John N. Gardi

Origins of the scalp recorded frequency-following response in the cat.

The frequency-following response (FFR) is a short-latency scalp-recorded evoked potential elicited by the presentation of low-frequency acoustic stimuli. It is thought to be the result of the synchronous electrical activity in brain stemauditory nuclei to each wave in the acoustic signal. The present investigation constitutes an attempt to determine the generators of the FFR in the cat by analysis of the response and by section of brain stem auditory nuclei and tracts. Among the results were the following: (1) the cochlear nuclei contribute approximately 50% of the amplitude of the scalp-recordedFFR in the cat. (2) The cochlea also makes a significant contribution, accounting for an average of nearly 25% of the response amplitude. (3) The superior olivary nuclei (and/or the nuclei of the lateral lemnisci) account for about 20% of the response amplitude. (4) The contributions from the inferior colliculi (contrary to earlier studies) were found to be relatively insignificant. (5) As a consequence of the existence of multiple generators, the FFR ample area of hair cell excitation for each stimulus frequency involved in the mediation of the FFR, suggestion that scalp-recorded FFRs could be used to ascertain low-frequency hearingsensitivity in uncooperative human subjects.

The 3-channel Lissajous' trajectory of the auditory brain-stem response. VI. Effects of lesions in the cat☆☆☆

Abstract Two discrete studies of lesions (sectioning the auditory nerve and mid-sagittal brain-stem) were performed in 5 adult cats to determine the relationship between planar-segment formation and underlying generator activity. The experiments gave 2 major results: (1) Section of the VIII nerve at the brain-stem resulted in loss of potentials after wave II and almost total loss of wave II in most traces. In the time interval of planar segments a1, a2 and b1, only a single planar loop remained after sectioning the VIII nerve, and its orientation corresponded to that of the pre-surgical planar segment a1. (2) Whereas mid-sagittal sectioning of the brain-stem considerably reduced waves 4, 5 and 7 in vector amplitude plots, planar analysis showed that planar segments c2, c3, c4 and d1 were reduced to the simpler configuration of a line in voltage-space, and that planar segments d2, d3 and d4 were reduced in size and shifted in orientation. These results suggest the following hypotheses: (1) planar segment a1 must represent activity arising from the VIII nerve peripheral to its entry point into the brain; (2) VIII nerve activity can extend into the time that the cochlear nuclear complex is active; (3) an additional generator, probably the cochlear nuclear complex, contributes to the negativity observed after wave I; (4) planar segments c2, c3, c4 and d1 represent activity from generators whose input is from the contralateral ear. Some of the contributors to planar segments d2, d3 and d4 are activated by pathways ipsilateral to the stimulated ear, while the remaining portion is contralaterally activated.

The use of kainic acid for studying the origins of scalp-recorded auditory brainstem responses in the guinea pig.

Kainic acid was injected into the medial nucleus of the trapezoid body (MNTB) of guinea pigs to evaluate its use in studying generator loci of the scalp-recorded auditory brain stem response (ABR). Sound-evoked near-field potentials from the MNTB and far-field ABRs were recorded before, during and up to 2 h after the injections. Two hours post-injection, small amounts of kainic acid (0.25 nmol in 0.1 microliters of Ringer solution) resulted in neuronal destruction which histologically appeared confined to the MNTB. Larger amounts (10 nmol in 1.0 microliters) produced more extensive lesions. Regardless of the dose of kainic acid, near-field activity evoked by contralateral ear stimulation was almost totally abolished and ABR wave III amplitude was reduced by as much as 60%. In future studies, the use of excitotoxic amino acids to produce lesions within complex nuclear subdivisions of the auditory pathway may yield valuable information as to the relative contributions that brainstem structures make to the various waves comprising the ABR and about the behavioral effects that axon sparing lesions produce.

The effect of high‐pass noise on the scalp‐recorded frequency following response (FFR) in humans and cats

Low‐frequency tone‐burst stimuli were used to elicit the scalp‐recorded frequency following response (FFR) from normal‐hearing adult human subjects and anesthetized adult cats. High‐pass masking stimuli (with variable low‐frequency cutoffs) were presented simultaneously with these tone‐burst stimuli. For a small group of human subjects, analogous psychophysical masking curves (threshold shift and binaural loudness balance) were conducted so that physiologically derived curves (masked FFR) could be related to these psychophysically derived curves. Among the results were the following: (1) human and cat FFR masking curves were very similar; (2) at moderate to high probe‐tone stimulus levels, the amplitude of the FFR masked with a 2‐kHz cutoff was approximately 70% to 75% of the amplitude of the unmasked FFR; (3) at probe‐tone stimulus levels near FFR threshold, the amplitude of the FFR masked with cutoffs of 1.5, 1.8, and 2.0 kHz was equivalent to the amplitude of the unmasked FFR; (4) a sizable fraction (1...

Planar‐curve analysis of auditory brain stem responses: Preliminary observations

When auditory brain stem responses (ABRs and/or FFRs) are collected from three orthogonally placed electrode pairs and voltage is plotted in three‐dimensional space, a three‐dimensional Lissajous figure is derived. Segments of the Lissajous figure (roughly corresponding to the beginning and ending points of individual waves in the classically recorded ABR or to the beginning and ending points of the plateau region in the scalp‐recorded FFR) form planar‐curves. ABRs elicited by click stimuli and FFRs elicited by low frequency tone bursts were collected from a group of adult animal (cat) and human subjects to investigate (in a preliminary manner) some of the consequences of this recording and analysis technique. Among the results were the following: (1) The formation of planes appears to be a consequence of a systematic change in the focus of synchronous input, as a function of time, and of the fact that auditory nuclei are for the most part geometrically ordered. (2) Planar‐curve position appears to be det...