The processing of verbs and nouns in neural networks: insights from synthetic brain imaging

Origins of Dissociations in the English Past Tense: A Synthetic Brain Imaging Model

Brain imaging studies of English past tense inflection have found dissociations between regular and irregular verbs, but no coherent picture has emerged to explain how these dissociations arise. Here we use synthetic brain imaging on a neural network model to provide a mechanistic account of the origins of such dissociations. The model suggests that dissociations between regional activation patterns in verb inflection emerge in an adult processing system that has been shaped through experience-dependent structural brain development. Although these dissociations appear to be between regular and irregular verbs, they arise in the model from a combination of statistical properties including frequency, relationships to other verbs, and phonological complexity, without a causal role for regularity or semantics. These results are consistent with the notion that all inflections are produced in a single associative mechanism. The model generates predictions about the patterning of active brain regions for different verbs that can be tested in future imaging studies.

Are imaging and lesioning convergent methods for assessing functional specialisation?: investigations using an artificial neural network

This article presents an investigation of the relationship between lesioning and neuroimaging methods of assessing functional specialisation, using synthetic brain imaging (SBI) and lesioning of a connectionist network of past-tense formation. The model comprised two processing 'routes': one was a direct route between layers of input and output units, while the other, indirect, route featured an intermediate layer of processing units. Emergent specialisation within the network was assessed (1) by lesioning either the direct or indirect route and measuring past-tense performance for regular and irregular verbs, and (2) by measuring functional activation in each route when processing each verb type (SBI). SBI and lesioning approaches failed to converge when network activation was summed over each route in our SBI approach. Examination of individual network solutions suggested that the verb types might be using the indirect route differently in terms of the pattern of activation across the route, rather than in terms of gross activation. A subsequent SBI analysis compared patterns of activation in the indirect route and confirmed that these patterns were more similar between regular-type verbs than between regular and irregular verbs. As the spatial and temporal resolution of neuroimaging techniques improves, the results of this investigation suggest that the key to finding functional specialisation will be to distinguish local coding differences across behaviours that are the results of developmental processes. Other analyses suggest that lesioning data may be limited because, with increasing damage, they reveal the resting activations of a computational system rather than a computational specialisation per se.

The mechanics of embodiment: a dialog on embodiment and computational modeling

Embodied theories are increasingly challenging traditional views of cognition by arguing that conceptual representations that constitute our knowledge are grounded in sensory and motor experiences, and processed at this sensorimotor level, rather than being represented and processed abstractly in an amodal conceptual system. Given the established empirical foundation, and the relatively underspecified theories to date, many researchers are extremely interested in embodied cognition but are clamoring for more mechanistic implementations. What is needed at this stage is a push toward explicit computational models that implement sensorimotor grounding as intrinsic to cognitive processes. In this article, six authors from varying backgrounds and approaches address issues concerning the construction of embodied computational models, and illustrate what they view as the critical current and next steps toward mechanistic theories of embodiment. The first part has the form of a dialog between two fictional characters: Ernest, the "experimenter," and Mary, the "computational modeler." The dialog consists of an interactive sequence of questions, requests for clarification, challenges, and (tentative) answers, and touches the most important aspects of grounded theories that should inform computational modeling and, conversely, the impact that computational modeling could have on embodied theories. The second part of the article discusses the most important open challenges for embodied computational modeling.

Nomen est omen: Investigating the dominance of nouns in word comprehension with eye movement analyses

Although nouns are easily learned in early stages of lexical development, their role in adult word and text comprehension remains unexplored thus far. To investigate the role of different word classes (open-class words: nouns, adjectives, verbs; closed-class words: pronouns, prepositions, conjunctions, etc.), 141 participants read a transposed German text while recording eye movements. Subsequently, participants indicated words they found difficult and reproduced the story. Then, participants were presented an untransposed text version while also tracking eye movements. Word difficulty, subjectively assessed by an interview and objectively by eye movement criteria (general fixation rate, number of fixations on specific words), text comprehension scores, and regressive fixations from one word class to another in the transposed text indicated that the noun was the most influential word class in enhancing the comprehension of other words. Developmental, intercultural, and neurophysiological aspects of noun dominance are discussed.

Experimental design and interpretation of functional neuroimaging studies of cognitive processes

This article discusses how the relation between experimental and baseline conditions in functional neuroimaging studies affects the conclusions that can be drawn from a study about the neural correlates of components of the cognitive system and about the nature and organization of those components. I argue that certain designs in common use--in particular the contrast of qualitatively different representations that are processed at parallel stages of a functional architecture--can never identify the neural basis of a cognitive operation and have limited use in providing information about the nature of cognitive systems. Other types of designs-such as ones that contrast representations that are computed in immediately sequential processing steps and ones that contrast qualitatively similar representations that are parametrically related within a single processing stage-are more easily interpreted.

Evolution of prehension ability in an anthropomorphic neurorobotic arm

In this paper, we show how a simulated anthropomorphic robotic arm controlled by an artificial neural network can develop effective reaching and grasping behaviour through a trial and error process in which the free parameters encode the control rules which regulate the fine-grained interaction between the robot and the environment and variations of the free parameters are retained or discarded on the basis of their effects at the level of the global behaviour exhibited by the robot situated in the environment. The obtained results demonstrate how the proposed methodology allows the robot to produce effective behaviours thanks to its ability to exploit the morphological properties of the robot's body (i.e. its anthropomorphic shape, the elastic properties of its muscle-like actuators and the compliance of its actuated joints) and the properties which arise from the physical interaction between the robot and the environment mediated by appropriate control rules.