Auditory Attention

Hillyard, S A, Hink, R F, Schwent, V L, & Picton, T W, 1973: Electrical Signs of Selective Attention in the Human Brain. Science, 182(4108):177-180.

Auditory evoked potentials were recorded from the cortex of subjects who listened selectively to a series of tone pips in one ear and ignored concurrent tone pips in the other ear. The negative component o f the evoked potential peaking at 80 to 110 milliseconds was substantially larger for the attended tones. This negative component indexed a stimiilus set mode of selective attention toward the tone pips in one ear. A late positive component peaking at 250 to 400 milliseconds reflected the response set established to recognize infrequent, higher pitched tone pips in the attended series.

Schröger, E, & Wolff, C, 1998: Behavioral and electrophysiological effects of task-irrelevant sound change: a new distraction paradigm. Cognitive Brain Research, 7(1):71-87.

A distraction paradigm was utilized that is suited to yield reliable auditory distraction on an individual level even with rather small frequency deviances (7%). Distraction to these tiny deviants was achieved by embedding task-relevant aspects and task-irrelevant, distracting aspects of stimulation into the same perceptual object. Event-related potential (ERP) and behavioral effects of this newly developed paradigm were determined. Subjects received tones that could be of short or long duration equiprobably. They were instructed to press a response button to long-duration tones (targets). In oddball blocks, tones could be of standard frequency or of low-probability (p = 0.1), deviant frequency. The task-irrelevant frequency deviants elicited MMN, N2b, and P3a components, and caused impoverished behavioral performance to targets. The usage of tiny distractors permits an interpretation of auditory distraction in terms of attention-switching due to a particular memory-related change-detection process. On the basis of the results from an additional condition in which tones were of 10 different frequencies (involving those frequencies which served as standard and deviant in oddball blocks), it is argued that one important prerequisite for linking the neural mechanisms reflected in change-related brain waves to behavioral distraction effects may be regarded as fulfilled. The robustness of the distraction effects to tiny deviations was confirmed in two control experiments.

Shucard, D W, Abara, J P, Cabe, D C M, Benedict, R B H, & Shucard, J L, 2004: The effects of covert attention and stimulus complexity on the P3 response during an auditory continuous performance task. Int'l J. Psychophysiology, 54(3):221-230.

This study examined the effects of motor responding and stimulus complexity on the event-related potential (ERP) P3 amplitude and latency during an auditory continuous performance task (A-CPT). Subjects were presented with undegraded and degraded syllables during two experimental conditions. In the motor attention (MA) condition participants performed a button press to target syllables. In the covert attention (CA) condition, participants listened for target syllables without responding. The ERP P3 amplitude for targets during MA and CA showed the expected anterior-to-posterior scalp topography, with the greatest amplitude at Pz. Although amplitudes across all scalp sites were greater for MA than CA target P3 responses, both MA and CA targets had greater P3 amplitudes than the P3 for the nontarget syllables (NT). There was no effect of stimulus complexity (degraded vs. undegraded) on P3 amplitude. However, stimulus complexity did affect P3 latency. Degraded syllables elicited longer P3 latency than undegraded syllables for both the MA and CA conditions. The amplitude and topography findings show that when stimulus probability is controlled through the use of a CPT paradigm, a reliable P3 component is present even when the task does not require a motor response to target stimuli.

Tactile Attention

Corneil, B D, Munoz, D P, Chapman, B B, Admans, T, & Cushing, S L, 2007: Neuromuscular consequences of reflexive covert orienting. Nature Neuroscience, 11:13-15.

Top of pageVisual stimulus presentation activates the oculomotor network without requiring a gaze shift. Here, we demonstrate that primate neck muscles are recruited during such reflexive covert orienting in a manner that parallels activity recorded from the superior colliculus (SC). Our results indicate the presence of a brainstem circuit whereby reflexive covert orienting is prevented from shifting gaze, but recruits neck muscles, predicting that similarities between SC and neck muscle activity should extend to other cognitive processes that are known to influence SC activity.

Eimer, M, & Forster, B, 2003: Modulations of early somatosensory ERP components by transient and sustained spatial attention. Experimental Brain Research, 151(1):24-31.

To investigate when and how spatial attention affects somatosensory processing, event-related brain potentials (ERPs) were recorded in response to mechanical tactile stimuli delivered to the left and right hand while attention was directed to one of these hands. The attended hand either remained constant throughout an experimental block (sustained attention), or was changed across successive trials (transient attention). Attentional modulations of the N140 component and a sustained 'processing negativity' for attended stimuli were observed in both attention conditions. However, attentional effects on earlier somatosensory components differed systematically. Sustained attention resulted in a contralateral negativity overlapping with the N80 component, while transient attention was reflected by a bilateral positivity overlapping with the P100 component. This dissociation indicates that sustained and transient attention affect different somatosensory areas. It is suggested that sustained attention can modulate tactile processing within primary somatosensory cortex (S1), while effects of transient attention are located beyond S1. Overall, results demonstrate that spatial selectivity in touch is mediated by activity modulations in modality-specific somatosensory cortex.

