Wednesday, May 2, 2012

Feeling blue, seeing gray: Reduced contrast sensitivity as a marker for depression : Neurophilosophy

I wonder if one day, eye doctors will be diagnosing depression?  I am particularly interested in the link between depression and contrast sensitivity as I suffer from suppressing one eye.  At least part of the reason that I have been suppressing one eye is due to contrast sensitivity, the ability to detect luminance (light) contrast. I have been working diligently on my RevitalVision program to correct this. 

Contrast sensitivity is a very important measure of visual function, especially in situations of low light, fog or glare, when the contrast between objects and their background often is reduced. Driving at night is an example of an activity that requires good contrast sensitivity for safety.

If you have low contrast sensitivity, you may have problems with night driving, including difficulty seeing pedestrians walking alongside poorly lit streets. Or you might notice that your eyes tire more easily while reading or watching television.

A person with normal visual acuity but poor contrast sensitivity might see the trees in the foreground clearly (high contrast), but have trouble seeing the contours of the mountains against the sky in the background (low contrast).
Poor contrast sensitivity also can increase your risk of a fall if you fail to see that you need to step down from a curb onto similarly colored pavement.

Contrast sensitivity has also been implicated in seasonal depression as well.  Clinical depression can enhance contrast sensitivity when stimuli elicit strong parvocellular responses. These enhancements implicate differences in retinal functionality. Mechanisms that link neuromodulatory activity to retinal signal processing are proposed.

Portrait of Suzanne Bloch (1904), from Picasso...
Portrait of Suzanne Bloch (1904), from Picasso's Blue Period. São Paulo Museum of Art collection (Photo credit: Wikipedia)

DEPRESSION has long been associated with vision - and to colour perception in particular - and the link between them is evident in everyday language. Depression is, of course, often referred to as "feeling blue", and those who suffer from it are sometimes told to "lighten up". The link can be found in art, too - Picasso's so-called "Blue Period", for example, which was brought on by the suicide of his close friend Carlos Casagemas, is characterised by a series of striking paintings in shades of cold blue, which express the deep melancholy he felt at the time. 

The interesting thing is that when patients recover from depression, they often say that the world is more colorful and more vibrant.  Could they be talking about what they actually see as opposed to describing a mental state in a metaphoric way?

Although the association between depression and colour is largely metaphorical, there is actually some evidence that they are closely linked. The most recent comes from a new study by German researchers published in the journal Biological Psychiatry. The study shows that depressed people have reduced sensitivity to contrast, and therefore that they may perceive the world differently from others. It also suggests that depression can be diagnosed by objective measurements of electrical activity in the eye.

Specifically, the researchers looked for differences between the two groups of participants in activity reflecting contrast gain, the process by which cells in the retina adapt to variance in the light intensity of the visual scene so that the amount of information extracted from it can be maximized. They found a significant difference in the contrast gain-related activity between the depressed patients and controls. The participants diagnosed with depression displayed a marked reduction in contrast gain when compared with the controls. The reduction was observed in both medicated and unmedicated patients. Those taking medication for their depression, however, had slightly lower depressivity scores and correspondingly better contrast gain than unmedicated patients. 

Furthermore, the reduction in contrast gain was strongly correlated with the severity of depression - the more severe the depression, the greater was the observed reduction in contrast gain. No difference was observed between patients with recurrent depression and those experiencing their first episode of the condition, or between depressed patients taking selective serotonin uptake inhibitors such as fluoxetine (Prozac) and those taking tricyclic antidepressants such as imipramine. The intensity of the treatment, or dose being taken, did not affect the reduction in contrast gain observed in the depressed patients. Finally, the researchers could predict, with an accuracy of greater than 90%, which of the participants had been diagnosed with depression on the basis of their electroretinographic recordings.

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