Monday, February 6, 2017

Highlights and new discoveries in Neuroscience (January 2017)

In the latest edition of this monthly digest series you can learn how dopamine cells influence our perception of time, why researchers are growing brains on a chip, whether split-brain patients have a split consciousness, and much more.

Split brain does not lead to split consciousness

Split brain is a lay term to describe the result of a corpus callosotomy, a surgical procedure—where a bundle of nerve fibers connecting the left and right brain hemispheres (i.e., the corpus callosum) is severed—used to alleviate severe epilepsy among patients. This condition was made famous by the work of Nobel laureates Roger Sperry and Michael Gazzaniga. In their canonical work, Sperry and Gazzaniga discovered that split-brain patients can only respond to stimuli in the right visual field with their right hand and vice versa. This was taken as evidence that severing the corpus callosum causes each hemisphere to gain its own consciousness.

Now, a new study led by University of Amsterdam psychologist Yair Pinto challenges the established view that so-called split-brain patients have a split consciousness. In their study, patients were placed in front of a screen, shown various objects displayed in several locations, and then asked to confirm whether an object appeared and to indicate its location. To the researchers’ surprise, the patients were able to respond to stimuli throughout the entire visual field with all the response types: left hand, right hand and verbally. "The patients could accurately indicate whether an object was present in the left visual field and pinpoint its location, even when they responded with the right hand or verbally," said Pinto. This was despite the fact that their cerebral hemispheres could hardly communicate with each other.

A depiction of the traditional view of the split brain syndrome (top) versus what the researchers actually found in two split-brain patients across a wide variety of tasks (bottom).

According to Pinto, the results present clear evidence for unity of consciousness in split-brain patients. "The established view of split-brain patients implies that physical connections transmitting massive amounts of information are indispensable for unified consciousness—that is, one conscious agent in one brain. Our findings, however, reveal that although the two hemispheres are completely insulated from each other, the brain as a whole is still able to produce only one conscious agent. This directly contradicts current orthodoxy and highlights the complexity of unified consciousness."


Dopamine cells influence our perception of time

Dopamine neurons in a brain region known as the substantia nigra pars compacta can alter the perception of time, according to new research published in Science. Stimulating or inhibiting these cells makes mice behave as if time is moving faster or slower, the researchers found. More specifically, boosting dopamine activity slows down the animals’ internal clock, leading them to underestimate time intervals.

Photo credit: Nicolas Tritsch

The study—the first to look so precisely at dopamine activity during timing judgments—provides a potential molecular mechanism for the perception that time flies when you're having fun, and that it slows to a trickle when you're bored. In their experiments, Joseph Paton (Champalimaud Centre for the Unknown, Portugal) and colleagues used fiber photometry to measure activity in dopamine neurons of the substantia nigra pars compacta as mice listened to two tones and then indicated whether the delay between them was shorter or longer than 1.5 seconds. Paton’s team found that dopamine activity varied but was correlated with the animal’s perception of time. When dopamine activity was high, the animal was more likely to judge an interval as short, and vice versa. The pattern was particularly strong for difficult trials, in which the interval between tones was close to 1.5 seconds. In a second set of experiments, the researchers used optogenetics to activate or silence the dopamine neurons. Consistent with the photometry measurements, altering neuronal activity made the animals under- or overestimate the duration of intervals.

One of dopamine’s primary roles in the brain is as a reinforcement signal when an animal gets an unexpected reward. This encourages the animal to seek out that situation again in the future. Time plays an important role in that process—a reward that comes earlier than expected triggers the dopamine signal. And time is a key component of the equations that describe reward error prediction.

The findings might also have implications for dopamine-linked disorders, such as attention deficit hyperactivity disorder (ADHD), which is linked to too little dopamine and is treated with dopamine drugs. People with the disorder also tend to be impulsive. “One dimension of impulsivity is the tendency or inability to promote delayed gratification,” Paton says. “If your dopamine levels are different and you sense time more slowly, it might be harder to wait for delayed gratification.”

The researchers next plan to look at other brain regions that produce dopamine, including the ventral tegmental area, which has been implicated in reward, reinforcement and addiction.

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Multi-regional brain on a chip

Harvard University researchers have developed a multi-regional brain on a chip that models the connectivity between three distinct regions of the brain: the amygdala, prefrontal cortex, and hippocampus. The in vitro model was used to extensively characterize the differences between neurons from different regions of the brain and to mimic the system’s connectivity.

Image of the in vitro model showing three distinct regions of the brain connected by axons. image is for credited to Disease Biophysics Group/Harvard University.