Forster, B, & Eimer, M, 2005: Covert attention in touch: Behavioral and ERP evidence for costs and benefits. Psychophysiology, 42(2):171-179.

To investigate the mechanism underlying tactile spatial attention, reaction times (RTs) and event-related potentials (ERPs) were recorded in response to mechanical stimuli delivered to the hands. At the start of each trial cues indicated either the correct (valid) or incorrect (invalid) tactile stimulus location or were uninformative (neutral). RT costs (suppression of invalid compared to neutral trials) were found to be larger than benefits (enhancement of valid compared to neutral trials). ERPs showed that costs and benefits contribute equally to attentional modulations of the somatosensory N140 component, whereas these were largely due to costs at longer latencies. These results differ from the pattern of attentional ERP modulations previously found for vision and audition, where costs precede benefits, and therefore suggest that the mechanisms of attentional selectivity in touch might be different from attentional processes in other modalities.

Visual Attention

Carrasco, M, & McElree, B, 2001: Covert attention accelerates the rate of visual information processing. Proc. Nat. Acad. Sci. 98(9):5363-5367.

Whenever we open our eyes, we are confronted with an overwhelming amount of visual information. Covert attention allows us to select visual information at a cued location, without eye movements, and to grant such information priority in processing. Covert attention can be voluntarily allocated, to a given location according to goals, or involuntarily allocated, in a reflexive manner, to a cue that appears suddenly in the visual field. Covert attention improves discriminability in a wide variety of visual tasks. An important unresolved issue is whether covert attention can also speed the rate at which information is processed. To address this issue, it is necessary to obtain conjoint measures of the effects of covert attention on discriminability and rate of information processing. We used the response-signal speed-accuracy tradeoff (SAT) procedure to derive measures of how cueing a target location affects speed and accuracy in a visual search task. Here, we show that covert attention not only improves discriminability but also accelerates the rate of information processing.

Carrasco, M, Williams, P E, & Yeshurun, Y, 2002: Covert attention increases spatial resolution with or without masks: Support for signal enhancement. Journal of Vision 2(6):467-479.

Visual attention can increase spatial resolution even when it leads to a decrease in performance. Whether this effect is mediated by reduction of external noise or by signal enhancement is an unsettled question. Although we previously demonstrated that attention can improve speed and accuracy in an acuity task, those experiments made use of a local postmask, which could be considered a source of external noise. In this work, a peripheral cue improved observers' abilities to indicate which side of a Landolt-square target had a gap whether or not a local postmask was used and with both central- and spread-neutral cues. In addition, we documented the presence of visual field inhomogeneities in a resolution task. Given that these experiments presented the target alone with no external noise added (i.e., without distracters or masks), our results indicate that transient attention enhanced the quality of the stimulus representation. Furthermore, because performance in the Landolt-square task indexes resolution, this attentional benefit indicates that transient attention can produce signal enhancement through finer spatial resolution.

Carrasco, M, & Yeshurun, Y, 1998: The contribution of covert attention to the set-size and eccentricity effects in visual search. J. Experim. Psychol.: Hum. Percept. Perform. 24(2):673-692.

To reexamine the role of covert attention in visual search, the authors directly manipulated attention by peripherally cueing the target location and analyzed its effects on the set-size and the eccentricity effects. Observers participated in feature and conjunction tasks. Experiment 1 used precues, and Experiment 2 used postcues in a yes-no task under valid-, invalid-, and neutral-cueing conditions. Experiments 3 and 4 used a 2-interval alternative forced-choice visual-search task under cued and neutral conditions. Precueing the target location improved performance in feature and conjunction searches; postcueing did not. For the cued targets, the eccentricity effect for features and conjunctions was diminished, suggesting that the attentional mechanism improves the quality of the sensory representation of the attended location. The conjunction set-size effect was reduced but not eliminated. This questions serial-search models that attribute a major role to covert attention in visual search.

Corbetta, M, Shulman, G L, 2002: Control of goal-directed and stimulus-driven attention in the brain. Nat. Rev. Neurosci. 3(3):201-215.

We review evidence for partially segregated networks of brain areas that carry out different attentional functions. One system, which includes parts of the intraparietal cortex and superior frontal cortex, is involved in preparing and applying goal-directed (top-down) selection for stimuli and responses. This system is also modulated by the detection of stimuli. The other system, which includes the temporoparietal cortex and inferior frontal cortex, and is largely lateralized to the right hemisphere, is not involved in top-down selection. Instead, this system is specialized for the detection of behaviourally relevant stimuli, particularly when they are salient or unexpected. This ventral frontoparietal network works as a 'circuit breaker' for the dorsal system, directing attention to salient events. Both attentional systems interact during normal vision, and both are disrupted in unilateral spatial neglect.

