Neural coding for the retrieval of multiple memory patterns.

We investigate the retrieval dynamics in a feature-based semantic memory model, in which the features are coded by neurons of the Hindmarsh-Rose type in the chaotic regime. We consider the retrieval process as consisting of the synchronized firing activity of the neurons coding for the same memory pattern. The retrieval dynamics is investigated for multiple patterns, with particular attention to the case of overlapping memories. In this case, we hypothesize a dynamical nontransitive mechanism based on synchronization, that allows for a shared feature to participate in multiple memory representations. The problem of the choice of a cognitive plausible time-scale for the retrieval analysis is investigated by analyzing the information that can be inferred from finite-time analyses. Different types of indicators are proposed in order to evaluate the temporal dynamics of the neurons engaged in the retrieval process. We interpret the simulation results as suggestive of a role for chaotic dynamics in allowing for flexible composition of elementary meaningful units in memory representations.

Modality-specific operations in semantic dementia.

A patient suffering from semantic dementia is described who consistently demonstrated the preserved ability to support specific types of semantic judgements from visual, but not from verbal, input. In addition the representations accessed from visual input were found to trigger complex behavioural schemata, while with verbal materials the patients performed almost invariably at chance level. A preliminary description is given of the nature of visual semantic representations, and the privileged relationship between this modality of input and some aspects of semantic knowledge is also explored. The richness of the semantic representations accessed from visual input can be accommodated within the "Multimodal Semantics" framework; alternative views, derived from the Identification Semantics and the Organized Unitary Content Hypothesis, are also discussed.