Covert visual spatial orienting and saccades: overlapping neural systems

The effect of FEF microstimulation on the responses of neurons in the lateral intraparietal area

The macaque FEFs and the lateral intraparietal area (LIP) are high-level cortical areas involved in both spatial attention and oculomotor behavior. Stimulating FEF at a level below the threshold for evoking saccades increases fMRI activity and gamma power in area LIP, but the precise effect exerted by the FEF on LIP neurons is unknown. In our study, we recorded LIP single-unit activity during a visually guided saccade task with a peripherally presented go signal during microstimulation of FEF. We found that FEF microstimulation increased the LIP spike rate immediately after the highly salient go signal inside the LIP receptive field when both target and go signal were presented inside the receptive field, and no other possible go cues were present on the screen. The effect of FEF microstimulation on the LIP response was positive until at least 800 msec after microstimulation had ceased, but reversed for longer trial durations. Therefore, FEF microstimulation can modulate the LIP spike rate only when attention is selectively directed toward the stimulated location. These results provide the first direct evidence for LIP spike rate modulations caused by FEF microstimulation, thus showing that FEF activity can be the source of top-down control of area LIP.

A model of proto-object based saliency

Organisms use the process of selective attention to optimally allocate their computational resources to the instantaneously most relevant subsets of a visual scene, ensuring that they can parse the scene in real time. Many models of bottom-up attentional selection assume that elementary image features, like intensity, color and orientation, attract attention. Gestalt psychologists, however, argue that humans perceive whole objects before they analyze individual features. This is supported by recent psychophysical studies that show that objects predict eye-fixations better than features. In this report we present a neurally inspired algorithm of object based, bottom-up attention. The model rivals the performance of state of the art non-biologically plausible feature based algorithms (and outperforms biologically plausible feature based algorithms) in its ability to predict perceptual saliency (eye fixations and subjective interest points) in natural scenes. The model achieves this by computing saliency as a function of proto-objects that establish the perceptual organization of the scene. All computational mechanisms of the algorithm have direct neural correlates, and our results provide evidence for the interface theory of attention.

A functional MRI study of a picture-sentence verification task: evidence of attention shift to the grammatical subject

Mapping the meaning of a sentence onto visual entities is a fundamental process of daily language use, but it is unclear how attention in the visual context influences sentence comprehension. Aiming to examine this problem, we conducted a picture-sentence matching experiment with scanning using functional MRI. In the experiment, a moving picture describing an event with two colored objects was presented on a screen. A visual cue was flashed at the position of an object's appearance just before the event presentation, and participants were instructed to pay attention to the visually cued object in the picture. They were then required to read a simple Japanese sentence and to verify whether it correctly described the previous event. To examine the effects of visual cueing, we defined two conditions on the basis of the relationship between the visually cued object in an event and the grammatical subject of the subsequent sentence. When comparing the conditions in which the visually cued object was incongruent with the grammatical subject to the congruent conditions, participants showed a lower hit rate, and the right frontal eye field, which is known to be the region related to attention shift, was more activated. These findings suggest that the attention was initially allocated to an object encoded as the grammatical subject in the process of linking the content of a sentence with a visual event. Therefore, the attention was shifted from the cued object to the other object under the conditions discussed above.

Differential roles of the frontal and parietal cortices in the control of saccades

Although externally as well as internally-guided eye movements allow us to flexibly explore the visual environment, their differential neural mechanisms remain elusive. A better understanding of these neural mechanisms will help us to understand the control of action and to elucidate the nature of cognitive deficits in certain psychiatric populations (e.g., schizophrenia) that show increased latencies in volitional but not visually-guided saccades. Both the superior precentral sulcus (sPCS) and the intraparietal sulcus (IPS) are implicated in the control of eye movements. However, it remains unknown what differential contributions the two areas make to the programming of visually-guided and internally-guided saccades. In this study we tested the hypotheses that sPCS and IPS distinctly encode internally-guided saccades and visually-guided saccades. We scanned subjects with fMRI while they generated visually-guided and internally-guided delayed saccades. We used multi-voxel pattern analysis to test whether patterns of cue related, preparatory and saccade related activation could be used to predict the direction of the planned eye movement. Results indicate that patterns in the human sPCS predicted internally-guided saccades but not visually-guided saccades in all trial periods and patterns in the IPS predicted internally-guided saccades and visually-guided saccades equally well. The results support the hypothesis that the human sPCS and IPS make distinct contributions to the control of volitional eye movements.

Spatial orienting in complex audiovisual environments

Previous studies on crossmodal spatial orienting typically used simple and stereotyped stimuli in the absence of any meaningful context. This study combined computational models, behavioural measures and functional magnetic resonance imaging to investigate audiovisual spatial interactions in naturalistic settings. We created short videos portraying everyday life situations that included a lateralised visual event and a co-occurring sound, either on the same or on the opposite side of space. Subjects viewed the videos with or without eye-movements allowed (overt or covert orienting). For each video, visual and auditory saliency maps were used to index the strength of stimulus-driven signals, and eye-movements were used as a measure of the efficacy of the audiovisual events for spatial orienting. Results showed that visual salience modulated activity in higher-order visual areas, whereas auditory salience modulated activity in the superior temporal cortex. Auditory salience modulated activity also in the posterior parietal cortex, but only when audiovisual stimuli occurred on the same side of space (multisensory spatial congruence). Orienting efficacy affected activity in the visual cortex, within the same regions modulated by visual salience. These patterns of activation were comparable in overt and covert orienting conditions. Our results demonstrate that, during viewing of complex multisensory stimuli, activity in sensory areas reflects both stimulus-driven signals and their efficacy for spatial orienting; and that the posterior parietal cortex combines spatial information about the visual and the auditory modality.

