The Measure of Madness: Philosophy of Mind, Cognitive Neuroscience, and Delusional Thought

4 The Default Mode Network

4 The Default Mode Network 4.1 Simulations as Narrative Elements The main task of this chapter is to explain the properties of the default mode network (DMN)—the system that is active and relatively unsupervised in delusion. The default mode of the DMN is a resting state of a powerful simulation system that evolved to allow humans to simulate experiences in the absence of an eliciting stimulus. This capacity released us from the stimulus-bound present, allowing us to recall past experiences and imagine possible futures. Mental time travel, as this type of simulation process is now called, is an adaptation for planning. We can review and preview possible scenarios and experience characteristic emotions before committing to action. These scenarios have a narrative structure that can range from very minimal to richly structured and elaborate. In mental time travel, narrative elements are highly organized by an overarching goal. Imagine mentally rehearsing a journey or a speech, for example. When not organized for problem solving, the DMN reverts to the screensaver mode we experience as daydreaming or mind-wandering. In this mode, there is no overarching goal to provide narrative structure. 68 Chapter 4 The DMN gets its name from its detection and mapping in the resting state: daydreaming is what the mind does in default mode in between episodes of simulation generated for cognitively demanding problem solving. Focusing on the default mode in isolation, however, turns attention away from three of its features , which are essential to understanding its role in delusion: (i) it is a simulation system; (ii) the simulations are fragments of autobiographical/personal narratives; and (iii) it is supervised by, and anticorrelated with, circuits specialized for decontextualized processing. The functioning of the default network is best understood in terms of its role in the processing hierarchy: supervising lowerlevel , more automatic processes and supervised by higher-level, decontextualized processes. We explain its properties in different conditions. The first is alert waking cognition in which it is in communication with other perceptual and executive systems. This is a familiar condition in which, for example, we rehearse possible future and actual past experience to help decide where to have lunch, where to send children to school, or whom to marry. Another condition that displays the properties of the default network is dreaming. When dreaming, the default network is disconnected from both the sensory periphery of the mind and high-level control systems deactivated during REM sleep. As a result, dreams are (relatively) cognitively incoherent. They are “dramatic simulations of our conceptions, concerns, and interests that occur when a specific constellation of neural regions is activated in a context where there is no engagement with the external world” (Domhoff 2011, 1172; my italics). The Default Mode Network 69 Once these conditions are described, we turn to the way in which the default network functions, relatively unsupervised, in delusion. Delusions are not dreams, or even waking dreams, but they have some of the properties of dreams because they depend on activity in default circuitry. Equally, they have some of the properties of “normal” waking cognition. They are simulations triggered by sensory and perceptual inputs that are subsequently incorporated into the agent’s psychology. The importance of this idea is that the falsity, fixity, and fabulous aspects of delusion result from the nature of default thought itself. Delusions are not caused by failures of reasoning, since default thinking is not intrinsically a reasoning process. Intrinsically it is an imaginative/simulative process precariously disciplined by the demands of narrative coherence. Neither imagination nor narrative is essentially governed by norms of rationality. In this respect, theorists who remark on similarities between dreams and delusions are correct. In neither case are the simulations tried before the tribunal of reality. 4.2 Mental Time Travel and the Default Network As Daniel Gilbert (2004) once put it, We are the only animals that can peer deeply into our futures—the only animal that can travel mentally through time, preview a variety of futures, and choose the one that will bring us the greatest pleasure and/or the least pain. This is a remarkable adaptation—which, incidentally, is directly tied to the evolution of the frontal lobe—because it means that we can learn from mistakes before we make them. Gilbert invokes the notion of mental time travel to capture the idea that imaginative rehearsal using information gathered in previous experience is the essence of planning...