Transient and sustained components of the pupil response evoked by achromatic spatial patterns

The present study provides new clues about visual processes underlying the human pupillary responses evoked by achromatic spatial patterns. The pupillary responses can be modeled as a combination of two processes, a temporally transient process which saturates with increasing contrast and a temporally sustained process which varies linearly with increasing grating contrast. The transient process has low-pass, whereas the sustained process has a middle band-pass spatial filter characteristic. the results support the hypothesis that the visual input to the pupillomotor nuclei is composed of phasic and tonic visual neurons that are functionally similar to those in the magno (M)- and parvo (P)- cellular layers in the lateral geniculate nucleus.