Abstracting Spatial Relations Among Goal Locations


Michael Brown
Villanova University

      

Varieties of Spatial Learning

Of all the constraints on nature, the most far reaching are imposed by space.  For space itself
has a structure that influences the shape of every existing thing.
(Stevens, 1974)

Tolman's (1948) original description of the construct of a cognitive map and his evidence for it are briefly described in Part IIA.  His ideas regained influence with the publication of O'keefe and Nadel's (1978) The Hippocampus as a Cognitive Map.  O'keefe and Nadel argued that the hippocampus is the site of spatial representations and that information stored by the hippocampus includes the spatial relations among locations in familiar environments.   Publication of this work marked the beginning of a wave of experimental and theoretical work aimed at understanding the physiological and cognitive mechanisms of spatial learning and memory. 

Several mechanisms have since been identified that allow animals to navigate accurately in two-dimensional and three-dimensional space.  A relatively simple mechanism is often referred to as beacon homing (Gallistel, 1990).  A beacon is a perceived landmark (visual, auditory, or chemical cue).  Beacon homing is simply moving toward a beacon, as illustrated below.  Of course, moving toward a beacon is not a trivial problem - there must be some means of determining when one is moving toward the beacon.  An increase in the apparent size of the perceived beacon, for example, has been identified as a mechanism by which some animals used visual stimuli as beacons.   Detecting an increase in the concentration of a chemical is a cue that can be used to use beacon homing to detect the source of a chemical, and is used by many animals to find food.

Figure B-1:
Beacon homing.  Animal moves toward (or away from) a perceived, localized cue.

 

Beacon Homing allows animals to locate a place, as long as there is a perceivable cue coincident with the goal location.  When cues are present, but not coincident with the goal location, many animals are able to locate the goal using landmarks that have a consistent spatial relation to the goal location and to each other (Gallistel, 1990).  In order to do so, the animal must have learned the spatial relations among the landmarks and goal location(s).  Such learned spatial relations correspond closely to the cognitive mapping process that Tolman described.

 

Figure B-2:
Piloting.  Animal moves toward (or away from) a location defined by its spatial relations to perceived, localized cue(s).

Cheng (1986) first proposed that a global spatial frame, within which the goal location(s) is consistently located, can serve as learned spatial cues.  The evidence for this view is described by Cheng and Newcombe elsewhere in this book.  The evidence indicates that animals acquire a representation of the spatial properties of the global space in which they search for food (e.g., a rectangular arena).  The representation includes the location of the goal(s) within that spatial framework.  The location of a food site or other important place is code in terms of its spatial relation position within the represented spatial frame. Cheng argued that this process constitutes a geometric module which functions somewhat independently of other forms of spatial control.  In the illustration below, an animal is navigating with a rectangular area (represented by the orange rectangle surrounding the shark).  A cognitive representation of the geometric properties of the area is  represented by the orange rectangle inside the animal.

 

Figure B-3:
Control by geometry.  Animal moves with reference to the perceived shape of a space (or a perceived cue that is determined by the shape of the space (see Cheng, this volume).

Beacon homing, landmark use, and control by geometry all rely on the presence of perceived visual (or auditory or chemical) cues.  In contrast, the spatial pattern learning that is the subject of the present chapter occurs in the absence of perceivable cues.  Rather, the context of the spatial pattern learning that we have studied is the configuration of hidden food locations.  No beacons or landmarks correspond to the correct food locations.   In our experiments, rats are foraging in a rectangular arena, similar to the ones used by Cheng (1986).  However, the food locations on any particular trial are not predictable in terms of location within the arena.  Thus, beacons, landmarks and geometric cues can not be involved in the ability of rats to find the baited poles in the pole box task.

Another mechanism known to be involved in animal navigation is dead reckoning (also known as path integration).   Internal movement cues (provided primarily by the vestibular system) allow the animal to integrate its position in space relative to a starting point (Biegler, 2000; Collett, Collett, Bisch, & Wehner, 1998; Etienne, Berlie, Georgakopoulos, & Maurer, 1998;  Mittelstaedt & Mittelstaedt, 1980; Wehner & Srinivasan, 1981).   Thus, when an animal moves a given distance and direction (indicated by the arrow behind the animal below), the vestibular system provides information about the distance and direction moved (relative to a starting location).  There is abundant evidence that this information allows the animal to find its way back to the starting location.  It has been argued (e.g., Gallistel, 1991) that this ability is mediated by a representation that has the form of a vector coding the distance and direction to the starting location (indicated by the arrow in the animal's head below).  This vector is continuously updated as the animal moves.

 

Figure B-4:
Dead Reckoning.  Animal moves according to internal cues that are determined by the direction and distance of its recent movement (e.g., vestibular cues).

In the pole box task, the baited locations have consistent spatial relations with each other.   Thus, it is possible that dead reckoning could be involved in the mechanism by which those relations are learned.  

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