Auditory attention affects two different areas in the human supratemporal cortex

The effect of selective attention on activity of the right human auditory cortex was studied with a 24-channel planar SQUID-gradiometer. Two conditions were used, favoring either a late attention effect following N100m, or an early effect, overlapping with N100m. In experiment 1 (15 subjects), a randomized tone sequence of 1 and 3 kHz tones was delivered to the left ear with a constant interstimulus interval (ISI) of 405 msec. The subjects' task was to count infrequent longer tones of one of these pitches among shorter standards. An attention effect, called magnetic difference (Md), was found when the responses to the irrelevant standards were subtracted from those to the relevant standards. Md peaked at about 220 msec for the 1 kHz tones and at 195 msec for the 3 kHz tones. The equivalent source of Md was in the supratemporal auditory cortex, about 1 cm anterior to the source of N100m, and in the same location as the source of P200m. In experiment 2 (8 subjects) the paradigm was similar, except that the 1 kHz and 3 kHz tones were led to different ears with a random ISI of 240-300 msec. In this case Md started already at 30-40 msec, adding to the N100m deflection, and the sources of N100m and Md overlapped. Present results show that attention can modify the activity of two different areas in the supratemporal auditory cortex. We interpret both attention effects as alterations of the exogenous evoked response components: the earlier effect as changed activity in neurons underlying N100m to relevant tones and the later effect as a modification of P200m to irrelevant tones.

Cerebral magnetic fields to lingual stimulation

We recorded cerebral magnetic fields to electric stimulation of the tongue in 7 healthy adults. The two main deflections of the response peaked around 55 msec (P55m) and 140 msec (N140m). During both of them the magnetic field pattern, determined with a 7- or 24-channel SQUID magnetometer, suggested a dipolar current source. The topography of P55m can be explained by a tangential dipole at the first somatosensory cortex (SI) in the posterior wall of the central sulcus. The equivalent source of N140m is, on average, about 1 cm lateral to the source of P55m. The reported method allows non-invasive determination of the cortical tongue representation area.

Magnetoencephalographic 10-Hz rhythm from the human auditory cortex

Spontaneous magnetoencephalographic activity was recorded with a 24-SQUID gradiometer over the lateral aspects of the head in 3 healthy adults. All subjects displayed 8-10 Hz rhythmic activity which was not affected by opening of the eyes but was occasionally dampened by auditory stimuli. The equivalent sources of the rhythm were in the supratemporal auditory cortex, and the activity may therefore represent 'idling' of the auditory cortex. Obviously each sensory projection cortex has its own local spontaneous rhythm.

Auditory evoked magnetic fields after ischemic brain lesions

Auditory evoked magnetic fields to noise/square-wave sequences, presented once every 2.2 seconds, were recorded in 8 patients who had ischemic lesions in the auditory cortex or in its vicinity. In 2 patients with large temporoparietal lesions, the magnetic 100-msec deflection (N100m) was absent over the damaged side. In 1 patient with a large but less deep frontotemporal lesion, a small N100m could be discerned on the defective side. Frontal lesions or small lesions in the vicinity of the supratemporal plane had no effect on N100m. Auditory evoked magnetic field recordings may be useful in clinical studies of auditory cortical functions.

Seeing speech: visual information from lip movements modifies activity in the human auditory cortex

Neuromagnetic responses were recorded over the left hemisphere to find out in which cortical area the heard and seen speech are integrated. Auditory stimuli were Finnish/pa/syllables presented together with a videotaped face articulating either the concordant syllable/pa/(84% of stimuli, V = A) or the discordant syllable/ka/(16%, V not equal to A). In some subjects the probabilities were reversed. The subjects heard V not equal to A stimuli as/ta/ or ka. The magnetic responses to infrequent perceptions elicited a specific waveform which could be explained by activity in the supratemporal auditory cortex. The results show that visual information from articulatory movements has an entry into the auditory cortex.

