Prefrontal Cortex and Neurological Impairments of Active Thought Tim

Prefrontal cortex and neurological impairments of active thought 1 2 Tim Shallice and Lisa Cipolotti 1. Institute of Cognitive Neuroscience, University College London, Gower Street, London WC1E 6BT, UK and SISSA, Trieste, Italy. email: [email protected] 2. Neuropsychology Department, National Hospital for Neurology and Neurosurgery, Queen Square, London WC1, UK and Dipartimento di Psicologia, Universita di Palermo, Italy. email: [email protected] Short title: Impairments of active thought Corresponding Author: Tim Shallice, Institute of Cognitive Neuroscience, University College London, Gower Street, London WC1E 6BT, UK. email: [email protected] 1 Table of Contents INTRODUCTION Overall perspective Why neuropsychology? The neuropsychological approach to frontal functions. BROAD BRUSH ASPECTS OF ACTIVE THOUGHT Dual system brain-based models of cognitive control Active thought and intelligence Active thought and language ESSENTIAL PREREQUISITES FOR ACTIVE THOUGHT Volition and concentration Set-Switching and response inhibition Active monitoring and checking Working memory FORMS OF ACTIVE THOUGHT Abstraction Deduction Reasoning in well-structured and less well-structured problem-spaces From lateral transformations to strategy shifts Planning for future action THEORETICAL CONCLUSIONS 2 Keywords Active thought, neuropsychology, prefrontal cortex, supervisory system, reasoning, lateralisation of function Abstract The paper reviews the effects of lesions to the frontal cortex in so far as they impact on the ability to carry out active thought, namely to reason, think flexibly, produce strategies and formulate and realise plans. How relevant neuropsychological studies should be carried out and why are discussed. The relation between active thought and each of intelligence and language are considered. The following basic processes necessary for effective active thought are reviewed: concentration, set-switching, inhibiting potentiated responses and monitoring and checking. Different forms of active thought are then addressed: abstraction, deduction, reasoning in well-structured and ill-structured problem spaces, novel strategy generation and planning. It is concluded that neuropsychological findings are valuable for providing information on systems rather than networks, and especially concerning prefrontal lateralisation of function. A synthesis is presented of the respective roles of the left and right lateral prefrontal cortex in active thought. 3 INTRODUCTION Overall Perspective This paper is concerned with what neuropsychological findings can tell us about the cognitive processes underlying active thinking. By active thinking we refer to mental processes that allow us to confront situations where we do not respond routinely to the environment but rather we effectively address problems which can be big or small. Active thinking entails a set of complex mental processes for example, those involved in abstraction, deduction and other forms of reasoning between alternative possibilities, switching lines of thought, strategy selection, inhibition of obvious responses and formulating and realising plans. For example, organizing a dinner party would entail many active thinking processes whereas day-dreaming or implicit processes like priming would not be considered active thinking. A number of well-known neuropsychological tests designed to assess prefrontal function require active thinking. Typical examples are tests such as Wisconsin Card-Sorting (switching lines of thought), Proverb Interpretation (abstraction), Stroop (inhibition) Tower of Hanoi (planning) and tests of fluid intelligence such as Progressive Matrices or Cattell Culture Fair (reasoning between alternative possibilities). Following frontal lobe lesions, performance on these tests is typically impaired. This suggests that the frontal lobes are critically involved in active thinking. Impairments in active thinking are also exemplified by a number of frontal lobe syndromes which involve release of irrelevant environmentally triggered actions. Examples include the grasp reflex, where the patient is repeatedly instructed not to grasp the doctor’s finger as the patient’s palm is stroked, and yet does so (De Renzi & Barbieri, 1992) or the somewhat 4 analogous situation where the patient should not move their eyes to a distracting light but does so (Paus et al. 1991). At a higher level, there is utilisation behavior, originally described by Lhermitte (1983). In this case the patient makes a standard afforded action to one of the objects surrounding him, such as dealing from a pack of cards, without being told to do so, or in the so-called incidental form, when explicitly told to do something else (Shallice et al, 1989). These examples highlight behaviors