In a new study, published in
eLife, we investigated how visual search templates are
reactivated to act as input filters for target detection. How the brain maintains a template of
the target of your search (your house keys, for example) has been a
much-debated topic in neuroscience for the past 30 years. Previous research has
indicated that neurons specialized for detecting the sought-after object when
it is in view are also pre-activated when we are seeking it. This would mean
that these ‘template’ neurons are active the entire time that we are searching.
We recorded brain activity from human volunteers using
magnetoencephalography (MEG) as they tried to detect when a particular shape
appeared on a computer screen. The patterns of brain activity could be analyzed
to identify the template that observers had in mind, and to trace when it became
active. This revealed that the template was only activated around the time when
a target was likely to appear, after which the activation pattern quickly
subsided again.
We also found that holding a template in mind largely corresponded
with different MEG patterns to those activated after a stimulus with the same
orientation appeared on a computer screen. This is contrary to the idea that
the same cells are responsible both for maintaining a template and for perceiving
its presence in our surroundings. The brief activation of the template suggests templates may come online just
in time to filter new sensory evidence to detect targets. This mechanism could
be advantageous because it lowers the amount of neural activity (and hence
energy) needed for the task. Although this points to a more efficient way in
which the brain searches for targets, these findings need to be replicated using
other methods and task settings to confirm whether the brain generally uses templates
in this way. For instance, we would like to know more about where in the brain
such a filter may be set up.
Reference:
Myers, N. E., G. Rohenkohl, V. Wyart, M. W. Woolrich, A. C. Nobre and M. G. Stokes (2015). "Testing sensory evidence against mnemonic templates." Elife 4.
