Armin Schnider

Rapid discrimination of visual and multisensory memories revealed by electrical neuroimaging.

Though commonly held that multisensory experiences enrich our memories and that memories influence ongoing sensory processes, their neural mechanisms remain unresolved. Here, electrical neuroimaging shows that auditory-visual multisensory experiences alter subsequent processing of unisensory visual stimuli during the same block of trials at early stages poststimulus onset and within visual object recognition areas. We show this with a stepwise analysis of scalp-recorded event-related potentials (ERPs) that statistically tested (1) ERP morphology and amplitude, (2) global electric field power, (3) topographic stability of and changes in the electric field configuration, and (4) intracranial distributed linear source estimations. Subjects performed a continuous recognition task, discriminating repeated vs. initial image presentations. Corresponding, but task-irrelevant, sounds accompanied half of the initial presentations during a given block of trials. On repeated presentations within a block of trials, only images appeared, yielding two situations-the images prior presentation was only visual or with a sound. Image repetitions that had been accompanied by sounds yielded improved memory performance accuracy (old or new discrimination) and were differentiated as early as approximately 60-136 ms from images that had not been accompanied by sounds through generator changes in areas of the right lateral-occipital complex (LOC). It thus appears that unisensory percepts trigger multisensory representations associated with them. The collective data support the hypothesis that perceptual or memory traces for multisensory auditory-visual events involve a distinct cortical network that is rapidly activated by subsequent repetition of just the unisensory visual component.

Recovery from spontaneous confabulations parallels recovery of temporal confusion in memory.

Background: In previous studies, the authors found that patients with spontaneous confabulation differ from those with nonconfabulating amnesia by 1) temporal context confusion (TCC) in memory based on an inability to suppress intrusions of currently irrelevant memory traces into ongoing thinking; and 2) lesions involving the orbitofrontal cortex, basal forebrain, or amygdala and perirhinal cortex. Objectives: To study the long-term clinical course of spontaneous confabulations, determine whether TCC in memory also parallels the clinical course of spontaneous confabulations, and study the impact of lesion site on clinical course. Methods: Eight patients with spontaneous confabulation were re-examined 18 months after onset. Tests of memory and executive functioning and measurement of TCC in memory were again applied. MRI according to a standard protocol was performed to determine areas of permanent damage. Results: Seven patients eventually stopped confabulating. TCC, but not common memory or executive tests, precisely paralleled the course of spontaneous confabulations. Patients with isolated, less extensive, orbitofrontal lesions stopped confabulating first and had the best neuropsychological outcome. Patients with basal forebrain lesions continued to confabulate for several months and remained amnesic. One patient with extensive orbitofrontal damage and perirhinal cortex damage continues to confabulate after more than 3 years, continuing to confuse memory traces. Conclusions: Temporal context confusion in memory is not only the sole feature reliably separating patients with spontaneous confabulation from those with nonconfabulating amnesia in the acute stage, it is also the only feature that precisely parallels the clinical course of spontaneous confabulations. Most patients eventually stop confabulating but duration of confabulations depends on the lesion site.

Memory without context: amnesia with confabulations after infarction of the right capsular genu.

OBJECTIVE--To explore the mechanism of an amnesia marked by confabulations and lack of insight in a patient with an infarct of the right inferior capsular genu. The confabulations could mostly be traced back to earlier events, indicating that the memory disorder ensued from an inability to store the temporal and spatial context of information acquisition rather than a failure to store new information. METHODS--To test the patients ability to store the context of information acquisition, two experiments were composed in which she was asked to decide when or where she had learned the words from two word lists presented at different points in time or in different rooms. To test her ability to store new information, two continuous recognition tests with novel non-words and nonsense designs were used. Recognition of these stimuli was assumed to be independent of the context of acquisition because the patient could not have an a priori sense of familiarity with them. RESULTS--The patient performed at chance in the experiments probing knowledge of the context of information acquisition, although she recognised the presented words almost as well as the controls. By contrast, her performance was normal in the recognition tests with non-words and nonsense designs. CONCLUSION--These findings indicate that the patients amnesia was based on an inability to store the context of information acquisition rather than the information itself. Based on an analysis of her lesion, which disconnected the thalamus from the orbitofrontal cortex and the amygdala, and considering the similarities between her disorder, Wernicke-Korsakoff syndrome, and the amnesia after orbitofrontal lesions, it is proposed that contextual amnesia results from interruption of the loop connecting the amygdala, the dorsomedial nucleus, and the orbitofrontal cortex.