that occur when active thinking processes are absent or impaired through brain injury. Interestingly, these syndromes have been most frequently described in patients with lesions involving medial frontal areas (see De Renzi & Barbieri, 1992). Prefrontal functions are involved in many different cognitive domains. They have been well reviewed fairly recently by Sczepanski & Knight (2014). This paper will therefore focus only on those cognitive domains we consider critical for active thinking. Thus we will discuss individual cognition rather than social and on-going reasoning rather than (long-term) memory, learning, motivation and emotion. Our paper is structured in the following fashion. The Introductory section will consider why neuropsychological evidence is chosen for review of the cognitive processes underlying active thinking, out of the many cognitive neuroscience techniques available. We then address the methodological approaches adopted for the neuropsychological investigation of prefrontal functions. The second section will briefly outline our theoretical framework for active thinking, which is largely based on the Norman and Shallice (1986) Supervisory System model of prefrontal cortex (PFC) functioning. We will also consider the relationship between active thinking and potentially overlapping cognitive domains such as intelligence and language. The third section will deal with processes that are prerequisites for active thinking, namely concentration, set-shifting, thought inhibition, and monitoring and checking. In the fourth section we will discuss different types of core active thinking 5 processes including abstraction, deduction, novel strategy selection, insight, and planning. The final section aims to produce an overall theoretical synthesis. Why neuropsychology? As cognitive processes become more abstract and distant from sensory and motor processes, it becomes increasingly difficult to investigate them adequately using behavioral means alone. Thus, taking theorists working on reasoning about syllogisms, Khemlani & Johnson- Laird (2012) wrote “…Thirty-five years ago they had only heuristic accounts that explained biases and errors, and so the domain appeared to be an excellent test case for cognitive science. There are now 12 sorts of theories of syllogisms and monadic inferences, and so skeptics may well conclude that cognitive science has failed…” (p.453). A more powerful source of empirical findings seems to be required. Obvious candidates are methodologies related to the brain. Within human cognitive neuroscience there are two main classes of methodologies. The oldest class are those derived from lesion studies of neurological patients, which have recently been supplemented by TMS, and somewhat more conceptually distant, by the cognitive effects of individual differences in brain structure across the normal population. The second class are those where on-line measures are taken of brain processes while normal subjects carry out tasks – PET, fMRI, EEG, MEG and so on. If one’s aim is to provide accurate anatomical correspondences for known cognitive processes or to provide real-time information on processing, the second class are much to be preferred. Despite this clear advantage of the second class of methodology, the first-class, especially neuropsychology, have complimentary advantages for the development of cognitive theory. This is for at least five reasons: 6 (i) If one takes cognitive theory to refer to models like classic box-and-arrow information-processing ones, then neuropsychological data can speak directly to cognitive theory. Appropriate inferences are derived from a set of simple assumptions, first formalised by Caramazza (1986). They are based on the idea of subtraction of components from an overall system. Of course, subtraction in reality is complicated by complex processes related to the recovery process (see Henson et al, 2016 for a good example). However, to a first approximation, subtraction is a plausible characterisation of the effect of a brain lesion. So, this approach was much used in the heyday of cognitive neuropsychology. Moreover, the same set of assumptions can be used to relate such data to connectionist models too (Shallice & Cooper, 2011). By contrast, methodologies of the second type require complex bridging assumptions, based on physics and physiology, to relate their data to cognitive theory. (ii) It is generally accepted that activation-based findings do not necessarily imply causal efficacy (see Giliae-Dotan et al, 2015 for a particularly clear example). This possibility is of particular concern for lateralisation of function. Thus, neuropsychological data show language functions to be strongly lateralised. Crossed aphasia is very rare. In a consecutive

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