Disparity-defined depth perception in face and non-face contexts

Abstract

Perception is modulated by context. For example, perceived colour is biased towards the familiar colour of the object. Moreover, perception of the lightness of a face is modulated by the racial category of the face. Furthermore, the judgment of the orientation of a line segment is subject to the influence of flanking contours. While depth perception is one of the most important abilities for survival and daily functions, studies on the contextual modulation of depth perception are extremely scarce. Here, we tested whether disparity-defined depth perception was modulated by the context of objects. Specifically, we constructed random-dot stereograms (RDSs) depicting faces (upright and inverted) and curvature-matched random shapes. In Experiment 1, we compared the thresholds of depth discrimination for these objects in a signal-in-noise (SNR) task and a feature difference task. In the SNR task, participants were required to judge the depth position of the central target relative to the surrounding region as the signal-to-noise ratio (SNR) varied between 0% and 100%. In the feature difference task, participants were required to judge which of two consecutively presented targets was nearer, when the disparity difference between the targets varied between 0 and 150 arcsec. Results indicated that thresholds were better for random shapes than upright faces in the SNR task, but better for upright faces than random shapes in the feature difference task. In Experiment 2, by means of functional magnetic resonance imaging (fMRI), we further examined the neural responses to faces and random shapes while participants were making depth judgments in the SNR task. Results of multivoxel pattern analysis (MVPA) showed that SVM classification accuracies of object types were significantly above chance in dorsal and ventral regions. Remarkably, there was robust correlation between the discrimination accuracy in FFA (fusiform face area) and the difference between the thresholds for face and non-face stimuli, suggesting that FFA might be the cortical locus for the modulation of depth discrimination ability. Overall, our findings reveal that the behavioural sensitivity and fMRI responses to disparity is context-dependent. Results of the depth discrimination tasks suggest different noise filtering and feature readout abilities for faces and non-face objects. Moreover, based on the fMRI data, we postulate that stereo and object recognition mechanisms interact with each other to optimize disparity-defined depth perception in natural context.