How the Posterior Parietal Cortex Makes Meaning Out of What We Sense

How does the human brain make sense of our busy, ever-changing environment?

It’s a question that has plagued neuroscientists since the birth of the field. Researchers now understand that our dominant senses—taste, touch, smell, sight, and hearing—take in information via specialized receptor cells and relay it as electrical signals to be processed by higher order areas of the brain. Such processing is critical to perception: Our memories and experience help put that sensory information into proper context. But where and how such contextual processing is taking place has remained an open question. Two studies recently published by researchers at Italy’s Scuola Internazionale Superiore Di Studi Avanzati (SISSA) suggest that a brain area called the posterior parietal cortex (PPC) is critical to adding meaning to sensory information by pulling in data from the past.

Understanding the prior and contraction bias

Imagine one of the smoke alarms in your home has a low battery, emitting an auditory warning beep every 30 seconds. At first, you aren’t sure which alarm in the house requires maintenance:  you’ll have to use the tell-tale chirp to locate it. As you scurry around the house, looking for the source of the annoying beep, your brain is doing important work. It is storing in memory the sound of the alert so, as you search for the offending alarm, you can compare the sound of the last beep to the current one to see if you are moving closer to the source of the sound. As you experience dozens of beeps during your search, the brain averages the memories of all those alarms into what cognitive psychologist’s call the “prior.”

“Priors are best guesses, in a sense. They are based on a history of values that are integrated to help us give a prediction for the next value,” says Mathew Diamond, director of the Tactile Perception and Learning Lab at SISSA. “When a stimulus ends, the brain keeps a shadow or echo of that stimulus. But it starts to fade away quite quickly. As it fades, it’s harder to call up the precise, exact memory of what you experienced. So the brain searches for the most likely representation, a sort of average of similar experiences.”

The act of brain conjuring that most likely representation, a regression to the mean of similar stimuli, is referred to as “contraction bias.” Such a bias can greatly help us put the world in proper context, says Takaki Komiyama, a neurobiologist at the University of California, San Diego.

“Sensory driven behavior is strongly biased by the history of the last minute or so,” he says. “Humans rely on the history of their senses, their actions, and their outcomes to decide what they want to do in the future. The prior helps us decide what we should do next, how to best adapt. They help us better make predictions about what’s going on in the environment.”

Previous research by Komiyama’s laboratory showed that the PPC affects bias when it is driven by choice outcomes. When the group inactivated the PPC in mice, they animals were no longer heavily influenced by the consequences of past choices. Those results were published in the December 2017 issue of Nature Communications.

But that study left open questions. Might the PPC be the brain area where the prior is generated from sensory information and maintained for comparison as well?

Where in the brain is the sensory prior?

To see if they might find evidence of the prior in the PPC, Athena Akrami, formerly of SISSA and now a Howard Hughes Medical Institute (HHMI) post-doctoral fellow at Princeton University, trained rats to discriminate between two similar auditory tones separated by a few seconds. Like so many other studies before, they found that rats, over hundreds of trials, shifted toward the “prior,” a mean representation of all previous stimuli when they made their choices. When the researchers used optogenetics to silence the PPC, however, they found that the rats performed significantly better on the discrimination task. This, Akrami says, surprised them.

“We expected to see impairment—or maybe not see any changes at all,” she says. “But we saw this strong effect. When the sound was not influenced by the memory of past trials, there was a lot less noise and the animals did better on the task because they could not use the prior.”

When Akrami and colleagues looked at electrophysiological recordings of PPC neurons, they also discovered that, as the rat experienced more trials of the discrimination task, the more it influenced their choices—suggesting PPC neurons are representing the prior rather than the last sound heard.

“The prior pulls the memory towards this averaged value,” says Diamond. “And it looks like the mechanism for pulling the memory of the stimulus towards the prior involves neurons in the PPC.”

What’s next?

There is still quite a bit of work to be done to understand how the brain takes in critical sensory information, integrates it, and then puts it in proper context, Akrami says. But she thinks the PPC is a great place to start those investigations. Komiyama agrees.

“These two studies are very compatible and show that whether contraction bias is based on outcome history of stimulus history, it seems to be funneled through PPC to influence the subsequent behavior,” he says.

Understanding priors may help offer new insights into disorders like dyslexia, a language disorder where people have difficulty reading as well as interpreting letters and symbols, Akrami says.

“Some studies suggest that dyslexic individuals don’t show a contraction bias. They do not seem to be impacted by past sensory input,” she says. “Maybe dyslexic individuals can’t build appropriate priors. That would explain a lot because, in reading, we are constantly having to update information about letters and words. If you don’t have those priors, you can’t predict what’s coming next or how all the words fit together to form an idea. It’s a compelling idea.”

Diamond says that understanding such a complex, cognitive function that is so important to everyday perception is worthwhile in its own right.

“The integration of history in such a way to provide context for the present is crucial to navigating the world,” he says. “We don’t experience the world as a sequence of isolated snapshots where we have to interpret each based only on the information we have in that instant. Each moment, as we understand it, is embedded in the past. Being able to understand how the cortex mediates all this is enormously important.”