Neural correlates of spatiotemporal properties of lightness induction

The lightness of a surface depends not only on the amount of light reflected off it but also on the context it is embedded in. 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 measuring fMRI responses to a dynamic version of a classical effect called simultaneous lightness induction (SLI), where a central disk appears lighter when embedded in a darker surround compared to when embedded in a lighter surround. During the fMRI experiment, we presented ten participants with a dynamic SLI stimulus by modulating the luminance of either the achromatic surround (surround-modulation condition) or the achromatic disk (disk-modulation condition) at four different frequencies ranging from 1 to 8 Hz. Behaviorally, when the surround luminance is modulated at low frequencies, participants perceive an illusory change in the lightness of the disk (lightness induction). In contrast, they perceive little or no induction at higher frequencies. Utilizing this temporal dependence and controlling for long-range responses to border contrast and luminance changes, we found that activity in the primary visual cortex (V1) correlates with lightness induction. However, such a correlation was not evident in extrastriate areas, V2, V3, and V4. These findings provide evidence for the involvement of V1 in the processing of context-dependent lightness.