Attentional selection of objects or features: evidence from a modified search task

Three experiments examined the domain of visual selective attention (i.e., feature-based selection vs. object-based selection). Experiment 1 extended the requirements of the visual search task by requiring a feature discrimination response to target elements presented for short durations (30-105 msec). Targets were embedded in 47 distractor elements and were defined by either a distinct color or a distinct orientation. Observers made a discrimination response to either the target's color or its orientation. When the target-defining feature and the feature to be discriminated were the same (matched conditions), accuracy was enhanced relative to when these features belonged to separate dimensions (mismatched conditions). In Experiment 2, similar results were found in a task in which the target-defining dimension varied from trial to trial and observers performed both color and orientation discriminations on every trial. The results from these two experiments are consistent with feature-based attentional selection, but not with object-based selection. Experiment 3 extended these findings by showing that the effect is rooted in the overlap between target and distractor values in the stimulus set. The results are discussed in the context of recent models of visual selective attention.     

Differential effect of one versus two hands on visual processing

Hand position in the visual field influences performance in several visual tasks. Recent theoretical accounts have proposed that hand position either (a) influences the allocation of spatial attention, or (b) biases processing toward the magnocellular visual pathway. Comparing these accounts is difficult as some studies manipulate the distance of one hand in the visual field while others vary the distance of both hands, and it is unclear whether single and dual hand manipulations have the same impact on perception. We ask if hand position affects the spatial distribution of attention, with a broader distribution of attention when both hands are near a visual display and a narrower distribution when one hand is near a display. We examined the effects of four hand positions near the screen (left hand, right hand, both hands, no hands) on both temporal and spatial discrimination tasks. Placing two hands near the display compared to two hands distant resulted in improved sensitivity for the temporal task and reduced sensitivity in the spatial task, replicating previous results. However, the single hand manipulations showed the opposite pattern of results. Together these results suggest that visual attention is focused on the graspable space for a single hand, and expanded when two hands frame an area of the visual field.     

On the rate of acquisition of visual information about space,time, and intensity

The temporal course of acquisition of visual information was mapped by measuring the speed and accuracy With which Ss could discriminate the position, intensity, or duration of a stimulus that was varied independently on these three binary dimensions. The speed-accuracy tradeoff was different for each of the three discrimination tasks, with performance substantially superior in the position discrimination task. In the intensity and duration discrimination tasks, there were systematic changes with RT in the relative influences of the three stimulus variables over the two alternative responses, suggesting .three phases of the corresponding perceptual processes: an initial dominance by position of the stimulus, a subsequent, joint influence by both intensity and duration, and finally an exclusive control by the correct dimension. The results demonstrate that Ss can selectively attend to various dimensions of the same stimulus, with concommitantly varying time constants. The perceptual processes being tapped by a discrimination task depend upon instructions and upon RT.     

Automatic attention lateral asymmetry in visual discrimination tasks

There is evidence that automatic visual attention favors the right side. This study investigated whether this lateral asymmetry interacts with the right hemisphere dominance for visual location processing and left hemisphere dominance for visual shape processing. Volunteers were tested in a location discrimination task and a shape discrimination task. The target stimuli (S2) could occur in the left or right hemifield. They were preceded by an ipsilateral, contralateral or bilateral prime stimulus (S1). The attentional effect produced by the right S1 was larger than that produced by the left S1. This lateral asymmetry was similar between the two tasks suggesting that the hemispheric asymmetries of visual mechanisms do not contribute to it. The finding that it was basically due to a longer reaction time to the left S2 than to the right S2 for the contralateral S1 condition suggests that the inhibitory component of attention is laterally asymmetric.     

Does Attention Modulate the Perception of Luminance Changes?

