Glutamate has an excitatory effect on SCN neurons, and PACAP appears to enhance the effects of glutamate in the hypothalamus. Other post synaptic targets of ipRGCs include: the intergenticulate leaflet (IGL), a cluster of neurons located in the thalamus, which play a role in circadian entrainment; the olivary pretectal nucleus (OPN), a cluster of neurons in the midbrain that controls the pupillary light reflex; the ventrolateral preoptic nucleus (VLPO), located in the hypothalamus and is a control center for sleep; the amygdala. Using various photoreceptor knockout mice, researchers have identified the role of ipRGCs in both the transient and sustained signaling of the pupillary light reflex (PLR). Transient PLR occurs at dim to moderate light intensities and is a result of phototransduction occurring in rod cells, which provide synaptic input onto ipRGCs, which in turn relay the information to the olivary pretectal nucleus in the midbrain. The neurotransmitter involved in the relay of information to the midbrain from the ipRGCs in the transient PLR is glutamate. At brighter light intensities the sustained PLR occurs, which involves both phototransduction of the rod providing input to the ipRGCs and phototransduction of the ipRGCs themselves via melanopsin. Researchers have suggested that the role of melanopsin in the sustained PLR is due to its lack of adaptation to light stimuli in contrast to rod cells, which exhibit adaptation. The sustained PLR is maintained by PACAP release from ipRGCs in a pulsatile manner. Experiments with rodless, coneless humans allowed another possible role for the receptor to be studied. In 2007, a new role was found for the photoreceptive ganglion cell. Zaidi and colleagues showed that in humans the retinal ganglion cell photoreceptor contributes to conscious sight as well as to non-image-forming functions like circadian rhythms, behaviour and pupillary reactions. Since these cells respond mostly to blue light, it has been suggested that they have a role in mesopic vision and that the old theory of a purely duplex retina with rod (dark) and cone (light) light vision was simplistic. Zaidi and colleagues' work with rodless, coneless human subjects hence has also opened the door into image-forming (visual) roles for the ganglion cell photoreceptor.