A 68-year-old woman from Mumbai presents with gradual loss of peripheral vision bilaterally over the past 6 months. She reports that she has difficulty navigating crowded spaces and frequently bumps into objects on either side. Visual acuity is 6/6 in both eyes. Fundoscopy shows optic disc cupping with a cup-to-disc ratio of 0.8 bilaterally. Intraocular pressure is 28 mmHg in both eyes. Visual field testing reveals bilateral concentric visual field defects. Which layer of the retina is primarily affected in this condition?