SPATIOTEMPORAL ANALYSIS OF PREPYRIFORM, VISUAL, AUDITORY, AND SOMATESTHETIC SURFACE EEGS IN TRAINED RABBITS

1. Spatial ensemble averages were computed for 64 traces of electroenc ephalograms (EEGs) simultaneously recorded from 8 x 8 arrays over the epidural surfaces of the prepyriform cortex (PPC) and visual, somatic, and auditory cortices. They revealed a common waveform across each ar ray. Examination of the spatial amplitude modulation (AM) of the wavef orm revealed classifiable spatial patterns in short time segments. The AM patterns varied within trials after presentation of identical cond itioned stimuli, and also between trials with differing stimuli. 2. PP C EEGs revealed strong correlates with the respiratory rhythm; neocort ical EEGs did not. 3. Time ensemble averaging of the PPC EEG attenuate d the oscillatory bursts, indicating that olfactory gamma oscillations (20-80 Hz) were not phase-locked to the times of stimulus delivery bu t instead to inhalations. Time ensemble averages of neocortical record ings across trials revealed average evoked potentials starting 30-50 m s after the arrival of the stimulus. 4. Average temporal fast Fourier transform (FFT) power spectral densities (PSDs) from pre- and poststim ulus PPC EEG segments revealed a peak of gamma activity in olfactory b ursts. 5. The logarithm of the average temporal FFT PSDs from pre- and poststimulus neocortical EEG segments, when plotted against log frequ ency, revealed 1/f-type spectra in both pre- and poststimulus segments for negative/aversive conditioned stimuli (CS-) and positive/rewardin g conditioned stimuli (CS+). The alpha'- and beta'-coefficients from t he regression of Eq. 2 onto the average PSDs were significantly differ ent between pre- and poststimulus segments, owing to the evoked potent ials, but not between CS- and CS+ stimulus segments. 6. Spatiotemporal patterns were invariant over all frequency bins in the 1/f domain (20 -100 Hz). Spatiotemporal patterns in the 2- to 20-Hz domain progressiv ely differed from the invariant patterns with decreasing frequency. 7. In the spatial frequency domain, the logarithm of the average spatial FFT power spectra from pre- and poststimulus neocortical EEG segments , when plotted against the log spatial frequency, fell monotonically f rom the maximum at the lowest spatial frequency, downwardly curving to a linear 1/f spectral domain. This curve in the 1/f spectral domain e xtended from 0.133 to 0.880 cycles/mm in the PPC and from 0.095 to 0.6 24 cycles/mm in the neocortices. 8. Methods of FFT and principal compo nent analysis (PCA) EEG decomposition were used to extract the broad-s pectrum waveform common to all 64 EEGs from an array. AM patterns for the FFT and PCA components were derived by regression. They were shown by cross-correlation to yield spatial patterns that were equivalent t o each other and to AM patterns from calculation of the 64 root-mean-s quare amplitudes of the segments. 9. Each spatial AM pattern was expre ssed by a 1 x 64 column vector and a point in 64-space. Similar patter ns formed clusters, and dissimilar patterns gave multiple clusters. A statistical test was devised to evaluate dissimilarity by a Euclidean distance metric in 64-space. 10. Significant spatial pattern classific ation of CS- versus CS+ trials (below the 1% confidence limit for 20 o f each) was found in discrete temporal segments of poststimulus data a fter digital temporal and spatial filter optimization. 11. Varying the analysis window duration from 10 to 500 ms yielded a window length of 120 ms as optimal for pattern classification. A 120-ms window was sub sequently stepped across each record in overlapping intervals of 20 ms . Windows in which episodic, significant CS+/CS- differences occurred lasted 50-200 ms and were separated by 100-200 ms in the poststimulus period. 12. Neocortical spatial patterns changed under reinforcement c ontingency reversal, showing a lack of invariance in respect to stimul i and a dependence on context and learning, as previously found for th e olfactory bulb and PPC. 13. The EEG data contributing to classificat ion were homogeneously distributed across wide temporal and spatial sp ectral bands and across the spatial array of electrodes. Patterns coul d be resolved with as few as 16 channels. No channel was more or less contributory to classification than any other. 14. The aperiodic wavef orms, the rapid global state changes, the context dependence of the AM patterns, and the homogeneous distribution of neural activity suggest that the neural events formed during perception are constructed by co operative population dynamics in both paleocortex and neocortex. These characteristics so far provide the most powerful evidence for spatial ly coherent, aperiodic oscillations manifesting macroscopic cortical s tates that are spatially continuous over areas >5 mm diam and that las t <200 ms.