Introduction

The presence of photoreceptors have been shown on the skin of blinds and therefore they cannot see the object even in the presence of bright light but as suggested in this review that they are benefited by the light therapy which is the only therapeutic method for seasonal affectiveness disorder (SAD).We have here discussed the reasons why partial blind respond less to this treatment compared to total blind subjects,We have also indicated that even  sighted subjects have extra visual function of the eye in humans. It looks like that blinds have retained this extra visual function of the eye.

When light falls on the object only then we can see the object. It's a paradoxical situation, where the blind subjects cannot see light, but they respond to light.  Light induces important physiological responses in the majority of animals, such as pupil constriction, pineal melatonin suppression, and the entrainment of circadian rhythms [1]. Light plays a fundamental role in the behavior of almost all organisms, with detection mechanisms that are likely to be universally mediated by opsin-based photopigments and the biochemical photocascades they activate. Skin is also able to detect light by using a Photoreceptor [2], probably blinds get information through photoreceptors (cryptochromes) on the skin.

Seasonal Affectiveness Disorder (SAD):

Human beings experienced predictable periods of daily light and dark driven by the solar day, which allowed for entrainment of intrinsic circadian rhythms to the environmental light–dark cycles [3]. Since the adoption of electric light, however, pervasive exposure to nighttime lighting has blurred the boundaries of day and night, making it more difficult to synchronize biological processes. Many systems are under circadian control, including sleep–wake behavior, hormone secretion, cellular function and gene expression. Circadian disruption by nighttime light perturbs those processes [4]. Seasonal Affectiveness Disorder (SAD) is believed to be related to light and nearly three times as many blind people suffer from SAD compared to people with normal vision. SAD is a form of depression that affects people during the winter months.SAD occurs in the cold, dark months of October to March in the northern hemisphere (5 Chronopharmacology). In other words, blind can also sense dark, the opposite of light. Sufferers experience mood changes and symptoms that are similar to depression, when there is less sunlight.

Causes for SAD:

It was puzzling  how some people who are completely blind—lacking the eye’s photoreceptors known as rods and cones—can experience SAD.It was recently found out that BECAUSE BLIND PEOPLE retain a newly discovered system of light-detecting cells, they, too, can suffer from seasonal affective disorder (SAD). We found [2]  that humans can detect light through their skin due to the presence of cryptochromes.This eye-like ability of skin to sense light triggers the production of melanin within hours Photoreceptors contain chemicals that change when they are hit by light. This causes an electrical signal, which is then sent to the brain along the nerve. Different types of photoreceptor allow us to see the full spectrum of light. Beyond mediating vision, light also has a crucial role in regulating circadian rhythms [5]. But when researchers in the Netherlands tested this idea by exposing just the skin of SAD patients to bright light, they found the treatment had no effect at all. How, then, are they detecting light?These observations led to the discovery of an additional photo­receptor system in the retina of humans and other mammals consisting of a small number of photosensi­­tive retinal ganglion cells (called pRGCs). These cells are most sensitive to blue light, and, significantly, blue light is most effective in alleviating the symptoms of SAD. We think that blind people can develop SAD because their other photoreceptor system—the pRGCs—remains intact. 

It was observed that mice lacking rods and cones were nonetheless able to synchronize their circadian rhythm to the light/dark cycle. These observations led to the discovery of an additional photo receptor system in the retina of humans (Extra visual functions of the eye) and other mammals consisting of a small number of photosensitve retinal ganglion cells (called pRGCs). These cells are most sensitive to blue light, and, significantly, blue light is most effective in alleviating the symptoms of SAD. We think that blind people can develop SAD because their other photoreceptor system—the pRGCs—remains intact. Likewise, although there are no known studies of light therapy in those who are visually impaired, we suspect light could be used to treat SAD symptoms in blind patients.

Recently it was established that the cause of a very complex disorder is actually   found in the retina of the eye--and not in the brain, it is also said   that those with poor vision suffer the most [6].   Fonken et al found that Dim light at night disrupts molecular circadian rhythms then the total darkness [7]. Light exerts a range of powerful biological effects beyond image vision, including mood and learning regulation. While the source of photic information affecting mood and cognitive functions is well established, viz. intrinsically photosensitive retinal ganglion cells (ipRGCs), the central mediators are unknown.

