Processing incoming sensory information and transforming this input into appropriate motor responses is a critical and ongoing aspect of our moment-to-moment interaction with the environment. While the neural mechanisms in the posterior parietal cortex (PPC) that support the transformation of sensory inputs into simple eye or limb movements has received a great deal of empirical attention – in part because these processes are easy to study in non-human primates – little work has been done on sensory-motor transformations in the domain of speech. Here, we used fMRI and multivariate analysis techniques to demonstrate that a region of the Planum Temporale (Spt) shows distinct spatial activation patterns during sensory and motor aspects of a speech task. This result suggests that just as the PPC supports sensorimotor integration for eye and limb movements, the Spt forms a sensory-motor integration circuit for the vocal tract.


Stimulus specific delay period activity in human primary visual cortex

J. Serences, E. Ester, E. Vogel, and E. Awh (2009, Psychological Science)

Working memory (WM) involves maintaining information in an “online” state. One emerging view is that information in WM is maintained via sensory recruitment, such that information is stored via sustained activity in the sensory areas that encode the to-be-remembered information. Using fMRI we observed that key sensory regions such as primary visual cortex (V1) showed little evidence of sustained increases in mean activation during a WM delay period, though such amplitude increases have typically been used to determine whether a region is involved in online maintenance. However, a multivoxel pattern analysis of delay period activity revealed a sustained pattern of activation in V1 that represented only the intentionally stored feature of a multi-feature object. Moreover, the pattern of delay activity was qualitatively similar to that observed during the discrimination of sensory stimuli, suggesting that WM representations in primary visual cortex are reasonable “copies” of those evoked during pure sensory processing.

Estimating the influence of attention on population codes in human visual cortex using voxel-based tuning functions

J. Serences, S. Saproo, M. Scolari, T. Ho, L.T. Muftuler (2009, Neuroimage)

In order to form stable perceptual representations, populations of sensory neurons must pool their output to overcome physiological noise; selective attention is then required to ensure that behaviorally relevant stimuli dominate these ‘population codes’ to gain access to awareness. However, the role that attention plays in shaping population response profiles has received little direct investigation, in part because most traditional neurophysiological methods cannot simultaneously assess changes in activity across large populations of sensory neurons. Based on single-unit recording studies, current theories hold that attending to a relevant feature sharpens the population response profile and improves the signal-to-noise ratio of the resulting perceptual representation. Here, we test this hypothesis using fMRI and an analysis approach that is able to estimate the influence of feature-based attentional modulations on population response profiles. We first derive orientation tuning functions for single voxels in human primary visual cortex, and then use these tuning functions to sort voxels according to their orientation preference. We then show that selective attention systematically biases population response profiles so that behaviorally relevant stimuli are represented in the visual system at the expense of behaviorally irrelevant stimuli. Collectively, the present results (1) provide a new approach for precisely characterizing feature-selective responses in human sensory cortices and (2) reveal how behavioral goals can shape population response profiles to support the formation of coherent perceptual representations.

 

Value-based modulations in human visual cortex

Supplemental Materials

Washington Post Article

J. Serences (2008, Neuron)

Economists and cognitive psychologists have long known that prior rewards bias decision making in favor of options with high expected value. Accordingly, value modulates the activity of sensorimotor neurons involved in initiating movements towards one of two competing decision alternatives. However, little is known about how value influences the acquisition and representation of incoming sensory information, or about the neural mechanisms that track the relative value of each available stimulus to guide behavior. Here, fMRI revealed value-related modulations throughout spatially selective areas of the human visual system in the absence of overt saccadic responses (including in V1). These modulations are primarily associated with the reward history of each stimulus and not to self-reported estimates of stimulus value. Finally, subregions of frontal and parietal cortex represent the differential value of competing alternatives and may provide signals to bias spatially-selective visual areas in favor of more valuable stimuli.

