June 12, 2024

Decoding the Role of the Superior Colliculus in the Continuity Illusion

A recent study conducted by a team at the Champalimaud Foundation has shed light on the often overlooked superior colliculus (SC), a deep-seated brain structure that plays a crucial role in how animals perceive motion and how the continuity illusion is formed. The continuity illusion is an essential perceptual process that impacts our daily activities, such as driving or watching movies. In this phenomenon, our brain perceives a rapid sequence of static frames as continuous and smooth motion.

The study, led by the Shemesh Lab at the Champalimaud Foundation, investigates how the continuity illusion is encoded in the brain. The team discovered that the speed at which flashes of light must occur for the brain to perceive them as continuous is known as the Flicker Fusion Frequency (FFF) threshold. Different animals have different FFF thresholds, with birds, for example, having a higher threshold than humans. Diseases and conditions like liver disorders or cataracts can also affect the FFF threshold.

The researchers used functional MRI (fMRI) brain scans, behavioral experiments, and electrical recordings of brain activity to understand how the continuity illusion process works. They found that the SC is vital in transitioning from perceiving individual flashes to seeing continuous motion. The SC may be a key component in creating the continuity illusion.

The study began as a conversation between two Ph.D. students at the Champalimaud Foundation. Rita Gil and Mafalda Valente developed a behavioral task that trained rats to distinguish between flickering and continuous light. They combined this behavioral data with fMRI and electrophysiological recordings of brain activity to measure and compare FFF thresholds using three distinct methods.

The fMRI experiments involved showing visual stimuli to rats at different frequencies. The researchers observed that the SC responded differently based on the frequency of visual stimuli. As the frequency increased, the SC’s response shifted from positive to negative fMRI signals, reflecting increased neural activity and potential suppression, respectively. This led the researchers to hypothesize that the transition from static to dynamic vision involves the suppression of activity in the SC.

To validate their hypothesis, the researchers conducted behavioral experiments in which rats learned to distinguish between flickering and continuous light. They observed that the change from positive to negative fMRI signals in the SC correlated with the frequencies at which the rats perceived the shift from flickering to continuous light. The SC showed the strongest correlation between behavior and fMRI data compared to other brain areas.

The researchers also recorded the electrical activity of neurons in the SC. They observed increased neural activity corresponding to individual flashes at low light frequencies, but this activity diminished as the frequency increased and the rats perceived continuous light. Instead, there were more pronounced responses at the start and end of the light stimulation, with suppression of neural activity in between.

These findings suggest that the SC acts as a novelty detector, processing each flash as a new event at low frequencies but suppressing activity when the stimulus is no longer considered new or noteworthy. This accounts for the pattern of increased activity at the beginning and end of high-frequency stimulation, with suppression in between.

The researchers believe that their findings have implications for clinical applications, such as assessing and treating visual dysfunctions in individuals with visual impairments, optic nerve diseases, autism, or stroke. By comparing FFF thresholds in these individuals with those in healthy populations, it may be possible to gauge the adaptability of specific brain regions and develop targeted therapeutic interventions.

Moving forward, the researchers aim to identify the specific cell types in the SC responsible for the observed activities. They also plan to combine experimental techniques such as targeted lesions or visual deprivation with MRI studies to gain a deeper understanding of the roles of various brain regions within the visual pathway.

This study highlights the intricate processes involved in the continuity illusion and offers new insights into how the brain perceives motion. By unraveling these processes, researchers may pave the way for advancements in visual perception and potential treatments for visual dysfunctions.

1. Source: Coherent Market Insights, Public sources, Desk research
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