Local computation of lightness on articulated surrounds

Abstract

Lightness of a grey target on a uniform light (or dark) surround changes by articulating the surround (articulation effect). To elucidate the processing of lightness underlying the articulation effect, the present study introduced transparency over a dark surround and investigated its effects on lightness of the target. The transparency was produced by adding a contiguous external field to the dark surround while keeping local stimulus configuration constant. Results showed that the target lightness did not change on the articulated surround when a dark transparent filter was perceived over the target, although it did on the uniform surround. These results suggest that image decomposition into a transparent filter and an underlying surface does not necessarily change lightness of the surface if the surface is articulated. Moreover, the present study revealed that articulating the surround does not always enhance lightness contrast; it can reduce the contrast effect when the target luminance is not the highest within the surround. These findings are consistent with the theoretical view that lightness perception on articulated surfaces is determined locally within a spatially limited region, and they also place a constraint on how the luminance distribution within the limited region is scaled.

Keywords: anchoring; articulation effect; image decomposition; lightness contrast; lightness perception; transparency.

Figure 1.

Lightness contrast and an articulation effect. (a) A classic lightness contrast phenomenon. Target lightness on a dark uniform surround is perceived lighter than that on a light surround. (b) The lightness difference between the targets on the light and dark surrounds changes (becomes larger in this case) on articulated surrounds, which demonstrates an articulation effect.

Figure 2.

Stimulus configuration and conditions in Experiment 1. (a) A schematic diagram of the stimulus display (contiguous condition). The light and dark surrounds were presented on the left side of the display and the matching stimulus was on the right side. The symbols ‘T’ and ‘M’ indicate the target and matching stimuli, respectively. The target was centred on the light or dark surround. (b) Stimulus conditions used in the experiment. The surrounds were either uniform (left) or articulated (right). In the contiguous condition (top), the perception of transparency was produced by adding a darker contiguous field to the dark surround. In the gapped condition (bottom), the perception of transparency was disrupted by making a gap at the border between the dark surround and the contiguous region.

Figure 3.

Results of Experiment 1. (a) Results of the lightness matching. The PSEs of the target lightness were plotted as a function of surround type (i.e., uniform vs. articulated). Different symbols indicate the results in different stimulus conditions as shown in the legend. Filled symbols denote the results in the contiguous condition, whereas open symbols designate those in the gapped condition. Error bars indicate ±1 SEM across observers. An arrow on the vertical axis denotes the target luminance used in the experiment. (b) Results of the transparency rating. Rating values were shown for the uniform and articulated surround conditions. Black and white bars indicate the results in the contiguous and the gapped conditions, respectively. Error bars indicate +1 SEM across observers.

Figure 4.

Results of Experiment 2. (a) Results of the lightness matching. The left and right panels show the results when the target luminance was 0.70 and 0.47 log cd/m², respectively. In each panel, the PSEs of the target lightness on the dark surround were plotted as a function of surround type. Different symbols denote the results in different configuration conditions as shown in the legend. (b) Results of the transparency rating. The rating values measured with different target luminances were pooled because a preliminary analysis showed no significant effect of target luminance. Black and white bars indicate the results in the contiguous and the gapped conditions, respectively. Other aspects of the figure are the same as those in Figure 3.