Attention modulates beta oscillations during prolonged tactile stimulation.

The inter-play between changes in beta-band (14-30-Hz) cortical rhythms and attention during somatosensation informs us about where and when relevant processes occur in the brain. As such, we investigated the effects of attention on somatosensory evoked and induced responses using vibrotactile stimulation and magnetoencephalographic recording. Subjects received trains of vibration at 23 Hz to the right index finger while watching a movie and ignoring the somatosensory stimuli or paying attention to the stimuli to detect a change in the duration of the stimulus. The amplitude of the evoked 23-Hz steady-state response in the contralateral primary somatosensory cortex (SI) was enhanced by attention and the underlying dipole source was located 2 mm more medially, indicating top-down recruitment of additional neuronal populations for the functionally relevant stimulus. Attentional modulation of the somatosensory evoked response indicates facilitation of early processing of the tactile stimulus. Beta-band activity increased after vibration offset in the contralateral primary motor cortex (MI) [event-related synchronization (ERS)] and this increase was larger for attended than ignored stimuli. Beta-band activity decreased in the ipsilateral SI prior to stimulus offset [event-related desynchronization (ERD)] for attended stimuli only. Whereas attention modulation of the evoked response was confined to the contralateral SI, event-related changes of beta-band activity involved contralateral SI-MI and inter-hemispheric SI-SI connections. Modulation of neural activity in such a large sensorimotor network indicates a role for beta activity in higher-order processing.

Schizophrenia: a neurophysiological evaluation of abnormal information processing.

Evoked potential indices of an early and late stage of attentional processing were recorded from schizophrenic and normal subjects during dichotic listening tasks. Despite slow and inaccurate detections, the schizophrenic subjects were able to focus selectively to different ears but only at a fast stimulation rate, showing integrity of the early selective stage. They showed an abnormal late stage, indicating inefficiency in processing information from detected targets. Marked deficits at a slow stimulation rate and during divided attention suggest that the schizophrenic attention disorder is one of control and maintenance of a selective processing strategy rather than of general slowness or absence of selectivity.

Human auditory attention: a central or peripheral process?

The click-evoked electrical responses of the human cochlear nerve were recorded from the external ear canal concurrently with the cortical evoked potentials from the scalp. Paying attention to the clicks during a discrimination task resulted in a highly significant enhancement of the cortical response but no change in the cochlear nerve response. Hence no evidence was obtained for the operation of a peripheral gating mechanism during attention in man.

Electrical signs of selective attention in the human brain.

Auditory evoked potentials were recorded from the vertex of subjects who listened selectively to a series of tone pips in one ear and ignored concurrent tone pips in the other ear. The negative component of the evoked potential peaking at 80 to 110 milliseconds was substantially larger for the attended tones. This negative component indexed a stimulus set mode of selective attention toward the tone pips in one ear. A late positive component peaking at 250 to 400 milliseconds reflected the response set established to recognize infrequent, higher pitched tone pips in the attended series.

Neurobiology of conscious experience.

Recent research and theory suggest that conscious experience involves a modelling process. The human brain generates an internal model to fit incoming information about the external world, and experiences the model rather than the information. The modelling occurs at the level of the neocortex and may be associated with specific oscillatory field potentials. Imagining exercises the modelling system independently of sensory information. General disorders of consciousness are caused by damage to those areas of the brain that energize and direct the modelling process Specific deficits of consciousness are caused by focal lesions to sensory or motor cortex that prevent the modelling of particular input and output patterns.

Correlates of eye blinking as determined by synthetic aperture magnetometry.

