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M Sams

Publications and source records attributed to M Sams.

At least 37 records · Page 2Linked to original sources
PubMed1993

Determinants of the auditory mismatch response.

The auditory mismatch field (MMF) is supposed to reflect a comparison process between an infrequent deviant stimulus and the memory trace left by frequent standard stimuli. Therefore, the MMF amplitude has been thought to depend on the strength of such a trace. We examined this hypothesis in records with a 24-channel planar SQUID magnetometer by varying the number of stimuli preceding each deviant, the interdeviant interval (IDI) and the interstimulus interval (ISI) just preceding the deviant (pISI). When a constant IDI was employed and the number of standards between two deviants varied in different sessions, MMF amplitude increased as the number of standards increased. However, MMF did not depend on the number of standards between two deviants when the number varied within a single session and ISI varied as well. MMF decreased slightly when pISI increased from 0.6 to 3.4 sec. When IDI increased and the ISI remained constant, MMF amplitude increased. Most results can be explained within the framework of the memory-trace hypothesis of MMF generation. However, the strengthening of the trace seems to be a complex process which is also affected by the temporal features of the stimulus sequence.

Adult↗
PubMed1993

Human auditory cortical mechanisms of sound lateralization: I. Interaural time differences within sound.

Neuromagnetic responses to 600-ms binaural click trains, presented once every 1.1 s, were recorded with a 24-channel gradiometer from 6 healthy humans. During the first 300 ms, the left-ear stimulus led the right by 0.7 ms and the sound was lateralized to the left ear. At 300 ms, the interaural time difference (ITD) changed and the lateralization moved to one of 5 different locations between the ears. An N100m response peaked about 110 ms after the sound onset and an N130mc response (c to stress a response to the change) about 135 ms after the ITD change. The source locations of N100m and N130mc agreed with activity in the supratemporal auditory cortex; this was confirmed in one subject by superimposing MEG results on MR images. The sources of N100m and N130mc did not differ statistically significantly from each other, nor were there differences in N130mc sources to various lateralization changes. N130mc grew larger when the ITD change increased, in parallel with the increase in the change of the perceived location. We suggest that N130mc is analogous to N100m, but is delayed due to postmasking induced by the early part of the sound.

Acoustic Stimulation↗
PubMed1993

Human auditory cortical mechanisms of sound lateralization: II. Interaural time differences at sound onset.

Neuromagnetic responses were recorded over the right temporal cortex using a 24-channel gradiometer. Stimuli were binaural click trains, presented with six separate interaural time differences (ITDs). N100m to sound onset was larger and earlier for stimuli presented with left- than with right-leading ITDs. With stimulus lateralization taken into account, monaural and binaural stimuli evoked responses of roughly equal amplitude. In selective adaptation and oddball experiments, stimuli presented with different ITDs excited overlapping neuronal populations, but the amount of overlap decreased as the ITD between the stimuli increased. There were no systematic differences in the cortical source locations of the N100m as a function of ITD, however. Thus it appears that ITD-sensitive neurons in the human auditory cortex are not organized into a large-scale, orderly representation, which could be resolved by MEG.

Acoustic Stimulation↗
PubMed1992

Event-related potentials to repetition and change of auditory stimuli.

The major intent of this study was to compare the role of stimulus repetition and change in the elicitation of the MMN, an ERP component specific to stimulus change, and N2b, usually partially overlapping the MMN when stimuli are attended. Event-related potentials were recorded in one set of conditions where subjects ignored the stimuli and read a book, and in another set of conditions where subjects counted stimuli designated as targets. Stimuli were delivered in 4 ways, the common feature between all these conditions being the occurrence of infrequent events at a probability of 0.20: (1) an oddball paradigm with 1 deviant, (2) an oddball paradigm with 2 deviants, each with a probability of 0.10, (3) a regular alternation of tones of 2 pitches where either of the 2 tones infrequently repeated (P = 0.20), and (4) a random presentation of tones of 5 different pitches, where any of the 5 tones infrequently repeated (P = 0.20). In the count conditions, the infrequent events were designated as targets. It was found that the MMN was elicited by stimulus change and not stimulus repetition in the ignore and count conditions, whereas the N2b was elicited by both stimulus changes and repetitions in the count conditions. It was also possible, in the count conditions, to disentangle the part of the late positive complex which is related to stimulus deviation and the part which is related to stimulus significance (target).

