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The role of the cortex in complex sensory processing
Neureka! Seminars at the Centre for Developmental Neurobiology, King's College London, had the great pleasure of hosting Prof. Carl Petersen from the EPFL in Lausanne for an online seminar on 23rd June 2020.
Prof. Petersen’s research aims to understand how sensory information is processed and communicated between different cortical regions. The specific questions Prof. Petersen’s research addresses include:
- How can different sensory cues be used to build a specific behaviour?
- What are the intermediate circuits that bridge sensory input and behavioural output?
From an evolutionary perspective, the neocortex is the most recent addition to the brain. It comprises, in particular, the motor cortex (instructs the generation of movements), the auditory cortex (processes auditory information), and the somatosensory cortex (processes touch information from the entire body).
Let's consider a specific example: 100m runners, like Usain Bolt, have to wait for and process several environmental cues before starting their race. They have to listen to the « on your marks » order from the referee to get ready, see the referee's arm ready to lift the starting gun and wait for the sound of the starting gun before starting to run. This follows a specific temporal sequence. Runners cannot start running before the starting gun is fired, or they would face disqualification (take again the striking example of Usain Bolt in the 100m World Championship final in 2011).
Prof. Petersen’s latest research builds upon a behavioural task where a mouse receives 3 different sensory cues in a particular temporal sequence, but the presence or absence of the intermediate cue will determine if the trial is rewarded or not. The behavioural trial starts with a visual cue and ends with an auditory cue. Animals are first pre-trained with these two cues, so they can learn the start and endpoints of the trial i.e. they must start paying attention when they see the visual stimulus, and give their motor response, such as licking a reward spout for a sweet reward, when the auditory cue is heard.
During training, a tactile whisker stimulation is introduced in between the visual and auditory cues. In « Go trials » where the whisker stimulation is applied, the mouse must lick the spout for reward following the auditory cue, when it does so, this is considered a “Hit”, whereas failure to do so is considered a “Miss”. In “NoGo trials” where no whisker stimulation is applied, the animal must not lick following the auditory cue, this is “Correct Rejection”, whereas licking on these trials is counted as a “False alarm”. It was observed that novice animals tended to make several false alarms, in other words, they licked on "NoGo trials". As training continued and animals transitioned from novices to experts, they learnt to lick only on « Go trials » and withhold licking on “NoGo trials”.
Prof. Petersen’s group then investigated cortical brain activity in behaving animals using widefield calcium imaging and extracellular electrophysiological recordings. The combination of these two techniques gave simultaneous access to the activity in many regions of the neocortex such as motor, visual, auditory somatosensory and higher-order cortices.
At the start (visual cue) and end (auditory cue) of the trial, visual and auditory cortices were activated respectively. On « Go trials », where the whisker stimulation was applied, primary and secondary somatosensory cortices were also activated. On trials where the animal licked following the auditory cue, a frontal region of the motor cortex, that controls tongue and jaw movements (tjM1) was activated.
Two major differences were observed in the brains of expert animals compared to novice animals. First, on “Go trials”, tjM1 was inhibited during the period between the whisker cue and the auditory cue in expert animals. It was only activated following the auditory cue to initiate licking. This was not observed in novice mice. Secondly, and most interestingly, a premotor area in the frontal cortex, called the anteriolateral motor cortex (ALM), was activated during the period between whisker and auditory stimulation. This activation resembled preparatory activity that builds up in frontal regions as the mouse prepares to lick. Moreover, optogenetic inactivation of tjM1 following the auditory cue led to fewer licks, which is likely due to the disruption of lick motor commands that originate from this area. Inactivation of ALM in the period between the whisker stimulus and the auditory cue also led to fewer licks, presumably due to the disruption of preparatory activity in this region.
Overall, this study from Prof. Carl Petersen’s laboratory gives us a glimpse of the several neocortical regions and intermediate circuits that are involved in complex sensory processing and sensory to motor transformations. It also highlights the adaptability of the brain and shows how circuits can be modulated by learning and be able to respond to ever-changing environmental cues.
For more details on this study, please see the paper preprint on Biorxiv.
Banner image: Esmaeili et al. Biorxiv 2020
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