Abstracts

Rationale: Neuronal adaptation is defined as a reduced neural response to a repeated stimulus. Brain is believed to exert the adaptation process to cope with sensory overload and save attention for novel stimuli. Neuronal adaptation can be demonstrated by reduced augmentation of event-related gamma-oscillations, but a previous study using ECoG failed to demonstrate such adaptation effects in the primary visual area by measuring visually-driven gamma-augmentation at <70 Hz (Privman et al., Cereb Cortex 2011). Here, we determined whether adaptation effects in human primary visual cortex can be demonstrated using gamma-oscillations at >80 Hz. We also determined whether pre-stimulus oscillatory measures can predict the degree of neural adaptation for an upcoming stimulus.Methods: We studied 13 epileptic patients who underwent extraoperative video-ECoG. ECoG was recorded with the sampling frequency at 1,000 Hz. Electrodes overlying seizure onset zones or MR lesions were excluded from analysis. Patients were given four blocks of flash stimuli; each block contained a series of 50 stimuli at a different inter-stimulus interval ranging from 0.2 to 2.0 sec. Using time-frequency analysis, the amplitude and phase of cortical oscillations were determined at a given moment. We determined whether the degree of visually-driven gamma-augmentation at >80 Hz was gradually reduced over trials. We also determined whether photic stimuli with shorter inter-stimulus intervals, compared to those with longer, would yield smaller augmentation of gamma oscillations at >80 Hz. Using multivariate regression models, we determined whether the number of prior stimuli, the length of inter-stimulus interval, and pre-stimulus oscillatory measures can independently predict the gamma-amplitude elicited in a given trial.Results: Stimuli elicited augmentation of gamma-oscillations at 80-150 Hz in the occipital cortex at about 50 to 100 msec, and such gamma-augmentation was most prominent in the medial occipital region. The degree of visually-driven gamma-augmentation was gradually reduced over trials. Photic stimuli with shorter inter-stimulus intervals yielded smaller gamma-augmentation. Gamma-augmentation was followed by lingering beta-augmentation and gamma-attenuation; thus, photic stimuli at short inter-stimulus intervals were given while beta-augmentation and gamma-attenuation were present. Multivariate analysis demonstrated that a larger number of prior stimuli, a shorter inter-stimulus interval, and an attenuated pre-stimulus gamma-amplitude independently predicted a reduced visually-driven gamma-augmentation in a given trial (p<0.01), while pre-stimulus beta-amplitude and delta-phase had no predictive value.Conclusions: Neural adaptation in the primary visual cortex was demonstrated as a gradual reduction of visually-driven gamma-augmentation over trials. Reduced neural responses independently associated with pre-stimulus gamma-attenuation can be explained by the notion that gamma-attenuation following visually-driven gamma-augmentation represents a refractory period.