Journal of Neuroscience Methods 167 (2008) 184–190 The fuzzy-boundary arena—A method for constraining an animal’s range in spatial experiments without using walls R.M.A. Hayman a,∗ , J.G. Donnett b,1 , K.J. Jeffery a,2 a Institute of Behavioural Neuroscience, Department of Psychology, University College London, 26 Bedford Way, London WC1H OAP, UK b Axona Ltd., Unit 4U, St. Albans Enterprise Centre, Long Spring, Porters Wood, St. Albans AL3 6EN, UK Received 30 April 2007; received in revised form 9 August 2007; accepted 13 August 2007 Abstract A method is described for confining an animal within an experimenter-defined area without the use of physical boundaries. The area of exploration is constrained by the presence of an aversive noise, triggered whenever the animal steps across a computer-controlled boundary. The radius of the invisible boundary is constantly reset so that the boundary becomes “fuzzy” and the animal cannot use it as a spatial localizing cue. The effectiveness of this technique is demonstrated both with behavioural data confirming reliable confinement, and also recordings of hippocampal place cells made from rats exploring the arena. The place cell data reveal that indeed, the cells did not appear to be controlled by the fuzzy boundary, in contrast with the strong control normally exerted by fixed boundaries. This technique is thus promising for studies of spatial behaviour in which the strong influence of walls needs to be removed in order to allow the study of more subtle processes such as landmark use and path integration. © 2007 Elsevier B.V. All rights reserved. Keywords: Spatial cognition; Exploration; Foraging; Spatial reinforcement; Virtual cues; Place cells; Hippocampus; Rat 1. Introduction The open field maze is one of the most widely used environ- ments for investigating spatial behaviour in both rats and mice (Hall, 1936a,b; see Choleris et al., 2001; Walsh and Cummins, 1976 for reviews). Although the details can change, the maze is typically a bounded arena, often lacking any intra-maze land- marks or objects. Physical and practical limitations constrain the available area the animal can explore; there are only a handful of examples where this is larger than 1 m 2 (Blanchard et al., 2001; Hafting et al., 2005; Zorner et al., 2003). Exploration is usually constrained by either raising the maze above floor level or by bounding it with walls. Because physical boundaries such as walls and edges provide a strong spatial cue (and indeed, the geometry of an environment can be used by mammals to orient their exploratory behaviour (Cheng, 2005; Cheng and Newcombe, 2005), their presence lim- its the range of questions that can be asked about the influence on ∗ Corresponding author. Tel.: +44 20 7679 5377. E-mail addresses: r.hayman@ucl.ac.uk (R.M.A. Hayman), donnett@axona.com (J.G. Donnett), k.jeffery@ucl.ac.uk (K.J. Jeffery). 1 Tel.: +44 1727 800 078. 2 Tel.: +44 20 7679 5308. behaviour of other cues such as extended surfaces, landmarks, surface topography and path integration. This limitation is prob- lematic given the current interest of spatial cognition researchers in the relative influence of boundaries vs. landmarks (Cheng and Gallistel, 2005; Graham et al., 2006) and path integration (Etienne and Jeffery, 2004). This report describes a method that allows an animal to be confined to a restricted area of the lab- oratory without the need for physical boundaries, allowing an uncontaminated investigation of the effects of other kinds of spatial cue. The use of sound as a negative reinforcer in a place preference task has been reported previously to guide navigation to a focal location (Kentros et al., 2004). The success of this procedure encouraged us to consider the possibility that a similar technique might be used to keep an animal out of a given area and keep it confined to a region in the centre of a room. More importantly, we wanted to do this using a technique that would prevent the noise barrier from becoming, itself, a spatial cue, so that influences on spatial cognition could be assessed independently of the strong contribution that walls are known to make. Here, we report the development of an experimental paradigm that allows computer-controlled constraint of the possible regions an animal can explore. The area of exploration is lim- ited by the onset of an aversive noise triggered whenever the 0165-0270/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jneumeth.2007.08.014