OBJECTIVE: To evaluate the spatiotemporal characteristics of ocular and cerebral current sources during voluntary eyeblinking. METHODS: Whole-head magnetoencephalographic (MEG) recordings were acquired during voluntary blinking in eight healthy adults and analysed using synthetic aperture magnetometry (SAM). RESULTS: Fronto-temporal MEG sensors showed a large slow wave lasting approximately 400 ms and a small burst of activity with frequencies above 30 Hz at the initiation of the blink. Group maps of blink-related oscillatory activity at frequencies between 1-18 Hz and 32-64 Hz showed increased activity in and around the orbits during the 400 ms following blink onset. Increased oscillatory activity occurred in occipital regions 200 ms after blink onset at frequencies between 18 and 64 Hz. CONCLUSIONS: Blink-related MEG signals are recorded in the regions of the eyes and in the occipital cortex. The anterior activation is likely a combination of muscle contraction and eyelid currents. Occipital activation likely represents neural processes concerned with re-establishing the visual image after transient ocular occlusion. SIGNIFICANCE: The possibility of eyeblink-related fields should be considered when interpreting frontal and occipital source activities during SAM analyses.

Event-related potentials during conscious and automatic memory retrieval.

The effects of study-test lags of between 0 and 32 items on conscious (C) and automatic (A) memory processes in a running word-completion task were investigated with event-related potentials (ERPs). The process dissociation procedure (PDP) can distinguish between C and A contributions to memory by comparing performance when subjects respond with either an old item (inclusion) or a new item (exclusion). C can be estimated by subtracting the probability of an intrusion of an old item during the exclusion task (due to A without C) from the probability of correctly producing an old item during the inclusion task (due to C and/or A). The behavioral results showed that C was stronger when the test item followed the studied word in the next trial or after a lag of one stimulus. The strength of A did not vary with lag. The ERP waveforms contained a broad parietal positive wave between 300 and 800 ms. This parietal wave distinguished between correctly recalled old and new words. The early portion of this old-new effect was significantly affected by lag. Subtracting waveforms to obtain a measure of C revealed an effect in the later portion of this wave, lateralized over the left hemisphere. A sustained frontal negativity occurred during all recordings and was larger during conscious retrieval. There was no consistent ERP effect related to automatic memory retrieval.

Attention and successful episodic encoding: an event-related potential study.

Event-related potentials (ERPs) were used to delineate the cerebral processes occurring when information is encoded into episodic memory and to determine how these processes are affected by divided attention. ERPs were recorded during encoding under focused or divided attention, and were selectively averaged on the basis of their retrieval during later free recall and recognition tests (with remember-know judgments). Items retrieved with conscious recollection of the encoding episode (remembered, recalled) were distinguished at encoding from later missed items by an enhanced left fronto-temporal negative wave (N340), a negative posterior sustained potential and a positive frontal sustained potential. These effects occurred independently of the level of attention. Items later retrieved on the basis of familiarity (known) elicited a larger N340 than missed items, but did not demonstrate the increased sustained potentials. We suggest that item-specific conceptual processing (N340) is sufficient to produce familiarity-based recognition, but additional elaborative processing (sustained interaction of frontal and posterior regions) is necessary for conscious recollection. The effect of divided attention on these processes was related to the difficulty of the secondary task, with the more difficult task causing greater and earlier interference.

Effects of spatial selective attention on the steady-state visual evoked potential in the 20-28 Hz range.

Steady-state visual evoked potentials (SSVEPs) were recorded from the scalp of subjects who attended to a flickering LED display in one visual field while ignoring a similar display (flickering at a different frequency) in the opposite visual field. The flicker frequencies were 20.8 Hz in the left-field display and 27.8 Hz in the right-field display. The SSVEP to the flicker in either field was enhanced in amplitude when attention was directed to its location. The scalp distribution of this SSVEP enhancement was narrowly focused over the posterior scalp contralateral to the visual field of stimulation. A source analysis using Variable Resolution Electromagnetic Tomography (VARETA) indicated that the source current densities for the SSVEP attention effect had a focal origin in the contralateral parieto-occipital cortex.

Event-related brain activity associated with auditory pattern processing.