On brain's magnetic responses to sensory stimuli

Magnetoencephalography (MEG) as a tool to study the brain's responses to external stimuli is briefly reviewed. The introduction of multichannel superconducting quantum interference device magnetometers with over 20 sensors is now revolutionizing MEG recordings, and data are starting to accumulate about the functions of different sensory systems. Magnetic evoked responses also can be used in clinical assessment of the integrity of sensory pathways.

Landau-Kleffner syndrome: epileptic activity in the auditory cortex

The Landau-Kleffner syndrome (LKS) is characterized by electroencephalographic spike discharges and verbal auditory agnosia in previously healthy children. We recorded magnetoencephalographic (MEG) spikes in a patient with LKS, and compared their sources with anatomical information from magnetic resonance imaging. All spikes originated close to the left auditory cortex. The evoked responses were contaminated by spikes in the left auditory area and suppressed in the right--the latter responses recovered when the spikes disappeared. We suggest that unilateral discharges at or near the auditory cortex disrupt auditory discrimination in the affected hemisphere, and lead to suppression of auditory information from the opposite hemisphere, thereby accounting for the two main criteria of LKS.

Bilateral electrical stimulation of a congenitally-deaf ear and of an acquired-deaf ear

Two identical multichannel intracochlear prostheses were implanted in the same patient. The first prosthesis, implanted in the congenitally-deaf right ear, elicited clear sound perception but no speech recognition. After 2 years, a second prosthesis, implanted in the acquired-deaf left ear, enabled the patient to understand speech without lip-reading. Brainstem and middle-latency evoked potentials were similar with electrical stimulation of both ears and resembled those evoked by acoustic stimuli in subjects with normal hearing. Cortical electric and magnetic responses differed for right- and left-sided electrical stimulation suggesting that stimulation of the congenitally-deaf ear elicited an abnormal activation of the auditory cortex. These results suggest that only cortical responses were affected by the different histories of deafness of the ears.

Activation of the human auditory cortex by speech sounds

Magnetic evoked responses were recorded to different speech sounds in healthy humans. (i) Short words consisting of fricative consonant/vowel combinations evoked strong responses at the auditory cortex about 100 ms after the vowel onset. The response is specific to acoustic rather than phonetic aspects of the sounds. (ii) In a categorization task, words elicited a transient response followed by a sustained field (SF). When the subject counted the number of target words, SF was clearly increased. There were no consistent differences between the hemispheres and a similar increase of SF was observed when the subject classified the duration of two tones. (iii) When tone 'probes' were presented randomly to either ear and speech sounds to one ear, the 100-ms response was dampened and delayed bilaterally. The dampening was not specific to speech masking but dependent on the amount of frequency and amplitude transitions in the masker. All these experiments suggest that the auditory system performs a very similar analysis of both speech signals and other sounds. (iv) In a recent study, more closely related to speech perception, visual input from articulatory movements of the speaker was found to affect the activity of the auditory cortex. It seems that MEG studies can be useful in the study of brain mechanisms underlying speech perception in intact humans.

Seeing faces activates three separate areas outside the occipital visual cortex in man

We have examined magnetic cortical responses of 15 healthy humans to 46 different pictures of faces. At least three areas outside the occipital visual cortex appeared to be involved in processing this input, 105-560 ms after the stimulus onset. The first active area was near the occipitotemporal junction, the second in the inferior parietal lobe, and the third in the middle temporal lobe. The source in the inferior parietal lobe was also activated by other simple and complex visual stimuli.

Magnetoencephalography in the study of epilepsy

A brief review is given about the basic principles of magnetoencephalography (MEG), a noninvasive brain research method in which weak magnetic fields are detected outside the human head with SQUID (Superconducting Quantum Interference Device) magnetometers. The active brain areas, producing the signal, are modelled by current dipoles, which are assumed to be situated in a spherically symmetric volume conductor. The locations of these "equivalent dipoles" can be found, in the optimal case, with a precision of a few millimeters. The new multichannel magnetometers allow measurements of spontaneous brain activity without EEG-triggered averaging. The 3-dimensional locations of superficial epileptogenic foci can be determined with respect to external landmarks on the skull and to known generator areas of evoked responses in the brain. Examples are given about MEG recordings of epileptic patients.