Cognitive performance in high-altitude climbers: a comparative study of saccadic eye movements and neuropsychological tests

Impairment of cognitive performance during and after high-altitude climbing has been described in numerous studies and has mostly been attributed to cerebral hypoxia and resulting functional and structural cerebral alterations. To investigate the hypothesis that high-altitude climbing leads to cognitive impairment, we used of neuropsychological tests and measurements of eye movement (EM) performance during different stimulus conditions. The study was conducted in 32 mountaineers participating in an expedition to Muztagh Ata (7,546 m). Neuropsychological tests comprised figural fluency, line bisection, letter and number cancellation, and a modified pegboard task. Saccadic performance was evaluated under three stimulus conditions with varying degrees of cortical involvement: visually guided pro- and anti-saccades, and visuo-visual interaction. Typical saccade parameters (latency, mean sequence, post-saccadic stability, and error rate) were computed off-line. Measurements were taken at a baseline level of 440 m and at altitudes of 4,497, 5,533, 6,265, and again at 440 m. All subjects reached 5,533 m, and 28 reached 6,265 m. The neuropsychological test results did not reveal any cognitive impairment. Complete eye movement recordings for all stimulus conditions were obtained in 24 subjects at baseline and at least two altitudes and in 10 subjects at baseline and all altitudes. Measurements of saccade performances showed no dependence on any altitude-related parameter and were well within normal limits. Our data indicates that acclimatized climbers do not seem to suffer from significant cognitive deficits during or after climbs to altitudes above 7,500 m. We demonstrated that investigation of EMs is feasible during high-altitude expeditions.

Separable networks for top-down attention to auditory non-spatial and visuospatial modalities

A central question for cognitive neuroscience is whether there is a single neural system controlling the allocation of attention. A dorsal frontoparietal network of brain regions is often proposed as a mediator of top-down attention to all sensory inputs. We used functional magnetic resonance imaging in humans to show that the cortical networks supporting top-down attention are in fact modality-specific, with distinct superior fronto-parietal and fronto-temporal networks for visuospatial and non-spatial auditory attention respectively. In contrast, parts of the right middle and inferior frontal gyri showed a common response to attentional control regardless of modality, providing evidence that the amodal component of attention is restricted to the anterior cortex.

Allocation of attention for dissociated visual and motor goals

In daily life, selecting an object visually is closely intertwined with processing that object as a potential goal for action. Since visual and motor goals are typically identical, it remains unknown whether attention is primarily allocated to a visual target, a motor goal, or both. Here, we dissociated visual and motor goals using a visuomotor adaptation paradigm, in which participants reached toward a visual target using a computer mouse or a stylus pen, while the direction of the cursor was rotated 45° counter-clockwise from the direction of the hand movement. Thus, as visuomotor adaptation was accomplished, the visual target was dissociated from the movement goal. Then, we measured the locus of attention using an attention-demanding rapid serial visual presentation (RSVP) task, in which participants detected a pre-defined visual stimulus among the successive visual stimuli presented on either the visual target, the motor goal, or a neutral control location. We demonstrated that before visuomotor adaptation, participants performed better when the RSVP stream was presented at the visual target than at other locations. However, once visual and motor goals were dissociated following visuomotor adaptation, performance at the visual and motor goals was equated and better than performance at the control location. Therefore, we concluded that attentional resources are allocated both to visual target and motor goals during goal-directed reaching movements.

Oscillatory alpha-band suppression mechanisms during the rapid attentional shifts required to perform an anti-saccade task

Neuroimaging has demonstrated anatomical overlap between covert and overt attention systems, although behavioral and electrophysiological studies have suggested that the two systems do not rely on entirely identical circuits or mechanisms. In a parallel line of research, topographically-specific modulations of alpha-band power (~8-14 Hz) have been consistently correlated with anticipatory states during tasks requiring covert attention shifts. These tasks, however, typically employ cue-target-interval paradigms where attentional processes are examined across relatively protracted periods of time and not at the rapid timescales implicated during overt attention tasks. The anti-saccade task, where one must first covertly attend for a peripheral target, before executing a rapid overt attention shift (i.e. a saccade) to the opposite side of space, is particularly well-suited for examining the rapid dynamics of overt attentional deployments. Here, we asked whether alpha-band oscillatory mechanisms would also be associated with these very rapid overt shifts, potentially representing a common neural mechanism across overt and covert attention systems. High-density electroencephalography in conjunction with infra-red eye-tracking was recorded while participants engaged in both pro- and anti-saccade task blocks. Alpha power, time-locked to saccade onset, showed three distinct phases of significantly lateralized topographic shifts, all occurring within a period of less than 1s, closely reflecting the temporal dynamics of anti-saccade performance. Only two such phases were observed during the pro-saccade task. These data point to substantially more rapid temporal dynamics of alpha-band suppressive mechanisms than previously established, and implicate oscillatory alpha-band activity as a common mechanism across both overt and covert attentional deployments.

