March 22, 2013
Brain’s Visual Cortex Activates For Reward Even When Stimulus Is Removed
Alan McStravick for redOrbit.com - Your Universe Online
Pavlov, you may know, was famous for conducting a study where a stimulus was introduced and the reward for that stimulus was slowly removed, with the aim of observing whether or not a physiological response would continue to be registered despite the lack of reward.
In this most recent study, neurophysiologists Wim Vanduffel and John Arsenault removed the stimulus and kept the reward. Their findings showed once rhesus monkeys had learned to associate a picture with a reward, the reward by itself became enough to alter the activity in the monkeys´ visual cortex. Their findings, supported through the use of functional brain scans, have been published in the journal Neuron.
According to the research team, our visual perception is not determined solely by retinal activity. As a point of fact, they state there are other factors important in influencing the processing of visual signals in the brain.
“Selective attention is one such factor,” according to Professor Vanduffel. “The more attention you pay to a stimulus, the better your visual perception is and the more effective your visual cortex is at processing that stimulus.”
Vanduffel continues, “Another factor is the reward value of a stimulus.” He claims when a visual signal becomes associated with a reward, it affects the processing of that visual signal. The purpose of this study was to investigate how a reward influences activity in the visual cortex.
As mentioned above, the team meant to create a response in their subjects, ala Pavlov, though completely opposite. Vanduffel, in describing their own study, first offered a general description of Pavlov´s study, saying, “Think of Pavlov giving a dog a treat after ringing a bell. The bell is the stimulus and the food is the reward. Eventually, the dogs learned to associate the bell with the food and salivated at the sound of the bell alone. Essentially, Pavlov removed the reward but kept the stimulus. In this study, we removed the stimulus but kept the reward.”
Arsenault, Vanduffel and colleagues presented images projected on a screen for their rhesus monkey subjects. Immediately following the image projection was a reward of juice for the monkeys. This is a standard in classical conditioning. However, later in the study, the monkeys would receive their juice reward while the screen was blank. Through the use of functional magnetic resonance imaging (fMRI) brain scans performed during the study, the researchers were able to determine that the visual cortex of each monkey was activated solely by being rewarded, despite the fact there was no image associated with the reward.
However, the visual cortex activations were not, as expected, spread through the whole visual system, but rather, were instead confined to the specific brain regions responsible for processing the exact stimulus used earlier in the conditioning. What the researchers drew from this information was that information about rewards was being sent to the visual cortex to indicate which stimuli were associated with the rewards.
Additonally, the team found these reward-only trials actually strengthened the cue-reward associations. As the researchers explained, this equates to giving an extra treat to Pavlov´s dog after a conditioning session and then noticing before the next conditioning session that the dog is salivating twice as much as before. Basically, their findings showed rewards could be associated with a particular stimuli over longer time scales than was previously believed possible.
While the results of their study clearly presented a “what," they did not definitively present a “why." The question of why the visual cortex reacts selectively in the absence of a visual stimulus on the retina is, as yet, unanswered. The team, however, has their theories.
One potential explanation has to do with the brain chemical, dopamine. “Dopamine is a signaling chemical (neurotransmitter) in nerve cells and plays an important role in processing rewards, motivation and motor functions," said Vanduffel.
“Dopamine´s role in reward signaling is the reason some Parkinson´s patients fall into gambling addiction after taking dopamine-increasing drugs. Aware of dopamine´s role in reward, we re-ran our experiments after giving the monkeys a small dose of a drug that blocks dopamine signaling. We found that the activations in the visual cortex were reduced by the dopamine blocker. What´s likely happening here is that a reward signal is being sent to the visual cortex via dopamine,” he concluded.