In the fourth issue of this monthly digest series you can learn what causes deja vu, whether consciousness comes in time slices, how the brain recognizes emotions in faces, why you should (not?) eat more fruit, and much more.
Consciousness in discrete time slices
We seem to experience the world as a continuous stream of percepts: one image or sound or smell or touch smoothly follows the other, providing us with a seamless understanding of the world around us. However, intriguing illusions and recent experiments suggest that the world is not continuously translated into conscious perception. Instead, perception seems to operate in a discrete manner, just like movies appear continuous although they consist of discrete images.
Scientists from EPFL and the universities of Ulm and Zurich, have now put forward a new model of how the brain processes unconscious information, suggesting that consciousness arises only in intervals up to 400 milliseconds, with no consciousness in between. One of the most striking examples against continuous theories is the color phi phenomenon (see image below). When two colored disks are presented spatially displaced in rapid succession, it appears as if one disk moves between the two positions and changes color in the middle of its trajectory. Logically it is impossible to experience the color change before having seen the second disk. The conscious percept must have been formed retrospectively, thus contradicting continuous theories. As an extreme example, under certain conditions, a stimulus that had been presented first can even be perceived occurring after a stimulus presented later in time.
The new model proposes a two-stage processing of information. First comes the unconscious stage: The brain processes specific features of objects, e.g. color or shape, and analyzes them quasi-continuously and unconsciously with a very high time-resolution. However, the model suggests that there is no perception of time during this unconscious processing. Even time features, such as duration or color change, are not perceived during this period. Instead, the brain represents its duration as a kind of "number," just as it does for color and shape.
Then comes the conscious stage: Unconscious processing is completed, and the brain simultaneously renders all the features conscious. This produces the final "picture," which the brain finally presents to our consciousness, making us aware of the stimulus.
The whole process, from stimulus to conscious perception, can last up to 400 milliseconds, which is a considerable delay from a physiological point of view. "The reason is that the brain wants to give you the best, clearest information it can, and this demands a substantial amount of time," explains Michael Herzog. "There is no advantage in making you aware of its unconscious processing, because that would be immensely confusing." This model focuses on visual perception, but the time delay might be different for other sensory information, e.g. auditory or olfactory.
Hippocampus makes maps of social space, too
Cells in a brain structure known as the hippocampus are known to be cartographers, drawing mental maps of physical space. But new studies show that this seahorse-shaped hook of neural tissue can also keep track of social space, auditory space and even time, deftly mapping these various types of information into their proper places.
In an experiment reported last year in Neuron, people went on a virtual quest to find a house and job by interacting with a cast of characters. Through these social interactions, the participants formed opinions about how much power each character held, and how kindly they felt toward him or her. These judgments put each character in a position on a "social space" map.
Neuroscientist Rita Tavares of Icahn School of Medicine at Mount Sinai in New York City described details of one of these new maps April 2 at the annual meeting of the Cognitive Neuroscience Society. It turns out that this social map depends on the traits of the person who is drawing it, says Tavares. People with more social anxiety tended to give more power to characters they interacted with. What's more, these people's social space maps were smaller overall, suggesting that they explored social space less. Tying these behavioral traits to the hippocampus may lead to a greater understanding of social behavior—and how this social mapping may go awry in psychiatric conditions.
The work emphasizes that the hippocampus is not just a mapper of space, Tavares says. Instead, it is a mapper of relationships. "It's relational learning," she says. "It's everything in perspective."
Source: Science News.
What causes déjà vu?
Most of us have had at least once in our lifetime a 'déjà vu' (French for 'already seen') experience. It is a mysterious feeling where time seems to pass by in slow-motion, where you perceive information in such a way as if you had already experienced the current situation sometime in the distant past. However, none of us can explain it, little understand it. Researchers have mentioned numerous ’causes’ ranging from paranormal disturbances and neurological disorders and even multiple universes coexisting with ours.
According to a study by the Department of Neuroscience and Experimental Therapeutics at the University of Texas A&M, this psychological phenomenon has occurred in about 70 percent of the general population. "Because there is no clear, identifiable stimulus that elicits a déjà vu experience (it is a retrospective report from an individual), it is very difficult to study déjà vu in a laboratory," said Michelle Hook, Ph.D., assistant professor in the Department of Neuroscience and Experimental Therapeutics, at the Texas A&M Health Science Center College of Medicine. Episodes of déjà vu may be closely related to how memory is stored in the brain. Retention of long-term memories, events and facts are stored in the temporal lobes, and, specific parts of the temporal lobe are also integral for the detection of familiarity, and the recognition of certain events. The takeaway: The temporal lobe is where you make and store your memories.
While déjà vu's connection to the temporal lobe and memory retention is still relatively unknown, clues about the condition were derived from people who suffer from temporal lobe epilepsy (a condition in which nerve cell activity in the brain is disturbed—causing seizures). Findings suggest that déjà vu events may be caused by an electrical malfunction in the brain.
