Sluggish auditory processing in dyslexics is not due to persistence in sensory memory.

The hypothesis that dyslexics show prolonged audible persistence was tested by an event-related brain response technique and rejected in favour of an attentional explanation.

Temporal characteristics of auditory sensory memory: neuromagnetic evidence.

We investigated the temporal dependencies of N100 m, the most prominent deflection of the auditory evoked response, using whole-head neuromagnetic recordings. Stimuli were presented singly or in pairs (tones in the pair were separated by 210 ms) at interstimulus intervals (ISIs) of 0.6-8.1 s. N100 m to single stimuli and to the first tone of the pair had similar temporal recovery functions, plateauing at ISIs of 6 s. N100 m to the second tone in the pair, which was smaller than that to the first except with short ISIs, plateaued with ISIs of about 4 s. Source analysis revealed that the N100 m could be decomposed into two sources separated by about 1 cm on the supratemporal plane. The recovery function of the posterior source was not affected by stimulus presentation, whereas that of the anterior source was. Activity in the anterior area appears to reflect the effects of temporal integration. We relate these results to auditory sensory memory.

Temporal integration in auditory sensory memory: neuromagnetic evidence.

The cortical mechanisms of auditory sensory memory were investigated by analysis of neuromagnetic evoked responses. The major deflection of the auditory evoked field (N100m) appears to comprise an early posterior component (N100mP) and a late anterior component (N100mA) which is sensitive to temporal factors. When pairs of identical sounds are presented at intervals less that about 250 msec, the second sound evokes N100mA with enhanced amplitude at a latency of about 150 msec. We suggest that N100mA may index the activity of two distinct processes in auditory sensory memory. Its recovery cycle may reflect the activity of a memory trace which, according to previous studies, can retain processed information about an auditory sequence for about 10 sec. The enhancement effect may reflect the activity of a temporal integration process, whose time constant is such that sensation persists for 200-300 msec after stimulus offset, and so serves as a short memory store. Sound sequences falling within this window of integration seem to be coded holistically as unitary events.

Persisting versus sustained neural activity: effects on transient N100m response.

Previous evidence from animal and human studies suggests that neural activity, both during a continuous tone and persisting after the offset of a first tone, can prolong the latency and enhance the amplitude of a transient response to a second tone. Our results showed that the latency of N100m to a second tone presented to the opposite ear was prolonged equally in both conditions. Unexpectedly, the effects on response strength strongly depended on stimulus laterality: the ipsilateral but not the contralateral N100m to the second tone was enhanced by the simultaneous presence of the first when compared with the effect of the preceding tone. This suggests that sustained neural activity during a continuous tone can release inhibition normally induced by ipsilateral stimulation.

Human auditory cortical mechanisms of sound lateralisation: III. Monaural and binaural shift responses.

Neuromagnetic responses were recorded over the whole head with a 122-channel gradiometer. A pair of 150-ms 1-kHz tones separated by an interval of 150 ms was presented to one ear every 2 s. The other ear received either no input, an identical pair simultaneous to the first, an identical pair alternating with the first or a continuous 600-ms tone. The 'monaural shift' condition in which stimuli alternated between ears produced a clear perception of changing lateralisation, but the evoked response could be explained as merely the sum of simple monaural onset and offset responses; thus we found no evidence for a separate response to interaural intensity difference in this condition. The 'binaural shift' condition, in which intensity changed in one ear while the other received a continuous tone, evoked a transient response (N130m) at a latency of about 130 ms. N130m was larger over the hemisphere contralateral to the direction of shift, and larger than the corresponding monaural response, whether to an onset or an offset. We concluded that N130m also was not a separate directional response, but was analogous to a simple monaural response, the prolonged latency being due to masking and the enhanced amplitude to facilitation by the sustained response to the continuous tone.

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.

Auditory evoked fields covary with perceptual grouping.

Abrupt acoustic events evoke a transient magnetic response (N100m) at the supratemporal plane. Such responses decrease in amplitude as the interval between successive stimuli decreases to about 1 s. However, when pairs of stimuli are separated by still smaller intervals the second stimulus evokes a larger response than the first. This enhancement depends on the duration of the pause between the offset of the first stimulus and onset of the second. The range over which enhancement of N100m is observed agrees quite well with the range over which subjects experience perceptual grouping of the two stimuli with loudness enhancement of the second. Recordings from multi-channel SQUID gradiometers show that the effect involves not only a change in source strength but also a change in source location within supratemporal cortex. The results suggest that inhibition induced by onset of the first stimulus may be disinhibited by its offset. Physiological and psychological implications are discussed.

