The detection of bilateral symmetry: effects of surrounding frames

Abstract The presence of visible frames can influence how one perceives ori entation and shape; this experiment investigated whether such frames affect the detection of bilateral symmetry in dot patterns. When obliquely or vertically oriented rectangular fram es were presented simultaneously surrounding the dot patterns, the frames influenced sym metry detection. Vertical symmetry detection was slower in the tilted frame than it w as in the vertical frame, and tilted symmetry was detected faster in the tilted frame than it was i n the vertical. When there was a time delay between frame and pattern onset, there was no measur able frame effect. The frames may have emphasized the orientation parallel to the long axi s of the frame, thereby increasing the likelihood that observers test for symmetry at that orien tation. The effect may also have arisen because of a disruption of the symmetry of the entire figur e when frame orientation and symmetry axis were misaligned. Symmetry detection was slowed wh en not in line with the long axis of the frame, whereas symmetry detection was not affecte d when the frame axis was aligned with the symmetry axis.Resume La presence de cadres visibles peut influencer comment le sujet perc oit l'orientation et la forme d'un contenu; le but de cette experience etait d'indiq uer si ces cadres influencent la perception d'axes de symetrie bilaterale de motifs en poin tilles. La presentation simultanee d'un cadre rectangulaire oblique ou vertical autour d es motifs en pointilles influencait la perception de symetrie. La perception d'une symetr ie verticale dans un cadre oblique etait plus lente que dans le cadre vertical; la perception d'une symetrie oblique etait plus rapide dans le cadre oblique que dans le cadre verti cal. Un retard dans l'imposition du cadre sur le motif eliminait tout un effet mesurable se rapportant au cadre. Il est possible que l'axe de longueur du cadre ait accentue la percep tion de l'orientation parallele des pointilles du motif, favorisant par le fait meme che z les observateurs un examen symetrique dans cette direction. L'effet peut aussi etre survenu par suite d'une perturbation de la symetrie de l'ensemble de la figure, au momen t ou l'orientation du cadre ne s'alignait plus sur l'axe de symetrie du motif. La vi tesse de detection de motifs pointilles symetriques etait moindre lorsque ceux - ci n'eta ient pas alignes sur la longueur du cadre, mais l'effet disparaissait si l'axe du cad re s'alignait sur l'axe des points symetriques.It is well known that the perceived shape or orientation of objects can be chang ed by the presence of other structures. For example, Palmer (1990) has demonstrated that the perception of equilateral triangles is influenced by their arrangement in a scene as well a s by the placement of a rectangular frame around them. Similarly, Palmer, Simone, and Kube (1988) found that arrangements of squares or diamonds that normally induce some ambiguity in shape descriptions generated by subjects would produce unambiguous shape reports when presented on a frame representing a plane receding in depth. Humphrey and Jolicoeur (1988, 1993) hav e shown that frames providing monocular depth cues helped subjects recognize line drawings of foreshortened views of objects. The rod and frame illusion also demonstrates the effect surro unding structure can have on perceived orientation (Wenderoth, 1977). For many years research has focussed on the question of how well bilateral (mirror) symmetry is detected when presented at different orientations within the fronto - parallel plane. Vertical symmetry has been found consistently to be detected most accurately and rapidly, whereas bilateral symmetry about an oblique axis or horizontal axis is more diff icult to detect (Barlow & Reeves, 1979; Corballis & Roldan, 1975; Fisher & Bornstein, 1982; Pash ler, 1990; Royer, 1981). Julesz (1971) and Corballis and Roldan (1975) have suggested thatnon - vertical symmetry is detected through mental rotation of the patterns to v ertical, allowing a process similar to that active in vertical symmetry detection to operate. …