Computational Studies of Lateralization of Phoneme Sequence Generation

Abstract

The mechanisms underlying cerebral lateralization of language are poorly understood. Asymmetries in the size of hemispheric regions and other factors have been suggested as possible underlying causal factors, and the corpus callosum (interhemispheric connections) has also been postulated to play a role. To examine these issues, we created a neural model consisting of paired cerebral hemispheric regions interacting via the corpus callosum. The model was trained to generate the correct sequence of phonemes for 50 monosyllabic words (simulated reading aloud) under a variety of assumptions about hemispheric asymmetries and callosal effects. After training, the ability of the full model and each hemisphere acting alone to perform this task was measured. Lateralization occurred readily toward the side having larger size, higher excitability, or higher-learning-rate parameter. Lateralization appeared most readily and intensely with strongly inhibitory callosal connections, supporting past arguments that the effective functionality of the corpus callosum is inhibitory. Many of the results are interpretable as the outcome of a "race to learn" between the model's two hemispheric regions, leading to the concept that asymmetric hemispheric plasticity is a critical common causative factor in lateralization. To our knowledge, this is the first computational model to demonstrate spontaneous lateralization of function, and it suggests that such models can be useful for understanding the mechanisms of cerebral lateralization.

Neonatal novelty exposure, dynamics of brain asymmetry, and social recognition memory

“…These parallel changes in asymmetry and learning along with results from the computational modeling literature (Kosslyn, Sokolov, & Chen, 1989;Levitan & Reggia, 2000;Reggia, Goodall, & Shkuro, 1998) provided further support for the hypothesis that the development of cerebral asymmetry may have a functional consequence for cognitive development (Güntürkün et al, 2000;Rogers, 2000). In a recent study, we found that neonatal novelty exposure led to prolongation of social recognition memory from less than 2 hr to at least 24 hr during adulthood and that individual differences in this form of adult recognition memory can be predicted by the turning asymmetry measured immediately prior to the social recognition test (Reeb et al, 2001;Tang et al, 2003).…”

An epigenetic induction of a right‐shift in hippocampal asymmetry: Selectivity for short‐ and long‐term potentiation but not post‐tetanic potentiation

“…From a computational point of view, a greater STP and LTP mean a greater rate of learning, which has been shown as one of the network properties critical for the development of functional lateralization in neural network models (Reggia et al, 1998). An initial asymmetry in hippocampal synaptic plasticity, even if very small, may serve to ''break'' the symmetry, providing differential rates of task acquisition between the two hippocampi during development, leading to asymmetric task-related activation.…”

Effects of callosal lesions in a model of letter perception

“…Most interesting is the observation that this diminished lateralization occurred regardless of the type of callosal influence (excitatory or inhibitory) assumed to be present. The postlesion loss of lateralization when inhibitory callosal influences were used is not surprising, given past models that have shown that transcallosal inhibition generally increases lateralization (Levitan & Reggia, 2000;Reggia et al, 1998;Shevtsova & Reggia, 1999). However, this finding is somewhat unexpected with excitatory callosal influences, and it suggests that even excitatory callosal influences can facilitate hemispheric specialization.Past arguments that hemispheric specialization depends on having inhibitory callosal influences lose some of their force in the context of these results.…”

“…In the context of this "callosal dilemma" about the excitatory/inhibitory nature of callosal connections, a number of computational models, consisting of paired left and right cortical regions, have recently been developed to study hemispheric interactions and cerebral functional asymmetries (Cook, 1999;Cook & Beech, 1990;Jacobs & Kosslyn, 1994;Levitan & Reggia, 1999Reggia, Gittens, & Chhabra, 2000;Reggia, Goodall, & Shkuro, 1998;Ringo, Doty, Demeter, & Simard, 1994;Shevtsova & Reggia, 1999Shkuro, Glezer, & Reggia, 2000). Among other things, these neural network models have been very successful in demonstrating that a variety of underlying hemispheric asymmetries (such as asymmetries in region size, excitability, receptive field size, feedback intensity, or synaptic plasticity) can, in theory, lead to hemispheric specialization.…”