Control of goal-directed and stimulus-driven attention in the brain

Reversal frequency in Caenorhabditis elegans represents an integrated response to the state of the animal and its environment

The locomotion of Caenorhabditis elegans consists of forward crawling punctuated by spontaneous reversals. To better understand the important variables that affect locomotion, we have described in detail the locomotory behavior of C. elegans and identified a set of parameters that are sufficient to describe the animal's trajectory. A model of locomotion based on these parameters indicates that reversal frequency plays a central role in locomotion. We found that several variables such as humidity, gravidity, and mechanostimulation influence reversal frequency. Specifically, both gentle and harsh touch can transiently suppress reversal frequency. Thus, reversal behavior is a model for the integration of information from numerous modalities reflecting diverse aspects of the state of an organism.

Visual attention: the where, what, how and why of saliency

Attention influences the processing of visual information even in the earliest areas of primate visual cortex. There is converging evidence that the interaction of bottom-up sensory information and top-down attentional influences creates an integrated saliency map, that is, a topographic representation of relative stimulus strength and behavioral relevance across visual space. This map appears to be distributed across areas of the visual cortex, and is closely linked to the oculomotor system that controls eye movements and orients the gaze to locations in the visual scene characterized by a high salience.

Spatial updating in human parietal cortex

Single neurons in monkey parietal cortex update visual information in conjunction with eye movements. This remapping of stimulus representations is thought to contribute to spatial constancy. We hypothesized that a similar process occurs in human parietal cortex and that we could visualize it with functional MRI. We scanned subjects during a task that involved remapping of visual signals across hemifields. We observed an initial response in the hemisphere contralateral to the visual stimulus, followed by a remapped response in the hemisphere ipsilateral to the stimulus. We ruled out the possibility that this remapped response resulted from either eye movements or visual stimuli alone. Our results demonstrate that updating of visual information occurs in human parietal cortex.

Functional organization of human intraparietal and frontal cortex for attending, looking, and pointing

We studied the functional organization of human posterior parietal and frontal cortex using functional magnetic resonance imaging (fMRI) to map preparatory signals for attending, looking, and pointing to a peripheral visual location. The human frontal eye field and two separate regions in the intraparietal sulcus were similarly recruited in all conditions, suggesting an attentional role that generalizes across response effectors. However, the preparation of a pointing movement selectively activated a different group of regions, suggesting a stronger role in motor planning. These regions were lateralized to the left hemisphere, activated by preparation of movements of either hand, and included the inferior and superior parietal lobule, precuneus, and posterior superior temporal sulcus, plus the dorsal premotor and anterior cingulate cortex anteriorly. Surface-based registration of macaque cortical areas onto the map of fMRI responses suggests a relatively good spatial correspondence between human and macaque parietal areas. In contrast, large interspecies differences were noted in the topography of frontal areas.

Identification of cerebral networks by classification of the shape of BOLD responses

Changes in regional blood oxygen level dependent (BOLD) signals in response to brief visual stimuli can exhibit a variety of time-courses. To demonstrate the anatomical distribution of BOLD response shapes during a match to sample task, a formal analysis of their time-courses is presented. An event-related design was used to estimate regional BOLD responses evoked by a cue word, which instructed the subject to attend to the motion or color of an upcoming target, and those evoked by a briefly presented moving target consisting of colored dots. Regional BOLD time-courses were adequately represented by the linear combination of three orthogonal waveforms. BOLD response shapes were then classified using a fuzzy clustering scheme. Three classes (sustained, phasic, and negative) best characterized cue responses. Four classes (sustained, sustained-phasic, phasic, and bi-phasic) best characterized target responses. In certain regions, the shape of the BOLD responses was modulated by the instruction to attend to the target's motion or color. A left frontal and a posterior parietal region showed sustained activity when motion was cued and transient activity when color was cued. A right thalamic and a left lateral occipital region showed sustained activity when color was cued and transient activity when motion was cued. Following the target several regions showed more sustained activity during motion than color trials. In summary, the effect of the task variable was focal following the cue and widespread following the target. We conclude that the temporal patterns of neural activity affected the shape of the BOLD signal.

