Is there feature-based attentional selection in visual search?

A new paradigm combines attentional cuing and rapid serial visual presentation to disentangle the effects of perceptual filtering and location selection. Observers search successive, superimposed arrays, in which feature values are alternated for a target numeral among letters. Two dimensions, size (small, large) and color (red, green) are tested. Selective attention to feature values is jointly manipulated by instructions, presentation probabilities, and payoffs. In Experiment 1, the attended feature provides temporal, not spatial, information; observers show no attentional costs or benefits in response accuracy. In Experiment 2, the attended feature indicates a unique location; observers show consistent attentional costs and benefits. Selective attention to a particular size or color does not cause perceptual exclusion or admission of items containing that feature; it acts by guiding search processes to spatial locations that contain the to-be-attended feature. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Independent practice association physician groups in California

Hemispatial neglect is a neurological disorder which entails a spatial bias that penalizes events occurring in the hemispace contralateral to a brain lesion. Mechanisms operating upon various stages ranging from perception to action have been invoked to explain neglect. The present study explores the contribution of a defective programming of arm movements towards the neglected hemispace to neglect behaviour. Two reaction time tasks -- a "perceptual" task and a "motor" task -- were performed by right brain-damaged (RBD) patients with left hemispatial neglect, RBD patients without signs of neglect and control subjects. The perceptual task consisted of lateralized visual stimuli and central motor responses, whereas the motor task consisted of visual stimuli presented on the vertical midline and hand responses to be produced in either hemispace. Neglect patients showed a rightward bias on the perceptual task, but only two RBD patients (showing no signs of severe neglect) were consistently slowed in producing leftward motor responses. Different reference frames may thus be used in perceptual tasks and tasks involving arm movements. We conclude that hemispatial neglect commonly results from attentional impairments operating upon a visual perceptual frame of reference; additional deficits appear to be necessary to produce a directional motor disorder.     

Visual size processing in spatial neglect

Evidence from the use of the landmark task and from two size-matching tasks shows that many patients with left-sided neglect systematically under-perceive visual extent in leftward parts of space. This perceptual distortion of size serves to explain the occurrence of rightward line-bisection errors in most neglect patients. One possibility is that attentional biases of a chronic nature might underlie these perceptual changes seen in neglect patients. But contrary to this idea, we have found that attentional cueing in the landmark task causes changes in neglect patients exactly opposite to those seen in healthy subjects. The distortions of size perception seen in neglect could instead be caused by a high-level alteration of visual processing rather than by an attentional bias. In order to explore which visual stream of cortical processing might be compromised in such a disorder, we have begun to examine neglect patients on visuomotor as well as perceptual tasks. We have found clear evidence in one patient for intact grip scaling for object size in the neglected half of space, despite gross perceptual underestimations of the same objects. This result suggests that neglect can occur without major disruption of the dorsal stream, and may result instead from damage to a cortical system whose predominant visual input comes from the ventral stream.     

Allocation of attention in the visual field.

Investigated in 2 experiments whether attentional resources can be simultaneously allocated to several locations in a visual display, whether the mode of processing (serial or parallel) can be switched within a trial, and the nature of the costs when attentional resouces are concentrated on an invalid location. In each of the experiments, 6 college students were required to determine which of 2 target letters was present in 8-letter circular displays. In precue conditions, a primary and a secondary target location were designated 150 msec before target onset by an indicator that varied in validity. In the control conditions no cue was provided. Exp II verified several assumptions that had been made in interpreting the data of Exp I. Modifications in J. Jonides's (see record 1984-08558-001) 2-process model are suggested in terms of a zoom lens model of attentional resources. Instead of 2 alternative processing modes, attentional resources are seen as capable of distribution over the visual field, but with low resolving power, or as continuously constricting to small portions of the visual field with a concomitant increase in processing power. (38 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Contingent capture for onsets and offsets: Attentional set for perceptual transients.

Four experiments were conducted to examine whether attentional set affects the ability of visual transients (onsets and offsets) to capture attention. In the experiments, visual search for an identity-defined target was conducted. In the first 3 experiments, the target display either onset entirely or was revealed by offsetting camouflaging line segments to reveal letters. Prior to the target display, there was a noninformative cue, either an onset or an offset, at one of the potential target locations. Cues that shared the same transient feature as the target display captured attention. The lack of predictable target transients led to attentional capture by all forms of transients. The final experiments with luminance changes without offsets or onsets showed attentional capture when the luminance changes were large. The results suggest that attentional set can be broadly or narrowly tuned to detect changes in luminance. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Repetition blindness for locations: Evidence for automatic spatial coding in an RSVP task.

The authors report a new phenomenon called repetition blindness (RB) for locations: When 3 or 4 letters are presented rapidly and sequentially at random locations within a spatial array, experimental participants have difficulty reporting pairs of letters appearing in the same location within 250 ms of each other. This deficit occurs both during report of letter identities and during report of the locations in which the letters appear; it can also be found using a partial report task. During letter report, the deficit is found for 4-location arrays bit not for 8-location arrays. In contrast, letter RB is not found during location report even when the letters are always chosen from a set of 4. These results indicate that a small number of locations-but not letters-can be encoded automatically even when they are not explicitly reported. The authors argue that RB for locations results from a difficulty individuating 2 tokens at the same spatial location. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Detecting targets in letter and non-letter arrays.

