Bertram Opitz

Differential Contribution of Frontal and Temporal Cortices to Auditory Change Detection: fMRI and ERP Results

The present study addresses the functional role of the temporal and frontal lobes in auditory change detection. Prior event-related potential (ERP) research suggested that the mismatch negativity (MMN) reflects the involvement of a temporofrontal network subserving auditory change detection processes and the initiation of an involuntary attention switch. In the present study participants were presented with repetitive spectrally rich sounds. Infrequent changes of either small (10% change), medium (30% change), or large (100% change) magnitude were embedded in the stimulus train. ERPs and fMRI measures were obtained in the same subjects in subsequent sessions. Significant hemodynamic activation in the superior temporal gyri (STG) bilaterally and the opercular part of the right inferior frontal gyrus was observed for large and medium deviants only. ERPs showed that small deviants elicited MMN when presented in silence but not when presented with recorded MR background noise, indicating that small deviants were hardly detected under fMRI conditions. The MR signal change in temporal lobe regions was larger for large than for medium deviants. For the right fronto-opercular cortex the opposite pattern was observed. The strength of the temporal activation correlated with the amplitude of the change-related ERP at around 110 ms from stimulus onset while the frontal activation correlated with the change-related ERP at around 150 ms. These results suggest that the right fronto-opercular cortex is part of the neural network generating the MMN. Three alternative explanations of these findings are discussed.

Combining electrophysiological and hemodynamic measures of the auditory oddball.

The neural mechanisms of deviancy and target detection were investigated by combining high density event-related potential (ERP) recordings with functional magnetic resonance imaging (fMRI). ERP and fMRI responses were recorded using the same paradigm and the same subjects. Unattended deviants elicited a mismatch negativity (MMN) in the ERP. In the fMRI data, activations of transverse/superior temporal gyri bilateral were found. Attended deviants generated an MMN followed by an N2/P3b complex. For this condition, fMRI activations in both superior temporal gyri and the neostriatum were found. These activations were taken as neuroanatomical constraints for the localization of equivalent current dipoles. Inverse solutions for dipole orientation provide evidence for significant activation close to Heschls gyri during deviancy processing in the 110-160-ms time interval (MMN), whereas target detection could be modeled by two dipoles in the superior temporal gyrus between 320 and 380 ms.

Broca's area in the human brain is involved in the selection of grammatical gender for language production: evidence from event- related functional magnetic resonance imaging

The neural correlates of the selection of grammatical gender during overt picture naming were investigated by event-related functional magnetic resonance imaging in the left hemisphere. Relative to simply naming a picture, the production of the definite determiner of the picture name (requiring gender selection) resulted exclusively in pronounced activation of a single region in the superior portion of Brocas area. This activation was not present in contrasts reflecting lexical access (naming a picture vs. saying jaja to a smiley) or articulation (saying jaja vs. rest). Rather, lexical access activated other inferior frontal regions, insula, fusiform and inferior temporal gyrus. Articulation involved insula, Rolandic operculum, motor and premotor cortex and superior temporal gyrus. The results are discussed with respect to data from studies investigating gender processing during language comprehension.

A MEG analysis of the P300 in visual discrimination tasks

Based on recent research that indicated that P300 scalp topography varies as a function of task and/or information to be processed, this study examined scalp-recorded magnetic fields correlated with the P300 by means of whole-head magnetoencephalography. Subjects performed two discrimination tasks, in which targets, defined on either object or spatial characteristics of the same visual stimuli, had to be discriminated. Based on the across-subject root mean square (RMS) functions a sequence of 4 components could be identified in both tasks, N1m, P3m, and two later components, which, based on their estimated neuronal sources, were classified as representing motor processes during and following the manual responses to target stimuli. Reliable between-task differences in source localization were obtained for the P3m component, but not for the other components. Inferior-medial sources were found for the P3m evoked by both spatial and object targets, with these sources being located about 3.5 cm more anterior for object targets. These results suggest that different neuronal sources, possibly located in subcortical regions in the vicinity of the thalamus, contribute to the P3m evoked by target stimuli defined by either object or spatial stimulus characteristics.

