Effects of feature and spatial attention on visual change detection

In event-related brain potential studies using a visual S1-S2 matching task, stimulus changes elicit change-related positivity, which reflects the detection of visual changes. To investigate the effects of attention on change detection, we tested the elicitation of change-related positivity in response to changes in color and spatial frequency under three attention conditions: (i) changes in an unattended feature at an attended location, (ii) in an attended feature at an unattended location, and (iii) in an unattended feature at an unattended location. The results suggest that stimulus changes can be detected even when both feature and spatial attention are withdrawn, but change detection can also be inhibited, which might be because of biased-competition determined by the combination of feature and spatial attention conditions.     

Posterior parietal mechanisms of visual attention

The posterior parietal cortex plays an important role in visual-spatial attention. Imaging studies reveal consistent parietal activation in attention tasks, while lesions to this area produce significant perceptual deficits. Non-human primate models have been particularly informative in investigating the neurophysiology of attention. The activity of posterior parietal neurons to the same operant stimuli is modulated by whether they are attended or not based on prior information, or whether they attract attention by virtue of their saliency. Posterior parietal neuronal activity appears to provide critical information for shifting attention between stimuli and spatial locations.     

Two separate mechanisms underlie auditory change detection and involuntary control of attention

We used behavioral and event-related potential (ERP) measures to study the neural mechanisms of involuntary attention switching to changes in unattended sounds. Our subjects discriminated two equiprobable sounds differing in frequency (fundamental frequency 186 or 196 Hz) while task-irrelevant intensity decrements or increments (-3, -6, -9, +3, +6, or +9 dB, standard intensity 60 dB HL) infrequently occurred in the same sounds. In line with the results of previous studies, discrimination performance deteriorated with increasing magnitude of the task-irrelevant intensity change. However, these distraction effects were dissimilar for intensity increments and decrements: while there were no differences in reaction time (RT) between intensity decrements and increments, hit rates (HR) were lower for large intensity increments than for large decrements. ERPs to task-irrelevant intensity increments and decrements were also distinctly different: the response to intensity increments consisted of an N1 enhancement, mismatch negativity (MMN), and P3a, while the response to intensity decrements consisted only of MMN. These results are consistent with the assumption that two separate mechanisms (indexed by N1 and MMN) underlie auditory change detection. However, the finding that distinct distraction effects were obtained for both intensity decrements and increments but that the P3a is elicited only by the intensity increments seems to suggest that P3a may not be regarded as a general index of attentional shift but rather it is only generated in conditions in which an enhanced N1 is elicited, too.     

Disruption of neural systems of visual attention in schizophrenia

BACKGROUND: Patients with schizophrenia show attention deficits. The frontal P2a and posterior N2b event-related potential components are early indices of activity in neural systems supporting attention and they are reduced in schizophrenia in auditory tasks. However, the auditory P300 is reduced as well. Thus, the P2a and N2b reductions may simply reflect a general event-related potential amplitude reduction. The visual P300, however, is often spared in schizophrenia. If neural systems supporting attention are specifically disrupted in schizophrenia, the attention-sensitive P2a and N2b should be differentially reduced in patients, compared with the P300, in a visual attention task. METHODS: We analyzed 64-channel event-related potentials from 14 schizophrenic patients and 14 control subjects in a visual object-spatial attention task. We examined the amplitude of the P2a, N2b, and P300 components in the target minus standard difference wave to see if there was a differential reduction of the P2a and N2b compared with the P300. RESULTS: Both the P2a and N2b waveforms were reduced in the patient group (81% [control mean, 1.99 microV; patient mean, 0.38 microV] and 95% [control mean, 0.55 microV; patient mean, 0.03 microV], respectively) while the P300 was not reduced. Measured at the peak of the frontal P2a, the N2b was larger dorsally in the spatial task and larger ventrally in the object task in the control group. CONCLUSIONS: The spatial distribution of the P2a and N2b was consistent with activity in the prefrontal cortex and modality-specific posterior cortex, respectively. The differential reduction of the P2a and N2b waveforms supports the hypothesis of specific disruption in neural systems of visual attention in schizophrenia.     

