Patricia Van Roon

Intracerebral Sources of Human Auditory Steady-State Responses

The objective of this study was to localize the intracerebral generators for auditory steady-state responses. The stimulus was a continuous 1000-Hz tone presented to the right or left ear at 70 dB SPL. The tone was sinusoidally amplitude-modulated to a depth of 100% at 12, 39, or 88 Hz. Responses recorded from 47 electrodes on the head were transformed into the frequency domain. Brain electrical source analysis treated the real and imaginary components of the response in the frequency domain as independent samples. The latency of the source activity was estimated from the phase of the source waveform. The main source model contained a midline brainstem generator with two components (one vertical and lateral) and cortical sources in the left and right supratemporal plane, each containing tangential and radial components. At 88 Hz, the largest activity occurred in the brainstem and subsequent cortical activity was minor. At 39 Hz, the initial brainstem component remained and significant activity also occurred in the cortical sources, with the tangential activity being larger than the radial. The 12-Hz responses were small, but suggested combined activation of both brainstem and cortical sources. Estimated latencies decreased for all source waveforms as modulation frequency increased and were shorter for the brainstem compared to cortical sources. These results suggest that the whole auditory nervous system is activated by modulated tones, with the cortex being more sensitive to slower modulation frequencies.

The correction of ocular artifacts: a topographic perspective

OBJECTIVE To evaluate the scalp topography of the potentials related to saccades and blinks. METHODS The scalp topographies of the potentials associated with saccades and blinks were recorded in 60 subjects. The topographies were analyzed using both source components and attenuation factors, with each factor representing the fraction of the potential recorded in peri-ocular electrodes that contributes to the EEG recorded from a particular scalp location. RESULTS Blinks and upward saccades generated potentials with very different topographies. Left and right saccades and up and down saccades generated equal but inverted fields except at peri-ocular locations where subtle inequalities occurred. The potentials associated with lateral saccades were consistently larger in female subjects than in male subjects. CONCLUSIONS The differences in the scalp topographies between blinks and vertical saccades can be explained by the different ways in which they are generated. Blink potentials are caused by the eyelids sliding down over the positively charged cornea, whereas saccade potentials are caused by changes in the orientation of the corneoretinal dipole. Any compensation procedure for ocular artifacts must take into account the topographic differences between blinks and upward saccades.

Multiple Auditory Steady-State Responses to AM and FM Stimuli

Multiple auditory steady-state responses were recorded using tonal stimuli that were amplitude-modulated (AM), frequency-modulated (FM) or modulated simultaneously in both amplitude and frequency (mixed modulation or MM). When MM stimuli combined 100% AM and 25% FM (12.5% above and below the carrier frequency) and the maximum frequency occurred simultaneously with maximum amplitude, the MM response was one third larger than the simple AM response. This enhancement occurred at intensities between 50 and 30 dB SPL and at carrier frequencies between 500 and 4000 Hz. The AM and FM components of a MM stimulus generate independent responses that add together to give the MM response. Since AM responses generally occur with a slightly later phase delay than FM responses, the largest MM response is recorded when the maximum frequency of the MM stimulus occurs just after the maximum amplitude.

The use of phase in the detection of auditory steady-state responses

OBJECTIVE To investigate how phase measurements might facilitate the detection of auditory steady-state responses. METHODS Multiple steady-state responses were evoked by auditory stimuli modulated at rates between 78 and 95 Hz and with intensities between 50 and 0 dB SPL. The responses were evaluated in 20 subjects after 1, 2, 4, and 6 min. The responses were analyzed in the frequency domain using 4 different detection protocols: (1) phase-coherence, (2) phase-weighted coherence, (3) F test for hidden periodicity, and (4) phase-weighted t test. The phase-weighted measurements were either based on the mean phase of a group of normal subjects or derived for each subject from the phase of the response at higher intensities. RESULTS Detection protocols based on both phase and amplitude (F test and phase-weighted t test) were more effective than those based on phase alone (phase coherence and phase-weighted coherence) although the difference was small. Protocols using phase-weighting were more effective than those without phase-weighting. The lowest thresholds for the steady-state responses were obtained using the phase-weighted t test. CONCLUSION Threshold detection can be improved by weighting the detection protocols toward an expected phase, provided that the expected phase can be reliably predicted.

Human auditory steady-state responses to tones independently modulated in both frequency and amplitude.

