The human brain plays a crucial role in interpreting our surroundings, primarily through our five senses: sight, hearing, touch, smell, and taste. However, these senses often provide incomplete information. For instance, many objects we perceive are only partially visible. Our brains utilize prior knowledge and expectations to bridge these gaps in perception, a process known as sensory reasoning.
We engage in sensory reasoning so frequently that it often goes unnoticed. Consider a coffee table: without sensory reasoning, recognizing it when you place your drink down would be challenging. Despite its commonplace nature, the mechanisms behind sensory reasoning remain unclear. Recently, a team from the University of California, Berkeley, embarked on a quest to uncover the brain processes that underpin sensory reasoning in mice.
Earlier studies have shown that mice, much like humans, experience phenomena such as the Kanizsa illusion. This optical illusion highlights sensory reasoning, displaying a white triangle that appears to be present, even though only three incomplete circles and angles are visible. Researchers have identified similar responses to such illusions in mice. The Berkeley team aimed to further this research by observing mouse brains to draw parallels with human sensory reasoning.
“Kanizsa Triangle” by Fibonacci is licensed under CC BY-SA 3.0. Most observers perceive a white triangle in the center rather than three incomplete circles.
To investigate sensory reasoning, researchers utilized two primary methods to monitor brain activity in mice. First, a device called Neuropixel was surgically implanted into the heads of 14 mice, facilitating the observation of numerous neurons simultaneously. The second method involved two-photon imaging, utilizing a specialized microscope to examine individual neuronal activity in four other mice.
These techniques offer complementary advantages and limitations. While Neuropixels provide a comprehensive overview of brain activity, two-photon imaging focuses on single neurons or small groups. The research team conducted experiments on two distinct groups of mice: one utilizing Neuropixels and the other employing two-photon imaging.
To decode sensory reasoning mechanisms, the researchers pinpointed neurons in mice that responded to the perceived white triangle in the Kanizsa illusion. They monitored brain activity while presenting two types of visuals: illusions and real shapes. They discovered that area V1, located at the back of the brain, exhibited similar activity patterns in response to both the illusion and actual shapes.
The study identified two distinct neuron types in area V1 contributing to sensory reasoning. The first type, known as optical illusion shape encoders, only activated upon viewing illusions—essentially shapes that don’t exist. The second neuron type, called segment responders, displayed consistent activity regardless of illusions, responding to specific shapes within the images.
Employing machine learning algorithms, the research team compared both neuron types. They found that optical illusion shape encoders, believed to facilitate the perception of illusions, have stronger connections to regions responsible for higher-level visual processing beyond V1. This insight implies that similar neurons may assist the brain in leveraging expectations to compensate for missing information, though the exact mechanisms remain unclear.
The researchers postulated that partial visual inputs could activate the optical illusion shape encoder, which, in turn, stimulates other neurons in V1, creating the sensation that an illusory shape genuinely exists. To validate this, they used a laser to stimulate the optical illusion shape encoders in resting mice, prompting activation across V1 and inducing the experience of viewing a tangible shape.
Their findings revealed that three interconnected circuits facilitate the experience of sensory reasoning in mice. Initially, segment responders detect shapes and alert higher processing regions of the brain regarding missing information. These advanced regions subsequently activate the optical illusion shape encoder, which completes the pattern and triggers the overall V1 activation, giving the impression of observing a real shape.
Although the study concentrated on illusions, the researchers posited that their discoveries are relevant to sensory reasoning more broadly. As our scientific grasp of brain functions like sensory reasoning evolves, future research may extend these findings to encompass additional cognitive processes, such as memory and language.
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Source: sciworthy.com
