Spatio-temporal analysis of feature-based attention.

The cortical mechanisms of feature-selective attention to color and motion cues were studied in humans using combined electrophysiological, magnetoencephalographic, and hemodynamic (functional magnetic resonance imaging) measures of brain activity. Subjects viewed a display of random dots that periodically either changed color or moved coherently. When attention was directed to the color change it elicited enhanced neural activity in visual area V4v, previously shown to be specialized for processing color information. In contrast, when dot movement was attended it produced enhanced activity in the motion-specialized area human MT. Parallel recordings of event-related electrophysiological and magnetoencephalographic responses indicated that the attention-related facilitation of neural activity in these specialized cortical areas occurred rapidly, beginning as early as 90-120 ms after stimulus onset. We conclude that selection of an entire feature dimension (motion or color) boosts neural activity in its specialized cortical module much more rapidly than does selection of one feature value from another (e.g., one color from another), as reported in previous electrophysiological studies. By combining methods with high spatial and temporal resolution it is possible to analyze the precise time course of feature-selective processing in specialized cortical areas.

Feature-selective attention enhances color signals in early visual areas of the human brain.

We used an electrophysiological measure of selective stimulus processing (the steady-state visual evoked potential, SSVEP) to investigate feature-specific attention to color cues. Subjects viewed a display consisting of spatially intermingled red and blue dots that continually shifted their positions at random. The red and blue dots flickered at different frequencies and thereby elicited distinguishable SSVEP signals in the visual cortex. Paying attention selectively to either the red or blue dot population produced an enhanced amplitude of its frequency-tagged SSVEP, which was localized by source modeling to early levels of the visual cortex. A control experiment showed that this selection was based on color rather than flicker frequency cues. This signal amplification of attended color items provides an empirical basis for the rapid identification of feature conjunctions during visual search, as proposed by "guided search" models.

Objects are highlighted by spatial attention.

Selective attention may be focused upon a region of interest within the visual surroundings, thereby improving the perceptual quality of stimuli at that location. It has been debated whether this spatially selective mechanism plays a role in the attentive selection of whole objects in a visual scene. The relationship between spatial and object-selective attention was investigated here through recordings of event-related brain potentials (ERPs) supplemented with functional magnetic brain imaging (fMRI). Subjects viewed a display consisting of two bar-shaped objects and directed attention to sequences of stimuli (brief corner offsets) at one end of one of the bars. Unattended stimuli belonging to the same object as the attended stimuli elicited spatiotemporal patterns of neural activity in the visual cortex closely resembling those elicited by the attended stimuli themselves, albeit smaller in amplitude. This enhanced neural activity associated with object-selective attention was localized by use of ERP dipole modeling and fMRI to the lateral occipital extrastriate cortex. We conclude that object-selective attention shares a common neural mechanism with spatial attention that entails the facilitation of sensory processing of stimuli within the boundaries of an attended object.

Effects of spatial congruity on audio-visual multimodal integration.

Spatial constraints on multisensory integration of auditory (A) and visual (V) stimuli were investigated in humans using behavioral and electrophysiological measures. The aim was to find out whether cross-modal interactions between A and V stimuli depend on their spatial congruity, as has been found for multisensory neurons in animal studies (Stein & Meredith, 1993). Randomized sequences of unimodal (A or V) and simultaneous bimodal (AV) stimuli were presented to right- or left-field locations while subjects made speeded responses to infrequent targets of greater intensity that occurred in either or both modalities. Behavioral responses to the bimodal stimuli were faster and more accurate than to the unimodal stimuli for both same-location and different-location AV pairings. The neural basis of this cross-modal facilitation was studied by comparing event-related potentials (ERPs) to the bimodal AV stimuli with the summed ERPs to the unimodal A and V stimuli. These comparisons revealed neural interactions localized to the ventral occipito-temporal cortex (at 190 msec) and to the superior temporal cortical areas (at 260 msec) for both same- and different-location AV pairings. In contrast, ERP interactions that differed according to spatial congruity included a phase and amplitude modulation of visual-evoked activity localized to the ventral occipito-temporal cortex at 100-400 msec and an amplitude modulation of activity localized to the superior temporal region at 260-280 msec. These results demonstrate overlapping but distinctive patterns of multisensory integration for spatially congruent and incongruent AV stimuli.

