What is anhedonia, and why is it so important?  

More severely
depressed patients take that a step father.  It becomes an
actual
distortion of perception.  They say that nothing tastes good,
everything tastes the same; perhaps they say that everything tastes
like cardboard.  In the most severe cases, a patient might
literally starve to death.

Likewise, impaired hedonic capacity occurs upon a spectrum of severity.
 If you question persons with severe depression, you find that
the
inability to experience pleasure is only part of
the problem.
 Reduced hedonic capacity is on the mild
end of the spectrum.  Anhedonia is on the
severe end of the spectrum.

A more complete understanding of anhedonia comes from an expanded
definition,
taking into account the full breadth of the spectrum.  I would
say
that anhedonia is not only the inability to experience pleasure, it is
the inability to experience any positive feeling whatsoever.
 It may seem like a slight distinction, but it is crucial to
understanding both the experience of the patient, and the impact that
this particular symptom has upon the patient.

In the most severe cases, patients are unable to experience anything
positive: no pleasure, no satisfaction, no fulfillment, no sense of
accomplishment, no sense of worth, no sense of meaningfulness.
 Just nothing.  

Many positive experiences have little, if anything, to do with
pleasure.  

Ordinarily, when a person goes through a day, there are innumerable
little positive feelings.  You wash the dishes after
breakfast, so
you know you’ll come home to a clean kitchen.  On the way to
work,  you notice how the
light filters through the trees.  You leave work, knowing you
have
done all your paperwork; it’s done.
 You stop and fill the tank with gas, and feel good knowing
you
won’t have to be inconvenienced by it tomorrow.  You have a
good
workout.  Your legs hurt, your arms hurt, you can hardly
breathe.
 It would be odd to call that pleasure,
but it can be a rewarding experience, even if it is not pleasurable.

I realize that different people may define “pleasure” in different
ways.  The point is not to get hung up on what is and what is
not
pleasure.  The point is that anhedonia, in its most severe
form,
involves the loss of all positive feelings.  In other words,
it
involves the complete loss of any kind of positive reinforcement.

One consequence of this, is that the person with anhedonia cannot do
anything to make himself or herself feel better.  This can
lead to
a sense of hopelessness and/or helplessness.  It also can lead
to
problems in relationships.  Other people do not understand why
the
patient is acting the way she or he is acting.  The patient
knows
that others do not understand, which adds to the experience of
alienation.  

Thus, we come to understand why anhedonia is one of the core symptoms
of depression.  The presence of anhedonia is likely to be the
cause, or part of the cause of many of the other symptoms: loss of
appetite, impaired libido, low motivation, loss of interest, and so
forth.  Furthermore, the presence of anhedonia ofter leads a
depressed person to do exactly the opposite of what he or she should do
to try to get out of depression.  

Aside from formal treatment with medication and psychotherapy, a
depressed person could benefit from eating right, getting some
exercise, spending time with supportive people, pursuing meaningful
activities, and getting the right amount of sleep at the right time.
 All of these are difficult or impossible in the presence of
anhedonia.  So it is one symptom that, once established, tends
to draw the patient further into the illness.

The neurochemical basis of anhedonia is being elucidated, but is beyond
the scope ofa “basic concepts” post.  Interested readers may
wish to visit this article: href="http://apt.rcpsych.org/cgi/content/full/9/6/446">Neural
systems underlying affective disorders.  A more
person, human-interest view can be found here:   href="http://www.mcmanweb.com/no_pleasure.htm">No Pleasure,
No Reward – Plenty of Depression: