Deciphering the Mysteries of the Brain: New Research Clarifies Puzzling Aspects of Visual Recognition Memory

A recent study resolves previous inconsistencies observed in the field of visual recognition memory (VRM), demonstrating that heightened visual evoked potentials (VEPs) during the perception of familiar objects act as an indicator of the brain’s swift identification process, subsequently leading to an overall reduction in neural activity.

Understanding whether what we see is new or familiar is critical for attention allocation. Over the years, neuroscientists have striven to comprehend why the brain excels in performing this crucial function.

Throughout their investigations, researchers have come across seemingly incongruous data. A newly published study, however, asserts that these apparent contradictions are essentially different facets of the same underlying phenomenon, thereby advancing the long-sought comprehension of visual recognition memory (VRM).

VRM is the capacity to swiftly identify known elements within scenes, which can then be assigned lower priority to allow focus on newer, potentially more significant, elements.

Consider an instance where you enter your home office in the evening to address an urgent email. As you take in your surroundings—your familiar furniture and gadgets—you also notice an intruder. VRM ensures that your attention zeroes in on the intruder rather than your usual setting.

The data presented in the study indicate a brief, yet significant, surge in neural activity—a visual evoked potential—when a specific stimulus pattern is displayed to a mouse at approximately 80 milliseconds. Interestingly, when a stimulus is recognized, neural activity diminishes considerably after that initial surge. Credit: Bear Lab/MIT Picower Institute

“While the foundational principles of this learning mechanism within the mammalian brain are not yet fully understood,” wrote Picower Professor Mark Bear and co-authors in the Journal of Neuroscience.

Historically, as far back as 1991, researchers discovered that when animals observed something they recognized, the cortical neurons—or neurons in the brain’s outer layer—were less activated compared to when they saw something new. Mark Bear’s MIT colleagues, Picower Professor Earl K. Miller and Doris and Don Berkey Professor Bob Desimone, were among the authors of that study.

However, in 2003, Bear’s laboratory observed a contrasting phenomenon: Neural activity in the mice’s primary visual cortex spiked when a known stimulus was presented. This increase in activity, known as a “visually evoked potential” (VEP), has been further shown by Bear’s lab to be a reliable marker of VRM.

The current research, led by former Bear Lab postdoctoral researchers Dustin Hayden and Peter Finnie, provides insights into how VEPs can rise even while there is an overall diminishing neural response to recognized stimuli, reconciling the findings of previous studies. The study also elucidates the underlying mechanisms of VRM—the transient increase in VEPs may trigger an overall inhibitory response.

Updated Perspectives

Bear’s team generated VEPs by showing mice a fluctuating black-and-white pattern. Over a period of time, as the mice became accustomed to this pattern, the VEPs increased, confirming the animals’ declining interest in the stimulus. For two decades, Bear’s laboratory has examined how synaptic changes contribute to VRM by studying a phenomenon termed “stimulus-selective response plasticity” (SRP).

Preliminary research proposed that SRP was concentrated among excitatory neurons in layer 4 of the visual cortex and specifically depended on the activation of their NMDA receptors. Subsequent experiments clarified that this activation was not confined to layer 4. The current study aims to understand VRM, SRP, and VEPs throughout the entire visual cortex.

The study found that key aspects of VRM, including VEPs, were prevalent in all layers of the cortex but were particularly reliant on NMDA receptors on excitatory neurons located in layer 6, not layer 4. The authors find this especially interesting because these neurons are closely linked to the thalamus and inhibitory neurons in layer 4.

Further, changes in brain wave oscillations were observed across different layers, reinforcing previous findings that as a stimulus becomes more familiar, the dominant brain wave frequency shifts from a higher “gamma” to a lower “beta.”

Brief Surge Amid Overall Decline

The study’s meticulous electrophysiology recordings revealed that VEPs are pronounced yet ephemeral spikes in neural electrical activity, embedded within an overall reduction in activity. “This finding indicates that both interpretations are valid,” stated Bear.

According to the new evidence, VEPs signify the brain’s rapid identification of a known stimulus, which then initiates a general reduction in neural activity related to that stimulus. “This is enlightening because it means that the encoding of familiarity isn’t due to the inhibition of excitatory synapses alone, but seems to be facilitated by the potentiation of excitatory synapses that then recruit inhibitory mechanisms in the cortex,” Bear explained.

Although the study significantly advances our understanding of VRM’s origins, open questions remain, including the exact neural circuits involved. “The precise role of the layer 6 circuit neurons is still undetermined,” added Bear, indicating that the research journey continues.

Reference: “Electrophysiological Signatures of Visual Recognition Memory Across All Layers of Mouse V1” by Dustin J. Hayden, Peter S.B. Finnie, Aurore Thomazeau, Alyssa Y. Li, Samuel F. Cooke, and Mark F. Bear, published on September 15, 2023, in JNeurosci. DOI: 10.1523/JNEUROSCI.0090-23.2023

In addition to Hayden, Finnie, and Bear, other authors of the study include Aurore Thomazeau, Alyssa Li, and Samuel Cooke.

The research was financially supported by The National Eye Institute of the National Institutes of Health, The Picower Institute for Learning and Memory, and The JPB Foundation.

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More about Visual Recognition Memory

• Journal of Neuroscience Publication
• Picower Institute for Learning and Memory
• National Eye Institute of the National Institutes of Health
• The JPB Foundation
• Study on NMDA Receptors and Visual Cortex