Dissociating age-related changes in cognitive strategy and neural efficiency using event-related fMRI

We used event-related fMRI to measure brain activity while younger and older adults performed an item-recognition task in which the memory-set size varied between 1 and 8 letters. Each trial was composed of a 4-second encoding period in which subjects viewed random letter strings, a 12-second retention period and a 2-second retrieval period in which subjects decided whether a single probe letter was or was not part of the memory set. For both groups, reaction time increased and accuracy decreased with increasing memory set-size. There were minimal age-related differences in activation patterns with increasing memory set-size in prefrontal cortex (PFC). Regression analyses of individual subjects' performance and cortical activity indicated that speed and accuracy accounted for considerable variance in dorsal and ventral PFC activity during encoding and retrieval. These results suggest that younger and older adults utilize similar working memory (WM) strategies to accommodate increasing memory demand. They support a model of cognitive slowing in which processing rate is related to neural efficiency.     

Brain dynamic mechanisms of scale effect in visual spatial attention

The dynamic mechanisms of the early event-related potential scale effect of different attentive regions in the brain was studied. The paradigm of this experiment is the precue-target visual search paradigm by event-related potential technique. The results showed that the reaction time was shortened with the reduction of cue scale, a cue to how big the search area would be, and fixed target stimulus, while the amplitudes of P1 and N1 components of event-related potentials increased. These results not only provided the electrophysiological evidences that supported the zoom-lens theory, but also indicated that the zoom-lens effect happened at the early selected attention period. The results also showed that there existed two kinds of separation in the P2 effect.     

Age-related changes in the preattentional detection of visual change

The ability to detect changes in the environment which occur outside the focus of current awareness is essential if the individual is to be able to divert attention to biologically salient stimuli. The preattentional mechanism underlying the automatic detection of stimulus change in the auditory modality has been extensively studied by recording an event-related potential known as the mismatch negativity. Recently a homologous response from the visual cortex has also been described. Ageing has been shown to affect the efficiency of preattentional processing in the auditory modality, a factor which may contribute to cognitive changes in the elderly. It is unclear whether a similar effect occurs in the visual system. To investigate this issue the visual mismatch negativity was recorded from 12 older adults and 24 younger adults. Whereas the younger adults displayed a robust visual MMN, that evoked in the older adults was significantly reduced in amplitude. The results are indicative of age-related deficits in automatic visual processing.     

Detecting effects of donepezil on visual selective attention using signal detection parameters in Alzheimer's disease

BACKGROUND: Attentional function is impaired in Alzheimer's disease (AD). Moreover, attention is mediated by acetylcholine. But, despite the widespread use of acetylcholinesterase inhibitors (AChE-I) to augment available acetylcholine in AD, measures of attentional function have not been used to assess the drug response. OBJECTIVES: We hypothesized that as cholinergic augmentation impacts directly on the attentional system, higher-order measures of visual selective attention would be sensitive to effects of treatment using an AChE-I (donepezil hydrochloride). We also sought to determine whether these attentional measures were more sensitive to treatment than other measures of cognitive function. METHODS: Seventeen patients with AD (8 untreated, 9 treated with donepezil) were contrasted on performance of a selective cancellation task. Two signal detection parameters were used as outcome measures: decision strategy (beta, beta) and discriminability (d-prime, d'). Standard screening and cognitive domain measures of vigilance, language, memory, and executive function were also contrasted. RESULTS: Treated patients judged stimuli more conservatively (p = 0.29) by correctly endorsing targets and rejecting false alarms. They also discriminated targets from distractors more easily (p = 0.58). The screening and neuropsychological measures failed to differentiate the groups. CONCLUSIONS: Higher-order attentional measures captured the effects of donepezil treatment in small groups of patients with AD. The results suggest that cholinergic availability may directly affect the attentional system, and that these selective attention measures are sensitive markers to detect treatment response.     

