Scientists at the Salk Institute for Biological Studies have demonstrated that sensory¹ regions in the brain develop in a fundamentally different way than previously² thought, a finding that may yield new insights into visual and neural³ disorders⁴. In a paper published June 7, in Science, Salk researcher Dennis O'Leary and his colleagues have shown that genes⁵ alone do not determine how the cerebral⁶ cortex grows into separate functional⁷ areas. Instead, they show that input⁸ from the thalamus（丘脑）, the main switching station in the brain for sensory information, is crucially required.

 

O'Leary has done pioneering studies in "arealization," the way in which the neo-cortex, the major region of cerebral cortex, develops specific areas dedicated⁹ to particular functions. In a landmark¹⁰ paper published in Science in 2000, he showed that two regulatory genes were critically responsible for the general pattern of the neo-cortex, and has since shown distinct roles for other genes in this process. In this new set of mouse experiments, his laboratory focused on the visual system, and discovered a new, unexpected twist to the story.

 

"In order to function properly, it is essential that cortical areas are mapped out correctly, and it is this architecture that was thought to be genetically¹¹ pre-programmed," says O'Leary, holder¹² of the Vincent J. Coates Chair in Molecular¹³ Neurobiology at Salk. "To our surprise, we discovered thalamic input plays an essential role far earlier in brain development."

 

Vision is relayed from the outside world into processing areas within the brain. The relay starts when light hits the retina, a thin strip of cells at the back of the eye that detects color and light levels and encodes the information as electrical and chemical signals. Through retinal ganglion cells, those signals are then sent into the Lateral¹⁴ Geniculate Nucleus¹⁵ (LGN), a structure in thalamus.

 

In the next important step in the relay, the LGN routes the signals into the primary visual area (V1) in the neo-cortex, a multi-layered structure that is divided into functionally¹⁶ and anatomically distinct areas. V1 begins the process of extracting visual information, which is further carried out by "higher order" visual areas in the neo-cortex that are vitally important to visual perception. Like parts in a machine, the functions of these areas are both individual and integrated. Damage in one tiny area can lead to strange visual disorders in which a person may be able to see a moving ball, and yet not perceive it is in motion.

 

Current dogma holds that this basic architecture is entirely¹⁷ genetically determined¹⁸, with environmental input only playing a role later in development. One of the most famous examples of this idea is the Nobel Prize-winning work of visual neuroscientists David Hubel and Torsten Wiesel, which showed that there is a "critical period" of sensitivity in vision. Their finding was commonly interpreted as a warning that without exposure to basic visual stimuli¹⁹ early in life, even an individual with a healthy brain will be unable to see correctly.

 

Later discoveries in neural plasticity more optimistically suggested that early deprivation²⁰ can be overcome, and the brain can even sprout²¹ new neurons in specific areas. Nevertheless, this still reinforced the idea that environmental influences might modify neural architecture, but only genetics could establish how cortical areas would be laid out.

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