BILATERAL SYMMETRY DETECTION - TESTING A CALLOSAL HYPOTHESIS

At the end of the 19th century Mach observed that vertical symmetry is more easily perceived than is symmetry at other orientations, and pro posed this resulted from bilateral symmetry in the visual system. Nume rous studies of symmetry detection have been conducted, but none has b een concentrated on Mach's proposal. Recent interpretations of Mach's hypothesis suggest the corpus callosum mediates the vertical-symmetry advantage. In this 'callosal' hypothesis it is suggested that the dete ctability of symmetry should be narrowly tuned around vertical, and th at presentation of patterns away from fixation should disrupt the vert ical advantage. We found that the vertical advantage was disrupted by presentation of patterns 1.2 deg from fixation, while detection of sym metry at other orientations was not disrupted. At fixation the orienta tion tuning was at least within +/-10 degrees of vertical. The detecti on of vertical symmetry at fixation was found to be anomalous in two s ubjects born without a corpus callosum as compared with controls, but relatively normal for presentation off fixation. The three experiments reported are in agreement with some of the predictions derived from t he callosal hypothesis. It appears that the callosal hypothesis may ac count for the relative advantage of vertical symmetry at fixation, but other mechanisms must operate to detect symmetry at other orientation s and positions.