Dimensional interaction in distance judgment.

Participants decided under speed stress whether or not the horizontal distances between pairs of frontal locations exceeded a criterion distance. The error rate reflected parallel effects of the horizontal and vertical distance between the locations. Whereas dimensional interaction in perceptual judgment has previously been attributed either to the perception of the stimulus or to the response decision concerning the stimulus, here dimensional interaction was attributed to the process of distance assessment regarding the test locations. Under the proposed account, the horizontal distance between the locations could not be assessed independently of the vertical distance. Only the overall distance between the locations could be assessed. However, because the horizontal and vertical positions of the locations could be independently assessed, the horizontal distance between the locations was available to the extent that the vertical positions of the locations were weighted so as to minimize vertical distance prior to the assessment of overall distance. In support of this account, parallel effects of horizontal and vertical distance were not observed when participants decided whether or not pairs of locations had the same horizontal position.