Visual word processing and experiential origins of functional selectivity in human extrastriate cortex

Concepts are more than percepts: the case of action verbs

Several regions of the posterior-lateral-temporal cortex (PLTC) are reliably recruited when participants read or listen to action verbs, relative to other word and nonword types. This PLTC activation is generally interpreted as reflecting the retrieval of visual-motion features of actions. This interpretation supports the broader theory, that concepts are comprised of sensory-motor features. We investigated an alternative interpretation of the same activations: PLTC activity for action verbs reflects the retrieval of modality-independent representations of event concepts, or the grammatical types associated with them, i.e., verbs. During a functional magnetic resonance imaging scan, participants made semantic-relatedness judgments on word pairs varying in amount of visual-motion information. Replicating previous results, several PLTC regions showed higher responses to words that describe actions versus objects. However, we found that these PLTC regions did not overlap with visual-motion regions. Moreover, their response was higher for verbs than nouns, regardless of visual-motion features. For example, the response of the PLTC is equally high to action verbs (e.g., to run) and mental verbs (e.g., to think), and equally low to animal nouns (e.g., the cat) and inanimate natural kind nouns (e.g., the rock). Thus, PLTC activity for action verbs might reflect the retrieval of event concepts, or the grammatical information associated with verbs. We conclude that concepts are abstracted away from sensory-motor experience and organized according to conceptual properties.

Neural representations of social status hierarchy in human inferior parietal cortex

Mental representations of social status hierarchy share properties with that of numbers. Previous neuroimaging studies have shown that the neural representation of numerical magnitude lies within a network of regions within inferior parietal cortex. However the neural basis of social status hierarchy remains unknown. Using fMRI, we studied subjects while they compared social status magnitude of people, objects and symbols, as well as numerical magnitude. Both social status and number comparisons recruited bilateral intraparietal sulci. We also observed a semantic distance effect whereby neural activity within bilateral intraparietal sulci increased for semantically close relative to far numerical and social status comparisons. These results demonstrate that social status and number comparisons recruit distinct and overlapping neuronal representations within human inferior parietal cortex.

Inter-subject variability in the use of two different neuronal networks for reading aloud familiar words

Cognitive models of reading predict that high frequency regular words can be read in more than one way. We investigated this hypothesis using functional MRI and covariance analysis in 43 healthy skilled readers. Our results dissociated two sets of regions that were differentially engaged across subjects who were reading the same familiar words. Some subjects showed more activation in left inferior frontal and anterior occipito-temporal regions while other subjects showed more activation in right inferior parietal and left posterior occipito-temporal regions. To explore the behavioural correlates of these systems, we measured the difference between reading speed for irregularly spelled words relative to pseudowords outside the scanner in fifteen of our subjects and correlated this measure with fMRI activation for reading familiar words. The faster the lexical reading the greater the activation in left posterior occipito-temporal and right inferior parietal regions. Conversely, the slower the lexical reading the greater the activation in left anterior occipito-temporal and left ventral inferior frontal regions. Thus, the double dissociation in irregular and pseudoword reading behaviour predicted the double dissociation in neuronal activation for reading familiar words. We discuss the implications of these results which may be important for understanding how reading is learnt in childhood or re-learnt following brain damage in adulthood.

Synaesthesia and cortical connectivity

Synaesthesia is a heritable condition of involuntary sensory cross-activation whereby the presentation of a particular stimulus elicits a secondary sensory-perceptual experience. It is thought to be caused by aberrant cross-activation of one cortical area by another, but models differ as to whether this reflects functional or structural differences in the brains of synaesthetes. Here we consider these models in light of recent experimental findings and argue for structural differences in the brains of synaesthetes, which might be more widespread than expected. We also discuss several plausible developmental mechanisms that could link a putative genetic variant to altered cortical connectivity and illustrate how synaesthesia could be an informative model to investigate how patterns of connectivity between cortical areas are established.

Interpreting fMRI data: maps, modules and dimensions

Neuroimaging research over the past decade has revealed a detailed picture of the functional organization of the human brain. Here we focus on two fundamental questions that are raised by the detailed mapping of sensory and cognitive functions and illustrate these questions with findings from the object-vision pathway. First, are functionally specific regions that are located close together best understood as distinct cortical modules or as parts of a larger-scale cortical map? Second, what functional properties define each cortical map or module? We propose a model in which overlapping continuous maps of simple features give rise to discrete modules that are selective for complex stimuli.

Nature versus nurture in ventral visual cortex: a functional magnetic resonance imaging study of twins

Using functional magnetic resonance imaging, we estimated neural activity in twins to study genetic influences on the cortical response to categories of visual stimuli (faces, places, and pseudowords) that are known to elicit distinct patterns of activity in ventral visual cortex. The neural activity patterns in monozygotic twins were significantly more similar than in dizygotic twins for the face and place stimuli, but there was no effect of zygosity for pseudowords (or chairs, a control category). These results demonstrate that genetics play a significant role in determining the cortical response to faces and places, but play a significantly smaller role (if any) in the response to orthographic stimuli.

Item-specific and generalization effects on brain activation when learning Chinese characters

Neural changes related to learning of the meaning of Chinese characters in English speakers were examined using functional magnetic resonance imaging (fMRI). We examined item specific learning effects for trained characters, but also the generalization of semantic knowledge to novel transfer characters that shared a semantic radical (part of a character that gives a clue to word meaning, e.g. water for lake) with trained characters. Behavioral results show that acquired semantic knowledge improves performance for both trained and transfer characters. Neuroimaging results show that the left fusiform gyrus plays a central role in the visual processing of orthographic information in characters. The left superior parietal cortex seems to play a crucial role in learning the visual-spatial aspects of the characters because it shows learning related decreases for trained characters, is correlated with behavioral improvement from early to late in learning for the trained characters, and is correlated with better long-term retention for the transfer characters. The inferior frontal gyrus seems to be associated with the efficiency of retrieving and manipulating semantic representations because there are learning related decreases for trained characters and this decrease is correlated with greater behavioral improvement from early to late in learning.