Biomedical subjects

R Hari

Publications and source records attributed to R Hari.

At least 181 records · Page 10Linked to original sources

PubMed1989

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)

Adult↗

PubMed1989

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.

Action Potentials↗

PubMed1989

Magnetic mu rhythm in man.

We report detection of magnetic mu rhythm in four subjects using a large-area seven-channel first-order superconducting quantum interference device gradiometer. The polarity of this activity was opposite at the upper and lower ends of the rolandic fissure, and during the sharp transients the field patterns could be satisfactorily explained by a current dipole model. The equivalent dipoles were located close to the sources of the early somatosensory evoked field component N20m, which suggests that the mu rhythm is generated mainly at the primary somatosensory hand projection area. The frequency spectrum of the mu had major peaks around 10 and 21 Hz in all subjects. The high-frequency activity was blocked by clenching of the fist, but not by opening of the eyes, in agreement with characteristics of the electric mu rhythm.

Adult↗

PubMed1989

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.

Acoustic Stimulation↗

PubMed1989

Neuromagnetic steady-state responses to auditory stimuli.

Steady-state magnetic responses to clicks presented at rates between 10 and 70 Hz have been recorded in healthy humans. The responses were highest in amplitude around 40 Hz. This amplitude enhancement is satisfactorily explained by summation of responses evoked by single clicks. The field maps suggest activation of the auditory cortex at all stimulus frequencies. Similar responses were obtained with gated noise bursts and by pauses in a series of clicks. The mean "apparent latency," determined from the phase lag at rates 30-70 Hz, was 54 ms. The physiological relevance of this quantity is shown to be questionable.

Acoustic Stimulation↗

PubMed1989

Interaural interaction in the human auditory cortex.

We studied the effect of binaural, contralateral and ipsilateral stimulation on middle- and long-latency auditory-evoked magnetic fields using trains of 40-Hz clicks. The stimuli evoked both a transient response (N100m) and a 40-Hz response, which presumably reflects coalescence of middle-latency responses. Binaural stimuli elicited significantly larger 40-Hz responses and sustained fields than contralateral stimuli. N100m amplitudes did not differ between binaural and contralateral stimulation; the dipole moments were even smaller to binaural than contralateral stimuli. Responses to the ipsilateral stimuli were always the smallest.

Acoustic Stimulation↗

PubMed1989

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.

Auditory Cortex↗

PubMed1988

Modification of neuromagnetic responses of the human auditory cortex by masking sounds.

We have studied the effects of masking sounds on auditory evoked magnetic fields (AEFs) of healthy humans. The AEFs were elicited by 25-ms tones presented randomly to the left or to the right ear, and the responses were recorded over the right auditory cortex. Without masking, the 100-ms deflection (N100m) was of somewhat higher amplitude and of shorter latency for contra- than ipsilateral stimuli. Continuous speech, music, or intermittent noise, delivered to the left ear, dampened N100m to stimulation of both ears without correlated changes in sensation. Intermittent noise had a weaker effect on N100m than speech or music. Continuous noise fed to the left ear dampened both the sensation of and the responses to the left-ear stimuli, with no significant effect on the responses to the right-ear stimuli. The results suggest that the masking effects of continuous noise, seen at the auditory cortex, derive mainly from the periphery whereas the effects of sounds with intensity and frequency modulations take place at more central auditory pathways.

Acoustic Stimulation↗

PubMed1988

Magnetic responses of the human auditory cortex to noise/square wave transitions.

We recorded evoked magnetic fields from the human auditory cortex to noise/square wave sequences. Two prominent deflections were observed: one 100 msec after the noise onset (N100m) and another 100 msec after the noise/square wave transition (N100m'). The amplitude of N100m' increased with decrease in square wave frequency from 2 kHz to 0.125 kHz and with increase in square wave duration from 4 msec to 200 msec. The latency of N100m' was on the average 23 msec longer for noise durations of 60 msec than 310 msec, whereas the amplitude of N100m' did not change. Increase in interstimulus interval from 1.1 to 8.8 sec enhanced the amplitude of N100m significantly more than that of N100m'. The small interaction between N100m and N100m' and their different recovery cycles suggest that different activation patterns underlie these two 100 msec responses at the auditory cortex.

Adult↗

PubMed1988

Neuromagnetic responses from a deaf subject to stimuli presented through a multichannel cochlear prosthesis.

Neuromagnetic responses to stimuli presented through a 4-channel cochlear prosthesis were recorded from a deaf subject. The topography of the responses agreed with activation of the supratemporal auditory cortex. The responses to tone pips, noise/square-wave sequences, and to intermittent frequency modulation of a continuous tone resembled those obtained from subjects with normal hearing, being consistent with the good performance of this subject with his implant.

Acoustic Stimulation↗

PubMed1987

Cortical origin of middle-latency auditory evoked responses in man.

We have recorded middle-latency magnetic evoked responses to 50-ms noise bursts, presented once every 0.9 s, over the right hemisphere of healthy humans. The measurements were carried out with a sensitive 7-channel SQUID gradiometer with a passband of 0.5-2000 Hz. The response consisted of peaks at about 30, 50 and 65 ms. The location of the equivalent source of the 30-ms deflection agrees with activation of the supratemporal auditory cortex, slightly anterior to the source area of the well-known 100-ms deflection.

Acoustic Stimulation↗

PubMed1987

Long-latency OFF-responses from the human sensorimotor cortex to tetanizing stimulation of thenar muscles.