Exogenous attentional selection of transparent superimposed surfaces modulates early event-related potentials.

Using a transparent motion paradigm, [Valdes-Sosa, M., Bobes, M. A., Rodriguez, V., & Pinilla, T. (1998). Switching attention without shifting the spotlight object-based attentional modulation of brain potentials, Journal of Cognitive Neuroscience, 10, 137-151; Valdes-Sosa, M., Cobo, A., & Pinilla, T. (2000). Attention to object files defined by transparent motion, Journal of Experimental Psychological: Human Perception and Performance, 26, 488-505] found that when attention is endogenously directed to one surface, observers can more reliably report the direction of a brief translation of the cued than the uncued surface. Using a similar design [Reynolds, J. H., Alborzian, S., & Stoner, G. R. (2003). Exogenously cued attention triggers competitive selection of surfaces, Vision Research, 43, 59-66] found that even in the absence of an endogenous cue, the first translation acted as a potent exogenous cue that impaired the observer's ability to discriminate a subsequent translation of the other surface. We investigated the neural basis of this exogenous cueing effect by recording visual event-related potentials (ERPs) elicited by translations of the cued and uncued surfaces. Subjects were given the task of judging whether or not the first and second translations were identical in direction, and their performance was impaired when the second translation occurred on the uncued, as compared to the cued surface. The posterior C1 (75-110 ms) and N1 (160-210 ms) components of the ERP elicited by the second translation of the cued surface were larger than those elicited by translation of the uncued surface. These behavioral and ERP cueing effects were present even when the two surfaces were identical in color and thus could not be attributed to attention-related modulations of the gain of color channels. These findings provide evidence that exogenous cueing results in preferential selection of the cued surface at both early and intermediate stages of visual-cortical processing.

Dynamics of feature binding during object-selective attention.

Objects in the environment may be attended selectively and perceived as unified ensembles of their constituent features. To investigate the timing and cortical localization of feature-integration mechanisms in object-based attention, recordings of event-related potentials and magnetic fields were combined with functional MRI while subjects attended to one of two superimposed transparent surfaces formed by arrays of dots moving in opposite directions. A spatiotemporal analysis revealed evidence for a rapid increase in neural activity localized to a color-selective region of the fusiform gyrus when the surface moving in the attended direction displayed an irrelevant color feature. These data provide support for the "integrated-competition" model of object-selective attention and point to a dynamic neural substrate for the rapid binding process that links relevant and irrelevant features to form a unified perceptual object.

Sustained division of the attentional spotlight.

By voluntarily directing attention to a specific region of a visual scene, we can improve our perception of stimuli at that location. This ability to focus attention upon specific zones of the visual field has been described metaphorically as a moveable spotlight or zoom lens that facilitates the processing of stimuli within its 'beam'. A long-standing controversy has centred on the question of whether the spotlight of spatial attention has a unitary beam or whether it can be divided flexibly to disparate locations. Evidence supporting the unitary spotlight view has come from numerous behavioural and electrophysiological studies. Recent experiments, however, indicate that the spotlight of spatial attention may be divided between non-contiguous zones of the visual field for very brief stimulus exposures (&<100 ms). Here we use an electrophysiological measure of attentional allocation (the steady-state visual evoked potential) to show that the spotlight may be divided between spatially separated locations (excluding interposed locations) over more extended time periods. This spotlight division appears to be accomplished at an early stage of visual-cortical processing.

An analysis of audio-visual crossmodal integration by means of event-related potential (ERP) recordings.