Blind people may unknowingly perceive light:

According to Hageman, it is possible that in some blind people, light still reaches the brain. This may help to explain why blind people seem to suffer less from winter depression compared to those with impaired vision.

In recent years, scientists have discovered cells in the retina, which have nothing to do with the conscious perception of light but are nonetheless capable of detecting light. Even if a person is completely blind and has no conscious perception of light, these cells may be sending light signals to the brain.

Blind people may benefit from light therapy:

The new research not only provides new information about the mechanisms of SAD. It also suggests that light therapy could be an effective treatment for blind people suffering from the condition. "The good news is that it makes sense to offer light therapy, and this is also true for the blind," says Hageman.

So long as the light capturing cells in the retina are at least partially functional, then the use of bright light as a therapy against the winter blues could be an effective treatment [8,9].

Biochemical interactions:
The neurobiology of circadian and seasonal adaptive mechanisms focusing on the suprachiasmatic nucleus (SCN), the indoleamines melatonin and serotonin, and the chronobiology of mood disorders.  Indolamines are known to transduce light signals into cells and organisms since early in evolution, and their role in signalling change of season is still preserved in humans [10]. Melatonin is synthesized primarily in the pineal gland and is the central hormone for internal clock circuitries. The melatonin precursor serotonin is known to modulate many behaviours that vary with season (Fig.1)

Figure 1: Neurotransmitter Serotonin in the presence of Dark getting converted into melatonin, through an intermediate compound N-acetylserotonin.

These photoreceptors are very important for synchronization of our internal biological clock to light and circadian rhythms [4], suggesting that some visually impaired people experience a bigger response in their photoreceptors, than those with normal vision. This suggests that visually impaired people are more sensitive to changes in light, despite their poor vision. One possible explanation could be that there’s an upwards adjustment of specific photoreceptors--cells in the retina that monitor brightness--in the visually impaired [11]

 The adult pineal gland weighs less than 0.2 g, and its principal function is to produce about 30 µg per day of melatonin, a hormone that regulates circadian rhythm through very high affinity interactions with melatonin receptors.The precursor to melatonin is serotonin, a neurotransmitter that itself is derived from the amino acid tryptophan. Within the pineal gland, serotonin is acetylated and then methylated to yield melatonin.

 It is clear that very minute concentrations of N, N-dimethyltryptamine have been detected in the brain, but they are not sufficient to produce psychoactive effects [10].  Seasonal changes impact the melatonin production   due to the photoperiodic alterations of the different seasons which impact the functions of pineal gland. The short photoperiod promotes pineal melatonin production [2]. As a result, during the winter, animals have significantly higher levels of melatonin than in summer. However, the seasonal changes also include temperature changes. Cold temperature per se can upregulate the expressions of gene AANAT and ASMTwhich are the important enzymes for melatonin   conversion from serotonin. Cold temperature is the major factor to induce melatonin production but increased temperature had limited influence on melatonin production The elevated melatonin levels  In addition, melatonin as a potent antioxidant and thermogenic agent would improve the survival chance of animals during cold weather [12] Although most totally visually blind individuals exhibit non entrained circadian rhythms due to an inability of light to entrain the circadian pacemaker, a small proportion retain photic circadian entrainment, melatonin suppression, and other nonimage-forming responses to light. It is thought that these responses to light persist because of the survival of melanospin-containing intrinsically photosensitive retinal ganglion cells (ipRGCs), which project primarily to the circadian pacemaker and are functionally distinct from the rod and cone photoreceptors that mediate vision [13]

It is possible that the special light receptors that subconsciously tell the brain how bright it is are partially destroyed in some visually impaired people. Bright-light therapy (BLT) for seasonal affective disorder is used at an intensity of 10,000 lux for 30-90 minutes daily, usually within 1 hour of arising in the morning. Like any effective antidepressant, BLT has the potential to precipitate a hypomanic or manic episode in susceptible individuals. For them, the brain may not receive enough light to maintain a stable mood, even though they can consciously perceive the difference in brightness [14]. The pathophysiology and clinical specifiers of SAD, which can be seen as a model disorder for chronobiological disturbances and the mechanism of action of BLT.