Independence of visual awareness from the scope of attention: an electrophysiological study

Recent brain imaging studies have revealed that increased neuralactivity along the ventral visual stream and parietal and frontalareas is associated with visual awareness. In order to studythe time-course and temporal aspects of awareness, we examinedelectrophysiological correlates of conscious vision in two maskingexperiments. The differences in event-related potentials (ERPs)between unmasked (consciously recognized) and masked (unrecognized)stimuli were considered to be electrophysiological correlatesof awareness. Two attentional conditions (global, local) wereincluded to examine the relationship between the scope of attentionand awareness. Two ERP-deflections were found to correlate withawareness. First, awareness was associated with a posteriornegative amplitude shift 130–320 ms after the stimulus.This effect was present in both attention conditions, suggestingthat it emerges independent of the scope of attention. Second,ERPs to unmasked stimuli became more positive as compared withmasked stimuli around 400 ms, peaking at parietal sites. Thiseffect was attenuated in the local attention condition, althoughthe participants were aware of the stimuli, suggesting thatthe late positivity does not directly correlate with visualawareness. The results imply that the earlier negativity isthe earliest and most direct correlate of visual awareness.     

Control of object-based attention in human cortex

Visual attention is a mechanism by which observers select relevant or important information from the current visual array. Previous investigations have focused primarily on the ability to select a region of space for further visual analysis. These studies have revealed a distributed frontoparietal circuit that is responsible for the control of spatial attention. However, vision must ultimately represent objects and in real scenes objects often overlap spatially; thus attention must be capable of selecting objects and their properties nonspatially. Little is known about the neural basis of object-based attentional control. In two experiments, human observers shifted attention between spatially superimposed faces and houses. Event-related functional magnetic resonance imaging (fMRI) revealed attentional modulation of activity in face- and house-selective cortical regions. Posterior parietal and frontal regions were transiently active when attention was shifted between spatially superimposed perceptual objects. The timecourse of activity provides insight into the functional role that these brain regions play in attentional control processes.     

The reentry hypothesis: the putative interaction of the frontal eye field, ventrolateral prefrontal cortex, and areas V4, IT for attention and eye movement

Attention is known to play a key role in perception, including action selection, object recognition and memory. Despite findings revealing competitive interactions among cell populations, attention remains difficult to explain. The central purpose of this paper is to link up a large number of findings in a single computational approach. Our simulation results suggest that attention can be well explained on a network level involving many areas of the brain. We argue that attention is an emergent phenomenon that arises from reentry and competitive interactions. We hypothesize that guided visual search requires the usage of an object-specific template in prefrontal cortex to sensitize V4 and IT cells whose preferred stimuli match the target template. This induces a feature-specific bias and provides guidance for eye movements. Prior to an eye movement, a spatially organized reentry from occulomotor centers, specifically the movement cells of the frontal eye field, occurs and modulates the gain of V4 and IT cells. The processes involved are elucidated by quantitatively comparing the time course of simulated neural activity with experimental data. Using visual search tasks as an example, we provide clear and empirically testable predictions for the participation of IT, V4 and the frontal eye field in attention. Finally, we explain a possible physiological mechanism that can lead to non-flat search slopes as the result of a slow, parallel discrimination process.     

Emotional perception: correlation of functional MRI and event-related potentials

Dense-array electrocortical and functional hemodynamic measures of human brain activity were collected to assess the relationship between 2 established neural measures of emotional reactivity. Recorded in parallel sessions, the slow-wave late positive potential (LPP) and visual cortical blood oxygen level-dependent (BOLD) signals were both modulated by the rated intensity of picture arousal. The amplitude of the LPP correlated significantly with BOLD intensity in lateral occipital, inferotemporal, and parietal visual areas across picture contents. Estimated strength of modeled regional sources did not correlate significantly with regional BOLD intensity. These data suggest that the enhanced positive slow wave seen over posterior sites during emotional picture processing represents activity in a circuit of visual cortical structures, reflecting a perceptual sensitivity to the motivational relevance of visual scenes.     

