Neural correlates of context-dependent lightness perception.

Objective: The lightness of a surface is not solely determined by the amount of light it reflects but it is also modulated by the surrounding context. Despite a long history of research, neural correlates of context dependent lightness perception remain a question of ongoing debate. The current study seeks to expand upon the existing literature by using a classical effect called simultaneous lightness induction (SLI) as stimulus where a central disk appears darker when embedded in a darker surround compared to when it is embedded in a lighter surround. Methods: We use functional magnetic resonance imaging (fMRI) to investigate locus of processing of context-dependent lightness in visual hierarchy. During the fMRI experiment, we presented ten participants with a dynamic stimulus by modulating the luminance of either achromatic surround (surroundmodulation condition) or achromatic disk (disk-modulation condition) at four different frequencies ranging from 1 to 8 Hz while they perform a demanding fixation task. Results: Behaviorally, when the surround luminance is modulated at low frequencies, participants perceive an illusory change in lightness of the disk (lightness induction). Whereas, they perceive the disk as having constant lightness at higher frequencies. Utilizing this temporal dependence of lightness induction and comparing the fMRI BOLD activity of the surround-modulation condition with that of the disk-modulation condition at different frequencies, while accounting for the effects of long-range luminance changes, we showed that activity in V1 correlates with perceived lightness. However, such a correlation was not evident in extrastriate areas, V2, V3, and V4. Conclusion: The findings of our study provide evidence for the involvement of the primary visual cortex in processing of context-dependent lightness information. [ABSTRACT FROM AUTHOR]