Localization of epileptic foci using a large-area magnetometer and functional brain anatomy

We used a large-area, 7-channel, first-order superconducting quantum interference device (SQUID) gradiometer to preoperatively determine the sites of epileptic foci in 2 patients with intractable temporal lobe seizures. The equivalent dipoles for the epileptic spikes were located with respect to external landmarks of the skull and in relation to the generation sites of magnetic auditory evoked responses. It was also possible, for the first time, to determine the location of the equivalent source using simultaneously measured data from seven locations only. The sites of the equivalent dipoles, in the right temporal lobe, agreed with the electrocorticographic and depth electrode recordings made during the operation.

Separate finger representations at the human second somatosensory cortex

We recorded neuromagnetic responses of the second somatosensory cortex in healthy humans. Cutaneous electrical stimulation of fingers elicited a response around 100 ms, with a field pattern agreeing with activation of the second somatosensory cortex in the upper bank of the Sylvian fissure. In an oddball paradigm, with standards presented to the thumb and deviants (10%) to the middle finger, or vice versa, the second somatosensory cortex responses to deviants were almost three times as high in amplitude as those to standards. A similar amplitude enhancement was obtained when the deviants were presented in the absence of the intervening standards but with the same interstimulus interval. The results indicate that an accurate functional representation of different body areas is maintained at the human second somatosensory cortex.

Omissions of Auditory Stimuli May Activate Frontal Cortex

We measured, with a 7 channel SQUID gradiometer, cerebral magnetic responses, time-locked to unpredictable and infrequent stimulus omissions (10% of stimuli) in an otherwise regular sequence of short tones, repeated once every 510 ms. In 10 out of 12 subjects broad responses with multiple peaks and with wide interindividual variability were detected. Attention clearly increased the observed signals. The magnetic field patterns evoked by stimulus omissions could be explained by single current dipoles, whose locations agree with activation of the posterolateral frontal cortex.

Reactions of human auditory cortex to a change in tone duration

Auditory evoked magnetic fields were measured using a stimulation sequence where repetitive identical tone bursts were randomly and infrequently replaced by shorter tones. The deviation in the stimulus duration evoked a specific response, the mismatch field, peaking about 130 ms after the end of the shorter stimulus. The response can be explained by neural activity at the supratemporal auditory cortex. The results suggest the existence of a neural mechanism specific to changes in sound duration.

Early deflections of cerebral magnetic responses to median nerve stimulation

We have recorded early components of somatosensory evoked magnetic fields with a sensitive 7-channel first-order gradiometer using a wide recording passband (0.05-2000 Hz) and high sampling frequency (8000 Hz). The left median nerve was stimulated at the wrist and responses were recorded over the right hemisphere. The responses typically consisted of a N20m peaking at 18-20 msec, a small P22m peaking at 21-23 msec and a P27m peaking at 29-31 msec. The topography of N20m could be explained by a tangential current dipole in the posterior wall of the central sulcus (probably in area 3b). The equivalent dipoles of P27m were located on average 10 mm antero-medially to the sources of N20m. This suggests that P27m may get a contribution from the anterior wall of the central sulcus. An increase of stimulus repetition rate from 2 to 5 Hz decreased the amplitude of P27m more than that of N20m, which implies that these two deflections are generated by different neural networks.

Evoked responses of human auditory cortex may be enhanced by preceding stimuli

We report enhancement of the 100 msec deflection N100m of the auditory evoked magnetic field in paired-stimulus paradigms. Noise bursts of 50 msec duration were delivered in pairs to the left ear at interpair intervals of 1.2-1.4 sec. Stimulus onset asynchrony (SOA) within the pair was either 70, 150, 230, 300, 370 or 500 msec, all intervals being presented randomly within the same block. Magnetic responses were recorded over the right hemisphere with a 7-channel first-order SQUID gradiometer. The mean amplitude of N100m to the second stimulus was maximal at an SOA of about 150 msec, decreasing at longer SOAs to an amplitude about equal to that of the N100m evoked by the first stimulus. Similar enhancement effects were elicited by noise bursts, square-wave tones and sinusoidal tones, by pauses in a continuous noise, and when the two stimuli of a pair were led to different ears.

