Brain & Cognition, KU Leuven, Leuven, Belgium.
Department of Psychology, Stanford University, Stanford, USA.
Sci Rep. 2018 May 3;8(1):6969. doi: 10.1038/s41598-018-24513-w.
Symmetry is a highly salient feature of the natural world which requires integration of visual features over space. The aim of the current work is to isolate dynamic neural correlates of symmetry-specific integration processes. We measured steady-state visual evoked potentials (SSVEP) as participants viewed symmetric patterns comprised of distinct spatial regions presented at two different frequencies (f and f). We measured intermodulation components, shown to reflect non-linear processing at the neural level, indicating integration of spatially separated parts of the pattern. We generated a wallpaper pattern containing two reflection symmetry axes by tiling the plane with a two-fold reflection symmetric unit-pattern and split each unit-pattern diagonally into separate parts which could be presented at different frequencies. We compared SSVEPs measured for wallpapers and control patterns for which both images were equal in terms of translation and rotation symmetry but reflection symmetry could only emerge for the wallpaper pattern through integration of the image-pairs. We found that low-frequency intermodulation components differed between the wallpaper and control stimuli, indicating the presence of integration mechanisms specific to reflection symmetry. These results showed that spatial integration specific to symmetry perception can be isolated through a combination of stimulus design and the frequency tagging approach.
对称是自然界中一个高度显著的特征，需要在空间上整合视觉特征。本研究的目的是分离对称特定整合过程的动态神经相关物。我们在参与者观看由不同空间区域组成的对称模式时测量了稳态视觉诱发电位（SSVEP），这些模式以两个不同的频率（f 和 f）呈现。我们测量了调制分量，这些分量反映了神经水平上的非线性处理，表明模式的空间分离部分的整合。我们通过用具有二倍反射对称性的单位图案平铺平面生成了一个具有两个反射对称轴的壁纸图案，并将每个单位图案对角线地分成可以在不同频率呈现的单独部分。我们比较了壁纸和控制图案的 SSVEP，控制图案在平移和旋转对称性方面与壁纸相同，但只有通过对图像对的整合，壁纸图案才能出现反射对称性。我们发现壁纸和控制刺激之间的低频调制分量不同，表明存在特定于反射对称性的整合机制。这些结果表明，通过刺激设计和频率标记方法的组合，可以分离出对称感知的空间整合。
