Neural correlates of attention
Main article: Psychoneurology of attention
Most experiments show that one neural correlate of attention is enhanced firing. Say a neuron has a certain response to a stimulus when the animal is not attending to that stimulus. When the animal attends to the stimulus, even if the physical characteristic of the stimulus remains the same the neurons response is enhanced. A strict criterion, in this paradigm of testing attention, is that the physical stimulus available to the subject must be the same, and only the mental state is allowed to change. In this manner, any differences in neuronal firing may be attributed to a mental state (attention) rather than differences in the stimulus itself.
In this context it is instructive, though slightly tangential, to mention the Necker cube illusion. This is a great example of the mental preception of a stimulus changing, even though the stimulus itself is unchanged. A recent neural study in monkeys claims to have found a neural correlate to the Necker cube illusion.