Spatial attention, precision, and Bayesian inference: a study of saccadic response speed

Inferring the environment's statistical structure and adapting behavior accordingly is a fundamental modus operandi of the brain. A simple form of this faculty based on spatial attentional orienting can be studied with Posner's location-cueing paradigm in which a cue indicates the target location with a known probability. The present study focuses on a more complex version of this task, where probabilistic context (percentage of cue validity) changes unpredictably over time, thereby creating a volatile environment. Saccadic response speed (RS) was recorded in 15 subjects and used to estimate subject-specific parameters of a Bayesian learning scheme modeling the subjects' trial-by-trial updates of beliefs. Different response models—specifying how computational states translate into observable behavior—were compared using Bayesian model selection. Saccadic RS was most plausibly explained as a function of the precision of the belief about the causes of sensory input. This finding is in accordance with current Bayesian theories of brain function, and specifically with the proposal that spatial attention is mediated by a precision-dependent gain modulation of sensory input. Our results provide empirical support for precision-dependent changes in beliefs about saccade target locations and motivate future neuroimaging and neuropharmacological studies of how Bayesian inference may determine spatial attention.

Differential frontal involvement in shifts of internal and perceptual attention

BACKGROUND

Perceptual attention enhances the processing of items in the environment, whereas internal attention enhances processing of items encoded in visual working memory. In perceptual and internal attention cueing paradigms, cues indicate the to-be-probed item before (pre-cueing) or after (retro-cueing) the memory display, respectively. Pre- and retro-cues confer similar behavioral accuracy benefits (pre-: 14-19%, retro-: 11-17%) and neuroimaging data show that they activate overlapping frontoparietal networks. Yet reports of behavioral and neuroimaging differences suggest that pre- and retro-cueing differentially recruit frontal and parietal cortices (Lepsien and Nobre, 2006).

OBJECTIVE/HYPOTHESIS

This study examined whether perceptual and internal attention are equally disrupted by neurostimulation to frontal and parietal cortices. We hypothesized that neurostimulation applied to frontal cortex would disrupt internal attention to a greater extent than perceptual attention.

METHODS

Cathodal transcranial direct current stimulation (tDCS) was applied to frontal or parietal cortices. After stimulation, participants completed a change detection task coupled with either pre- or retro-cues.

RESULTS

Cathodal tDCS across site (frontal, parietal) hindered performance. However, frontal tDCS had a greater negative impact on the retro-cued trials demonstrating greater frontal involvement during shifts of internal attention.

CONCLUSIONS

These results complement the neuroimaging data and provide further evidence suggesting that perceptual and internal attention are not identical processes. We conclude that although internal and perceptual attention are mediated by similar frontoparietal networks, the weight of contribution of these structures differs, with internal attention relying more heavily on the frontal cortex.

Dissociated α-band modulations in the dorsal and ventral visual pathways in visuospatial attention and perception

Modulations of occipito-parietal α-band (8-14 Hz) power that are opposite in direction (α-enhancement vs. α-suppression) and origin of generation (ipsilateral vs. contralateral to the locus of attention) are a robust correlate of anticipatory visuospatial attention. Yet, the neural generators of these α-band modulations, their interdependence across homotopic areas, and their respective contribution to subsequent perception remain unclear. To shed light on these questions, we employed magnetoencephalography, while human volunteers performed a spatially cued detection task. Replicating previous findings, we found α-power enhancement ipsilateral to the attended hemifield and contralateral α-suppression over occipito-parietal sensors. Source localization (beamforming) analysis showed that α-enhancement and suppression were generated in 2 distinct brain regions, located in the dorsal and ventral visual streams, respectively. Moreover, α-enhancement and suppression showed different dynamics and contribution to perception. In contrast to the initial and transient dorsal α-enhancement, α-suppression in ventro-lateral occipital cortex was sustained and influenced subsequent target detection. This anticipatory biasing of ventro-lateral extrastriate α-activity probably reflects increased receptivity in the brain region specialized in processing upcoming target features. Our results add to current models on the role of α-oscillations in attention orienting by showing that α-enhancement and suppression can be dissociated in time, space, and perceptual relevance.

Cortical control of saccades in Parkinson disease and essential tremor

A number of studies suggest that some features of essential tremor (ET) and Parkinson disease (PD) overlap. Besides tremor, also some cognitive features have been implicated in ET and PD. There is recent evidence that a common genetic mutation occurs in ET and PD. Saccadic eye movements could provide an easily quantifiable procedure to help in the differential diagnosis in early PD and ET. Being able to distinguish early on the two diseases may help in tailoring therapy. Cortical control of saccades and antisaccades as they pertain to the potential discrimination of PD and ET is reviewed. Imaging and electrophysiological studies are highlighted; however, there are still few studies. Hopefully this review will stimulate further research, in particular in the direction of differences and similarities in the neural circuits involved in PD and ET.

Paying Attention through Eye Movements: A Computational Investigation of the Premotor Theory of Spatial Attention

Growing evidence indicates that planning eye movements and orienting visuospatial attention share overlapping brain mechanisms. A tight link between endogenous attention and eye movements is maintained by the premotor theory, in contrast to other accounts that postulate the existence of specific attention mechanisms that modulate the activity of information processing systems. The strong assumption of equivalence between attention and eye movements, however, is challenged by demonstrations that human observers are able to keep attention on a specific location while moving the eyes elsewhere. Here we investigate whether a recurrent model of saccadic planning can account for attentional effects without requiring additional or specific mechanisms separate from the circuits that perform sensorimotor transformations for eye movements. The model builds on the basis function approach and includes a circuit that performs spatial remapping using an “internal forward model” of how visual inputs are modified as a result of saccadic movements. Simulations show that the latter circuit is crucial to account for dissociations between attention and eye movements that may be invoked to disprove the premotor theory. The model provides new insights into how spatial remapping may be implemented in parietal cortex and offers a computational framework for recent proposals that link visual stability with remapping of attention pointers.

