Sources of the Scalp-Recorded Amplitude-Modulation Following Response

Abstract

The scalp-recorded amplitude-modulation following response (AMFR)” is gaining recognition as an objective audiometric tool, but little is known about the neural sources that underlie this potential. We hypothesized, based on our human studies and single-unit recordings in animals, that the scalp-recorded AMFR reflects the interaction of multiple sources. We tested this hypothesis using an animal model, the unanesthetized rabbit. We compared AMFRs recorded from the surface of the brain at different locations and before and after the administration of agents likely to enhance or suppress neural generators. We also recorded AMFRs locally at several stations along the auditory neuraxis. We conclude that the surface-recorded AMFR is indeed a composite response from multiple brain generators. Although the response at any modulation frequency can reflect the activity of more than one generator, the AMFRs to low and high modulation frequencies appear to reflect a strong contribution from cortical and subcortical sources, respectively.

Cochlear neuropathy and the coding of supra-threshold sound

“…However, brainstem SSSRs can be distinguished from responses generated at the cortical level by virtue of their relatively high frequency content; practically speaking, cortical and SSSR responses can be extracted from the same raw scalp recordings by appropriate filtering (e.g., see Krishnan et al, 2012 ; Bharadwaj and Shinn-Cunningham, 2014 ). The responses that specifically phase lock to the envelope of amplitude modulated (AM) sounds have been referred to as EFRs or amplitude modulation following responses (AMFRs; Dolphin and Mountain, 1992 ; Kuwada et al, 2002 ). In the recent literature, SSSRs are most commonly referred to as frequency following responses (FFRs), a term originally used to denote responses phase locked to pure tones (Marsh et al, 1975 ).…”

“…First, they represent neural activity that is the sum of a large population of neurons, filtered by layers of brain tissue, skull, and scalp. Depending on the stimulus parameters, thousands of neurons in each of multiple subcortical nuclei may contribute to the EFR (Kuwada et al, 2002 ). Neurons from several regions along the tonotopic axis could contribute to the EFR for high-level sounds due to spread of excitation, even for narrow-band sounds.…”

“…Based on single-unit data, reversible inactivation studies, irreversible lesion studies, and studies analyzing EFR group delay, it has been argued that the dominant generators of the EFR move from caudal (AN and CN) to rostral (inferior colliculus or IC) as modulation frequency decreases (Sohmer et al, 1977 ; Dolphin and Mountain, 1992 ; Kiren et al, 1994 ; Herdman et al, 2002 ; Kuwada et al, 2002 ). These studies provide evidence that the IC dominates EFRs at modulation frequencies between about 70 and 200 Hz, in all species tested.…”

“…These studies provide evidence that the IC dominates EFRs at modulation frequencies between about 70 and 200 Hz, in all species tested. Changes in the slope of the response phase vs. input modulation frequency can be used to calculate apparent latency of the sources and thereby infer changes in the relative strengths of different neural generators in the mixture (Kuwada et al, 2002 ); regions where the slope is constant indicate regions where the mixture of generators is constant. Above 200 Hz, the pattern of these changes varies across species, probably due to differing head sizes and shapes.…”

Visual load does not decrease the auditory steady‐state response to 40‐Hz amplitude‐modulated tones

“…To examine whether attentional filtering can occur earlier or later in the auditory pathway, future studies should use modulation frequencies other than 40 Hz. For faster modulations (e.g., 80 Hz), ASSRs seem to originate mostly from the brainstem (Giraud et al., 2000; Herdman et al., 2002; Kuwada et al., 2002; Luke et al., 2017; Picton et al., 2003; Plourde et al., 1991), whereas, for slower modulations (below 40 Hz), ASSRs seem to originate mostly from the auditory cortex (Giraud et al., 2000; Kuwada et al., 2002; Luke et al., 2017). Accordingly, faster modulations may be informative about earlier stages of the auditory pathway (brainstem), and slower modulations may be informative about later processing stages.…”