 

On-line response-selection and the attentional blink: multiple-processing channels

J. Serences, M. Scolari, E. Awh (2008, Visual Cognition)

Robust interference often arises when multiple targets (T1 and T2) are discriminated in rapid succession (the attentional blink or AB). The AB has been observed for a wide range of stimuli, and is often thought to reflect a central capacity limitation in working memory consolidation, attentional engagement, and/or online response selection. However, recent evidence challenges the existence of unitary bottleneck during post-perceptual processing. Awh et al (2004) found no AB interference when a digit target preceded a face target, presumably because these stimuli could be processed by means of separable processing channels. Using a modified AB procedure, recent studies have also demonstrated that speeded response selection of T1 leads to an AB effect for T2 identification, supporting the conclusion that response selection induces the same processing limitations that typically gives rise to an AB. The present research tests this hypothesis by examining the effects of response selection on the identification of faces. Although we replicated previous demonstrations that online response selection of a digit disrupts the identification of T2 letters, we found no interference in the identification of T2 faces. However, robust AB interference was once again observed when a speeded response to a T1 face was required, confirming that faces are not simply immune to central interference. These results dispute the existence of a unitary post-perceptual capacity limitation that gives rise to the AB.

Sleep-Dependent Learning and Practice-Dependent Deterioration in an Orientation Discrimination Task

S. Mednick, S. Drummond, G.M. Boynton, E. Awh, J. Serences (2008, Behavioral Neuroscience)

Learning new information requires practice, and is associated with cortical plasticity. Studies show that the degree of learning can be influenced by the amount of practice and whether or not subjects sleep between sessions. However, over-practice can sometimes also lead to deterioration in performance. Little work has been done to examine the relationship between practice-dependent deterioration and sleep-dependent learning, so it is not known how these factors interact. The present study examines this question by asking A) whether the amount of practice before a sleep episode alters learning and B) whether a prior sleep episode protects against deterioration. Two groups (N=33) were tested three times across two days on an orientation discrimination task. The High practice group was tested twice before a night of sleep and once after, at 9AM, 7PM and 9AM. The Low practice group was tested once before a night of sleep and twice after, at 7PM, 9AM and 7PM. Overall, both groups showed 1) deterioration with repeated, within-day testing, 2) performance improvement only after a night of sleep, 3) similar amounts of sleep-dependent learning and practice-dependent deterioration. In summary, we found that sleep resets visual contrast thresholds to a lower baseline (i.e., produces learning), but does not prevent over-practice deterioration effects. Likewise, over-practice deterioration does not influence the magnitude of overnight learning on this task.

The impact of temporal regularization on estimates of the BOLD hemodynamic response function: a comparative analysis

R. Casanova, S. Ryali, J. Serences, L. Yang, R Kraft, P.J. Laurienti, JA Maldjian (2008, Neuroimage)

In fMRI data analysis it has been shown that for a wide range of situations the hemodynamic response function (HRF) can be reasonably characterized as the impulse response function of a linear and time invariant system. An accurate and robust extraction of the HRF is essential to infer quantitative information about the relative timing and strength of the neuronal events in different brain regions. When no assumptions are made about the HRF shape, it is most commonly estimated using time windowed averaging or a least-squares estimated general linear model based on either Fourier or delta basis functions. Recently, regularization methods have been employed to increase the estimation efficiency of the HRF; typically these methods produce more accurate HRF estimates than the least squares approach (Goutte, et al. 2000). Here, we use simulations to clarify the relative merit of temporal regularization based methods compared to the least squares methods with respect to the accuracy of estimating certain characteristics of the HRF such as time to peak (TTP), amplitude (HR) and width (W) of the response. Two methods were employed for selecting the regularization parameter; a Bayesian approach proposed by Marrelec et al. (2001, 2003) and its deterministic counterpart based on a combination of Tikhonov regularization (Tikhonov and Arsenin 1977) and generalized cross-validation (GCV) (Wahba,1990). The performance of both methods is compared with least-squared estimates as a function of temporal resolution, color and strength of the noise, and the type of stimulus sequence used. In almost all situations, the regularization-based techniques more accurately characterize the HRF compared to the least-squared method, and our results clarify the effects of temporal resolution, noise color, and experimental design on the accuracy of HRF estimation.

