Orientation Specificity in Long-Term-Memory for Environmental Spaces

Orientation Specificity in Long-Term-Memory for Environmental Spaces Tobias Meilinger (tobias.meilinger@tuebingen.mpg.de) Bernhard E. Riecke (bernhard.riecke@tuebingen.mpg.de) Heinrich H. Bulthoff (heinrich.buelthoff@tuebingen.mpg.de) Max-Planck-Institute for Biological Cybernetics Spemannstr. 38, 72076 Tubingen, Germany each theory would independently predict different outcomes. A detailed description of each of these three theoretical positions will now be discussed. Abstract This study examined orientation specificity in long-term human memory for environmental spaces. Twenty participants learned an immersive virtual environment by walking a multi-segment route in one direction. The environment consisted of seven corridors within which target objects were located. In the testing phase, participants were teleported to different locations in the environment and were asked to identify their location and heading and then point towards previously learned targets. As predicted by view- dependent theory, participants pointed more accurately when oriented in the direction in which they originally learned each corridor. No support was found for a global reference direction underlying the memory of the whole layout or for an exclusive orientation-independent memory. We propose a “network of reference frames” theory to integrate elements of the different theoretical positions. Spatial memory is orientation independent. An orientation independent representation has mainly been argued for by Sholl and colleagues (e.g., Easton & Sholl, 1995; Holmes & Sholl, 2005; Sholl, 2001). They propose an allocentric organization of environmental knowledge. Essentially this means that object-to-object relations are stored in memory, as opposed to self-to-object relations. The defining characteristic of this theory is it assumes that memory content can be accessed equally well, independently of one’s current position within the environment and/or facing direction. According to this theory, performance measures should not differ systematically when participants are asked to imagine a previously-learned environment from different perspectives. As such, this theoretic position is thus referred to as orientation independent. According to this approach, additional egocentric reference systems are assumed to exist in which space is not represented in object-to-object relations, but in self-to-object relations. Orientation independence is thought to only occur in well learned environments. Keywords: Reference frame; environmental space; spatial memory; allocentric; egocentric; reference direction; view- dependent; self-localization; pointing; virtual environment; head-mounted display; navigation; spatial orientation Introduction Spatial memory is crucial for our lives as mobile organisms. Without having the capacity to orient oneself in space (which is largely reliant on spatial memory) we would have to search for our bathroom every morning and use aids to find the supermarket. Even when simply having to walk around a corner we would get lost, as is observed, for example in many patients suffering from Morbus Alzheimers. Of specific interest is how locations and spatial layouts are stored in memory. Spatial memory is orientation dependent with respect to a reference direction. Reference direction theory also assumes an allocentric (i.e. object-to-object) memory for space. The objects however, are encoded with respect to one or two reference directions like “north” or the main axis of a room (e.g., Mou, McNamara, Valiquette & Rump, 2004; Rump & McNamara, in press; McNamara & Valiquette 2004). The axes of coordinate systems which define spatial locations might also be interpreted as reference directions (e.g., O’Keefe, 1991). According to this theory, retrieving information from memory should be easiest when aligned with one of the reference directions. For example, imagining a certain position and orientation within a previously- learned scene should be easiest when the to-be-imagined orientation is aligned with one of the reference directions. This facilitating effect is expected to be reflected in improved performance measures such as faster response times and/or decreased errors. The resultant representation is consequently said to be orientation-dependent with respect to one or more reference directions. Such a reference direction is proposed to originate either from the initial exposure to an environment (e.g., the first view of a room), Theories about the organization of spatial memory There is an abundance of different and partially conflicting theories about the nature of spatial memory in humans and other animals. (e.g., Burgess, 2006; Mallot & Gillner, 2000; O’Keefe, 1991; Sholl, 2001; McNamara & Valiquette, 2004; Wang & Spelke, 2002). These different theories can roughly be categorized with respect to their assumptions regarding how people represent spatial information in long- term memory. More specifically, these theories assume that we store spatial information either: (1) in an orientation independent manner, (2) in an orientation dependent manner with respect to one or more reference directions, or (3) in an orientation dependent manner with respect to different experienced orientations. Our goal for the current study was to distinguish between these three theories by designing an experiment in which