Abstracts

Rationale: First described by Adrian in 1936, afterdischarges (ADs) are runs of rhythmic focal epileptiform activity induced by cortical electrical stimulation (CES). Observations in experimental preparations (e.g. Pinsky & Burns, J Neurophysiol 25:359-79, 1962) and humans undergoing extraoperative mapping (e.g. Blume et al, Clin Neurophysiol 115(4):982-9, 2004) document sufficient stimulus intensity for ADs to occur. Other factors predictive of ADs are brain location and low-frequency power in the background electrocorticogram (ECoG) at the stimulus site (Lesser et al, Brain 131(6):1528-39, 2008). However, it remains unclear why and how ADs occur following CES. We propose a mechanism here, based on human ECoG data. Methods: Extraoperative CES ECoG data over the left temporal lobe were reviewed on four (N=4) patients undergoing presurgical subdural grid evaluation. 1079 bipolar stimuli ranging from 2-10 mA were delivered intermittently over several hours in each patient during standardized motor and language testing. Forty-two ADs were observed, 32 of which lasted ≥ 8 seconds and were analyzed (n=32). Power spectra of AD segments were compared to those from length-matched segments of baseline ECoG and following subthreshold stimuli. Results: All four morphologies of AD described by Blume et al (2004) were observed: rhythmic waves, spike-waves, polyspike bursts, and sequential spikes; 12/32 (37.5%) of ADs had features of more than one type. Power spectra of background ECoG epochs from all tested channels - including those subsequently exhibiting ADs - were broadband and multimodal, with peaks in one or more of the classical (α, β, δ, etc.) EEG bands. Power spectra of ADs bore a consistent relation to background ECoG spectra from the same channel, in one of three ways that depended on Blume type: (i) a ‘condensing' or ‘clumping' relation, comprising lessening of the breadth of the spectrum and AD spectral peaks appearing at the same locations, but less dispersed than corresponding peaks from the background ECoG; (ii) condensation plus harmonics, where in addition, peaks appeared at selected harmonics of condensed peaks and (iii) harmonic-dominant, where there was minimal condensed peak spectral power, with harmonic content dominating. Moreover, power spectra of epochs following subthreshold stimuli differed from baseline spectra in a similar though less prominent fashion. Conclusions: Power spectral analysis of ADs following CES indicate that ADs, rather than representing de novo change, arise fundamentally from a ‘reorganization' of background brain rhythms. Reorganization takes the form of ‘condensation' of pre-existing peaks, as well as a process that generates higher harmonics of the condensed spectral clusters. Together, these features synchronize the baseline multifrequency ECoG into the coherent rhythm of an AD, with their relative contributions determining Blume type. Putative underlying neurophysiological mechanisms will be discussed; a larger study including quantitative modeling of the phenomena seen here is under way.