Summary: A new multimodal study reveals how the brain transforms a rapid sequence of flashes into the perception of smooth, continuous motion. This work identifies the superior colliculus (SC) as a central hub in switching vision from discrete, static events to a continuous, dynamic mode—an essential process for how mammals, including humans, interpret movement in the world.
Using functional MRI, controlled behavioural tests, and direct electrophysiological recordings, researchers mapped how different visual frequencies alter brain responses and pinpointed the SC as a critical structure underlying the continuity illusion.
By integrating whole-brain MRI with behavioural performance and neural recordings, the team showed that SC activity changes predictably with stimulus frequency. These changes align with when animals stop perceiving flicker and begin to perceive continuous light, offering new insight into perception and potential pathways for evaluating and treating visual disorders.
Key Facts:
- The Flicker Fusion Frequency (FFF) threshold—the frequency at which flashing light is perceived as continuous—varies across species and is closely linked to processing in the superior colliculus.
- A combination of fMRI, behavioural tasks, and electrophysiology demonstrates that SC responses shift as the visual input moves from discrete flashes to continuous perception, matching behavioural thresholds.
- At frequencies above the behavioural FFF, neural activity in the SC is suppressed between pronounced onset and offset responses, implicating the SC’s suppression as a mechanism for flicker fusion.
Source: Champalimaud Centre for the Unknown
Imagine watching a film: the smooth motion you experience is created from many static frames shown in rapid succession.
This perceptual effect—the continuity illusion—occurs when the brain interprets a sequence of rapid flashes as uninterrupted motion. It is an essential property of visual systems across mammals, helping animals detect and track moving objects. The new study from the Shemesh Lab at the Champalimaud Centre, published in Nature Communications, investigates how this illusion is encoded in the brain.
The frequency at which flashes blend into a continuous percept is called the Flicker Fusion Frequency (FFF). FFF varies by species—for example, animals that require fast vision, like some birds, have higher thresholds than humans. FFF is relevant not only to perception but also to survival behaviours such as predator–prey interactions, and it can change in disease states affecting the eye or nervous system.
Different measurement methods—behavioural testing, retinal recordings, or cortical electrophysiology—can yield different FFFs, suggesting that multiple brain regions contribute to the transition from flicker to continuity. To resolve where and how this transition is encoded, the researchers combined whole-brain functional MRI with behavioural assays and direct recordings of neuronal activity in rats.
A three-pronged experimental strategy
The project grew from discussions among PhD students in collaborating labs and developed into a coordinated effort. Researchers first mapped brain-wide responses with fMRI while presenting visual stimuli across a range of temporal frequencies. Rats were lightly sedated for fMRI to reduce motion while preserving sensory responses. fMRI detects blood-flow changes linked to neural activity and allowed the team to identify candidate regions whose signal patterns shifted with stimulus frequency.
The superior colliculus stood out: fMRI responses in the SC changed from positive (increased signal) at low frequencies to negative (decreased signal) at higher frequencies that matched the behavioural transition to continuous perception. To test whether these fMRI signs reflected neural activation and suppression, the team trained rats in a behavioural task to report whether a light sequence appeared flickering or continuous. Behavioural thresholds averaged 18 ± 2 Hz.
Comparing behaviour and fMRI revealed a strong alignment in the SC but not in other regions. The researchers then recorded electrical activity in SC neurons under conditions similar to the fMRI experiments. Electrophysiology showed robust neural responses to each flash at low frequencies, while at higher frequencies—those perceived as continuous—the per-flash responses diminished. Instead, neural activity became concentrated at the onset and offset of stimulation, with clear suppression between these peaks.
Additional experiments disabling the cortex showed that the SC’s signature persisted, indicating that the SC generates these patterns intrinsically rather than inheriting them from cortical input. Together, the data support a model in which the SC acts as a novelty detector: it responds strongly to discrete, novel flashes but reduces ongoing responses when stimuli occur faster than a threshold, producing the percept of continuity.
Implications and future directions
The study demonstrates the value of using fMRI as a first-pass tool to locate brain regions of interest and then applying targeted electrophysiology to understand circuit mechanisms. Clinically, these findings may inform assessments of visual processing in conditions such as optic nerve disease, cataract, stroke, or neurodevelopmental disorders, where FFF measurements and SC function could reveal impaired temporal processing.
Future work will aim to identify the specific SC cell types responsible for the observed dynamics and to map how each visual-area contribution changes with experience or injury. Combining targeted manipulations with imaging and behaviour should refine our understanding of how different brain regions cooperate to produce stable, continuous perception from rapidly changing inputs.
Next time you enjoy a movie’s smooth motion, consider the brain processes—from onset responses to suppression between flashes—that create the continuity illusion and the research unraveling those mechanisms.
About this visual neuroscience research news
Author: Hedi Young ([email protected])
Source: Champalimaud Centre for the Unknown
Contact: Hedi Young – Champalimaud Centre for the Unknown
Image: The image is credited to Neuroscience News
Original Research: Open access. “Rat superior colliculus encodes the transition between static and dynamic vision modes” by Noam Shemesh et al., Nature Communications
Abstract
Rat superior colliculus encodes the transition between static and dynamic vision modes
The visual continuity illusion reflects a shift from static to dynamic vision when stimuli arrive at high temporal frequency, a change that is essential for detecting moving objects. How this shift is represented across the visual pathway has been unclear, because retinal, cortical, and behavioural frequency thresholds do not always match, suggesting other brain regions participate. Using a multimodal approach—behavioural testing, whole-brain fMRI, and electrophysiology—the study finds a behavioural threshold near 18 ± 2 Hz in rats. fMRI signals in the superior colliculus switch from positive to negative at the behaviourally defined threshold, a transition not seen in thalamic or cortical areas. Electrophysiological data indicate that these fMRI transitions correspond to neural activation and suppression, and lesions of primary visual cortex show the effect originates within the superior colliculus, modulated by cortical gain. The results highlight the superior colliculus as a key structure encoding temporal frequency shifts and the switch from static to dynamic vision modes.