Naomi Bleich

The auditory P50 component to onset and offset of sound

OBJECTIVE The auditory Event-Related Potentials (ERP) of component P50 to sound onset and offset have been reported to be similar, but their magnetic homologue has been reported absent to sound offset. We compared the spatio-temporal distribution of cortical activity during P50 to sound onset and offset, without confounds of spectral change. METHODS ERPs were recorded in response to onsets and offsets of silent intervals of 0.5 s (gaps) appearing randomly in otherwise continuous white noise and compared to ERPs to randomly distributed click pairs with half second separation presented in silence. Subjects were awake and distracted from the stimuli by reading a complicated text. Measures of P50 included peak latency and amplitude, as well as source current density estimates to the clicks and sound onsets and offsets. RESULTS P50 occurred in response to noise onsets and to clicks, while to noise offset it was absent. Latency of P50 was similar to noise onset (56 ms) and to clicks (53 ms). Sources of P50 to noise onsets and clicks included bilateral superior parietal areas. In contrast, noise offsets activated left inferior temporal and occipital areas at the time of P50. Source current density was significantly higher to noise onset than offset in the vicinity of the temporo-parietal junction. CONCLUSIONS P50 to sound offset is absent compared to the distinct P50 to sound onset and to clicks, at different intracranial sources. P50 to stimulus onset and to clicks appears to reflect preattentive arousal by a new sound in the scene. Sound offset does not involve a new sound and hence the absent P50. SIGNIFICANCE Stimulus onset activates distinct early cortical processes that are absent to offset.

Short latency visual evoked potentials in man

Contrary to auditory and somatosensory evoked potentials, surface recorded visual evoked potentials which arise in subcortical neural elements have rarely been described. Considerable disagreement exists between the reports in the literature on such visual potentials. In this study, flash stimuli were used to evoke the potentials which were recorded from the skin overlying the infra-orbital ridge, outer canthus, middle of the forehead, vertex, mastoid ipsilateral to the stimulated eye and inion, using a non-cephalic reference. The potentials were amplified in a band which was chosen to omit slow retinal and cortical potentials, and to enhance activity which might include compound neural activity. Potentials were recorded from 9 subjects (13 eyes), and for each one the effects of eye position and stimulus intensity were studied. The results indicate that the series of components recorded within the first 100 msec following photic stimulation were volume-conducted activity generated by a subset of the visual system which is activated by luminosity changes. The generators of the first 4 or 5 components seem to be situated within the retina, the subsequent components seem to be generated in the optic nerve or tracts, and the later components may be thalamo-cortical in origin. These potentials may complement pattern evoked potentials in a more accurate definition of sites of lesions along the visual pathway.

A comparison of auditory evoked potentials to acoustic beats and to binaural beats

The purpose of this study was to compare cortical brain responses evoked by amplitude modulated acoustic beats of 3 and 6 Hz in tones of 250 and 1000 Hz with those evoked by their binaural beats counterparts in unmodulated tones to indicate whether the cortical processes involved differ. Event-related potentials (ERPs) were recorded to 3- and 6-Hz acoustic and binaural beats in 2000 ms duration 250 and 1000 Hz tones presented with approximately 1 s intervals. Latency, amplitude and source current density estimates of ERP components to beats-evoked oscillations were determined and compared across beat types, beat frequencies and base (carrier) frequencies. All stimuli evoked tone-onset components followed by oscillations corresponding to the beat frequency, and a subsequent tone-offset complex. Beats-evoked oscillations were higher in amplitude in response to acoustic than to binaural beats, to 250 than to 1000 Hz base frequency and to 3 Hz than to 6 Hz beat frequency. Sources of the beats-evoked oscillations across all stimulus conditions located mostly to left temporal lobe areas. Differences between estimated sources of potentials to acoustic and binaural beats were not significant. The perceptions of binaural beats involve cortical activity that is not different than acoustic beats in distribution and in the effects of beat- and base frequency, indicating similar cortical processing.