Culham, J C, & Kanwisher, N G, 2001: Neuroimaging of cognitive functions in human parietal cortex. Curr. Opin. Neurobiol. 11(2):157-163.

Functional neuroimaging has proven highly valuable in mapping human sensory regions, particularly visual areas in occipital cortex. Recent evidence suggests that human parietal cortex may also consist of numerous specialized subregions similar to those reported in neurophysiological studies of non-human primates. However, parietal activation generalizes across a wide variety of cognitive tasks and the extension of human brain mapping into higher-order 'association cortex' may prove to be a challenge.

Hawkins, H L, Hillyard, S A, Luck, S J, Mouloua, M, Downing, C J, & Woodword, D P, 1990: Visual attention modulates signal detectability. J. Experim. Psychol.: Hum. Percept. Perform. 16(4):802-811..

The mechanism by which visual-spatial attention affects the detection of faint signals has been the subject of considerable debate. It is well known that spatial cuing speeds signal detection. This may imply that attentional cuing modulates the processing of sensory information during detection or, alternatively, that cuing acts to create decision bias favoring input at the cued location. These possibilities were evaluated in 3 spatial cuing experiments, with a total of 22 university students. Peripheral cues were used in Exp 1, and central cues were used in Exps 2 and 3. Cuing similarly enhanced measured sensitivity, P(A) and d', for simple luminance detection in all 3 experiments. Under some conditions it also induced shifts in decision criteria (beta). These findings indicate that visual-spatial attention facilitates the processing of sensory input during detection either by increasing sensory gain for inputs at cued locations or by prioritizing the processing of cued inputs.

Heinze, H J, & Mangun, G R, 1995: Electrophysiological signs of sustained and transient attention to spatial locations. Neuropsychologia, 33(7):889-908.

Event-related potentials were elicited by bilateral and unilateral stimulus arrays flashed in rapid sequence in order to investigate both focused attention and attentional orienting. Subjects attended selectively to the stimuli on one side of the bilateral arrays and were required to discriminate infrequent target stimuli on either the attended side (no switch of attentional focus) or unattended side of the array (switch of attentional focus). The ERPs to the bilateral stimuli elicited an occipital P1 component that was larger in amplitude over scalp regions contralateral to the attended visual halffield. The ERPs to the unilateral stimuli on the attended side also showed an amplitude enhancement of early P1 components, followed by a positive shift that lasted until 200 msec latency over the contralateral occipital scalp. No enhancement of the N1 component was observed to attended-side stimuli. These patterns were not different for conditions requiring or not requiring a spatial switch of the attentional focus. In conjunction with ERP signs of focused spatial attention, significant differences in discrimination performance (d') were obtained for the attended vs unattended-side targets; no changes in measures of criterion (ß) were obtained. These data support the idea that the early occipital P1 attention effect represents a facilitation of visual inputs that occur at attended locations in the visual field.

Joseph, J S, Chun, M M, & Nakayama, K, 1997: Attentional requirements in a 'preattentive' feature search task. Nature, 387(6635):805-807.

It is commonly assumed that certain features are so elementary to the visual system that they require no attentional resources to be perceived. Such 'preattentive' features are traditionally identified by visual search performance, in which the reaction time for detecting a feature difference against a set of distractor items does not increase with the number of distractors. This suggests an unlimited capacity for the perception of such features. We provide evidence to the contrary, demonstrating that detection of differences in a simple feature such as orientation is severely impaired by additionally imposing an attentionally demanding rapid serial visual presentation task involving letter identification. The same visual stimuli exhibit non-increasing reaction time versus set-size functions. These results demonstrate that attention can be critical even for the detection of so-called 'preattentive' features.

Kastner, S, & Ungerleider, L G, 2000: Mechanisms of visual attention in the human cortex. Ann. Rev. Neurosci. 23:315-341.

A typical scene contains many different objects that, because of the limited processing capacity of the visual system, compete for neural representation. The competition among multiple objects in visual cortex can be biased by both bottom-up sensory-driven echanisms and top-down influences, such as selective attention. Functional brain imaging studies reveal that, both in the absence and in the presence of visual stimulation, biasing signals due to selective attention can modulate neural activity in visual cortex in several ways. Although the competition among stimuli for representation is ultimately resolved within visual cortex, the source of top-down biasing signals derives from a network of areas in frontal and parietal cortex.

Kelley, T A, Serences, J T, Giesbrecht, B, & Yantis, S, 2008: Cortical Mechanisms for Shifting and Holding Visuospatial Attention. Cereb. Cortex, 18(1):114-125.