Receptive amusia: temporal auditory processing deficit in a professional musician following a left temporo-parietal lesion

This study examined the musical processing in a professional musician who suffered from amusia after a left temporo-parietal stroke. The patient showed preserved metric judgement and normal performance in all aspects of melodic processing. By contrast, he lost the ability to discriminate or reproduce rhythms. Arrhythmia was only observed in the auditory modality: discrimination of auditorily presented rhythms was severely impaired, whereas performance was normal in the visual modality. Moreover, a length effect was observed in discrimination of rhythm, while this was not the case for melody discrimination. The arrhythmia could not be explained by low-level auditory processing impairments such as interval and length discrimination and the impairment was limited to auditory input, since the patient produced correct rhythmic patterns from a musical score. Since rhythm processing was selectively disturbed in the auditory modality, the arrhythmia cannot be attributed to a impairment of supra-modal temporal processing. Rather, our findings suggest modality-specific encoding of musical temporal information. Besides, it is proposed that the processing of auditory rhythmic sequences involves a specific left hemispheric temporal buffer.

The Dorsal Attention Network Mediates Orienting toward Behaviorally Relevant Stimuli in Spatial Neglect

Experimental neurophysiology and functional neuroimaging have identified a dorsal attention network that encodes neural signals related to the behavioral significance of a stimulus. The core anatomical areas of this network are the frontal eye fields and the posterior parietal cortex, which are interconnected by the superior longitudinal fasciculus. Here, we show that damage or disconnection of this network predicts the extent to which task-relevant stimuli capture attention of human stroke patients with spatial neglect. Healthy volunteers, right-hemisphere-damaged control participants, and patients with left neglect reacted to peripheral targets defined by their color, which were preceded by a brief distracter stimulus. The position of the distracter and its relevance for the current trial were independently varied. In neglect patients with damage including the frontal eye fields and the superior longitudinal fasciculus, ipsilesional distracters impaired orienting into contralesional space regardless of their relevance for the current task. In contrast, patients with sparing of these regions were only impaired when distracters were task-relevant. These findings indicate that the dorsal attention network controls spatial orienting by modulating the saliency of distracter stimuli according to current action goals.

The behavioral significance of coherent resting-state oscillations after stroke

Stroke lesions induce not only loss of local neural function, but disruptions in spatially distributed areas. However, it is unknown whether they affect the synchrony of electrical oscillations in neural networks and if changes in network coherence are associated with neurological deficits. This study assessed these questions in a population of patients with subacute, unilateral, ischemic stroke. Spontaneous cortical oscillations were reconstructed from high-resolution electroencephalograms (EEG) with adaptive spatial filters. Maps of functional connectivity (FC) between brain areas were created and correlated with patient performance in motor and cognitive scores. In comparison to age matched healthy controls, stroke patients showed a selective disruption of FC in the alpha frequency range. The spatial distribution of alpha band FC reflected the pattern of motor and cognitive deficits of the individual patient: network nodes that participate normally in the affected functions showed local decreases in FC with the rest of the brain. Interregional FC in the alpha band, but not in delta, theta, or beta frequencies, was highly correlated with motor and cognitive performance. In contrast, FC between contralesional areas and the rest of the brain was negatively associated with patient performance. Alpha oscillation synchrony at rest is a unique and specific marker of network function and linearly associated with behavioral performance. Maps of alpha synchrony computed from a single resting-state EEG recording provide a robust and convenient window into the functionality and organization of cortical networks with numerous potential applications.

Why do we yawn

Yawning is a phylogenetically old behaviour that can be observed in most vertebrate species from foetal stages to old age. The origin and function of this conspicuous phenomenon have been subject to speculations for centuries. Here, we review the experimental evidence for each of these hypotheses. It is found that theories ascribing a physiological role to yawning (such as the respiratory, arousal, or thermoregulation hypotheses) lack evidence. Conversely, the notion that yawning has a communicative function involved in the transmission of drowsiness, boredom, or mild psychological stress receives increasing support from research in different fields. In humans and some other mammals, yawning is part of the action repertoire of advanced empathic and social skills.

