Reconciling one-dimensional valence with distinct mechanisms for reward/punishment It is a widespread notion in psychology that there are two dimensions to emotion: valence (aversiveness or attractiveness of some entity) and intensity. However, recent experimental studies in neuropsychology (e.g. those listed below) have given strong evidence for two distinct pathways which process reward and punishment associated with some input. This in turn compellingly suggests that valence is in fact made up of two independent dimensions, and I'm guessing there are behavioral studies reaching similar conclusions (i.e. that persons are able to have "mixed feelings"). How do we reconcile these views? Are we forced to reject one-dimensional valence on the basis of the evidence?

Distinct pathways for reward/punishment:


Paton, Joseph J., and Kenway Louie. "Reward and punishment illuminated." Nature neuroscience 15.6 (2012): 807.
(http://www.researchgate.net/publication/225043743_Reward_and_punishment_illuminated/file/e0b495227b6d325c3d.pdf)
Fujiwara, Juri, et al. "Segregated and integrated coding of reward and punishment in the cingulate cortex." Journal of Neurophysiology 101.6 (2009): 3284-3293.
Monosov, Ilya E., and Okihide Hikosaka. "Regionally distinct processing of rewards and punishments by the primate ventromedial prefrontal cortex." The Journal of Neuroscience 32.30 (2012): 10318-10330.
A meta-analysis of 397 neuroimaging studies was just published looking at this very question (see here).

They tested three hypotheses: 


Bipolarity - negative and positive are on the same continuum
Bivalence - negative and positive are independent
Affective workspace - valence is supported by a "flexible set of valence-general regions".


Their analyses are rather complicated, but they didn't find much support for the bipolarity or bivalence hypotheses.  Instead, in support of the affective workspace hypothesis, they conclude (emphasis mine):


  These findings suggest that, at the level of regional brain activity, there is no single region or even voxel that uniquely represents positivity or negativity. Limbic tissue, including the anterior insula, rostral ACC/ventromedial prefrontal cortex, dorsal ACC, amygdala, ventral striatum, as well as several other regions including the thalamus and occipitotemporal cortex, appears to contain cells that are part of the brain’s valence-general affective workspace or “affective neural reference space


But they note that some brain regions have preferences for negativity or positivity, meaning that these brain regions may tend to represent negativity or positivity over the other.

This meta-analysis isn't definitive, but it's the best we have right now.