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M Sams

Publications and source records attributed to M Sams.

At least 55 records · Page 3Linked to original sources
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

Event-related brain potentials reflecting processing of relevant and irrelevant stimuli during selective listening.

Event-related brain potentials were recorded from the human scalp during selective listening to tone pips differing in location and/or pitch from irrelevant tones. The subjects' task was to discriminate infrequent deviant tones of lower intensity appearing among designated (relevant) tones. A large processing negativity was observed in the event-related potentials to relevant tones differing from the irrelevant tones in location even when both tones randomly varied in pitch. Similarly, a large processing negativity was elicited by the relevant tones differing from the irrelevant tones in pitch even when the location of both tones varied randomly. The results support the theory that the processing negativity to relevant stimuli reflects a match of these stimuli with an "attentional trace," an actively maintained neuronal representation of the physical feature(s) of relevant stimuli that distinguish these stimuli from the irrelevant stimuli. Furthermore, the infrequent lower-intensity tones appearing among irrelevant tones elicited a mismatch negativity similar to the mismatch negativity elicited by target tones, equivalent lower-intensity tones appearing among relevant tones. This indicates that these infrequent stimulus changes were automatically discriminated by the generator mechanism associated with mismatch negativity.

Adult↗
PubMed1988

Frequency and location specificity of the human vertex N1 wave.

Test tones of 1000 Hz subjectively located in the middle of the head were randomly presented with equiprobable intervening tones. The latter stimulus was constant within a stimulus block. The frequency of the intervening stimulus varied between different blocks from 578 Hz to 1728 Hz and its location varied in parallel with the frequency along the left-right dimension through 7 different locations. The constant inter-stimulus interval was 460 msec. The EEG was recorded at Cz and Fz. The N1 wave elicited by the test stimuli was smaller the smaller was the separation between the two stimuli in frequency or location. These results were interpreted in terms of stimulus-specifically adapted detector activity. The more the test and intervening stimuli resemble each other the greater is the overlap between the respective feature-detector populations activated and, therefore, the smaller is the N1 amplitude. Thus the sensory-specific component of the N1 wave generated in the primary auditory areas at least in part reflects detector activity. The selective frequency adaptation was much more specific in the present study than in the previous ones and suggested that the N1 component recorded was generated by highly frequency-specific neurons. The frequency and location effects were independent, i.e., the frequency effect was rather similar for different location separations, and vice versa. Thus, evidence for separate detectors for frequency and location of an auditory stimulus was obtained.

Adult↗
PubMed1987

Brain mechanism of selective listening reflected by event-related potentials.

Stimulus selection during selective listening on the basis of simple physical stimulus features is reflected by an event-related potential (ERP) component called the processing negativity (PN). PN has been proposed to indicate a matching or comparison process between the physical features of the stimulus and an 'attentional trace,' an actively formed and maintained temporary neuronal representation of the features defining the relevant stimuli. According to this theory, the smaller is the difference between the eliciting stimulus and that represented by the attentional trace, the longer time is the stimulus processed, and thus the larger in amplitude and longer in duration is the PN elicited. The relevant stimuli, perfectly matching with the attentional trace, and therefore eliciting the largest and longest-duration PN, are selected for further processing. In the present study, the relevant and irrelevant stimuli differed in pitch, and the magnitude of this pitch separation was varied between different stimulus blocks. The results support the afore-mentioned matching or comparison hypothesis of selective attention by showing that PN is not elicited only by the relevant stimuli but even by irrelevant stimuli, and further that the latter PN is larger in amplitude and longer in duration the more similar the irrelevant stimuli are to the relevant stimuli. This PN, however, was smaller than that to the relevant stimuli even for very small separations, reflecting high accuracy of the discrimination function of the attentional trace mechanism proposed to underly selective listening. The termination of the PN to the irrelevant stimuli was followed by a positivity which thus partly explained the difference (Nd) between the ERPs to the relevant and irrelevant stimuli.

Adult↗
PubMed1987

Stimulus selection during auditory spatial attention as expressed by event-related potentials.

Selective listening to stimuli with certain physical features, such as pitch or location, is reflected by an event-related brain-potential (ERP) component called the processing negativity (PN). PN may reflect a matching or comparison process between the sensory inflow and the hypothesized 'attentional trace'. The latter is an actively formed and maintained neuronal representation of the physical features defining the relevant stimulus. According to this matching theory of selective attention, the better the sensory input corresponds to the attentional trace the longer the stimulus is processes (as reflected by a larger and longer PN). This theory is supported by the experiment in which the relevant and irrelevant stimuli were separated from one another in pitch. It was found that even the irrelevant stimuli elicited PN, and further that this PN was larger the smaller the pitch separation between the eliciting stimuli and the relevant stimuli. The largest PN was elicited by the relevant stimuli. The present study finds analogous results when the relevant and irrelevant stimuli are separated in spatial origin rather than in pitch.

Adult↗
PubMed1985

Cerebral neuromagnetic responses evoked by short auditory stimuli.

