Researchers establish the role of the hippocampus in detecting novel stimuli.
The ability to detect and respond to novel stimuli is essential to survival. A change in the environment may, for example, signal the approach of a predator, a vehicle, or some other danger. The brain detects novel stimuli by continuously monitoring its environment, ignoring stimuli that remain unchanged but immediately attending to anything that changes. This is easily demonstrated: if you grasp an object and keep your hand perfectly motionless, you will, after a short period of time stop being aware of the sensation of the object in your hand, because the sensory inputs from the hand remain unchanged. This mechanism evolved to prevent the brain from expending its limited resources to repeatedly process the same information. But any slight movement of your hand will result in a change of sensory inputs, and your brain will alert you to this change very quickly.
As well as detecting stimulus novelty, the brain also attends to what is called associative novelty: familiar objects or stimuli that have been arranged in unfamiliar configurations, such as the rearrangement of furniture in a room, or a friend’s new hairdo. Central to the brain’s ability to detect both stimulus novelty and associative novelty is a structure called the hippocampus, situated on the inner surface of the temporal lobe. The circuitry in the hippocampus is believed to carry out computations that compare past experiences with present ones. Predictions of how experiences will pan out are generated by the hippocampal circuitry. These predictions are based on past experiences, and are compared with new experiences. Changing stimuli are thought to be detected if there is a mismatch between the prediction and the actual sequence of events. The mismatch sets off alarm bells, indicating that there is something novel about the information being processed.
Although many studies investigating the role of the hippocampus in detecting novel stimuli, very few have addressed its role in associative novelty, and there is no direct evidence that it actually generates predictive models of how experiences will unfold. Now, a neuroimaging study by Dharshan Kumaran and Elaine Maguire, of the Institute of Neurology‘s Wellcome Department of Imaging Neuroscience in London, investigates the role of the hippocampus in detecting novel stimuli. The study was published earlier this week in PLoS Biology.
The 17 participants involved in the study were repeatedly presented with the same sets of four pictures, which were shown sequentially. Images of household objects, animals and cars, but not faces, were used. This is because faces are known to processed differently from other images. The image sets were re-presented under three different conditions: in one, the images were shown in exactly the same sequence as they had been in the first instance; in the second, two of the four images were shown in a different order; and, in the third, the four images were presented in a completely different order:
While the participants viewed the image sequences, activity in their hippocampi was monitored by functional magnetic resonance imaging (fMRI). Because of the way in which the experiment was designed, a mismatch between the predicted and the actual sequence of images arises only upon presentation of the third image in the second condition, in which the order of the final two images is reversed. It was indeed found that a burst of activity in the hippocampus accompanied the presentation of the third image in the second experimental condition. Thus, the first sequence of images is what the participants came to expect; the third image in the second image set does not match these expectations, and results in increased hippocampal activity.
In contrast, activity in the entorhinal cortex, which is adjacent to the hippocampus, was seen to increase during the presentation of the second and third sets of images. The activity in the entorhinal cortex may reflect the phenomenon of repetition suppression, alluded to in the first paragraph, whereby neural activity is reduced in response to the presentation of familiar stimuli.
The findings therefore support the hypothesis that the hippocampus functions as an associative mismatch detector, whereas the entorhinal cortex is involved in detecting stimulus novelty per se.
A recent hypothesis is that the perception of an initial sensory stimulus triggers the recall of stored representations of previous, similar events. This recalled event, presumed to be generated by the CA3 region of the hippocampus, is the basis of the prediction with which the new event is compared. The recalled sequence of events is transmitted to the CA1 region, where, according to the model, associative mismatch detection takes place. This study provides more insight into the computations performed in the hippocampus, and it may also shed some light on the involvement of this region of the brain in episodic (or declarative) memory.