Abstract
Representations in the brain are encoded as patterns of activity of large populations of neurons. The science of population encoded representations, also known as parallel distributed processing (PDP), achieves neurological verisimilitude and has been able to account for a large number of cognitive phenomena in normal people, including reaction times (and reading latencies), stimulus recognition, the effect of stimulus salience on attention, perceptual invariance, simultaneous egocentric and allocentric visual processing, top-down/bottom-up processing, language errors, the effect of statistical regularities of experience, frequency, and age of acquisition, instantiation of rules and symbols, content addressable memory and the capacity for pattern completion, preservation of function in the face of noisy or distorted input, inference, parallel constraint satisfaction, the binding problem and gamma coherence, principles of hippocampal function, the location of knowledge in the brain, limitations in the scope and depth of knowledge acquired through experience, and Piagetian stages of cognitive development. PDP studies have been able to provide a coherent account for impairment in a variety of language functions resulting from stroke or dementia in a large number of languages and the phenomenon of graceful degradation observed in such studies. They have also made important contributions to our understanding of attention (including hemispatial neglect), emotional function, executive function, motor planning, visual processing, decision making, and neuroeconomics. The relationship of neural network population dynamics to electroencephalographic rhythms is starting to emerge. Nevertheless, PDP approaches have scarcely penetrated major areas of study of cognition, including neuropsychology and cognitive neuropsychology, as well as much of cognitive psychology. This article attempts to provide an overview of PDP principles and applications that addresses a broader audience.
Highlights
In 1980, it was not possible to imagine how a brain composed of 100 billion highly interconnected, lipid-encased, reticular electrochemical devices could possibly support complex neural functions like language, memory, visuospatial, emotional and executive function
THE STRUCTURE OF POPULATION ENCODING NETWORKS. It has been known for some time that representations in the central nervous system (CNS) are population encoded, that is, encoded as patterns of activity involving very large numbers of highly interconnected neurons in one or more neural networks extending over large expanses of the brain (O’Keefe and Nadel, 1979; Georgopoulos et al, 1982; Churchland and Sejnowski, 1992; Rolls and Treves, 1998; Zhang et al, 1998; Zhang and Sejnowski, 1999; Rolls and Deco, 2002; Behrmann and Plaut, 2013; Rolls, 2016; Lebedev and Nicolelis, 2017)
Hippocampal knowledge that cannot be cortically encoded at all remains hippocampally dependent indefinitely and is lost with hippocampal lesions, the most dramatic example being the loss of autobiographical memory with anoxic injury (Vargha-Khadem et al, 1997). This understanding of hippocampal function reveals how the neurodynamical operations of the heavily interconnected cerebral cortex are transformed into a fundamentally different mathematical form in order to meet several very specific goals while avoiding limitations of and consequences for its operations if such a transformation did not occur. These include (1) sparcification of representations by the dentate-CA3 process so that new episodic memories are highly specific to certain cortical representations and are not mistakenly generalized to large domains, e.g., animals in lieu of my pet cat; (2) capacity for formation of arbitrary associations between objects, times and places by the CA3 autoassociator network; (3) retention of knowledge as long-loop corticocortical connectivity until, to the extent that features are shared, it can gradually be integrated into cortical connectivity in the process of memory consolidation while not obliterating old knowledge in the process; (4) the critical facility for full retrieval of a hippocampally dependent memory given only a cue, and (5) the facility for lifetime retention of episodic memories that cannot be integrated into cerebral cortical connectivity
Summary
PDP studies have been able to provide a coherent account for impairment in a variety of language functions resulting from stroke or dementia in a large number of languages and the phenomenon of graceful degradation observed in such studies. They have made important contributions to our understanding of attention (including hemispatial neglect), emotional function, executive function, motor planning, visual processing, decision making, and neuroeconomics.
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