Auditory and visual gratings elicit distinct gamma responses

Abstract

Sensory stimulation is often accompanied by fluctuations at high frequencies (>30Hz) in brain signals. These could be "narrowband" oscillations in the gamma band (30-70 Hz) or non-oscillatory "broadband" high-gamma (70-150 Hz) activity. Narrowband gamma oscillations, which are induced by presenting some visual stimuli such as gratings and have been shown to weaken with healthy aging and the onset of Alzheimers Disease, hold promise as potential biomarkers. However, since delivering visual stimuli is cumbersome as it requires head stabilization for eye tracking, an equivalent auditory paradigm could be useful. Although simple auditory stimuli have been shown to produce high-gamma activity, whether specific auditory stimuli can also produce narrowband gamma oscillations is unknown. We tested whether auditory ripple stimuli, which are considered an analogue to visual gratings, could elicit narrowband oscillations in auditory areas. We recorded 64-channel EEG from male and female (18 each) subjects while they either passively fixated on the monitor while viewing static visual gratings, or listened to stationary and moving ripples, played using loudspeakers, with their eyes open or closed. We found that while visual gratings induced narrowband gamma oscillations with suppression in the alpha band (8-12Hz), auditory ripples did not produce narrowband gamma but instead elicited very strong broadband high-gamma response and suppression in the beta band (14-26Hz). Even though we used equivalent stimuli in both modalities, our findings indicate that the underlying neuronal circuitry may not share ubiquitous strategies for stimulus processing. Significance statementIn the visual cortex, gratings can induce robust narrowband gamma oscillations (30-70Hz). These visual stimulus-induced oscillations can further be used as a biomarker for diagnosing neuronal disorders. However, tasks used to elicit these oscillations are challenging for elderly subjects, and therefore, we tested if we could use auditory stimuli instead. We hypothesized that auditory ripple stimuli, which are analogous to visual gratings, may elicit these narrowband oscillations. We found that ripples induce a broadband high-gamma response (70-150Hz) in human EEG, unlike visual gratings that produce robust gamma. Thus, the underlying neural circuitry in the two areas may not be canonical.