A pupillometric correlate of scotopic visual acuity.

While not easily fit into the classic descriptions of the pupillary light reflex, previous studies reported that changes in the spatial composition of the retinal image can evoke a pupillary response. The present study extends this observation by showing that the pupil constricts in response to scotopic as well as photopic spatial patterns. Moreover, the amplitude of the scotopic response decreases with increasing spatial frequencies suggesting a pupillary spatial acuity of about 3 c/deg. The scotopic pupil acuity is similar to the scotopic perceptual visual acuity measured in the same observers.