spatial frequencies; 2) demonstrated prolonged response latencies both in reading (pure alexia) and object naming; and 3) were especially sensitive to visual complexity and similarity when discriminating between novel visual patterns. These results suggest that the patients' dual reading and non-orthographic recognition impairments have a common underlying mechanism and reflect the loss of high spatial frequency visual information normally coded in the left pFG