A new way of assessing the strength of a figure-ground cue

Methods to assess the strength of figure-ground cues generally fall into two categories: (1) Subjective reports about which part of a display is seen as figure. (2) Reaction time measures to stimuli that probe a part of a display. Both methods have drawbacks. Subjective reporting manipulates the process under investigation. Studies with probes can superimpose a new figure on the figure-ground display, possibly changing the representation, or fail to control what subjects do during the presentation of figure-ground displays (when probes follow the display). In our approach we try to increase control over the behaviour of subjects using a visual short term matching task (Driver and Baylis, 1996), while refraining from explicit figure-ground instructions. We biased figure-ground assignment by presenting the first half of an ambiguous figure-ground display before adding the second half. The first half will initially become figure, since it is presented on a large background. If the first half contains a strong figure cue, it should be resistent against the distraction caused by the onset of the second half, 100 msec later. If, however, the second half contains a strong figure cue it self, this may cancel the initial figure assignment to the first half. Figure assignment was probed afterwards and subjects decided which of two mirror images was part of the figure-ground display. In our experiment, we used the lower-region cue (Vecera, Vogel and Woodman, 2002). We report an interaction between region presented first, and whether this region is subsequently probed. For upper regions presented first and probed, reaction times are slower than for lower regions presented first and probed. This illustrates the viability of our approach: a strong figure cue will overcome the disadvantage of being presented second, whilst also holding on to the advantage of being presented first