A neuroconstructivist investigation of object ...

A neuroconstructivist investigation of object knowledge and action in development and in developmental disorders. Caspar Addyman Supervisors: Denis Mareschal, Michael Thomas March 21, 2005

The aim of this research will be to use connectionist modeling techniques together with experimental data from infants and atypical populations to better understand both the development of (seeming) cognitive modularity and the causes and dynamics of developmental disorders. Specically, I will extend the connectionist dual-route model ('where'/'what') of Mareschal et al. (1999) to make further predictions about infant object knowledge and action. Then I will investigate if disruption of this model produces decits in spatio-temporal processing (dorsal route) analogous to those found in autism (Spencer et al., 2000), dyslexia (Hansen et al., 2001) and Williams Syndrome (Paul et al., 2002). The divison of visual information into dorsal and ventral streams of processing is a well established feature of the mature primate visual system (Ungerleider and Mishkin, 1982; Van Essen et al., 1992). However, much less is known about the early development of these pathways. Results from object permanence tests, A not B tasks and scanning studies suggest that perception, cognition and action in infants are not the consequence of inexperienced or immature versions of adult modules but are best explained in terms of genuine developmental change. (See Bremner, 2000; Johnson and Mareschal, 2001). This neuroconstructivist viewpoint requires adult architecture to be end point of processes of interactive specialization and representational redescription acting on a relatively undierentiated neural substrate in the presence of a small number of innate constraints on development. (See Karmilo-Smith, 1992) Two important features of this approach are that it takes advantage of connectionist techniques and explanations (Elman et al., 1996) and that it provides a developmental account of developmental disorders (Karmilo-Smith, 1998; Thomas and Karmilo-Smith, 2002). 1

There is a developmental dichotomy between knowledge and action. In contrast to the classic position of Piaget (1952) more recent studies have shown that infants can have object knowledge (as found by looking time measures) without the ability to act on these objects (Baillargeon et al., 1985). For example, infants at six months being unable to reach for an occluded object even though they possess the sensorimotor ability to perform the necessary action (Von Hofsten, 1989). Mareschal et al. (1999) is a widely cited connectionist model of the dual-route visual system that explains this developmental delay in object directed action. It provides a computational account of object-oriented behaviour which is both motivated by cortical constraints and incorporates a response integration network and a trajectory prediction network. The initial stage of my investgation will be to replicate this model and test its applicability to more recent experimental data. An important constraint in neuroconstructivist accounts of development is that there is much initial plasticity and that modularity is emergent. In the visual system, this is highlighted by the fact that the functional distinction between dorsal and ventral streams is not reective of any clear dierences in neuroanatomy (Van Essen et al., 1992). A study by O'Reilly and McClelland (1992) shows how a dual-route system with spatially-invariant and spatio-temporal specializations can emerge as a result of self-organization in a network caused by small dierences in activation rates in an otherwise homogenous network. This suggests a developmental mechanism that might derive an architecture like that of Mareschal et al. (1999). An aim of the present study will be to attempt to combine these to approaches. Having just completed a modelling exercise which attempted to match infant performance on a visual statistical learning task (Kirkham et al., 2002), I became aware of several major short-comings that are generally associated with network models; Most of the interpretation takes place outside the model itself and decisions about architecture are often arbitrary. It is up to the experimenter to read the activations and choose how to relate the state of the network to a mental state or (far more rarely) an action. Input representations, learning regimes and architectural changes are similarly the arbitary choice of the modeller, leading to frequent criticism that the solution is an artefact of encoding or training (Marcus, 1998). More recently this seems to be changing. For example, the HAB (Habituation, Autoassociation and Brain) model of Sirois and Mareschal (2004) derives its architecture and learning rules from neurological consideration of the hippocampal and thalamic pathways thought to be involved in habituation and demonstrates the seven features of habituation (Sirois and Mareschal, 2002). 2

Similarly, Mareschal et al. (1999) provides a computational account which is both motivated by cortical constraints and incorporates relevant pathways. To some extent this approach also emphasizes the role of the infant as an autonomous agent rather than as a passive computational information processor. I think this embodied approach is essential for any comprehensive approach to infant behaviour and can often be more parsimonious too (Schlesinger, 2003) Applying the neuroconstructivist framework to developmental disorders leads to a number of predictions; Decits will not be localized but will be manifest across a range of cognitive functions since decits are thought to be the consequence of abnormal neuronal ring or atypical local or global connectivity. Apparently normal behaviours in developmental disorders could be the result of atypical processes and there may be compensation in other modules. Disorders lie on a continuum with normal functioning. (Karmilo-Smith, 1998) Only when these theoretical predictions are applied to specic cortical systems and particular disorders can the value of the framework be assessed. The extensive developmental data on the functioning of the visual system and the wide neuropsychological literature on developmental disorders such as autism and Williams syndrome make this an excellent area for such research. Moreover, by starting from an explicit connectionist implementation of the development of dorsal and ventral processing, it will be possible to make experimentally testable predictions. Therefore, this study will be likely to include looking time and habituation studies with infants using carefully selected stimuli to discriminate performance for dierent object types and relationships, such as in Mareschal and Johnson (2003) and will also be likely to adopt experimental paradigms for testing motion and shape coherence detection found in Spencer et al. (2000). But in addition, modern experimental techniques make possible the brain-level investigation of localisation of function even in very young infants. They can reveal the brain states associated with particular aspects of the object knowledge. For example, a EEG study by Kaufman et al. (2003) found bursts of gamma-band activity in the temporal lobe to a neural correlate of object permanence in six month old infants. Studies of this kind are highly pertinant to the approach of this proposed investigation and the facilities and expertise in the Centre for Brain and Cognitive Development make Birkbeck the ideal place to conduct such research.

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