Frontal eye field microstimulation induces task-dependent gamma oscillations in the lateral intraparietal area

Macaque frontal eye fields (FEF) and the lateral intraparietal area (LIP) are high-level oculomotor control centers that have been implicated in the allocation of spatial attention. Electrical microstimulation of macaque FEF elicits functional magnetic resonance imaging (fMRI) activations in area LIP, but no study has yet investigated the effect of FEF microstimulation on LIP at the single-cell or local field potential (LFP) level. We recorded spiking and LFP activity in area LIP during weak, subthreshold microstimulation of the FEF in a delayed-saccade task. FEF microstimulation caused a highly time- and frequency-specific, task-dependent increase in gamma power in retinotopically corresponding sites in LIP: FEF microstimulation produced a significant increase in LIP gamma power when a saccade target appeared and remained present in the LIP receptive field (RF), whereas less specific increases in alpha power were evoked by FEF microstimulation for saccades directed away from the RF. Stimulating FEF with weak currents had no effect on LIP spike rates or on the gamma power during memory saccades or passive fixation. These results provide the first evidence for task-dependent modulations of LFPs in LIP caused by top-down stimulation of FEF. Since the allocation and disengagement of spatial attention in visual cortex have been associated with increases in gamma and alpha power, respectively, the effects of FEF microstimulation on LIP are consistent with the known effects of spatial attention.

Hand-foot motor priming in the presence of temporary inability to use hands

To verify if the link between observed hand actions and executed foot actions found in aplasics is essentially induced by the constant use of foot substituting the hand, we investigated if the vision of a grasping hand is able to prime a foot response in normals. Participants were required to detect the time-to-contact of a hand grasping an object either with a suitable or a less suitable movement, an experimental paradigm known to induce a priming effect. Participants responded either with the hand or the foot, while having free or bound hands. Results showed that for hand responses motor priming effect was stronger when the hands were free, whereas for foot responses it was stronger when the hands were bound. These data are interpreted as a further evidence that a difficulty to move affects specific cognitive functions and that the vision of a grasping hand may prime a foot response.

Toward a unified theory of visual area V4

Visual area V4 is a midtier cortical area in the ventral visual pathway. It is crucial for visual object recognition and has been a focus of many studies on visual attention. However, there is no unifying view of V4's role in visual processing. Neither is there an understanding of how its role in feature processing interfaces with its role in visual attention. This review captures our current knowledge of V4, largely derived from electrophysiological and imaging studies in the macaque monkey. Based on recent discovery of functionally specific domains in V4, we propose that the unifying function of V4 circuitry is to enable selective extraction of specific functional domain-based networks, whether it be by bottom-up specification of object features or by top-down attentionally driven selection.

Cell-type-specific synchronization of neural activity in FEF with V4 during attention

Shifts of gaze and shifts of attention are closely linked and it is debated whether they result from the same neural mechanisms. Both processes involve the frontal eye fields (FEF), an area which is also a source of top-down feedback to area V4 during covert attention. To test the relative contributions of oculomotor and attention-related FEF signals to such feedback, we recorded simultaneously from both areas in a covert attention task and in a saccade task. In the attention task, only visual and visuomovement FEF neurons showed enhanced responses, whereas movement cells were unchanged. Importantly, visual, but not movement or visuomovement cells, showed enhanced gamma frequency synchronization with activity in V4 during attention. Within FEF, beta synchronization was increased for movement cells during attention but was suppressed in the saccade task. These findings support the idea that the attentional modulation of visual processing is not mediated by movement neurons.

The role of the pulvinar in distractor processing and visual search

The pulvinar nuclei of the thalamus are hypothesized to coordinate attentional selection in the visual cortex. Different models have, however, been proposed for the precise role of the pulvinar in attention. One proposal is that the pulvinar mediates shifts of spatial attention; a different proposal is that it serves the filtering of distractor information. At present, the relation between these possible operations and their relative importance in the pulvinar remains unresolved. We address this issue by contrasting these proposals in two fMRI experiments. We used a visual search paradigm that permitted us to dissociate neural activity reflecting shifts of attention from activity underlying distractor filtering. We find that distractor filtering, but not the operation of shifting attention, is associated with strong activity enhancements in dorsal and ventral regions of the pulvinar as well as in early visual cortex areas including the primary visual cortex. Our observations indicate that distractor filtering is the preponderant attentional operation subserved by the pulvinar, presumably mediated by a modulation of processing in visual areas where spatial resolution is sufficiently high to separate target from distractor input.