Dissociation of stimulus relevance and saliency factors during shifts of visuospatial attention

The control of visuospatial attention entails multiple processes, including both voluntary (endogenous) factors and stimulus-driven (exogenous) factors. Exogenous processes can be triggered by visual targets presented at a previously unattended location, thus capturing attention in a stimulus-driven manner. However, little is known about the relative role of stimulus salience and behavioral relevance for this type of spatial reorienting. Here, we directly assessed how salience and relevance affect activation of the frontoparietal attentional system, using either low-salience but task-relevant target stimuli or salient but task-irrelevant flickering checkerboards. We compared event-related functional magnetic resonance imaging responses for stimuli presented at the unattended versus attended side (invalid minus valid trials), separately for the 2 categories of visual stimuli. We found that task-relevant invalid targets activated the frontoparietal attentional network, demonstrating that this system engages when target stimuli are presented at an unattended location, even when these have a low perceptual salience. Conversely, the presentation of high-salience checkerboards in one hemifield while endogenous attention was engaged elsewhere did not activate the attentional network. These findings indicate that task relevance is critical for stimulus-driven engagement of the attentional network when attentional resources are endogenously allocated somewhere else.     

A frontoparietal network for spatial attention reorienting in the auditory domain: a human fMRI/MEG study of functional and temporal dynamics

Several studies have identified a supramodal network critical to the reorienting of attention toward stimuli at novel locations and which involves the right temporoparietal junction and the inferior frontal areas. The present functional magnetic resonance imaging (fMRI)\magnetoencephalography (MEG) study investigates: 1) the cerebral circuit underlying attentional reorienting to spatially varying sound locations; 2) the circuit related to the regular change of sound location in the same hemifield, the change of sound location across hemifields, or sounds presented randomly at different locations on the azimuth plane; 3) functional temporal dynamics of the observed cortical areas exploiting the complementary characteristics of the fMRI and MEG paradigms. fMRI results suggest 3 distinct roles: the supratemporal plane appears modulated by variations of sound location; the inferior parietal lobule is modulated by the cross-meridian effect; and the inferior frontal cortex is engaged by the inhibition of a motor response. MEG data help to elucidate the temporal dynamics of this network by providing high-resolution time series with which to measure latency of neural activation manipulated by the reorienting of attention.     

Dorsal anterior cingulate cortex resolves conflict from distracting stimuli by boosting attention toward relevant events

In everyday life, we often focus greater attention on behaviorally relevant stimuli to limit the processing of distracting events. For example, when distracting voices intrude upon a conversation at a noisy social gathering, we concentrate more attention on the speaker of interest to better comprehend his or her speech. In the present study, we investigated whether dorsal/caudal regions of the anterior cingulate cortex (dACC), thought to make a major contribution to cognitive control, boost attentional resources toward behaviorally relevant stimuli as a means for limiting the processing of distracting events. Sixteen healthy participants performed a cued global/local selective attention task while brain activity was recorded with event-related functional magnetic resonance imaging. Consistent with our hypotheses, greater dACC activity during distracting events predicted reduced behavioral measures of interference from those same events. dACC activity also differed for cues to attend to global versus local features of upcoming visual objects, further indicating a role in directing attention toward task-relevant stimuli. Our findings indicate a role for dACC in focusing attention on behaviorally relevant stimuli, especially when the achievement of our behavioral goals is threatened by distracting events.     

A contralateral preference in the lateral occipital area: sensory and attentional mechanisms

Here we examined the level of the lateral occipital (LO) area within the processing stream of the ventral visual cortex. An important determinant of an area's level of processing is whether it codes visual elements on both sides of the visual field, as do higher visual areas, or prefers those in the contralateral visual field, as do early visual areas. The former would suggest that LO, on one side, combines bilateral visual elements into a whole, while the latter suggests that it codes only the parts of forms. We showed that LO has a relative preference for visual objects in the contralateral visual field. LO responses were influenced by attention. However, relative changes in LO activity caused by changes in object location were preserved even when attention was shifted away from the objects to moving random dot patterns on the opposite side. Our data offer a new view on LO as an intermediate, but not a high-level, visual area in which neurons are driven by visual input and spatial attention in a multiplicative fashion.     

