Cite as:
Cham J, Hayes A, 2005, "The 'encirclement effect' in an orientation search task" Perception 34 ECVP Abstract Supplement

The 'encirclement effect' in an orientation search task

J Cham, A Hayes

Visual search for a tilted line segment amongst a background of vertical line segments is very efficient, but the efficiency can be severely diminished when all line segments are encircled. We investigated how features of the circles, such as polarity and structure, affect the efficiency of detecting an oriented line segment. The stimuli were black (9.71 cd m^-2) vertical line segments (0.33 deg) on a gray background (29.2 cd m^-2), with one line segment tilted 30° from vertical, encircled or not depending on the experimental condition. The numbers of line segments in a display were 4, 8, 12, or 16. Observers were required to indicate as quickly as possible whether a tilted line segment was present in the display. We found that for line segments without encirclement the search was 'efficient', since reaction time remained largely unchanged as a function of set size. However, search became 'inefficient' when black circles surrounded all line segments, such that an additional 8 ms was needed for each extra line segment added to the stimulus. We tested the effect of polarity by placing white circles around black line segments. The efficiency of visual search did not change, and performance was almost the same as when black circles were used. However, adding white patches around black line segments, and replacing circles with broken circles, did affect search efficiency. In the latter condition, search efficiency was maximally affected when the broken circle was rotated to 30°. The results, in combination, suggest that masking may be the cause of the encirclement effect.

[Supported by Grant HKU7146/02H from the Research Grants Council of Hong Kong.]

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