Biomedical subjects

Walter Ritter

Publications and source records attributed to Walter Ritter.

At least 19 recordsLinked to original sources

PubMed2006

Rapid targeting followed by sustained deployment of visual spatial attention.

We investigated preparatory attention processes when a spatial discrimination was required at a cued location, by measuring electroencephalography following a central symbolic cue to deploy spatial attention. Electroencephalography activity in response to the cue revealed three cue-related activations: an early-onset positivity following the P1 at posterior scalp sites contralateral to the cued location, followed by cue-related frontal scalp activity and later-onset sustained activity at posterior scalp sites contralateral to the cued location. The early contralateral positivity may reflect rapid targeting of the cued location. Our results also extend the findings of cue-related frontal activity followed by posterior activity contralateral to the cued location, found with nonspatial feature discriminations, to a task requiring a spatial discrimination.

Adult↗

PubMed2006

Seeing voices: High-density electrical mapping and source-analysis of the multisensory mismatch negativity evoked during the McGurk illusion.

Seeing a speaker's facial articulatory gestures powerfully affects speech perception, helping us overcome noisy acoustical environments. One particularly dramatic illustration of visual influences on speech perception is the "McGurk illusion", where dubbing an auditory phoneme onto video of an incongruent articulatory movement can often lead to illusory auditory percepts. This illusion is so strong that even in the absence of any real change in auditory stimulation, it activates the automatic auditory change-detection system, as indexed by the mismatch negativity (MMN) component of the auditory event-related potential (ERP). We investigated the putative left hemispheric dominance of McGurk-MMN using high-density ERPs in an oddball paradigm. Topographic mapping of the initial McGurk-MMN response showed a highly lateralized left hemisphere distribution, beginning at 175 ms. Subsequently, scalp activity was also observed over bilateral fronto-central scalp with a maximal amplitude at approximately 290 ms, suggesting later recruitment of right temporal cortices. Strong left hemisphere dominance was again observed during the last phase of the McGurk-MMN waveform (350-400 ms). Source analysis indicated bilateral sources in the temporal lobe just posterior to primary auditory cortex. While a single source in the right superior temporal gyrus (STG) accounted for the right hemisphere activity, two separate sources were required, one in the left transverse gyrus and the other in STG, to account for left hemisphere activity. These findings support the notion that visually driven multisensory illusory phonetic percepts produce an auditory-MMN cortical response and that left hemisphere temporal cortex plays a crucial role in this process.

Acoustic Stimulation↗

PubMed2006

Preattentively grouped tones do not elicit MMN with respect to each other.

To promote preattenive grouping of two sets of tones, one set of tones with a combination of frequency and ear of delivery was intermixed with another set of tones with a different combination of frequency and ear of delivery. The ERPs elicited by tones delivered to one ear that were preceded by three or four tones delivered in a row to the other ear were associated with an enhanced N1, due to the changes in frequency and ear of delivery with respect to the immediately preceding tones. However, no mismatch negativity (MMN) was obtained, even though these tones differed from the previious tones on the two dimensions of frequency and ear of delivery. The data were interpreted to signify that preattentively grouped sets of tones do not elicit MMN with respect to one another. This implies that once acoustic input has been preattentively grouped, the MMN system is dedicated to detecting changes that occur within but not between preattentively grouped stimuli.

Acoustic Stimulation↗

PubMed2005

Brain potentials to sexually suggestive whistles show meaning modulates the mismatch negativity.

Electroencephalographic data suggest that spoken words produce an enhanced output of the brain's automatic deviance detection system, as reflected by the mismatch negativity. Using meaningful and nonmeaningful whistles, we sought to distinguish the effect of semantic content on the brain's deviance detection system from language-specific stimulus features. In the meaningful condition, study participants heard a human 'wolf whistle', which is commonly interpreted as an unsolicited expression of sexual attention. In the nonmeaningful condition participants heard an acoustically identical, but digitally rearranged, version of the wolf whistle. The mismatch negativity amplitude was significantly larger when the infrequent stimulus was meaningful than when it was meaningless. These data suggest that enhanced mismatch negativity magnitude was due to the semantic valence of the eliciting deviant.

Acoustic Stimulation↗

PubMed2005

Aging effects on the ERP correlates of involuntary attentional capture in speech sound analysis.

