A new taxonomy for perceptual filling-in

Perceptual filling-in occurs when structures of the visual system interpolate information across regions of visual space where that information is physically absent. It is a ubiquitous and heterogeneous phenomenon, which takes place in different forms almost every time we view the world around us, such as when objects are occluded by other objects or when they fall behind the blind spot. Yet, to date, there is no clear framework for relating these various forms of perceptual filling-in. Similarly, whether these and other forms of filling-in share common mechanisms is not yet known. Here we present a new taxonomy to categorize the different forms of perceptual filling-in. We then examine experimental evidence for the processes involved in each type of perceptual filling-in. Finally, we use established theories of general surface perception to show how contextualizing filling-in using this framework broadens our understanding of the possible shared mechanisms underlying perceptual filling-in. In particular, we consider the importance of the presence of boundaries in determining the phenomenal experience of perceptual filling-in.     

Using afterimages to test neural mechanisms for perceptual filling-in.

Many theories of visual perception propose that brightness information spreads from edges to define the perceived intensity of the interior of visual surfaces. Several theories of visual perception have hypothesized that this filling-in process is similar to a diffusion of information where the signals coding brightness spread to nearest neighbors. This paper shows that diffusive mechanisms fail to account for the characteristics of certain afterimage percepts that seem to be dependent on the filling-in process. A psychophysical experiment tests a key property of diffusion-based filling-in mechanisms and finds data that rejects this class of models. A non-diffusive based filling-in mechanism is proposed and is shown to act much like the diffusive based mechanism in many instances, but also produces afterimage percepts that match the experimental data.     

Effects of lunch on selective and sustained attention

Post-lunch impairments in performance efficiency have been found in both real-life settings and the laboratory. The first aim of the present experiment was to determine whether post-lunch changes in performance depend on eating a meal, or whether they are also found in subjects who abstain from eating. A second aim was to examine which aspects of performance are most likely to show post-lunch impairments. The results showed that the ability to maintain attention and react quickly to the appearance of a new stimulus was impaired following consumption of the meal. The extent of this effect did not vary with time on task, and was unaltered by changes in the time at which lunch was eaten. In contrast to this, the ability to resist distraction was unaffected by eating lunch. Other types of performance, such as movement time, varied as a function of time of day, but were not impaired by the meal.     

fMRI Measures of perceptual filling-in in the human visual cortex

Filling-in refers to the tendency of stabilized retinal stimuli to fade and become replaced by their background. This phenomenon is a good example of central brain mechanisms that can selectively add or delete information to/from the retinal input. Importantly, such cortical mechanisms may overlap with those that are used more generally in visual perception. In order to identify cortical areas that contribute to perceptual filling-in, we used functional magnetic resonance imaging to image activity in the visual cortex while subjects experienced filling-in. Nine subjects viewed an achromatic disc with slightly higher luminance than the background and indicated the presence or absence of filling-in by a keypress. The disc was placed in either the upper or lower left quadrant. Similar high-contrast stimuli were used to map out the retinotopic representation of the disc. Unexpectedly, the lower-field high-contrast stimulus produced more parietal cortex activation than the upper-field condition, indicating preferential representation of the lower field by attentional control mechanisms. During perceptual filling-in, we observed significant contralateral reductions in activation in lower-tier retinotopic areas V1 and V2. In contrast, increased activation was consistently observed in visual areas V3A and V4v, higher-level cortex in the intraparietal sulcus, posterior superior temporal sulcus, and the ventral occipital-temporal region, as well as the pulvinar. The filling-in activation pattern was remarkably similar for both the upper- and lower-field conditions. Behaviorally, filling-in was reported to be easier for the lower visual field, and filling-in periods were longer for the lower than the upper quadrant. We suggest this behavioral asymmetry may be partially due to the preferential parietal representation of the lower field. The results lead us to propose that perceptual filling-in recruits high-level control mechanisms to reconcile competing percepts, and alters the normal image-related signals at the first stages of cortical processing. Moreover, the overall pattern of activation during filling-in resembles that seen in other studies of perceptually bistable stimuli, including binocular rivalry, indicating common control mechanisms.     

Effects of frontal lesions on a selective attention task

We set out to test the hypothesis that patients with frontal damage are specifically disabled in carrying out tasks requiring a high level of controlled attention. A group of patients with frontal lesions and another group of patients with retrorolandic lesions were tested for selective attention on a computerized task designed to produce a conflict situation between automatic and controlled processes. Frontal patients proved to be significantly more prone to errors of commission (false alarms) than retrorolandic patients.

