哺乳類動物的晝夜節律，是由一個位於大腦視交叉上核(SCN)的時鐘所管轄。越來越多證據顯示，一種新發現的視網膜光感受器—自發性感光視網膜神經節細胞(ipRGC)，在調節和同步晝夜節律上的重要性。這些ipRGCs對光照反應並不依賴傳統的視桿和視錐細胞，而是透過被稱為黑視素(melanopsin)，並投射於SCN的獨特感光色素進行。此以黑視素為基礎的信號傳導路徑，統稱為非成像(NIF)視覺系統，負責調節同步晝夜週期，瞳孔的持續光反射以及松果體褪黑激素的抑制。近來，一個突破性的研究發現，NIF視覺系統可能和偏頭痛畏光的病生理機制有關。因此，本文將提供NIF視覺系統的功能及其特性的概要。

The circadian system of mammals is governed by a master clock at the suprachiasmatic nucleus (SCN) of the brain. More and more evidences have revealed the significance of a novel photoreceptor of the retina, the intrinsically photosensitive retinal ganglion cell (ipRGC), in mediating the light entrainment of the circadian cycle. The ipRGCs respond to light independently of the traditional rod and cone cells and express a unique photopigment known as melanopsin that projects mainly to the SCN. This melanopsin-based signaling pathway, collectively known as the non-image forming (NIF) visual system, regulates the photoentrainment of the circadian cycle, the sustained pupillary light reflex as well as the inhibition of pineal melatonin. Recently, a ground-breaking study suggests that the NIF visual system may be implicated in the pathophysiology of photophobia in migraine. Accordingly, this article will provide an overview of the functions and distinct properties of the NIF visual system.