Neural mechanisms of top-down control during spatial and feature attention

Theories of visual selective attention posit that both spatial location and nonspatial stimulus features (e.g., color) are elementary dimensions on which top-down attentional control mechanisms can selectively influence visual processing. Neuropsychological and neuroimaging studies have demonstrated that regions of superior frontal and parietal cortex are critically involved in the control of visual-spatial attention. This frontoparietal control network has also been found to be activated when attention is oriented to nonspatial stimulus features (e.g., motion). To test the generality of the frontoparietal network in attentional control, we directly compared spatial and nonspatial attention in a cuing paradigm. Event-related fMRI methods permitted the isolation of attentional control activity during orienting to a location or to a nonspatial stimulus feature (color). Portions of the frontoparietal network were commonly activated to the spatial and nonspatial cues. However, direct statistical comparisons of cue-related activity revealed subregions of the frontoparietal network that were significantly more active during spatial than nonspatial orienting when all other stimulus, task, and attentional factors were equated. No regions of the frontal-parietal network were more active for nonspatial cues in comparison to spatial cues. These findings support models suggesting that subregions of the frontal-parietal network are highly specific for controlling spatial selective attention.

Representation of change: separate electrophysiological markers of attention, awareness, and implicit processing

Awareness of change within a visual scene only occurs in the presence of focused attention. When two versions of a complex scene are presented in alternating sequence separated by a blank mask, unattended changes usually remain undetected, although they may be represented implicitly. To test whether awareness of change and focused attention had the same or separable neurophysiological substrates, and to search for the neural substrates of implicit representation of change, we recorded event-related brain potentials (ERPs) during a change blindness task. Relative to active search, focusing attention in the absence of a change enhanced an ERP component over frontal sites around 100-300 msec after stimulus onset, and in posterior sites at the 150-300 msec window. Focusing attention to the location of a change that subjects were aware of, replicated those attentional effects, but also produced a unique positive deflection in the 350-600 msec window, broadly distributed with its epicenter in mediocentral areas. The unique topography and time course of this latter modulation, together with its dependence on the aware perception of change, distinguishes this "awareness of change" electrophysiological response from the electrophysiological effects of focused attention. Finally, implicit representation of change elicited a distinct electrophysiological event: Unaware changes triggered a positive deflection at the 240-300 msec window, relative to trials with no change. Overall, the present data suggest that attention, awareness of change, and implicit representation of change may be mediated by separate underlying systems.

A cortical mechanism for triggering top-down facilitation in visual object recognition

The majority of the research related to visual recognition has so far focused on bottom-up analysis, where the input is processed in a cascade of cortical regions that analyze increasingly complex information. Gradually more studies emphasize the role of top-down facilitation in cortical analysis, but it remains something of a mystery how such processing would be initiated. After all, top-down facilitation implies that high-level information is activated earlier than some relevant lower-level information. Building on previous studies, I propose a specific mechanism for the activation of top-down facilitation during visual object recognition. The gist of this hypothesis is that a partially analyzed version of the input image (i.e., a blurred image) is projected rapidly from early visual areas directly to the prefrontal cortex (PFC). This coarse representation activates in the PFC expectations about the most likely interpretations of the input image, which are then back-projected as an "initial guess" to the temporal cortex to be integrated with the bottom-up analysis. The top-down process facilitates recognition by substantially limiting the number of object representations that need to be considered. Furthermore, such a rapid mechanism may provide critical information when a quick response is necessary.