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Sommario Il presente lavoro si propone di verificare l'ipotesi di una specifica perturbazione di soggetti con lesioni frontali nella esecuzione di compiti richiedenti un livello elevato di controllo attentivo volontario. Due gruppi di pazienti, rispettivamente con lesioni frontali e retrorolandiche, sono stati sottoposti ad un esperimento di attenzione selettiva assistito dal computer, costruito in modo da realizzare una situazione conflittuale tra processi di tipo automatico e volontario. I risultati hanno evidenziato una significativa tendenza dei soggetti con lesione frontale a commettere un maggiore numero di errori di commissione (falsi allarmi) rispetto ai soggetti con lesione retrorolandica.

     

The effects of interstimulus interval on event-related indices of attention: an auditory selective attention test of perceptual load theory.

OBJECTIVE: We examined the impact of perceptual load by manipulating interstimulus interval (ISI) in two auditory selective attention studies that varied in the difficulty of the target discrimination. METHODS: In the paradigm, channels were separated by frequency and target/deviant tones were softer in intensity. Three ISI conditions were presented: fast (300ms), medium (600ms) and slow (900ms). Behavioral (accuracy and RT) and electrophysiological measures (Nd, P3b) were observed. RESULTS: In both studies, participants evidenced poorer accuracy during the fast ISI condition than the slow suggesting that ISI impacted task difficulty. However, none of the three measures of processing examined, Nd amplitude, P3b amplitude elicited by unattended deviant stimuli, or false alarms to unattended deviants, were impacted by ISI in the manner predicted by perceptual load theory. CONCLUSIONS: The prediction based on perceptual load theory, that there would be more processing of irrelevant stimuli under conditions of low as compared to high perceptual load, was not supported in these auditory studies. SIGNIFICANCE: Task difficulty/perceptual load impacts the processing of irrelevant stimuli in the auditory modality differently than predicted by perceptual load theory, and perhaps differently than in the visual modality.     

Auditory perceptual grouping and attention in dyslexia

Despite dyslexia affecting a large number of people, the mechanisms underlying the disorder remain undetermined. There are numerous theories about the origins of dyslexia. Many of these relate dyslexia to low-level, sensory temporal processing deficits. Another group of theories attributes dyslexia to language-specific impairments. Here, we show that dyslexics perform worse than controls on an auditory perceptual grouping task. The results show differences in performance between the groups that depend on sound frequency and not solely on parameters related to temporal processing. Performance on this task suggests that dyslexics' deficits may result from impaired attentional control mechanisms. Such deficits are neither modality nor language-specific and may help to reconcile differences between theories of dyslexia.     

Early attention modulates perceptual interpretation of multisensory stimuli

Perceptual interpretation of the same multisensory stimuli may rely upon influence of attention on multisensory integration. We tested this hypothesis by recording the brain activity using magnetoencephalography in a passing-bouncing illusion with sound. Early activation of the attention-related brain areas and subsequent involvement of the multisensory areas were associated with the bouncing percept. Early activation of the unimodal sensory areas and later involvement of the attention-related areas were associated with the passing percept. Thus, the bouncing percept occurs when early attentional deployment facilitates multisensory integration modulating visual perception. The passing percept results from hierarchical sequence of the perceptual processes: early activation of the unimodal areas and late attentional deployment. Alternation of the perceptual interpretations may depend on spontaneous fluctuation of attention.     

Schizophrenia and selective attention

A comparative study of selective attention was made on 22 schizophrenic inpatients, 18 non schizophrenic inpatients and a control of 22 non psychiatric adults. Signs of schizophrenia were quantified through the BPRS (Brief Psychiatric Rating Scale) and selective attention was expressed as the capacity to resist a distracting stimulus dimension in the Stroop word-colour test. The results indicated higher distractibility in schizophrenic patients, and a positive, significative relation was found between this interference and the "activation" cluster.     

Effects of selective attention on the late components of evoked potentials in man.

Vertex and specific evoked potentials to visual and cutaneous stimuli were recorded in 6 subjects. The delivery of roughly equal numbers of flashes and shocks was paced by the subjects themselves. They also had to choose which stimulus they would be given next; 9 times out of 10 the stimulus matched the subject's expectation and stimuli from the unexpected modality were given randomly 1 time out of 10 on the average. No task was required after the stimulus occurrence. Cortical evoked potentials (CEPs) to unexpected stimuli were averaged and compared with CEPs the major difference consisted in large N2 and P3 waves in response to unexpected stimuli; no difference was observed in the amplitude of the N1 wave. In specific CEPs unexpected stimuli also elicited N2 and P3 waves; in addition N1 showed a slight tendency to be higher in CEPs to unexpected stimuli,     

Effects of selective attention on visual evoked potentials in depressed patients and healthy controls

The study of the attention process during depression has been realized with the recording of visual evoked potentials (VEP) by simple pattern reversal checkerboard, and in sequences where a rare stimulus appears randomly, with of without task. The recordings obtained on fifty patients before antidepressive treatment (D0) have been compared to the VEP obtained on the same patients at the end of the treatment (D28) and to those obtained on fifteen reference subjects. The results indicate that some perturbations, characteristic of the depressive state, should happen at both levels of attentional selection, early (components P1 and N1) and late (component P3). The responses of the depressive patient appear to reflect rather the intervention of automatic attention process. Electrophysiological and behavioral data seem to indicate a positive effect of the psychotropic treatment.     

