New insights into the perceptual process Evidence suggests that the interaction of feed-forward and feedback information plays a key role in cognitive processing, helping to shape the way we experience the world. We spoke to Professor Matthew Larkum of the ActiveCortex project about his group’s work in testing a hypothesis for explaining the perceptual process The first results
Feed-forward information Approximately 80 percent of the cortex is made up of these pyramidal cells, which stretch vertically across the layers of the cortex, and play an important role in processing information. Interestingly, the cortex conserves this architectural aspect everywhere, which Larkum interprets as meaning that understanding the role of pyramidal neurons is the key to unlocking the secrets of the cerebral cortex and therefore the basis of mammalian intelligence. The cortex is wired such that information from the outside world and internal information predominantly arrive at opposite ends of the pyramidal neurons. “For example, when seeing a tiger, your retina delivers the visual information that hits the bottom of your pyramidal neurons near the cell body, and causes them to fire in a steady, low-frequency way,” continues Professor Larkum. On the other hand, if you just think about a tiger but don’t see it, then information from your internal representation of the tiger causes signals to be transmitted predominantly to the top of your pyramidal neurons. “If you had very strong thoughts about the tiger, you might even get a sporadic highfrequency response from these cells,” continues Professor Larkum.
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Chris Schwarz/Shutterstock However, what’s really special about these neurons is that they act like coincidence detectors and completely change their output when information arrives at both ends of the neurons
India, you might suspect there’s a tiger, whereas the same orange color in the local park would not necessarily bring tigers to mind. This ‘bringing to mind’, Larkum argues, is nothing other than putting the right contextual information to the end of the orange neurons and seeing if there’s a match. “In our latest study, we basically looked for calcium in the dendrites – the trigger for coincidence detection – and saw that it does in fact correlate with the moment of perception,” he says. “The clincher was that when we suppressed the coincidence detection mechanism in the dendrites, the animal no longer perceived anything at the same stimulus strength.” This is very exciting because it suggests that higher brain function can be investigated and even manipulated by
We looked for calcium in the dendrites and saw that it does correlate with the threshold for perception. At the point where we know the rodent has recognised the object, we see calcium in the dendrites simultaneously. In this way, Larkum proposes, the pyramidal neuron is crucial for cognition because it acts as an associative device that draws together new information that relates to previously learned information. In other words, if you see an orange object move behind the green grass while in the wild, say, in
focusing on this cellular switch. In another paper published in 2016, his team were able to show that transcranial magnetic stimulation suppresses this cellular switch, suggesting that it could be used as a non-invasive tool for investigating this mechanism in humans during cognitive tasks.
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from this ERC project suggest they have identified a powerful model for understanding the cerebral cortex. The crucial feature, says Professor Matthew Larkum, is an explosive ‘hot spot’ strategically located in the thin dendrites of the main neurons of the cerebral cortex. As unlikely as it may have seemed at first, their most recent data published in the journal Science show that they can alter perception by targeting this hot spot. “The cerebral cortex processes things like orientation, colour, shape and movement in different regions, which all have pyramidal neurons,” he explains.
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How to see a tiger. New information from the retina or previous knowledge about tigers alone have only a subtle influence on neuronal firing, whereas the combination is dramatically different due to activation of a dendritic “hot spot”.
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