In an auditory oddball paradigm, 16 old and 16 young adults were asked to ignore binaurally presented disyllabic speech sounds and to watch a silent movie while event-related potentials (ERPs) were recorded. Four types of phonetic deviants occurred on, respectively: (1) the Stressed-First syllable, (2) the Unstressed-First syllable, (3) the Stressed-Second syllable and (4) the Unstressed-Second syllable. The nature of the phonetic change was the same across the four deviants. A P3a was seen only for deviant 3 in the elderly while observed for deviants 1 and 3 in the young. The mismatch negativities (MMNs) to deviants 1 and 2 were reduced compared with those to the other two deviants in both age groups. It is concluded that the ability to use salient prosody to involuntarily capture attention for speech sounds is preserved in normal aging. The lack of P3a response to deviant 1 in the elderly may result from the combined effects of backward masking and age-related temporal encoding inefficiency as indicated by reduced MMNs for the elderly.

Adult↗

PubMed2005

ERP correlates of involuntary attention capture by prosodic salience in speech.

This study addressed whether temporally salient (e.g., word onset) or prosodically salient (e.g., stressed syllables) information serves as a cue to capture attention in speech sound analysis. In an auditory oddball paradigm, 16 native English speakers were asked to ignore binaurally presented disyllabic speech sounds and watch a silent movie while ERPs were recorded. Four types of phonetic deviants were employed: a deviant syllable that was either stressed or unstressed and that occurred in either the first or second temporal position. The nature of the phonetic change (a change from a voiced consonant to its corresponding unvoiced consonant) was kept constant. MMNs were observed for all deviants. In contrast, the P3a was only seen when the deviance occurred on stressed syllables. The sensitivity of the P3a to the stress manipulation suggests that prosodic rather than temporal salience captures attention in unattended speech sounds.

Acoustic Stimulation↗

PubMed2004

Grabbing your ear: rapid auditory-somatosensory multisensory interactions in low-level sensory cortices are not constrained by stimulus alignment.

Multisensory interactions are observed in species from single-cell organisms to humans. Important early work was primarily carried out in the cat superior colliculus and a set of critical parameters for their occurrence were defined. Primary among these were temporal synchrony and spatial alignment of bisensory inputs. Here, we assessed whether spatial alignment was also a critical parameter for the temporally earliest multisensory interactions that are observed in lower-level sensory cortices of the human. While multisensory interactions in humans have been shown behaviorally for spatially disparate stimuli (e.g. the ventriloquist effect), it is not clear if such effects are due to early sensory level integration or later perceptual level processing. In the present study, we used psychophysical and electrophysiological indices to show that auditory-somatosensory interactions in humans occur via the same early sensory mechanism both when stimuli are in and out of spatial register. Subjects more rapidly detected multisensory than unisensory events. At just 50 ms post-stimulus, neural responses to the multisensory 'whole' were greater than the summed responses from the constituent unisensory 'parts'. For all spatial configurations, this effect followed from a modulation of the strength of brain responses, rather than the activation of regions specifically responsive to multisensory pairs. Using the local auto-regressive average source estimation, we localized the initial auditory-somatosensory interactions to auditory association areas contralateral to the side of somatosensory stimulation. Thus, multisensory interactions can occur across wide peripersonal spatial separations remarkably early in sensory processing and in cortical regions traditionally considered unisensory.

Adult↗

PubMed2004

The neural circuitry of pre-attentive auditory change-detection: an fMRI study of pitch and duration mismatch negativity generators.

Electrophysiological studies have revealed a pre-attentive change-detection system in the auditory modality. This system emits a signal termed the mismatch negativity (MMN) when any detectable change in a regular pattern of auditory stimulation occurs. The precise intracranial sources underlying MMN generation, and in particular whether these vary as a function of the acoustic feature that changes, is a matter of some debate. Using functional magnetic resonance imaging, we show that anatomically distinct networks of auditory cortices are activated as a function of the deviating acoustic feature--in this case, tone frequency and tone duration--strongly supporting the hypothesis that MMN generators in auditory cortex are feature dependent. We also detail regions of the frontal and parietal cortices activated by change-detection processes. These regions also show feature dependence and we hypothesize that they reflect recruitment of attention-switching mechanisms.

Adult↗

PubMed2004

Multisensory visual-auditory object recognition in humans: a high-density electrical mapping study.

Multisensory object-recognition processes were investigated by examining the combined influence of visual and auditory inputs upon object identification--in this case, pictures and vocalizations of animals. Behaviorally, subjects were significantly faster and more accurate at identifying targets when the picture and vocalization were matched (i.e. from the same animal), than when the target was represented in only one sensory modality. This behavioral enhancement was accompanied by a modulation of the evoked potential in the latency range and general topographic region of the visual evoked N1 component, which is associated with early feature processing in the ventral visual stream. High-density topographic mapping and dipole modeling of this multisensory effect were consistent with generators in lateral occipito-temporal cortices, suggesting that auditory inputs were modulating processing in regions of the lateral occipital cortices. Both the timing and scalp topography of this modulation suggests that there are multisensory effects during what is considered to be a relatively early stage of visual object-recognition processes, and that this modulation occurs in regions of the visual system that have traditionally been held to be unisensory processing areas. Multisensory inputs also modulated the visual 'selection-negativity', an attention dependent component of the evoked potential this is usually evoked when subjects selectively attend to a particular feature of a visual stimulus.

