Relationship of transient and steady-state auditory evoked fields

Abstract

SummaryTransient and steady-state auditory evoked fields (AEFs) to brief tone pips were recorded over the left hemisphere at 7 different stimulus rates (0.125-39 Hz) using a 37-channel biomagnetometer. Previous observations of transient auditory gamma band response (GBR) activity were replicated. Similar rate characteristics and equivalent dipole locations supported the suggestion that the steady-state response (SSR) at about 40 Hz represents the summation of successive overlapping (10 Hz) middle latency responses (MLRs). On the other hand, differences in equivalent dipole locations and habituation effects suggest that the magnetically recorded GBR is a separate phenomenon which occurs primarily at low stimulus rates and is unrelated to either the magnetically recorded MRL or SSR.Key words: Auditory evoked fields; Steady-state response; Gamma band; Magnetoencephalography; 40 Hz auditory evoked potential Magnetoencephalography (MEG) has become an es-. tablished method for the noninvasive study of the macroscopic activity of the human cortex (for a recent review see, e.g., Hari 1990). The main sources of cortical auditory evoked magnetic fields are intracellular currents flowing tangentially to the skull, originating in similarly oriented pyramidal cells in the walls of the sylvian fissure. Magnetic studies of the human auditory cortex have demonstrated functional relationships similar to those observed earlier in invasive animal and human studies and have also provided new information about the functional organization of the human auditory cortex (Romani et al. 1982;Pantev et al. 1989), and about the physiology of auditory feature extraction (Hari 1990;Csepe et al. 1992).Responses evoked by stimuli which follow each other at sufficiently long interstimulus intervals (ISIs) for the auditory system to return (mostly or completely) to its initial state before the next stimulus occurs (Buchwald and Huang 1975) are called transient evoked responses. If the ISI is shortened to such an extent that the transient response to one stimulus has not died away before the next stimulus is delivered, the com- pound response that appears is generally referred to as a steady-state response (SSR) (Regan 1982) or steadystate field (SSF) in magnetic recording. For a linear system, transient and steady-state descriptions of the system's behaviour are equivalent, and a simple superposition of transient responses with the appropriate time lags perfectly predicts the amplitude and form of the steady-state response. In this case, both phenomena may be regarded as alternative versions of the same response. However, the auditory system shows several types of non-linear behaviour, and therefore transient and steady-state field recordings may provide complementary information about auditory function.The transient auditory evoked field (AEF) consists of several components which are labelled according to their latency: the brain-stem AEF (latency 1-10 msec (Erne et al. 1987)), the middle latency AEF (latency 10-50 msec ), the gamma Mnd AEF (20-...

Blind separation of auditory event-related brain responses into independent components

“…This ICA decomposition of the auditory SSR into three spatially fixed components contrasted sharply with a previously proposed interpretation that the nonstationary SSR reflects a moving crest of activity sweeping through cortex every 25 ms (30). By itself, however, ICA cannot be used to decide between these or other source models (26,31).…”

The Rapid Extraction of Gist—Early Neural Correlates of High-level Visual Processing

“…This possibility was already discussed in previous studies for the auditory modality (Müller, Keil, Kissler, & Gruber, 2001;Pantev et al, 1993;Başar, Rosen, Başar-Eroglu, & Greitschus, 1987). According to this debate, the eGBR could reflect band-pass filtered portions of the middlelatency components.…”

Generation of human auditory steady-state responses (SSRs). II: Addition of responses to individual stimuli

“…It has been suggested that this response does not participate in the generation of the 40 Hz-SSR since these neural activities originate from separate cortical sources (Pantev et al, 1993). Moreover, the GBR has been proven to be strongly enhanced by stimulus rates lower than 1 Hz (Makeig and Galambos, 1989;Pantev et al, 1993) and this makes it unlikely that this response appears at the rates used in our stimulation paradigm.…”