Effects of same- and different-modality cues in a Posner task: extinction-type, spatial, and non-spatial deficits after right-hemispheric stroke

The response delay to left target stimuli preceded by right-side cues, first described by Posner et al. [J. Neurosci. 4 (1984) 1863-1874] appears to be a stable marker of right-parietal injury. However, only few studies compared patients' performance to age-matched controls. Furthermore, only few studies compared visual and auditory stimuli in this task. Therefore, two groups of right-hemisphere stroke patients, with and without left visual hemineglect, and a healthy control group were studied in three versions of Posner's paradigm. Visual or auditory target stimuli were presented to the subject's left or right, following a visual or auditory cue by 150 ms. The classical 'extinction-type' effect, an increase in missing responses for right visual cue/left visual target, was specifically observed in neglect patients. In the same condition, an 'extinction-type' response delay was present in patients with neglect and in those without neglect. No such delay occurred in any group when cues were auditory. Specifically in neglect patients, response times were generally longer for left than for right visual targets, regardless of cue side and of cue modality. Response times were generally prolonged in neglect patients regardless of target modality. This suggests that three components impair neglect patients' performance in this paradigm: a non-spatial, supramodal deficit, a global, neglect-type deficit of the contralesional hemi-field, and the extinction-type impairment. The latter two deficits appear to be most marked within the visual domain.

Top-down attentional processing enhances auditory evoked gamma band activity

In contrast to animal studies, relatively little is known about the functional significance of the early evoked gamma band activity in humans. We investigated whether evoked and induced 40 Hz activity differentiate automatic, bottom-up aspects of attention from voluntary, top-down related attentional demands. An auditory novelty-oddball task was applied to 14 healthy subjects. As predicted, more evoked gamma was found for the target condition than in the two task-irrelevant conditions. Since gamma band activity was not enhanced for novel stimuli, the evoked gamma response cannot be explained with a simple concept of stimulus arousal. Neither induced gamma nor the degree of 40 Hz phase-locking were different between the experimental conditions. Taken together, our data emphasize the role of evoked gamma band activity for top-down attentional processing.

Cortical mechanisms of space-based and object-based attentional control

Visual attention, the mechanism by which observers select relevant or important information from scenes, can be deployed to locations in space or to spatially invariant object representations. Studies have examined both the modulatory effects of attention on the strength of extrastriate cortical representations, and the control of attention by parietal and frontal cortical circuits. Subregions of parietal and frontal cortex are transiently active when attention is voluntarily shifted between spatial locations or object representations. This transient activity may reflect an abrupt shift in the attentional set of the observer, complementing sustained signals that are thought to maintain a given attentive state.

Neural coding of behavioral relevance in parietal cortex

Flexible control of behavior requires the selective processing of task-relevant sensory information and the appropriate linkage of sensory input to action. A great deal of evidence suggests a central role for the parietal cortex in these functions. Recent results from neurophysiological studies in non-human primates and neuroimaging experiments in humans illuminate the importance of parietal cortex for attention, and suggest how parietal neurons might allow the dynamic representation of behaviorally relevant information.

Graspable objects grab attention when the potential for action is recognized

Visually guided grasping movements require a rapid transformation of visual representations into object-specific motor programs. Here we report that graspable objects may facilitate these visuomotor transformations by automatically grabbing visual spatial attention. Human subjects viewed two task-irrelevant objects--one was a 'tool', the other a 'non-tool'--while waiting for a target to be presented in one of the two object locations. Using event-related potentials (ERPs), we found that spatial attention was systematically drawn to tools in the right and lower visual fields, the hemifields that are dominant for visuomotor processing. Using event-related fMRI, we confirmed that tools grabbed spatial attention only when they also activated dorsal regions of premotor and prefrontal cortices, regions associated with visually guided actions and their planning. Although it is widely accepted that visual sensory gain aids perception, our results suggest that it may also have consequences for object-directed actions.

