Steady-state evoked potentials possibilities for mental-state estimation

The use of the human steady-state evoked potential (SSEP) as a possible measure of mental-state estimation is explored. A method for evoking a visual response to a sum-of-ten sine waves is presented. This approach provides simultaneous multiple frequency measurements of the human EEG to the evoking stimulus in terms of describing functions (gain and phase) and remnant spectra. Ways in which these quantities vary with the addition of performance tasks (manual tracking, grammatical reasoning, and decision making) are presented. Models of the describing function measures can be formulated using systems engineering technology. Relationships between model parameters and performance scores during manual tracking are discussed. Problems of unresponsiveness and lack of repeatability of subject responses are addressed in terms of a need for loop closure of the SSEP. A technique to achieve loop closure using a lock-in amplifier approach is presented. Results of a study designed to test the effectiveness of using feedback to consciously connect humans to their evoked response are presented. Findings indicate that conscious control of EEG is possible. Implications of these results in terms of secondary tasks for mental-state estimation and brain actuated control are addressed

Transducer influence on Auditory Steady State Evoked Potentials

Preliminary studies have stirred the hope that sound-field stimulation through auditory steady state evoked potentials can be used to assess aided thresholds in the difficult-to-test population. Before the introduction of ASSEP into the clinical field, as a technique for the prediction of aided thresholds in the difficult-to-test population, a question arises concerning its clinical validation. The application of ASSEP through sound field stimulation, in the determination of aided thresholds and for the evaluation of amplification fittings, is dependent on the determination of unaided responses. Subsequently the estimation of unaided thresholds in the hearing impaired population is dependent on the establishment of normative data from the normal hearing population. The aim of this study was to determine the influence of insert earphones and sound field speaker presentation on threshold estimations using monotic auditory steady state evoked potentials, in a group of normal hearing adults. To achieve the aim of the study, a comparative, within-group experimental design was selected. The results of the current study indicated that the monotic single ASSEP technique under both insert earphone- and sound field conditions provided a reasonable estimation (25-35 dB HL for inset earphones; 20-33 dB HL for sound field speaker presentation) of the behavioural pure tone thresholds. The minimum response levels obtained under insert earphone conditions differed significantly from those obtained under sound field conditions for all the frequencies tested except 2 kHz (p < 0.01). Subsequently, the current study indicates that minimum response levels obtained using a specific transducer should serve as the basis of comparison with behavioural thresholds obtained under the same transducer. Therefore, behavioural pure tone thresholds obtained under insert earphone conditions will not suffice as a basis of comparison for minimum response levels obtained for the ASSEP technique under sound field conditions, and vice versa. This research endeavour concluded that the monotic ASSEP technique under both insert earphone and sound field conditions provide useful information for the estimation of frequency specific thresholds, but that the results are transducer specific and that comparison across transducers should be avoided.Dissertation (M (Communication Pathology))--University of Pretoria, 2006.Speech-Language Pathology and AudiologyUnrestricte

Assessing childrens visual acuity with steady state evoked potentials

The majority of children attending ophthalmology clinics require a visual acuity assessment. The optimal technique depends on age as well as the ability to cooperate with testing. Most acuity assessments are performed subjectively by an orthoptist. Objective acuity assessment by Visual Evoked Potential (VEP) provides a complementary assessment in those subjects who cannot complete subjective tests. The aim of this study was to develop and evaluate a rapid, objective visual acuity assessment. The technique was named the step_ VEP and is based on the real-time analysis of steady-state VEPs (ssVEP). It presents high contrast checkerboard stimuli of sizes 0.4 to 3.0 LogMAR with a successive approximation algorithm. Speed of response detection, specificity and sensitivity were optimised by investigation of recording montage and analysis techniques in a group of normal children and adults (N=102). The success, duration and outcome of step_ VEP acuity assessment was compared to transient VEP (t-VEP) acuity assessment and subjective acuity assessment in a group of paediatric patients (N=218). I-D Laplacian analysis of three occipital electrodes was significantly faster than conventional recording and analysis (Oz-Fz) at detecting ssVEP responses near visual acuity threshold (3' checks) from three years upwards, and at detecting responses to 6' and 9' checks in the 7-9 year age group. A lateral electrode site at 15% of the half-head circumference was fastest most often in adults. Step_ VEPs were 16% more successful than t-VEPs and 9% more successful than subjective tests in providing a complete acuity assessment. Subjective acuity scores were systematically higher than VEP acuity scores in subjects who successfully completed both assessments. A closer agreement with subjective acuity scores was found for step_ VEPs than t-VEPs. The disparity between step_ VEP acuity score and subjective acuity score was shown to reduce with age

Exploration of nociceptive cortical processing with steady-state evoked potentials

The periodic presentation of a sensory stimulus induces, at certain frequencies of stimulation, a sustained electroencephalographic response known as steady-state evoked potentials (SS-EP). SS-EPs are considered to reflect entrainment of cortical sensory networks resonating at the frequency of stimulation. In the present study we characterize and compare SS-EPs elicited by the selective electrical activation of nociceptive Aδ-fibers and non-nociceptive Aβ-fibers. Nine subjects took part in the experiment. Ten second trains of nociceptive (intra-epidermal electrical stimulation) and non-nociceptive (transcutaneous electrical stimulation) stimuli were applied to the left and right hand in separate blocks. Trains consisted of 0.5 ms constant-current pulses modulated at 3, 7, 13, 23 and 43 Hz. Consistent nociceptive and non-nociceptive SS-EPs were recorded at all stimulation frequencies. Whereas non-nociceptive SS-EPs were maximal over the parietal region contralateral to the stimulated side, nociceptive SS-EPs were maximal at the vertex and symmetrically distributed over both hemispheres, thus indicating that the two responses reflect the entrainment of distinct neuronal populations. The recording of nociceptive and non-nociceptive somatosensory SS-EPs offers a unique opportunity to study the cortical representation of nociception and touch in humans