Cortical mechanisms for shifting and holding visuospatial attention

T.A. Kelley, J. Serences, B. Giesbrecht, S. Yantis (2008, Cerebral Cortex).

Access to visual awareness is often determined by covert, voluntary deployments of visual attention. Voluntary orienting without eye movements requires decoupling attention from the locus of fixation, a shift to the desired location and maintenance of attention at that location. We used event-related fMRI to dissociate these components while observers shifted attention among three streams of letters and digits, one located at fixation and two in the periphery. Compared to holding attention at the current location, shifting attention between the peripheral locations was associated with transient increases in neural activity in the superior parietal lobule (SPL) and frontal eye fields (FEF), as in previous studies. The supplementary eye fields (SEF) and separate portions of SPL and FEF were more active for decoupling attention from fixation than for shifting attention to a new location. Large segments of precentral sulcus (PreCS) and posterior parietal cortex (PPC) were more active when attention was maintained in the periphery than when it was maintained at fixation. We conclude that distinct subcomponents of the dorsal frontoparietal network initiate redeployments of covert attention to new locations and disengage attention from fixation, while sustained activity in lateral regions of PPC and PreCS represents sustained states of peripheral attention.

The representation of behavioral choice for motion in human visual cortex

J. Serences, G.M. Boynton (2007, Journal of Neuroscience)

UCI press release

Single unit recording studies have demonstrated a close link between neural activity in the middle temporal area (MT) and motion perception. In contrast, researchers using functional magnetic resonance imaging (fMRI) and Multivoxel Pattern Analysis (MVPA) methods have recently documented direction-specific responses within many regions of the visual system (e.g. V1-V4v) not normally associated with motion processing. Our goal was to determine how these direction-selective response patterns directly relate to the conscious perception of motion. We dissociated neuronal responses associated with the perceptual experience of motion from the physical presence of motion in the display by asking observers to report the perceived direction of an ambiguous stimulus. Activation patterns in the human MT complex closely matched the reported perceptual state of the observer, whereas patterns in other visual areas did not. These results suggest that even when selective responses to a given feature are distributed relatively broadly across the visual system, the conscious experience of that feature may be primarily based on activity within specialized cortical areas.

Human Adult Cortical Reorganization and Consequent Visual Distortion

D. Dilks, J. Serences, B. Rosenau, S. Yantis, M. McCloskey (2007, Journal of Neuroscience)  

MIT Press release

Neural and behavioral evidence for cortical reorganization in the adult somatosensory system following loss of sensory input (e.g., amputation) has been well documented. In contrast, evidence for reorganization in the adult visual system is far less clear: neural evidence is the subject of controversy, behavioral evidence is sparse, and studies combining neural and behavioral evidence have not previously been reported. Here we report converging behavioral and neuroimaging evidence for cortical reorganization in the adult human visual system from a stroke patient (BL). BL’s stroke spared primary visual cortex (V1), but destroyed fibers that normally provide input to V1 from the upper left visual field (LVF). As a consequence, BL is blind in the upper LVF, and exhibits distorted perception in the lower LVF: Stimuli appear vertically elongated, toward and into the blind upper LVF. For example, a square presented in the lower LVF is perceived as a rectangle extending upward. We hypothesized that the perceptual distortion was a consequence of cortical reorganization. Extensive behavioral testing supported our hypothesis, and functional magnetic resonance imaging (fMRI) confirmed V1 reorganization. Together the behavioral and fMRI data show that loss of input to V1 following a stroke leads to cortical reorganization in the adult human visual system, and provide the first evidence that reorganization of the adult visual system affects visual perception. These findings contribute to our understanding of the human adult brain’s capacity to change, and has implications for topics ranging from learning to recovery from brain damage.

