Spatial updating and the maintenance of visual constancy
We tested this internal updating during head rotation in roll, exploiting the rod-and-frame effect: The illusory tilt of a vertical line surrounded by a tilted visual frame.If line orientation is stored relative to gravity, these distortions should also affect the updating process.Alternatively, if coding is head- or frame-centered, updating errors should be related to the changes in their orientation.Ten subjects were instructed to memorize the orientation of a briefly flashed line, surrounded by a tilted visual frame, then rotate their head, and subsequently judge the orientation of a second line relative to the memorized first while the frame was upright.If the orientation of the line is stored in a gravity-based frame of reference, the corresponding memory will be affected by the perceived distortion of this frame.
Finally, it could be considered that the updating is not restricted to using a single reference frame, but that multiple reference frames are used in the maintenance of line orientation (cf. Here, we investigated the role of allocentric and egocentric reference frames in the updating of line orientation across head rotations.The brain can use multiple reference frames to code line orientation, including head-, object-, and gravity-centered references.If these frames change orientation, their representations must be updated to keep register with actual line orientation.The stimulus was a gray 1-mm–wide line with a length of 25 cm (8° visual angle), presented with a random noise overlay and a Gaussian blur to prevent aliased edges of the stimulus from giving additional cues about the orientation of the line on screen.A square frame with sides of 45 cm (15° visual angle, line width 0.3 cm) was presented around the stimulus line.We conclude that the brain relies primarily on a gravity-based reference to remember line orientation during head roll.Maintaining a veridical percept of objects in the world around us requires knowledge about the spatial relationships between objects in the environment, and between objects and ourselves (Burgess, 2006; Doeller, King, & Burgess, 2008; Filimon, 2015; Lambrey, Doeller, Berthoz, & Burgess, 2012; Mou, Mc Namara, Rump, & Xiao, 2006; Wang & Spelke, 2000). If one of the references changes, the spatial relationship needs to be recomputed to maintain a correct registry with its true spatial orientation in the world.To test between these reference frames, we exploited the systematic error that is observed when aligning a visual line to the gravitational vertical in the presence of panoramic visual cues—for example, when the line is surrounded by a square frame (Witkin & Asch, 1948).When the head is upright, line settings indicate a bias in the perception of gravity direction, which cyclically modulates with the orientation of the frame (Beh, Wenderoth, & Purcell, 1971).Results showed that updating errors were mostly related to the amount of subjective distortion of gravity at both the initial and final head orientation, rather than to the amount of intervening head rotation.In some subjects, a smaller part of the updating error was also related to the change of visual frame orientation.