Subjective Contours along Truncated Letters

The truncation of upper-case words can result in a subjective contour along the truncated ends of the letters. We explored this effect in two experiments designed to tease apart the processes responsible: in particular, the possible role of letter recognition. Such a role would indicate an unprecedented involvement of “high-level vision” in the genesis of subjective contours. In experiment 1, we confirmed the basic effect of word truncation in eliciting a subjective contour, using only letters where truncation does not eliminate any critical features. In experiment 2, we showed that the effect is not confined to words/letters but is equally strong for controlled non-letter stimuli that, like words, have many forms that have an inflection or an intersection near the centre. Truncation at one end then introduces proportional imbalance between upper and lower sections of the figures. We conclude from the two experiments that part of the effect is due to vertical shortening per se and the rest to the proportional imbalance introduced by the truncation. The effect of proportional imbalance, a novel determinant of subjective contours, may result from experience with letters, although the effect is not “high level” in requiring the recognition of specific letters.