Agnosic vision is crowded

Visual agnosia is a neuropsychological impairment of visual object recognition. In spite of over a century of research, little is known about the nature of the functional damage underlying this deficit. We propose that the agnosic deficit is visual crowding: The central vision of agnosic patients is crowded, like the peripheral vision of normally sighted observers. To examine our hypothesis, for each patient who took multiple object-recognition tests, we converted each test score into an equivalent eccentricity, i.e. the eccentricity at which our normally sighted “standard” observer performs as poorly as the centrally-viewing patient. Our standard observer took 15 different screening tests for the diagnosis of visual agnosia at various eccentricities in his periphery. In normal peripheral vision, perception of a simple image (e.g. an isolated letter) is limited by acuity, and perception of a complex image (e.g. a face or a word) is limited by crowding. Our crowding hypothesis proposes that each apperceptive agnosia patient is limited by a degree of crowding that consistently corresponds to one equivalent eccentricity, across all complex images. Analyzing the published data of 32 apperceptive agnosia patients and a group of 14 Posterior Cortical Atrophy (PCA) patients, we find that acuity is spared, and each patient’s pattern of object recognition deficits is well characterized by one number, the equivalent eccentricity (for crowding) at which our standard observer’s peripheral vision is like the central vision of the agnosic patient. In other words, each agnosic patient’s equivalent eccentricity is conserved across tasks. Across patients, equivalent eccentricity ranges from 4 to 40 deg. This indicates that the visual impairment of apperceptive agnosia is crowding. In concert with Song, Levi, & Pelli (2014), we report a double dissociation of acuity and crowding: apperceptive agnosia worsens crowding while sparing acuity, and anisometropic amblyopia worsens acuity while sparing crowding.