Attentional set for external information activates the right intraparietal area

Visual attention can be allocated to a location or an object by using two different types of information: internal information and external information. The results of recent psychological studies [Bagon and Egeth, Percept. Psychophys. 55 (1994) 485] suggest that an observer's attentional set determines how these two kinds of information are used in visual tasks. In this study, we measured brain activities during two modes of visual search; one is the feature search mode, in which an attentional set for knowledge of a target item (internal information) is used, and the other is the singleton detection mode, in which an attentional set for oddness in the visual scene (external information) is used. We found extended activation in the frontal and parietal areas for both search modes. In addition, a direct comparison of brain activity during the singleton detection mode and the feature search mode revealed that the areas around the right intraparietal sulcus were more involved in the attentional set for oddness. These results suggest that the human right intraparietal cortex is related to the attentional set for external information.

Spatial cognition: evidence from visual neglect

Recent work on human attention and representational systems has benefited from a growing interplay between research on normal attention and neuropsychological disorders such as visual neglect. Research over the past 30 years has convincingly shown that, far from being a unitary condition, neglect is a protean disorder whose symptoms can selectively affect different sensory modalities, cognitive processes, spatial domains and coordinate systems. These clinical findings, together with those of functional neuroimaging, have increased knowledge about the anatomical and functional architecture of normal subsystems involved in spatial cognition. We provide a selective overview of how recent investigations of visual neglect are beginning to elucidate the underlying structure of spatial processes and mental representations.

Preparatory states in crossmodal spatial attention: spatial specificity and possible control mechanisms

We used event-related functional magnetic resonance imaging to study the neural correlates of endogenous spatial attention for vision and touch. We examined activity associated with attention-directing cues (central auditory pure tones), symbolically instructing subjects to attend to one hemifield or the other prior to upcoming stimuli, for a visual or tactile task. In different sessions, subjects discriminated either visual or tactile stimuli at the covertly attended side, during bilateral visuotactile stimulation. To distinguish cue-related preparatory activity from any modulation of stimulus processing, unpredictably on some trials only the auditory cue was presented. The use of attend-vision and attend-touch blocks revealed whether preparatory attentional effects were modality-specific or multimodal. Unimodal effects of spatial attention were found in somatosensory cortex for attention to touch, and in occipital areas for attention to vision, both contralateral to the attended side. Multimodal spatial effects (i.e. effects of attended side irrespective of task-relevant modality) were detected in contralateral intraparietal sulcus, traditionally considered a multimodal brain region; and also in the middle occipital gyrus, an area traditionally considered purely visual. Critically, all these activations were observed even on cue-only trials, when no visual or tactile stimuli were subsequently presented. Endogenous shifts of spatial attention result in changes of brain activity prior to the presentation of target stimulation (baseline shifts). Here, we show for the first time the separable multimodal and unimodal components of such preparatory activations. Additionally, irrespective of the attended side and modality, attention-directing auditory cues activated a network of superior frontal and parietal association areas that may play a role in voluntary control of spatial attention for both vision and touch.

Neural mechanisms for detecting and remembering novel events

The ability to detect and respond to novel events is crucial for survival in a rapidly changing environment. Four decades of neuroscientific research has begun to delineate the neural mechanisms by which the brain detects and responds to novelty. Here, we review this research and suggest how changes in neural processing at the cellular, synaptic and network levels allow us to detect, attend to and subsequently remember the occurrence of a novel event.

Frontal and parietal components of a cerebral network mediating voluntary attention to novel events