Evidence for a link between the experiential allocation of saccade preparation and visuospatial attention

Whether a link exists between the two orienting processes of saccade preparation and visuospatial attention has typically been studied by using either sensory cues or predetermined rules that instruct subjects where to allocate these limited resources. In the real world, explicit instructions are not always available and presumably expectations shaped by previous experience play an important role in the allocation of these processes. Here we examined whether manipulating two experiential factors that clearly influence saccade preparation--the probability and timing of saccadic responses--also influences the allocation of visuospatial attention. Occasionally, a visual probe was presented whose spatial location and time of presentation varied relative to those of the saccade target. The proportion of erroneous saccades directed toward this probe indexed saccade preparation, and the proportion of correct discriminations of probe orientation indexed visuospatial attention. Overall, preparation and attention were significantly correlated to each other across these manipulations of saccade probability and timing. Saccade probability influenced both preparation and attention processes, whereas saccade timing influenced only preparation processes. Unexpectedly, discrimination ability was not improved in those trials in which the probe triggered an erroneous saccade despite particularly heightened levels of saccade preparation. To account for our results, we propose a conceptual dual-purpose threshold model based on neurophysiological considerations that link the processes of saccade preparation and visuospatial attention. The threshold acts both as the minimum activity level required for eliciting saccades and a maximum level for which neural activity can provide attentional benefits.

Dynamic activation of frontal, parietal, and sensory regions underlying anticipatory visual spatial attention

Although it is well established that multiple frontal, parietal, and occipital regions in humans are involved in anticipatory deployment of visual spatial attention, less is known about the electrophysiological signals in each region across multiple subsecond periods of attentional deployment. We used MEG measures of cortical stimulus-locked, signal-averaged (event-related field) activity during a task in which a symbolic cue directed covert attention to the relevant location on each trial. Direction-specific attention effects occurred in different cortical regions for each of multiple time periods during the delay between the cue and imperative stimulus. A sequence of activation from V1/V2 to extrastriate, parietal, and frontal regions occurred within 110 ms after cue, possibly related to extraction of cue meaning. Direction-specific activations ∼300 ms after cue in frontal eye field (FEF), lateral intraparietal area (LIP), and cuneus support early covert targeting of the cued location. This was followed by coactivation of a frontal-parietal system [superior frontal gyrus (SFG), middle frontal gyrus (MFG), LIP, anterior intraparietal sulcus (IPSa)] that may coordinate the transition from targeting the cued location to sustained deployment of attention to both space and feature in the last period. The last period involved direction-specific activity in parietal regions and both dorsal and ventral sensory regions [LIP, IPSa, ventral IPS, lateral occipital region, and fusiform gyrus], which was accompanied by activation that was not direction specific in right hemisphere frontal regions (FEF, SFG, MFG). Behavioral performance corresponded with the magnitude of attention-related activity in different brain regions at each time period during deployment. The results add to the emerging electrophysiological characterization of different cortical networks that operate during anticipatory deployment of visual spatial attention.

Neuro-robotics study on integrative learning of proactive visual attention and motor behaviors

The current paper proposes a novel model for integrative learning of proactive visual attention and sensory-motor control as inspired by the premotor theory of visual attention. The model is characterized by coupling a slow dynamics network with a fast dynamics network and by inheriting our prior proposed multiple timescales recurrent neural networks model (MTRNN) that may correspond to the fronto-parietal networks in the cortical brains. The neuro-robotics experiments in a task of manipulating multiple objects utilizing the proposed model demonstrated that some degrees of generalization in terms of position and object size variation can be achieved by organizing seamless integration of the proactive object-related visual attention and the related sensory-motor control into a set of action primitives in the distributed neural activities appearing in the fast dynamics network. It was also shown that such action primitives can be combined in compositional ways in acquiring novel actions in the slow dynamics network. The experimental results presented substantiate the premotor theory of visual attention.

Hemispheric differences over frontal theta-band power discriminate between stimulus- versus memory-driven saccadic eye movement

Although several electrophysiological studies have demonstrated the role of theta band during the execution of different visuospatial attention tasks, this study is the first to directly investigate the role of theta power during the planning, execution and cognitive control of saccadic eye movements (SEMs). The current study aims at addressing this issue by investigating absolute theta power over the frontal cortex during the execution of random and fixed SEMs. Twelve healthy volunteers, performed two tasks involving different conditions in the planning, execution and cognitive control of SEMs while their brain activity pattern was recorded using quantitative electroencephalography. We found an interaction between SEM condition and electrode (F3, F4, Fz), and a main effect of time point and electrode. Our key finding revealed that the stimulus presentation induces different patterns over frontal theta power increase between the left and right hemisphere. We conclude that right and left frontal regions are an important factor to discriminate between memory- versus stimulus-driven SEMs, and speculate on their different contributions to visuospatial attention.

Common capacity-limited neural mechanisms of selective attention and spatial working memory encoding

One characteristic feature of visual working memory (WM) is its limited capacity, and selective attention has been implicated as limiting factor. A possible reason why attention constrains the number of items that can be encoded into WM is that the two processes share limited neural resources. Functional magnetic resonance imaging (fMRI) studies have indeed demonstrated commonalities between the neural substrates of WM and attention. Here we investigated whether such overlapping activations reflect interacting neural mechanisms that could result in capacity limitations. To independently manipulate the demands on attention and WM encoding within one single task, we combined visual search and delayed discrimination of spatial locations. Participants were presented with a search array and performed easy or difficult visual search in order to encode one, three or five positions of target items into WM. Our fMRI data revealed colocalised activation for attention-demanding visual search and WM encoding in distributed posterior and frontal regions. However, further analysis yielded two patterns of results. Activity in prefrontal regions increased additively with increased demands on WM and attention, indicating regional overlap without functional interaction. Conversely, the WM load-dependent activation in visual, parietal and premotor regions was severely reduced during high attentional demand. We interpret this interaction as indicating the sites of shared capacity-limited neural resources. Our findings point to differential contributions of prefrontal and posterior regions to the common neural mechanisms that support spatial WM encoding and attention, providing new imaging evidence for attention-based models of WM encoding.