In a previous study assessing the modulation of visual processing by attention, Bonnel, Possami, and Schmitt showed that, when discriminating line-length, subjects precisely shared processing resources between two pairs of lines presented to the left and right of fixation. In a close replication requiring the detection of luminance increments instead of line-length differences, subjects were unable to follow the instructions and to allocate attention differentially, thus supporting the claim that light detection is fundamentally different from shape discrimination. In a subsequent experiment, we tested and rejected the possibility that luminance perception was not open to modulation by attention due to its physical nature. Replacing brightness detection by brightness identification allowed voluntary control on the quality of processing to be evidenced. The similarity between the latter results and the data from line-length discrimination suggests that task requirements may be crucial in determining the distribution of attention.     

Dividing attention across feature dimensions in statistical processing of perceptual groups

Statistical processing has been shown in the perception of several visual dimensions, including size, speed, direction of motion, and orientation. Chong and Treisman (2005) found no decrement when people simultaneously averaged two sets on a single dimension, size. What happens when attention is divided between different dimensions? In two experiments, we investigated judgments of mean size and speed, either within the same objects or in two separate sets. In a third, we examined judgments of mean size and orientation in two different sets. All three experiments suggest a decrement in performance when attention is shared between two dimensions, especially when they are carried by two different sets of objects.     

A dual-task investigation of automaticity in visual word processing

An analysis of activation models of visual word processing suggests that frequency-sensitive forms of lexical processing should proceed normally while unattended. This hypothesis was tested by having participants perform a speeded pitch discrimination task followed by lexical decisions or word naming. As the stimulus onset asynchrony between the tasks was reduced, lexical-decision and naming latencies increased dramatically. Word-frequency effects were additive with the increase, indicating that frequency-sensitive processing was subject to postponement while attention was devoted to the other task. Either (a) the same neural hardware shares responsibility for lexical processing and central stages of choice reaction time task processing and cannot perform both computations simultaneously, or (b) lexical processing is blocked in order to optimize performance on the pitch discrimination task. Either way, word processing is not as automatic as activation models suggest.     

The effect of spatial attention on invisible stimuli

The influence of selective attention on visual processing is widespread. Recent studies have demonstrated that spatial attention can affect processing of invisible stimuli. However, it has been suggested that this effect is limited to low-level features, such as line orientations. The present experiments investigated whether spatial attention can influence both low-level (contrast threshold) and high-level (gender discrimination) adaptation, using the same method of attentional modulation for both types of stimuli. We found that spatial attention was able to increase the amount of adaptation to low- as well as to high-level invisible stimuli. These results suggest that attention can influence perceptual processes independent of visual awareness.     

Impaired temporal discrimination within the attentional blink

Yeshurun and Levy (2003) reported that temporal discrimination performance of a visual stimulus benefits when spatial attention is oriented away from its location. In the present study, we investigated whether this negative influence of attention on temporal discrimination performance is restricted to transient spatial attention or might generalize to other paradigms of attention. We employed the attentional blink (AB) paradigm and required either a spatial (Experiment 1) or a temporal discrimination task (Experiment 2). The results of both experiments revealed a performance decrement if attention was temporally unavailable during the AB and a recovery with increasing attentional availability. Thus, contrary to the results of Yeshurun and Levy, the absence of attention decreased temporal discrimination performance in this paradigm. We hypothesize that attention which operates at different processing levels might exert differential effects on temporal stimulus processing.     

Similarity between concurrent visual discriminations: Dimensions and objects

Accuracy is of ten reduced when two visual discriminations must be made concurrently (“divided attention”). According to a hypothesis originally proposed by Treisman (1969) and Aliport (1971), this result should depend on the similarity of required discriminations. When discriminations concern different visual dimensions, they should be made in somewhat separate visual subsystems, reducing interference between them. This prediction was tested in two experiments, involving discriminations of shape, size, orientation, and spatial frequency. In differentconditions of divided attention, concurrent discriminations concerned either the same or different dimensions, and either one or two objects. The results showed that performance depends only on the number of relevant objects, not on the number or similarity of required discriminations. They suggest that selective attention to an object is a coordinated state in which the outputs of multiple visual subsystems are made concurrently available for control of behavior.     

