Responses to lightness variations in early human visual cortex

Abstract

Lightness is the apparent reflectance of a surface, and it depends not only on the actual luminance of the surface but also on the context in which the surface is viewed [1-10]. The cortical mechanisms of lightness processing are largely unknown, and the role of early cortical areas is still a matter of debate [11-17]. We studied the cortical responses to lightness variations in early stages of the human visual system with functional magnetic resonance imaging (fMRI) while observers were performing a demanding fixation task. The set of dynamically presented visual stimuli included the rectangular version of the classic Craik-O'Brien stimulus [3, 18, 19] and a variant that led to a weaker lightness effect, as well as a pattern with actual luminance variations. We found that the cortical activity in retinotopic areas, including the primary visual cortex (V1), is correlated with context-dependent lightness variations.

Figure 1. Stimuli used in the behavioral and fMRI experiments.

Behavioral and fMRI responses to the lightness effect caused by the “Illusory” Craik-O'Brien (CO) stimulus were measure and compared with responses to the “Real” condition in which flanks physically differed. The response to the “Illusory” stimulus was also compared with that to the “Control” stimulus that caused a weak lightness effect. Luminance profile along a horizontal cross-section is shown below each stimulus.

Figure 2. Behavioral measurement of the lightness effect.

The contrast of the subjectively equivalent “Real” stimulus was determined in an adaptive two-interval forced-choice experiment (2IFC). Observers were asked to indicate the interval in which the luminance difference between the flanks, within the square frames, was larger while maintaining fixation at a central mark. The same measurement method was repeated for the “Control” stimulus.

Figure 3. Design of the fMRI experiment.

Cortical responses were measured to dynamically presented versions of the stimuli shown in Figure 1. The contrast of the “Real” stimulus was adjusted to match the lightness effect for each observer based on his or her behavioral data. (A) Each stimulus was first presented statically for 18 seconds followed by a square wave modulated counterphase flicker for 12 seconds. During the entire scan observers performed a fixation task that required them to detect a target letter (X) among distracters during rapidly changing presentation of these letters. (B) Region of interest (ROI) definitions. Counterphase flickering checker patterns were used to localize ROIs. The left panel shows dimensional parameters of the “Illusory” stimulus and the positions of the ROI localizers. The right panel shows the areas of the visual cortex activated by the localizers for one observer (p(corr.)<10⁻⁴). Red lower ROIs, blue upper ROIs. Because the cortical areas corresponding to upper ROIs were much closer to the vertical meridian and therefore to the contrast border in the stimuli, only MR data in lower ROI pairs are presented here (See Supplemental Figure 1 for results from both ROI pairs).

Figure 4. FMRI results.

(A) V1, V2, and V3 responses averaged across observers. The average response to the “Illusory” condition was smaller than that to the “Real” condition. However this difference was statistically significant only in V1. Importantly, the MR signal to “Illusory” condition was larger than that to the “Control” stimulus in V1, V2 and V3. The difference between the MR signals to “Illusory” and “Control” conditions suggest that the activity in “Illusory” condition can not be explained as a direct response (neural or vascular) to distant contrast and luminance variations. (See Supplemental Figure 1 for the analysis of MR data in both ROI pairs). (B) Individual responses in V1. For two observers out of five, MR signals to “Real” condition was statistically significantly larger than that to the “Illusory” condition. For all observers except one, the “Illusory” stimulus evoked a significantly larger MR signal than the “Control” stimulus. Error bars represent one standard error of the mean (s.e.m.). Statistically significant differences are indicated (* p<0.05, ** p<0.01, *** p<0.001).