From the swamp to the cocktail party

Longin

“We can tell which brain parts are likely engaged in a task, but we still do not know how that task is actually accomplished by cells.”

– André Longtin

It is among our most impressive everyday skills. Standing in a crowded room, surrounded by background music and a variety of conversations, we think nothing of being able to make sense of what the individual in front us is saying.

It is easy to take this ability for granted, but it is no less impressive than the fact that hundreds of frogs croaking in a swamp at night seem to identify one another by the fre­quency and number of their calls. This behaviour seems to originate from a middle portion of the brain called the torus semicircularis, which processes signals from the ears. The same structure is found in the brains of frogs and humans, as well as those of other animals whose survival depends on being able to sort out complex arrays of sound.

Physicist André Longtin dubs this elaborate filtering mechanism the “cocktail party algorithm.” The brain function essentially sorts what is a useful signal from a meaningless, noisy or simply redun­dant background.

“No one knows how it works,” he confesses. “We are extremely good at it. Frogs are good at it. Most animals are good at it, just to survive. But the substrate is not clear.”

Longtin has spent the past three decades pondering the distinctions between noise and deterministic patterns in the brain, as well as physical systems in general. His passion began in the 1980s with mathematical models of the human auditory reflex. This work, and the technology used to explore the workings of the nervous system, have progressed significantly since then. But many of the underlying challenges have persisted.

“We’ve just figured out how the neuroplasticity of a fish’s brain can remove redundant activity,” he explains. “We and others are now also getting at the circuitry that seems to underlie voluntary action.”

He notes the success of methods like functional magnetic resonance imaging, which can track brain activity in subjects as they conduct tasks such as identifying words or expressing emotions. The results are often tantalizing, and regularly accompanied by a great deal of hype suggesting brain scans can reveal such things as when we are lying. Longtin responds with one of the harsher words in the physics vocabulary: messy.

“You’ve got messy data, few data points, huge error bars,” he argues. “We can tell which brain parts are likely engaged in a task, but we still do not know how that task is actually accomplished by cells. We don’t know what the ‘neural computation’ is.”

He adds that his own academic background builds on the research foundation laid by biology and biochemistry. “Physics and applied math can give you some of the tools to go and find the organizing measures that can tell you what’s changing and predict what will happen next.”

In his search for those measures, Longtin has turned to brains much simpler than ours, such as those of frogs or fish. There it should be easier to find links between the signals essential to particular func­tions, like communication or navigation, and the actions of particular arrangements of cells. Nevertheless, things get complicated very quickly, even as researchers move from the study of a single cell to small groups of cells.

“Nature uses resources in ways you wouldn’t think are intelligent at first glance,” he explains, pointing to how parts of the brain seem to handle information in much the same way computers conduct parallel processing.

In fact, the natural world could offer valuable contributions to fields such as information technology. If we learn how frogs have solved their signal processing problems in the swamp, for example, engineers may be able to refine the challenges they face on computer networks.

To find answers to such complicated questions, Longtin points to the need to strike up enduring interdisciplinary partnerships. After an extensive collaboration, Longtin and Leonard Maler, a professor in the Department of Cellular and Molecular Medicine, founded the University’s Centre for Neural Dynamics in 2004. This virtual orga­nization has built bridges between investigators in areas ranging from psychology and mathematics to systems biology and surgery.

More recently, Longtin has been working with Georg Northoff, a psychiatrist at the Royal Ottawa Hospital who studies how individuals react when their mind is unfocused and wandering, and the brain might be assumed to be “resting.” Under these conditions, the brain may not have to cope with the cacophony of a cocktail party, but it nevertheless remains ready to sort out a wide variety of sights and sounds.

“We’re trying to figure out what is it about this resting state, with its fluctuating neural activity, that makes you sensitive to certain stimuli and not others, and how is it altered in disease,” says Longtin. “There might be something worth pursuing there, from a modelling perspective, something very exciting.” 

 
by Tim Lougheed

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