Cortical evoked potentials to an auditory illusion: Binaural beats

OBJECTIVE To define brain activity corresponding to an auditory illusion of 3 and 6Hz binaural beats in 250Hz or 1000Hz base frequencies, and compare it to the sound onset response. METHODS Event-Related Potentials (ERPs) were recorded in response to unmodulated tones of 250 or 1000Hz to one ear and 3 or 6Hz higher to the other, creating an illusion of amplitude modulations (beats) of 3Hz and 6Hz, in base frequencies of 250Hz and 1000Hz. Tones were 2000ms in duration and presented with approximately 1s intervals. Latency, amplitude and source current density estimates of ERP components to tone onset and subsequent beats-evoked oscillations were determined and compared across beat frequencies with both base frequencies. RESULTS All stimuli evoked tone-onset P(50), N(100) and P(200) components followed by oscillations corresponding to the beat frequency, and a subsequent tone-offset complex. Beats-evoked oscillations were higher in amplitude with the low base frequency and to the low beat frequency. Sources of the beats-evoked oscillations across all stimulus conditions located mostly to left lateral and inferior temporal lobe areas in all stimulus conditions. Onset-evoked components were not different across stimulus conditions; P(50) had significantly different sources than the beats-evoked oscillations; and N(100) and P(200) sources located to the same temporal lobe regions as beats-evoked oscillations, but were bilateral and also included frontal and parietal contributions. CONCLUSIONS Neural activity with slightly different volley frequencies from left and right ear converges and interacts in the central auditory brainstem pathways to generate beats of neural activity to modulate activities in the left temporal lobe, giving rise to the illusion of binaural beats. Cortical potentials recorded to binaural beats are distinct from onset responses. SIGNIFICANCE Brain activity corresponding to an auditory illusion of low frequency beats can be recorded from the scalp.

Auditory-evoked potentials to frequency increase and decrease of high- and low-frequency tones

OBJECTIVE To define cortical brain responses to large and small frequency changes (increase and decrease) of high- and low-frequency tones. METHODS Event-Related Potentials (ERPs) were recorded in response to a 10% or a 50% frequency increase from 250 or 4000 Hz tones that were approximately 3 s in duration and presented at 500-ms intervals. Frequency increase was followed after 1 s by a decrease back to base frequency. Frequency changes occurred at least 1 s before or after tone onset or offset, respectively. Subjects were not attending to the stimuli. Latency, amplitude and source current density estimates of ERPs were compared across frequency changes. RESULTS All frequency changes evoked components P(50), N(100), and P(200). N(100) and P(200) had double peaks at bilateral and right temporal sites, respectively. These components were followed by a slow negativity (SN). The constituents of N(100) were predominantly localized to temporo-parietal auditory areas. The potentials and their intracranial distributions were affected by both base frequency (larger potentials to low frequency) and direction of change (larger potentials to increase than decrease), as well as by change magnitude (larger potentials to larger change). The differences between frequency increase and decrease depended on base frequency (smaller difference to high frequency) and were localized to frontal areas. CONCLUSIONS Brain activity varies according to frequency change direction and magnitude as well as base frequency. SIGNIFICANCE The effects of base frequency and direction of change may reflect brain networks involved in more complex processing such as speech that are differentially sensitive to frequency modulations of high (consonant discrimination) and low (vowels and prosody) frequencies.

Three modality evoked potentials in Charcot-Marie-Tooth disease (HMSN-1).

Sixteen patients with dominant hereditary motor-sensory neuropathy type I (HMSN I), members of 5 families, underwent trimodality evoked potential studies. All patients had clinically normal optic nerves. History of deafness was present in 3 patients and sensory-neural hearing defect was found in 5 of 7 patients in whom audiometry was obtained. In 43.7 percent of the subjects significant prolongation of P100 of the VEP was found. Prolongation of N19 of the SEP was found in all 12 subjects examined. Significant bilateral prolongation of peak I of the ABEP was found in 37.5 percent of the subjects and in 50 percent of the ears examined: these findings indicated that in addition to peripheral nerves, the myelin of the optic and cochlear nerves is also affected in HMSN type I.

Band-pass specific contributions of multiple generators to the auditory 40-Hz steady state potentials

Objective The purpose of this study was to separate the composite components of the auditory 40 Hz steady-state potentials (40 Hz SSP), by differentially augmenting them with filtering at different low passes, and to compare them with their counterparts in the transient-evoked auditory middle-latency evoked potentials (AMEP). Methods Transient-evoked AMEP to 3.3/sec clicks and 40 Hz SSP to 40/sec clicks were recorded from 18 subjects using three orthogonally positioned electrode pairs. Each type of potentials was filtered with a 100 Hz and with a 50 Hz low pass. Equivalent dipoles of components were estimated using Three-channel Lissajous’ Trajectories and compared between filter settings (50 and 100 Hz low pass) and between the transient-evoked and the steady-state potentials. Results With a band pass of 3 to 100 each period of the 40 Hz SSP consisted of a brain stem (V) and four cortical (P0, Na, Pa1, Pa2, and Nb) components. The lower-frequency components of the 40-Hz response corresponded in latency and equivalent dipole orientation to the later transient-evoked cortical AMEP components, whereas the higher-frequency components corresponded to the earlier, brain stem and primary cortical components of transient-evoked AMEP. Band-pass filtering at 3 to 50 Hz resulted in fewer components, as early brain stem and primary cortical components diminished. Conclusions A band pass of 3 to 100 Hz for recording the 40 Hz SSP results in a composite waveform comprising of distinct brain stem and cortical generators with different orientations of their equivalent dipoles. The relative contributions of the multiple constituents are affected by the acquisition filter low pass: brain stem and primary cortical generators mostly contribute the high frequencies and later cortical contributions dominate the lower frequencies.

