The optic quality of the eyes is, at least in part, determined by pupil size. Large pupils let more light enter the eyes, but degrade the point spread function, and thus the spatial resolution that can be achieved (Campbell & Gregory, 1960). In natural conditions, the pupil is mainly driven by the luminance (and possibly the color and contrast) at the gazed location, but is also modulated by attention and cognitive factors. Whether changes in eyes' optics related to pupil size modulation by luminance and attention impacts visual processing was assessed in two experiments. In Experiment 1, we measured pupil size using a constantly visible display made of four disks with different luminance levels, with no other task than fixating the disks in succession. The results confirmed that pupil size depends on the luminance of the gazed stimulus. Experiment 2, using similar settings as Experiment 1, used a two-interval forced-choice design to test whether discriminating high spatial frequencies that requires covert attention to parafoveal stimuli is better during the fixation of bright disks that entails a small pupil size, and hence better eyes' optics, as compared to fixating dark disks that entails a large pupil size, and hence poorer eyes' optics. As in Experiment 1, we observed large modulations of pupil size depending on the luminance of the gazed stimulus, but pupil dynamics was more variable, with marked pupil dilation during stimulus encoding, presumably because the demanding spatial frequency discrimination task engaged attention. However, discrimination performance and mean pupil size were not correlated. Despite this lack of correlation, the slopes of pupil dilation during stimulus encoding were correlated to performance, while the slopes of pupil dilation during decision-making were not. We discuss these results regarding the possible functional roles of pupil size modulations.

Pupillary responses to light offer a convenient and objective way to quickly assess the functional health of the anterior afferent visual pathways. We here present a proof of concept of an innovative one minute pupillary test consisting in 9 visual sub-regions simultaneously modulated in luminance at 9 different temporal frequencies. The spectral power of the sustained pupillary responses evoked by this display over 45 seconds of passive fixation distinguishes patients with retinal and optic nerve diseases from healthy participants with remarkable sensitivity and specificity, at both global and local scales. Reliable and fast, this test could ease patient care and allow screening for, and following-up, chronic ophthalmic diseases whose prevalence worryingly increases worldwide.

Pupillary responses to light offer a convenient and objective way to quickly assess the functional health of the anterior afferent visual pathways. We here present the characteristics of Pupil Cycle Time (PCT) obtained with a computerized biofeedback setting in patients with retinal and optic nerve diseases. The spectral analysis of the sustained pupillary oscillations elicited over 45 seconds of passive fixation of colored displays with different spatial configurations provides relevant information that allow distinguishing patients from healthy participants with good sensitivity and specificity. PCT measures done with this method could complement the current functional examination of chronic ophthalmic diseases whose prevalence worryingly increases worldwide.