Effects of psycho-educational training and stimulant medication on visual perceptual skills in children with attention deficit hyperactivity disorder

Attention deficit hyperactivity disorder (ADHD) is treated with stimulants and psycho-educational remedial programs despite limited literature support for the latter. This study aimed to examine changes in a “Test of Visual Perceptual Skills” (TVPS) that has not been previously reported in children with ADHD enrolled in such a program.     

Voluntary attention changes the speed of perceptual neural processing

While previous studies in psychology demonstrated that humans can respond more quickly to the stimuli at attended than unattended locations, it remains unclear whether attention also accelerates the speed of perceptual neural activity in the human brain. One possible reason for this unclarity would be an insufficient spatial resolution of previous electroencephalography (EEG) and magnetoencephalography (MEG) techniques in which neural signals from multiple brain regions are merged with each other. Here, we addressed this issue by combining MEG with a novel stimulus-presentation technique that can focus on neural signals from higher visual cortex where the magnitude of attentional modulation is prominent. Results revealed that the allocation of spatial attention induces both an increase in neural intensity (attentional enhancement) and a decrease in neural latency (attentional acceleration) to the attended compared to unattended visual stimuli (Experiment 1). Furthermore, an attention-induced behavioural facilitation reported in previous psychological studies (Posner paradigm) was closely correlated with the neural 'acceleration' rather than 'enhancement' in the visual cortex (Experiment 2). In addition to bridging a gap between previous psychological and neurological findings, our results demonstrated a temporal dynamics of attentional modulation in the human brain.     

Neural substrates of perceptual enhancement by cross-modal spatial attention

Orienting attention involuntarily to the location of a sudden sound improves perception of subsequent visual stimuli that appear nearby. The neural substrates of this cross-modal attention effect were investigated by recording event-related potentials to the visual stimuli using a dense electrode array and localizing their brain sources through inverse dipole modeling. A spatially nonpredictive auditory precue modulated visual-evoked neural activity first in the superior temporal cortex at 120-140 msec and then in the ventral occipital cortex of the fusiform gyrus 15-25 msec later. This spatio-temporal sequence of brain activity suggests that enhanced visual perception produced by the cross-modal orienting of spatial attention results from neural feedback from the multimodal superior temporal cortex to the visual cortex of the ventral processing stream.     

Hemispheric asymmetry for selective attention

A letter-identification task, previously demonstrated to show activation of the pulvinar nucleus of the thalamus with fluodeoxyglucose position emission tomography, was administered to 20 normal volunteers. The letter to be detected could appear alone as a big stimulus or as a small one stimulus surrounded by flanking letters. To test for a hemispheric specialization for filtering processes, the stimuli were displayed horizontally, either in the left or in the right hemifield, or vertically, either above or below the fixation point. In addition, to study the effect of cognitive processes on selective attention resources, we varied the feedback conditions, by delivering or not delivering a blue flash in cases of misses or mistakes. The results show a significant interaction between the type of stimulus (alone or surrounded by flankers) and the hemifield of presentation (left or right) only in the condition where the subjects were presented stimuli horizontally without any feedback. In this condition, reaction times (RTs) were shorter in the left hemifield than in the right hemifield for single stimuli, whereas for stimuli surrounded by flankers, the opposite pattern was observed, that is, shorter RT in the right hemifield than in the left one. The present findings suggest a hemispheric specialization for selective attention, in particular at the subcortical level.     

Effects of focused selective attention tasks on event-related potentials in autistic and normal individuals.

Event-related potentials (ERPs) and behavioral responses were recorded from autistic and normal subjects under two focused selective attention conditions. In each condition, subjects were presented with an identical stimulus paradigm--a random sequence of 50 msec sounds and flashes occurring at interstimulus intervals ranging between 0.5 and 1.5 sec. The sequence consisted of rare auditory (12.5%), rare visual (12.5%), standard auditory (37.5%) and standard visual (37.5%) stimuli. In the focal auditory condition, subjects pressed a button to the rare auditory target, and in a focal visual condition, they pressed a button to the rare visual stimuli. When normal subjects detected target stimuli in a given attended modality, all stimuli in the attended modality produced enhanced negative ERP responses at frontal electrode sites (i.e., auditory Nde, Ndl, and Nc; visual N270 and Nc) and enhanced positive ERP responses at posterior electrode sites (i.e., P3b and visual P400). In the autistic subjects, in contrast, all auditory and visual attention-related negativities recorded in the auditory and visual focused attention tasks were not in evidence. P3b was significantly diminished in size. The results suggest that abnormalities in the neurophysiological mechanisms of selective attention may underlie the cognitive deficits in autism. The present report and its companion papers are the first reports of the neurophysiological correlates of selective attention in autism.     

