Modulation Of Visual Signals On Responsiveness Of Auditory Cortical Neurons In Cue-discrimination Tasks

In the nervous system,sensory information of different sensory modalities can interact with each other,but our understanding of it is very limited,especially the role of sensory cortices(such as auditory and visual cortex)in this complex process.In this study,we used the multi-channel recording technique to record the responsiveness of auditory cortical neurons to sensory stimuli(auditory,visual and audiovisual combined stimuli)in rats who were free to move and completed the task of perceptual discrimination.We studied the modulation of visual signals on auditory cortex neurons.Firstly,we study how visual information modulates the response of neurons to different target auditory stimuli in the task of distinguishing auditory cues.In this task,animals first need to trigger a target stimulus(high frequency sound,low frequency sound,high frequency sound + flash of light,low frequency sound + flash of light)and then make choice selection.The results showed that after nearly two months of training,animals could perform this task well.By recording and analyzing the response of animals to different target stimuli in the process of performing tasks,the results showed that most of the auditory cortex neurons recorded exhibited obvious preference for target sound(i.e.,there was a significant difference between auditory responses).The proportion of neurons with showing auditory preference was much higher than that of animals in anesthetized state.In this task,the visual component of audiovisual stimulus does not provide any clues related to the choice selection,but it greatly affected the auditory responses.This effect was mainly achieved by differential modulation modes,that is,the response to preferred auditory stimulus was facilitated,while the response to non-preferential auditory stimulus is affected less or inhibited.This differential modulation enhances the neuron's capability to discriminating related stimuli.At the same time,through this differential modulation,some neurons with no discrimination ability to target sound could also show the capability of discriminating joint auditory-visual stimuli(high frequency sound + flash of light vs.low frequency sound + flash of light).However,this modulation effect is greatly affected by task-context.When a single visual stimulus(i.e.,flash of light)was added to the pool of target stimuli,which increased the complexity of the task,modulation mode was changed.In more neurons,visual information inhibited rather than facilitate preferred auditory response.Thus,in this task,the number of neurons that can distinguish two audiovisual stimuli is significantly less than the number of neurons that can distinguish two auditory stimuli.The results indicated that the interaction between different senses is greatly influenced by the complexity of the task and other interference factors.In experiments shown above,we also found that the response of auditory cortical neurons to target auditory stimuli during perceptual tasks is low,so we did some additional experiments to examine it.We found that the response of neurons to target auditory stimuli is much smaller than that of non-target auditory stimuli with frequency spectrum closed to target sound.This response pattern is only expressed in perceptual tasks,and is not observed during the anesthetized state.The completion of behavioral manipulation in the task has no influence for it.We guess attention might play the critical role in this process.This study in rat auditory cortex will help us to re-understand the function of sensory cortex in cross-modal integration and provide important experimental evidence for elucidating the neural mechanism of multisensory information processing in the brain.It is worth mentioning that our experiment was carried out under the condition that rats were free to move and completed perceptual tasks.Thus,the experimental results will better reflect the mechanism of brain information processing.