Early visual detection of oriented line targets: spatial characteristics, and reference frames of early visual orientation-processing

  • Laura M. Doherty

Student thesis: Doctoral ThesisDoctor of Philosophy


The orientations of lines and edges are important in defining the structure of the visual environment, and observers can detect differences in line orientation within
the first few hundred milliseconds of scene viewing. The present work is a
psychophysical investigation of the mechanisms of early visual orientation-processing.
In experiments with briefly presented displays of line elements, observers
indicated whether all the elements were uniformly oriented or whether a uniquely
oriented target was present among uniformly oriented nontargets. The minimum
difference between nontarget and target orientations that was required for effective target-detection (the orientation increment threshold) varied little with the number of elements and their spatial density, but the percentage of correct responses in detection of a large orientation-difference increased with increasing element density. The differing variations with element density of thresholds and percent-correct scores may indicate the operation of more than one mechanism in early visual orientation-processIng.
Reducing element length caused threshold to increase with increasing number
of elements, showing that the effectiveness of rapid, spatially parallel orientation-processing depends on element length.
Orientational anisotropy in line-target detection has been reported previously: a coarse periodic variation and some finer variations in orientation increment
threshold with nontarget orientation have been found. In the present work, the
prominence of the coarse variation in relation to finer variations decreased with
increasing effective viewing duration, as if the operation of coarse orientation-processing mechanisms precedes the operation of finer ones.
Orientational anisotropy was prominent even when observers lay horizontally and viewed displays by looking upwards through a black cylinder that excluded all
possible visual references for orientation. So, gravitational and visual cues are not
essential to the definition of an orientational reference frame for early vision, and such a reference can be well defined by retinocentric neural coding, awareness of body-axis orientation, or both.
Date of AwardSept 1997
Original languageEnglish
SupervisorDavid H. Foster (Supervisor)


  • vision
  • psychophysics
  • orientation
  • preattentive search

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