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Frederick Kingdom, Roger Watt; An after-effect of perceived length. Journal of Vision 2008;8(6):937. doi: 10.1167/8.6.937.
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Aim. Both single-unit recording studies in V1 and psychophysical studies have revealed orientation-selective mechanisms whose length selectivity is partially separable from their width or spatial frequency selectivity. This suggests that length might be a spatial dimension coded early in vision. If so, one would expect line length to be adaptable, with adaptation producing shifts in the perceived length of subsequently presented lines. Method. Subjects adapted to horizontal d.c.-balanced lines of various lengths, presented either singly or in grids. After adaptation, the perceived length of horizontal test lines that were either shorter or longer than the adaptors was measured using a conventional staircase procedure. To test whether any after-effect of perceived length was a manifestation of the well-known size or spatial-frequency after-effect, we also used vertically-oriented square-wave grating adaptors whose bars were equal in width to the length of the adaptor lines. Results. Adaptation to line length made shorter lines appear shorter and longer lines appear longer, analogous to the repulsion effects found with other spatial dimensions such as orientation and spatial frequency. With square-wave grating adaptors however, the after-effect was much smaller. Conclusion. Line length is an adaptable dimension and the resulting line-length after-effect is not simply a manifestation of the size or spatial-frequency after-effect. Line length appears to be a spatial dimension that is likely coded through the population response of neurons tuned to similar widths but different lengths.
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