Crossmodal integration was studied in humans by presenting random sequences of auditory (brief noise bursts), visual (flashes), and audiovisual (simultaneous noise bursts and flashes) stimuli from a central location at irregular intervals between 600 and 800 ms. The subjects' task was to press a button to infrequent and unpredictable (P=0.15) target stimuli that could be either a more intense noise burst, a brighter flash, or a combination of the two. In accordance with previous studies, behavioral data showed that bimodal target stimuli were responded to much faster and were identified more accurately than the unimodal target stimuli. The neural basis of this crossmodal interaction was investigated by subtracting the ERPs to the auditory (A) and the visual (V) stimuli alone from the ERP to the combined audiovisual (AV) stimuli (i.e. interaction=AV-(A+V)). Using this approach, we replicated previous reports of both early (at around 40 ms) and late (after 100 ms) ERP interaction effects. However, it appears that the very early interaction effects can be largely accounted for by an anticipatory ERP that precedes both the unimodal and bimodal stimuli. In calculating the ERP interaction this slow shift is subtracted twice, resulting in an apparent shift of the opposite polarity that may be confounded with actual crossmodal interactions.

Electrophysiological analysis of cortical mechanisms of selective attention to high and low spatial frequencies.

OBJECTIVES: This study investigated whether short-latency (<100ms) event-related potential (ERP) components were modulated during attention to spatial frequency (SF) cues. METHODS: Sinusoidally modulated checkerboard stimuli having high (5 cycles per degree (cpd)) or low (0.8cpd) SF content were presented in random order at intervals of 400-650ms. Subjects attended to either the high or low SF stimuli, with the task of detecting targets of slightly higher or lower SF, respectively, than the above standards. ERPs were recorded from 42 scalp sites during task performance and spatio-temporal analyses were carried out on sensory-evoked and attention-related components. RESULTS: Attended high SF stimuli elicited an early negative difference potential (ND120) starting at about 100ms, whereas attended low SF stimuli elicited a positivity (PD130) in the same latency range. The neural sources of both effects were estimated with dipole modeling to lie in dorsal, extrastriate occipital areas. Earlier evoked components evoked at 60-100ms that were modeled with striate and extrastriate cortical sources were not affected by attention to SF. Starting at 150ms, attended stimuli of both SFs elicited a broad selection negativity (SN) that was localized to ventral extrastriate visual cortex. The SN was larger over the left/right cerebral hemisphere for attended stimuli of high/low SF. CONCLUSIONS: These results support the view that attention to SF does not involve a mechanism of amplitude modulation of early-evoked components prior to 100ms. Attention to high and low SF information involves qualitatively different and hemispherically specialized neural processing operations.

Electrophysiological evidence for the "missing link" in crossmodal attention.

Orienting attention involuntarily to the location of a sensory event influences responses to subsequent stimuli that appear in different modalities with one possible exception: orienting attention involuntarily to a sudden light sometimes fails to affect responses to subsequent sounds (e.g., Spence & Driver, 1997). Here we investigated the effects of involuntary attention to a brief flash on the processing of subsequent sounds in a design that eliminates stimulus-response compatibility effects and criterion shifts as confounding factors. In addition, the neural processes mediating crossmodal attention were studied by recording event-related brain potentials. Our data show that orienting attention to the location of a spatially nonpredictive visual cue modulates behavioural and neural responses to subsequent auditory targets when the stimulus onset asynchrony is short (between 100 and 300 ms). These findings are consistent with the hypothesis that involuntary shifts of attention are controlled by supramodal brain mechanisms rather than by modality-specific ones.

Putting spatial attention on the map: timing and localization of stimulus selection processes in striate and extrastriate visual areas.

This study investigated the cortical mechanisms of visual-spatial attention in a task where subjects discriminated patterned targets in one visual field at a time. Functional magnetic imaging (fMRI) was used to localize attention-related changes in neural activity within specific retinotopic visual areas, while recordings of event-related brain potentials (ERPs) traced the time course of these changes. The earliest ERP components enhanced by attention occurred in the time range 70-130 ms post-stimulus onset, and their neural generators were estimated to lie in the dorsal and ventral extrastriate visual cortex. The anatomical areas activated by attention corresponded closely to those showing increased neural activity during passive visual stimulation. Enhanced neural activity was also observed in the primary visual cortex (area V1) with fMRI, but ERP recordings indicated that the initial sensory response at 50-90 ms that was localized to V1 was not modulated by attention. Modeling of ERP sources over an extended time range showed that attended stimuli elicited a long-latency (160-260 ms) negativity that was attributed to the dipolar source in area V1. This finding is in line with hypotheses that V1 activity may be modulated by delayed, reentrant feedback from higher visual areas.