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 functional magnetic resonance imaging to dissociate these components while observers shifted attention among 3 streams of letters and digits, one located at fixation and 2 in the periphery. Compared with 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 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.

Kelly, S P, Lalor, E, Finucane, C, & Reilly, R B, 2004: A Comparison of Covert and Overt Attention as a Control Option in a Steady-State Visual Evoked Potential-based Brain Computer Interface. Proc. 26th Annual Int'l Conf. IEEE EMBS.

EEG data were recorded from occipital scalp regions of subjects who attended to an alternating checkerboard stimulus in one visual field while ignoring a similar stimulus of a different frequency in the opposite visual field. Classification of left/right spatial attention is attempted by extracting Steady-State Visual Evoked Potentials (SSVEPs) elicited by the stimuli to assess the potential use of such a spatial selective attention paradigm in a Brain Computer Interface (BCI). Experimental setup and analysis procedure in a previous study in which eye movement is permitted are replicated in order to quantify differences in classification performance using overt and covert attention. Four variations of the basic paradigm, involving both feedback and addition of extra mental load, are studied for comparison. The average accuracy is found to be reduced by ~20% in the switch from overt to covert attention when no other specifications of the task are changed.

Luck, S J, Hillyard, S A, Mouloua, M, Woldorff, M G, Clark, V P, & Hawkins, H L, 1994: Effects of spatial cuing on luminance detectability: Psychophysical and electrophysiological evidence for early selection. J. Experim. Psychol.: Hum. Percept. Perform. 20(4):887-904.

Three experiments were conducted to determine whether attention-related changes in luminance detectability reflect a modulation of early sensory processing. Exps 1 and 2 used peripheral cues to direct attention and found substantial effects of cue validity on target detectability; these effects were consistent with a sensory-level locus of selection but not with certain memory- or decision-level mechanisms. In Exp 3, event-related brain potentials were recorded in a similar paradigm using central cues, and attention was found to produce changes in sensory-evoked brain activity beginning within the 1st 1 msec of stimulus processing. These changes included both an enhancement of sensory responses to attended stimuli and a suppression of sensory responses to unattended stimuli; the enhancement and suppression effects were isolated to different neural responses, indicating that they may arise from independent attentional mechanisms.

Mack, A, Tang, B, Tuma, R, Kahn, S, & Rock, I, 1992: Perceptual organization and attention. Cogn. Psychol. 24(4):475-501.

It is widely assumed that the grouping of the visual field first described by the Gestalt psychologists and the related phenomenon of texture segregation occur very early in the processing of visual information and involve preattentive processes. All the recent evidence supporting this assumption comes from visual search experiments in which the subject is actively looking for a target and attending to the stimulus. The question at issue is whether these kinds of patterns are perceived under conditions of inattention, i.e., when observers are not searching for them. We performed six experiments to determine whether texture segregation and grouping by similarity or proximity are perceived under conditions of inattention. On the first two trials subjects were asked to report the longer arm of a briefly presented cross which was surrounded by a pattern of ungrouped small elements. On the third trial and subsequent control trials these elements were configured into grouping patterns and subjects queried about them immediately following their line length reports. The results establish that neither texture segregation nor grouping by similarity of lightness or proximity are perceived under conditions of inattention. They support the conclusion that there is an earlier stage of processing than that referred to as preattentive.

Mangun, G R, 1995: Neural mechanisms of visual selective attention. Psychophysiology 32(1):4-18.

Visual selective attention improves our perception and performance by modifying sensory inputs at an early stage of processing. Spatial attention produces the most consistent early modulations of visual processing, which can be observed when attention is voluntarily allocated to locations. These effects of spatial attention are similar when attention is cued in a trial-by-trial, or sustained, fashion and are manifest as changes in the amplitudes, but not the latencies, of evoked neural activity recorded from the intact human scalp. This modulation of sensory processing first occurs within the extrastriate visual cortex and not within the striate or earlier subcortical processing stages. These relatively early spatial filters alter the inputs to higher stages of visual analysis that are responsible for feature extraction and ultimately object perception and recognition, and thus provide physiological evidence for early precategorical selection during visual attention. Moreover, the physiological evidence extends early selection theories by providing neurophysiologically precise information about the stages of visual processing affected by attention.

Moore, T, Armstrong, K M, & Fallah, M, 2003: Visuomotor origins of covert spatial attention. Neuron. 40(4):671-683.

Covert spatial attention produces biases in perceptual performance and neural processing of behaviorally relevant stimuli in the absence of overt orienting movements. The neural mechanism that gives rise to these effects is poorly understood. This paper surveys past evidence of a relationship between oculomotor control and visual spatial attention and more recent evidence of a causal link between the control of saccadic eye movements by frontal cortex and covert visual selection. Both suggest that the mechanism of covert spatial attention emerges as a consequence of the reciprocal interactions between neural circuits primarily involved in specifying the visual properties of potential targets and those involved in specifying the movements needed to fixate them.