Specific impairment of visual spatial covert attention mechanisms in Parkinson's disease

Visual deficits in early and high level processing nodes have been documented in Parkinson's disease (PD). Non-motor high level visual integration deficits in PD seem to have a cortical basis independently of a low level retinal contribution. It is however an open question whether sensory and visual attention deficits can be separated in PD. Here, we have explicitly separated visual and attentional disease related patterns of performance, by using bias free staircase procedures measuring psychophysical contrast sensitivity across visual space under covert attention conditions with distinct types of cues (valid, neutral and invalid). This further enabled the analysis of patterns of dorsal-ventral (up-down) and physiological inter-hemispheric asymmetries. We have found that under these carefully controlled covert attention conditions PD subjects show impaired psychophysical performance enhancement by valid attentional cues. Interestingly, PD patients also show paradoxically increased visual homogeneity of spatial performance profiles, suggesting flattening of high level modulation of spatial attention. Finally we have found impaired higher level attentional modulation of contrast sensitivity in the visual periphery, where mechanisms of covert attention are at higher demands.These findings demonstrate a specific loss of attentional mechanisms in PD and a pathological redistribution of spatial mechanisms of covert attention.     

Viewing the body modulates neural mechanisms underlying sustained spatial attention in touch

Cross-modal links between vision and touch have been extensively shown with a variety of paradigms. The present event-related potential (ERP) study aimed to clarify whether neural mechanisms underlying sustained tactile-spatial attention may be modulated by visual input, and the sight of the stimulated body part (i.e. hands) in particular. Participants covertly attended to one of their hands throughout a block to detect infrequent tactile target stimuli at that hand while ignoring tactile targets at the unattended hand, and all tactile non-targets. In different blocks, participants performed this task under three viewing conditions: full vision; hands covered from view; and blindfolded. When the participants' hands were visible attention was found to modulate somatosensory ERPs at early latencies (i.e. in the time range of the somatosensory P100 and the N140 components), as well as at later time intervals, from 200 ms after stimulus onset. By contrast, when participants were blindfolded and, crucially, even when only their hands were not visible, attentional modulations were found to arise only at later intervals (i.e. from 200 ms post-stimulus), while earlier somatosensory components were not affected by spatial attention. The behavioural results tallied with these electrophysiological findings, showing faster response times to tactile targets under the full vision condition compared with conditions when participants' hands were covered, and when participants were blindfolded. The results from this study provide the first evidence of the profound impact of vision on mechanisms underlying sustained tactile-spatial attention, which is enhanced by the sight of the body parts (i.e. hands).     

Serial attention mechanisms in visual search: a direct behavioral demonstration

In visual search, inefficient performance of human observers is typically characterized by a steady increase in reaction time with the number of array elements-the so-called set-size effect. In general, set-size effects are taken to indicate that processing of the array elements depends on limited-capacity resources, that is, it involves attention. Contrasting theories have been proposed to account for this attentional involvement, however. While some theories have attributed set-size effects to the intervention of serial attention mechanisms, others have explained set-size effects in terms of parallel, competitive architectures. Conclusive evidence in favor of one or the other notion is still lacking. Especially in view of the wide use of visual search paradigms to explore the functional neuroanatomy of attentional mechanisms in the primate brain, it becomes essential that the nature of the attentional involvement in these paradigms be clearly defined at the behavioral level. Here we report a series of experiments showing that highly inefficient search indeed recruits serial attention deployment to the individual array elements. In addition, we describe a number of behavioral signatures of serial attention in visual search that can be used in future investigations to attest a similar involvement of serial attention in other search paradigms. We claim that only after having recognized these signatures can one be confident that truly serial mechanisms are engaged in a given visual search task, thus making it amenable for exploring the functional neuroanatomy underlying its performance.     