Assessing language and visuospatial functions with one task: a "dual use" approach to performing fMRI in children

In order to increase the rate of successful functional MR studies in children it is helpful to shorten the time spent in the scanner. To this effect, assessing two cognitive functions with one task seems to be a promising approach. The hypothesis of this study was that the control condition of an established language task (vowel identification task, VIT) requires visuospatial processing and that the control condition (VIT(CC)) therefore may also be applicable to localize visuospatial functions. As a reference task, a visual search task (VST, previously established for use in children) was employed. To test this hypothesis, 43 children (19 f, 24 m; 12.0±2.6, range 7.9 to 17.8 years) were recruited and scanned using both tasks. Second-level random effects group analyses showed activation of left inferior-frontal cortex in the active condition of the VIT, as in previous studies. Additionally, analysis of the VIT(CC) demonstrated activation in right-dominant superior parietal and high-frontal brain regions, classically associated with visuospatial functions; activation seen in the VST was similar with a substantial overlap. However, lateralization in the parietal lobe was significantly more bilateral in the VST than in the VIT(CC). This suggests that the VIT can not only be applied to assess language functions (using the active>control contrast), but also that the control>active condition is useful for assessing visuospatial functions. Future task design may benefit from such a "dual use" approach to performing fMRI not only, but also particularly in children.

Cortical network for gaze control in humans revealed using multimodal MRI

Functional magnetic resonance imaging (fMRI) techniques allow definition of cortical nodes that are presumed to be components of large-scale distributed brain networks involved in cognitive processes. However, very few investigations examine whether such functionally defined areas are in fact structurally connected. Here, we used combined fMRI and diffusion MRI-based tractography to define the cortical network involved in saccadic eye movement control in humans. The results of this multimodal imaging approach demonstrate white matter pathways connecting the frontal eye fields and supplementary eye fields, consistent with the known connectivity of these regions in macaque monkeys. Importantly, however, these connections appeared to be more prominent in the right hemisphere of humans. In addition, there was evidence of a dorsal frontoparietal pathway connecting the frontal eye field and the inferior parietal lobe, also right hemisphere dominant, consistent with specialization of the right hemisphere for directed attention in humans. These findings demonstrate the utility and potential of using multimodal imaging techniques to define large-scale distributed brain networks, including those that demonstrate known hemispheric asymmetries in humans.

Covert motor activity on NoGo trials in a task sharing paradigm: evidence from the lateralized readiness potential

Previous studies on task sharing propose that a representation of the co-actor's task share is generated when two actors share a common task. An important function of co-representation seems to lie in the anticipation of others' upcoming actions, which is essential for one's own action planning, as it enables the rapid selection of an appropriate response. We utilized measures of lateralized motor activation, the lateralized readiness potential (LRP), in a task sharing paradigm to address the questions (1) whether the generation of a co-representation involves motor activity in the non-acting person when it is other agent's turn to respond, and (2) whether co-representation of the other's task share is generated from one's own egocentric perspective or from the perspective of the actor (allocentric). Results showed that although it was the other agent's turn to respond, the motor system of the non-acting person was activated prior to the other's response. Furthermore, motor activity was based on egocentric spatial properties. The findings support the tight functional coupling between one's own actions and actions produced by others, suggesting that the involvement of the motor system is crucial for social interaction.

Saccadic latency is prolonged in Spinocerebellar Ataxia type 2 and correlates with the frontal-executive dysfunctions

Data on saccadic latency in patients with Spinocerebellar Ataxia 2 (SCA2) are sparse and contradictory. In order to determine whether saccadic latency is definitely prolonged, identify its possible determinants and evaluate it as disease biomarker we assessed the saccadic latency by electronystagmography in 110 SCA2 patients and their paired controls. Mean saccadic latencies were significantly longer in patients when compared to controls for all tested target displacements. Forty-six percent of SCA2 patients had saccadic latencies above the normal range. Reciprobit plots of saccadic latency demonstrated a skewed distribution in the direction of longer latencies for the patients compared to controls. As saccadic latency increased, the velocity and amplitude of saccades significantly decreased in SCA2 subjects but not in controls. Saccadic latency was not influenced by any demographical, clinical or molecular SCA2 variables, but it showed a significant correlation with the performance of the Stroop test, the verbal fluency test and the Wisconsin Card Sorting Test in SCA2 patients. This paper demonstrated that saccadic latency is prolonged in SCA2 patients and it significantly correlates with the performance of frontal-executive functions, thus this parameter could be a useful biomarker to evaluate the efficiency of future therapeutical options on these dysfunctions.

Focused visual attention distorts distance perception away from the attentional locus

Several lines of evidence show that visual perception is altered at the locus of visual attention: detection is faster, performance better and spatial resolution increased. It is however not known whether attention can affect visual perception further away from its locus. In the present study, we specifically question whether and how visual attention influences spatial perception away from its locus, independently from any saccadic preparation. We use a landmark task in which subjects have to estimate the location of a bisection stimulus relative to two landmark stimuli 15° apart, while fixating one of them. This task is combined with a highly demanding discrimination task performed on one of the two landmarks. This allows us to test for the effect of spatial attention allocation on distance perception, as measured by the subject estimation of the landmarks midpoint. We show that the estimated midpoint is displaced towards the attentional locus, both when attention is instructed on the central landmark or on the peripheral landmark. These results suggest an overrepresentation of space around the attentional locus that can affect perception up to 8° away, and question the existence of an objective spatial representation. They are in line with reports of spatial distortion in hemineglect patients while they strikingly contrast with the spatial compression reported around the time of saccadic execution.

