Role of the melanopsin photoreceptor
I am concerned with the relationship between sensory processes and higher level of brain function, especially through colour and brightness processing. I also focus on the function of intrinsically photosensitive retinal ganglion cells, (ipRGCs: melanopsin cells) (*1) which play an important role in the brain. Our goal is to answer the question of how these functions operate, how they are combined, and how they are represented in the brain. We have used neurological and psychophysical techniques.
After the discovery of melanopsin cells in 1999, many scientists initially thought that they contributed only to non-image forming functions such as circadian rhythm regulation and pupillary light reflex. These functions do not require pattern/image information that is different from conventional visual functions. However, recent studies suggest that melanopsin cells have been underestimated. They may also play an important role in vision. The melanopsin cells influence pattern and brightness perception. Further evidence reveals that they seem to enable ambient light to influence cognitive processes such as attention, mood, learning and memory.
The challenge in melanopsin-cell research stems primarily from the need to selectively stimulate the melanopsin cells while avoiding the classical visual photoreceptors; rod and cone. The silent substitution known in experimental psychology provides a theoretical basis for achieving independent stimulation of cones (L-, M- and S-cone types), rods and melanopsin cells.
To this end we have developed a multi-primary stimulation system for humans and mice that can stimulate cone, rod and melanopsin photoreceptors independently (Tsujimura et al., Proc Roy Soc Lond. B. Bio 2010).
The following are some of my current research projects on how melanopsin cells contribute to the cognitive process (many of which involve research students and / or colleagues):
1. Brightness perception: How do we feel brightness in the brain?
2. Pattern vision and melanopsin cells: effects of melanopsin on functional contrast acuity.
3. Rod and melanopsin contributions to colour perception.
How does blue light influence our body?
Looking at bright white computer screens often causes difficulties falling asleep at night or waking up in the morning. Using PCs and smart phones late at night, can disturb the body's circadian rhythm, making it hard to sleep. Many studies have reported that the blue light from the white screens influences the wake-sleep cycle.
A new type of photoreceptor that is sensitive to blue light was recently discovered. The cells containing photopigment melanopsin is called intrinsically photosensitive retinal ganglion cells, (ipRGCs: melanopsin cells). The melanopsin cells play an important role in regulating the circadian rhythm as well as regulating pupil size. Since these two streams are not involved in pattern/image vision it is known as a non-image forming pathway.
Since melanopsin cells are sensitive to blue region wavelengths, many scientists have reported an effect on the body, influenced by blue light. However, since blue light stimulates cones as well as melanopsin cells, it is hard to know how melanopsin cells influence the body. The responses to blue light is a mixture of responses of the cone, rod and melanopsin cells.
In our lab we selectively stimulate a target photoreceptor (e.g. melanopsin cell) while keeping the other photoreceptors silent using a silent-substitution paradigm. Therefore, the response to the stimulation should come from the targeted photoreceptor.
Our particular interest is how signals from melanopsin cells and from the classical photoreceptors; cone and rod, are integrated in the brain. To this end, we focus on pupil response that is regulated by melanopsin cells as well as cone and rod photoreceptors.
The specific title of project and a brief description are as follows.
“Identification of neural correlation of cone, rod and melanopsin cells in the non-image forming pathway”
The pupil is a rich source of information, and often involves studies in patients with selective damage to visual pathways. It was found that melanopsin cells (ipRGCs) have a primary role for the regulation of pupil size. We propose studies to investigate the interaction of cone, rod and melanopsin photoreceptor signals in the eye. Although the proposed studies focus on fundamental research, the findings may have long term clinical significance. The clinical usefulness of pupil examination in neuro-ophthalmology may well be enhanced by making pupil responses more specific indicators of disease or recovery of visual function.