Close your eyes, and try to imagine a purple polar bear.
If you were at all successful, you were witness to a miracle of brain function. You read the words “purple”, “polar” and “bear” embedded in the context of an everyday sentence, and up popped a real image of an animal that has never been seen in reality. If you were particularly good at what psychologists call imagery, it might have seemed so real that you could have reached out to touch it.
Imagery is one example of top-down control – there is no visual input from a purple polar bear, and yet you manage to recruit the same neural resources that would give rise to you seeing a purple polar bear, should one exist. How is this trickery of the mind achieved?
In an experiment recently carried out by researchers at CalTech, Moran Cerf and his colleagues were able to record neural activity in real time during the process of imagery. Patients with severe epilepsy are sometimes treated through a surgical operation to remove the part of the brain causing the seizures. However, the surgeons do not always know which part should be removed. In order to find out, they first implant electrodes into the brain to monitor the patient and pinpoint the focus of the seizure when it occurs. For the most part, these electrodes are picking up otherwise normal brain activity. And it is during these periods of quiescence that Moran and his colleagues asked patients if they would like to take part in a ground-breaking experiment.
In a previous paper, the same team showed that individual neurons in the medial temporal lobe (a structure deep in the brain involved in storing memories) had surprisingly specific properties. One neuron, for instance, only fired to pictures of Halle Berry, but not other actresses. This cell also fired to Halle Berry’s name printed on the screen – in other words, the cell was concerned with the concept of Halle Berry, but not with how that concept was triggered. In their new study, they recorded similar neurons, found out which concept they liked, and then connected them up to a decoder.
The crucial twist was to use the decoded activity to control noisy pictures of the concepts favoured by different neurons, and then feed these pictures back to the patient on a computer monitor. The experimenters chose one of these concepts as a “target”, and one as a “distractor”. If the activity of the target concept increased, the target image was made more prominent in the display. If the activity of the distractor increased, its image was made clearer. The patient’s task was to control the activity of these single neurons in order to “fade in” the target image on the display.
Let’s pause for a moment to re-read the previous sentence. The patient’s task was to control the activity of single neurons. There are several 100 billion neurons in the human brain. How can the patient begin to know which neuron needs to increase in activity to complete the task? The researchers left this part up to the patients, letting them explore strategies until amazingly, they succeeded. On trials in which they had to bring Marilyn to the fore, they managed to drive up the activity of the Marilyn neuron in order for the decoder to give them more of Marilyn on the screen. This can be seen in the image above – when Marilyn was the target, then the pattern of activity in the volunteer’s medial temporal lobe (represented by the horizontal traces) became more Marilyn-like. When Josh Brolin was the target, the activity diverged in the opposite direction. By the power of thought alone, the patients were able to instruct their medial temporal lobes to bring forth a category at will, and produce an image on the screen.
Admittedly, the experiment was designed to make the patient’s job a bit easier. When the category Marilyn was activated, this produced a bit more Marilyn on the screen, which presumably led to more Marliyn category activation, thus more Marilyn on the screen, and so on. This positive feedback process might have made thought-control easier than if the same concepts were used to drive something more abstract, such as the position of a cursor. Still, the question needs to be asked – how were they doing it? The researchers suggest that because the medial temporal lobe has many neurons dedicated to a single category, “cognitive control strategies such as object-based selective attention permit subjects to voluntarily, rapidly and differentially up- and downregulate the firing activities of distinct groups of spatially interdigitated neurons”.
In other words, by focusing on the purple polar bear, you alter the activity of neurons representing particular visual properties, such as purple, bear, etc. But if one part of your brain is being focused, which is the part doing the focusing? Where is the “you” in control? Things get even more complicated when we learn that certain areas of prefrontal cortex (a region traditionally involved in self-control) can themselves be self-controlled.
An alternative view is that “exerting control” just is the dynamic interplay of ongoing brain activity, visual input, and the experimenter’s instructions. How these interactions play out in real time is currently beyond the grasp of brain science. By decoding the concept “Marilyn Monroe” from deep within a person’s brain, the CalTech team has taken us one step closer to that goal.