Deficit of inhibition motor control in untreated patients with schizophrenia: further support from visually guided saccade paradigms

In addition to classical delusional, negative, and cognitive deficit, schizophrenia has consistently been associated with impairments in saccadic eye movements, e.g., an increased error rate in the antisaccade task. We hypothesized that a deficit in inhibitory control is a core defect in untreated patients with schizophrenia leading to impairment in different oculomotor paradigms. Ten drug-free or drug-naïve patients with schizophrenia were matched in age and gender to 11 healthy controls with no psychoactive substance use or abuse. They were explored using reflexive saccades with unpredictable targets with or without the gap procedure, predictive saccades and a fixation/distracter paradigm. Patients with schizophrenia displayed shorter latency in reflexive and predictive saccades. In the GAP condition, patients made more anticipatory saccades, fewer regular saccades, and had a shorter latency of express saccades than controls. In addition, patients had an increased error rate in the fixation/distracters task. Altogether, these results provide new evidence of reduced prefrontal inhibitory regulation of subcortical and brainstem systems involved in the control of saccades.

Specificity of human parietal saccade and reach regions during transcranial magnetic stimulation

Single-unit recordings in macaque monkeys have identified effector-specific regions in posterior parietal cortex (PPC), but functional neuroimaging in the human has yielded controversial results. Here we used on-line repetitive transcranial magnetic stimulation (rTMS) to determine saccade and reach specificity in human PPC. A short train of three TMS pulses (separated by an interval of 100 ms) was delivered to superior parieto-occipital cortex (SPOC), a region over the midposterior intraparietal sulcus (mIPS), and a site close to caudal IPS situated over the angular gyrus (AG) during a brief memory interval while subjects planned either a saccade or reach with the left or right hand. Behavioral measures then were compared to controls without rTMS. Stimulation of mIPS and AG produced similar patterns: increased end-point variability for reaches and decreased saccade accuracy for contralateral targets. In contrast, stimulation of SPOC deviated reach end points toward visual fixation and had no effect on saccades. Contralateral-limb specificity was highest for AG and lowest for SPOC. Visual feedback of the hand negated rTMS-induced disruptions of the reach plan for mIPS and AG, but not SPOC. These results suggest that human SPOC is specialized for encoding retinally peripheral reach goals, whereas more anterior-lateral regions (mIPS and AG) along the IPS possess overlapping maps for saccade and reach planning and are more closely involved in motor details (i.e., planning the reach vector for a specific hand). This work provides the first causal evidence for functional specificity of these parietal regions in healthy humans.

Prepare for conflict: EEG correlates of the anticipation of target competition during overt and covert shifts of visual attention

When preparing to make a saccadic eye movement in a cued direction, perception of stimuli at the target location is enhanced, just as it is when attention is covertly deployed there. Accordingly, the timing and anatomical sources of preparatory brain activity accompanying shifts of covert attention and saccade preparation tend to exhibit a large degree of overlap. However, there is evidence that preparatory processes are modulated by the foreknowledge of visual distractor competition during covert attention, and it is unknown whether eye movement preparation undergoes equivalent modulation. Here we examine preparatory processes in the electroencephalogram of human participants during four blocked versions of a spatial cueing task, requiring either covert detection or saccade execution, and either containing a distractor or not. As in previous work, a typical pattern of spatially selective occipital, parietal and frontal activity was seen in all task versions. However, whereas distractor presence called on an enhancement of spatially selective visual cortical modulation during covert attention, it instead called on increased activity over frontomedial oculomotor areas in the case of overt saccade preparation. We conclude that, although advance orienting signals may be similar in character during overt and covert conditions, the pattern by which these signals are modulated to ameliorate the behavioral costs of distractor competition is highly distinct, pointing to a degree of separability between the overt and covert systems.

Functionally dissociating temporal and motor components of response preparation in left intraparietal sulcus

To optimise speed and accuracy of motor behaviour, we can prepare not only the type of movement to be made but also the time at which it will be executed. Previous cued reaction-time paradigms have shown that anticipating the moment in time at which this response will be made (“temporal orienting”) or selectively preparing the motor effector with which an imminent response will be made (motor intention or “motor orienting”) recruits similar regions of left intraparietal sulcus (IPS), raising the possibility that these two preparatory processes are inextricably co-activated. We used a factorial design to independently cue motor and temporal components of response preparation within the same experimental paradigm. By differentially cueing either ocular or manual response systems, rather than spatially lateralised responses within just one of these systems, potential spatial confounds were removed. We demonstrated that temporal and motor orienting were behaviourally dissociable, each capable of improving performance alone. Crucially, fMRI data revealed that temporal orienting activated the left IPS even if the motor effector that would be used to execute the response was unpredictable. Moreover, temporal orienting activated left IPS whether the target required a saccadic or manual response, and whether this response was left- or right-sided, thus confirming the ubiquity of left IPS activation for temporal orienting. Finally, a small region of left IPS was also activated by motor orienting for manual, though not saccadic, responses. Despite their functional independence therefore, temporal orienting and manual motor orienting nevertheless engage partially overlapping regions of left IPS, possibly reflecting their shared ontogenetic roots.