Longitudinal comparison of auditory steady-state evoked potentials in preterm and term infants : the maturation process

Introduction Preterm neonates are at risk of changes in their auditory system development, which explains the need for auditory monitoring of this population. The Auditory Steady-State Response (ASSR) is an objective method that allows obtaining the electrophysiological thresholds with greater applicability in neonatal and pediatric population. Objective The purpose of this study is to compare the ASSR thresholds in preterm and term infants evaluated during two stages. Method The study included 63 normal hearing neonates: 33 preterm and 30 term. They underwent assessment of ASSR in both ears simultaneously through insert phones in the frequencies of 500 to 4000Hz with the amplitude modulated from 77 to 103Hz. We presented the intensity at a decreasing level to detect the minimum level of responses. At 18 months, 26 of 33 preterm infants returned for the new assessment for ASSR and were compared with 30 full-term infants. We compared between groups according to gestational age. Results Electrophysiological thresholds were higher in preterm than in full-term neonates (p 0.05) in all the variables described. Conclusion In the first evaluation preterm had higher thresholds in ASSR. There was no difference at 18 months of age, showing the auditory maturation of preterm infants throughout their development

Smart helmet: wearable multichannel ECG & EEG

Modern wearable technologies have enabled continuous recording of vital signs, however, for activities such as cycling, motor-racing, or military engagement, a helmet with embedded sensors would provide maximum convenience and the opportunity to monitor simultaneously both the vital signs and the electroencephalogram (EEG). To this end, we investigate the feasibility of recording the electrocardiogram (ECG), respiration, and EEG from face-lead locations, by embedding multiple electrodes within a standard helmet. The electrode positions are at the lower jaw, mastoids, and forehead, while for validation purposes a respiration belt around the thorax and a reference ECG from the chest serve as ground truth to assess the performance. The within-helmet EEG is verified by exposing the subjects to periodic visual and auditory stimuli and screening the recordings for the steady-state evoked potentials in response to these stimuli. Cycling and walking are chosen as real-world activities to illustrate how to deal with the so-induced irregular motion artifacts, which contaminate the recordings. We also propose a multivariate R-peak detection algorithm suitable for such noisy environments. Recordings in real-world scenarios support a proof of concept of the feasibility of recording vital signs and EEG from the proposed smart helmet

How attention and beat perception modulate neural entrainment to rhythm

Recently, steady-state evoked potentials (SS-EPs) at the frequency of the beat have been observed in electroencephalograms (EEG; Nozaradan et al., 2011, 2012). Previous studies involved participants actively attending to isochronous sequences and repeating rhythms. Here we assessed whether neural enhancement of SS-EPs at beat-related frequencies occurred when (1) participants did not attend to the rhythms, and (2) the rhythm was novel and did not repeat. When participants listened to rhythms that contained a beat SS-EP enhancement was larger during attended rhythms than when participants were distracted by another task, although SS-EPs were still present in all conditions. SS-EP enhancement therefore occurs in non-repeating rhythms, providing further evidence of SS-EPs as a marker of beat perception. Greater response in attended conditions suggests that attention may be a necessary component of beat perception

Steady-state evoked potentials distinguish brain mechanisms of self-paced versus synchronization finger tapping

Sensorimotor synchronization (SMS) requires aligning motor actions to external events and represents a core part of both musical and dance performances. In the current study, to isolate the brain mechanisms involved in synchronizing finger tapping with a musical beat, we compared SMS to pure self-paced finger tapping and listen- only conditions at different tempi. We analyzed EEG data using frequency domain steady-state evoked potentials (SSEPs) to identify sustained electrophysiological brain activity during repetitive tasks. Behavioral results revealed different timing modes between SMS and self-paced finger tapping, associated with distinct scalp topographies, thus suggesting different underlying brain sources. After subtraction of the listen-only brain activity, SMS was compared to self-paced finger tapping. Resulting source estimations showed stronger activation of the left inferior frontal gyrus during SMS, and stronger activation of the bilateral inferior parietal lobule during self-paced finger tapping. These results point to the left inferior frontal gyrus as a pivot for perception–action coupling. We discuss our findings in the context of the ongoing debate about SSEPs interpretation given the variety of brain events contributing to SSEPs and similar EEG frequency responses

A crossmodal crossover: opposite effects of visual and auditory perceptual load on steady-state evoked potentials to irrelevant visual stimuli

Mechanisms of attention are required to prioritise goal-relevant sensory events under conditions of stimulus competition. According to the perceptual load model of attention, the extent to which task-irrelevant inputs are processed is determined by the relative demands of discriminating the target: the more perceptually demanding the target task, the less unattended stimuli will be processed. Although much evidence supports the perceptual load model for competing stimuli within a single sensory modality, the effects of perceptual load in one modality on distractor processing in another is less clear. Here we used steady-state evoked potentials (SSEPs) to measure neural responses to irrelevant visual checkerboard stimuli while participants performed either a visual or auditory task that varied in perceptual load. Consistent with perceptual load theory, increasing visual task load suppressed SSEPs to the ignored visual checkerboards. In contrast, increasing auditory task load enhanced SSEPs to the ignored visual checkerboards. This enhanced neural response to irrelevant visual stimuli under auditory load suggests that exhausting capacity within one modality selectively compromises inhibitory processes required for filtering stimuli in another