 

Feature-based attentional modulations in the absence of direct visual stimulation

J. Serences, G.M. Boynton (2007, Neuron)

Accompanying commentary by Treue and Martinez-Trujillo: Attending to Features inside and outside the Spotlight of Attention

UCI Press release, articles in Washington Post and Scientific American

When faced with a crowded visual scene observers must selectively attend to behaviorally relevant objects to avoid sensory overload. Often this selection process is guided by prior knowledge of a target-defining feature (e.g. the color red when looking for an apple), which enhances the firing rate of visual neurons that are selective for the attended feature. Here, we used functional magnetic resonance imaging and a pattern classification algorithm to predict the attentional state of human observers as they monitored a visual feature (one of two directions of motion). We found that feature-specific attention effects spread across the visual field - even to regions of the scene that did not contain a stimulus. This spread of feature-based attention to empty regions of space may facilitate the perception of behaviorally relevant stimuli by increasing sensitivity to attended features at all locations in the visual field.

Spatially-selective representations of voluntary and stimulus-driven attentional priority in human occipital, parietal, and frontal cortex

J. Serences, S. Yantis (2007, Cerebral Cortex)

When multiple objects are present in a visual scene, they compete for cortical processing in the visual system; selective attention biases this competition so that representations of behaviorally relevant objects enter awareness and irrelevant objects do not. Deployments of selective attention are often guided by prior knowledge of the location of a relevant object, or by knowledge of a target-defining attribute such as color or form. Here we use functional magnetic resonance imaging to show that activity within spatially-selective regions of human occipital, parietal, and frontal cortex reflects both the sensory strength and the behavioral relevance of objects presented in the contralateral visual field. Neural activity within these regions varies along a continuum such that early visual areas (e.g. V1) are primarily driven by the low-level sensory properties of the display, whereas later regions (e.g. parietal and frontal cortex) are increasingly modulated by deployments of selective attention.

Neural System for Controlling the Contents of Object Working Memory in Humans

J.K. Roth, J. Serences, S.M. Courtney (2006, Cerebral Cortex)

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.

Selective attention and perceptual coherence

J. Serences, S. Yantis (2006, Trends in Cognitive Science)

Conscious perception of the visual world depends on neural activity at all levels of the visual system from the retina to regions of parietal and frontal cortex. Neurons in early visual areas have small spatial receptive fields (RFs) and code basic image features; neurons in later areas have large RFs and code abstract features such as behavioral relevance. This hierarchical organization presents challenges to perception: objects compete when they are presented in a single RF, and component object features are coded by anatomically distributed neuronal activity. Recent research has shown that selective attention coordinates the activity of neurons to resolve competition and link distributed object representations. We refer to this ensemble activity as a coherence field, and propose that voluntary shifts of attention are initiated by a transient control signal that nudges the visual system from one coherent state to another.

Retinotopic mapping in the human visual cortex using vascular space occupancy-dependent functional magnetic resonance imaging

H. Lu, G. Basso, J. Serences, S. Yantis, X. Golay, P.C. van Zijl (2005, Neuroreport)

Recently, we introduced a new methodology, vascular space occupancy functional magnetic resonance imaging, which detects brain activation on the basis of blood volume changes in parenchymal microvasculature and may provide higher spatial specificity than the blood oxygenation level-dependent method. To study whether this technique can be used for advanced brain mapping applications, we performed retinotopic mapping using alternating horizontal and vertical wedges that stimulate different portions of the visual field. The results using vascular space occupancy functional magnetic resonance imaging showed clear boundaries for V1/V2/VP/V4v in the ventral areas and V1/V2/V3/V3A in the dorsal areas, similar to the maps obtained using blood oxygenation level-dependent functional magnetic resonance imaging. Vascular space occupancy functional magnetic resonance imaging is a useful addition to the other neuroimaging techniques. Disadvantages of vascular space occupancy functional magnetic resonance imaging include lower contrast-to-noise ratio (about 1/3 of that of blood oxygenation level-dependent method) and limited volume coverage (nine slices for TR=3 s).

