Guido Conti

Generation of human auditory steady-state responses (SSRs). I: Stimulus rate effects

Auditory evoked responses were recorded in 16 normally hearing subjects in order to investigate the mechanisms underlying the generation of the 40 Hz steady-state response (SSR). In the first part of our study, auditory potentials were evoked by 0.1 ms clicks presented at 105 dB p.e. SPL with repetition rates of 7.9 (to obtain middle latency response, MLR), 20, 30, 40, 50, 60 Hz. In each subject predictions of the responses recorded at stimulus repetition rates of 30, 40, 50, 60 Hz were synthesized by superimposing MLRs at suitable time intervals. The calculated mean amplitude/rate and phase/rate functions behaved similarly for the recorded and predicted curves, showing the highest amplitude at 40 Hz and a linear increase of phase values when increasing the stimulus rate. Nevertheless the synthetic curves closely predicted amplitude and phase values of the recorded responses only at 40 Hz. At frequencies below 40 Hz, the mean amplitude of the predicted curve was lower than that of the recorded one while at frequencies above 40 Hz the mean amplitude was higher. Predicted phase values were found lagging at 30 Hz, and leading at 50 Hz and 60 Hz in comparison to phase values calculated on the recorded responses. Our findings suggest that a model based on the linear addition of transient MLRs is not able to adequately predict steady-state responses at stimulus rates other than at 40 Hz. Other mechanisms related to the recovery cycle of the activated system come into play in the steady-state response generation causing a decrease in amplitude and an increase in phase lag when increasing the stimulus repetition rate.

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

In order to investigate the generation of the 40 Hz steady-state response (SSR), auditory potentials evoked by clicks were recorded in 16 healthy subjects in two stimulating conditions. Firstly, repetition rates of 7.9 and 40 Hz were used to obtain individual middle latency responses (MLRs) and 40 Hz-SSRs, respectively. In the second condition, eight click trains were presented at a 40 Hz repetition rate and an inter-train interval of 126 ms. We extracted from the whole train response: (1) the response-segment taking place after the last click of the train (last click response, LCR), (2) a modified LCR (mLCR) obtained by clearing the LCR from the amplitude enhancement due to the overlapping of the responses to the clicks preceding the last within the stimulus train. In comparison to MLRs, the most relevant feature of the evoked activity following the last click of the train (LCRs, mLCRs) was the appearance in the 50-110 ms latency range of one (in 11 subjects) or two (in 2 subjects) additional positive-negative deflections having the same periodicity as that of MLR waves. The grand average (GA) of the 40 Hz-SSRs was compared with three predictions synthesized by superimposing: (1) the GA of MLRs, (2) the GA of LCRs, (3) the GA of mLCRs. Both the MLR and mLCR predictions reproduced the recorded signal in amplitude while the LCR prediction amplitude resulted almost twice that of the 40 Hz-SSR. With regard to the phase, the MLR, LCR and mLCR closely predicted the recorded signal. Our findings confirm the effectiveness of the linear addition mechanism in the generation of the 40 Hz-SSR. However the responses to individual stimuli within the 40 Hz-SSR differ from MLRs because of additional periodic activity. These results suggest that phenomena related to the resonant frequency of the activated system may play a role in the mechanisms which interact to generate the 40 Hz-SSR.

Origin and distribution of P13 and P14 far-field potentials after median nerve stimulation. Scalp, nasopharyngeal and neck recording in healthy subjects and in patients with cervical and cervico-medullary lesions

We studied median nerve SEPs in 10 healthy subjects, by means of simultaneous recording over the scalp, around the neck and near the ventral surface of the medulla using a nasopharyngeal (NP) electrode. This recording technique enabled us to clearly differentiate P13 and P14 potentials. The former was always found in NP records, while the latter was more evident in scalp traces. The same technique was used to study 9 patients with various lesions of the cervical cord or cervico-medullary junction. Patients with high cervical lesions demonstrated abnormalities of both P13 and P14 potentials, while patients with lesions of the cervico-medullary junction demonstrated a clear dissociation between normal P13 in scalp and NP traces, and abnormal scalp P14. Patients with lower cervical lesions, selectively involving the central grey matter, showed normal P13 and P14 potentials, in spite of abnormal N13 cervical responses. Our findings strongly suggest that both scalp and NP P13 have the same generators in higher segments of the cervical cord, and that NP more than scalp records are effective in analyzing the P13 response. We suggest that the selective recording of the P13 potential could be useful in the assessment of focal lesions of the higher cervical cord or of the cervico-medullary junction.