One of the basic properties of the auditory system is the ability to analyse complex temporal patterns. Here, we investigated the neural activity associated with auditory pattern processing using event-related brain potentials. Participants were presented with a continuously repeating sequence of four tones with rare changes in either the frequency or timing of one of the tones. Both frequency- and time-deviant sounds generated mismatch negativity (MMN) waves that peaked at midline central electrode sites and inverted in polarity at inferior temporal and occipital sites, consistent with generators in the supratemporal plane. The MMN scalp topography was similar for the frequency- and time-deviant stimuli, suggesting that both spectral and temporal relations among elements of an auditory pattern are encoded in a unified memory trace.

Effects of visual attentional load on auditory processing.

Auditory evoked potentials were recorded while participants attended to visually presented digits. The difficulty of the visual task was manipulated by requiring participants to process only the current digit (0-back) or both the current and the preceding digit (1-back). Tones deviating in frequency from standard tones elicited a frontal mismatch negativity peaking around 200 ms which did not vary with visual task. However, decreasing the visual task load enhanced a right-temporal positive wave peaking around 200 ms when tones were presented slowly, and a frontocentral negative wave peaking around 450 ms when tones were presented more rapidly. The degree to which task-irrelevant sounds are processed therefore depends on the degree to which a visual task engages attentional resources.

Event-related brain activity associated with auditory pattern processing.

One of the basic properties of the auditory system is the ability to analyze complex temporal patterns. Here, we investigated the neural activity associated with auditory pattern processing using event-related brain potentials. Participants were presented with a continuously repeating sequence of four tones with rare changes in either the frequency or timing of one of the tones. Both frequency- and time-deviant sounds generated mismatch negativity (MMN) waves that peaked at midline central electrode sites and inverted in polarity at inferior temporal and occipital sites, consistent with generators in the supratemporal plane. The MMN scalp topography was similar for the frequency- and time-deviant stimuli, suggesting that both spectral and temporal relations among elements of an auditory pattern are encoded in a unified memory trace.

Fast temporal interactions in human auditory cortex.

The temporal resolution of the human primary auditory cortex (AC) was studied using middle-latency evoked fields. Paired sounds with either the same or different spectral characteristics were presented with gaps between the sounds of 1, 4, 8 and 14 ms. Spatio-temporal modelling showed (1) that the response to the second sound was recognizable with gaps of 1 ms and rapidly increased in amplitude with increasing gap durations, (2) an enhanced N40m amplitude at gaps > 4 ms, (3) delayed N19m-P30m latencies when the stimuli were different. The median psychoacoustical thresholds were 1.6 ms for the same stimuli and 2.5 ms for different stimuli, confirming the electrophysiological evidence for rapid pattern-specific temporal processing in human primary auditory cortex.

Weighted averaging of steady-state responses.

OBJECTIVE: To compare weighted averaging and artifact-rejection to normal averaging in the detection of steady-state responses. METHODS: Multiple steady-state responses were evoked by auditory stimuli modulated at rates between 78 and 95 Hz. The responses were evaluated after recording periods of 3, 6 and 10 min, using 5 averaging protocols: (1) normal averaging; (2) sample-weighted averaging; (3) noise-weighted averaging; (4) amplitude-based artifact-rejection; and (5) percentage-based artifact rejection. The responses were analyzed in the frequency domain and the signal-to-noise ratio was estimated by comparing the signals at the modulation-frequencies to the noise at adjacent frequencies. RESULTS: Weighted averaging gave the best signal-to-noise ratios. Artifact-rejection was better than normal averaging but not as good as weighted averaging. Responses that were not significant with normal averaging became significant with weighted averaging much more frequently than vice versa. False alarm rates did not significantly differ among the protocols. The advantage of weighted averaging was especially evident when stimuli were presented at lower intensities or when smaller amounts (e.g. only 3 or 6 min) of data were evaluated. Weighted averaging was most effective when the background noise levels were variable. Weighted averaging underestimated the amplitude of the responses by about 2%. CONCLUSION: Weighted averaging should be used instead of normal averaging for detecting steady-state responses.