Involuntary orienting to sound improves visual perception.

To perceive real-world objects and events, we need to integrate several stimulus features belonging to different sensory modalities. Although the neural mechanisms and behavioural consequences of intersensory integration have been extensively studied, the processes that enable us to pay attention to multimodal objects are still poorly understood. An important question is whether a stimulus in one sensory modality automatically attracts attention to spatially coincident stimuli that appear subsequently in other modalities, thereby enhancing their perceptual salience. The occurrence of an irrelevant sound does facilitate motor responses to a subsequent light appearing nearby. However, because participants in previous studies made speeded responses rather than psychophysical judgements, it remains unclear whether involuntary auditory attention actually affects the perceptibility of visual stimuli as opposed to postperceptual decision and response processes. Here we provide psychophysical evidence that a sudden sound improves the detectability of a subsequent flash appearing at the same location. These data show that the involuntary orienting of attention to sound enhances early perceptual processing of visual stimuli.

The role of spatial selective attention in working memory for locations: evidence from event-related potentials.

We investigated the hypothesis that the covert focusing of spatial attention mediates the on-line maintenance of location information in spatial working memory. During the delay period of a spatial working-memory task, behaviorally irrelevant probe stimuli were flashed at both memorized and nonmemorized locations. Multichannel recordings of event-related potentials (ERPs) were used to assess visual processing of the probes at the different locations. Consistent with the hypothesis of attention-based rehearsal, early ERP components were enlarged in response to probes that appeared at memorized locations. These visual modulations were similar in latency and topography to those observed after explicit manipulations of spatial selective attention in a parallel experimental condition that employed an identical stimulus display.

Guidelines for using human event-related potentials to study cognition: recording standards and publication criteria.

Event-related potentials (ERPs) recorded from the human scalp can provide important information about how the human brain normally processes information and about how this processing may go awry in neurological or psychiatric disorders. Scientists using or studying ERPs must strive to overcome the many technical problems that can occur in the recording and analysis of these potentials. The methods and the results of these ERP studies must be published in a way that allows other scientists to understand exactly what was done so that they can, if necessary, replicate the experiments. The data must then be analyzed and presented in a way that allows different studies to be compared readily. This paper presents guidelines for recording ERPs and criteria for publishing the results.

Spatial attention to central and peripheral auditory stimuli as indexed by event-related potentials.

Young adult subjects attended selectively to brief noise bursts delivered in free-field via central and peripheral arrays of four loudspeakers each that were arranged along a semi-circle extending from the midline to 90 degrees right of center. Frequent "standard" stimuli (90%) and infrequent "target/deviant" stimuli (10%) of increased bandwidth were delivered at a fast rate in random order and equiprobably from all eight speakers. In separate runs, the subject's task was to selectively attend to the center or rightmost speaker, and to press a button to the infrequent "target" stimuli occurring at the designated (spatial) location. Behavioral detection rates and concurrently recorded event-related potentials (ERPs) indicated that auditory attention was deployed as a finely tuned gradient around the attended source. The attentional gradients were steeper for the central than the peripheral array, indicating that attention can be more sharply focused upon sound sources directly in front of the listener. The ERP data suggested that selection for location is accomplished in two distinct stages, with an initial broadly tuned filtering, followed by a more narrowly focused selection of attended-location deviants.

Intra-modal and cross-modal spatial attention to auditory and visual stimuli. An event-related brain potential study.

This study investigated cross-modal interactions in spatial attention by means of recording event-related brain potentials (ERPs). Noise bursts and light flashes were presented in random order to both left and right field locations separated by 60 degrees in free-field. One group of subjects was instructed to attend selectively to the noise bursts (attend-auditory group), and a second group attended only to the flashes (attend-visual group). On different runs attention was directed to either the right or left field stimuli of the designated modality. In the attend-auditory group, noise bursts at the attended location elicited a broad, biphasic negativity (Nd) beginning at 70 ms. The cross-modal spatial attention effect on the auditory ERPs in the attend-visual group was very similar in morphology, but the Nd was reduced in amplitude relative to the intra-modal effect. In the attend-visual group, flashes at the attended location elicited enhanced early (100-200 ms) and late (200-350 ms) ERP components relative to unattended-location flashes. The cross-modal effect in the attend-auditory group included small but significant enhancements of early components of the visual ERPs. It was concluded that spatial attention has a multi-modal organization such that the processing of stimuli at attended locations is facilitated at an early, sensory level, even for stimuli of an unattended modality.