Conscious recollection and illusory recognition: an event-related fMRI study

In this event‐related functional magnetic resonance imaging (fMRI) study we examined the neuronal correlates of the subprocesses underlying recognition memory. In an explicit memory task, participants had to discriminate studied (‘old’) words from semantically related and unrelated ‘new’ (unstudied) words. We examined whether the correct rejection of semantically related words which were similar to old words, which had elicited correct responses, was based on conscious recollection of study phase information. In this task, false‐positive responses to semantically related new words can be assumed to result from the assessment of the semantic similarity between test words and studied words with minimal recollection. For correct identification of old words and correct rejection of new related words we found common activation in a variety of brain areas that have been shown to be involved in conscious recollection, among them the left middle frontal gyrus, the precuneus, the retrosplenial cortex, the left parahippocampal gyrus and the thalamus. For correct responses to old words, the frontomedian wall, the posterior cingulate cortex and the nucleus accumbens were additionally activated, suggesting an emotional contribution to these judgements. Correct rejections of related new words were associated with additional activation of the right middle frontal gyrus, suggesting higher monitoring demands for these more difficult recognition judgements. False‐positive responses to semantically related new words were associated with enhanced activation in the frontomedian wall. The results point to an important role of the prefrontal cortex as well as medial temporal and medial parietal regions of the brain in successful memory retrieval and conscious recollection.

Semantic aspects of novelty detection in humans

The ability to detect unusual and novel events is an important prerequisite for storage of information in memory. Non-tonal novel sounds that deviate from an ongoing auditory environment elicit a positive event-related potential (ERP) component, the so-called novel P3. Though there is converging evidence on the neuronal network engaged in novelty detection, little attention has been paid to the properties of novel sounds, such as their typicality or relationship to mental concepts. Here we report the ERPs evoked by two types of generically novel stimuli, namely identifiable (meaningful) and non-identifiable (non-meaningful) novel sounds. The ERP analysis revealed a novel P3 for both types of sounds. However, when subjects actively attended to the stimuli only identifiable novel sounds evoked a right-lateralized negativity (N4) that peaked shortly after the novel P3. We conclude that novelty detection not only includes the registration of deviancy but also fast access and identification of related semantic concepts.

A new concept for EEG/MEG signal analysis : Detection of interacting spatial modes

We propose a new concept for analyzing EEG/MEG data. The concept is based on a projection of the spatiotemporal signal into the relevant phase space and the interpretation of the brain dynamics in terms of dynamical systems theory. The projection is obtained by a simultaneous determination of spatial modes and coefficients of differential equations. The resulting spatiotemporal model can be characterized by stationary points and corresponding potential field maps. Brain information processing can be interpreted by attraction and repulsion of spatial field distributions given by these stationary points. This allows an objective and quantitative characterization of the brain dynamics. We outline this concept and the underlying algorithm. Results of the application of this method to an event related potential (ERP) study of auditory memory processes are discussed. Hum. Brain Mapping 6:137–149, 1998.

Ansätze und Methoden der kognitiven Neurowissenschaften

Approaches and Methods in Cognitive Neurosciences. New approaches in cognitive neurosciences are methods with high temporal but poor spatial resolution such as event-related potentials (ERP). A precise neuroanatomical localization is provided by neuroimaging techniques such as functional magnetic resonance imaging (fMRI). Despite the high spatial resolution neuroimaging techniques do not provide the temporal resolution required to make inferences about cognitive processes. The neuroanatomically constrained dipole analysis is a new approach to overcome the intrinsic limitations by combining both methods. The present report demonstrates that this new approach of combined analysis of electrophysiological and hemodynamic brain activity in an auditory discrimination task provides a new opportunity to describe the temporal aspects of neural activation in a distributed network underlying cognitive processes.