Despite the important role that attending to novel events plays in human behavior, there is limited information about the neuroanatomical underpinnings of this vital activity. This study investigated the relative contributions of the frontal and posterior parietal lobes to the differential processing of novel and target stimuli under an experimental condition in which subjects actively directed attention to novel events. Event-related potentials were recorded from well-matched frontal patients, parietal patients, and non-brain-injured subjects who controlled their viewing duration (by button press) of line drawings that included a frequent, repetitive background stimulus, an infrequent target stimulus, and infrequent, novel visual stimuli. Subjects also responded to target stimuli by pressing a foot pedal. Damage to the frontal cortex resulted in a much greater disruption of response to novel stimuli than to designated targets. Frontal patients exhibited a widely distributed, profound reduction of the novelty P3 response and a marked diminution of the viewing duration of novel events. In contrast, damage to posterior parietal lobes was associated with a substantial reduction of both target P3 and novelty P3 amplitude; however, there was less disruption of the processing of novel than of target stimuli. We conclude that two nodes of the neuroanatomical network for responding to and processing novelty are the prefrontal and posterior parietal regions, which participate in the voluntary allocation of attention to novel events. Injury to this network is indexed by reduced novelty P3 amplitude, which is tightly associated with diminished attention to novel stimuli. The prefrontal cortex may serve as the central node in determining the allocation of attentional resources to novel events, whereas the posterior parietal lobe may provide the neural substrate for the dynamic process of updating one's internal model of the environment to take into account a novel event.

Shifts of attention in light and in darkness: an ERP study of supramodal attentional control and crossmodal links in spatial attention

Crossmodal links in spatial attention, uncovered by recent behavioural and electrophysiological studies, have been interpreted as evidence for supramodal processes controlling shifts of attention. However, previous experiments have usually been conducted in illuminated environments. Continuously available visuo-spatial information might result in shifts of attention being primarily guided by visible information, even when another modality is task-relevant. The present ERP study evaluated this. A symbolic auditory cue directed attention to the left or right hand. Participants had to detect infrequent tactile targets delivered to the cued hand, while ignoring any visual stimuli. Stimuli were presented either in a lit environment or in darkness. Although continuous ambient visuo-spatial information was eliminated in the latter condition, processing of task-irrelevant visual events was still modulated by spatial attention for the tactile task. Moreover, ERP correlates of attentional shifts in the cue-target interval were similar for both illumination conditions. This was further confirmed in a follow-up experiment where the darkness condition was repeated without any peripheral visual stimulation ever occurring. These findings demonstrate that the ERP correlates of crossmodal attention (both preparatory effects in the cue-target interval, and also modulations of stimulus-evoked components) do not depend on selection being guided by ambient visible information in a lit environment. They suggest instead that spatial shifts of attention are controlled supramodally.

Preparatory set associated with pro-saccades and anti-saccades in humans investigated with event-related FMRI

Previous studies have shown that the BOLD functional MRI (fMRI) signal is increased in several cortical areas when subjects perform anti-saccades compared with pro-saccades. It remains unknown, however, whether this increase is due to an increased cortical motor signal for anti-saccades or due to differences in preparatory set between pro- and anti-saccade trials. To address this question, we measured event-related fMRI in a paradigm that allowed us to separate instruction-related brain activity from saccade-related brain activity. In this paradigm, the instruction to either generate a pro-saccade or an anti-saccade was conveyed by a switch in the color of the central fixation stimulus and preceded the presentation of a peripheral stimulus by either 6, 10, or 14 s. Cortical areas were functionally mapped using the general linear model comparing standard pro- and anti-saccade blocks with fixation blocks. When the trials were aligned on the onset of the instruction stimulus, bilateral frontal eye fields and right hemisphere dorsolateral prefrontal cortex showed an increased signal during the instruction period on anti-saccade trials as compared with pro-saccade trials. When the trials were aligned on the movement stimulus and the instruction period activity was subtracted, there were no differences between pro- and anti-saccades. This finding suggests that the increased cortical activation found in previous blocked designs originates predominately from differences in preparatory set and not from differences in the motor signal between pro- and anti-saccades.

Visual and cognitive control of attention in smooth pursuit

In this chapter, we describe the role of attention in the control of smooth pursuit eye movements. As a voluntary and continuous eye movement, smooth pursuit is driven by both visual and cognitive signals. Here we show that whereas the entire process of smooth pursuit requires visual attention, the post-onset phase of the initiation and the maintenance smooth pursuit are under an additional sustained non-visual cognitive attention control. The temporal dynamics of these complementary controls of visual and non-visual cognitive attention support the continuous generation of smooth pursuit so that eye tracking of a moving target can be prompt and accurate.

