Can we really do two things at once?

We are becoming multitasking creatures: we work on computers while listening to music and waiting for that next text message to arrive. A recent study has uncovered new findings on our ability to select relevant information from multiple sources. What mechanism is responsible for filtering several sources of perceptual information? How does our brain choose the right information? Mice have shed new light on the topic...

Depending on the context, we are constantly choosing where to focus our attention. Though scientists were generally in agreement about the role of the prefrontal cortex and the thalamus in focusing attention, it had until now never been proven. But now a team of researchers from NYU's Neuroscience Institute have done just that by conducting a study, the results of which have been published in Nature. Using mice, the scientists managed to create a task that forced them to focus their attention. The circuits underlying this process were then analyzed. Let’s take a look at this incredible experiment.

First, take genetically modified mice and subject them to optogenetic testing. This new area of research makes neurons sensitive to light, allowing them to be activated by light stimulation. We can thus “turn on” or “turn off” certain brain circuits in these mice. Second, with the help of binaural sounds (sounds that appear in the brain due to physical stimulation), the mouse is trained to wait for a reward (milk). Two modalities are used to do so: the mouse is conditioned (by playing a "blue" sound at 11 KHz) to wait for an audio cue and is distracted by a flash of light; or the mouse is conditioned (by playing a “brown” sound at 10 KHz) to wait for a visual cue and is distracted by a sound. In both cases, the mouse must choose the appropriate sensory input and ignore the parasitic input. Thus, using optogenetics, the researchers were able to intentionally disrupt the prefrontal cortex (PFC) to emphasize it role in the selection of auditory and visual stimuli. The electrical activity was measured in this area of the brain, but also in a specific area of the thalamus, the thalamic reticular nucleus (TRN). What did the results show?

First, when the mouse has to focus on the sound and ignore the light to receive the reward, the neurons of the TRN that control hearing become more active than those controlling vision. In the reverse situation, when the mouse must focus its attention on the light rather than the sound, the TRN neurons controlling vision are more active. Next, using optogenetics to “turn on” or "turn off” the brain areas being studied, the researchers were able to determine the workings of the mechanism that controls both attention (focusing on the right perceptual visual or auditory stimulus) and inhibition (neutralization of the irrelevant stimulus). Indeed, when they disrupted (turned off) the PFC, the mouse's ability to appropriately choose contradictory visual/auditory stimuli decreased.

Let’s look at the instance where the mouse has just heard a "blue" sound. It “knows” that if it wants its milk reward, it must concentrate on the sound rather than the flash of light. What happens in the brain? The prefrontal cortex gives the order to the thalamic reticular nucleus to choose the auditory information. Then, the TRN, serving as a switchboard, controls attention by activating the neurons that control hearing and "deactivating" those that control sight.

Finally, this research confirms that we are unable to correctly perform two tasks at the same time since “the anticipation of one task disrupts the performance of the task at hand.” But if modern man is doomed to be more and more “attentive” to multiple and simultaneous sensory inputs, won’t our TRNs eventually become saturated?
Source: Ralf D. Wimmer, L. Ian Schmitt1, Thomas J. Davidson, Miho Nakajima1, Karl Deisseroth & Michael M. Halassa : Thalamic control of sensory selection in divided attention

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