The effect of category learning on the representation of shape: dimensions can be biased but not differentiated

Recent studies have suggested a profound influence of category learning on visual perception, resulting in independent processing of previously integral dimensions. The authors reinvestigate this issue for shape dimensions. They first extend previous findings that some shape dimensions (aspect ratio and curvature) are processed in a separable way, whereas others (radial frequency components) are not. They then show that a category-learning phase improved the discrimination of a relevant with respect to an irrelevant dimension, but only for separable dimensions. No similar effect was found on the relative sensitivity for integral shape dimensions. Thus, category learning is capable of biasing separable shape dimensions but does not alter the status of dimensions in the visual system as either separable or integral.     

Spatial Repetition Blindness Is Modulated by Selective Attention to Color or Shape

Subjects reported either the colors or shapes of two simultaneous masked letters. Our first study found that they were less accurate when the reported features were identical ("repetition blindness," or RB), while repetition along the unreported dimension had no effect. Three follow-up studies confirmed that when the same dimension was judged (overtly or covertly) for both stimuli, performance was only affected by repetition along that dimension. However, when different dimensions were judged for the two stimuli, performance was affected by repetition on both dimensions. These findings support new conclusions about both RB and visual attention. First, RB depends critically on visual attention, rather than simply on the stimulus presented or the overt response required. Second, while attention can be restricted to a single visual dimension, this is efficient only when the same dimension is selected for both objects. Selecting the color of one object and the shape of another simultaneous object results in both dimensions being accessed for both objects.     

The Lower Visual Search Efficiency For Conjunctions Is Due To Noise and Not Serial Attentional Processing

Models of human visual processing start with an initial stage with parallel independent processing of different physical attributes or features (e.g. color, orientation, motion). A second stage in these models is a temporally serial mechanism (visual attention) that combines or binds information across feature dimensions. Evidence for this serial mechanism is based on experimental results for visual search. I conducted a study of visual search accuracy that carefully controlled for low-level effects: physical similarity of target and distractor, element eccentricity, and eye movements. The larger set-size effects in visual search accuracy for briefly flashed conjunction displays, compared with feature displays, are quantitatively predicted by a simple model in which each feature dimension is processed independently with inherent neural noise and information is combined linearly across feature dimensions. The data are not predicted by a temporally serial mechanism or by a hybrid model with temporally serial and noisy processing. The results do not support the idea that a temporally serial mechanism, visual attention, binds information across feature dimensions and show that the conjunction-feature dichotomy is due to the noisy independent processing of features in the human visual system.     

Attention alters the appearance of spatial frequency and gap size

Transient attention is the automatic and short-lasting preferential processing of an area in visual space initiated by sudden stimulation in the same vicinity. Transient attention enhances early visual processing in a variety of dimensions, increasing contrast sensitivity, spatial resolution, and acuity. A recent study established that the increase in contrast sensitivity is accompanied by an increase in apparent contrast. In the present study, we investigated whether the effects of transient attention on spatial resolution and acuity are accompanied by corresponding phenomenological changes in these dimensions. The data indicate that transient attention increases the apparent spatial frequency of Gabor stimuli (Experiment 1) and increases apparent gap size in a Landolt-square acuity task (Experiment 2). Transient attention not only affects basic visual processing-it changes what one experiences.     

Multidimensional shape similarity in the development of visual object classification

The current work examined age differences in the classification of novel object images that vary in continuous dimensions of structural shape. The structural dimensions employed are two that share a privileged status in the visual analysis and representation of objects: the shape of discrete prominent parts and the attachment positions of those parts. Experiment 1 involved a triad classification task in which participants at each of three different ages (5 years, 8 years, and adult) classified object images from two distinct stimulus sets. Across both sets, the youngest children demonstrated a systematic bias toward the shape of discrete parts during their judgments. With increasing age, participants increasingly came to select both the shape and the position of parts when classifying the images. The findings from Experiment 2 indicate that the local shape bias observed in young children's classifications is not merely a consequence of a discrimination advantage for that dimension. Results are discussed in relation to corresponding age-related changes in other functional contexts of visual processing.     

Asymmetric interactions in the processing of the visual dimensions of position, width, and contrast of bar stimuli.