Improved auditory spatial tuning in blind humans.

Despite reports of improved auditory discrimination capabilities in blind humans and visually deprived animals, there is no general agreement as to the nature or pervasiveness of such compensatory sensory enhancements. Neuroimaging studies have pointed out differences in cerebral organization between blind and sighted humans, but the relationship between these altered cortical activation patterns and auditory sensory acuity remains unclear. Here we compare behavioural and electrophysiological indices of spatial tuning within central and peripheral auditory space in congenitally blind and normally sighted but blindfolded adults to test the hypothesis (raised by earlier studies of the effects of auditory deprivation on visual processing) that the effects of visual deprivation might be more pronounced for processing peripheral sounds. We find that blind participants displayed localization abilities that were superior to those of sighted controls, but only when attending to sounds in peripheral auditory space. Electrophysiological recordings obtained at the same time revealed sharper tuning of early spatial attention mechanisms in the blind subjects. Differences in the scalp distribution of brain electrical activity between the two groups suggest a compensatory reorganization of brain areas in the blind that may contribute to the improved spatial resolution for peripheral sound sources.

Involvement of striate and extrastriate visual cortical areas in spatial attention.

We investigated the cortical mechanisms of visual-spatial attention while subjects discriminated patterned targets within distractor arrays. Functional magnetic resonance imaging (fMRI) was used to map the boundaries of retinotopic visual areas and to localize attention-related changes in neural activity within several of those areas, including primary visual (striate) cortex. Event-related potentials (ERPs) and modeling of their neural sources, however, indicated that the initial sensory input to striate cortex at 50-55 milliseconds after the stimulus was not modulated by attention. The earliest facilitation of attended signals was observed in extrastriate visual areas, at 70-75 milliseconds. We hypothesize that the striate cortex modulation found with fMRI may represent a delayed, re-entrant feedback from higher visual areas or a sustained biasing of striate cortical neurons during attention. ERP recordings provide critical temporal information for analyzing the functional neuroanatomy of visual attention.

The gradient of spatial auditory attention in free field: an event-related potential study.

Young adult subjects attended selectively to brief noise bursts delivered in free field via a horizontal array of seven loudspeakers spaced apart by 9 degrees of angle. Frequent "standard" stimuli (90%) and infrequent "target/deviant" stimuli (10%) of increased bandwidth were delivered at a fast rate in a random sequence equiprobably from each speaker. In separate runs, the subjects' task was to selectively attend to the leftmost, center, or rightmost speaker and to press a button to the infrequent "target" stimuli occurring at the designated spatial location. Behavioral detection rates and concurrently recorded event-related potentials (ERPs) indicated that auditory attention was deployed as a finely tuned gradient around the attended sound source, thus providing support for gradient models of auditory spatial attention. Furthermore, the ERP data suggested that the spatial focusing of attention was achieved in two distinct stages, with an early more broadly tuned filtering of inputs occurring over the first 80-200 msec after stimulus onset, followed by a more narrowly focused selection of attended-location deviants that began at around 250 msec and closely resembled the behavioral gradient of target detections.

Sensory gain control (amplification) as a mechanism of selective attention: electrophysiological and neuroimaging evidence.

Both physiological and behavioral studies have suggested that stimulus-driven neural activity in the sensory pathways can be modulated in amplitude during selective attention. Recordings of event-related brain potentials indicate that such sensory gain control or amplification processes play an important role in visual-spatial attention. Combined event-related brain potential and neuroimaging experiments provide strong evidence that attentional gain control operates at an early stage of visual processing in extrastriate cortical areas. These data support early selection theories of attention and provide a basis for distinguishing between separate mechanisms of attentional suppression (of unattended inputs) and attentional facilitation (of attended inputs).