The human brain is a signal processing system. Input streams contain sensory, motor, reward and possibly more signals mixed. I wonder if neuro transmitters associated with reward increase or decrease memorization of those streams. That would result in a tendency to memorize streams co-occurring with high reward over those co-occurring with low reward. Thus, attractors would settle for better streams and we would perform actions optimizing the reward.
The effect of dopamine signaling is one of the areas where computational neuroscience has provided insight into brain mechanisms, specifically via Reinforcement Learning (RL) models.
Dopamine neurons are activated by rewarding events that are better than predicted, remain uninfluenced by events that are as good as predicted, and are depressed by events that are worse than predicted. By signaling rewards according to a prediction error, dopamine responses have the formal characteristics of a teaching signal postulated by reinforcement learning theories.
To answer your question directly, it does seem the speed/plasticity of learning is modulated by dopamine neurons.
A bit of context
In mammalian brains, a main neurotransmiter linked with reward is Dopamine. This molecule is produced in the Ventral Tegmental Area (VTA) and in the Substantia Nigra (SN).
In a very famous study, Schultz recorded neurons in this two regions (at the time, we didn't really make the difference between them), and realised that the activity of these neurons was linked with reward. More precisely, neurons were coding for Reward Prediction Error, a feature that is implemented in Reinforcement Learning models.
Recently, some researchers decided to adress your question. To do this, they used optogenetics, wich is a technique that permit you to modulate the activity of neurons with light.
You can read the paper here : McNamara et al. Nat. Neurosci. (2014)
What are the main results ?
Mice with increased dopamine neuron's activity had better performance in a spatial task. They were better at learning a new maze. On top of that, the increased activity of VTA neurons induced a higher amount of specific oscillations (called Sharp Wave Ripples) in the Hippocampus, a structure important in spatial learning. We start to have strong evidence that this oscillations are important for memory, as disrupting them impair memory (see Jadhav et al. Science (2012) and Girardeau et al. Nat. Neuosci. (2009) if you are interested in the subject)
Note that this study was done in mice. Their is probably papers making a link between the activity of the VTA and learning using FMRi on humans, but I don't know the litterature enough.