To demonstrate the chip’s efficacy in modeling disease, the team doped different regions of the brain with the drug Phencyclidine hydrochloride—commonly known as PCP—which simulates schizophrenia. The brain-on-a-chip allowed the researchers for the first time to look at both the drug’s impact on the individual regions as well as its downstream effect on the interconnected regions in vitro. "When the cells are communicating with other regions, the cellular composition of the culture changes, the electrophysiology changes, all these inherent properties of the neurons change," said Ben Maoz, co-first author of the paper and postdoctoral fellow in the Disease Biophysics Group in the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS). "This shows how important it is to implement different brain regions into in vitro models, especially when studying how neurological diseases impact connected regions of the brain."

The brain-on-a-chip could be useful for studying any number of neurological and psychiatric diseases, including drug addiction, post traumatic stress disorder, and traumatic brain injury.

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Researchers at the RIKEN Center for Developmental Biology in Kobe, Japan have shown that retinal transplants derived from induced pluripotent stem cells (iPSCs) can restore visual function in mice. Published in Stem Cell Reports, the study shows that neurons in the implanted retinal sheets integrate well with the host, respond to light, and send their signals to the brain as judged by behavioral experiments. (via NeuroscienceNews)

New research from the University of Maryland suggests that an overload in neural connections early in life (possibly even before birth) are involved in developing autism symptoms. So-called subplate neurons form the first connections in the developing brain, building a network of scaffolding thought to support other neurons grow. If their early progress is impaired, hyperconnections in the developing cortex could result in the neural pathologies observed in human autism. (Cell)

Children of obese parents may be at risk for developmental delays, according to a study by the National Institutes of Health (NIH). Investigators found that children of obese mothers were more likely to fail tests of fine motor skill—the ability to control movement of small muscles, such as those in the fingers and hands. Children of obese fathers were more likely to fail measures of social competence, and those born to extremely obese couples also were more likely to fail tests of problem solving ability. (Pediatrics via NIH)

In a new study of more than 2,400 older black and white adults enrolled in the prospective Health, Aging, and Body Composition program by NIH, UCSF researchers found that poor odor discrimination might be a simple way to predict dementia a decade before symptoms occur. (Neurology via NeuroscienceNews)

The difference between an old brain and a young brain isn’t so much the number of neurons but the presence and function of supporting cells called glia. By examining post-mortem brain samples from 480 individuals ranging from age 16 to 106, researchers found that the state of someone’s glia is so consistent through the years that it can be used to predict someone’s age. The work lays the foundation to better understand glia’s role in late-in-life brain disease. (Cell Reports via NeuroscienceNews)

Biologists at The Scripps Research Institute (TSRI) have identified a brain hormone that appears to trigger fat burning in the gut, called Tachykinin. Their findings, based on digestion in C. elegans roundworms, explains how serotonin in the brain triggers gut neurons to produce the fat-burning hormone in the gut, which could have implications for future pharmaceutical development in humans. (via NeuroscienceNews)

A little-known synesthetic ‘hearing-motion’ phenomenon in which people hear faint sounds following totally silent visual flashes may be more common than previously thought, according to a new study from City, University of London. Although most synesthesias are rare (in ~4% of the population), hearing-motion synesthesia occurred in 22% of the people tested—possibly occurring subliminally, disrupting detection of real auditory signals. (Consciousness and Cognition via NeuroscienceNews)

Long-term heavy use of alcohol in adolescence alters cortical excitability and functional connectivity in the brain, according to a new study from the University of Eastern Finland and Kuopio University Hospital. These alterations were observed in physically and mentally healthy but heavy-drinking adolescents, who nevertheless did not fulfill the diagnostic criteria for a substance abuse disorder. The use of alcohol may be more detrimental to a developing brain than previously thought, although it takes time for alcohol-related adverse effects to manifest in a person’s life. (Addiction Biology)

Scientists at UC Berkeley, Nanyang Technological University in Singapore, Universit√© Paris Descartes and Dartmouth College have found that blinking does more than lubricate dry eyes and protect them from irritants. They found that when we blink, our brain re-positions our eyeballs so we can stay focused on what we’re viewing. When our eyeballs roll back in their sockets during a blink, they don’t always return to the same spot when we reopen our eyes. This misalignment prompts the brain to activate the eye muscles to realign our vision. (Current Biology)

Researchers from the University of Zurich, Switzerland found that LSD consumption alters perception via the serotonin 2A receptor type. A coherent self depends on a functioning network of the cortical mid-brain structure. Under the influence of LSD, this network is disrupted, leading to excessive or exaggerated attributions of meaning to experiences and environmental stimuli. (Current Biology)