Undercounting features and missing features: evidence for a high-level deficit in strabismic amblyopia

Abnormal visual development in strabismic amblyopia drastically affects vis ual perception and properties of neurons in primary visual cortex (V1). To test the notion that amblyopia also has consequences for higher visual area s, we asked humans with amblyopia to count briefly presented features. Usin g the amblyopic eye, strabismic amblyopes counted inaccurately, markedly un derestimating the number of features. This inaccuracy was not due to low-le vel considerations (blur, visibility crowding, undersampling or topographic al jitter), as they also underestimated the number of features missing from a uniform grid. Rather, counting deficits in strabismic amblyopes reflecte d a higher-level limitation in the number of features the amblyopic Visual system can individuate.