Efficient search for size targets on a background texture gradient: Is detection guided by discontinuities in the retinal-size gradient of items?

Six visual search experiments were carried out to investigate the processin g of size information in early vision. The apparent size of display items w as manipulated independently of their retinal size by placing items on a te xtured surface which altered the perceived distance in depth of the items. Overall, these experiments demonstrate that a target item differing from no n-target items in terms of apparent size can be detected efficiently. Howev er, the pattern of results indicates that, rather than deriving apparent-si ze information, target detection is guided by discontinuities in the 'retin al-size gradient' of items, in particular between items at the same 'depth' Although the arrangement of items on the texture surface strongly influenc ed search, this was largely due to the retinal size of items and the retina l separation between items. The implications of these experiments for the n ature of the pre-attentive representation of size are discussed.