Spatially distributed encoding of covert attentional shifts in human thalamus

Spatial attention modulates signal processing within visual nuclei of the thalamus—but do other nuclei govern the locus of attention in top-down mode? We examined functional MRI (fMRI) data from three subjects performing a task requiring covert attention to 1 of 16 positions in a circular array. Target position was cued after stimulus offset, requiring subjects to perform target detection from iconic visual memory. We found positionally specific responses at multiple thalamic sites, with individual voxels activating at more than one direction of attentional shift. Voxel clusters at anatomically equivalent sites across subjects revealed a broad range of directional tuning at each site, with little sign of contralateral bias. By reference to a thalamic atlas, we identified the nuclear correspondence of the four most reliably activated sites across subjects: mediodorsal/central-intralaminar (oculomotor thalamus), caudal intralaminar/parafascicular, suprageniculate/limitans, and medial pulvinar/lateral posterior. Hence, the cortical network generating a top-down control signal for relocating attention acts in concert with a spatially selective thalamic apparatus—the set of active nuclei mirroring the thalamic territory of cortical “eye-field” areas, thus supporting theories which propose the visuomotor origins of covert attentional selection.

Cortical processing of head- and eye-gaze cues guiding joint social attention

Previous fMRI experiments showed an involvement of the STS in the processing of eye-gaze direction in joint attention. Since head-gaze direction can also be used for the assessment of another person's attentional focus, we compared the mechanisms underlying the processing of head- and eye-gaze direction using a combined psychophysical and fMRI approach. Subjects actively followed the head- or eye-gaze direction of a person in a photograph towards one of seven possible targets by moving their eyes. We showed that the right posterior superior temporal sulcus (STS) as well as the right fusiform gyrus (FSG) were involved in both processing of head- as well as eye-gaze direction. Another finding was a bilateral deactivation of a distinct area in the middle STS (mSTS) as well as the left anterior STS (aSTS), that was stronger when subjects followed eye-gaze direction than when they followed head-gaze direction. We assume that this deactivation is based on an active suppression of information arising from the distracting other directional cue, i.e. head-gaze direction in the eye-gaze direction task and eye-gaze direction in the head-gaze direction task. These results further support the hypothesis that the human equivalent of the gaze sensitive area in monkeys lies in more anterior parts of the STS than previously thought.

Hemispheric asymmetries of motor versus nonmotor processes during (visuo)motor control

Language and certain aspects of motor control are typically served by the left hemisphere, whereas visuospatial and attentional control are lateralized to the right. Here a (visuo)motor tracing task was used to identify hemispheric lateralization beyond the general, contralateral organization of the motor system. Functional magnetic resonance imaging (fMRI) was applied in 40 male right-handers (19-30 yrs) during line tracing with dominant and nondominant hand, with and without visual guidance. Results revealed a network of areas activating more in the right than left hemisphere, irrespective of the effector. Inferior portions of frontal gyrus and parietal lobe overlapped largely with a previously described ventral attention network responding to unexpected or behaviourally relevant stimuli. This demonstrates a hitherto unreported functionality of this circuit that also seems to activate when spatial information is continuously exploited to adapt motor behaviour. Second, activation of left dorsal premotor and postcentral regions during tracing with the nondominant left hand was more pronounced than that in their right hemisphere homologues during tracing with the dominant right hand. These activation asymmetries of motor areas ipsilateral to the moving hand could not be explained by asymmetries in skill performance, the degree of handedness, or interhemispheric interactions. The latter was measured by a double-pulse transcranial magnetic stimulation paradigm, whereby a conditioning stimulus was applied over one hemisphere and a test stimulus over the other. We propose that the left premotor areas contain action representations strongly related to movement implementation which are also accessed during movements performed with the left body side.

Nicotine modulation of information processing is not limited to input (attention) but extends to output (intention)

RATIONALE

Nicotine influences many cognitive processes, especially those requiring high attentional loads, yet the impact of nicotine on all aspects of information processing has not been well delineated.

OBJECTIVE

The aim of the study was to determine the relative behavioral and functional effects of nicotine on dissociable aspects of information processing (i.e., selective attention and motor intention).

METHODS

Adult smokers (N = 25) and healthy controls (N = 23) performed the intention/attention task (IAT) twice, during functional magnetic resonance imaging. The IAT assesses the relative differences in performance evoked by prime stimuli that provide information regarding either the correct hand with which to respond (i.e., intentional primes) or the likely location of a target stimulus (i.e., attentional primes). Smokers were scanned 2 h after nicotine (21 mg) or placebo patch placement. The order of nicotine and placebo sessions was randomized and counter-balanced. Controls were also scanned twice, with no patch placement in either session.

RESULTS

While drug condition had no significant effect on reaction time, smokers were overall more accurate than controls. Moreover, nicotine significantly increased the response to intentional primes in brain regions known to mediate response readiness, e.g., inferior parietal lobe, supramarginal gyrus, and striatum.

CONCLUSIONS

While limited to participant accuracy, these data suggest that the behavioral effects of nicotine in smokers are not only limited to information processing input (i.e., selective attention) but are also generalizable to output functions (i.e., motor intention). Moreover, nicotine's effects on intention appear to be mediated by a facilitation of function in brain regions associated with information processing output.