Functional Specialisation of the Left Hemisphere in Local Visual Processing: Evidence from a Lesional Study.

While classical studies have established that lightness illusions, such as the gradient-induced lightness contrast effect, are enhanced by specific visual contexts (Agostini & Galmonte, 2002) and that achromatic appearance illusions - including brightness/luminosity effects - engage occipito-temporal cortical regions (Leonards et al., 2005), a complete neuroanatomical and mechanistic account remains elusive. Current models, including adaptive sensory coding frameworks, still struggle to fully explain these phenomena, a gap underscored by recent analyses highlighting persistent theoretical challenges (Tyler, 2022; Zhang et al., 2024). To clarify the neural substrates of achromatic contrast, we conducted a lesion study and tested patients with acquired unilateral brain injuries. We replicated the paradigm by Agostini & Galmonte (2002) in two patient groups - one with left-hemisphere (LH) lesions and one with right-hemisphere (RH) lesions - alongside healthy controls. Results revealed that controls and patients with RH lesions experienced the illusion similarly. In contrast, patients with LH lesions did not exhibit the illusion, perceiving target lightness accurately regardless of background contrast. This finding suggests that damage to LH regions disrupts the mechanisms underlying the gradient-induced lightness illusion . Overall, the results point to a preferential involvement of LH structures in achromatic contrast processing, while warranting cautious interpretation given the sample size and lesion heterogeneity .