Newswise — A curious phenomenon emerges among over a quarter of all stroke victims - they experience an unusual disorder where they lose conscious awareness of half of the visual information perceived by their eyes.
Following a stroke in the right half of the brain, for instance, individuals may exhibit intriguing behaviors, such as choosing to eat only the food on the right side of the plate due to their unawareness of the left side. In a similar manner, they might glance at a photo and solely perceive its right half, disregarding the presence of any person or object on their left side.
Remarkably, despite the limited conscious awareness of half the world, these stroke victims retain the ability to emotionally react to the entire photo or scene. It appears that their brains are still processing the complete visual information, even though their conscious perception is restricted to just half of it.
This puzzling condition, known as unilateral neglect, brings attention to a longstanding question in brain science: What distinguishes perceiving something from being consciously aware of perceiving it? For instance, you might not consciously register passing a shoe store while scrolling through your Instagram feed, but later find yourself searching online for shoe sales. This demonstrates that your brain is capable of recording information even when you are not consciously aware of it.
Excitingly, neuroscientists from the Hebrew University of Jerusalem and the University of California, Berkeley, have made progress in identifying the specific brain region responsible for retaining sustained visual images that we perceive for a few seconds. Their research findings, recently published in the journal Cell Reports, shed light on this fascinating aspect of visual perception.
Gal Vishne, the lead author of the paper and a graduate student from Hebrew University, emphasized the significance of consciousness, particularly visual experience, as the most fundamental aspect of human perception. From the moment people open their eyes in the morning to when they go to sleep, it is an integral part of their daily lives. Vishne stated that their study delves into the realm of ordinary human experiences, exploring the intricate nature of consciousness that each individual encounters throughout their daily routines.
Although the current findings may not fully elucidate the mechanisms behind our unawareness of certain perceptions, studies like these hold promise for practical applications in the future. For instance, they might enable doctors to assess a coma patient's brain activity and determine if the individual remains aware of the external world, offering potential avenues for improvement. Moreover, gaining a deeper understanding of consciousness could aid doctors in developing treatments for disorders of consciousness, further enhancing their ability to provide better care and support for patients facing such conditions.
Senior author Leon Deouell, a professor of psychology at Hebrew University and a member of the Edmond and Lily Safra Center for Brain Research, revealed that his scientific passion stems from observing patients with stroke who suffer from unilateral neglect, leading them to disregard half of their visual world. This experience sparked his keen interest in the question of conscious awareness. He wonders how it is possible for individuals to possess information without subjectively experiencing it, leading them to not act upon it, move their eyes towards it, or grasp it. Understanding the intricacies of this phenomenon would be invaluable in identifying what is lacking in the cognitive system and the brains of patients with this condition. Ultimately, such knowledge could offer crucial insights into the underlying mechanisms of this syndrome and potentially pave the way for more effective treatments in the future.
Robert Knight, another senior author of the study and a professor of psychology at UC Berkeley, as well as a member of the Helen Wills Neuroscience Institute, emphasized the significance of their research in advancing our understanding of consciousness. He described their findings as a valuable addition to the puzzle of consciousness, specifically in deciphering how certain visual information persists in the mind's eye, allowing individuals to take actions based on it. Their work contributes to unraveling this complex aspect of human cognition and perception, bringing us closer to comprehending the underlying mechanisms of consciousness.
The brain has a transient and a sustained response
Deouell pointed out that, for nearly six decades, electrical studies of the human brain have primarily focused on the initial surge of activity that occurs after perceiving something. However, this spike in brain activity typically subsides after around 300 or 400 milliseconds, while our conscious awareness of certain stimuli can persist for seconds or even longer. This disparity between the brief electrical responses and the sustained conscious awareness has been a significant gap in our understanding of brain function. The study's findings aim to bridge this gap and shed light on the brain processes involved in retaining sustained visual information beyond the initial neural response.
"That leaves a whole lot of time which is not explained in neural terms," he said.
In their pursuit of studying longer-lasting brain activity, the neuroscientists obtained consent from ten individuals who were undergoing procedures involving the placement of electrodes on their brain surface to monitor epileptic seizures. During this process, the researchers recorded neural activity from the electrodes while presenting various images to the patients on a computer screen for different durations, ranging up to 1.5 seconds. To ensure the patients were actively paying attention, they were asked to press a button whenever they spotted an occasional item of clothing within the presented images. This experimental setup allowed the scientists to examine the sustained brain activity associated with visual perception and consciousness over extended periods, providing valuable insights into how the brain retains and processes visual information beyond the initial moment of perception.
