The Review of Metaphysics

This book advances a brain-based account of the mind focusing on the neural mechanisms explaining higher-level mental phenomena. It's a bold attempt at providing a robust, unified explanation of mental phenomena using the semantic pointer architecture developed by Chris Eliasmith's 2013 How to Build a Brain: A Neural Architecture for Biological Cognition. Overall, Brain-Mind is best interpreted as developing an abductive argument where Thagard displays the explanatory power of semantic pointers in order to justify their introduction as a theoretical posit within cognitive neuroscience.

Chapter 2 introduces the central neural mechanisms. Thagard begins with a common idea that mental representation emerges from interactions of patterns of firing both within and across groups of neurons. He then describes how Eliasmith's semantic pointers are created from binding several representations together into a new mental representation via a mathematical process of "circular convolution." Roughly, the idea is that information from several mental representations gets combined and compressed into a new mental representation (a new pattern of neural firing across several groups of neurons). Semantic pointers supposedly have several features that aid Thagard in explaining a diverse set of mental phenomena. First, convolution can be performed recursively. Second, semantic pointers have "modal retention": they retain information specific to "the particular sensory or motor modality that produced them." Third, despite modal retention, semantic pointers are independent from the sensory representations that give rise to them and so can function as symbols in inference and language. Fourth, semantic pointers can be unpacked to extract information included in the representations from which they were originally formed. Chapter 2 concludes with brief discussions of Hebbian learning and of parallel constraint satisfaction using semantic pointers. Both are crucial to the explanations provided throughout the book. This chapter is dense, but Thagard's presentation is admirably clear and easy to understand; his analogies for explaining semantic pointers are exceptionally well crafted. The chapter also includes an appendix discussing the mathematical operation of convolution in more detail and compares the semantic pointer architecture with other prominent theories such as those appealing to Bayesian inference, standard connections models, and so on.

Thagard often relies on the idea that semantic pointers can incorporate both linguistic and nonlinguistic representations to argue that this framework provides a more robust account of several psychological operations. Chapter 3 reviews experimental evidence from cognitive science that much of our thinking uses imagery rather than information converted into a linguistic format. Thagard argues that semantic pointers can provide neural mechanisms responsible for this imagery. Sensory representations can be used to form new semantic pointers that can then be retrieved and recalled later by reactivation of the neural firing pattern. The modal retention of semantic pointers explains how the imagistic [End Page 831] representations maintain their sensory like character. However, these nonlinguistic representations are still independent of the specific sensory representations from which they are formed and can thereby function in symbollike manipulation such as intensification, zooming, rotation, juxtaposition, and so on.

Similarly, Thagard appeals to semantic pointers' ability to incorporate verbal and nonverbal representation to provide a more substantive account of analogical thought in chapter 6. Semantic pointers combine and compress verbal information along with information from sensory representations, motor information, expectations, rules, intentions, and emotions. As such, the semantic pointers that serve as the source and targets of an analogy combine and compress information from several sources while managing to retain information particular to their different sensory modalities. This allows for a more robust thought process where analogical thought can simultaneously draw on verbal and nonverbal representations along with syntactic, semantic, and pragmatic features unified within the semantic pointer.

Chapter 7 uses semantic pointers to bring together three seemingly competing theories of emotions: emotions as cognitive appraisals, bodily perceptions, or social constructions. Cognitive appraisals of situations, bodily perceptions, and representations of social context are understood as semantic pointers. Thagard then identifies emotions with semantic pointers that bind these different representations and thereby...