Dr Jon Simons
Jon Simons explores how the brain helps you keep a grip on reality.
During gatherings of old friends, it rarely takes long for people to start reminiscing about the past. You can find yourself mentally transported back in time, re-living a fondly-remembered episode as if it were playing out in front of you once again. Except – how do you know that you were actually there when the event originally took place? How can you be sure that you’re remembering a faithful representation of what happened, as opposed to a fictitious recollection of an event that might have been entirely imagined? In short, how do we determine whether our memories are real?
My research group has spent the last few years pondering these questions, seeking answers by undertaking experiments using cognitive neuroscience methods like functional brain imaging of healthy volunteers and studies of neurological and psychiatric disorders, as well as of normal aging. Our aim is to understand how the brain supports our capacity to distinguish what is real from what we imagined, an ability termed “reality monitoring” that is vital for maintaining confidence in our memories, and in understanding ourselves as individuals with a past and a future. In characterising how these processes might be organised in the brain, we can better understand the way in which they may break down in disorders like schizophrenia, in which perceptions of reality can be altered. We can also improve our understanding of how our memories change as we get older, and why some people seem to be better than others at accurately recalling the past.
Among the research methods used in my group is the brain imaging technique of functional MRI. This method provides the ability to observe changes in brain activity that occur when people undertake a cognitive task such as remembering the context in which a previous event was experienced. We have used the technique to isolate the brain regions that are involved when people try to remember whether an event was previously imagined or did actually occur. One brain area that has emerged as playing a key role in discriminating imagination from reality is anterior prefrontal cortex. This is a region right at the front of the brain, just behind your forehead. It is an area that, in relative terms, is roughly twice as large in the human brain as in even our closest non-human cousins, the great apes. It is thought to be among the last areas to fully develop achieve myelination, the neurodevelopmental process that continues into adolescence and enables nerve cells to transmit information more rapidly, allowing for more complex cognitive abilities. As such, although the functions performed by this area are not well understood, they have generally been considered likely to be among the “higher” levels of human complex cognition.
In the last few years, we have tested whether anterior prefrontal cortex is involved in distinguishing real from imagined events. In one experiment, for example, we presented volunteers either with well-known word-pairs like “Laurel and Hardy” or with the first word of a word-pair and a question mark (“Laurel and ?”). In the latter condition, participants were instructed to imagine the second word of the word-pair. Later, we scanned participants’ brains using functional MRI while they tried to remember whether they had seen or imagined the second word of each previously-encountered word-pair. A number of brain areas showed activity that could be related to general memory retrieval processes. But the one region consistently to emerge across a number of similar experiments as contributing to the distinction between seen and imagined information has been anterior prefrontal cortex.
One of the applications of this work has been to inform understanding of the cognitive dysfunction seen in clinical disorders, such as schizophrenia. Although schizophrenia can vary in its presentation, among the positive symptoms often observed are hallucinations, whereby patients report, for example, hearing voices when none are present. It has been suggested that these symptoms may result from a difficulty in discriminating between information that is perceived in the external world and information that is imagined. For example, you might imagine a voice conveying a message, but misattribute that voice as real, coming from another person.
We have tested a number of the predictions that arise from this suggestion. First, individuals with schizophrenia have been shown to be impaired on the kinds of seen vs. imagined memory tasks that elicit anterior prefrontal cortex activity. Second, the anterior prefrontal area we have identified overlaps closely with an area that tends to be functionally disrupted in schizophrenia. Finally, healthy volunteers who exhibit reduced levels of activity in this region tend to make more of the misattribution errors typically observed in schizophrenia, mistakenly endorsing imagined items as having been seen.
Recently, we have extended these findings to try to explain the variability in reality monitoring performance that is typically seen even in apparently healthy individuals. Given the same task to distinguish seen from imagined words or objects, some people will score highly, correctly attributing 80% or more to the correct source, whereas others will struggle to perform that well. We have uncovered evidence that this variability may be linked to the presence or absence of a particular brain structural folding pattern in the anterior prefrontal cortex, known as the paracingulate sulcus. This structural variation is one of the last cortical folds to develop in utero and for this reason varies greatly in size between individuals in the general population. We have discovered that healthy adults whose MRI scans indicate an absence of the paracingulate sulcus appear to demonstrate reduced reality monitoring ability compared with other participants.
Thus, although there is much work to do before we can claim to understand the functions supported by anterior prefrontal cortex, evidence is mounting that one of its key roles may be to help us keep a firm grip on reality.