Differential Roles Of Insula Subregions In Auditory Signal Processing

The insula is a hidden part of the cerebral cortex and relates to many cognitive processes and mental illness of humans.Previous studies regarding functions of the insula have mainly focused on its role in the processing of interoception information.There has been little research on the processing of external sensory information(e.g.,auditory)within the region.In addition,due to limited temporal resolution of currently available techniques,studying of detailed mechanism regarding the flow of information processing in the sub regions of the insula remains incomplete.Taking advantage of the high spatiotemporal resolution of direct intracerebral recordings with intracranial stereotactic electroencephalography(sEEG)techniques,this study aims to explore the mechanism of auditory information processing in the insula during auditory oddball tasks.Local field potentials were recorded from thirty-one insular sites in 3 epileptic patients with surgically implanted electrodes.Classical auditory oddball paradigm was used in the experiments.The sequence of auditory stimulation consists of a standard stimulation at 800 Hz or 1500 Hz with an intensity of 80 dB and a novel stimulation with the same intensity at 1500 Hz or 800 Hz.The analysis of event-related potentials indicated that all the pure tones evoked an early negative wave N1 with a latency of around 130 ms in the insula.In addition,a large positive wave with a latency of approximately 300 ms,P300,was also observed in the anterior insula and most part of the posterior insula following the auditory stimulations with novel frequencies.Both the latency and the amplitude of the N1 component recorded from either the anterior or the posterior insula changed significantly with different frequencies of the auditory stimulations.The difference in the N1 amplitude between the standard and novel stimuli was also statistically significantly in both subregions.On the other hand,there existed some difference in the responses to the auditory stimulations between the anterior and posterior insula.The N1 latency tended to be shorter(p=0.06)in the posterior than in the anterior region.The N1 amplitude was significantly different between the two regions.Moreover,the difference in the N1 amplitude caused by low and high frequencies of stimuli was smaller in the anterior than in the posterior insula.But such a difference was not observed with standard and novel stimulations.These findings suggest that both the anterior and posterior insula are involved in the encoding of the physical properties(i.e.,frequency)of sound,as well as in the identifying of novel stimulations.ANOVA analysis of P300 revealed that both the latency and amplitude of the P300 changed with the alteration of the auditory frequency.When the latencies of P300 were compared between the anterior and the posterior insula,no difference was found.Interesting,both N1 and P300 showed an interaction between recording sites and auditory frequencies.Furthermore,the linear regression analysis between the latencies,or amplitudes,and y-axis MNI coordinates showed that the latencies of both N1 and P300 became longer as the recording site moved from posterior to anterior.The amplitudes of the two components were the other way around.Taken together,our findings indicate that pure tones can evoke an early auditory eventrelated potential N1 in the human insula,while novel auditory stimuli can induce a later P300.The short latency and the frequency-responsive property of the N1 component suggest that the component may be related to the encoding of sensory information.The trend of a linear change from posterior to anterior in the latency and amplitude of the potential suggests that the insula encodes auditory information by a sequential process from posterior to anterior.