Acoustic Stimulation↗
PubMed1992

Neuromagnetic mismatch fields to single and paired tones.

In 8 subjects we recorded multichannel magnetic responses to pitch changes in single 50 msec tones and in tone pairs (oddball paradigm; interstimulus interval 745 msec). Either "A" (1 kHz) was standard (90%) and "B" (1.2 kHz) deviant (10%), or "AA" was standard and "AB" deviant in pairs with onset asynchrony of 75 msec. Subject did not pay attention to the tones. A mismatch field (MMF) was evident in responses to deviants with an equivalent source in the supratemporal auditory cortex, about 1 cm anterior to that for N100m. The MMF source was 3-fold stronger for tone pairs than for single tones.

Adult↗
PubMed1991

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.

Adult↗
PubMed1991

Neuromagnetic responses of the human auditory cortex to short frequency glides.

Neuromagnetic responses were recorded to 33-ms frequency-glide tones with a 7-channel SQUID gradiometer. Ninety per cent of the tones were of rising/falling frequency and 10% otherwise similar tones but presented backwards, i.e. comparable falling/rising frequency glides. The infrequent stimuli elicited a specific response, the mismatch field (MMF), whose equivalent source suggested a generation in the supratemporal auditory cortex. Responses to rising and falling infrequent glides showed no consistent asymmetry. The MMF amplitude correlated positively with the glide magnitude. It is suggested that the generation of MMF to changes in the direction of frequency glides involves neuronal networks tuned to frequency glides.

Adult↗
PubMed1991

Right hemisphere dominance of different mismatch negativities.

Auditory stimulus blocks were presented to 10 reading subjects. Each block consisted of 2 types of stimulus, standard (P = 90%) and deviant (P = 10%), delivered in a random order with short constant inter-stimulus intervals. The standard stimuli were 600 Hz. 80 dB SPL 50 msec sine wave bursts. In different blocks, the deviant stimuli differed from the standards either in frequency (650 Hz), intensity (70 dB) or duration (20 msec). Left- and right-ear stimulations were used in separate blocks. Event-related brain potentials (ERPs) were recorded with 16 electrodes over both hemispheres. All the different types of deviant stimuli elicited an ERP component called the mismatch negativity (MMN). The MMN was larger over the right hemisphere irrespective of the ear stimulated whereas the N1 component, elicited by both standards and deviants, was larger over the hemisphere contralateral to the ear stimulated. The results provide further evidence for the view that the MMN reflects a neural mismatch process with a memory trace which automatically codes the physical features of the repetitive stimuli.

Acoustic Stimulation↗
PubMed1991

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.

Acoustic Stimulation↗
PubMed1991

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.

Adult↗
PubMed1991

Cortical activity elicited by changes in auditory stimuli: different sources for the magnetic N100m and mismatch responses.

Magnetic responses to frequent and infrequent auditory stimuli, all presented in the same stimulus block in randomized order, were recorded. The standard stimuli, comprising 90% of all the stimuli, were 100-ms, 1000 Hz, 90dB sinusoidal tone bursts. There were three deviant tones, each presented at a probability of 3.3%, which differed from the standard tone on one dimension only: frequency deviant (1500 Hz), intensity deviant (67dB SPL), or duration deviant (50 ms). All mismatch fields, i.e., responses elicited by different deviants, as well as N100m to the standards and deviants, could be explained by neural activity in the supratemporal auditory cortex. The source of N100m to standards and deviants was significantly posterior to the sources for the three different mismatch fields. The mean locations of the equivalent dipoles for the different mismatch fields did not differ significantly from each other, but some differences were found for individual subjects.

Acoustic Stimulation↗
PubMed1991

Cortical responses to changes in auditory stimuli. Magnetoencephalographic studies.

Any infrequent change ("deviant") in the parameters of a repetitive auditory stimulus ("standard") elicits both in electro- and magnetoencephalogram a specific "mismatch response". Identical infrequent stimuli do not elicit it; the presence of standards is necessary. Magnetic measurements have shown that the mismatch response has its neural source at the supratemporal auditory cortex. This source is about 10 mm anterior to the source of the N100m deflection, suggesting that these two waves are generated at different cytoarchitectonic areas. It is suggested here that the mismatch response reflects the activity in "change detectors", whereas activity in auditory feature maps underlies the N100m deflection. A model is proposed where the activity from feature maps converges on change detectors; the presence of a mismatch response is suggested to indicate that feature maps have been previously activated.