Parietal mechanisms of switching and maintaining attention to locations, features, and objects (Review Book Chapter)

J. Serences, T. Liu, S. Yantis (in Neurobiology of Attention, Eds. Laurent Itti, Geraint Rees, and John Tsotsos, 2005)

Two distinct components of attentional control have been documented within subregions of parietal cortex. First, broad regions of intraparietal sulcus (IPS) and frontal eye fields (FEF) are tonically active when attention is directed to a particular location, feature, or object in a visual scene. This tonic activity in IPS may be the source of a signal to maintain the current state of attention in visual cortex. Second, regions of superior parietal lobule (SPL), IPS, and precuneus are transiently active when attention is shifted between attentive states. This transient activity may reflect an attentional control signal that initiates abrupt changes of attentional state in sensory areas of visual cortex.

Coordination of voluntary and stimulus-driven attentional control in human cortex

J. Serences, S. Shomstein, A.B. Leber, X. Golay, H.E. Egeth, S. Yantis (Psychological Science, 2005)

Visual attention may be voluntarily directed to particular locations or features (voluntary control), or it may be captured by salient stimuli, such as the abrupt appearance of a new perceptual object (stimulus-driven control). Most often, however, the deployment of attention is the result of a dynamic interplay between voluntary attentional control settings (e.g., based on prior knowledge about a target's location or color) and the degree to which stimuli in the visual scene match these voluntary control settings. Consequently, nontarget items in the scene that share a defining feature with the target of visual search can capture attention, a phenomenon termed contingent attentional capture. We used functional magnetic resonance imaging to show that attentional capture by target-colored distractors is accompanied by increased cortical activity in corresponding regions of retinotopically organized visual cortex. Concurrent activation in the temporoparietal junction and ventral frontal cortex suggests that these regions coordinate voluntary and stimulus-driven attentional control settings to determine which stimuli effectively compete for attention.

Preparatory activity in visual cortex indexes distractor suppression during covert spatial orienting

J. Serences, S. Yantis, A. Culberson, E. Awh (Journal of Neurophysiology, 2004)

The deployment of spatial attention induces retinotopically specific increases in neural activity that occur even before a target stimulus is presented. Although this preparatory activity is thought to prime the attended regions, thereby improving perception and recognition, it is not yet clear whether this activity is a manifestation of signal enhancement at the attended locations or suppression of interference from distracting stimuli (or both). We investigated the functional role of these preparatory shifts by isolating a distractor suppression component of selection. Behavioral data have shown that manipulating the probability that visual distractors will appear modulates distractor suppression without concurrent changes in signal enhancement. In 2 experiments, functional magnetic resonance imaging revealed increased cue-evoked activity in retinotopically specific regions of visual cortex when increased distractor suppression was elicited by a high probability of distractors. This finding directly links cue-evoked preparatory activity in visual cortex with a distractor suppression component of visual selective attention.

Control of object based attention in human cortex

J. Serences, J. Schwarzbach, S.M. Courtney, X. Golay, S. Yantis (Cerebral Cortex, 2004)

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.

A comparison of methods for estimating the event-related BOLD signal in rapid fMRI