Auditory steady-state responses to click trains from the rat temporal cortex

In order to investigate the mechanisms underlying the generation of steady-state responses (SSRs), auditory evoked potentials elicited by click trains presented at several stimulation rates (30, 40, 50, 60 Hz) were recorded in 7 awake rats by means of epidural electrodes placed over the temporal cortex. Mean amplitude-rate function calculated on the recorded responses appeared almost flat and showed the maximum value at 50 Hz, while mean phases showed a linear increase when increasing the stimulation rate. In each rat, predictions of the recorded responses at 30, 40, 50 and 60 Hz were synthesized by superimposing middle-latency auditory evoked potentials (MAEPs) at suitable time intervals at each rate. Mean amplitudes calculated on the predicted curves decreased linearly when increasing the stimulation rate and appeared higher in comparison to those obtained from the recorded SSRs. Predicted phases showed a linear increase when increasing the stimulation rate and were leading with respect to corresponding phase values calculated for recorded SSRs. Our findings indicate that the MAEP superimposition mechanism does not adequately predict the generation of temporal recorded SSRs in rats. This was explained by admitting that phenomena related to the recovery cycle and, to a lesser extent, to rate-dependent facilitating effects come into play.

Severe form of Freeman-Sheldon syndrome associated with brain anomalies and hearing loss

We describe a child with whistling face and multiple contractures, including ulnar deviation of fingers, compatible with a diagnosis of Freeman-Sheldon syndrome (FSS). This patient also presented severe hypertonicity, multiple episodes of pneumonia, difficulty in swallowing, and poor weight gain, which are characteristic of the most severe cases of FSS. A brain CT scan showed cerebellar and brainstem atrophy. Auditory brainstem responses were absent. The child died at 5 months of respiratory failure. This case suggests the possibility that, especially in the most severe forms, brain abnormalities may be responsible for some of the clinical manifestations of this syndrome, i.e., respiratory problems, difficulty in swallowing and severe hypertonicity. We assume that there is more than one pathogenetic mechanism (muscular, skeletal, and neurological) underlying FSS, which, together with the genetic heterogeneity and the wide range of clinical symptoms leads us to suggest that it is more appropriate to speak of a Freeman-Sheldon spectrum rather than syndrome and that thorough investigation for CNS and auditory abnormalities should be part of the initial work-up of these patients.

Predictive Role of Audiological and Clinical Features for Functional Results after Stapedotomy

Objective: To analyze and compare the preoperative factors that potentially influence the outcome of stapedotomy in our study group. Materials and Methods: 161 cases were enrolled. Clinical variables considered to influence functional results – air conduction (AC) and bone conduction (BC) pure-tone average (PTA), air-bone gaps (ABG), sensorineural hearing loss (SNHL), ABG gain and ΔSNHL – were gender, age, case type (unilateral vs. bilateral), ear side (right vs. left), pregnancy, vascular disease and family history of otosclerosis. The audiometric variables were preoperative AC- and BC-PTA, SNHL and ABG. Results: Univariate logistic regression analysis showed that the probability of obtaining a ≥10 dB gain is significantly affected by the following factors: age <50 years, AC-PTA ≥50 dB and preoperative ABG ≥30 dB. All the other factors included into the registration (gender, familiarity, side, bilateral vs. unilateral, pregnancy, vascular diseases and preoperative BC-PTA) were not found to significantly affect postoperative gain (p > 0.05). Nevertheless, multivariate logistic regression analysis maintained a statistically significant correlation only between gain ≥10 dB and both preoperative ABG ≥30 dB and age <50 years. Conclusions: The accurate knowledge of predictive factors is a valuable tool that permits the surgeon to plan surgery with a better case selection as well as assisting in counseling the patient with regard to the likelihood of success of the procedure.