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.

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.

Deconvolution of 40 Hz steady-state fields reveals two overlapping source activities of the human auditory cortex.

Steady-state auditory evoked fields were recorded from 15 subjects using a whole head MEG system. Stimuli were 800 ms trains of binaural clicks with constant stimulus onset asynchrony (SOA). Seven different SOA settings (19, 21, 23, 25, 27, 29 and 31 ms) were used to give click rates near 40 Hz. Transient responses to each click were reconstructed using a new algorithm that deconvoluted the averaged responses to the different trains. Spatio-temporal multiple dipole modelling in relation to 3D MRI scans revealed two overlapping source components in both the left and right auditory cortex. The primary sources in the medial part of Heschl's gyrus exhibited a N19-P30-N40 m pattern. The secondary, weaker sources at more lateral sites on Heschl's gyrus showed a N24-P36-N46 m pattern. When applied to transient middle latency auditory evoked fields (MAEFs) recorded at SOAs of 95-135 ms, the primary sources imaged activities similar to the deconvoluted steady-state responses, but the secondary source activities were inconsistent. Linear summation of the deconvoluted source waveforms accounted for more than 96% of the steady-state variance. This indicates that the primary activity of the auditory cortex remains constant up to high stimulation rates and is not specifically enhanced around 40 Hz.

Bottom-up and top-down influences on auditory scene analysis: evidence from event-related brain potentials.

The physiological processes underlying the segregation of concurrent sounds were investigated through the use of event-related brain potentials. The stimuli were complex sounds containing multiple harmonics, one of which could be mistuned so that it was no longer an integer multiple of the fundamental. Perception of concurrent auditory objects increased with degree of mistuning and was accompanied by negative and positive waves that peaked at 180 and 400 ms poststimulus, respectively. The negative wave, referred to as object-related negativity, was present during passive listening, but the positive wave was not. These findings indicate bottom-up and top-down influences during auditory scene analysis. Brain electrical source analyses showed that distinguishing simultaneous auditory objects involved a widely distributed neural network that included auditory cortices, the medial temporal lobe, and posterior association cortices.

The P300 wave of the human event-related potential.

The P300 wave is a positive deflection in the human event-related potential. It is most commonly elicited in an "oddball" paradigm when a subject detects an occasional "target" stimulus in a regular train of standard stimuli. The P300 wave only occurs if the subject is actively engaged in the task of detecting the targets. Its amplitude varies with the improbability of the targets. Its latency varies with the difficulty of discriminating the target stimulus from the standard stimuli. A typical peak latency when a young adult subject makes a simple discrimination is 300 ms. In patients with decreased cognitive ability, the P300 is smaller and later than in age-matched normal subjects. The intracerebral origin of the P300 wave is not known and its role in cognition not clearly understood. The P300 may have multiple intracerebral generators, with the hippocampus and various association areas of the neocortex all contributing to the scalp-recorded potential. The P300 wave may represent the transfer of information to consciousness, a process that involves many different regions of the brain.

Event-related potentials recorded during the discrimination of improbable stimuli.

In three separate experiments using auditory stimuli, subjects detected improbable targets in an on-going train of standard stimuli. In experiment I there were two equally improbable target stimuli, one difficult to discriminate from the standard stimuli and one easy to discriminate. Experiment II investigated the effects of discrimination difficulty using four different targets, each equal in probability. Experiment III evaluated the effects of both target discriminability and probability on the target-evoked potentials. The subjects in all three experiments consistently displayed delayed reaction times and higher percentages of misses with the more difficult targets. The amplitude of the N1 wave to target stimuli increased as target discriminability was increased. The latencies of the target-evoked N2 and P3 waves increased and the amplitude of the P3 decreased as target discriminability decreased. The amplitude of the N2 wave increased as the target discrimination became more difficult. The amplitude of the N2 wave elicited by an easy target was enhanced by making the target highly improbable whereas probability did not affect the N2 elicited by a difficult target.