What spatial reference frame is used for priming in visual search?

Abstract

The perception-action model proposes that vision for perception and vision for action are subserved by two separate cortical systems, the ventral and dorsal streams respectively (Milner & Goodale, 1995, 2006). It is argued that the dorsal stream codes spatial information egocentrically, that is, relative to the observer, and that these representations are highly transient. The purpose of this thesis was to investigate whether short-term spatial memory can use egocentric information. This was achieved by using spatial priming in visual search. Spatial priming manifests itself in speeded detection times for a target when that target appears in the same location that it previously appeared in (Shore & Klein, 2001). Target locations can be defined in either egocentric (i.e. relative to the body or a specific part of the body) or allocentric (i.e. relative to other items in the display) frames of reference. However, it is unclear which of these are used in spatial priming. It was found that both allocentric and egocentric cues were used, and that egocentric cues were the most effective for short-term priming (Study A). In exploring the nature of the egocentric frame of reference further, it was found that target location was not coded using an eye-centred frame of reference but rather it was coded relative to the observer’s body (Study B). Moreover, when participants moved to a new location and the relationship with the target was maintained, priming effects were still observed (Study C). The availability of egocentric information was tested further, and the priming effects were still observed after a minimum delay of eight seconds between the first and second presentations of a target location (Study D). Finally, it was found that the application of transcranial magnetic stimulation to the posterior parietal cortex did not modulate either allocentric or egocentric spatial priming effects (Study E). While the perception-action model predicts that egocentric information is short lived, the findings from the experiments presented in this thesis suggest that egocentric information can persist for several seconds.