neurobiology - Do we understand the non-subjective mechanisms behind pleasure and pain?

Neurobiology is not my field of expertise, but this paper seems relevant:

Kent C. Berridge, Chao-Yi Ho, Jocelyn M. Richard, Alexandra G. DiFeliceantonio (2010) The tempted brain eats: Pleasure and desire circuits in obesity and eating disorders. Brain Research, 1350, 43-64.

What we eat, when and how much, all are influenced by brain reward mechanisms that
generate “liking” and “wanting” for foods. As a corollary, dysfunction in reward circuits
might contribute to the recent rise of obesity and eating disorders. Here we assess brain
mechanisms known to generate “liking” and “wanting” for foods and evaluate their
interaction with regulatory mechanisms of hunger and satiety, relevant to clinical issues.
“Liking” mechanisms include hedonic circuits that connect together cubic-millimeter
hotspots in forebrain limbic structures such as nucleus accumbens and ventral pallidum
(where opioid/endocannabinoid/orexin signals can amplify sensory pleasure). “Wanting”
mechanisms include larger opioid networks in nucleus accumbens, striatum, and amygdala
that extend beyond the hedonic hotspots, as well as mesolimbic dopamine systems, and
corticolimbic glutamate signals that interact with those systems. We focus on ways in
which these brain reward circuits might participate in obesity or in eating disorders.

You may also be interested in these two book chapters:

Smith, Kyle, Stephen V. Mahler, Susana Pecina, and Kent C. Berridge. “Hedonic Hotspots:
Generating Sensory Pleasure in the Brain.” In Pleasures of the Brain, edited by Morten
L. Kringelbach and Kent C. Berridge, 27–49. New York: Oxford University Press, 2009.

A vital question concerning sensory pleasure
is how brain mechanisms cause stimuli
to become pleasurable and liked. Pleasure
is not an intrinsic feature of any stimulus,
but instead refl ects an affective evaluation
added to the stimulus by the brain. That is,
as Frijda expresses it (Frijda, this volume;
Frijda, 2006), a pleasure gloss or hedonic
value must be actively ‘painted’ on sweet
or other sensations to make them pleasant.
Brain mechanisms of pleasure, whatever
they are, must take a mere sensory signal
and transform it into a hedonic and ‘liked’
reward.

Finding the brain mechanisms responsible
for painting a pleasure gloss is a major
challenge for affective neuroscience (Barrett
and Wager, 2006; Berridge, 2003b; Damasio,
1999; Davidson, this volume; Davidson and
Irwin, 1999; Kringelbach, 2005; Kringelbach,
this volume; LeDoux, 1996; Panksepp,
1991; Peciña et al., 2006). Fortunately,
progress on fi nding hedonic generators in
the brain is being made. In this chapter we
focus specifi cally on the neuroanatomical
hedonic hotspots in the brain where neurochemical
signals actually contribute causally
to the generation of pleasure.

Aldridge, J. Wayne, and Kent C. Berridge. “Neural Coding of Pleasure: ‘Rose-tinted Glasses’
of the Ventral Pallidum.” In Pleasures of the Brain, edited by Morten L. Kringelbach
and Kent C. Berridge, 62–72. New York: Oxford University Press, 2009.

Pleasure is not a sensation. What is it then? Nico Frijda's answer in the "pleasure questions" section of this book (which he suggested a number of years ago) epitomizes an emerging consensus among many psychologists and neuroscientists (Frijda, Chapter 6, this book). He notes that pleasure "is a 'pleasantness gloss' added to whatever is pleasant". [...]

Here we ask: how is a "pleasure gloss" encoded in brain activity? Where in the brain is this glossing operation performed and how does it work? Is it possible for neuroscientists to recognize the signature patterns of neural activity that represent a pleasure gloss? These are difficult questions that are only beginning to be addressed. The "pleasure gloss" metaphor, applied to the transformation of neural signals for a stimulus, is like a varnish that is applied on top of a dull object to transform it into a shiny one. Adding hedonic tone to the signal passed on to downstream structures, the neural gloss effectively gives the entire brain a "rose-tinted" hedonic perception of the stimulus as pleasant.

In the context of neural firing signals, our idea is that a particular pattern of neuronal spikes or action potentials in crucial neurons may apply a glaze of pleasure on what might otherwise be an ordinary sensation or action signal.