Annekathrin Weise

Omission mismatch negativity builds up late.

The mismatch negativity (MMN) event-related potential (ERP) is elicited by irregular sounds in a regular sound sequence. MMN depends on the length of the preceding regular stimulation: in repetitive sequences, changes following only two or three repetitions may elicit MMN. Thus, MMN builds up rapidly. Infrequent omissions in a repetitive sound sequence may also elicit an MMN-like waveform. In this study, the build-up of this omission-related ERP was investigated. Trains of two to nine tones with omissions at the penultimate stimulus positions were presented. Results showed no consistent omission-related ERP. In contrast, when omissions occurred with 9% probability among continuously presented tones, the omission-related waveform was elicited. Thus, the omission-related ERP builds up later than MMN.

A temporal constraint for automatic deviance detection and object formation: A mismatch negativity study

The automatic detection of deviations within a constant sine wave tone is confined to the initial part of approximately 350 ms. When a deviation occurs beyond this critical limit, the mismatch negativity (MMN) - a deviance-related component of the event-related potential - is largely attenuated or even absent. However, for time-variant acoustic stimuli such as speech sounds or tonal patterns, MMN is also obtained for deviations beyond the initial 350 ms. We consider two hypotheses that can explain the MMN to time-variant sounds. One is that the terminal part of those sounds is represented as the spectral information varies over time (spectral-variation hypothesis). The other is that transients, occurring in time-variant signals, help to segment the long sounds into smaller units, each being not larger than the critical 350 ms (segmentation hypothesis). We measured MMN to duration shortenings (deviants) embedded in a sequence of 1000 ms long standard tones of increasing frequency (sweeps). The sweeps did or did not contain a noise burst. Results reveal a lack of MMN to the duration deviant in the sweep without a noise burst, which rules out the spectral-variation hypothesis. The presence of MMN to the duration deviant in the sweep with a noise burst supports the segmentation hypothesis. Thus, the results suggest a temporal constraint inherent to the processing of unstructured/unsegmented long tones. We argue that transients within a sound act as segmentation cues providing an automatic sound representation for which deviations can be detected.

An asymmetry in the automatic detection of the presence or absence of a frequency modulation within a tone: a mismatch negativity study

The infrequent occurrence of a transient feature (deviance; e.g., frequency modulation, FM) in one of the regular occurring sinusoidal tones (standards) elicits the deviance related mismatch negativity (MMN) component of the event-related brain potential. Based on a memory-based comparison, MMN reflects the mismatch between the representations of incoming and standard sounds. The present study investigated to what extent the infrequent exclusion of an FM is detected by the MMN system. For that purpose we measured MMN to deviances that either consisted of the exclusion or inclusion of an FM at an early or late position within the sound that was present or absent, respectively, in the standard. According to the information-content hypothesis, deviance detection relies on the difference in informational content of the deviant relative to that of the standard. As this difference between deviants with FM and standards without FM is the same as in the reversed case, comparable MMNs should be elicited to FM inclusions and exclusions. According to the feature-detector hypothesis, however, the deviance detection depends on the increased activation of feature detectors to additional sound features. Thus, rare exclusions of the FM should elicit no or smaller MMN than FM inclusions. In passive listening condition, MMN was obtained only for the early inclusion, but not for the exclusions nor for the late inclusion of an FM. This asymmetry in automatic deviance detection seems to partly reflect the contribution of feature detectors even though it cannot fully account for the missing MMN to late FM inclusions. Importantly, the behavioral deviance detection performance in the active listening condition did not reveal such an asymmetry, suggesting that the intentional detection of the deviants is based on the difference in informational content. On a more general level, the results partly support the “fresh-afferent” account or an extended memory-comparison based account of MMN.

Which kind of transition is important for sound representation? An event-related potential study