Prefrontal interactions reflect future task operations

When task instructions are given, the human brain establishes a task set before the task is actually performed. By introducing a delay between the instruction and the task, we have identified the neural correlates of task sets using functional magnetic resonance imaging (fMRI). Subjects were instructed to remember a sequence of positions or letters, either in the order presented or in the reverse order. Spatial or verbal processing areas were active during the delay, depending on whether positions or letters were to be remembered, whereas the anterior region of the prefrontal cortex (PFC) was active regardless of the domain of the items. Furthermore, the nature of the interaction between the anterior PFC and the domain-specific posterior prefrontal areas (superior frontal sulcus and left inferior frontal gyrus) depended on whether the items were to be remembered in the forward or backward order. Thus we have identified inter-regional interactions that reflect preparation for task performance.

Functional reorganization and stability of somatosensory-motor cortical topography in a tetraplegic subject with late recovery

The functional organization of somatosensory and motor cortex was investigated in an individual with a high cervical spinal cord injury, a 5-year absence of nearly all sensorymotor function at and below the shoulders, and rare recovery of some function in years 6-8 after intense and sustained rehabilitation therapies. We used functional magnetic resonance imaging to study brain activity to vibratory stimulation and voluntary movements of body parts above and below the lesion. No response to vibratory stimulation of the hand was observed in the primary somatosensory cortex (SI) hand area, which was conversely recruited during tongue movements that normally evoke responses only in the more lateral face area. This result suggests SI reorganization analogous to previously reported neuroplasticity changes after peripheral lesions in animals and humans. In striking contradistinction, vibratory stimulation of the foot evoked topographically appropriate responses in SI and second somatosensory cortex (SII). Motor cortex responses, tied to a visuomotor tracking task, displayed a near-typical topography, although they were more widespread in premotor regions. These findings suggest possible preservation of motor and some somatosensory cortical representations in the absence of overt movements or conscious sensations for several years after spinal cord injury and have implications for future rehabilitation and neural-repair therapies.

Visual detection is gated by attending for action: evidence from hemispatial neglect

We report observations in patients with visual extinction demonstrating that detection of visual events is gated by attention at the level of processing at which a stimulus is selected for action. In one experiment, three patients reported the identity of numerical words and digits presented either in the ipsilesional field, the contralesional field, or both fields. On the critical bilateral trials, extinction was greater when the competing items shared the same meaning and response, regardless of whether the items were visually different (e.g., ONE + 1), or identical (e.g., 1 + 1). A fourth patient was tested in a second experiment in which the competing items on bilateral trials were either different (e.g., ONE + TWO), identical (e.g., ONE + ONE) or homophones that were visually and semantically different but shared the same response (e.g., ONE + WON). Homophones and identical items caused similar extinction with less extinction occurring on different item trials.

Attentional control of the processing of neural and emotional stimuli

A typical scene contains many different objects that compete for neural representation due to the limited processing capacity of the visual system. At the neural level, competition among multiple stimuli is evidenced by the mutual suppression of their visually evoked responses and occurs most strongly at the level of the receptive field. The competition among multiple objects can be biased by both bottom-up sensory-driven mechanisms and top-down influences, such as selective attention. Functional brain imaging studies reveal that biasing signals due to selective attention can modulate neural activity in visual cortex not only in the presence but also in the absence of visual stimulation. Although the competition among stimuli for representation is ultimately resolved within visual cortex, the source of top-down biasing signals likely derives from a distributed network of areas in frontal and parietal cortex. Competition suggests that once attentional resources are depleted, no further processing is possible. Yet, existing data suggest that emotional stimuli activate brain regions "automatically," largely immune from attentional control. We tested the alternative possibility, namely, that the neural processing of stimuli with emotional content is not automatic and instead requires some degree of attention. Our results revealed that, contrary to the prevailing view, all brain regions responding differentially to emotional faces, including the amygdala, did so only when sufficient attentional resources were available to process the faces. Thus, similar to the processing of other stimulus categories, the processing of facial expression is under top-down control.

