Lower Internal Additive Noise and Better Perceptual Template Characterize Binocular Contrast Sensitivity Summation

Binocular summation is generally defined as the superiority of binocular over monocular performance. Here, we investigated how external noise modulates the effect of binocular summation on the contrast sensitivity function (CSF) and clarified the corresponding mechanisms with a perceptual template model (PTM). The contrast sensitivity (CS) over 10 spatial frequencies and three external noise levels was assessed under one binocular and two monocular viewing conditions. The binocular summation ratio (BSR) was calculated by dividing the area under the log CSF (AULCSF), or the CS of using both eyes, by that of only using the “good eye” (BSRG) or the “bad eye” (BSRB), respectively. We found that: (1) based on the AULCSF, the BSRB was higher than the BSRG; (2) based on the AULCSF, the BSR was more pronounced under zero-noise than under low-noise conditions, but the BSR was not higher than 1 under high-noise conditions due to a large individual difference; (3) based on the CS, with increasing spatial frequencies, the BSRB steadily increased; (4) both decreased internal additive noise and an improved perceptual template accounted for the gain in binocular summation. These results help us better understand the features of binocular CS and shed light on the clinical studies on populations with monocular CS loss.

Perceptual Transparency From Cast Shadow

This study examined how a shadow contributes to the perception of a transparent surface. As stimuli, we used computer graphics images in which a transparent surface with a color-mosaic pattern casts a shadow onto a background surface. We manipulated two parameters: (a) the spatial heterogeneity of the transmittance of the transparent surface and (b) the size of the light source shining on the transparent surface and its background. The latter parameter determined the blurriness of shadows. Observers judged whether the stimulus image contained a transparent surface or not. We found that the proportion of reports identifying a transparent surface was dependent on both parameters we tested. Specifically, a high spatial heterogeneity of transmittance decreased the proportion of reports of a transparent surface; this was possibly because globally defined X-junctions, which were one of the cues to perceptual transparency, perceptually broke down. On the other hand, blurred shadows were effective even when the global X-junctions were not effective. Locally defined X-junctions only moderately contributed to perceptual transparency. The results indicate that in addition to global and local X-junctions, blurred shadows are image features that elicit the perception of transparency from a cast shadow. A large individual difference as to which information each participant used as a cue to perceptual transparency was also discussed.