Auditory transients (such as sound onset or a frequency transition within a continuous sound) are assumed to parse the auditory input into smaller units enabling the formation of unitary sound representations separately for each segment. This was discovered by using the mismatch negativity (MMN) component of the event-related potential (ERP) that taps into auditory sensory memory representations. For unstructured sounds, MMN amplitude decreased or even vanished with increasing the temporal distance of an irregular feature (deviance, e.g. duration decrement) relative to the onset of an otherwise regularly occurring sound, whereas for sounds that were segmented by a transient, MMN persisted. It has been speculated that the P1-N1-P2 complex, indexing the sensory encoding of the transient, determines the temporal units of the acoustic input that are represented by the information processing system. To test this hypothesis, we utilized a previously reported asymmetry in the sensory encoding of physically identical but time-reversed transitions between segments of constant and gliding frequency. In separate blocks, we regularly presented 1400-ms sounds with a centered constant-to-glide or glide-to-constant transition. Occasionally and unpredictably, one of the regularly occurring sounds was shortened in duration to 910 ms. We found larger transition-related P1-N1-P2 potentials accompanied by larger deviance-related MMN amplitudes for sounds with constant-to-glide transition than for sounds with glide-to-constant transition. This provides evidence that it is the precise sensory encoding of the transition which is crucial for automatically parsing the auditory input into smaller units, thus enabling the formation of unitary sound representations even for late segments.

The processing of concurrent sounds based on inharmonicity and asynchronous onsets: an object-related negativity (ORN) study.

This study addresses the processing of concurrent sounds based on inharmonicity and onset asynchrony cues. We used harmonic complex sounds with one component starting marginally (40 ms) or considerably (500 ms) earlier than the complex and being slightly (3%) or strongly (13%) inharmonic. To index sound segregation of concurrent events, we measured the object-related negativity (ORN) component of the event-related potential. We contrasted two hypotheses: According to the concurrent-segregation hypothesis, increased onset asynchrony is assumed to promote segregation of the leading partial from the harmonic complex, which should be reflected in increased ORN amplitudes. That is, even with large onset asynchronies concurrent events would be processed by a simultaneous sound segregation mechanism. According to the sequential-integration hypothesis, however, with increased onset asynchrony concurrent cues are assumed to be less considered by simultaneous grouping processes, which should be reflected in attenuated ORN amplitudes for long onset asynchronies. This assumption is based on the notion that due to sequential integration, a stable percept of the leading partial has been developed within ~350 ms after sound onset, thus less processing is required from scene analysis mechanisms based on concurrent cues. Indeed, with increased onset asynchrony ORN was found to decrease, which supports the sequential-integration hypothesis. In line with previous data, ORN was also found to increase with increased inharmonicity. The absence of an inharmonicity×onset asynchrony interaction further supports the assumption that both cues are used in different ways for simultaneous sound segregation.

Differential processing of terminal tone parts within structured and non-structured tones

Recent studies utilizing the mismatch negativity (MMN) event-related potential (ERP) revealed that when a repetitive sequence of sinusoidal tones is presented, the occasional insertion of a short deviation into some of the tones leads to the elicitation of an MMN only if it occurs during the initial 300 ms, but not beyond. In contrast, deviations occurring in speech sounds elicit MMN even beyond 300 ms. We conducted two experiments to resolve this conflict. We hypothesised that an additional transient within an otherwise unstructured tone may overcome this limitation. First, we tested for MMN to a deviance at the terminal part of a 650 ms tone which did or did not contain a gap. Only when the tone included the gap, MMN was obtained. Second, we compared the gap condition with two noise conditions, in which the gap was replaced by modulated white noise. The noise conditions differed with respect to the saliency of the perceived interruption of the tone. In all three conditions, MMN was elicited. These results demonstrate that structuring a sinusoidal tone by a gap or a noise interval is sufficient to regain MMN. It is suggested that the introduction of an additional transient triggers a new integration window overcoming the temporal constraints of automatic tone representation. This resolves the seeming contradiction between MMN studies using tonal and speech sounds.

Auditory event-related potentials reflect dedicated change detection activity for higher-order acoustic transitions

The processing of auditory changes at cortical level relies partly on dedicated change-detectors whose activity is reflected in the elicitation of the N1 and P2 event-related potentials (ERPs). In previous studies, N1 and P2 have been found only for first-order frequency transitions (i.e. constant-to-glide) but not for higher-order transitions (i.e. glide-to-constant). We tested whether this asymmetry is due to the complete lack, or the smaller number of dedicated higher-order change detectors compared to first-order change detectors by recording ERPs to constant-to-glide and glide-to-constant frequency transitions within pure and complex tones. For constant-to-glide transitions ERP amplitudes increased with the rate of frequency change and spectral complexity. Importantly, for glide-to-constant transitions, N1 was elicited, even though only for spectrally rich tones when the frequency-change rate was fastest. Thus, the asymmetry in auditory change-related N1 elicitation is attributable not to the lack of higher-order change detectors, but to their relatively low number.

