Localization of Neuronal Gain Control in the Pupillary Response

Area of Science:

• Neuroscience
• Ophthalmology
• Visual Perception

Background:

• Multifocal pupillographic objective perimetry (mfPOP) is an emerging alternative to standard visual perimetry.
• mfPOP extracts pupil responses to stimuli, with gain control influenced by stimulus density.
• The precise location of this gain control within the pupillary pathway requires localization.

Purpose of the Study:

• To determine the specific neural locus of gain control within the pupillary pathway during mfPOP testing.
• To differentiate the roles of the retina, pretectal olivary nuclei (PON), and Edinger-Westphal nuclei (EWN) in pupillary response modulation.

Main Methods:

• Eight subjects underwent mfPOP testing with 14 stimulus variants.
• Stimulus temporal density was manipulated at the retinal, PON, and EWN levels by altering presentation rates and restricting stimulus locations.
• Pupil constriction amplitudes were measured and compared across variants with differing signal densities at specific neural levels.

Main Results:

• No significant differences in pupil constriction amplitude were found when retinal or PON signal density varied, provided EWN density was matched.
• Significant reductions in pupil constriction amplitude occurred when EWN signal density differed, even with matched retinal and PON densities.
• These effects were consistent across different stimulus field restrictions (homonymous hemifield, nasal, temporal).

Conclusions:

• The majority of gain control in the subcortical pupillary pathway during mfPOP testing resides at the level of the Edinger-Westphal nuclei (EWN).
• This finding helps refine the understanding of the neural circuitry underlying objective perimetry and pupillary light reflexes.