Social Working Memory: Memory for Another Rat's Spatial Choices Can Increase or Decrease Choice Tendencies

Article excerpt

In two experiments using a radial-arm maze, pairs of rats made choices among eight maze locations, each containing a large quantity of one of two food types. The choices made by 1 rat affected the choices made by the other rat. Under most conditions, visits by 1 rat increased the tendency of the other rat to subsequently choose that maze location. However, the effect depended on the quality of the food available in a particular location. When it was possible for the rats to observe each other on the maze arms and a rat had experienced that a location contained the less preferred food type, a previous visit to that location by the foraging partner decreased the tendency to visit that location. These effects are attributed to working memory for the spatial choices of another rat, and they indicate that memory produced by a rat's own visit to a maze location is integrated with memory for the behavior of another rat to determine spatial choice.

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Since the report of Olton and Samuelson (1976), numerous studies using the radial-arm maze have confirmed the strong tendency of rats to avoid revisits to spatial locations (see Foreman & Ermakova, 1998, for a review). The phenomenon is reminiscent of earlier work on spontaneous alternation (Dember & Richman, 1989), but research involving the radial maze has focused on the use of a working- memory system that stores dynamic information about spatial locations, such as whether particular locations have recently been visited (Olton, 1978). The spatial locations themselves may be represented in memory as discrete items or in some kind of integrated spatial representation. There is long-standing debate about the properties and structure of spatial representations (e.g., Brown, 1992; Brown & Cook, 2006; Brown, Rish, VonCulin, & Edberg, 1993; O'Keefe & Nadel, 1978; Poucet, 1993; Tolman, 1948). Regardless of how spatial representations are structured, an important aspect of the memory used in the radial-arm maze and related tasks is the need to maintain information about the status of multiple locations as their status changes. For example, as a rat visits locations, the content of the memory used to discriminate locations not yet visited from locations yet to be visited must change in correspondence with those visits (Cook, Brown, & Riley, 1985). This dynamic quality is a core property of working memory.

Brown, Farley, and Lorek (2007) recently reported that rats also use working memory to avoid visits to locations that had been visited by another rat. The tendency to avoid locations that had already been visited by a foraging partner was not as robust as the well-known tendency of rats to avoid revisits to locations they have already visited, but it was found in several different versions of the radial-maze task. Brown et al. (2007) ruled out odor trails or other physical traces of visits made by the other rat as an explanation for this tendency and concluded that its mechanism is working memory for spatial locations chosen by the other rat.

The present experiments were designed so that we may further examine the conditions under which the spatial choices of 1 rat (the focal rat) affect the subsequent spatial choices of another rat (the nonfocal rat). Both experiments involved an eight-arm radial maze, in which 2 rats (cage mates) made choices simultaneously. It was a standard radial-arm maze, except that the maze arms were constructed of tubes, which allow 2 rats to pass each other on maze arms (Brown et al., 2007). Pairs of rats were placed in the central arena at the beginning of each trial and allowed to make choices. We examined whether the tendency to choose locations by the focal rat was affected by previous visits to that same arm by the nonfocal rat.

A focus of the present experiments was the nature of the reinforcement available at the ends of maze arms. Each location was baited with one of two types of food: grain pellets or sucrose pellets. …