The chronometrics of cortical excitation as explored with auditory probes

In a series of studies we examined sensory evoked potentials to stimuli presented while subjects performed a primary task. The primary tasks were designed to induce changes in slow cortical potentials (SCP) such as P300, contingent negative variation (CNV), and postimperative negative variation (PINV). Brief auditory stimuli were presented before, during or after the primary task. The interactions between the two types of event-related potentials, SCP and the probe-evoked N 1/P2, served to further explore the nature of SCP, particularly in its relation to excitability in cortical neuronal networks. The functional state indicated by SCP was also probed in patients with a chronic schizophrenic disorder, who are known to exhibit deviant SCP. Increased amplitudes of the probe-evoked vertex potential (N 1/P2) and faster motor responses to probes during the CNV, and reduced N 1/P2-omplitudes and delayed motor responses during positive waves (P300) support the hypothesis that anticipatory cortical negativity indicates excitation in cortical neuronal networks while positive-going waves may indicate widespread cortical suppression (with focused islands of cortical excitability). Increased amplitudes of the N 1/P2 during the CNV but not during the PINV suggest a different functional significance of these two negative-going slow waves. Although schizophrenic patients displayed different amplitudes and scalp distribution, particularly of the PINV, suggesting a deviant spatia-temporal regulation of cortical excitability, the functional relation between SCP and probe-related activity was similar in schizophrenic patients and control subjects. 'J Introduction ROd theoretical backgronnd Slow cortical potentials (SCP) can be measured from the surface of the scalp when tens of thousands of cortical pyramidal cells are synchronously depolarized. The excitatory postsynaptic potentials that depolarize the apical dendritic trees of cortical pyramidal cells result in surface-negative potentials (Caspers, Speckmann, & Lehmenkiihler, 1984, 1987; Speckmann, Caspers, & EIger, 1984). A negative potential over larger cortical surfaces will generally result in a surface negativity also visible on the scalp. A state characterized by surface-negative SCPs might therefore generally indicate a state of higher neural excitability or activation of the underlying neuronal networks (Elbert, 1993; Elbert & Rockstroh, 1987; RosIer, 1991). When an afferent volley reaches such an excitable cortical region, cell assemblies therein should ignite more easily compared to conditions when the same input reaches the network during states of reduced depolarization. The latter states would be characterized by relative surface positivity. Consequently, facilitated responding to an afferent input is to be expected, i. e., the evoked potential should be larger in amplitude when it is elicited during a surface negative shift in regions where the stimulus is processed, but smaller during positive potentials. We have developed a model ofregulation of cortical excitability (Elbert, 1993; Elbert & Rockstroh, 1987; Rockstroh, Elbert, Canavan, Lutzenberger, & Birbaumer, 1989; Birbaumer, Elbert, Canavan, & Rockstroh, 1990) suggesting that the extent of depolarization in the various cortical regions must be controlled in order to regulate the spread of activation. At any given instant in time, gross levels of cortical activation are measured through descending information to the striatum and the basal ganglia which in