Hemispheric asymmetries for different components of global/local attention occur in distinct temporo-parietal loci

Data from brain-damaged and neurologically intact populations indicate hemispheric asymmetries in the temporo-parietal cortex for discriminating an object's global form (e.g. the overall shape of a bicycle) versus its local parts (e.g. the spokes in a bicycle tire). However, it is not yet clear whether such asymmetries reflect processes that (i) bias attention toward upcoming global versus local stimuli and/or (ii) attend/identify global versus local stimuli after they are presented. To investigate these possibilities, we asked sixteen healthy participants to perform a cued global/local attention task while their brain activity was recorded using event-related functional magnetic resonance imaging (fMRI). The results indicated a novel double dissociation. Hemispheric asymmetries for deploying attention toward expected global versus local object features were specific to the intraparietal sulcus (iPs). However, hemispheric asymmetries for identifying global versus local features after they were presented were specific to the inferior parietal lobe/superior temporal gyrus (IPL/STG). This double dissociation provides the first direct evidence that hemispheric asymmetries associated with different components of global/local attention occur in distinct temporo-parietal loci. Furthermore, it parallels an analogous dissociation reported in a recent fMRI study of spatial orienting, suggesting that global/local attention and spatial attention might rely on similar cognitive/neural mechanisms.     

Activation of area MT/V5 and the right inferior parietal cortex during the discrimination of transient direction changes in translational motion

The perception of changes in the direction of objects that translate in space is an important function of our visual system. Here we investigate the brain electrical phenomena underlying such a function by using a combination of magnetoencephalography (MEG) and magnetic resonance imaging. We recorded MEG-evoked responses in 9 healthy human subjects while they discriminated the direction of a transient change in a translationally moving random dot pattern presented either to the right or to the left of a central fixation point. We found that responses reached their maximum in 2 main regions corresponding to motion processing area middle temporal (MT)/V5 contralateral to the stimulated visual field, and to the right inferior parietal lobe (rIPL). The activation latencies were very similar in both regions ( approximately 135 ms) following the direction change onset. Our findings suggest that area MT/V5 provides the strongest sensory signal in response to changes in the direction of translational motion, whereas area rIPL may be involved either in the sensory processing of transient motion signals or in the processing of signals related to orienting of attention.     

Brain mechanisms mediating auditory attentional capture in humans

The ability to detect and preferentially process salient auditory stimuli, even when irrelevant to a current task, is often critical for adaptive behavior. This stimulus-driven allocation of processing resources is known as "attentional capture." Here we used functional magnetic resonance imaging in humans to investigate brain activity and behavioral effects related to such auditory attentional capture. Participants searched a sequence of tones for a target tone that was shorter or longer than the nontarget tones. An irrelevant singleton feature in the tone sequence resulted in behavioral interference (attentional capture) and activation of parietal and prefrontal cortices only when the singleton was associated with a nontarget tone (nontarget singleton) and not when associated with a target tone (target singleton). In contrast, the presence (vs. absence) of a singleton feature in the sequence was associated with activation of frontal and temporal loci previously associated with auditory change detection. These results suggest that a ventral network involving superior temporal and inferior frontal cortices responds to acoustic variability, regardless of attentional significance, but a dorsal frontoparietal network responds only when a feature singleton captures attention.     

The integration of colour and motion by the human visual brain

Objects in the visual scene are defined by different cues such as colour and motion. Through the integration of these cues the visual system is able to utilize different sources of information, thus enhancing its ability to discriminate objects from their backgrounds. In the following experiments, we investigate the neural mechanisms of cue integration in the human. We show, using functional magnetic resonance imaging (fMRI), that both colour and motion defined shapes activate the lateral occipital complex (LOC) and that shapes defined by both colour and motion simultaneously activate the anterior-ventral margins of this area more strongly than shapes defined by either cue alone. This suggests that colour and motion cues are integrated in the LOC and possibly a neighbouring, more anterior, region. We support this result using an fMR adaptation technique, demonstrating that a region of the LOC adapts on repeated presentations of a shape regardless of the cue that is used to define it and even if the cue is varied. This result raises the possibility that the LOC contains cue-invariant neurons that respond to shapes regardless of the cue that is used to define them. We propose that such neurons could integrate signals from different cues, making them more responsive to objects defined by more than one cue, thus increasing the ability of the observer to recognize them.     

