Exploring how the brain helps you keep a grip on reality

This is based on a piece about our research that I was asked to write for a thing after recently being selected to receive the Experimental Psychology Society Prize. The EPS is one of the most venerable and respected learned societies in my field, and it was a real honour for our work to be recognised by them.

Whenever old friends get together, it rarely takes long for people to start reminiscing about the past. Amusing (sometimes, bawdy) tales might be told about events that may have occurred many years ago. As a listener, 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?

We have 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 that Marcia Johnson has termed “reality monitoring” which 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.

View of brain showing
anterior prefrontal cortex

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 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 field of memory research, scientists sought to use functional brain imaging techniques to identify brain areas that were active when people undertook complex memory tasks like recollecting the context in which previous events were experienced, but found it difficult to characterise what role anterior prefrontal cortex might play. Some studies reported memory-related activation there whereas other, equally well conducted, apparently very similar studies failed to identify activity in that region. We hypothesised that the discrepancy between studies might be because the kinds of information participants were being asked to remember differed according to whether it had been generated by internal cognitive functions such as thought and imagination, or derived from the outside world by perceptual processes.

Anterior prefrontal cortex (L)
contributes with areas like the
medial temporal lobe (R)
to reality monitoring

In the last few years, we have tested this hypothesis in a number of experiments. We have shown, for example, that activity in anterior prefrontal cortex differentiates between stimuli that were previously seen or merely imagined. To demonstrate this, in one experiment 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 magnetic resonance imaging (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 from our laboratory and others 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 by Chris Frith and others 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.

Regions involved in reality monitoring
overlap with areas dysfunctional
in schizophrenia

We have tested a number of the predictions that arise from this suggestion. First, individuals with schizophrenia are impaired on the kinds of seen vs. imagined memory tasks that we have shown to elicit anterior prefrontal cortex activity. Second, the anterior prefrontal area we have identified overlaps closely with an area that is among those that tend to be functionally disrupted in schizophrenia. Third, 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. Finally, colleagues in San Francisco have shown that patients with schizophrenia exhibit reduced activity in anterior prefrontal cortex during performance of reality monitoring tasks.

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 grip on reality.