Humans Utilize An Orthogonal Internal Reference Frame On Angle Perception

Angle perception is one of the most important mid-level visual processes,and it plays an essential role in perceiving shapes.The discussion of angle perception has largely hinged on whether the angle is represented as a holistic feature(Gestalt perspective)or the combination of two lines(Structuralism).Here,we developed a new computational model,demonstrating that the human visual system estimates the size of an angle feature using an orthogonal internal reference frame(IRF).The model included three stages: 1)orientation encoding: the primary visual cortex encodes the orientations of two lines;2)coordinate transformation: higher-level cortical areas align one of the bounding lines with its nearest reference axis;and 3)angle estimation: the brain calculates the orientation difference between the other bounding line and the nearest axis.The IRF model fits well with the data from previous studies,which neither Structuralism nor Gestalt theory could explain.We subsequently conducted a series of experiments to verify the necessity of the three stages and explore the underlying mechanisms of the IRF.We found that angle discrimination thresholds varied when the same angle was tilted in different directions,suggesting that the sensitivity to the orientation of bounding lines contributes to angle perception.Experiment II showed that the transformation of IRFs might have little effect on JNDs or RTs in angle.Further analysis suggested that the IRF was aligned with the prior knowledge of angle distribution in a Bayesian decision framework,given a relatively large proportion of angles at 0°,90° and 180° in the natural environment.Moreover,we applied virtual reality technology to explore external cues affecting the IRF.By rotating participants’ body positions in a flight simulator and visual contexts in a head-mounted display,we dissociated the contributions of egocentric,gravity and visual context cues to the IRF.We found that the IRF for angle perception mainly relied on egocentric cues and was slightly modulated by visual-contextual and gravitational cues.In sum,the study provides a new computational framework for angle discrimination,resolving a long-standing debate between Structuralism and the Gestalt perspective on angle perception.