Carmel Mevorach

Opposite biases in salience-based selection for the left and right posterior parietal cortex.

Visual selection is determined in part by the saliency of stimuli. We assessed the brain mechanisms determining attentional responses to saliency. Repetitive transcranial magnetic stimulation (rTMS) was applied to the left and right posterior parietal cortices (PPC) immediately before participants were asked to identify a compound letter. rTMS to the right PPC disrupted the guidance of attention toward salient stimuli, whereas rTMS to the left PPC affected the ability to bias selection away from salient stimuli. We conclude that right and left PPC have opposite roles in biasing selection to and from salient stimuli in the environment.

The Diversity of Attention Deficits in ADHD The Prevalence of Four Cognitive Factors in ADHD Versus Controls

The performance of participants with attention-deficit/hyperactivity disorder (ADHD) relative to control participants was measured on four tasks uniquely assessing the functions of selective attention, executive attention, sustained attention, and orienting of attention. The results showed that deficits in sustained attention were the most pronounced, characterizing most participants with ADHD and deficits in each of the other three functions characterized more than half of these participants. Different participants with ADHD revealed different clusters of attentional deficits. These results call for a revision of leading theories of ADHD that identify the core of the pathology as a sole deficit in executive functions.

Ignoring the elephant in the room: a neural circuit to downregulate salience.

How do we ignore stimuli that are salient but irrelevant when our task is to select a lower salient stimulus? Since bottom–up processes favor high saliency, detection of a low-salient target in the presence of highly salient distractors requires top–down attentional guidance. Previous studies have demonstrated that top–down attention can modulate perceptual processing and also that the control of attention is driven by frontoparietal regions. However, to date, there is no direct evidence on the cause and effect relationship between control regions and perceptual processing. Here, we report the first evidence demonstrating a neural circuit for the downregulation of salient distractors when a low-salient target is selected, combining brain imaging using functional magnetic resonance imaging with brain stimulation by transcranial magnetic stimulation. Using these combined techniques, we were able to identify a cause and effect relationship in the suppression of saliency, based on an interaction between the left intraparietal sulcus (IPS) and a region implicated in visual processing in our task (the left occipital pole). In particular, low-salient stimuli were selected by the left IPS suppressing early visual areas that would otherwise respond to a high-saliency distractor in the task. Apart from providing a first documentation of the neural circuit supporting selection by saliency, these data can be critical for understanding the underlying causes of problems in ignoring irrelevant salience that are found in both acquired and neurodevelopmental disorders (e.g., attention deficit/hyperactivity disorder or autism).

The neural mechanisms of visual selection: the view from neuropsychology

In this review, we discuss how neuropsychological impairments in visual selection can inform us about how selection normally operates. Using neuroanatomical and behavioral evidence on the disorders of neglect, extinction, and simultanagnosia, we propose functional and anatomical links between different aspects of visual selection and distinct sites in the posterior parietal cortex (PPC). This includes linking: (i) bottom‐up attentional capture and the right temporo‐parietal junction (TPJ); (ii) top‐down segmentation of displays and the medial PPC; (iii) grouping, individuation and identification, and the inferior intra‐parietal sulcus (IPS) bilaterally; and (iv) the suppression of saliency and the left IPS. In addition, when neuropsychological studies are combined with fMRI, there is evidence that these regions of the PPC interact with striate and extra‐striate cortical areas, which respond to specific properties of stimuli. Selection should be viewed as an emergent property of a network of areas involving both ventral and dorsal cortex.

Driven to Less Distraction: rTMS of the Right Parietal Cortex Reduces Attentional Capture in Visual Search

In visual search, the presence of a highly salient color singleton can slow or facilitate search for a shape target depending on whether the singleton is a distractor or coincides with the target. This is consistent with an attentional shift (attentional capture) to the salient item. This attentional capture can be driven by bottom-up or top-down processes or both. We investigated the role of the parietal cortex in attentional capture by a singleton using repetitive transcranial magnetic stimulation. Following disruption to the right posterior parietal cortex by sustained transcranial magnetic stimulation, the reaction time (RT) cost of the singleton distractor was reduced. At least part of this lessening of singleton distraction was due to the elimination of priming (top-down) effects between target and distractor singletons on consecutive trials. In Experiment 2, we presented the different conditions in separate blocks meaning any effects of the distractor can most likely be attributed to bottom-up processes. Nevertheless, there was still a decrease in RT interference from the distractor so that a reduction in priming cannot provide a full account of the results. The data are consistent with previous work positing that the right parietal cortex directs attention to salient stimuli (e.g., Constantinidis 2005, Mevorach et al. 2006), while also suggesting a role for the right parietal cortex in the integration of bottom-up salience information with memories for salient features on prior trials.