But, what is the basis for déjà vu in healthy people without epilepsy? Some researchers describe it as a 'glitch' in the brain—when the neurons for recognition and familiarity fire—allowing the brain to mistake the present for the past. Instances of déjà vu in healthy individuals may also be attributed to a 'mismatch' in the brain's neural pathways. This could be because the brain is constantly attempting to create whole perceptions of the world around us with limited input. For example, it only takes a small amount of sensory information -- like a familiar smell -- for the brain to create a detailed recollection. Déjà vu could be linked to discrepancies in the memory systems of the brain, leading the sensory information to by-pass short-term memory and reach long-term memory instead. This may produce the unsettling feeling that we've experienced a new moment before.
"Some suggest that when a difference in processing occurs along these pathways, the perception is disrupted and is experienced as two separate messages. The brain interprets the second version, through the slowed secondary pathway—as a separate perceptual experience—and thus the inappropriate feeling of familiarity occurs," Hook said.
Others have drastically different views of the phenomenon. For example, according to Dr. Michio Kaku, an American futurist, theoretical physicist and popularizer of science, parallel universes can explain the mysterious phenomenon. That's right. Kaku states that quantum physics may provide the necessary details which suggest déjà vu might be caused by your ability to "flip between different universes." Looks like there will be some time before scientists will agree.
Researchers from the Ohio State University have pinpointed the area of the brain responsible for recognizing human facial expressions. It's on the right side of the brain behind the ear, in a region called the posterior superior temporal sulcus (pSTS). Further, the researchers have discovered that neural patterns within the pSTS are specialized for recognizing movement in specific parts of the face. One pattern is tuned to detect a furrowed brow, another is tuned to detect the upturn of lips into a smile, and so on. (ScienceDaily)
Cortical connectivity might be optimized to store a large number of attractor states in a robust fashion, argues Dr. Nicolas Brunel from the University of Chicago. In a theoretical study, Brunel characterized the statistics of synaptic connectivity matrices that try to maximize the number of patterns that can be stored in the connections. This organizational principle was sufficient to explain a number of properties typically associated with cortical synapses: Connections are sparse and often bidirectional, where bidirectionally connected pairs of neurons have stronger synapses than unidirectionally connected pairs. (Nature Neuroscience)
Researchers from the University of Oxford and Chinese Academy of Medical Sciences found that people who eat fresh fruit on most days (and live in China) are at lower risk of heart attack and stroke than people who rarely eat fresh fruit. This conclusion was drawn from tracking the health of 500,000 adults from 10 urban and rural localities in China over 7 years. In China, fresh fruit consumption is much lower than in high-income countries, which might explain the strong correlation between fruit consumption and cardiovascular risk, says Dr. Huaidong Du from the University of Oxford. Also, fruit in China is almost exclusively consumed raw, whereas much of the fruit in high-income countries is processed, and many previous studies combined fresh and processed fruit. (ScienceDaily)
Ironically, while people in China should eat more fruit, people in the West should consume less fructose. A new study by UCLA life scientists has found that hundreds of genes can be damaged by fructose, a sugar that's common in the Western diet, in a way that could lead to diseases ranging from diabetes to cardiovascular disease, and from Alzheimer's disease to attention deficit hyperactivity disorder. However, they discovered good news as well: an important omega-3 fatty acid known as DHA seems to reverse the harmful changes produced by fructose. (ScienceDaily)
Researchers from Harvard and Caltech were surprised to find that mice can see color via rod-cone opponency. Like most mammals, the mouse has one type of rod and two types of cone photoreceptors, with absorption maxima in the ultraviolet (S pigment) and green (M pigment) region of the spectrum. Although the mouse retina contains a green-sensitive cone, the ON response instead originates in rods. Rods and cones both contribute to the response over several decades of light intensity. Remarkably, the rod signal in this circuit is antagonistic to that from cones. For rodents, this UV-green channel may play a role in social communication, as suggested by spectral measurements from the environment. In the human retina, all of the components for this circuit exist as well, and its function can explain certain experiences of color in dim lights, such as a "blue shift" in twilight. (Nature)
An interesting review article has been published that focuses on a quite bizarre motor speech disorder called Foreign Accent Syndrome (FAS), where patients suddenly pick up a foreign accent as a result of a stroke, head trauma, migranes, or developmental problems. Despite an unconfirmed news report in 2010 that a Croatian speaker has gained the ability to speak fluent German after emergence from a coma, there has been no verified case where a patient's foreign language skills have improved after a brain injury. The goal of the review was to analyze the main features of psychogenic FAS in order to shed more light on this taxonomic variant and facilitate the diagnosis in clinical practice. (Frontiers)
Researchers from the Boston University School of Medicine have identified a new set of genes that may be responsible for the two most common and disabling neurological conditions, stroke and dementia. They identified a new gene called FOXF2 which increased the risk of having a stroke due to small vessel disease in the brain. No previous study has identified a gene for the common type of small vessel disease stroke, which is a major contributor to dementia risk, and is associated with gait problems and depression. (Neuroscience News)
Things to look out for next month:
- Vision Sciences Society Annual Meeting, May 13-18, St. Pete Beach, Florida.
- 6th Annual Traumatic Brain Injury Conference, May 11-12, Washington, DC.
- Second International Conference on Mathematical NeuroScience (ICMNS), May 30 - June 1, Antibes, France.
Anything I missed? Sound away in the comment section! Have something of interest or want your discovery to be considered for next month's issue? Let me know via mbeyeler (at) uci (dot) edu.