Binocular retinal signal integration in non-image-forming visual pathways

Dr Maydel Fernández-Alonso is a postdoctoral researcher in the Sensory and Circadian Neuroscience Group at the Max Planck Institute for Biological Cybernetics in Tübingen. Her work bridges vision science, visual display technology, and chronobiology. In this seminar, she will share recent findings from her research on binocular integration and melanopsin signalling in humans.

Abstract

The intrinsically photosensitive retinal ganglion cells mediate multiple non-image-forming functions through the retinohypothalamic tract, which connects the retina to the Suprachiasmatic nucleus (SCN). Through this pathway, light influences circadian physiology, including the regulation of melatonin, a hormone produced by the Pineal gland in the evening and night, whose secretion is suppressed by light exposure in a dose-dependent manner. It remains unclear how signals from the two eyes are combined in the SCN. Previous human and animal studies have hinted at the existence of a binocular facilitation effect.

We present the results of a study examining binocular retinal signal integration in non-image-forming visual function. 55 healthy adults (31 female, 26±5 years old) completed a 3-week circadian stabilisation protocol. They attended four 6-hour in-laboratory evening light exposure sessions, with measures of salivary melatonin, core body temperature, psychomotor vigilance, and subjective alertness taken throughout. Different light stimuli were presented using a calibrated virtual reality headset two hours before habitual bedtime.

We found strong evidence of binocular facilitation in melatonin suppression: binocular exposure to 90 lux per eye produced 2x suppression than monocular exposure to 180 lux, despite equivalent total input. To investigate the temporal dynamics of this mechanism, we employed in-phase and counter-phase flicker at multiple frequencies, and found evidence that binocular combination in the SCN operates over a slow timescale. Together, these findings provide evidence for a nonlinear binocular combination in human circadian photoreception.