Harvey et al At present, we have very limited understanding of the mechanism by which psychological stress can “convert” into physical symptoms. The concept of “konversion” was first introduced by Freud and Breuer. Breuer speculated on possible neurological mechanisms of conversion symptoms when he discussed the “abnormal excitability” of the nervous system and “weakness of resistances” in particular paths of conduction (Freud and Breuer 1978). However, it is Freud’s psychodynamic conceptualization of conversion disorder that remains inextricably linked with the diagnosis in current classifications. The lack of understanding of the neural mechanism by which psychological stressors can unconsciously result in physical symptoms is an important Table 1 Definitions and explanations of important terminology reason for the ongoing controversy and stigma surrounding the diagnosis. Empirical research in this area has tended to lag behind theoretical speculation (Halligan, Bass, et al 2000). However, advances in functional imaging and neuro- psychological testing offer the opportunity to narrow the gap between psychoanalytical theory and neurobiological explanations of conversion disorder. One difficulty facing research in this field is the complexity of the conceptual issues and variable ways in which terminology has been used. To try to address this issue, we have attempted to define the way in which important terms have been used in this paper as precisely as possible (Table 1). Consciousness and awareness “conscious” “unconscious” “awareness” “explicit/implicit” Attention Volition, willed action, and intentions Hierarchical processing “higher” or “downstream” processing Inhibition, excitation, modulation P300 potential Blindsight Neglect 14 The term “consciousness” can be used in several ways (Zeman 2001), but in this paper we will refer to consciousness as the qualitative, subjective dimension of experience.Thus, mental processes of which we are aware (eg, perceiving an object, forming an intention to move) would be said to be “conscious” or “explicit”, whereas mental processes of which we are not aware (eg, a pupillary light reflex,“blindsight”, implicit memory) would be said to be “unconscious” or “implicit”. In this sense, consciousness is inextricably linked with attention and volition. William James provided a seminal definition of attention as “the taking possession by the mind, in clear and vivid form, of one out of what seem several simultaneously possible objects or trains of thought. Focalization, concentration, of consciousness are of its essence” (James 1890). Current neuropsychological models see attention as a set of processes that serve to enhance sensory, motor, and cognitive processing (Gitelman 2003). Deficits in attention may be relevant to sensory conversion symptoms. Volition or “will” is the faculty of consciously making a choice or selecting an action (James 1890). “Willed actions” (eg, walking on a slippery surface) can be differentiated from stereotyped, routine actions (eg, walking normally) by the fact that we consciously attend to them and select them (Jahanshahi and Firth 1998).To perform a willed action we first form an “intention” and then execute the action. Evidence from functional imaging and neurophysiology suggests that these stages have a distinct neurological basis (Lau et al 2004; Waszak et al 2004). Deficits in volition, or translating intentions into actions, may be relevant to motor conversion symptoms. Hierarchical processing refers to perception occurring via a stepwise process in which representations of sensory information are initially simple and gradually become more abstract, holistic, and multimodal (Grill-Spector and Malach 2004).Thus we may refer to “early” processing (eg, the primary visual cortex) and “higher” or “downstream” processing (eg, response to faces in the fusiform gyrus). Several functionally specialized parallel “streams” of hierarchical processing may occur (eg, the dorsal and ventral streams in the visual system for spatial localization and object recognition respectively). It is not known at what stage in this process “awareness” or “consciousness” arises. Terms such as “inhibition”,“excitation”, and “modulation” are often used rather imprecisely in the literature to refer to the effect that one brain region or pathway may have on another. In fact, cortical and subcortical regions have rich, reciprocal connections to other structures and function within complex networks and circuits that are not yet fully understood. However, simplified models of the important ways in which specific regions may interact are useful to help us form testable hypotheses about mechanisms of conversion symptoms. The P300 potential is a late positive deflection in an event-related response, which occurs when subjects detect a “target” stimulus, but not when they ignore it or fail to detect it. It is therefore thought to reflect conscious processing of the stimulus (Picton 1992). Blindsight occurs uncommonly in patients with damage to the primary visual cortex. Despite having no conscious awareness of visual perception, these patients can perform simple visual discrimination tasks in forced choice (Weiskrantz 1996).This phenomenon is thought to reflect implicit visual processing in extrastriatal pathways. Neglect occurs in patients with nondominant inferior parietal lobe damage. It is characterized by reduced awareness of stimuli in the hemispace contralateral to the lesion.This reduced awareness is more marked when stimuli are present in the unaffected hemispace. The neglect can sometimes be improved by drawing patients’ attention to the stimulus (Driver and Mattingley 1998). Neuropsychiatric Disease and Treatment 2006:2(1)