Auditory processing in infancy: do early abnormalities predict disorders of language and cognitive development?

Increasing attention has been devoted to the maturation of sensory processing in the first year of life. While the development of cortical visual function has been thoroughly studied, much less information is available on auditory processing and its early disorders. The aim of this paper is to provide an overview of the assessment techniques for early auditory processing. While otoacoustic emissions and auditory brainstem responses are well‐established tools for neonatal screening of hearing loss, there have been less consistent results for neurophysiological assessments of central auditory processing in clinical practice. Early auditory event‐related potentials could provide valuable diagnostic information, but their use as a possible clinical screening method is still limited and should be further assessed. Behavioural tests are few and are greatly needed in young infants as they could provide a more easily used tool for detecting the preconditions of early cerebral auditory impairment.

Sudden onset sensorineural hearing loss caused by meningeal carcinomatosis secondary to occult malignancy: Report of two cases

Meningeal carcinomatosis (MC) is an uncommon form of metastasis of solid tumors. In the absence of clinical meningeal or parenchymal involvements, the sensorineural hearing loss (SNHL) as the starting symptom of MC is very infrequent. We report the history of two patients affected by MC who presented first with progressive SNHL. In both cases the early magnetic resonance imaging (MRI) finding mimicked bilateral masses in the cerebellopontine angle (CPA). Only the histopathologic result and surgical biopsy in cases 1 and 2, respectively, identified masses secondary to occult malignancy. However the available investigations could not discover the primary site of metastatic carcinoma. Despite the poor prognosis, because of the rarity and severity of MC we consider important to make known our experience in order to consider metastatic tumors in the differential diagnosis for sudden SNHL.

Steady state auditory evoked potentials in normal hearing subjects: evaluation of threshold and testing time.

Background: Steady state responses (SSRs), between 75 and 110 Hz, evoked by auditory amplitude modulated single or multiple tone stimuli, may be used to estimate objective hearing threshold. Methods: The aim of this study was to compare SSRs and click-evoked auditory brainstem responses (ABRs) in both ears of 20 adults (10 males and 10 females, aged between 24 and 36 years) with normal hearing threshold. Results: Mean ABR threshold was found at 21.25 (±5.9) dB nHL. Mean SSR threshold was found at 15.6 (±9.6) dB nHL after a single frequency stimulus (1 kHz); at 10.5 (±18.2) dB nHL and at 7.1 (±12.4) dB nHL after bifrequency stimulation (0.5 and 2 kHz). SSR thresholds after multifrequency stimulation (0.5, 1, 2 and 4 kHz) were found, respectively, at 12.1 (±12.9) dB nHL, 12.2 (±12.8) dB nHL, 12.3 (±8.3) dB nHL and 18.9 (±17.2) dB nHL. Mean duration of the recording session was 6 min in the case of ABRs, while it was 25 min in the single frequency condition and 29 min in the multifrequency condition in the case of SSRs. Conclusion: SSRs can be used for frequency-specific objective audiometry. The multifrequency stimulation greatly reduces the whole testing time.

Acute auditory agnosia as the presenting hearing disorder in MELAS

MELAS is commonly associated with peripheral hearing loss. Auditory agnosia is a rare cortical auditory impairment, usually due to bilateral temporal damage. We document, for the first time, auditory agnosia as the presenting hearing disorder in MELAS. A young woman with MELAS (A3243G mtDNA mutation) suffered from acute cortical hearing damage following a single stroke-like episode, in the absence of previous hearing deficits. Audiometric testing showed marked central hearing impairment and very mild sensorineural hearing loss. MRI documented bilateral, acute lesions to superior temporal regions. Neuropsychological tests demonstrated auditory agnosia without aphasia. Our data and a review of published reports show that cortical auditory disorders are relatively frequent in MELAS, probably due to the strikingly high incidence of bilateral and symmetric damage following stroke-like episodes. Acute auditory agnosia can be the presenting hearing deficit in MELAS and, conversely, MELAS should be suspected in young adults with sudden hearing loss.