Near and far space: Understanding the neural mechanisms of spatial attention

Visuospatial neglect is a multicomponent syndrome, and one dissociation reported is between neglect for near (peripersonal) and far (extrapersonal) space. Owing to patient heterogeneity and extensive lesions, it is difficult to determine the precise neural mechanisms underlying this dissociation using clinical methodology. In this study, transcranial magnetic stimulation was used to examine the involvement of three areas in the undamaged brain, while participants completed a conjunction search task in near and far space. The brain areas investigated were right posterior parietal cortex (rPPC), right frontal eye field (rFEF), and right ventral occipital cortex (rVO), each of which has been implicated in visuospatial processing. The results revealed a double dissociation, whereby rPPC was involved for search in near space only, whilst rVO only became necessary when the task was completed in far space. These data provide clear evidence for a dorsal and ventral dissociation between the processing of near and far space, which is compatible with the functional roles previously attributed to the two streams. For example, the involvement of the dorsal stream in near space reflects its role in vision for action, because it is within this spatial location that actions can be performed. The results also revealed that rFEF is involved in the processing of visual search in both near and far space and may contribute to visuospatial attention and/or the control of eye‐movements irrespective of spatial frame. We discuss our results with respect to their clear ramifications for clinical diagnosis and neurorehabilitation. Hum Brain Mapp, 2013. © 2011 Wiley Periodicals, Inc.     

Within-subject reproducibility of category-specific visual activation with functional MRI

We used functional magnetic resonance imaging (fMRI) to investigate within-subject reproducibility of activation in higher level, category-specific visual areas to validate the functional localization approach widely used for these areas. The brain areas investigated included the extrastriate body area (EBA), which responds selectively to human bodies, the fusiform face area (FFA) and the occipital face area (OFA), which respond selectively to faces, and the parahippocampal place area (PPA), which responds selectively to places and scenes. All six subjects showed significant bilateral activation in the four areas. Reproducibility was very high for all areas, both within a scanning session and between scanning sessions separated by 3 weeks. Within sessions, the mean distance between peak voxels of the same area localized by using different functional runs was 1.5 mm. The mean distance between peak voxels of areas localized in different sessions was 2.9 mm. Functional reproducibility, as expressed by the stability of T-values across sessions, was high for both within-session and between-session comparisons. We conclude that within subjects, high-level category-specific visual areas can be localized robustly across scanning sessions.     

A functional MRI study on the neural substrates for writing

Functional neuroanatomy of writing is relatively unknown compared to that of other linguistic processes. This study aimed at identifying brain regions crucial to the process of writing. Using functional magnetic resonance imaging (fMRI), brain hemodynamic activity was examined during three conditions that differentially engaged visual, linguistic, and/or motor functions: (1) writing names of pictures with the right index finger, (2) naming pictures silently, and (3) visually cued finger tapping. A writing minus naming comparison and a writing minus tapping comparison were performed, and brain regions commonly activated in these two contrasts were detected. Our main finding was that such common activation was observed in the anterior part of the left superior parietal lobule, the posterior part of the middle and superior frontal gyri, and the right cerebellum. The parietal and frontal regions were considered to subserve the process of writing as separated from that of naming and finger movements, which is consistent with the classical notion mainly proposed by studies of selective writing deficits called pure agraphia. The right cerebellar activation, on the other hand, was interpreted as the reflection of the execution of complex finger movements required for writing.     

