A 58-year-old man from Delhi presents with progressive loss of his peripheral vision over 6 months. He reports that he bumps into objects on the sides while walking straight. Visual acuity is 6/6 in both eyes. Confrontation visual field testing reveals bilateral concentric field constriction. Fundoscopy shows normal optic disc and macula. Optical coherence tomography (OCT) of the macula is normal. Which layer of the retina is primarily affected in this condition?