Posner, M I, 1980: Orienting of attention. Quarterly J. Experim. Psychol., 32:3-25.

This is one of the classic papers on the subject; would be nice to have.

Posner, M I, Cohen, Y, & Rafal, R D, 1982: Neural systems control of spatial orienting. Phil. Trans. Roy. Soc. Lond., B, Biol. 298(1089):187-198.

A peripheral visual cue in an empty field (1) often summons head or eyes, or both, (2) improves efficiency at the cued position while attention is directed to it, even without overt movements, and (3) reduces processing efficiency at the cued position once attention is withdrawn. We have studied the time course and the effects of mid-brain and cortical damage on these components of orienting. The facilitation arises from shifts in covert attention. In cases of mid-brain degeneration due to progressive supranuclear palsy, saccadic movements were abolished, while covert orienting still occurs. However, covert orienting was found to be delayed in directions in which eye movements were most affected, suggesting a role for mid-brain pathways in covert orienting. Parietal lesions can cause massive loss in detection contralateral to the lesion. This is especially true when attention has been directed to the opposite side. These findings relate aspects of covert orienting of attention to neural control systems.

Posner, M I, Walker, J A, Friedrich, F J, & Rafal, R D, 1984: Effects of parietal injury on covert orienting of attention. J. Neurosci. 4(7):1863-1874.

The cognitive act of shifting attention from one place in the visual field to another can be accomplished covertly without muscular changes. The act can be viewed in terms of three internal mental operations: disengagement of attention from its current focus, moving attention to the target, and engagement of the target. Our results show that damage to the parietal lobe produces a deficit in the disengage operation when the target is contralateral to the lesion. Effects may also be found on engagement with the target. The effects of brain injury on disengagement of attention seem to be unique to the parietal lobe and do not appear to occur with our frontal, midbrain, and temporal control series. These results confirm the close connection between parietal lobes and selective attention suggested by single cell recording. They indicate more specifically the role that parietal function has on attention and suggest one mechanism of the effects of parietal lesions reported in clinical neurology.

Rock, I, Linnett, C M, Grant, P, & Mack, A, 1992: Perception without attention: results of a new method. Cogn. Psychol. 24(4):502-534.

Having found by the use of a new method for examining perception without attention that grouping and texture segregation do not seem to occur (see Mack, Tang, Tuma, Kahn, & Rock (1992) Cognitive Psychology, 24) we go on to ask what is perceived without attention using this new method. Our subjects receive only one inattention trial in a sequence of trials involving a visual distraction task. In addition to the distraction task in the inattention trial, subjects received a stimulus of which they had no prior knowledge or expectation and were questioned or tested directly afterward for their perception of that stimulus. Two subsequent trials containing test stimuli serve as within-subject controls. The results of a series of experiments indicate that the presence of one or more stimulus objects and their locations are preattentively perceived, as is their color, but shape is not. Because individual items are detected without attention, we conclude that perceptual organization is initially based on a principle in which connected regions of uniform stimulation are inferred to be discrete units (the principle of uniform connectedness). One striking, unexpected finding is that without attention many subjects have no awareness at all of the stimulus object, an effect we call inattentional blindness.

Salillas, E, Yagoubi, R E, & Semenza, C, 2007: Sensory and cognitive processes of shifts of spatial attention induced by numbers: an ERPs study. Cortex, In Press.

The relationship between space and number has become a focus of intensive investigation (Hubbard et al., 2005; Walsh, 2003). The present paper aims to explore the nature of attentional shifts induced by the perception of irrelevant numbers as it was shown by Fischer and collaborators (2003). We measured the event related potentials induced by the perception of visual lateralized targets cued by numbers that differed in their magnitude. Congruent trials were defined as those where a target presented in the right visual field followed a large number and those where a target presented in the left visual field followed a small number. Numbers generate a modulation of evoked potentials on targets as soon as 80 msec. after the presentation of the target: congruency of the target determined the amplitude on perceptual P100 and cognitive P300 in both sides of presentation of the target. Although a typical distribution of the components was found, effects of congruency were distributed around anterior and centro-parietal sites. Due to the functional properties of the mentioned components, the present data suggests that, in fact, perception of numbers does affect the location of attention to external space. Moreover, the distribution of the congruency effect signals that the representational nature of numbers makes a difference with respect to the stimuli classically used in cueing studies of visual attention to location. The role of top-down control generated by numbers is discussed.

Sannita, W G, Bandini, F, Beelke, M, De Carli, F, Carozzo, S, Gesino, D, Mazzella, L, Ogliastro, C, & Narici, L, 2001: Time dynamics of stimulus- and event-related gamma band activity: contrast-VEPs and the visual P300 in man. Clin. Neurophysiol., 112(12):2241-2249.