Conjunctive Continuous Performance Task (CCPT)—A pure measure of sustained attention ☆

Among the large variety of attentional tasks that have been used to study sustained attention, the Continuous Performance Task (CPT) is perhaps the most widely used. Despite substantial differences in task characteristics and demands, all CPT paradigms have been referred to as measures of sustained attention. In the present study we introduce a new variant of CPT, which minimizes perceptual and memory components while maximizing the sustained attention components of the task. In addition, we tested the contention that the ability to sustain attention should not be overly dependent on the specific stimuli and task-modality. To this end, we used a new visual Conjunctive CPT (CCPT) developed by Tsal, Shalev, & Mevorach (2005) and its auditory analogue. Using a Multi-Trait-Multi-Method (MTMM) analysis investigating reliability coefficients, convergent validity coefficients and divergent (discriminant) validity coefficients, we established that the new CCPT is a valid measure of sustained attention. In particular, high correlations were obtained between mean RT and SD-RT within each sensory modality. High correlations were also found between performance across sensory modalities (mean convergent validity: .71). Finally, low correlations (mean correlation of .18) were found between performance in the CCPT tasks and performance in two additional visuospatial attention tasks, which do not rely on sustained attention. These findings suggest that independent of sensory modality, the two CCPT tasks used here tap the same stable construct, namely, sustained attention. We conclude that when a measure of sustained attention is required, researchers should be aware of the caveats of standard CPTs and should be careful in selecting a proper task. Moreover, clinicians should appreciate that some CPT measures may reflect a combination of different cognitive operations rather than pure sustained attention.

The left intraparietal sulcus modulates the selection of low salient stimuli

Neuropsychological and functional imaging studies have suggested a general right hemisphere advantage for processing global visual information and a left hemisphere advantage for processing local information. In contrast, a recent transcranial magnetic stimulation study [Mevorach, C., Humphreys, G. W., & Shalev, L. Opposite biases in salience-based selection for the left and right posterior parietal cortex. Nature Neuroscience, 9, 740–742, 2006b] demonstrated that functional lateralization of selection in the parietal cortices on the basis of the relative salience of stimuli might provide an alternative explanation for previous results. In the present study, we applied a whole-brain analysis of the functional magnetic resonance signal when participants responded to either the local or the global levels of hierarchical figures. The task (respond to local or global) was crossed with the saliency of the target level (local salient, global salient) to provide, for the first time, a direct contrast between brain activation related to the stimulus level and that related to relative saliency. We found evidence for lateralization of salience-based selection but not for selection based on the level of processing. Activation along the left intraparietal sulcus (IPS) was found when a low saliency stimulus had to be selected irrespective of its level. A control task showed that this was not simply an effect of task difficulty. The data suggest a specific role for regions along the left IPS in salience-based selection, supporting the argument that previous reports of lateralized responses to local and global stimuli were contaminated by effects of saliency.

Effects of saliency, not global dominance, in patients with left parietal damage

Neuropsychological and functional imaging studies have shown a general right hemisphere advantage for processing global visual information and a left hemisphere advantage for processing local information. There is also evidence for the left parietal lobe being important for switching attention between local and global levels. Here we examined whether the left parietal lobe is associated with another aspect of attentional control over hierarchical visual processing; namely, ignoring the irrelevant aspect of the stimulus when it is more salient than the target attribute. In experiment 1 a group of left parietal patients were abnormally affected by a salient local stimulus that significantly interfered with their ability to identify global shapes. This effect was reversed in experiment 2 when small sized compound letters were used (where the global shape was more salient than the local letters). The patients then had difficulty ignoring the global shape. Experiments 3 and 4 demonstrated that the failure to accurately identify global form in experiment 1 could not be attributed to a difficulty in spreading attention across a large area. Finally, in experiment 4 the effect of a salient irrelevant stimulus was significantly attenuated when the irrelevant level did not map onto a response. The data indicate that damage to the left parietal lobe disrupts the ability to select attributes of stimuli that have low salience when other attributes have high salience for the task.

The Salient Self: The Left Intraparietal Sulcus Responds to Social as Well as Perceptual-Salience After Self-Association

Perceptual learning is associated with experience-based changes in stimulus salience. Here, we use a novel procedure to show that learning a new association between a self-label and a neutral stimulus produces fast alterations in social salience measured by interference when targets associated with other people have to be selected in the presence of self-associated distractors. Participants associated neutral shapes with either themselves or a friend, over a short run of training trials. Subsequently, the shapes had to be identified in hierarchical (global-local) forms. The data show that giving a shape greater personal significance by associating it with the self had effects on visual selection equivalent to altering perceptual salience. Similar to previously observed effects linked to when perceptually salient distractors are ignored, effects of a self-associated distractor also increased activation in the left intraparietal cortex sulcus. The results show that self-associations to sensory stimuli rapidly modulate neural responses in a manner similar to changes in perceptual saliency. The self-association procedure provides a new way to understand how personal significance affects behavior.

Global processing of compound letters in a patient with Balint's syndrome

We report data on the ability of a patient with Balints syndrome (GK) to process global information from compound letters. As with other patients with Balints syndrome, GK was impaired at respond-ing to large, global letters. In Experiment 1 we show that this was due to local capture rather than the absolute size of the stimuli. Also, despite his impairment with global letters, GK showed global inter-ference on local judgements, indicating that some implicit processing took place at the global level. Interestingly, the inability to perceive large global letters was overcome when GK identified a solid, large prime letter prior to the onset of the compound figure (Experiment 2). This priming effect was temporary, and decreased as the interval between the prime and the compound letter increased (Experiment 3). When the prime was an English letter, the effect was maintained even when GK only had to identify the primes colour, provided a colour-identification block of trials followed rather than preceded a block of trials where prime shapes had to be identified (Experiment 4). In contrast, there was no priming when GK had to identify the colour of English letter primes in a trial block following a block where the task was to identify the colour of Hebrew letter primes (Experiment 5). Overall the data indicate that local capture in Balints syndrome can be overcome by actively priming a wide attentional window. The results can be interpreted in terms of an interaction between spatial attention and grouping processes that subserves the perception of global compound letters.