Visual inspection of three-dimensional objects by human observers

Eye movements are an important aid in active visual exploration of the environment and in performing behavioural tasks. Eye movements might also play a role in human perception of three-dimensional (3-D) objects. Eye-movement strategies were investigated when humans inspected and memorised 3-D objects. Subjects were instructed to memorise the 3-D structure of parts of statues of human figures placed on a turntable free to rotate through 360 degrees. Eye movements and turning behaviour were recorded. Different turning and eye-movement strategies could be observed. Subjects showed individual turning behaviours that were reproducible between trials. Turning strategies ranged from focusing on only a limited number of perspective views to almost continuously rotating the object with only short stops. On average twelve-thirteen views were inspected during memorising. Eye movements also revealed individual strategies. Fixation locations within each inspection view ranged from either closely spaced on isolated parts of the object to distributed over the whole view with large saccades in between. Eye movements were often directed to the same details from different perspectives. The differences in turning and viewing strategy also resulted in differences in the ability to recognise parts of the object later on. In general, successful later recognition required that the subject actually fixated the part to be recognised. A strategy of thoroughly inspecting the object with a series of closely spaced fixations from only a limited number of viewpoints led to best recognition rates. This was especially true for two subjects trained in fine arts with prior experiences in modelling. The results support models of viewpoint-dependent object recognition with viewer-centred, two-dimensional representations of 3-D objects.