The attention network of the human brain: relating structural damage associated with spatial neglect to functional imaging correlates of spatial attention

Functional imaging studies of spatial attention regularly report activation of the intraparietal sulcus (IPS) and dorsal premotor cortex including the frontal eye fields (FEF) in tasks requiring overt or covert shifting of attention. In contrast, lesion-overlap studies of patients with spatial neglect - a syndrome characterized by severe impairments of spatial attention - show that the critical damage concerns more ventral regions, comprising the inferior parietal lobule, the temporal-parietal junction (TPJ), and the superior temporal gyrus. We performed voxel-based lesion-symptom mapping of 29 right-hemisphere stroke patients, using several performance indices derived from a cueing task as measures of spatial attention. In contrast to previous studies, we focused our analyses on eight regions of interest defined according to results of previous functional imaging studies. A direct comparison of neglect with control patients revealed that neglect was associated with damage to the TPJ, the middle frontal gyrus, and the posterior IPS. The latter region was also a significant predictor of the degree of contralesional slowing of target detection and the extent to which ipsilesional distracters captured attention of neglect patients. Finally, damage to the FEF and posterior IPS was negatively correlated with the tendency of neglect patients to orient attention toward behaviourally relevant distracters. These findings support the results of functional imaging studies of spatial attention and provide evidence for a network account of neglect, according to which attentional selection of relevant environmental stimuli and the reorienting of attention result from dynamic interactions between the IPS, the dorsal premotor cortex, and the TPJ.

Cortical and subcortical anatomy of chronic spatial neglect following vascular damage.

BackgroundThe role of the inferior parietal lobule (IPL) and superior temporal gyrus (STG) or subcortical pathways as possible anatomical correlates of spatial neglect is currently intensely discussed. Some of the conflicting results might have arisen because patients were examined in the acute stage of disease.MethodsWe examined the anatomical basis of spatial neglect in a sample of patients examined in the post-acute stage following right-hemispheric vascular brain damage. Lesions of 28 patients with chronic spatial neglect were contrasted to lesions of 22 control patients without neglect using lesion subtraction techniques and voxel-wise comparisons.ResultsThe comparisons identified the temporo-parietal junction (TPJ) with underlying white matter, the supramarginal gyrus, the posterior STG, and the insula as brain regions damaged significantly more often in neglect compared to non-neglect patients. In a subgroup of neglect patients showing particularly large cancellation bias together with small errors on line bisection damage was prevalent deep in the frontal lobe while damage of patients with the reverse pattern was located in the white matter of the TPJ.ConclusionConsidering our results and the findings of previous studies, spatial neglect appears to be associated with a network of regions involving the TPJ, inferior IPL, posterior STG, the insular cortex, and posterior-frontal projections. Frontal structures or projections may be of particular relevance for spatial exploration, while the IPL may be important for object-based attention as required for line bisection.

Subcortical Loop Activation during Selection of Currently Relevant Memories

Clinical studies on spontaneous confabulation and imaging studies with healthy subjects indicate that the anterior limbic system, in particular, the orbitofrontal cortex (OFC), is necessary to adjust thought and behavior to current reality. It appears to achieve this by continuously suppressing activated memories that do not pertain to ongoing reality, even before their content is consciously recognized. In the present study, we explored through what anatomical connections the OFC exerts this influence. Healthy subjects were scanned with H2 15O PET as they performed four blocks of continuous recognition tasks, each block composed of a different type of stimuli (meaningful designs, geometric designs, words, nonwords). Within each block, three runs composed of exactly the same picture series, arranged in different order each time, were made. Subjects were asked to indicate item recurrences only within the currently ongoing run and to disregard familiarity from previous runs. In the combined first runs, in which all items were initially new and responses could be based on familiarity judgement (with repeated items) alone, we found medial temporal and right orbitofrontal activation. In the combined third runs, when all items were already known and selection of currently relevant memories was required, we found left orbitofrontal activation contingent with distinct activation of the ventral striatum, head and body of the caudate nucleus, substantia nigra, and medial thalamus. The study indicates that the OFC influences the cortical representation of memories through subcortical connections including the basal ganglia and the thalamus. The data are compatible with a role of the dopaminergic reward system in the monitoring of ongoing reality in thinking.