Many conventional methods used to record neural activity in humans, like functional MRI (fMRI) or electroencephalography (EEG), provide valuable insights into either the location or timing of brain activity but not both simultaneously. However, the innovative approach taken by the researchers from Hebrew University and UC Berkeley, involving the use of implanted electrodes within the skull, allowed them to overcome this limitation. By employing this technique, they were able to precisely track and analyze both the spatial and temporal aspects of brain activity, providing a more comprehensive understanding of how the brain processes and retains information over extended periods. This methodological breakthrough represents a significant advancement in the study of consciousness and neural processing in the human brain.
Following data analysis using machine learning techniques, the research team made a fascinating discovery. Contrary to earlier studies that suggested a brief burst of brain activity upon perceiving something new, the visual areas of the brain demonstrated a sustained level of low activity, retaining information about the perceived image for an extended period. This continuous pattern of neural activity exhibited striking similarities to the initial burst of activity, and it underwent changes whenever the individuals viewed different images. These findings challenge the conventional understanding of how the brain processes visual information, providing new insights into the mechanisms underlying conscious awareness and retention of visual stimuli.
"This stable representation suggests a neural basis for stable perception over time, despite the changing level of activity," Deouell said.
In contrast to certain earlier studies, the research team observed that the prefrontal and parietal cortexes in the front of the brain only became active when perceiving something novel. Surprisingly, this brain activity vanished entirely within a remarkably short duration of half a second (500 milliseconds), even for stimuli that lasted much longer. This finding suggests that these regions of the brain play a specific role in processing new and unfamiliar visual information, with their activity quickly subsiding after the initial perception.
Additionally, the occipitotemporal area of the visual cortex in the posterior part of the brain also exhibits brief but intense activity, lasting approximately 300 milliseconds, before transitioning to a sustained but lower level of activity, typically ranging from 10% to 20% of the initial spike. Interestingly, the pattern of activity in this region does not dissipate entirely; instead, it endures unaltered for the duration that a person views the image. This sustained and consistent neural activity in the occipitotemporal area suggests its critical role in continuously processing and maintaining visual information throughout the period of perception.
"The frontal cortex is involved in the detection of something new," Deouell explained. "But you also see an ongoing representation in the higher-level sensory regions."
The sequence of events in the brain could be interpreted in various ways. Knight and Vishne lean toward the idea that conscious awareness comes when the prefrontal cortex accesses the sustained activity in the visual cortex. Deouell suspects that consciousness arises from connections among many areas of the brain, the prefrontal cortex being just one of them.
The team's research findings have been independently validated by a group known as the Cogitate Consortium. Although the consortium's results are currently awaiting peer review, they were presented during a June event in New York City, which was framed as a face-off between two prominent theories of consciousness. Interestingly, both the published results in Cell Reports and the unpublished results from the Cogitate Consortium align well with either theory of consciousness. This convergence of findings adds further credibility and significance to the research, contributing to our growing understanding of the complex nature of consciousness and its neural underpinnings.
"That adversarial collaboration involves two theories out of something like 22 current theories of consciousness," Deouell cautioned. "Many theories usually means that we don't understand."
Nevertheless, the two studies and other ongoing studies that are part of the adversarial collaboration initiated by the Templeton Foundation could lead to a true, testable theory of consciousness.
According to Vishne, the team's research has shown that the predictions of both tested theories of consciousness are correct. However, when considering the broader scope, neither theory, in its current form, fully explains all aspects of consciousness, even though each theory contains some elements of truth. Given the considerable unknowns surrounding the neural basis of consciousness, the researchers believe that more data should be gathered before a new and more comprehensive understanding of consciousness can emerge. They liken this process to a phoenix rising from the ashes of previous theories, with the hope that future research will eventually shed light on the intricate nature of consciousness and its underlying neural mechanisms.
In their upcoming research endeavors, Deouell and Knight have outlined plans to investigate the electrical activity linked to consciousness in various other regions of the brain. This includes exploring brain areas involved in memory and emotions. By extending their investigations beyond the visual cortex, they aim to gain a more comprehensive understanding of how different brain regions contribute to conscious awareness and the interplay between various cognitive processes. These future studies hold the promise of unraveling further complexities of consciousness, paving the way for a more holistic view of the neural mechanisms underlying our subjective experiences.
Edden Gerber is also a co-author of the paper. The study was supported by the U.S.-Israel Binational Science Foundation (2013070) and the National Institute of Neurological Disorders and Stroke of the National Institutes of Health (R01 NS021135).
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