Summary: A new study investigates visual masking, an illusion in which a quickly presented second image prevents conscious perception of a first image, revealing how the brain constructs awareness.
Researchers have now demonstrated this visual masking effect in mice and shown that it mirrors human perception, supporting the idea that similar neural processes underlie the phenomenon across species. By training animals to report what they saw and combining behavior with targeted manipulations, the team identified brain regions required for the illusion and clarified how sensory signals become conscious perception.
The results point to the visual cortex and downstream cortical areas as key sites where awareness of visual stimuli is generated or lost.
Key Facts:
- Visual masking occurs when a briefly presented image is not consciously perceived because it is quickly followed by another image; this study shows the effect in both humans and mice.
- Experimental results indicate the visual cortex (primary visual area V1) and cortical regions downstream are essential for the masking effect, suggesting these areas play a central role in conscious perception.
- The similarity between mouse and human responses implies conserved mechanisms for converting retinal input into conscious visual experience, narrowing the search for neural circuits of awareness.
Source: Allen Institute
A new study published in Nature Neuroscience examines how visual masking makes us “unsee” things and what that reveals about the neural origins of conscious perception.
Visual masking describes a situation in which an initial image fails to reach conscious awareness if a second image follows it very quickly. Timing is critical: the first image must be brief, and the following mask must appear within a short interval (on the order of tens of milliseconds) for the initial stimulus to be suppressed from awareness.
Shawn Olsen, Ph.D., an investigator at the Allen Institute, together with colleagues, investigated the circuit basis of this illusion and demonstrated for the first time that mice experience visual masking in a way that closely resembles human behavior. The team trained animals to report the position of a briefly flashed image and then introduced a masking stimulus immediately afterward, which disrupted the animals’ ability to report the original target.
“This is an interesting observation, where what is present in the world is not accurately reflected in your perception,” Olsen said. “Like other visual illusions, we think it tells us something about how the visual system operates and about the neural circuits that underlie visual awareness.”
Although visual masking was first documented in the 19th century, the neural mechanisms that cause a stimulus to be processed without reaching conscious awareness have remained unclear. This study narrows the potential loci of awareness generation, implicating visual cortex circuitry as a critical stage where target and mask signals interact.
When photons strike the retina, visual information follows a set pathway from the eye through multiple brain regions before arriving at higher cortical areas. Earlier work has shown that neurons in the retina and early visual pathway can respond to stimuli even when the subject is not consciously aware of them, indicating that early sensory activity alone does not guarantee perception.
To investigate when unconscious sensory signals become conscious perception and guide behavior, the researchers trained 16 mice to turn a small wheel toward the direction of a brief target image to receive a reward for correct responses. The team then presented a masking image immediately on both sides of the display after the target. With the mask in place, animals’ performance declined, consistent with the animals no longer perceiving the original target.
Because visual masking had not been tested in mice before, the researchers designed a task tailored to the animals’ behavioral capabilities; stimulus parameters therefore differed from those commonly used in human studies. To validate the cross-species relevance, the same masking paradigm was adapted for 16 human participants who reported target location with keystrokes. Human participants showed reduced subjective visibility and behavioral deficits that matched the pattern seen in mice.
These parallel results support the conclusion that the conversion from unconscious sensation to conscious perception takes place in or after the primary visual cortex (V1). The study also used optogenetic suppression of V1 to compare the effects of removing cortical activity with the effects produced by masking, showing that both manipulations impair performance over similar timescales but in different ways.
About this visual neuroscience research news
Author: Peter Kim
Source: Allen Institute
Contact: Peter Kim – Allen Institute
Image: The image is credited to Neuroscience News
Original Research: Closed access.
“Backward masking in mice requires visual cortex” by Shawn Olsen et al. Nature Neuroscience
Abstract
Backward masking in mice requires visual cortex
Visual masking reveals the temporal dynamics of perception, but the circuit mechanisms behind it are not fully understood.
This work introduces a backward masking task implemented in both mice and humans in which the spatial location of a brief stimulus is effectively masked by a subsequent image.
Human subjects report reduced subjective visibility that aligns with measurable behavioral deficits. In mice, both backward masking and optogenetic silencing of primary visual cortex (V1) impair task performance over similar time windows, although they produce distinct effects on response rates and accuracy.
Neuronal activity in V1 corresponds with masked behavior when analyzed over longer time windows but not over very short windows. A dual-accumulator model reproduces key features of both mouse and human behavior, suggesting that the earliest spikes in V1 can be sufficient to trigger a correct response while subsequent V1 activity driven by the mask can degrade performance.
Consistent with this interpretation, optogenetic suppression of mask-evoked activity in V1 restores accurate behavior. Together, the findings show that mice, like humans, are susceptible to backward masking and that the initial confounding of target and mask information arises downstream of V1.