Objectives: To investigate the time dynamics and phase relationship with the stimulus of the onset/offset visual evoked potentials (VEPs), P300 and gamma band oscillatory responses to visual (contrast) stimulation. Gamma band oscillatory activity mediates in sensory and cognitive operations, with a role in stimulus-related cortical synchronization, but is reportedly reduced in the time window of the P300 response.

Methods: Ten healthy volunteers were studied. VEPs and P300 were obtained in a stimulus condition combining standard contrast stimulation and a visual odd-ball paradigm. Visual stimuli were gratings with a sinusoidal luminance profile (9.0° central retina; 1.3 cycles/degree; 70% contrast) that were presented monocularly in onset/offset mode, with vertical orientation (frequent stimulus; 80%) or with a 15° rotation to the right (infrequent, target stimulus). The total signal activity (temporal spectral evolution), the activity phase-locked to the stimulus onset (rectified integrated average), and the 'locking index' (ratio of the activity phase-locked to the stimulus to the total signal activity) were computed over time and across frequencies on the signals recorded at occipital (visual responses) and central locations (P300).

Results: Oscillatory activity centered around 20.0-35.0 Hz and phase-locked to the stimulus was recorded at occipital locations with time dynamics anticipating the conventional VEPs. Phase-locking was higher after frequent than in response to target stimuli and after the stimulus offset compared to onset, while the phase-locking of the VEP frequency components was higher after the stimulus onset. The low frequency components of the P300 recorded at Cz (below 8.0-10.0 Hz) were almost totally phase-locked to the stimulus, while the gamma band activity at the P300 location did not vary over time in amplitude or phase-locking and was mostly non-locked to the target stimulus.

Conclusions: These observations add to the evidence of a role of the gamma band oscillatory responses (centered at 20.0-35.0 Hz) in visual information processing and suggest that the increment in gamma band activity during cognitive operations also depends on task characteristics, vigilance or selective attention, and brain functional state. The visual P300 appears to reflect low frequency synchronization mechanisms.

Serences, J T, & Yantis, S, 2006: Selective visual attention and perceptual coherence. Trends Cogn. Sci. 10(1):38-45.

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.

Silver, M A, Ress, D, & Heeger, D J, 2005: Topographic maps of visual spatial attention in human parietal cortex. J. Neurophysiol. 94(2):1358-1371.

Functional magnetic resonance imaging (fMRI) was used to measure activity in human parietal cortex during performance of a visual detection task in which the focus of attention systematically traversed the visual field. Critically, the stimuli were identical on all trials (except for slight contrast changes in a fully randomized selection of the target locations) whereas only the cued location varied. Traveling waves of activity were observed in posterior parietal cortex consistent with shifts in covert attention in the absence of eye movements. The temporal phase of the fMRI signal in each voxel indicated the corresponding visual field location. Visualization of the distribution of temporal phases on a flattened representation of parietal cortex revealed at least two distinct topographically organized cortical areas within the intraparietal sulcus (IPS), each representing the contralateral visual field. Two cortical areas were proposed based on this topographic organization, which we refer to as IPS1 and IPS2 to indicate their locations within the IPS. This nomenclature is neutral with respect to possible homologies with well-established cortical areas in the monkey brain. The two proposed cortical areas exhibited relatively little response to passive visual stimulation in comparison with early visual areas. These results provide evidence for multiple topographic maps in human parietal cortex.

Slavutskaya, M, & Shulgovskii, V V, 2007: Presaccadic Brain Potentials in Conditions of Covert Attention Orienting. Spanish J. Psychology, 10(2):277-284.

Twelve healthy subjects underwent investigation of averaged (electroencephalogram) EEG potentials during preparation for motor activity and in the latent period (LP) of visually evoked saccades by presentation of stimuli using Posner's (1980) design of "cost-benefit." It has been shown that covert spatial attention orientation leads to an increase in amplitude and decrease in latency of presaccadic initiation potential peaks within the saccadic latent period (LP) (P-100, N -50). Processes of covert orientation of attention during the interstimulus interval period of anticipation of the target stimulus correlate with the increase of slow negativity of fronto-parietal-temporal localization. Spatial-temporal changes of presaccadic potentials are evidence of the fact that orientation of attention during motor preparation and saccadic initiation is reflected in intensification of fronto-parietal networks of saccadic control and attention, activating the fronto-medio-thalamic and thalamo-parietal modulating systems.

Taylor, M J, 2002: Non-spatial attentional effects on P1. Clin. Neurophysiol., 113(12):1903-1908.

Objective: It has been suggested that P1, the earliest endogenous visual potential, is influenced primarily by spatial location. However, we have found that attention to non- spatial visual features can affect both the latency and amplitude of this component.

