Recruitment of an invisible depth cue

Cue recruitment occurs when a sensory signal is put into correlation with trusted cues and subsequently influences perceptual interpretation as the trusted cues do. In all cue recruitment experiments to date, the signal has been well above detection threshold and was easily visible. For example, it has been shown that object position and motion can be recruited as a cue to influence the interpretation of the ambiguous Necker Cube (Haijiang et al., 2006). Here we asked whether a signal that is not visible on its own could be recruited as a cue. Vertical size ratio (VSR, the ratio of vertical angles subtended by an object at the two eyes) is normally used to correct for relative position of the head when interpreting horizontal disparity (Gillam Lawergren, 1983; Backus et al 1999) but it is not visible in displays consisting of horizontal lines only because there are no horizontal discontinuities. We manipulated VSR of displays simulating a cylinder composed of horizontal lines that rotated about a horizontal axis. On training trials, the rotation direction of the cylinder was unambiguously specified by horizontal disparity and occlusion cues and these trusted cues were correlated with the VSR cue to be recruited. On test trials, the display did not contain horizontal disparity or occlusion, so that the rotation direction specified by the trusted cues was ambiguous. If participants however made use of the VSR cue in the test display rotation direction could become unambiguous after training. For 8 out of 9 participants, apparent rotation on test trials became contingent on the value of VSR. We conclude that a signal need not have perceptual consequences by itself for the system to assign it a new use during the construction of appearances