Our memories and sensory impressions of the world aren’t formed in a vacuum; rather, they are heavily influenced by our recent experiences. If you try to recall the size of a baseball after playing with marbles, the baseball will seem smaller than it really is. But if you look at basketballs first instead of marbles, you will remember the baseball as bigger. Our sensory history essentially ‘contracts’ perception, pushing it toward the average of what we have experienced before.
This phenomenon, known as contraction bias, was first described more than a century ago. But the neurobiological underpinnings of how prior stimuli affect perception are largely unknown. New research, published in Nature in February, suggests that a brain region called the posterior parietal cortex is involved in this process. “This gives us a foothold into unraveling the rest of the neural circuit,” says Carlos Brody, a neuroscientist at Princeton University and an investigator with the Simons Collaboration on the Global Brain, who led the study.
Brody’s team didn’t set out to study contraction bias. Athena Akrami, a postdoctoral researcher in the lab and the study’s first author, was investigating working memory. She trained rats to identify the louder of two sounds presented one after the other. As the rats performed the task, she silenced the posterior parietal cortex using optogenetics.
The posterior parietal cortex has long been thought to regulate working memory. The researchers expected that silencing it would hobble the rats’ ability to identify the louder sound, because the earlier sound must be held in working memory to be compared with the later one. But animals whose posterior parietal cortex was impaired actually improved at the task. “That was a complete surprise,” Brody says. “We scratched our heads for a long time about it.”
To understand what had happened, the researchers computationally modeled the animals’ behavior. The analysis revealed that silencing the posterior parietal cortex interfered with the animals’ use of previous experience in assessing the sound in the task.
Akrami then recorded the electrophysiological activity of neurons in the posterior parietal cortex while the animals performed the task. Each trial consists of two sounds separated by a delay. The neurons’ firing rate during the delay was affected much more strongly by the volume of sounds played in previous trials than by the volume of that first sound — showing that past experience, rather than working memory, is represented in the posterior parietal cortex.
“Once we realized that the brain area we set out to study was really related to the effects of sensory history on working memory, we also realized that we were getting on to answering this hundred-year-old puzzle,” Brody says.
The results suggest that expectations about an experience are encoded in a different region of the brain from the one where the sensory experience itself is processed, says Yonatan Loewenstein, a neuroscientist at the Hebrew University of Jerusalem who was not involved in the study.
Although contraction bias has been found in humans and nonhuman primates, the current study is the first to reveal it in rodents. “The thing that rodents bring to the table is all the tools that are available to study neural circuitry,” Brody says.
With these tools, researchers can start to examine how the regions that project to the posterior parietal cortex or receive projections from it participate in this neural process. Elucidating how this circuit affects behavior may have especially interesting implications for brain disorders such as dyslexia, in which contraction bias is reduced, Lowenstein says.