Methods: A series of studies are reviewed, starting with 4 using simple geometric forms, and either serial presentation of single stimuli or presentation of stimulus arrays followed by two studies using natural complex images.

Results: With simple stimuli, latency and amplitude effects are seen on the P1, but differ among the paradigms, depending on the demands of the task. The data further showed a facilitation effect and that binding occurs in parallel with single feature processing. For complex stimuli we found P1 shorter to faces than inverted faces, eyes or non-face stimuli, and larger to animal than non-animal pictures. The above effects were present in children as well as in adults.

Conclusions: These findings demonstrate that very early stages of processing can be modified by top-down attentional influences across a range of ages and experimental paradigms, concordant with visual processing models showing very rapid and dispersed activation with feedback at early cortical levels.

Thompson, K G, Biscoe, K L, & Sato, T R, 2005: Neuronal basis of covert spatial attention in the frontal eye field. J. Neurosci. 25(41):9479-9487.

The influential "premotor theory of attention" proposes that developing oculomotor commands mediate covert visual spatial attention. A likely source of this attentional bias is the frontal eye field (FEF), an area of the frontal cortex involved in converting visual information into saccade commands. We investigated the link between FEF activity and covert spatial attention by recording from FEF visual and saccade-related neurons in monkeys performing covert visual search tasks without eye movements. Here we show that the source of attention signals in the FEF is enhanced activity of visually responsive neurons. At the time attention is allocated to the visual search target, nonvisually responsive saccade-related movement neurons are inhibited. Therefore, in the FEF, spatial attention signals are independent of explicit saccade command signals. We propose that spatially selective activity in FEF visually responsive neurons corresponds to the mental spotlight of attention via modulation of ongoing visual processing.

Thut, G, Nietzel, A, Brandt, S A, & Pascual-Leone, A, 2006: Alpha-Band Electroencephalographic Activity over Occipital Cortex Indexes Visuospatial Attention Bias and Predicts Visual Target Detection. J. Neurosci. 26(37):9494-9502.

Covertly directing visual attention toward a spatial location in the absence of visual stimulation enhances future visual processing at the attended position. The neuronal correlates of these attention shifts involve modulation of neuronal "baseline" activity in early visual areas, presumably through top-down control from higher-order attentional systems. We used electroencephalography to study the largely-unknown relationship between these neuronal modulations and behavioral outcome in an attention orienting paradigm. Covert visuospatial attention shifts to either a left or right peripheral position in the absence of visual stimulation resulted in differential modulations of oscillatory a-band (8-14 Hz) activity over left versus right posterior sites. These changes were driven by varying degrees of a-decreases being maximal contralateral to the attended position. When expressed as a lateralization index, these a-changes differed significantly between attention conditions, with negative values (a_right < a_left) indexing leftward and more positive values (a_left £ a_right) indexing rightward attention. Moreover, this index appeared deterministic for processing of forthcoming visual targets. Collapsed over trials, there was an advantage for left target processing in accordance with an overall negative bias in a-index values. Across trials, left targets were detected most rapidly when preceded by negative index values. Detection of right targets was fastest in trials with most positive values. Our data indicate that collateral modulations of posterior a-activity, the momentary bias of visuospatial attention, and imminent visual processing are linked. They suggest that the momentary direction of attention, predicting spatial biases in imminent visual processing, can be estimated from a lateralization index of posterior a-activity.

Womelsdorf, T, Fries, P, Mitra, P P, & Desimone, R, 2006: Gamma-band synchronization in visual cortex predicts speed of change detection. Nature, 439(7077):733-736.

Our capacity to process and respond behaviourally to multiple incoming stimuli is very limited. To optimize the use of this limited capacity, attentional mechanisms give priority to behaviourally relevant stimuli at the expense of irrelevant distractors. In visual areas, attended stimuli induce enhanced responses and an improved synchronization of rhythmic neuronal activity in the gamma frequency band (40-70 Hz)1-11. Both effects probably improve the neuronal signalling of attended stimuli within and among brain areas1,12-16. Attention also results in improved behavioural performance and shortened reaction times. However, it is not known how reaction times are related to either response strength or gamma-band synchronization in visual areas. Here we show that behavioural response times to a stimulus change can be predicted specifically by the degree of gamma-band synchronization among those neurons in monkey visual area V4 that are activated by the behaviourally relevant stimulus. When there are two visual stimuli and monkeys have to detect a change in one stimulus while ignoring the other, their reactions are fastest when the relevant stimulus induces strong gamma-band synchronization before and after the change in stimulus. This enhanced gamma-band synchronization is also followed by shorter neuronal response latencies on the fast trials. Conversely, the monkeys' reactions are slowest when gamma-band synchronization is high in response to the irrelevant distractor. Thus, enhanced neuronal gamma-band synchronization and shortened neuronal response latencies to an attended stimulus seem to have direct effects on visually triggered behaviour, reflecting an early neuronal correlate of efficient visuo-motor integration.

