Studies On Neurons' Activities In Rat Medial Prefrontal Cortex In Stimuli-guided Decision Making Task

We use the multiple sensory inputs such as visual,auditory and tactile as well as their combinations to sense the outside world.To effectively and correctly respond to stimuli or events in an usually complex and dynamically changing environment,our brain must accurately encode them at first.Encoding sensory information is extremely critical not just for decision-making but for behavioral activities,learning,and social interactions etc.Despite that,we know little about the neural mechanism underlying how the brain differentiates sensory cues and then rely on it to perceptually form the corresponding decision making.Medial prefrontal cortex(mPFC)is the high-order cortex and receives sensory inputs from all sensory cortices such as visual,auditory and tactile cortices.mPFC is believed to play important role in sensory encoding,learning,memory and decision making.In this study,we design a novel behavioral task in which rats need to learn differentiation among 3KHz pure tone,10KHz pure tone,flash of light and their combinations(3KHz+light,10KHz+light).If the stimulus triggered by animal's head porting into the central port hole(see figure 1)is 3KHz pure tone,animals should go right to get water reward.Reversely,if triggered stimulus is 10KHz+Light,animals should go left to get reward.When other stimuli occurred,animals should make no behavioral choice.Once well trained,animals got a surgery to be implanted multiple channel electrodes in mPFC.Through recording and analyzing mPFC neurons' signals in the behavioral task,we tended to reveal how mPFC involves in sensory encoding and decision-making.The experiment was conducted on 8 rats.Through a 4-6 months of training period,all of them could complete the task well and the general correct rate can reach above 75%.We totally recorded 508 neurons from these 8 animals,and 328 of them appeared to be involved in this task.108 neurons responded to sensory stimulus including 74 neurons had strong spike activities to both auditory and auditory-visual stimuli and 34 neurons responded to not only auditory but also visual stimuli alone.Behavioral training seemed to make some part of neurons establish strong capabilities to differentiate those 5 stimuli presented in this behavioral task,which is true especially in those 74 neurons.Generally,they show strong activities to stimuli associated with reward,that is,responses to 3KHz pure tone and to 10KHz+Light were much stronger than those to 3KHz+Light and 10KHz pure tone.The reason might be that mPFC neurons' activities not just reflect sensory signals but merge the signals such as decision-making or reward etc.That's why neurons' responses to reward-associated stimuli were much greater.However,excluding factors such as water reward and behavior choice,we find that the neurons selectively exhibit more robust acti,vities to 3KHz pure tone than to lOKHz+light,suggesting that neurons still kept strong discernable capability to them.This result indicates that the way mPFC neurons encoded multisensory stimuli in the behavioral task state is different from what we saw before.In contrast,34 neurons who responded to all of three sorts of stimuli only showed weak discernable capability.We analyzed 140 neurons who did not showed obvious sensory response in detail,and found those neurons showed the strong directional preference between left and right direction.Meanwhile,it was noted that the decision-signal in cross-modal stimulus condition was formed earlier than in 3KHz pure tone condition,the phenomenon which is consistent with previous studies conducted in both animals and humans.The results in this study disclosed how mPFC neurons responded to different sorts of stimuli with reward and non-reward association,and how they formed the decision making in the behavioral task,which theoretically enriches our understanding of the neural mechanism regarding how the brain processes sensory signals and form/change decision-making dynamically in the complex environment.Considering mPFC as higher-order cortices plays the critical rode in conception and behavioral decision-making as found here,our result could provide the theoretical knowledge for clinical examination of diseases involving the damage of prefrontal cortex.