Objective Independent amplitude and frequency modulation (IAFM) of a carrier tone uses two different modulating frequencies, one for amplitude modulation (AM) and one for frequency modulation (FM). This study measured the human steady-state responses to multiple IAFM tones. The first question was whether the IAFM responses could be recorded without attenuation of the AM and FM components. The second question was whether IAFM stimuli would provide a more effective demonstration of responses at intensities near threshold than the responses to AM tones. The third question was whether the responses to multiple IAFM stimuli would relate to the discrimination of words at different intensities. Design Multiple AM, FM, or IAFM stimuli were presented simultaneously. Responses were recorded between the vertex and the neck and analysed in the frequency domain. The first experiment compared IAFM responses with AM and FM responses. The second experiment compared IAFM responses with AM responses between intensities 20 to 50 dB SPL. The third experiment related the IAFM responses to the discrimination of monosyllabic words at intensities between 20 and 70 dB SPL. Results Steady-state responses to the individual component of the IAFM stimuli were clearly recognizable although attenuated a little (14%) from the responses to AM or FM alone. Using IAFM stimuli was not different than simply using AM stimuli when trying to recognize responses at low intensities. The number of responses detected during multiple IAFM stimulation and the amplitudes of these responses correlated significantly with word discrimination. Conclusions IAFM of a carrier using two different modulating frequencies (one for AM and one for FM) elicits separate AM and FM responses that are relatively independent of each other. These separate responses can be used to detect whether a particular carrier has been processed in the cochlea, but they are not as effective as measuring responses to carriers that have been modulated in both amplitude and frequency at the same modulation frequency (mixed modulation). The detectability of eight different responses (four AM and four FM) to an IAFM stimuli relates well to the ability of subjects to discriminate words. IAFM stimuli therefore show promise as an objective test for assessing suprathreshold hearing.

Species sensitivity of early face and eye processing.

Humans are better at recognizing human faces than faces of other species. However, it is unclear whether this species sensitivity can be seen at early perceptual stages of face processing and whether it involves species sensitivity for important facial features like the eyes. These questions were addressed by comparing the modulations of the N170 ERP component to faces, eyes and eyeless faces of humans, apes, cats and dogs, presented upright and inverted. Although all faces and isolated eyes yielded larger responses than the control object category (houses), the N170 was shorter and smaller to human than animal faces and larger to human than animal eyes. Most importantly, while the classic inversion effect was found for human faces, animal faces yielded no inversion effect or an opposite inversion effect, as seen for objects, suggesting a different neural process involved for humans faces compared to faces of other species. Thus, in addition to its general face and eye categorical sensitivity, the N170 appears particularly sensitive to the human species for both faces and eyes. The results are discussed in the context of a recent model of the N170 response involving face and eye sensitive neurons (Itier et al., 2007) where the eyes play a central role in face perception. The data support the intuitive idea that eyes are what make animal head fronts look face-like and that proficiency for the human species involves visual expertise for the human eyes.

Frontal EEG/ERP correlates of attentional processes, cortisol and motivational states in adolescents from lower and higher socioeconomic status

Event-related potentials (ERPs) and other electroencephalographic (EEG) evidence show that frontal brain areas of higher and lower socioeconomic status (SES) children are recruited differently during selective attention tasks. We assessed whether multiple variables related to self-regulation (perceived mental effort) emotional states (e.g., anxiety, stress, etc.) and motivational states (e.g., boredom, engagement, etc.) may co-occur or interact with frontal attentional processing probed in two matched-samples of fourteen lower-SES and higher-SES adolescents. ERP and EEG activation were measured during a task probing selective attention to sequences of tones. Pre- and post-task salivary cortisol and self-reported emotional states were also measured. At similar behavioural performance level, the higher-SES group showed a greater ERP differentiation between attended (relevant) and unattended (irrelevant) tones than the lower-SES group. EEG power analysis revealed a cross-over interaction, specifically, lower-SES adolescents showed significantly higher theta power when ignoring rather than attending to tones, whereas, higher-SES adolescents showed the opposite pattern. Significant theta asymmetry differences were also found at midfrontal electrodes indicating left hypo-activity in lower-SES adolescents. The attended vs. unattended difference in right midfrontal theta increased with individual SES rank, and (independently from SES) with lower cortisol task reactivity and higher boredom. Results suggest lower-SES children used additional compensatory resources to monitor/control response inhibition to distracters, perceiving also more mental effort, as compared to higher-SES counterparts. Nevertheless, stress, boredom and other task-related perceived states were unrelated to SES. Ruling out presumed confounds, this study confirms the midfrontal mechanisms responsible for the SES effects on selective attention reported previously and here reflect genuine cognitive differences.

Noise-induced increase in human auditory evoked neuromagnetic fields

Noise is usually detrimental to auditory perception. However, recent psychophysical studies have shown that low levels of broadband noise may improve signal detection. Here, we measured auditory evoked fields (AEFs) while participants listened passively to low‐pitched and high‐pitched tones (Experiment 1) or complex sounds that included a tuned or a mistuned component that yielded the perception of concurrent sound objects (Experiment 2). In both experiments, stimuli were embedded in low or intermediate levels of Gaussian noise or presented without background noise. For each participant, the AEFs were modeled with a pair of dipoles in the superior temporal plane, and the effects of noise were examined on the resulting source waveforms. In both experiments, the N1m was larger when the stimuli were embedded in low background noise than in the no‐noise control condition. Complex sounds with a mistuned component generated an object‐related negativity that was larger in the low‐noise condition. The results show that low‐level background noise facilitates AEFs associated with sound onset and can be beneficial for sorting out concurrent sound objects. We suggest that noise‐induced increases in transient evoked responses may be mediated via efferent feedback connections between the auditory cortex and lower auditory centers.

