The cortical electromicrophysiology of pathological delta waves in the electroencephalogram of cats

Abstract

Pathological delta waves produced by lesions of the subcortical white matter, the mesencephalic reticular formation or of the thalamus show similar microphysiological features. Computer analysis of the relationship between these delta waves and cortical unit discharges demonstrate that the latter are statistically correlated with surface positivity of the delta waves, while surface negativity is correlated with a significant reduction of probability of discharge of cortical neurons. These findings suggest that surface positive delta waves may represent the sums of deep intracortical EPSPs, while surface negative waves may represent the sums of deep intracortical IPSPs. The unit-slow wave relationships for delta waves induced by thalamic lesions were, however, more variable and often less clearcut than for those induced by lesions of the white matter or of the mesencephalic reticular formation. Also the thalamically induced delta waves occasionally showed increased probability of unit discharge with surface negativity and a decrease with surface positivity. Laminar profiles of the three types of delta waves were plotted by means of a computer averaging program. Delta waves produced by white matter lesions showed phase reversal at a mean cortical depth of 965 ± 160 μm. The corresponding figure for delta waves produced by mesencephalic reticular formation lesions was 1380 ± 375 μm. (The two values are significantly different, P > 0.005). For slow waves induced by thalamic lesions phase reversals occurred at more variable depths and were often difficult to establish. Generally, delta waves decreased greatly in amplitude with increasing depth in the cortex; this was also true for phase inverted waves in the depth. In about 1 out of 4 instances, the amplitude reduction in the depth was such that the delta wave was no longer detectable in the deeper cortical layers and no phase inversion could be established. Tangential surface recordings of delta waves showed that there was no significant current flow over small distances. These findings indicate that delta waves are generated by radially oriented cellular elements. Pyramidal neurons, particularly those of the Vth cortical layer, qualify for this role. In the light of our findings a significant contribution by glial cells to delta wave production can be ruled out.