'Rhythm beats volume': How the brain preserves our perception of familiarity

The brain's remarkable adaptability—its plasticity—is fundamental to our ability to learn and adapt. Neurons can modify their activity in response to new stimuli, and even their responses to the same stimuli can drift over time—a process known as representational drift. Despite this neural variability, our perception of the world remains remarkably stable day after day. This intriguing phenomenon has long puzzled scientists.

To investigate the mechanisms behind this stability, researchers from Rice University have developed ultra-thin, flexible probes called nanoelectronic threads (NETs). These probes are thousands of times thinner than a human hair and can embed seamlessly into brain tissue, enabling high-precision, long-term recordings of neuronal activity. By applying these probes to the visual cortex of mice over a period of 15 days, the team tracked how neurons responded as the mice viewed thousands of images, ranging from simple line patterns to complex natural scenes.

One key discovery was that the stability of visual representations in the brain is better understood through the lens of milliseconds-long neuronal rhythms, or the temporal code, rather than by simply counting how often neurons fire, known as the firing rate or volume. Traditional recording methods, which are too slow, tend to suggest that neural responses are quite variable. However, using the fast electrical recordings of NETs, the researchers found that the brain relies heavily on these precise timing patterns to maintain consistent perception.

Hanlin Zhu, a postdoctoral associate at Rice and the study's first author, explained, "Previous studies mainly relied on slower measures of brain activity, which made neural responses appear more 'drifty' than they truly are. Our technology allows us to directly read the rhythms and discover that time-based coding is crucial for keeping our perceptual world stable."

This research not only provides insight into fundamental neuroscience but also opens new avenues for developing brain-computer interfaces, sensory prostheses, and advanced neurological therapies. The NET technology is also being employed to explore spinal cord circuits and improve brain stimulation techniques.

In their experiments, the scientists observed that neurons’ responses based on firing rate alone appeared unreliable. However, when analyzing the temporal patterns, they found that the preferences of individual cells remained consistent across days, especially for those that seemed inconsistent when judged only by volume. Additionally, by examining the relationships between neurons firing together—the functional connectivity—they uncovered that a shared network effort underpins perceptual stability, with timing rather than firing loudness acting as the primary stabilizer.

Using these high-resolution recordings, the team also trained computer models to identify what stimulus the mice observed, even days later, with greater accuracy and less drift. This demonstrates the power of temporal coding in maintaining stable representations over time.

Leaders of the study, Chong Xie and Lan Luan, emphasized the importance of advanced recording tools in revealing the brain's organizing principles. They highlighted that sustained investment in neuroscience research, such as the proposed Texas Dementia Prevention and Research Institute, is critical for transformative discoveries that can positively impact lives. The research showcases how deep technological insights are vital for future innovations in neural interfaces and therapeutic strategies.