A computational exploration on the role of semantic memory in episodic future thinking

A computational exploration on the role of semantic memory in episodic future thinking Yuichi Ito (ito.yuichi@nagoya-u.jp), Taiji Ueno (taijiueno7@gmail.com), Shinji Kitagami (kitagami@cc.nagoya-u.ac.jp), and Jun Kawaguchi (kawaguchijun@nagoya-u.jp) Department of Psychology, Graduate School of Environmental Studies, Nagoya University, Furo-cho, Chikusa-ku, Nagoya City, Aichi 4648601, JAPAN Abstract Episodic future thinking refers to a human cognitive process which generates successive predictions of events that are likely to occur in a cue-specific context in the future. An emerging view is that semantic memory as well as episodic memory contributes to this process, but the exact mechanism remains unclear. We built a computational model that learned to predict the next event upon a presented event (sequence prediction model). After learning the statistical structure in the training sequence, the model was tested for generating successive self-predictions of events triggered by a cue. The generated sequence of events captured some phenomenological features of patients with semantic dementia when the semantic system of the model was damaged. The role of semantics in episodic future thinking and the usefulness of a sequence prediction model are discussed. Keywords: episodic future thinking; semantics; parallel- distributed processing model; sequence learning Introduction We can project ourselves into the future despite the fact that we have never experienced it. The term episodic future thinking refers to a human ability to envision a plausible future event in a specific time and place (i.e., a specific context) (Atance & O’Neill, 2001; Schacter, Addis & Buckner, 2008). Over the last decade, researchers from various fields, including psychology, neuropsychology, and neuroimaging, have investigated episodic future thinking, focusing mainly on the contribution of episodic memory to constructing episodic future thought. More recently, data from patients with semantic dementia have suggested that semantic memory may also play a role (Irish, Addis, Hodges, & Piguet, 2012). The current study used a computational model to investigate the mechanism by which semantic memory supports episodic future thinking. Role of Episodic Memory The role of episodic memory has been suggested in various studies. For example, some neuroimaging studies have revealed a common neural network involved in the remembering of past, and in imagining future events (Szpunar, Watson, & McDermott, 2007). These data are consistent with neuropsychological studies with amnesic patients (e.g., hippocampal amnesia or Alzheimer’s disease) who showed simultaneous impairments in both remembering past episodes and imagining future events (e.g., Irish et al., 2012). Based on these findings, the constructive episodic simulation hypothesis was proposed, which assumes that imagining future events requires a system that can retrieve detailed information stored in episodic memory and flexibly recombine them into coherent representations of future events (Schacter, Addis, & Buckner, 2008). Further support for this idea comes from experimental psychology. For example, both retrieving an episode and imagining a future event are affected by a temporal distance factor in the same manner. Specifically, Addis, Wong, & Schacter (2008) collected both the past events that participants recalled and the future events they generated, and classified detailed information in these outputs as either internal or external. Internal details are “episodic” information, meaning specific in time and place and related to the central events (i.e., the main event described by the participant). In contrast, external details are not specific in time and place. It was found that, in both recalling of past episodes and thoughts about future episodes, internal details lessened as participants were required to produce farther events from the present in both directions. This means that as episodic future thinking goes farther in terms of temporal distance from the present, the time and place (context) of the generated events deviates from those of the central events (central topic). Role of Semantic Memory More recently, the role of semantic memory in episodic future thinking has also captured attention (Irish et al., 2012). D’Argembeau and Mathy (2011) suggest that construction of episodic future thought typically involves progressive conversion from general to more specific information such that access to general knowledge (semantics) precedes retrieval of time-specific episodic information. In other words, semantic memory provides a “framework” for construction of episodic future event representations, and then episodic information from the past is integrated to form a coherent and elaborated sequence of future events. A key support for this idea came from a study with neurological patients with semantic dementia, characterized by the progressive and insidious loss of conceptual knowledge about objects, facts and the meaning of words, yet preserved non-verbal episodic memory (Irish et al., 2012). Specifically, Irish et al. (2012) found that although their patients were as good at remembering past episodes as controls, their episodic future thoughts lacked internal details. In other words, the sequence of events they generated did not maintain the time and place information (context) that was