Timing matters: the processing of pitch relations

The human central auditory system can automatically extract abstract regularities from a variant auditory input. To this end, temporarily separated events need to be related. This study tested whether the timing between events, falling either within or outside the temporal window of integration (~350 ms), impacts the extraction of abstract feature relations. We utilized tone pairs for which tones within but not across pairs revealed a constant pitch relation (e.g., pitch of second tone of a pair higher than pitch of first tone, while absolute pitch values varied across pairs). We measured the mismatch negativity (MMN; the brain’s error signal to auditory regularity violations) to second tones that rarely violated the pitch relation (e.g., pitch of second tone lower). A Short condition in which tone duration (90 ms) and stimulus onset asynchrony between the tones of a pair were short (110 ms) was compared to two conditions, where this onset asynchrony was long (510 ms). In the Long Gap condition, the tone durations were identical to Short (90 ms), but the silent interval was prolonged by 400 ms. In Long Tone, the duration of the first tone was prolonged by 400 ms, while the silent interval was comparable to Short (20 ms). Results show a frontocentral MMN of comparable amplitude in all conditions. Thus, abstract pitch relations can be extracted even when the within-pair timing exceeds the integration period. Source analyses indicate MMN generators in the supratemporal cortex. Interestingly, they were located more anterior in Long Gap than in Short and Long Tone. Moreover, frontal generator activity was found for Long Gap and Long Tone. Thus, the way in which the system automatically registers irregular abstract pitch relations depends on the timing of the events to be linked. Pending that the current MMN data mirror established abstract rule representations coding the regular pitch relation, neural processes building these templates vary with timing.

The detection of higher-order acoustic transitions is reflected in the N1 ERP

The auditory system features various types of dedicated change detectors enabling the rapid parsing of auditory stimulation into distinct events. The activity of such detectors is reflected by the N1 ERP. Interestingly, certain acoustic transitions show an asymmetric N1 elicitation pattern: whereas first-order transitions (e.g., a change from a segment of constant frequency to a frequency glide [c-to-g change]) elicit N1, higher-order transitions (e.g., glide-to-constant [g-to-c] changes) do not. Consensus attributes this asymmetry to the absence of any available sensory mechanism that is able to rapidly detect higher-order changes. In contrast, our study provides compelling evidence for such a mechanism. We collected electrophysiological and behavioral data in a transient-detection paradigm. In each condition, a random (50%-50%) sequence of two types of tones occurred, which did or did not contain a transition (e.g., c-to-g and constant stimuli or g-to-c and glide tones). Additionally, the rate of pitch change of the glide varied (i.e., 10 vs. 40 semitones per second) in order to increase the number of responding neural assemblies. The rate manipulation modulated transient ERPs and behavioral detection performance for g-to-c transitions much stronger than for c-to-g transitions. The topographic and tomographic analyses suggest that the N1 response to c-to-g and also to g-to-c transitions emerged from the superior temporal gyrus. This strongly supports a sensory mechanism that allows the fast detection of higher-order changes.

The representation of unattended, segmented sounds: a mismatch negativity (MMN) study.

The detection of an irregular, potentially relevant change (deviance) in the regular, unattended acoustic environment is ensured by the automatic deviance detection mechanism. It underlies the formation of a regularity representation and a comparison of an incoming sound with this representation. A mismatch outcome of this comparison evokes the mismatch negativity (MMN) of the event-related potential. For unattended pure tones the automatic deviance detection mechanism operates most efficiently for initial sound parts, which is why these are suggested to contribute more to sound representation than later parts. A transient that physically segments the sound can overcome this temporal constraint in sound representation. Whether the resulting individual (initial and terminal) sound segments or the joined two-segments give rise to the regularity representation is addressed here. We took advantage that the MMN attenuation to the second of two successive deviances (deviance-repetition effect) is more pronounced when the deviances belong to the same unit of representation. We measured MMN for two deviances (frequency modulations) within segmented sounds that either occurred within the initial or the terminal segment, or that were split across both segments. Unexpectedly, we did not obtain a deviance-repetition effect. Instead, we obtained a temporal distance effect: With increasing temporal distance from deviance-onset relative to segment-onset the MMN amplitude decreased. Furthermore, this effect did not depend on whether the deviance occurred in the initial or in the terminal segment. Thus, (for the current approach) we suggest that the regularity representation is based on the individual rather than joined segments.