Exploring eye movement analysis as a measure of selective visual attention in brain injured individuals

Primary objective: To investigate the utility of using a new method of assessment for deficits in selective visual attention (SVA).

Methods and procedures: An independent groups design compared six participants with brain injuries with six participants from a non-brain injured control group. The Sensomotoric Instruments Eye Movement system with remote eye-tracking device (eye camera) and two sets of eight stimuli were employed to determine if the camera would be a sensitive discriminator of SVA in these groups.

Main outcomes and results: The attention profile displayed by the brain injured group showed that they were slower, made more errors, were less accurate and more indecisive than the control group.

Conclusions: The utility of eye movement analysis as an assessment method was established, with implications for rehabilitation requiring further development.     

Attention to simultaneous unrelated auditory and visual events: behavioral and neural correlates

The cognitive and neural bases of the ability to focus attention on information in one sensory modality while ignoring information in another remain poorly understood. We hypothesized that bimodal selective attention results from increased activity in corresponding sensory cortices with a suppression of activity in non-corresponding sensory cortices. In a functional magnetic resonance imaging (fMRI) study, we presented melodies and shapes alone (unimodal) or simultaneously (bimodal). Subjects monitored for changes in an attended modality while ignoring the other. Subsequently, memory for both attended and unattended stimuli was tested. Subjects remembered attended stimuli equally well in unimodal and bimodal conditions, and significantly better than ignored stimuli in bimodal conditions. When a subject focused on a stimulus, the blood-oxygen-level-dependent (BOLD) response increased in sensory cortices corresponding to that modality in both unimodal and bimodal conditions. Additionally, the BOLD response decreased in sensory cortices corresponding to the non-presented modality in unimodal conditions and the unattended modality in bimodal conditions. We conclude that top-down attentional effects modulate the interaction of sensory cortical areas by gating sensory input. This interaction between sensory cortices enhances processing of one modality at the expense of the other during selective attention, and subsequently affects memory encoding.     

Large-scale gamma-band phase synchronization and selective attention

Explaining the emergence of a coherent conscious percept and an intentional agent from the activity of distributed neurons is key to understanding how the brain produces higher cognitive processes. Gamma-band synchronization has been proposed to be a mechanism for the functional integration of neural populations that together form a transitory, large-scale, task- and/or percept-specific network. The operation of this mechanism in the context of attention orienting entails that cortical regions representing attended locations should show more gamma-band synchronization with other cortical areas than would those representing unattended locations. This increased synchronization should be apparent in the same time frame as that of the deployment of attention to a particular location. In order to observe this effect, we made electroencephalogram recordings while subjects attended to one side or the other of the visual field (which we confirmed by event-related potential analysis) and calculated phase-locking statistics between the signals recorded at relevant electrode pairs. We observed increased gamma-band phase synchronization between visual cortex contralateral to the attended location and other, widespread, cortical areas approximately 240-380 ms after the directional cue was presented, confirming the prediction of a large-scale gamma synchronous network oriented to the cued location.     

Forgetting as an active process: an FMRI investigation of item-method-directed forgetting

Using event-related functional magnetic resonance imaging (fMRI), we examined the blood oxygen level-dependent response associated with intentional remembering and forgetting. In an item-method directed forgetting paradigm, participants were presented with words, one at a time, each of which was followed after a brief delay by an instruction to Remember or Forget. Behavioral data revealed a directed forgetting effect: greater recognition of to-be-remembered than to-be-forgotten words. We used this behavioral recognition data to sort the fMRI data into 4 conditions based on the combination of memory instruction and behavioral outcome. When contrasted with unintentional forgetting, intentional forgetting was associated with increased activity in hippocampus (Broadmann area [BA] 35) and superior frontal gyrus (BA10/11); when contrasted with intentional remembering, intentional forgetting was associated with activity in medial frontal gyrus (BA10), middle temporal gyrus (BA21), parahippocampal gyrus (BA34 and 35), and cingulate gyrus (BA31). Thus, intentional forgetting depends on neural structures distinct from those involved in unintentional forgetting and intentional remembering. These results challenge the standard selective rehearsal account of item-method directed forgetting and suggest that frontal control processes may be critical for directed forgetting.     

