Archive for 13/12/2009

Include the thalamus

ScienceDaily has a report of two studies on the thalamus from M. Sherman’s lab by B. Theyel and D. Llano and by C. Lee. (here).

Two new studies show that the thalamus-the small central brain structure often characterized as a mere pit-stop for sensory information on its way to the cortex-is heavily involved in sensory processing, and is an important conductor of the brain’s complex orchestra. …”The thalamus really hasn’t been a part of people’s thinking of how cortex functions,” said Sherman, “It’s viewed as a way to get information to cortex in the first place and then its role is done. But the hope is these kinds of demonstrations will start putting the thalamus on the map.”… information makes a stopover in the thalamus before being sent to the visual cortex of the brain to be processed. Similarly, auditory and somatosensory (touch) information is routed through the thalamus before traveling to cortex for more complex processing. …Once sensory information reaches the cortex, it is thought to remain segregated there as it moves from primary cortex to secondary cortex and higher-order areas. But when Theyel severed the direct connection between primary and secondary cortical regions, stimulating primary somatosensory cortex still activated secondary cortex as well as the thalamus, suggesting a robust pathway from cortex to thalamus and back. Only when the thalamus itself is interrupted does the activation of secondary cortex fail. The observation that at least a portion of sensory information passes back through the thalamus on its travels between cortical areas refutes the notion of the thalamus as a passive, one-time relay station, Theyel and Sherman said. “The ultimate reality is that without thalamus, the cortex is useless, it’s not receiving any information in the first place,” …. “But that may be because as a bottleneck, it provides a convenient way to control the flow of information. It is a very strategically organized structure.” … “These are two parallel streams serving different functions,” Lee said. “The thalamus is also the central hub for transferring information between cortical areas. Rather than carrying information, this second pathway winds up modulating information being sent between cortical areas.”… Both papers newly characterize the complexity of the thalamus and its role in shaping sensory information both before and after that information reaches higher cortical regions — not a crossroads, but a conductor. … “People who study how the cortex functions now have to take the thalamus into account. This can’t be ignored.”

I like to think of the neo-cortex as the thalamus’ on-line computer. The thalamus-cortex loop is certainly part of the neurological basis of consciousness.