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.

Potentials evoked by temporal deviance.

Infrequent stimuli deviating qualitatively from a train of standard stimuli appear to evoke two 'N2' potentials: A modality-specific 'mismatch negativity' (N2a), and a sharp peak forming part of a biphasic vertex response (N2b/P3a). Previous investigations leave it uncertain whether the same pattern of potentials is elicited by temporal deviance. In the present experiment a chequerboard was flashed at a standard ISI of 2000 msec, with occasional flashes occurring early at ISIs of 500, 1000 or 1500 msec. Detected deviants evoked N2b/P3a at an amplitude which increased with degree of deviance, but it was also evoked, at lower amplitude, by missed deviants and by standards. Its amplitude seems to depend upon the discriminability of the deviant stimuli, rather than upon whether they are actually detected. Moreover deviance from a repetitive background does not appear to be a necessary condition, since the complex can be elicited by isolated stimuli presented at irregular intervals. These considerations suggest that temporal uncertainty is an important factor. Temporal deviance produced no discernible mismatch negativity, modality-specific or otherwise, suggesting that it may involve processes different from those engaged in qualitative deviance.

Stimulus deviance and evoked potentials.

In many studies on the effect of selective attention and stimulus significance on evoked potentials, the target, or otherwise significant, stimuli were also infrequent stimuli. Repetitive homogeneous auditory stimuli were presented at short constant intervals. One of two deviant stimuli, one slightly lower and the other slightly higher in pitch than the repetitive stimulus ('standard'), randomly replaced it at the same low probability. One at a time of these two physically equally deviant and equally infrequent stimuli was designated as a target and the subject's task was to count its number in a run and to ignore the other deviant stimulus. The two deviant stimuli elicited very similar potentials; hence, no target effect was obtained. On the other hand, both potentials were much larger and more complex than those elicited by standard stimuli. Additionally, a 'probe' stimulus, a widely deviant auditory stimulus randomly replaced, with a very small probability, a standard stimulus in these conditions. Even this irrelevant, physically widely deviant, stimulus elicited a wave complex basically similar to that elicited by slightly deviant stimuli, but of much larger amplitude. The comparison of these brain-wave sequences to those elicited by the same stimuli in reading subjects led to the conclusion that in detection conditions, deviant stimuli elicit two overlapping sequences of brain events: exogeneous and endogeneous. The former sequence, mainly including the processes producing the N1 and the N2 (neuronal mismatch) components, is an automatic, inflexible set of brain processes which appears as if providing a central-level stimulus to the endogeneous sequence. The latter seems to include a triphasic frontocentral complex overlapping the mismatch N2 and preceding the parietal late positive component and frontal late negative component. This endogeneous set of brain waves was regarded as a sign of detection of stimulus deviance. Consequently, it did not occur in response to the slightly deviant stimuli in reading, but the widely deviant stimuli which were also (involuntarily) perceived by the subject tended to elicit it in attenuated and delayed form.

Event-related brain potentials in selective response.

Event-related potentials were recorded during performance of a simple reaction task and two selective response tasks differing in the intensity of the non-key stimulus. Known differences in reaction between these tasks were confirmed, but there were no corresponding differences in amplitude of the slow potential shift which develops in the EEG in anticipation of the imperative signal. Since this anticipatory potential is not affected by event uncertainty, it seems to be related to motor preparation rather than to expectancy. Performance differences between simple and selective response tasks appear to depend not upon differential preparation, but upon selective processing of the imperative signal, which is reflected in the N120 component of the visual evoked response to that signal.

The impact of warning signal intensity on reaction time and components of the contingent negative variation.

Slow EEG potentials were recorded during performance of a simple reaction task in which warning signal intensity was varied from trial to trial under foreperiod durations of 1, 3 and 8 sec. As shown by speed of reaction, the warning signal had an activating effect which increased with its intensity and decreased with foreperiod duration. This effect was related to the amplitude of a slow potential which appears in the EEG shortly after presentation of the warning signal. This potential is interpreted as a component of the orienting response regulating sensitivity to subsequent stimulation, so that reaction time is affected through change in the effective intensity of the imperative signal.