Evoked and Event-Related Potentials: Has Evolution Altered Assembly Responses?
Electrical responses to stimuli or to cognitive events of known onset time commonly cause more or less localized activity in the brain. Evoked potentials and their subclass of event related potentials are recorded as compound field potentials whose origin is not completely understood. They are believed to result from spikes, synaptic potentials and other forms of activity of parts of neurons and perhaps also other generators, including neuroglia, all vector-summed in the extracellular medium. The form, time course and dynamic properties of the compound field potentials are poorly predicted or explained from data derived from single units, whether intracellular or extracellular; the compound waves have little energy in the high frequency part of the power spectrum and are mainly slow.
A wide variety in form and dynamics is found in different parts of the brain. Some late, slow (long duration) waves and some early, fast (brief) waves vary strikingly with the brain state; these are largely endogenous but influenced to a degree by the parameters of the stimulus. Others, especially early, fast waves are mainly exogenous—dependent upon stimulus parameters almost independently of brain state. The former class are called cognitive event related potentials; the term evoked potentials embraces both classes.
Evoked potentials are often more sensitive to feeble stimuli than single units and to subtle differences, for example in cognitive events. They can be the best means of distinguishing subsystems, for example within the auditory pathways, or of testing for the presence of a sense modality, for example electroreception in an unknown species, or for the ability to discriminate between two stimuli, for example slightly different sound frequencies or click repetition rates or sound sources.
Temporal organization. Abrief, physiological stimulus can elicit a succession of waves; a short pulse to a cutaneous sense organ can cause a sequence of four or five peaks and valleys in its primary brainstem nucleus, extending to at least 130 ms. A single light flash can cause a sequence of waves in the optic tectum which can extend to more than one second. The spectrum of evoked potentials, from sharp waves of synchronized unit firing, no wider than a nerve impulse, to slow waves of many hundreds of milliseconds duration can be found already at early brainstem levels. At the level of the cerebral cortex also a wide range of durations occurs but rarely the sharpest, narrow waves. The presumably diverse underlying cellular mechanisms, in terms of synchrony, of circuit time constants and of intracellular slow events or oscillations are poorly known. They probably include important contributions from inhibitory processes that are so small in extracellular current as to be invisible in the field potential. The hypothesis that evoked potentials are largely due to a phase resetting of the ongoing EEG waves may well be true for a limited class but is difficult to prove, whereas it is readily shown in many cases that new energy must be invoked; probably both mechanisms operate, either together or separately. The dynamics of trial to trial variability has been little studied.
Spatial organization. This is less well known; clearly they can be quite local and they can be quite widespread. The equivalent dipole is only a first approximation. Sometimes a phase reversal depth can be found, but often not. Micromapping has revealed unexpectedly fine structure in some places and should be extended not only topographically but in respect to the dynamics using iterative stimuli. Input–output relations are known only spottily. Intensity functions need to be permuted with recent history, repetition rate, temporal patterns of stimuli and concurrent activation of other systems.
Dynamic characterization of each locus is insightful, over and above the similar characterization of single units from the same population. Evoked potentials are not a hopeless quagmire but an empirical domain rich in information about neural organization. A number of powerful tools are still little used, including ethological and multimodal stimuli, endogenous events like the cognitive stimuli used in research with human subjects, multielectrode arrays with wideband recording, patterned trains of stimuli, micromapping, current source density mapping, temporal structure of coherence and of higher moments such as bispectrum and bicoherence. If we make use of adequate methods, this observer expects we will find an evolutionary story: significant differences in assembly responses between animal taxa with relatively simple brains and those with more complex brains. This will represent a major advance in understanding how brains evolved, in terms of physiological organization.
KeywordsEvent Related Potential Visual Evoke Potential Auditory Brainstem Response Brain State Optic Tectum
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