Recording and interpretation of cerebral magnetic fields

Contemporary brain research progresses along two main lines: the microlevel approach explores single neurons and subcellular elements, while macrolevel studies focus on more complex cerebral functions, including behavior. This review presents results obtained mainly in our laboratory by means of an intermediate method, magnetoencephalography (MEG), which reflects cortical activity of neuronal populations at the level fo cytoarchitectonic areas. Because it is completely noninvasive, MEG can be used to study brain functions that are characteristically human.

Neuromagnetic responses of human auditory cortex to interruptions in a steady rhythm

We have recorded, with a 7-channel SQUID gradiometer, evoked magnetic responses of 6 healthy humans to interruptions of a steady rhythm of 50 ms 'standard' tone bursts repeated once every 610 ms. Ten percent of the tones occurred 'too early', 410 ms after the preceding stimulus. The response to standards peaked, on average, at 90 ms and that to the early tones at 148 ms. Field patterns were dipolar during both responses and the equivalent sources agreed with activation of the supratemporal auditory cortex, at slightly different locations. The dipole moments were more than twice as strong for the early tones as for the standards. The results emphasize the importance of temporal stimulation patterns in activating the human auditory cortex.

Multichannel detection of magnetic compound action fields of median and ulnar nerves

We have recorded magnetic compound action fields, evoked by median and ulnar nerve stimulation at the wrist, with a 7-channel 1st-order SQUID gradiometer. Responses at cubita, elbow and brachial plexus were usually monophasic, with polarities and field patterns corresponding to the intracellular current flow in the leading edge of the action potential volley.

Selective listening modifies activity of the human auditory cortex

We have studied the effect of selective listening on the neuromagnetic evoked activity of the human auditory cortex. In the word categorization experiment the stimuli were 5-letter words, each beginning with (k). Half of them were targets, i.e., names of animals or plants, and half other meaningful Finnish words. In the duration discrimination experiment equiprobable tones of 425 ms (targets) or 600 ms duration were presented. In both experiments the interstimulus interval (ISI) was 2.3 s and the stimuli of the two classes were presented randomly. Subjects either ignored the stimuli (reading condition) or counted the number of targets (listening condition). The magnetic field over the head was measured with a 7-channel 1st-order SQUID-gradiometer. The stimuli evoked a transient response followed by a sustained field. The transient response did not differ between the two conditions but the sustained field was significantly larger in the listening than reading condition; the increase began 120-200 ms after stimulus onset and continued for several hundred milliseconds. The equivalent source locations of both transient and sustained responses agreed with activation of the supratemporal auditory cortex. In the dichotic listening experiment 25-ms square-wave stimuli were presented randomly and equiprobably either to the left or to the right ear at an ISI of 0.8-1 s, either alone or in presence of a speech masker. Counting the stimuli of either ear resulted in differences between responses to relevant and irrelevant sounds. The difference began 140-150 ms after stimulus onset and peaked at 200-240 ms. During monaural speech masking, N100m was larger for attended than ignored stimuli.(ABSTRACT TRUNCATED AT 250 WORDS)

Cerebral magnetic responses to noise bursts and pauses of different durations

We compared magnetic-evoked responses of human auditory cortex to short (5, 10, 20, 40, 80 and 160 ms) noise bursts and to pauses of identical durations in continuous noise. Onsets of both stimuli evoked responses with the most prominent deflection (N100m) peaking at about 100 ms. Both field maps could be explained by current dipoles, which agree with activity at the supratemporal cortex at slightly different locations. At the shortest 5-ms duration the noise bursts evoked a clear N100m whereas pauses elicited very low-amplitude responses or no response at all. For both stimuli, N100m increased in amplitude when the stimulus duration was increased from 5 up to 20-40 ms. The latencies were 10-20 ms longer for pauses than noise bursts with the longest latencies at the shortest stimulus durations. The differences in amplitudes and latencies as a function of stimulus duration and the slightly different source areas indicate that the generators of the on- and off-responses are not identical.