The processing of different dimensions of a single stimulus may be either integral or separable. Dimensions are called integral if correlated variation of one improves discrimination on the basis of the other and random variation of one interferes with discrimination on the basis of the other. For separable dimensions on the other hand, subjects can attend to one dimension and disregard variations in the other. These discrimination tests were used to find the interactions between the processing of the visual dimensions of position, width, and contrast of a light bar stimulus. The relations between these dimensions were found to be asymmetric: judgments of position and width are independent of contrast variations, but variations in these dimensions influence contrast discriminations. Furthermore, position variations influence width judgements more than vice versa. The data were analyzed for repetition effects, and it was found that this model is not sufficient to explain all the interaction phenomena. The asymmetries found may be related to the different ways these dimensions are mapped onto cortical neuron responses.     

工作记忆负荷对无意注意的影响

通过探讨工作记忆负荷对无意注意的影响,考察工作记忆的认知控制功能对选择性注意的作用机制。为了避免工作记忆和选择注意的目标或者干扰子之间的知觉注意资源的竞争,采用言语工作记忆任务和视觉空间位置辨别任务,同时视觉干扰刺激采用被试无法识别的韩语。结果发现,在高负荷条件下,干扰效应显著;而在低和中负荷条件下,没有观察到干扰效应。说明在高负荷条件下,被试没有额外的资源来抑制干扰加工,从而出现了显著的干扰效应。由此,本研究支持负荷理论的观点：工作记忆在选择性注意中执行认知控制功能。     

Intersensory redundancy guides attentional selectivity and perceptual learning in infancy

This study assessed an intersensory redundancy hypothesis, which holds that in early infancy information presented redundantly and in temporal synchrony across two sense modalities selectively recruits attention and facilitates perceptual differentiation more effectively than does the same information presented unimodally. Five-month-old infants' sensitivity to the amodal property of rhythm was examined in 3 experiments. Results revealed that habituation to a bimodal (auditory and visual) rhythm resulted in discrimination of a novel rhythm, whereas habituation to the same rhythm presented unimodally (auditory or visual) resulted in no evidence of discrimination. Also, temporal synchrony between the bimodal auditory and visual information was necessary for rhythm discrimination. These findings support an intersensory redundancy hypothesis and provide further evidence for the importance of redundancy for guiding and constraining early perceptual learning.     

Altered vision near the hands

The present study explored the manner in which hand position may affect visual processing. We studied three classic visual attention tasks (visual search, inhibition of return, and attentional blink) during which the participants held their hands either near the stimulus display, or far from the display. Remarkably, the hands altered visual processing: people shifted their attention between items more slowly when their hands were near the display. The same results were observed for both visible and invisible hands. This enhancement in vision for objects near the hands reveals a mechanism that could facilitate the detailed evaluation of objects for potential manipulation, or the assessment of potentially dangerous objects for a defensive response.     

Spatial constraints on visual-tactile cross-modal distractor congruency effects

Across three experiments, participants made speeded elevation discrimination responses to vibrotactile targets presented to the thumb (held in a lower position) or the index finger (upper position) of either hand, while simultaneously trying to ignore visual distractors presented independently from either the same or a different elevation. Performance on the vibrotactile elevation discrimination task was slower and less accurate when the visual distractor was incongruent with the elevation of the vibrotactile target (e.g., a lower light during the presentation of an upper vibrotactile target to the index finger) than when they were congruent, showing that people cannot completely ignore vision when selectively attending to vibrotactile information. We investigated the attentional, temporal, and spatial modulation of these cross-modal congruency effects by manipulating the direction of endogenous tactile spatial attention, the stimulus onset asynchrony between target and distractor, and the spatial separation between the vibrotactile target, any visual distractors, and the participant’s two hands within and across hemifields. Our results provide new insights into the spatiotemporal modulation of crossmodal congruency effects and highlight the utility of this paradigm for investigating the contributions of visual, tactile, and proprioceptive inputs to the multisensory representation of peripersonal space.