Neuromagnetic recordings revealed a new type of OFF-response from the human sensorimotor cortex 170-235 ms after the end of tetanization of the thenar muscles with 10-240 Hz electrical pulse trains, delivered directly to the thenar eminence or to the median nerve at the wrist. No OFF-responses were discerned when the skin of the thumb was stimulated. The role of muscle afferents in the generation of the OFF-response is discussed.

Adult↗

PubMed1987

Responses of the human auditory cortex to vowel onset after fricative consonants.

Neuromagnetic responses to different auditory stimuli (noise bursts and short speech stimuli) were mapped over both hemispheres of seven healthy subjects. The results indicate that a particular acoustic feature of speech, vowel onset after voiceless fricative consonants, evokes a prominent response in the human supratemporal auditory cortex. Although the observed response seems to be specific to acoustic rather than phonetic characteristics of the stimuli, it might reflect feature detection essential for further speech processing.

Auditory Cortex↗

PubMed1987

Evidence for cortical origin of the 40 Hz auditory evoked response in man.

We have recorded cerebral magnetic fields to trains of 40 Hz clicks in 6 subjects to study the neural generators of the 40 Hz auditory evoked response (AER). The largest deflection, N100m (magnetic counterpart of the electric N100), was preceded by a low voltage P40m and followed by P200m. A sustained field, SF, was seen at 300-500 msec. A rhythmic 40 Hz fluctuation was superimposed on the whole response. All deflections reversed polarity between the approximated ends of the sylvian fissure and the field patterns were dipolar during the peaks. The field patterns of the 40 Hz response suggested activation of the auditory cortex at the supratemporal plane, 2-3 cm beneath the scalp, close to the sources of P40m, N100m and SF. The equivalent source of SF was 0.5-1.5 cm anterior to the source of N100m in all subjects. The results suggest a cortical origin for the magnetic 40 Hz response, also suggesting that the electric 40 Hz AER is, at least in part, generated within the auditory cortex.

Brain Mapping↗

PubMed1987

Cerebral magnetic responses to stimulation of ulnar and median nerves.

We have compared spatial patterns of somatosensory evoked magnetic fields (SEFs) to stimulation of the ulnar and median nerves at the wrist. An oddball paradigm was used additionally to examine whether an infrequent change in the stimulation site would alter the field pattern. The response consisted of 3 parts: an early small deflection at 22-28 msec, a large deflection peaking between 34 and 86 msec, and a late deflection at 110-180 msec. The wave forms and amplitudes of the responses to ulnar and median nerve stimulation were similar, without any additional deflections for the infrequent stimuli. The field patterns, which were interpreted in terms of the dipole model, could be explained by activation of the primary sensorimotor cortex during all peaks of the response. For the early parts of the response at 22-46 msec, the locations of the equivalent sources for median and ulnar nerve stimulation differed from each other, in agreement with the known somatotopy of SI. No somatotopical order was found for the sources of the later deflections.

Adult↗

PubMed1987

Cerebral magnetic responses to stimulation of tibial and sural nerves.

We report somatosensory evoked magnetic fields (SEFs) to stimulation of the mixed posterior tibial nerve (PTN) and the sensory sural nerve (SN) in 6 healthy subjects. The first peak of the responses occurred at 39-50 ms (P40m), with a 2-3 ms longer latency for SN than PTN stimulation. Within 200 ms several other deflections followed, with interindividually varying waveforms and latencies. Magnetic field mappings indicated that the source of P40m for PTN can be modelled by a single equivalent current dipole at the primary sensorimotor cortex; for the smaller responses to SN stimulation the single dipole model was less applicable. Field patterns for later deflections differed from those of P40m, indicating that several current sources within or near the primary foot projection area are sequentially activated after stimulation of both a mixed and a sensory lower limb nerve. These late deflections could not always be satisfactorily explained by single current dipoles, suggesting more complex geometries for the underlying neural activity.

Adult↗

PubMed1987

Different analysis of frequency and amplitude modulations of a continuous tone in the human auditory cortex: a neuromagnetic study.

We have measured auditory evoked magnetic fields to intermittent frequency and amplitude modulations (FMs and AMs) of a continuous tone in 6 healthy humans. The stimuli were presented in pairs separated by 500 ms in four different combinations (FM-AM, FM-FM, AM-FM and AM-AM). Both modulations elicited neuromagnetic responses of similar waveforms: the largest deflection, N100m (magnetic counterpart of the electric N100), was preceded by a low amplitude P60m and followed by P200m. For stimuli of different types, the decrease of N100m from the first to the second response was less than expected from the recovery cycle of the responses, estimated from the pairs of similar stimuli. We interpret these results as evidence for different processing of amplitude and frequency modulations in the auditory pathways up to the level of supratemporal auditory cortex.

Auditory Cortex↗

PubMed1987

Crossmodal interaction is reflected in vertex potentials but not in evoked magnetic fields.

Magnetic responses and slow vertex potentials evoked by tone bursts and median nerve shocks were recorded in healthy humans to study crossmodal interaction. Stimuli were presented either alone, simultaneously or in pairs separated by 0.5 s; the interstimulus interval was 5 s. Simultaneous or preceding stimuli of another modality did not influence the amplitudes of the magnetic responses, which are known to be generated at modality specific projection areas. However, the electric vertex potentials decreased in amplitude in similar situations, suggesting that the electric and magnetic responses reflect neural events of at least partly different brain areas. Our results do not give support for crossmodal interaction at the sensorimotor or auditory cortices.

Acoustic Stimulation↗