The temporal dynamics of the effects in occipital cortex of visual-spatial selective attention

The temporal dynamics of the effects of lateralized visual selective attention within the lower visual field were studied with the combined application of event-related potentials (ERPs) and positron emission tomography (15O PET). Bilateral stimuli were rapidly presented to the lower visual field while subjects either passively viewed them or covertly attended to a designated side to detect occasional targets. Lateralized attention resulted in strongly enhanced PET activity in contralateral dorsal occipital cortex, while ERPs showed an enhanced positivity (P1 effect, 80-160 ms) for all stimuli (both non-targets and targets) over contralateral occipital scalp. Dipole modeling seeded by the dorsal occipital PET foci yielded an excellent fit for the peak P1 attention effect. However, more detailed ERP modeling throughout the P1 latency window (90-160 ms) suggested a spatial-temporal movement of the attention-related enhancement that roughly paralleled the shape of the dorsal occipital PET attention-related activations-likely reflecting the sequential attention-related enhancement of early visual cortical areas. Lateralized spatial attention also resulted in a longer-latency contralateral enhanced negativity (N2 effect, 230-280 ms) with a highly similar distribution to the earlier P1 effect. Dipole modeling seeded by the same dorsal occipital PET foci also yielded an excellent fit. This pattern of results provides evidence for re-entrance of attention-enhanced activation to the same retinotopically organized region of dorsal extrastriate cortex. Finally, target stimuli in the attended location elicited an additional prolonged enhancement of the longer-latency negativity over contralateral occipital cortex. The combination of PET activation and dipole modeling suggested contribution from a ventral-occipital generator to this target-related activity.

The neuropsychological profile of early and continuously treated phenylketonuria: orienting, vigilance, and maintenance versus manipulation-functions of working memory

In this paper, we review neuropsychological test results of early and continuously treated Phenylketonuria (PKU) patients. To increase insight into the neuropsychological profile of this population, we have attempted to place the results within an attentional network model [Images of the mind, 1994], which proposes interacting but dissociable attentional networks for orienting, vigilance, and executive control of attention. Executive control of attention is discussed against the background of the process-specific theory of working memory (WM) [Handbook of neuropsychology, 1994], which postulates a distinction between the 'maintenance'-function of WM and the 'manipulation and monitoring'-function. Neuropsychological results are presented for 67 early and continuously treated PKU patients and 73 controls aged 7-14 years. Four neuropsychological tasks were employed to measure orienting, mnemonic processing, interference suppression, and top-down control in visual search. No differences were found in orienting and the maintenance-function of WM. In addition to previously reported impairments in sustained attention/vigilance and inhibition of prepotent responding, PKU patients exhibited deficits when top-down control was required in a visual search task, but showed no impairment when interference suppression was required. It is discussed how the specific neuropsychological impairments in PKU may be a consequence of mid-dorsolateral prefrontal cortex (DLPFC) dysfunctioning due to deficiencies in catecholamine modulation.

Transient neural activity in human parietal cortex during spatial attention shifts

Observers viewing a complex visual scene selectively attend to relevant locations or objects and ignore irrelevant ones. Selective attention to an object enhances its neural representation in extrastriate cortex, compared with those of unattended objects, via top-down attentional control signals. The posterior parietal cortex is centrally involved in this control of spatial attention. We examined brain activity during attention shifts using rapid, event-related fMRI of human observers as they covertly shifted attention between two peripheral spatial locations. Activation in extrastriate cortex increased after a shift of attention to the contralateral visual field and remained high during sustained contralateral attention. The time course of activity was substantially different in posterior parietal cortex, where transient increases in activation accompanied shifts of attention in either direction. This result suggests that activation of the parietal cortex is associated with a discrete signal to shift spatial attention, and is not the source of a signal to continuously maintain the current attentive state.