Short latency visual evoked potentials to flashes from light-emitting diodes.

Short latency visual evoked potentials (SVEPs) have been described in response to high-intensity, strobe flashes. High-intensity flashes can now be generated from goggle-mounted light emitting diodes (LEDs) and the SVEPs to such flashes have been shown to be reproducible across subjects, avoiding photic spread to the examination room and acoustical artifacts from the strobe stimulator. In this study, SVEPs from multichannel records are described in terms of normative latencies and amplitudes, as well as scalp distributions, to explore their generators. Potentials were recorded from 10 young male subjects, from 16 scalp locations, in response to flashes from goggle-mounted LEDs. Flashes were presented to each eye in turn, as well as binocularly. The latencies, scalp distributions and intersubject variabilities of the LED evoked SVEPs were similar to those obtained with strobe flashes. SVEP components were divided into 3 groups, according to their latency and the electrodes at which they were recorded with the largest amplitudes: periocular (under 40 msec latency), fronto-central (40-55 msec) and parieto-occipital (55-80 msec latency). The scalp distributions observed in this study suggest subcortical generators along the visual pathway, beginning at the retina. The use of goggle-mounted LEDs should promote routine evaluation of the integrity of the visual pathway between retina and cortex using SVEPs.

The combined effects of forward masking by noise and high click rate on monaural and binaural human auditory nerve and brainstem potentials

OBJECTIVE To study effects of forward masking and rapid stimulation on human monaurally- and binaurally-evoked brainstem potentials and suggest their relation to synaptic fatigue and recovery and to neuronal action potential refractoriness. METHODS Auditory brainstem evoked potentials (ABEPs) were recorded from 12 normally- and symmetrically hearing adults, in response to each click (50 dB nHL, condensation and rarefaction) in a train of nine, with an inter-click interval of 11 ms, that followed a white noise burst of 100 ms duration (50 dB nHL). Sequences of white noise and click train were repeated at a rate of 2.89 s(-1). The interval between noise and first click in the train was 2, 11, 22, 44, 66 or 88 ms in different runs. ABEPs were averaged (8000 repetitions) using a dwell time of 25 micros/address/channel. The binaural interaction components (BICs) of ABEPs were derived and the single, centrally located equivalent dipoles of ABEP waves I and V and of the BIC major wave were estimated. RESULTS The latencies of dipoles I and V of ABEP, their inter-dipole interval and the dipole magnitude of component V were significantly affected by the interval between noise and clicks and by the serial position of the click in the train. The latency and dipole magnitude of the major BIC component were significantly affected by the interval between noise and clicks. Interval from noise and the clicks serial position in the train interacted to affect dipole V latency, dipole V magnitude, BIC latencies and the V-I inter-dipole latency difference. Most of the effects were fully apparent by the first few clicks in the train, and the trend (increase or decrease) was affected by the interval between noise and clicks. CONCLUSIONS The changes in latency and magnitude of ABEP and BIC components with advancing position in the click train and the interactions of click position in the train with the intervals from noise indicate an interaction of fatigue and recovery, compatible with synaptic depletion and replenishing, respectively. With the 2 ms interval between noise and the first click in the train, neuronal action potential refractoriness may also be involved.

Equivalent dipoles of the Binaural Interaction components and their comparison with binaurally evoked human auditory 40 Hz steady-state evoked potentials

Objective The purpose of this study was to acquire the Binaural Interaction (BI) components of the auditory middle-latency steady-state 40 Hz potentials, compare them with those of the binaurally evoked 40 Hz response and with transient-evoked Auditory Middle Latency Evoked Potentials (AMEP) and suggest possible contributors and generators of the composite 40 Hz BI. Methods Potentials were recorded from 15 normal-hearing adults in response to 40/sec clicks. BI was derived by subtracting the binaurally evoked potentials from the algebraic sum of the evoked potentials to left and to right ear stimulation. Latencies, magnitudes and orientations of the dipole equivalents of 40 Hz components were compared with their BI counterparts, as estimated by three-channel Lissajous’ trajectories. Comparison of the transient AMEP to binaural stimulation with the BI of the steady-state 40 Hz response was also conducted to elucidate the contributions of different levels along the auditory pathway to the 40 Hz BI responses. Results Each cycle of the BI of the steady-state 40 Hz AMEP included four components that corresponded in latency, amplitude, and dipole orientation to their counterparts in the binaurally evoked waveform. Amplitudes of BI components were 50 to 60% of the respective values in the binaurally evoked potentials. Orientations of BI components matched those of the cortical components in the transient-evoked AMEP. Conclusions The results suggest that the main contribution to the 40 Hz BI is from rate resistant thalamo-cortical neurons. The results also suggest that the binaural cortical neurons contributing to the 40 Hz BI are less affected by increased rate than monaural neurons.