| The visual image is the visual entity which is got from real world by human visual system, and acts directly or indirectly on human eyes to produce visual perception. It is well known that the human visual system is highly sensitive to the symmetry property(especially the bilateral symmetry) of the visual image, which draws a lot of attention to the cognitive psychology researchers. These studies found that symmetry perception could play an important role in Chinese characters recognition, shape perception and other cognitive processes. And the symmetry perception itself also has many attractive properties. For example, the researchers found that the vertical symmetry is more sensitivity to horizontal symmetry in symmetry perception, and the latter is more sensitivity to other symmetry forms. Yet a limitation of nearly all theoretical and experimental studies of symmetry perception is that they consider two-dimensional bilateral symmetry in fronto-parallel view, normally with a vertical axis of symmetry. Self-evidently, bilaterally symmetrical patterns and objects do not normally produce a symmetrical retinal image. Two main types of transformations are possible: skewed projections caused by viewing a symmetrical two-dimensional pattern out of the fronto-parallel plane, and the more complex transformations produced by rotating a 3 dimensional symmetrical object. It is relevant here that the 3 dimensional structure can often be inferred from a 2 dimensional view, given a prior assumption that an object is symmetrical. The 2D retinal image of a symmetric 3D shape is itself symmetric only if the line of sight lies on the symmetry plane. For other viewing orientations, the 2D retinal image is asymmetric and will be called 3D skewed symmetry.The prior studies showed that the human observer can reliably recognize the same symmetric 3D shape from different viewpoints. The reliable performance of 3D shape recognition requires veridical perception of the 3D shape from each 2D image. And what the role of symmetry perception played in this process still remains unknown. According to the Marr‘s 3D shape perception, there are two competing hypotheses: one is the retinal structure hypothesis, which means the symmetry perception process in 3 dimensional skew symmetry occurs in the processing stage of retinal image mapping, this hypothesis is supported by the researches of van der Vloed et al; another hypothesis is the perspective normalization assumption, which means symmetry perception is a postconstancy process, following active normalization based on other visual cues that allow slant to be determined, and it is supported by the research of Szlyk et al.To remedy this problem, Our research use the ERP technology to investigate the cognitive process of symmetry perception with realistic 3 dimensional stimuli. The stimuli set are separated into two classes: symmetric object images and asymmetric object images, both are 2.5D image of real world objects, which means the visual images of both classes objects are asymmetric. And other visual properties are balanced between the two classes. Subjects are asked to perform key-press reaction under different kinds of tasks, and the EEG signals from each subject’s scalp surface are recorded during the image presentation.Experiment 1: we use object category discrimination task to investigate whether symmetry perception processing is involved in early visual processing during the object recognition phases, that is to say whether symmetry is processing automatically in the retinal image level. Subjects are asked to make a force choice between artificial object and natural object when the object image is presented, and the EEG signals from each subject’s scalp surface are recorded during the image presentation. Recently studies have explored visual symmetry processing by measuring event related potentials and neural oscillatory activity. There is a sustained posterior negativity(SPN) related to the presence of symmetry. Therefore, we use the SPN component as the physiological index of the symmetry perception processing to explore the time course of symmetry processing. The event related desynchronization(ERD) also included.Experiment 2: we use symmetry detection task to investigate whether symmetry processing is involved in the early visual processing and the later visual information integration during the symmetry perception. Subjects are asked to make a force choice between symmetric object or asymmetric object when the object image is presented, and the EEG signals from each subject’s scalp surface are recorded during the image presentation. Same as experiment 1, the onset time and amplitude difference of SPN component are used as the objective index to reflect the symmetry of perceptual processing. And the time frequency analyses are also executed to get ERD components.The results show that skew symmetric object pictures cause more negative SPN component than asymmetric object pictures in the object category discrimination task,, but the statistical results is marginal significant. After adjusting the time window for post hoc statistical analysis, we find that the difference(of SPN component) is statistical significant during 250 ms and 650 ms time window, and the difference disappears after 650 ms onset. However, in the symmetry detection task skew symmetric object pictures caused more negative SPN component than asymmetric object pictures and the statistical results is significant. And the difference of SPN is significant during the adjusted 250ms-650 ms time window, which disappears after 650 ms onset. To our surprise, we also observe the right hemisphere lateralization phenomenon which means the ERP in the right hemisphere is more negative than the ERP in the left after 650 ms.According to the experimental results, we can draw the following conclusions: 1) symmetry perception occurs in early visual processing during the 3 dimensional shape perception, which means our results are support the retinal structure hypothesis; 2) symmetry perception only occurs in early visual processing during the process of 3D object recognition. In the later stage of visual processing which the 3 dimensional representation of the object is established, the process of other clues such as depth may be dominant and cause the processing of symmetry perception decaying; 3) in the symmetry detection of 3 dimensional objects, SPN displays a right brain lateralization phenomenon, which means right brain has an advantage in symmetry perception; 4) event related desynchronization(ERD) is not sensitive to skew symmetry, which means ERD component may related to activation of the exogenous attention network in symmetry perception, and to give more details about the mechanism, further studies are needed. Thus, this study reveals the processing of symmetry perception under the 3 dimensional stereoscopic vision circumstance, and provides theoretical explanation and empirical evidence to solve the disagreement on the process time of symmetry perception. |
