Configuration specificity in bisection acuity

Crucial for the perception of form are the spatial relationships between th e elements of a visual stimulus. To investigate the mechanisms involved in coding the distance between visual stimuli, thresholds for detecting whethe r a central marker accurately bisects a spatial interval were compared for a variety of configurations. Thresholds are best when all three members of the bisection configuration are identical. Performance is impaired, often b y as much as a factor of two, when the outer delimiters of the spatial inte rval differ from the central marker in either length, orientation or contra st polarity. Illusory contours act poorly as borders for bisection by a cen tral line. Disparity thresholds are not affected by orientation differences between test and flanking lines. Because in peripheral vision bisection ac uity improves with practice, transfer of training between configurations ca n be used to gauge overlap of neural processing mechanisms. Transfer is com plete only between patterns where all markers are similar, reduced when the outer markers differ by 20 degrees in orientation and absent when they are orthogonal. The dependence of bisection discrimination on similarity betwe en the elements of the stimulus demonstrates that the encoding of spatial l ocation and spatial extent are coupled to the coding of other stimulus prop erties (C) 2001 Elsevier Science Ltd. All rights reserved.