We presented 30 msec sinusoidal tone bursts to the subject's left ear once every 1300 msec. The number of standard tones (1000 Hz) between deviants (1030 Hz) varied randomly from 3 to 15 (even distribution) so that the probability of the standards was 0.9 and that of the deviants 0.1. During stimulation the subject was reading a book. Magnetic responses to the standards and deviants were measured at the estimated ends of the sylvian fissure and electrical responses at Fpz, Fz and Cz. In addition, extensive field maps over the right hemisphere were made from 60 to 75 points in 3 subjects. A least-squares fit was performed to find out the parameters of the equivalent current dipole in a spherically symmetrical head model. Field maps suggested that the source of magnetic response at 100 msec (N100m) can be approximated by a current dipole at the supratemporal plane, possibly at the primary auditory cortex. In two subjects the location and/or the orientation of the equivalent dipole changed during N100m, possibly due to change in the size of the activated cortical area. The deviant stimuli elicited in addition to N100m a second deflection, MMNm, peaking at about 200 msec. This response was regarded as specific to stimulus change. On the basis of field maps it was also concluded that MMNm got a contribution from activity at the supratemporal plane.

Adult↗
PubMed1985

Auditory frequency discrimination and event-related potentials.

Auditory stimulus blocks were presented to 6 subjects. 80% of the stimuli in each block were standards of 1000 Hz and 20% were deviants of either 1002 Hz, 1004 Hz, 1008 Hz, 1016 Hz or 1032 Hz, one deviant type in each block. The constant interstimulus interval was 1 sec and the order of the stimuli was randomized. The subject was instructed either to ignore the deviant stimuli (ignore condition) or to press a response key to them (discrimination condition). In the ignore condition, an ERP component called the mismatch negativity (MMN), with a peak latency of approximately 170 msec, was elicited by those deviants exceeding the discrimination threshold (1016 Hz and 1032 Hz) and also those at the threshold (1008 Hz) tended to elicit a small MMN. In the discrimination condition, in addition to MMN, another negative component, N2b, was elicited by the detected deviants. This component had a somewhat longer latency than, and its midline distribution was posterior to, the MMN. The present results are in line with the hypothesis according to which the MMN component reflects the activation of cerebral mechanisms of passive discrimination, those which cause us to become aware of occasional changes in unattended stimulus sequences. In the discrimination condition, N2b and the slow parietal positivity were dominant features of the ERPs elicited by the detected suprathreshold deviants. The data obtained at the discrimination threshold specifically associate the parietal positivity with becoming aware of stimulus change since those deviants which were detected elicited this positivity whereas there was none to those (physically identical) deviants which remained undetected.

Acoustic Stimulation↗
PubMed1983

Sequential effects on the ERP in discriminating two stimuli.

Two auditory stimuli differing in pitch were presented in random order and equal probability with the constant inter-stimulus interval of 1 sec. The subject's task was to count one of these stimuli. The event-related brain potentials (ERPs) to each stimulus were averaged according to the immediately preceding stimulus sequence. It was found that when a few consecutive repetitions of one stimulus occurred, the ERP to the other stimulus immediately after those repetitions included features resembling those of the ERP to the infrequent stimulus usually observed in the so called 'oddball paradigm'. These features included, among other things, the mismatch negativity usually regarded as a scalp reflection of the neuronal mismatch process with an existing neuronal model. The mismatch negativity was accompanied by an 'N2b', a sharper and later negativity. Interestingly, N2b only occurred when the not-to-be-counted stimulus succeeded (one or several) counted stimuli but not when the order was reversed. This suggests that N2b reflects template mismatch, the occurrence of a stimulus mismatching with the mental image of the target stimulus voluntarily held by the subject.

Adult↗
PubMed1979

Auditory evoked transient and sustained potentials in the human EEG: I. Effects of expectation of stimuli.

The characteristics of auditory evoked transient and sustained potentials were recorded using trains of four-tone stimuli of 1-second duration (interstimulus interval = 1 second) presented once every minute. The subject either attentively expected the stimuli or ignored them while reading. The electroencephalogram was recorded from derivations Cz-Al and Fz-Al. Expectation of the stimuli was associated with increased amplitudes of the transient responses both at the first stimulus of the train and during stimulus repetition. In contrast, the sustained potential at the first stimulus of the train was unchanged or smaller when the subject expected the stimuli. During stimulus repetition, however, the amplitude of the potential was enhanced by expectation of the stimuli. The results support the hypothesis of two sustained potential components and stress the importance of stimulus repetition rate when sustained potentials are studied.

Acoustic Stimulation↗
PubMed1979

Auditory evoked transient and sustained potentials in the human EEG: II. Effects of small doses of ethanol.

The effect of small doses of ethanol (0.4 g/kg) on auditory evoked transient and sustained potentials was studied. Tones of 1-second duration were presented in trains of four stimuli (interstimulus interval = 1 second; intertrain interval = 1 minute). The electroencephalogram was recorded from derivation Cz-Al. Ethanol depressed the transient responses both at the first stimulus of the train and during repeated stimuli. The sustained potentials elicited by the first stimuli of the train were not affected by ethanol, whereas the sustained potentials elicited by repeated stimuli were larger in amplitude under the influence of ethanol than during control experiments. It is suggested that the decrease of the transient responses under the influence of ethanol is mainly due to depression of the reticular formation, whereas the increase of sustained potentials reflects ethanol-induced release of intracortical inhibition.

Acoustic Stimulation↗