J Vision
-
Studies have shown that displacing specular highlights from their natural locations in images reduces perceived surface gloss. Here, we assessed the extent to which perceived gloss depends on congruence in the position and orientation of specular highlights relative to surface shape and the diffuse shading from which surface shape is recovered. The position and orientation congruence of specular highlights with diffuse shading was altered while preserving their compatibility with physical surface shape (Experiment 1). ⋯ When we disrupted the consistency in the local position and orientation of specular highlights with respect to the diffuse shading and local surface meso-structure, a decline in perceived gloss was still observed (Experiment 2). This decline in perceived gloss caused by misaligning the positions and orientations of specular highlights relative to diffuse surface shading could not be explained by differences in orientation fields alone (Experiments 3 and 4). These results suggest the visual system assesses both position and orientation congruence between specular highlights and diffuse shading to estimate surface gloss.
-
During observation of an ambiguous Necker cube, our percept changes spontaneously although the external stimulus does not. An EEG paradigm allowing time-resolved EEG measurement during endogenous perceptual reversals recently revealed a chain of ERP correlates beginning with an early occipital positivity at around 130 ms (Reversal Positivity, "RP"). In order to better understand the functional role of this RP, we investigated its relation to the P100, which is spatiotemporally close, typically occurring 100 ms after onset of a visual stimulus at occipital electrodes. ⋯ The main results are: (1) The P100 amplitude increases monotonically with stimulus size but is independent of the participants' percept. (2) The RP, in contrast, is percept-related and largely unaffected by stimulus size. (3) A similar pattern to RP was found for reaction times: They depend on the percept but not on stimulus size. We speculate that the P100 reflects processing of elementary visual features, while the RP is related to a processing conflict during 3D interpretation that precedes a reversal. The present results indicate that low-level visual processing (related to stimulus size) and (relative) high-level processing (related to perceptual reversal) occur in close spatial and temporal vicinity.
-
The perception of surface gloss depends on specular highlights but little is understood about how the visual system distinguishes specular highlights from other luminance maxima generated by variations in pigmentation or illumination. It has been argued that diffuse shading gradients provide information for identifying specular highlights. Specular highlights typically share the orientation of the diffuse shading locally. ⋯ Rotations reduced perceived gloss more than translations, even though translations displaced highlights into darker regions than rotations. The same reductions in perceived gloss occurred when highlights were matched in perceived contrast across conditions (Experiment 2b). The results provide evidence that the perception of gloss depends on highlight distance from the luminance maxima of the surrounding intensity gradient (brightness congruence) in addition to the shared orientation of highlights with their surrounds (orientation congruence).