New research suggests the brain uses a specific type of neuron in the neocortex to selectively focus attention on specific sensory inputs, while ignoring others.

The neurons are one of the reasons humans can hold a conversation in a crowded, noisy room.

Previous studies showed the electric patterns of neural activity in the neocortex changes when humans begin to focus their attention. Neurons stop firing in synch and begin to fire independently. The cholinergic system and its production of acetylcholine plays an important role in triggering the desynchronization.

The disruption of synchronized activity allowed different neural pathways to respond differently to various inputs, some more important than others.

"Destruction of the cholinergic system in animals profoundly degrades cognition, and the formation of memory," Stephen Williams, professor at the Queensland Brain Institute, said in a news release. "Importantly, in humans a progressive degeneration of the cholinergic system occurs in devastating diseases that blunt cognition and memory, such as Alzheimer's disease."

Williams and his research partner Lee Fletcher want to find out if there was a link between the cholinergic system and layer 5 B-pyramidal neurons.

"The output neurons of the neocortex perform computations that are thought to underlie our perception of the world," Williams said.

The researchers tested their hypothesis by altering the genes of mice so that their cholinergic system only produced acetylcholine when activated with a flash of blue light.

Tests showed that the flash of blue light failed to trigger much activity when layer 5 B-pyramidal neurons weren't activated. But when the output neurons received sensory inputs, the flash of blue light released a flood of neural activity.

"It's as if the cholinergic system has given a 'go' signal," Fletcher said.

Because the activity was isolated among the output neurons receiving excitatory input, scientists confirmed the link between layer 5 B-pyramidal neurons and the cholinergic system as essential to focusing attention.

"We have known for some time that the dendrites of the output neurons of the neocortex only become active when animals are actively performing a behavior, and that this activity is correlated with perception and task performance," Williams said.

Though the new research -- published in the journal Neuron -- only involved mice, Williams and Fletcher think humans also rely on layer 5 B-pyramidal neurons and the cholinergic system's electric switching mechanism for selective focus.

"We suggest that this switch also occurs in the human neocortex, allowing us to rapidly switch our state of vigilance and attention," Williams said. "Our work therefore provides important insight into how the progressive degeneration of the cholinergic system in disease blunts human cognition."

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