Biomedical subjects

Walter Ritter

Publications and source records attributed to Walter Ritter.

8 recordsLinked to original sources

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.

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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.

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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.

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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.

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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.

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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.

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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.

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PubMedDate not supplied

Age-related changes in neural trace generation of rule-based auditory features.

Young (M = 22) and elderly volunteers (M = 74) participated in an ERP study that examined rule-based feature ion in the auditory modality. Stimuli were either a frequent ascending tone pair or an infrequent descending tone pair. Tone-pairs were presented under three conditions. Physical feature monaural (1 tone pair), feature monaural (10 tone pairs), and feature binaural (10 tone pairs). Volunteers watched a silent movie during ERP recordings. After completion of the ERP session all volunteers participated in a behavioral discrimination task. MMNs were elicited under all three conditions for the young. For the elderly, MMNs were elicited under both monaural conditions but not under binaural conditions. Behavioral discrimination was high under the physical feature condition but fell to near chance under the two rule-based feature conditions for both age groups. Thus rule-based neural representations were generated for both age groups under monaural conditions, but only for the young under binaural conditions, suggesting that there is an age-related decline in the efficacy of integrating multiple sources into a single auditory stream.

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