Search

Though motivational value is a recognized trigger of approach and avoidance behavior, less is known about the potential of reward to capture attention. We here explored whether positive or negative reward modulates the characteristic deficit of patients with left spatial neglect to disengage attention from an ipsilesional distracter. We built our study on recent observations showing that the disengagement deficit is exaggerated for distracters with target-defining features, indicating that task-relevance captures attention. Patients with left neglect and matched healthy controls were asked to react to lateralized, colored targets preceded by a peripheral cue. Crucially, the cue either possessed the color of the target and was thus task-relevant, or was followed by a positive, negative, or neutral symbolic reward. Neglect patients only exhibited a disengagement deficit when cues were task-relevant or were followed by a negative reward. This finding indicates that attentional selection is driven by task-relevance and negative reward, possibly through interactions between limbic and attention networks.

Patients with visual object agnosia fail to recognize the identity of visually presented objects despite preserved semantic knowledge. Object agnosia may result from damage to visual cortex lying close to or overlapping with the lateral occipital complex (LOC), a brain region that exhibits selectivity to the shape of visually presented objects. Despite this anatomical overlap the relationship between shape processing in the LOC and shape representations in object agnosia is unknown. We studied a patient with object agnosia following isolated damage to the left occipito-temporal cortex overlapping with the LOC. The patient showed intact processing of object structure, yet often made identification errors that were mainly based on the global visual similarity between objects. Using functional Magnetic Resonance Imaging (fMRI) we found that the damaged as well as the contralateral, structurally intact right LOC failed to show any object-selective fMRI activity, though the latter retained selectivity for faces. Thus, unilateral damage to the left LOC led to a bilateral breakdown of neural responses to a specific stimulus class (objects and artefacts) while preserving the response to a different stimulus class (faces). These findings indicate that representations of structure necessary for the identification of objects crucially rely on bilateral, distributed coding of shape features.

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.

Electrophysiological correlates of recovery from anomia were analysed in four aphasic patients in the post-acute stage. Event-related potentials (ERPs) were recorded during picture naming at baseline and after a period of therapy for anomia. All patients had severe anomia at baseline assessment and improved significantly in naming during the study period. Waveform analyses and temporal segmentation were carried out on the ERPs of each patient in comparison with 15 healthy control subjects. Normalisation as well as an increase of abnormal electrophysiological correlates accompanied recovery. An increase of abnormal amplitudes appeared in a patient with semantic impairment during the first 300 ms after picture onset, while only normalisation of amplitudes and topographic maps accompanied recovery in the three patients with lexical-phonological impairment in this early time-window. Abnormal amplitudes and topographic maps emerged during recovery in the patients with lexical-phonological impairment in later time-windows, starting between 250 and 300 ms. Follow-up ERP recordings carried out 6 months later in two of them showed normalisation of amplitudes and persistence of abnormal maps. The results suggest that electrophysiological changes accompanying recovery from anomia in the post-acute stage are observed in specific time-windows, probably corresponding to different encoding processes and that recovery correlates with normalisation of EEG patterns as well as with the emergence of abnormalities, which presumably indicates compensation mechanisms of specific encoding processes.

Confabulation denotes the emergence of memories of experiences and events which never took place. Whether there are distinct forms with distinct mechanisms is still debated. In this study, we explored 4 forms of confabulation and their mechanisms in 29 amnesic patients. Patients performed tests of explicit memory, executive functions, and two test of orbitofrontal reality filtering (memory selection and extinction capacity in a reversal learning task) previously shown to be strongly associated with confabulations that patients act upon and disorientation. Results indicated the following associations: (1) Intrusions in a verbal memory test (simple provoked confabulations) dissociated from all other forms of confabulation and were not associated with any specific cognitive measure. (2) Momentary confabulations, defined as confabulatory responses to questions and measured with a confabulation questionnaire, were associated with impaired mental flexibility, a tendency to fill gaps in memory, and with one measure of reality filtering. Momentary confabulations, therefore, may emanate from diverse causes. (3) Behaviourally spontaneous confabulation, characterized by confabulations that the patients act upon and disorientation, was strongly associated with failure in the two reality filtering tasks. Behaviourally spontaneous confabulation may be seen as a specific instance of momentary confabulations with a distinct mechanism. (4) A patient producing fantastic confabulations with nonsensical, illogical content had wide-spread cognitive dysfunction and failed in the reality filtering tasks. The results support the presence of truly or partially dissociable types of confabulation with different mechanisms.

Neglect dyslexia--a peripheral reading disorder generally associated with left spatial neglect--is characterized by omissions or substitutions of the initial letters of words. Several observations suggest that neglect dyslexia errors are independent of viewer-centered coordinates; the disorder is therefore thought to reflect impairment at the level of object-centered representations. This hypothesis is indirectly supported by lesion studies connecting object-centered neglect errors with damage to posterior cortical regions lying in the ventral visual stream. Here, we performed a lesion-symptom mapping study of 40 patients with spatial neglect asked to read words presented at different positions relative to a viewer-centered coordinate frame. We found that the frequency of object-centered reading errors was constant across horizontal positions, whereas the frequency of entirely neglected words (reflecting a page-centered deficit) linearly increased from right to left. Damage to the intraparietal sulcus and the angular and middle temporal gyri was the best predictor of object-centered errors. We discuss these findings with reference to a role of the posterior parietal lobe in adapting the size of the attentional focus and biasing object representations elaborated in the ventral visual stream.