Distinctive neural mechanisms supporting visual object individuation and identification

Many everyday activities, such as driving on a busy street, require the encoding of distinctive visual objects from crowded scenes. Given resource limitations of our visual system, one solution to this difficult and challenging task is to first select individual objects from a crowded scene (object individuation) and then encode their details (object identification). Using functional magnetic resonance imaging, two distinctive brain mechanisms were recently identified that support these two stages of visual object processing. While the inferior intraparietal sulcus (IPS) selects a fixed number of about four objects via their spatial locations, the superior IPS and the lateral occipital complex (LOC) encode the features of a subset of the selected objects in great detail (object shapes in this case). Thus, the inferior IPS individuates visual objects from a crowded display and the superior IPS and higher visual areas participate in subsequent object identification. Consistent with the prediction of this theory, even when only object shape identity but not its location is task relevant, this study shows that object individuation in the inferior IPS treats four identical objects similarly as four objects that are all different, whereas object shape identification in the superior IPS and the LOC treat four identical objects as a single unique object. These results provide independent confirmation supporting the dissociation between visual object individuation and identification in the brain.     

Automatic versus contingent mechanisms of sensory-driven neural biasing and reflexive attention

Recent studies have generated debate regarding whether reflexive attention mechanisms are triggered in a purely automatic stimulus-driven manner. Behavioral studies have found that a nonpredictive "cue" stimulus will speed manual responses to subsequent targets at the same location, but only if that cue is congruent with actively maintained top-down settings for target detection. When a cue is incongruent with top-down settings, response times are unaffected, and this has been taken as evidence that reflexive attention mechanisms were never engaged in those conditions. However, manual response times may mask effects on earlier stages of processing. Here, we used event-related potentials to investigate the interaction of bottom-up sensory-driven mechanisms and top-down control settings at multiple stages of processing in the brain. Our results dissociate sensory-driven mechanisms that automatically bias early stages of visual processing from later mechanisms that are contingent on top-down control. An early enhancement of target processing in the extrastriate visual cortex (i.e., the P1 component) was triggered by the appearance of a unique bright cue, regardless of top-down settings. The enhancement of visual processing was prolonged, however, when the cue was congruent with top-down settings. Later processing in posterior temporal-parietal regions (i.e., the ipsilateral invalid negativity) was triggered automatically when the cue consisted of the abrupt appearance of a single new object. However, in cases where more than a single object appeared during the cue display, this stage of processing was contingent on top-down control. These findings provide evidence that visual information processing is biased at multiple levels in the brain, and the results distinguish automatically triggered sensory-driven mechanisms from those that are contingent on top-down control settings.     

Neural mechanisms of visual attention: object-based selection of a region in space

Objects play an important role in guiding spatial attention through a cluttered visual environment. We used event-related functional magnetic resonance imaging (ER-fMRI) to measure brain activity during cued discrimination tasks requiring subjects to orient attention either to a region bounded by an object (object-based spatial attention) or to an unbounded region of space (location-based spatial attention) in anticipation of an upcoming target. Comparison between the two tasks revealed greater activation when attention selected a region bounded by an object. This activation was strongly lateralized to the left hemisphere and formed a widely distributed network including (a) attentional structures in parietal and temporal cortex and thalamus, (b) ventral-stream object processing structures in occipital, inferior-temporal, and parahippocampal cortex, and (c) control structures in medial- and dorsolateral-prefrontal cortex. These results suggest that object-based spatial selection is achieved by imposing additional constraints over and above those processes already operating to achieve selection of an unbounded region. In addition, ER-fMRI methodology allowed a comparison of validly versus invalidly cued trials, thereby delineating brain structures involved in the reorientation of attention after its initial deployment proved incorrect. All areas of activation that differentiated between these two trial types resulted from greater activity during the invalid trials. This outcome suggests that all brain areas involved in attentional orienting and task performance in response to valid cues are also involved on invalid trials. During invalid trials, additional brain regions are recruited when a perceiver recovers from invalid cueing and reorients attention to a target appearing at an uncued location. Activated brain areas specific to attentional reorientation were strongly right-lateralized and included posterior temporal and inferior parietal regions previously implicated in visual attention processes, as well as prefrontal regions that likely subserve control processes, particularly related to inhibition of inappropriate responding.