Acoustic Stimulation↗
PubMed1990

Human somatosensory evoked potentials to mechanical pulses and vibration: contributions of SI and SII somatosensory cortices to P50 and P100 components.

Somatosensory evoked potentials (SEPs) were measured to short tactile pulses and vibratory stimuli applied to the fingertip to determine the characteristics and scalp topography of different early and late SEP components to these types of stimulus. The measurements were obtained from 3 homologous contra- and ipsilateral locations and from the vertex. In 2 subjects the SEPs were measured from 23 recording locations. The subjects were reading during the experiments. The first distinct contralateral response was an anteriorly negative and centrally as well as posteriorly positive peak at about 50 msec latency (P50). Largest P50 responses with shortest peak latencies were measured to single tactile pulses. We suggest that P50 is probably generated in the contralateral SI cortex. The P50 was followed by a distinct negative deflection (N70) in the middle and posterior recording locations on the contralateral hemisphere, which reversed its polarity in the frontal records. This peak was also seen ipsilaterally. At about 100 msec latency a distinct bilateral positive P100 peak was obtained. This peak was most prominent to vibratory stimuli, and especially to high frequency vibration. Comparisons with recent intracortical SEP studies in primates and MEG studies in humans suggest that P100 might be best accounted for by bilateral generators in SII cortices. The early components were followed by a negative N140 wave and by a slow, positive wave with a maximum at about 300 msec. Both waves had an asymmetrical distribution. The N140 wave occurred bilaterally, but was largest contralaterally, and often had two peaks at posterior recording locations. The slow positivity was largest at the vertex and at mid-posterior recording sites.

Adult↗
PubMed1990

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.

Adult↗
PubMed1990

The effect of small variation of the frequent auditory stimulus on the event-related brain potential to the infrequent stimulus.

We investigated whether the mismatch process between a rare stimulus and the trace of the frequent stimulus, which generates the mismatch-negativity component of the event-related potential, can tolerate a small variation in the intensity of the frequent stimulus. Series of short tone pips were presented to 10 subjects while they were reading a book and ignoring the auditory stimuli. The intensity (mean 80dB) of the frequent stimulus (600 Hz) varied within a range that was different in different blocks. The probability of the infrequent stimuli which were, in different blocks, either intensity deviants (600 Hz/70dB) or frequency deviants (650 Hz/80dB) was 10%. Both deviant stimuli elicited mismatch negativity even when the intensity of the frequent stimulus varied, although the amplitude of this component decreased with the increasing variability of the frequent stimulus. These results show that the generator process of mismatch negativity tolerates some variation in the repetitive stimulus, thus indicating that this process is also activated in ecologically more valid conditions. This is crucial to the interpretation of the generator process of mismatch negativity as a biologically vital warning mechanism.

Adult↗
PubMed1990

Event-related brain potentials in selective listening to frequent and rare stimuli.

Our previous event-related brain potential (ERP) results suggest that during selective listening, relevant stimuli are selected for further processing by comparing each stimulus to an "attentional trace," a neuronal representation of the physical features of the relevant stimuli that distinguish them from the irrelevant stimuli. This comparison process is reflected by the early component of the processing negativity (PN), which is largest and longest to the relevant stimuli (perfectly matching with the trace). In the present study, the subjects selectively listened to designated tone stimuli which randomly appeared among irrelevant tones of a different pitch. The probability of relevant stimuli in a block was varied. The processing negativity elicited by relevant stimuli was smaller the less frequent they were. The results support the attentional-trace theory of selective attention, which proposes that, in addition to active maintenance, the trace also depends on the rate of sensory reinforcement provided by the relevant stimuli.

Acoustic Stimulation↗
PubMed1989

Do event-related potentials reveal the mechanism of the auditory sensory memory in the human brain?

Event-related brain potentials (ERP) to task-irrelevant tone pips presented at short intervals were recorded from the scalp of normal human subjects. Infrequent decrements in stimulus intensity elicited the mismatch negativity (MMN) which was larger in amplitude and shorter in latency the softer the deviant stimulus was. The results obtained imply memory representations which develop automatically and accurately represent the physical features of the repetitive stimulus. These memory traces appear to be those of the acoustic sensory memory, the 'echoic' memory. When an input does not match with such a trace the MMN is generated.

Acoustic Stimulation↗