J. Serences (Neuroimage, 2004)

Information about the shape and temporal duration of the blood oxygenation level dependent (BOLD) response can inform both functional neuroanatomy and psychological theory. However, the BOLD response evolves over 20 s or more, making it difficult to distinguish the unique characteristics of the response evoked by temporally adjacent stimuli. Fortunately, event-related BOLD signals can be extracted given that there is adequate variance in the distribution of inter-stimulus intervals (ISI). Unfortunately, the ISI distribution that yields the highest statistical efficiency is not always optimal from a psychological perspective; variability in the stimulus timing may complicate the interpretation of neuroimaging data in terms of underlying cognitive operations. In the present paper, Monte Carlo simulations are used to evaluate two techniques for estimating the event-related BOLD timeseries-event-related averaging and deconvolution using the Ordinary Least Squares estimate -with respect to maintaining acceptable levels of statistical power and experimental validity. While the unbiased deconvolution technique more robustly estimates the shape of the BOLD response functions, both methods succeed in accurately re-producing known differences between evoked BOLD responses when the stimulus ordering is randomized. However, the deconvolution method is more effective at preserving differences when there are sequential dependencies in the stimulus presentation order and restricted ISI distributions are used; particularly if the second of two sequentially dependent stimuli is omitted on some portion of the trials. Importantly, the successful re-production of the evoked BOLD response using restricted ISI distributions often maximizes the ability to make psychologically valid experimental conclusions.

Evidence against a central bottleneck during the attentional blink: Multiple channels for configural and featural processing

E. Awh, J. Serences, P. Laurey, H. Dhaliwal, T. van der Jagt, P. Dassonville (Cognitive Psychology, 2004).

When a visual target is identified, there is a period of several hundred milliseconds when the processing of subsequent targets is impaired, a phenomenon labeled the attentional blink (AB). The emerging consensus is that the identification of a visual target temporarily occupies a limited attentional resource that is essential for all visual perception. The present results challenge this view. With the same digit discrimination task that impaired subsequent letter discrimination for several hundred milliseconds, we found no disruption of subsequent face discrimination. These results suggest that all stimuli do not compete for access to a single resource for visual perception. We propose a multi-channel account of interference in the AB paradigm.

Cortical mechanisms of feature-based attentional control

T. Liu, S. Slotnick, J. Serences, S. Yantis (Cerebral Cortex, 2003).

A network of fronto-parietal cortical areas is known to be involved in the control of visual attention, but the representational scope and specific function of these areas remains unclear. Recent neuroimaging evidence has revealed the existence of both transient (attention-shift) and sustained (attention-maintenance) mechanisms of space-based and object-based attentional control. Here we investigate the neural mechanisms of feature-based attentional control in human cortex using rapid event-related functional magnetic resonance imaging (fMRI). Subjects viewed an aperture containing moving dots in which dot color and direction of motion changed once per second. At any given moment, observers attended to either motion or color. Two of six motion directions and two of six colors embedded in the stimulus stream cued subjects either to shift attention from the currently attended to the unattended feature or to maintain attention on the currently attended feature. Attentional modulation of the blood oxygenation level dependent (BOLD) fMRI signal was observed in early visual areas that are selective for motion and color. More importantly, both transient and sustained BOLD activity patterns were observed in different fronto-parietal cortical areas during shifts of attention. We suggest these differing temporal profiles reflect complementary roles in the control of attention to perceptual features.

Top-Down Control of Biased Competition During Covert Spatial Orienting

E. Awh, M. Matsukura, J. Serences (Journal of Experimental Psychology-Human Perception and Performance, 2003

Larger benefits of spatial attention are observed when distractor interference is prevalent, supporting the view that spatial selection facilitates visual processing by suppressing distractor interference. The present work shows that cuing effects with identical visual displays can grow substantially as the probability of distractor interference increases. The probability of interference had no impact on spatial cuing effects in the absence of distractors, suggesting that the enlarged cuing effects were not caused by changes in signal enhancement or in the spatial distribution of attention. These findings suggest that attentional control settings determine more than where spatial attention is directed; top-down settings also influence how attention affects visual processing, with increased levels of distractor exclusion when distractor interference is likely.

Neural mechanisms of space-based and object based attentional control

S. Yantis, J. Serences (Current Opinion in Neurobiology, 2003).

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.

Transient neural activity in human parietal cortex during spatial attention shifts

S. Yantis, J. Schwarzbach, J. Serences, R.L. Carlson, M.A. Steinmetz, J.J. Pekar, S.M. Courtney (Nature Neuroscience, 2002).

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.