Adult↗

PubMed2004

Repetition and semantic priming of nonwords: implications for theories of N400 and word recognition.

ERPs were elicited by two types of orthographically legal, pronounceable nonwords. One nonword set was derived from and resembled real words, whereas the other set did not. Nonwords derived from related root words elicited N400 semantic priming effects similar to those obtained for words, indicating semantic activation of the root words. N400 repetition priming effects from nonderived nonwords were similar to those obtained for words. The elicitation of N400 by only derived nonwords would have suggested it was generated by the activation of word meanings, per se. However, both types of nonwords produced an N400, and an N400 priming effect. Because nonderived nonwords are not associated with meaning, the N400 cannot be generated by semantic activation per se. Rather, the N400 appears to be generated by orthographic/phonological analysis and is attenuated by the top-down feed of semantic information to the orthographic/phonological level.

Adult↗

PubMed2004

Feature versus gestalt representation of stimuli in the mismatch negativity system of 7- to 9-year-old children.

We examined preattentive auditory change detection in 7- to 9-year-old children. The question of interest was whether the preattentive comparison of stimuli indexed by the scalp-recorded mismatch negativity (MMN) was performed on representations of individual stimulus features or on gestalt representations of their combined attributes. The design of the study, based on a work by D. Deacon, J. Nousak, M. Pilotti, W. Ritter, and C. Yang (Psychophysiology, 1998), was such that both feature and gestalt representations could have been available to the comparator mechanism generating the MMN. The data indicated that for the majority of the children-those that exhibited an inverse relationship between the amplitude of the MMN and the probability of the deviant-the MMN was based on feature-specific information. This study also provides a method to obtain MMNs to deviants in three different features in the time usually required to obtain an MMN to a single acoustic feature.

Acoustic Stimulation↗

PubMed2003

Auditory selective attention in young and elderly adults: the selection of single versus conjoint features.

The effects of aging on the behavioral and neurophysiological correlates of auditory selective attention were investigated when selection was between either unidimensional or multidimensional stimuli. Attentional processes were studied by recording event-related brain potentials (ERPs) from 16 young (M = 22 years) and 16 elderly adults (M = 74 years) while they detected target tones based on a single location cue or a conjunction of location and pitch cues. Performance declined from the single- to the conjoint-cue task for both age groups but more so for the elderly. The ERP data showed that both age groups used a hierarchical processing strategy to perform the conjoint-cue task, but processing of the pitch dimension took longer for the elderly than for the young. The ERP data also showed that the scalp distribution of a late aspect of the waveform was more restricted in both anterior and posterior directions for the elderly. This suggests that frontal-lobe dependent attentional processes may be less efficacious with aging.

Acoustic Stimulation↗

PubMed2003

Preattentive auditory context effects.

The effects of auditory context on the preattentive and perceptual organization of tone sequences were investigated. Two sets of experiments were conducted in which the pitch of contextual tones was varied, bringing about two different contextual manipulations. Preattentive auditory organization was indexed by the mismatch negativity event-related potential, which is elicited by violations of auditory regularities even when participants ignore the sounds (e.g., by reading a book). The perceptual effects of the contextual manipulations on auditory grouping were assessed using target-detection and order-judgment tasks. The close correspondence found between the effects of auditory context on the perceptual and preattentive measures of auditory grouping suggests that a large part of contextual processing is preattentive.

Acoustic Stimulation↗

PubMed2002

Top-down effects can modify the initially stimulus-driven auditory organization.

We recorded event-related potentials (ERPs) and magnetic fields (ERFs) of the human brain to determine whether top-down control could modulate the initial organization of sound representations in the auditory cortex. We presented identical sound stimulation and manipulated top-down processes by instructing participants to either ignore the sounds (Ignore condition), to detect pitch changes (Attend-pitch condition), or to detect violations of a repeating tone pattern (Attend-pattern condition). The ERP results obtained in the Attend-pattern condition dramatically differed from those obtained with the other two task instructions. The magnetoencephalogram (MEG) findings were fully compatible, showing that the neural populations involved in detecting pattern violations differed from those involved in detecting pitch changes. The results demonstrate a top-down effect on the sound representation maintained in auditory cortex.

Acoustic Stimulation↗

PubMed2002

Multisensory auditory-visual interactions during early sensory processing in humans: a high-density electrical mapping study.