Brain activation dissociates during the repeated performance of memory tasks with decreasing medial temporal and increasing orbitofrontal activation. The impact of such adaptations on a baseline task is unknown. In this study, we used H 2 15 O positron emission tomography (PET) in two groups of subjects performing a continuous recognition task and a baseline task. The group performing the baseline task after the main task showed significant medial temporal activation in the subtraction (recognition task–baseline). The group performing the baseline task at the beginning showed right orbitofrontal activation. These differences appeared to result primarily from different activations during the baseline task. It thus appears that the temporal context of a baseline task may fundamentally alter cognitive requirements and substantially influence apparent brain activation during a memory task. We suggest that the automatic filtration of memories according to their relevance for ongoing reality, a capacity mediated by the orbitofrontal cortex, is one such influence on apparent activation.

Although word-finding difficulties have been largely studied from a theoretical and a rehabilitation point of view, recovery mechanisms and especially the fact that patients with similar anomic patterns may exhibit different recovery, are still not fully understood. In the first part of the present study we investigated the word retrieval curve during therapy and the psycholinguistic variables affecting word-finding recovery patterns in three anomic subjects (PG, AH and TM). Despite the fact that all patients had similar anomia at baseline, they presented different recovery patterns during an identical therapy program. The progress during therapy and the number of sessions necessary to reach satisfactory improvement was similar in two patients (AH and TM), but differed in the third patient (PG), who needed more treatment sessions. Moreover, these two different patterns were affected by different psycholinguistic variables: words age of acquisition predicted improvement in AH and TM, whereas phonological neighbourhood predicted improvement in PG. Following the observation that phonological neighbourhood density affected the slower progress during therapy, in the second study we analysed whether this variable also predicts pseudo-word learning in healthy controls and in anomic subjects. Indeed, phonological neighbourhood predicted pseudo-word learning speed in controls and in some anomic patients. We suggest that the analysis of progress during therapy for anomia and the comparison of the variables affecting learning and recovery may provide information about the underlying nature of the anomic deficit that is not available through the simple assessment of performance.

One of the most constant findings of studies about selective attention is that detection of visual stimuli is enhanced when a visual cue is presented at the position of the upcoming target. In healthy participants, comparable benefits were reported when the cue was presented in a different modality than the target. The aim of this study was to examine spatial and temporal dynamics of visual attention following auditory cues in patients with spatial neglect. Twelve healthy subjects and five patients with left-sided neglect were asked to react to a small vertical line presented randomly at one of four positions. The target appeared 150 or 1000 ms after an auditory cue that was either static (continuous 380 Hz tone presented to the left or right ear) or dynamic (380 Hz tone moving from the left to the right ear or vice versa). The reaction time pattern of healthy participants was unaffected by the different tones. In contrast, reaction times of neglect patients were significantly faster to left targets following a dynamic tone moving from right to left in comparison to a tone moving from left to right. Interestingly, static unilateral tones modulated visual attention of neglect patients to a lesser degree than dynamic tones. The modulation of visual attention by dynamic auditory cues was of short duration and disappeared after 1000 ms. These results demonstrate a fast automatic shift of spatial attention in the direction of a moving tone, suggesting strong dynamic links between visual and auditory attention in patients with a severe spatial deficit.

The orbitofrontal cortex (OFC) processes the occurrence or omission of anticipated rewards, but clinical evidence suggests that it might serve as a generic outcome monitoring system, independent of tangible reward. In this positron emission tomography (PET) study, normal human subjects performed a series of tasks in which they simply had to predict behind which one of two colored rectangles a drawing of an object was hidden. While all tasks involved anticipation in that they had an expectation phase between the subject's prediction and the presentation of the outcome, they varied with regards to the uncertainty of outcome. No comment on the correctness of the prediction, no record of ongoing performance, and no reward, not even a score, was provided. Nonetheless, we found strong activation of the OFC: in comparison with a baseline task, the left anterior medial OFC showed activation in all conditions, indicating a basic role in anticipation; the left posterior OFC was activated in all tasks with some uncertainty of outcome, suggesting a role in the monitoring of outcomes; the right medial OFC showed activation exclusively during guessing. The data indicate a generic role of the human OFC, with some topical specificity, in the generation of hypotheses and processing of outcomes, independent of the presence of explicit reward.

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.

We have previously demonstrated that spontaneous confabulations and disorientation in amnesia emanate from temporal context confusion in memory characterised by intrusion of previously acquired information into ongoing thinking. This failure appears to be due to an inability to suppress previously activated memory traces of events dating back hours, weeks or even years. In the present study, the possibility was explored that spontaneous confabulators also have a problem in processing information in the very short-term range of the psychological present (the ‘now’), which has been defined as the duration of an experiential process. Tasks assessing the processing of time within the psychological present were devised to test this possibility. In experiment 1, spontaneous confabulators, in comparison with other amnestic patients, failed to discriminate brief, visually presented intervals in the now. In experiment 2, they failed to make motion-associated time estimations within the now. These deficits also correlated with disorientation. Although these results do not prove a causal relationship, they indicate that spontaneously confabulating and disorientated patients typically do fail to process information within the ‘now’.