Behavioural Processes 72 (2006) 23–31 Twenty-three generations of mice bidirectionally selected for open-field thigmotaxis: Selection response and repeated exposure to the open field Pia K. Lepp¨ anen a,∗ , N. Ravaja b , S.B.M. Ewalds-Kvist a a Department of Psychology, University of Turku, Assistentinkatu 7, 20500 Turku, Finland b M.I.N.D. Lab, Center for Knowledge and Innovation Research, Helsinki School of Economics, Finland Received 1 October 2004; received in revised form 12 October 2005; accepted 26 November 2005 Abstract We examined: (a) the response to bidirectional selection for open-field (OF) thigmotaxis in mice for 23 generations and (b) the effects of repeated exposure (during 5 days) on different OF behaviors in the selectively bred high OF thigmotaxis (HOFT) and low OF thigmotaxis (LOFT) mice. A total of 2049 mice were used in the study. Prior to the testing in the selection experiment, the mice were exposed to the OF apparatus for approximately 2 min on each of 4 consecutive days. Thus, the selection was based on the scores registered on the 5th day after the four habituation periods. The HOFT mice were more thigmotactic than the LOFT mice in almost each generation. The HOFT mice also tended to rear less than the LOFT mice, which was explained by the inverse relationship between emotionality and exploratory tendencies. The lines did not generally differ in ambulation. Sex differences were found in thigmotaxis, ambulation, and rearing. In the repeated exposure experiment, the development of nine different OF behaviors across the 5 days of testing was addressed. Both lines ambulated, explored, and reared most on the 1st, 4th, and 5th days. Grooming and radial latency decreased and thigmotaxis increased linearly across the testing days. Line differences were found in ambulation, exploration, grooming, and rearing, while sex differences were manifested in ambulation and exploration. The line difference in thigmotaxis was evident only on the 5th day. Temporal changes were partially at variance with the general assumptions. OF thigmotaxis was found to be a powerful characteristic for producing two diverging lines of mice. © 2005 Elsevier B.V. All rights reserved. Keywords: Emotionality; HOFT and LOFT mice; Open field; Selective breeding; Thigmotaxis 1. Introduction The open field (OF) is a traditional test in animal psychol- ogy for studying rodent behavior. The method was originally developed by Hall for measuring the natural or spontaneous direction of rat behavior, with an emphasis on emotionality (Hall, 1934a,b; Hall and Ballachey, 1932). In general, the OF test comprises an enclosed open area in which the animal is placed and subjected to various behavioral recordings (Choleris et al., 2001; Ossenkopp et al., 1994). Rodents, such as mice and rats, have a natural aversion to novelty (Boissy, 1995) as well as to lit and open places (Calatayud and Belzung, 2001; Russell, 1979). On the other hand, novel environments also elicit exploratory behavior (Crawley, 1985; Crusio, 2001; see also Blois-Heulin and Belzung, 1995). Consequently, the OF ∗ Corresponding author. Present address: Sauvontie 3 A 1, FI-00280 Helsinki, Finland. Tel.: +358 505015454. E-mail address: pkleppanen@hotmail.com (P.K. Lepp¨ anen). test is based upon rodents’ contradictory tendencies to avoid aversive stimuli and to explore new areas (Cl´ ement et al., 2002; Mathis et al., 1994). To put it in another way, the behavior of a rodent in the OF test is motivated by competition between exploratory tendencies and fear (Crusio, 2001; Morgan and Pfaff, 2001; see also Ramos and Morm` ede, 1998), which can also be seen as a form of approach/avoidance conflict (Lister, 1990). When an animal is placed into an OF, it tends at first to ambu- late in the peripheral area near the wall and to avoid the center of the field (Archer, 1973; Ramos and Morm` ede, 1998). This wall- seeking behavior called thigmotaxis refers to the propensity of a rodent to stay in close contact with the OF wall, owing to the underlying tendency to avoid open, unknown, and potentially dangerous places (see Choleris et al., 2001). Thigmotaxis is con- sidered to belong to the category of phylogenetically prepared fear reactions for the purpose of avoiding predators (Grossen and Kelley, 1972; Treit and Fundytus, 1989). It is assumed that the more thigmotactic, the more emotional (or anxious or fearful) an animal is (Valle, 1970). In other words, a highly 0376-6357/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.beproc.2005.11.010