Integration of information from multiple senses is fundamental to perception and cognition, but when and where this is accomplished in the brain is not well understood. This study examined the timing and topography of cortical auditory-visual interactions using high-density event-related potentials (ERPs) during a simple reaction-time (RT) task. Visual and auditory stimuli were presented alone and simultaneously. ERPs elicited by the auditory and visual stimuli when presented alone were summed ('sum' ERP) and compared to the ERP elicited when they were presented simultaneously ('simultaneous' ERP). Divergence between the 'simultaneous' and 'sum' ERP indicated auditory-visual (AV) neural response interactions. There was a surprisingly early right parieto-occipital AV interaction, consistent with the finding of an earlier study [J. Cogn. Neurosci. 11 (1999) 473]. The timing of onset of this effect (46 ms) was essentially simultaneous with the onset of visual cortical processing, as indexed by the onset of the visual C1 component, which is thought to represent the earliest cortical visual evoked potential. The coincident timing of the early AV interaction and C1 strongly suggests that AV interactions can affect early visual sensory processing. Additional AV interactions were found within the time course of sensory processing (up to 200 ms post stimulus onset). In total, this system of AV effects over the scalp was suggestive of both activity unique to multisensory processing, and the modulation of 'unisensory' activity. RTs to the stimuli when presented simultaneously were significantly faster than when they were presented alone. This RT facilitation could not be accounted for by probability summation, as evidenced by violation of the 'race' model, providing compelling evidence that auditory-visual neural interactions give rise to this RT effect.

Acoustic Stimulation↗

PubMed2002

Temporal integration: intentional sound discrimination does not modulate stimulus-driven processes in auditory event synthesis.

OBJECTIVE: Our previous study showed that the auditory context could influence whether two successive acoustic changes occurring within the temporal integration window (approximately 200ms) were pre-attentively encoded as a single auditory event or as two discrete events (Cogn Brain Res 12 (2001) 431). The aim of the current study was to assess whether top-down processes could influence the stimulus-driven processes in determining what constitutes an auditory event. METHODS: Electroencepholagram (EEG) was recorded from 11 scalp electrodes to frequently occurring standard and infrequently occurring deviant sounds. Within the stimulus blocks, deviants either occurred only in pairs (successive feature changes) or both singly and in pairs. Event-related potential indices of change and target detection, the mismatch negativity (MMN) and the N2b component, respectively, were compared with the simultaneously measured performance in discriminating the deviants. RESULTS: Even though subjects could voluntarily distinguish the two successive auditory feature changes from each other, which was also indicated by the elicitation of the N2b target-detection response, top-down processes did not modify the event organization reflected by the MMN response. CONCLUSIONS: Top-down processes can extract elemental auditory information from a single integrated acoustic event, but the extraction occurs at a later processing stage than the one whose outcome is indexed by MMN. SIGNIFICANCE: Initial processes of auditory event-formation are fully governed by the context within which the sounds occur. Perception of the deviants as two separate sound events (the top-down effects) did not change the initial neural representation of the same deviants as one event (indexed by the MMN), without a corresponding change in the stimulus-driven sound organization.

Acoustic Stimulation↗

PubMed2002

Memory reactivation or reinstatement and the mismatch negativity.

Previous studies have suggested that the memory underlying the mismatch negativity (MMN) can be deactivated by a period of silence and later reactivated. An alternative is that the memory becomes inapplicable due to a period of silence and subsequently reinstated. Both interpretations rest on the absence of a MMN for a deviant in Position 1 of a train (showing deactivation or inapplicability of the memory) and the presence of a MMN in Position 2 of the train (evidencing reactivation or reinstatement of the memory). In these studies, the standards were fixed across all trains of a given condition. Hence, it is not clear if the MMN elicited in Position 2 requires presentation of multiple trains with identical standards or only a single train. Experiment 1 showed that a single train is sufficient. With data from recent studies, Experiment 2 showed that MMNs in Position 2 of trains are due to reinstatement rather than reactivation of the memory.

Acoustic Stimulation↗

PubMed2002

Auditory-somatosensory multisensory processing in auditory association cortex: an fMRI study.

Using high-field (3 Tesla) functional magnetic resonance imaging (fMRI), we demonstrate that auditory and somatosensory inputs converge in a subregion of human auditory cortex along the superior temporal gyrus. Further, simultaneous stimulation in both sensory modalities resulted in activity exceeding that predicted by summing the responses to the unisensory inputs, thereby showing multisensory integration in this convergence region. Recently, intracranial recordings in macaque monkeys have shown similar auditory-somatosensory convergence in a subregion of auditory cortex directly caudomedial to primary auditory cortex (area CM). The multisensory region identified in the present investigation may be the human homologue of CM. Our finding of auditory-somatosensory convergence in early auditory cortices contributes to mounting evidence for multisensory integration early in the cortical processing hierarchy, in brain regions that were previously assumed to be unisensory.

Acoustic Stimulation↗