Cross-Modal Effects

Busse, L, Roberts, K C, Crist, R E, Weissman, D H, & Woldorff, M G, 2005: The spread of attention across modalities and space in a multisensory object. Proc. Nat. Acad. Sci. 102(51):18751-18756.

Attending to a stimulus is known to enhance the neural responses to that stimulus. Recent experiments on visual attention have shown that this modulation can have object-based characteristics, such that, when certain parts of a visual object are attended, other parts automatically also receive enhanced processing. Here, we investigated whether visual attention can modulate neural responses to other components of a multisensory object defined by synchronous, but spatially disparate, auditory and visual stimuli. The audiovisual integration of such multisensory stimuli typically leads to mislocalization of the sound toward the visual stimulus (ventriloquism illusion). Using event-related potentials and functional MRI, we found that the brain's response to task-irrelevant sounds occurring synchronously with a visual stimulus from a different location was larger when that accompanying visual stimulus was attended versus unattended. The event-related potential effect consisted of sustained, frontally distributed, brain activity that emerged relatively late in processing, an effect resembling attention-related enhancements seen at earlier latencies during intramodal auditory attention. Moreover, the functional MRI data confirmed that the effect included specific enhancement of activity in auditory cortex. These findings indicate that attention to one sensory modality can spread to encompass simultaneous signals from another modality, even when they are task-irrelevant and from a different location. This cross-modal attentional spread appears to reflect an object-based, late selection process wherein spatially discrepant auditory stimulation is grouped with synchronous attended visual input into a multisensory object, resulting in the auditory information being pulled into the attentional spotlight and bestowed with enhanced processing.

Eimer, M, van Velzen, J, & Driver, J, 2004: Evidence for Cross-Modal Audiovisual Effects of Endogenous Spatial Attention within Hemifields. J. Cognitive Neuroscience, 16(2):272-288.

Previous ERP studies have uncovered cross-modal interaction in endogenous spatial attention. Directing attention to one side to judge stimuli from one particular modality can modulate early modality-specific ERP components not only for that modality, but also for other currently irrelevant modalities. However, past studies could not determine whether the spatial focus of attention in the task-irrelevant secondary modality was similar to the primary modality, or was instead diffuse across one hemifield. Here, auditory or visual stimuli could appear at any of four location (two on each side). In different blocks, subjects judged stimuli at only one of these four locations, for an auditory (Experiment 1) or visual (Experiment 2) task. Early attentional modulations of visual and auditory ERPs were found for stimuli at the currently relevant location, compared with those at the irrelevant location within the same hemifield, thus demonstrating within-hemifield tuning of spatial attention. Crucially this was found not only for the currently relevant modality, but also for the currently irrelevant modality. Moreover, these within-hemifield attention effects were statistically equivalent regardless of the task relevance of the modality, for both the auditor and visual ERP data. These results demonstrate that within-hemifield spatial attention for one task- relevant modality can transfer cross-modally to a task-irrelevant modality, consistent with spatial selection at a multimodal level of representation.

Kennett, S, Eimer, M, Spence, C, & Driver, J, 2001: Tactile-Visual Links in Exogenous Spatial Attention Under Different Postures: Convergent Evidence from Psychophysics and ERPs. J. Cognitive Neuroscience, 13(4):462-478.

Tactile-visual links in spatial attention were examined by presenting spatially nonpredictive tactile cues to the left or right hand, shortly prior to visual targets in the left or right hemifield. To examine the spatial coordinates of any crossmodal links, different postures were examined. The hands were either uncrossed, or crossed so that the left hand lay in the right visual field and vice versa. Visual judgments were better on the side where the stimulated hand lay, though this effect was somewhat smaller with longer intervals between cue and target, and with crossed hands. Event-related brain potentials (ERPs) showed a similar pattern. Larger amplitude occipital N1 components were obtained for visual events on the same side as the preceding tactile cue, at ipsilateral electrode sites. Negativities in the Nd2 interval at midline and lateral central sites, and in the Nd1 interval at electrode Pz, were also enhanced for the cued side. As in the psychophysical results, ERP cueing effects during the crossed posture were determined by the side of space in which the stimulated hand lay, not by the anatomical side of the initial hemispheric projection for the tactile cue. These results demonstrate that crossmodal links in spatial attention can influence sensory brain responses as early as the N1, and that these links operate in a spatial frame-of- reference, that can remap between the modalities across changes in posture.

Schürmann, M, Grumbt, M, Heide, W, & Verleger, R, 2003: Effects of same- and different-modality cues in a Posner task: extinction-type, spatial, and non-spatial deficits after right-hemispheric stroke. Cognitive Brain Research, 16(3):348-358.

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.