Voluntary attention increases the phenomenal length of briefly flashed lines  [presentation, ppt, 487 kB]

Masin S. C.
Università di Padova, Padova, Italy

In 1956, Fraisse, Ehrlich, and Vurpillot found that subjects judged that lines were longer when voluntary attention was focused on the lines than when attention was distracted from the lines. There have been many attempts to repeat these results, but so far none has been able to ascertain whether the effect of attention on reported line length was a phenomenal effect. In the present study, 46 subjects were shown stimuli consisting of pairs of horizontal or vertical briefly flashed lines with a fixation cross placed equidistant between the lines, far from each of the lines. A change in colour of one arm of the cross was used as a cue to focus the subject's voluntary attention on one line. The results showed that attention increased the judged length of the attended lines. Since this effect of attention also occurred when the subjects were absolutely certain that they saw that the stimulus lines differed in length, this effect indicates that attention increased the phenomenal length of the attended lines. This lengthening was quite small: it involved a maximum mean increase of about 0.15 in the probability of responding that the attended line was longer. This effect occurred in the horizontal dimension and was almost absent in the vertical dimension. In agreement with data indicating that flashed lines expand phenomenally by activating motion detectors and that focused attention makes neural motion responses increase in amplitude, the present results suggest that focused attention makes attended lines look longer because it makes these lines expand phenomenally more rapidly.