Supratemporal Auditory Cortex Research Articles

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.

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.

Cortical Activity Elicited by Changes in Auditory Stimuli: Different Sources for the Magnetic N1OOm 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.

Event-Related Potentials to Infrequent Changes in Synthesized Phonetic Stimuli

Event-related potentials (ERPs) to synthetic consonant-vowel syllables were recorded. Infrequent changes in such a syllable elicited a "mismatch negativity" as well as an enhanced N100 component of the ERP even when subjects did not pay attention to the stimuli. Both components are probably generated in the supratemporal auditory cortex suggesting that in these areas there are neural networks that are automatically activated by speech-specific auditory stimulus features such as formant transitions.

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.

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.

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)

Sequential mapping favours the hypothesis of distinct generators for Na and Pa middle latency auditory evoked potentials

The temporo-spatial organization of Na and Pa middle latency auditory components evoked by monaural clicks delivered separately to right and left ears was assessed by sequential mapping of scalp potentials. The potential field distribution was found to be different for the two components and was assessed by calculating the maximal potential differences in the Na/Pa time period. These data are compatible with the hypothesis that distinct generators are responsible for the two components. Scalp potential field configuration observed for Na suggests a deep generator, which could be situated at the mesencephalic or diencephalic level. Bilateral cortical generators tangentially orientated satisfactorily account for the distribution of the Pa potential field, which could be related to simultaneous activation of both supratemporal auditory cortices in response to monaural stimulation.

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.

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.

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.