*New Worlds: The brain’s 3D compass*

Weizmann Institute of Science scientists have now for the first time demonstrated the existence of such a 3D compass in the mammalian brain.

March 28, 2015 22:11
Human brain

An image of the human brain. (photo credit: REUTERS)

Pilots are trained to guard against vertigo – a sudden loss of the sense of vertical direction that renders them unable to tell up from down and sometimes even leads to crashes. Coming up out of a subway station can produce similar confusion: For a few moments, you are unsure which way to go, until regaining your sense of direction.

In both cases, the disorientation is thought to be caused by a temporary malfunction of a brain circuit that operates as a three-dimensional (3D) compass.

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Weizmann Institute of Science scientists have now for the first time demonstrated the existence of such a 3D compass in the mammalian brain.

As recently reported in Nature, the researchers showed that bat brains contain neurons that sense which way the bat’s head is pointed and could therefore support the animal’s navigation in 3D space.

Navigation relies on spatial memory – past experience of different locations. This memory is formed primarily in a deep-seated brain structure called the hippocampal formation.

In mammals, three types of brain cells located in different areas of the hippocampal formation form key components of the navigation system. They are “place” and “grid” cells, which work like a GPS, allowing animals to keep track of their position; and “head-direction” cells, which respond whenever the animal’s head points in a specific direction, and act like a compass.

Much research has been conducted on place and grid cells, whose discoverers were awarded the 2014 Nobel Prize in Physiology or Medicine, but until recently, head-direction cells have been studied only in two-dimensional (2D) settings, in rats, and very little was known about the encoding of 3D head direction in the brain.

To study the functioning of head-direction cells in three dimensions, Weizmann scientists developed a tracking apparatus that allowed them to video-monitor all the three angles of head rotation – in flight terminology, yaw, pitch and roll – and to observe the movements of freely-behaving Egyptian fruit bats. At the same time, the bats’ neuronal activity was monitored via micro-electrodes implanted in their heads. Recordings made with the help of these micro-electrodes revealed that in a specific sub-region of the hippocampal formation, neurons are tuned to a particular 3D angle of the head; certain neurons became activated only when the animal’s head was pointed at that 3D angle.

Though the study was conducted in bats, the scientists believe their findings should also apply to non-flying mammals, including squirrels and monkeys that jump between tree branches, as well as humans.

The study also revealed for the first time how the brain computes a vertical direction, integrating it with the horizontal. It turns out that in the neural compass, these directions are computed separately, at different levels of complexity. The Rehovot scientists found that head-direction cells in one region of the hippocampal formation became activated in response to the bat’s orientation relative to the horizontal surface, that is, facilitating the animal’s orientation in two dimensions, whereas cells responding to the vertical component of the bat’s movement – that is, a 3D orientation – were located in another region.

The researchers believe that the 2D head-direction cells could serve for locomotion along surfaces, as happens in humans when driving a car, whereas the 3D cells could be important for complex maneuvers in space, such as climbing tree branches or, in the case of humans, moving through multi-story buildings or piloting an aircraft.

By further experimenting on bats hanging head-down, the scientists were able to clarify exactly how the head-direction signals are computed in the bat brain. It turned out that these computations are performed in a way that can be described by an exceptionally efficient system of mathematical coordinates. Thanks to this computational approach used by their brains, the bats can efficiently orient themselves in space whether they are moving head up or down.

This research supports the idea that head-direction cells in the hippocampal formation serve as a 3D neural compass.

CAN’T SING? DO IT MORE OFTEN If you’ve ever been told that you’re “tone deaf” or “can’t carry a tune,” don’t give up; there is hope. New research from Northwestern University in Illinois suggests that singing accurately is not so much a talent as a learned skill that can decline over time if not used. The ability to sing on key may have more in common with the kind of practice that goes into playing an instrument than people realize, said lead researcher and music education Prof. Steven Demorest.

“No one expects a beginner on violin to sound good right away; it takes practice, but everyone is supposed to be able to sing,” Demorest said. “When people are unsuccessful, they take it very personally, but we think if you sing more, you’ll get better.”

Published in a recent issue of the journal Music Perception, the study compared the singing accuracy of three groups – kindergarteners, sixth graders and college-aged adults. One test asked the volunteers to listen to four repetitions of a single pitch and then sing back the sequence, while another asked them to sing back at intervals.

The study showed considerable improvement in accuracy from kindergarten to late elementary school, when most children receive regular music instruction. But in the adult group, the gains were reversed – to the point that college students performed at the level of the kindergarteners on two of the three tasks, suggesting the “use it or lose it” effect. Singing on key is likely easier for some people than others, “But it’s also a skill that can be taught and developed, and much of it has to do with using the voice regularly,” Demorest said. “Our study suggests that adults who may have performed better as children lost the ability when they stopped singing.”

By eighth grade, only 34 percent of US children participate in elective music instruction, Demorest said. That number declines as they near high school graduation.

Children who have been told they can’t sing well are even less likely to engage with music in the future and often vividly remember the negative experience well into adulthood. Being called “tone deaf” can have devastating effects on a child’s self-image, the researchers wrote in the study.

Demorest worries that singing can serve as a barrier to other musical activities. “So much of elementary school music revolves around singing, but that’s only one way to measure musicality,” he said. “Everyone should be able to have music as a part of his life.”

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