Have you ever wondered what happens in our brains that allows us not only to see, but also to recognize what is familiar and novel? Scientists have pondered this question, as well. Two research projects have studied this, and here is what scientists have learned.
How Vision Contributes Working Memory
Working memory involves briefly retaining information, like the words in this sentence, until your mind puts them together to form the meaning of entire sentence. Researchers at New York University showed that working memory relies not just on what is stored in memory but the why—namely, the purpose of storing something in the first place.
The study focused on how our brain stores the visual properties of our memories in the occipital lobe, which is where the visual system resides It explored how the neural codes that store memories change as a person prepares a response that depends on the memory. The study itself had subjects look where they remembered an object that disappeared after a few seconds.
Most theories on working memory state that the storage codes remain stable over time. This means the pattern of brain activity storing a specific visual memory remains the same as when it was first seen and encoded. Recent animal studies show that the neural patterns dynamic, and the memory codes change over time. To delve deeper into this phenomenon, researchers developed methods to measure the changing dynamics in the brain and to make them interpretable.
They presented to subjects a visual target. Neural activity in the primary visual cortex and a high-level visual area showed a bump when the subject saw the target. Interestingly, the activity bump remained at the target area during the memory delay. However, the activity in the primary visual cortex evolved during the memory delay, corresponding to where the subject was looking and where they would move their eyes after the delay.
Scientists believe that this reflects the path of the shift of gaze that is being prepared in the subject’s mind but hasn’t been done yet. These finding show that the dynamics reflect on the change from past sensory events, what we have just seen, into future behaviors that are guided by memory.
Knowing What is Familiar & What is Novel
Telling the difference between what is familiar and what is new or novel isn’t as mundane as it sounds. Understanding these distinctions helps us prioritize where our attention should be directed. For instance, imagine you’re driving down a familiar road, and suddenly you see someone dart across. Because the sight of someone running across the road is novel, your instinct is to slow down.
As you can guess, scientists have spent years studying how our brains are so good at telling the difference from the familiar and the novel. One study at the Picower Institute at the Massachusetts Institute of Technology (MIT), shed light on their similarities and paved the way for deeper insights into visual recognition memory. Visual recognition memory is the ability to recognize the familiar in a scene, allowing us to allocate less attention to them and focus on more critical aspects at a given moment. An example of this is seeing someone suddenly run across a familiar road while you are driving on that road.
In 1991, researchers at MIT learned that when animals viewed a familiar object, neurons in the cortex exhibited less activation compared to when they encountered a new object. However, in 2003, MIT researchers observed the opposite pattern. Mice displayed a surge in neural activity in the primary visual region of the cortex when a familiar item was briefly presented in front of them. This spike in activity is known as a visually evoked potential, serving as an indicator of visual recognition memory.
In this study, researchers induced visually evoked potentials by exposing mice to a black-and-white striped grating. The stripes periodically switch their shade, so that the pattern appears to reverse. Over several days, as the mice viewed this pattern, their visually evoked potentials increased, showing their growing familiarity with the visual stimulus. Researchers looked at how the visually evoked potential, stimulus-selective response plasticity, and visual recognition memory operate across the layers of the visual cortex.
What they found was that signs of visual recognition memory were observed in all layers of the visual cortex. Yet, it depended on NMDA receptors on a group of excitatory neurons in layer 6. What’s great about this finding is that neurons in this layer are connected to the thalamus, which is a deeper brain region that relays sensory information. They also interact with inhibitory neurons in layer 4, where the initial measurement of visually evoked potentials occurred. The study also measured changes in brain waves in each layer and this confirmed a previous finding: That when a stimulus is new, the brain waves are in a high frequency that depends on a particular kind of inhibitory neuron. Still, as the stimulus becomes familiar, the waves shift to a lower frequency that depend on a different inhibitory population.
The data from this study shows that the visually evoked potential represent pronounced, but transient spikes of electrical activity in the brain. These spikes occur amidst a broader, overall lull in neural activity. Interestingly, previous studies failed to detect these brief spikes due to limitations in their measurement capabilities. So, current understanding suggests that the visually evoked potential serve as a sign of brain activity associated with recognizing the familiar and triggering an inhibition of related neural processes.
From shifts in brain waves to visually evoked potentials, we are able to form memories from what we see and react to novel sights, such as a person running across a road. As scientists continue to study this, more is learned about memory and perception and these findings can lead to applications ranging from neural science to Artificial Intelligence.