Directed attention and perceptual asymmetry to monaurally presented tones

Forty-eight right-handed Ss were tested for simple reaction time (RT) to a monaurally presented 1000 Hz tone of 250 msec duration. Each S was tested in 3 series comprising 7, 11 and 15 stimulus presentations (trials). In each series all stimuli but one (probe) were presented to a single ear. Ss were divided equally into right or left-ear attention groups on the basis of the ear receiving the non-probe trials. Right-ear attention resulted in equally good RTs to either attended or non-attended ear. Left-ear attention resulted in slower RTs than right-ear attention and showed a significant discrepancy between ears in favor of the non-attended ear. The results are interpreted as showing that neither simulus competition nor language processing are necessary conditions for eliciting auditory asymmetry. The data indicate that a cortically-mediated process of selective attention operates in the production of auditory asymmetries for some non-language sounds and that, as with language perception, it is the left cerebral hemisphere that is primarily involved.     

Neurobiological measures of human selective attention

Selective attention allows people to process some stimuli more thoroughly than others. This is partly under voluntary control, and partly determined by stimulus salience. Selective attention has been studied with psychological methods for many years, but recent cognitive neuroscience studies using brain-imaging methods (and other neurobiological measures) have transformed the topic. Such studies have demonstrated that sensory processing can be strongly modulated by attention throughout perceptual networks, including even processing in primary sensory cortex. They have shown that some of these modulations can be anticipatory, arising prior to stimulus presentation, while other components reflect a changed neural response to an incoming stimulus. Recent imaging studies have also examined the mechanisms involved in controlling such attentional modulation of sensory processing. In addition to answering many long-standing questions about selective attention, such research also raises new questions. The various contributions in this volume provide an overview of the spectacular recent advances in attention research using neurobiological measures, and they outline critical issues to be resolved in the future.     

Orienting attention to locations in perceptual versus mental representations

Extensive clinical and imaging research has characterized the neural networks mediating the adaptive distribution of spatial attention. In everyday behavior, the distribution of attention is guided not only by extrapersonal targets but also by mental representations of their spatial layout. We used event-related functional magnetic resonance imaging to identify the neural system involved in directing attention to locations in arrays held as mental representations, and to compare it with the system for directing spatial attention to locations in the external world. We found that these two crucial aspects of spatial cognition are subserved by extensively overlapping networks. However, we also found that a region of right parietal cortex selectively participated in orienting attention to the extrapersonal space, whereas several frontal lobe regions selectively participated in orienting attention within on-line mental representations.     

Pre-saccadic perceptual facilitation can occur without covert orienting of attention

The pre-motor theory of attention suggests that the mechanisms involved in target selection for eye movements are the same as those for spatial attention shifts. The pre-saccadic facilitation of perceptual discrimination at the location of a saccadic goal (paradigm of Deubel and Schneider, 1996) has been considered as an argument for this theory. We compared letter discrimination performance in a saccade (overt attention - pre-saccadic facilitation) and a fixation (covert attention) task in a patient with right posterior parietal damage and 4 controls.In the overt attention condition, the patient was instructed by a central cue to make a saccade to a target located at a peripheral location. During the saccade latency (in a period of time of 250 msec following the presentation of the cue), a letter was presented at the target location. Accuracy of leftward saccades was impaired compared to rightward saccades. To evaluate letter discrimination performance in this saccade task (i.e., the presence of pre-saccadic facilitation), we selected only those leftward saccades that were equivalent in accuracy (and latency) to the rightward ones. Within these selected trials, the patient was able to discriminate letters equally well in both visual fields. In contrast, he performed at chance level during the fixation task (covert attention condition) for letters presented at the same peripheral location with the same timing with respect to the cue presentation. The patient could thus discriminate the letter presented at 8° of visual eccentricity while he was preparing a saccade, whereas he was unable to perceive the letter in the fixation task.Remarkably, in the left visual field, letter discrimination was impossible even when a letter was presented as close as 2.5° of visual eccentricity in the fixation task. Altogether, these results suggest that pre-saccadic perceptual facilitation does not rely on the same processes as those of covert attention, as tested by fixation task. Instead, we propose that pre-saccadic perceptual facilitation results from a form of attention specific to action, which could correspond to a pre-saccadic remapping process.