Working memory (WM) is known to be impaired in patients with stroke experiencing unilateral spatial neglect (USN). Here, we examined in a systematic manner three WM components: memory of object identity, memory of object location, and binding between object identity and location. Moreover, we used two different retention intervals to isolate maintenance from other mnemonic and perceptual processes. Fourteen USN first-event stroke patients with right-hemisphere damage were tested in two different WM experiments using long and short retention intervals and an analog response scale. Patients exhibited more identification errors for items displayed on the contralesional side. Localization errors were also more prominent in the contralesional side, especially after a long retention interval. These localization errors were often a result of swap errors, that is, erroneous localizations of correctly identified contralesional objects in correctly memorized locations of ipsilesional objects. We conclude that a key WM deficit in USN is a lateralized impairment in binding between the identity of an object and its spatial tag.

Stroke patients with ideomotor apraxia (IMA) have difficulties controlling voluntary motor actions, as clearly seen when asked to imitate simple gestures performed by the examiner. Despite extensive research, the neurophysiological mechanisms underlying failure to imitate gestures in IMA remain controversial. The aim of the current study was to explore the relationship between imitation failure in IMA and mirror neuron system (MNS) functioning. Mirror neurons were found to play a crucial role in movement imitation and in imitation-based motor learning. Their recruitment during movement observation and execution is signaled in EEG recordings by suppression of the lower (8–10 Hz) mu range. We examined the modulation of EEG in this range in stroke patients with left ( n = 21) and right ( n = 15) hemisphere damage during observation of video clips showing different manual movements. IMA severity was assessed by the DeRenzi standardized diagnostic test. Results showed that failure to imitate observed manual movements correlated with diminished mu suppression in patients with damage to the right inferior parietal lobule and in patients with damage to the right inferior frontal gyrus pars opercularis—areas where major components of the human MNS are assumed to reside. Voxel-based lesion symptom mapping revealed a significant impact on imitation capacity for the left inferior and superior parietal lobules and the left post central gyrus. Both left and right hemisphere damages were associated with imitation failure typical of IMA, yet a clear demonstration of relationship to the MNS was obtained only in the right hemisphere damage group. Suppression of the 8–10 Hz range was stronger in central compared with occipital sites, pointing to a dominant implication of mu rather than alpha rhythms. However, the suppression correlated with De Renzi's apraxia test scores not only in central but also in occipital sites, suggesting a multifactorial mechanism for IMA, with a possible impact for deranged visual attention (alpha suppression) beyond the effect of MNS damage (mu suppression).

The syndrome of unilateral spatial neglect (USN) after right-hemisphere damage is characterized by failure of salient left-sided stimuli to activate an orienting response, attract attention, and gain access to conscious awareness. The explicit failure processing left-sided visual information is not uniform, however, and patients seem to be more successful performing certain visual tasks than others. The source of this difference is still not clear. We focus on processing of visual scene statistical properties, asking whether, in computing the average size of an array of objects, USN patients give appropriate weight to objects on the left; disregard left-side objects entirely; or assign them an intermediate, lower weight, in accord with their tendency to neglect these objects. The interest in testing this question stems from a series of studies in healthy individuals that led Chong and Treisman [Chong, S. C., & Treisman, A. Statistical processing: Computing the average size in perceptual groups. Vision Research, 45, 891–900, 2005a; Chong, S. C., & Treisman, A. Attentional spread in the statistical processing of visual displays. Perception & Psychophysics, 67, 1–13, 2005b] to propose that processing of statistical properties (like the average size of visual scene elements) is carried out in parallel, with no need for serial allocation of focal attention to the different scene elements. Our results corroborate this suggestion, showing that objects in the left (“neglected”) hemispace contribute to average size computation, despite a marked imbalance in spatial distribution of attention, which leads to a reduced weight of left-side elements in the averaging computation. This finding sheds light on the nature of the impairment in USN and on basic mechanisms underlying statistical processing in vision. We confirm that statistical processing depends mainly on spread-attention mechanisms, which are largely spared in USN.

Patients with unilateral spatial neglect (USN) often show impaired performance in spatial working memory tasks, apart from the difficulty retrieving “left-sided” spatial data from long-term memory, shown in the “piazza effect” by Bisiach and colleagues. This study's aim was to compare the effect of the spatial position of a visual object on immediate and delayed memory performance in USN patients. Specifically, immediate verbal recall performance, tested using a simultaneous presentation of four visual objects in four quadrants, was compared with memory in a later-provided recognition task, in which objects were individually shown at the screen center. Unlike healthy controls, USN patients showed a left-side disadvantage and a vertical bias in the immediate free recall task (69% vs. 42% recall for right- and left-sided objects, respectively). In the recognition task, the patients correctly recognized half of “old” items, and their correct rejection rate was 95.5%. Importantly, when the analysis focused on previously recalled items (in the immediate task), no statistically significant difference was found in the delayed recognition of objects according to their original quadrant of presentation. Furthermore, USN patients were able to recollect the correct original location of the recognized objects in 60% of the cases, well beyond chance level. This suggests that the memory trace formed in these cases was not only semantic but also contained a visuospatial tag. Finally, successful recognition of objects missed in recall trials points to formation of memory traces for neglected contralesional objects, which may become accessible to retrieval processes in explicit memory.

The performance of patients with lesions involving the basal ganglia (BG) was compared to that of patients with prefrontal (PFC) lesions, thalamic (TH) lesions, and age-matched controls in order to examine the specific role of the BG within the frontal-subcortical circuits (FSCC) in task switching. All the BG patients and none of the other participants showed a marked increase in error rate in incongruent trials where correct responses depended upon the choice of the correct task rule. Some BG patients erred in failing to switch tasks and others failed despite their attempt to switch tasks. Additionally, reaction time results indicate abnormal response repetition effects among the BG patients; failure in benefiting from advance task information among all the patients; and increased task mixing costs following PFC lesions. The authors conclude that although the frontal-subcortical circuits jointly determine some behaviors (such as benefiting from preparation), the BG play a unique role within the FSCC in action selection and/or the inhibition of irrelevant information.

The present research examines the effect of spatial (object-centered) attentional constraints on pattern recognition. Four normal subjects and two right-hemisphere-damaged patients with left visual neglect participated in the study. Small, letterlike, prelearned patterns served as stimuli. Short exposure time prevented overt scanpaths during stimulus presentation. Attention was attracted to a central (midsagittal) hation point by precuing this location prior to each stimulus presentation. Minute (up to 1.5° of visual angle) rightward and leftward stimulus shifts caused attention to be allocated each time to a different location on the object space, while remaining in a fixed central position in viewercentered coordinates. The task was to decide which of several prelearned patterns was presented in each trial. In the normal subjects, best performance was achieved when the luminance centroid (LC; derived from the analysis of low-spatial frequencies in the object space) of each pattern coincided with the spatial position of the precue. In contrast, the patients with neglect showed optimal recognition performance when precuing attracted attention to locations within the object space, to the left of the LC. The normal performance suggests that the LC may serve as a center of gravity for attention allocation during pattern recognition. This point seems to be the target location where focal attention is normally directed, following a primary global analysis based on the low spatial frequencies. Thus, the LC of a simple pattern may serve as the origin point for an object-centered-coordiate-frame (OCCF), dividing it into right and left. This, in turn, serves to create a prototype description of the pattern, in its own coordinates, in memory, to be addressed during subsequent recognition tasks. The best match of the percept with the stored description may explain the observed advantage of allocating attention to the LC. The performance of the brain- damaged patients can be explained in terms of neglect operating in the OCCE