Neural basis for priming of pop-out during visual search revealed with fMRI

Maljkovic and Nakayama first showed that visual search efficiency can be influenced by priming effects. Even "pop-out" targets (defined by unique color) are judged quicker if they appear at the same location and/or in the same color as on the preceding trial, in an unpredictable sequence. Here, we studied the potential neural correlates of such priming in human visual search using functional magnetic resonance imaging (fMRI). We found that repeating either the location or the color of a singleton target led to repetition suppression of blood oxygen level-dependent (BOLD) activity in brain regions traditionally linked with attentional control, including bilateral intraparietal sulci. This indicates that the attention system of the human brain can be "primed," in apparent analogy to repetition-suppression effects on activity in other neural systems. For repetition of target color but not location, we also found repetition suppression in inferior temporal areas that may be associated with color processing, whereas repetition of target location led to greater reduction of activation in contralateral inferior parietal and frontal areas, relative to color repetition. The frontal eye fields were also implicated, notably when both target properties (color and location) were repeated together, which also led to further BOLD decreases in anterior fusiform cortex not seen when either property was repeated alone. These findings reveal the neural correlates for priming of pop-out search, including commonalities, differences, and interactions between location and color repetition. fMRI repetition-suppression effects may arise in components of the attention network because these settle into a stable "attractor state" more readily when the same target property is repeated than when a different attentional state is required.     

Right temporoparietal cortex activation during visuo-proprioceptive conflict

The conflict between vision and proprioception has been proposed to explain why healthy subjects perform worse than proprioceptively deafferented patients in conditions with optical displacement, e.g. novel mirror drawing. It is not known which brain processes depend upon the successful integration of visual and proprioceptive information and are therefore impaired when these modalities disagree. With fMRI in healthy subjects we compared brain activity across two conditions with similar visual and proprioceptive stimulation and similar task demands that differed by the congruence of movement showed by the two modalities. Subjects felt the passive movement of the right index finger on a rectangular field and watched a cursor moving on a computer screen. Cursor and finger locations either mapped onto each other (congruent condition) or did not (incongruent condition). Monitoring incongruent compared with congruent movement activated the premotor area bilaterally and the right temporoparietal junction. These brain areas have previously been associated with shifts in the attended location in the visual space. These findings suggest an interaction between vision and proprioception in orienting to spatial locations.     

Attentional modulation of object representations in working memory

We investigated the role of object-based attention in modulating the maintenance of faces and scenes held online in working memory (WM). Participants had to remember a face and a scene, while cues presented during the delay instructed them to orient their attention to one or the other item. Event-related functional magnetic resonance imaging revealed that orienting attention in WM modulated the activity in fusiform and parahippocampal gyri, involved in maintaining representations of faces and scenes respectively. Measures from complementary behavioral studies indicated that this increase in activity corresponded to improved WM performance. The results show that directed attention can modulate maintenance of specific representations in WM, and help define the interplay between the domains of attention and WM.     

Neural system for controlling the contents of object working memory in humans

Working memory (WM), the active maintenance of currently relevant information, is a flexible system allowing for fast and frequent goal-directed changes of rehearsed information. Successful WM maintenance prevents interference from distracting stimuli while allowing new task-relevant information to update the contents of WM. We used functional magnetic resonance imaging to show that when WM contents were updated, regardless of stimulus type (faces or houses), a frontoparietal network showed transient increases in activation. Some of these regions are highly similar to those identified in studies of shifting attention, supporting the idea that updating WM involves a change in the attentional priority afforded to the current perceptual input. A region within the mid-ventrolateral prefrontal cortex, near the junction of the inferior frontal sulcus and precentral sulcus (inferior frontal junction), that has previously been implicated in cognitive control, demonstrated transient increases in activity during updating as well as sustained maintenance activity. A more anterior prefrontal region, middle frontal gyrus, previously implicated in protecting the contents of WM from interfering stimuli during maintenance, demonstrated transient increases in activity during updating. The current study suggests that updating WM results from a combination of increased attention to the visual stimulus and a change in the system's interference protection state.