Multiple auditory steady state responses (80-101 Hz): effects of ear, gender, handedness, intensity and modulation rate.

Objective: To evaluate how the amplitudes and latencies of auditory steady state responses (ASSRs) to multiple stimuli presented at rates between 80 and 101 Hz vary with the ear of stimulation, the handedness or gender of a subject, and the rate and intensity of the stimuli. Design: ASSRs were recorded in a group of 56 young adults (27 females, 13 left handed) using several stimulus conditions. In the two main conditions, four sinusoidally amplitude-modulated tones (each uniquely modulated using rates between 80 and 105 Hz) with carrier frequencies of 500, 1000, 2000, and 4000 Hz, were presented concurrently to each ear (eight total). In the first condition the modulation rates for the left ear were slower than those for the right and in the second condition this relationship was reversed. Other conditions evaluated the responses to single stimuli, to multiple stimuli presented in one ear only and to multiple stimuli presented dichotically (four in each ear) with rates that decreased rather than increased with increasing carrier frequency. Stimuli were presented at an intensity of 73 dB SPL except in two conditions wherein the intensity was 53 dB SPL. Results: At 73 dB SPL, multiple-stimulus ASSRs were significantly reduced (monotic or dichotic) compared with single-stimulus ASSRs, especially at 1000 and 2000 Hz. There were significant differences between monotic and dichotic stimulation. When the stimuli were presented dichotically, the amplitude of the response varied with the relative rates of modulation for the stimuli presented in each ear. ASSRs were larger in the ear with the higher rate when the carrier frequencies were 500 and 1000 Hz and when the modulation rates were <90 Hz. There were no consistent effects of gender or ear of stimulation. There were also no significant effects of handedness. Conclusions: Presenting multiple stimuli at 73 dB SPL in the same ear decreases the amplitude of the ASSR compared with when the stimuli are presented singly. This is caused by the masking effect of low on higher carrier frequencies and some other inhibitory effect of high on lower frequencies. Dichotic stimulation can increase the amplitude of the response to stimuli modulated more rapidly (and concomitantly decrease the responses to the stimuli modulated more slowly). This effect occurs only for carrier frequencies <2000 Hz and for modulation frequencies <90 Hz. Dichotic stimulation also causes a small but highly significant decrease in the latency of the response compared with monotic stimulation.

Human auditory steady-state responses during sweeps of intensity.

Objective: To record steady-state responses to amplitude-modulated tones that change their intensity over time and to see how well behavioral thresholds can be estimated from such responses. Design: The intensity of the stimuli used in this experiment increased from 25 to 75 dB SPL for 8 sec and then decreased back to 25 dB HL during the subsequent 8 sec. Responses to this intensity sweep were averaged and then analyzed using a short-time Fast-Fourier Transform to measure how the amplitude and phase of the responses changed with intensity. One experimental condition presented single 2-kHz tones to the left ear; a second condition examined the use of simultaneously presented multiple tones (0.5, 1, 2, and 4 kHz) to the left ear; a third condition used multiple tones presented dichotically; and a fourth condition presented the multiple dichotic tones in masking noise to simulate either low-frequency (less than 1400 Hz) or high-frequency (greater than 1400 Hz) hearing loss. Physiological thresholds were determined using six different algorithms and the relations between physiological and behavioral thresholds were evaluated to see how well behavioral thresholds could be estimated. Results: The amplitude-intensity functions for the 1 and 2 kHz responses both demonstrated a plateau at higher intensities in the multiple-stimulus conditions but not in the single-stimulus condition. The slope of the amplitude-intensity functions varied significantly with the carrier frequency of the stimulus: 1.30 at 500 Hz, 0.87 at 1000 Hz, 0.75 at 2000 Hz, and 1.40 at 4000 Hz. The slope of the phase-intensity function averaged 1.16 degrees per dB and did not vary with carrier frequency. Estimates of latency, however, indicated that latency increased with decreasing carrier frequency and with decreasing intensity. The performance of the threshold estimating algorithms differed between normal hearing and simulated hearing loss, since the amplitude- and phase-intensity functions in the latter condition were not linear. Physiological-behavioral threshold differences were generally greater for normal hearing than for simulated hearing loss. Linear regression provided the least physiological-behavioral difference but was quite variable during simulated hearing loss. Simply defining threshold as the lowest intensity above which all responses were significantly different from residual EEG noise was the most accurate method in terms of yielding the least standard deviation of the physiological-behavioral difference with an average standard deviation of 10 dB, provided EEG noise levels were low enough in the normal hearing condition. Conclusions: Thresholds can be estimated using intensity sweeps with about the same accuracy as recording separate responses to discrete intensities. Sweep recordings provide additional information about the responses at suprathreshold intensities by clearly determining amplitude- and phase- intensity functions at these intensities.