Three experiments with 48 right-handed undergraduates examined the difference between the processing of letter and nonletter arrays using visual search. Results show that (1) letters were processed differently from other shapes; specifically, detection latencies were relatively short for the ends of letter arrays and increased left to right yielding a sloping M-shaped function, whereas the function for nonletter arrays was U-shaped; (2) this result was not restricted to any one nonletter character set, nor is visual familiarity or nameability crucial; and (3) digits produced results comparable to letters. Mechanisms common to word and number processing are advanced to account for this latter finding. The basic letter–shape contrast shows that the letter position function is constrained by processing strategy, not by structural limitations. (French abstract) (28 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Assessment of sensorimotor neglect after occlusion of the middle cerebral artery in the rat

Evaluating the efficacy of neuroprotective drugs in rat models of focal cerebral ischemia has involved histological and behavioral batteries to examine treatment outcome. However, the behavioral tests used to date provide little insight into the nature of the neurological impairments. To provide an analysis of a possible "neglect" syndrome after occlusion of the middle cerebral artery, M. I. Posner's (1980) visual attentional paradigm was adapted for use in the rat. A paw-reaching task and a test of somatosensory "neglect" also were used to assess forelimb sensorimotor function. The lesion group displayed unilateral deficits; however, there was no evidence of attentional dysfunction. Results are consistent with the conclusion that the behavioral deficits identified arise from a somatosensory deficit rather than hemineglect due to dysfunctional spatial attention.     

Visuospatial neglect: the ultimate deconstruction?

Unilateral visuospatial neglect is now widely acknowledged to be a highly heterogeneous condition: The overt manifestations of visual neglect can vary as a function of task, spatial domain, and mode of response (at least). Double dissociations (sometimes of the strong form) have already been reported between most of the components of what was originally thought to be a relatively stable construct within the visual modality. Nonetheless, throughout successive fractionations of neglect, reported cases of bidirectional task-specific neglect after unilateral brain damage are rare. We now report two such cases. After right hemisphere stroke, the first patient reliably showed severe left neglect on cancellation but right neglect on line bisection. After left hemisphere stroke, the second case showed right neglect on cancellation but left neglect on line bisection. Extensive investigation of case 1 confirmed our previous conjecture that the crucial distinction between these tasks lies in the presence or the absence of an overt target. In contrast to cancellation, line bisection demands the internal computation of the location of the "target" (the midpoint), followed by executing a motor response toward the precise location of that "imaginary" midpoint. The relative attentional and premotor contributions of the intact and damaged hemispheres to these forms of bidirectional neglect are also assessed.     

Vertical orienting control: Evidence for attentional bias and "neglect" in the intact brain.

Unilateral neglect may be due to attentional bias: an exaggerated tendency to orient in an ipsilesional direction. Likewise, cases of vertical neglect suggest the existence of vertical attentional biases. This article reports evidence of upward biases in neurologically intact observers. In 5 experiments, observers performed a visual line-bisection task by judging the location of a gap along a tachistoscopically presented vertical line. The upward bias was influenced by attentional cues and was stronger in the right visual field than in the left visual field. Instructions designed to foster the use of an object-based, categorical encoding strategy were associated with an upward bias, whereas those fostering spatial, between-object comparisons were not. The results are discussed in terms of hemispheric differences in attention and specializations along the dorsal-ventral axis of the brain that influence directional orienting. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Abrupt visual onsets and selective attention: Voluntary versus automatic allocation.

The hypothesis that abrupt visual onsets capture attention automatically, as suggested by S. Yantis and J. Jonides (see record 1985-19225-001), was tested in four experiments. A centrally located cue directed attention to one of several stimulus positions in preparation for the identification of a target letter embedded in an array of distractor letters. In all experiments, one stimulus (either the target or one of the distractors) had an abrupt onset; the remaining letters did not. The effectiveness of the cue was manipulated (varying either its duration or its predictive validity) to test whether abrupt onsets capture attention even when subjects are in a highly focused attentional state. Results showed that onsets do not necessarily capture attention in violation of an observer's intentions. A mechanism for partially automatic attentional capture by abrupt onset is proposed, and the diagnosticity of the intentionality criterion for automaticity is discussed. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Attentional blink for color.

Colored letters were presented by means of rapid serial visual presentation (RSVP). Two letters (T1 and T2) were targets that required a speeded (T1) or delayed (T2) response. T1 was shown on half of the trials. Two tasks were performed in the experimental conditions. Task1 required a discrimination between 2 letters (when T1 was shown) or an indication that T1 was absent. In Experiment 1, Task2 was to detect the presence of a unique color in the RSVP stream (e.g., green in a stream of alternating red and gray). In Experiment 2, Task2 was to report the color of the first colored letter to appear in the RSVP stream. The lag between T1 and T2 in the RSVP stream was manipulated. Accuracy of color reports in Task2 was poor at shorter lags and improved as the lag was lengthened in both experiments, demonstrating an attentional blink for chromatic information. Theoretical implications of the results are discussed. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Types and tokens in visual letter perception.

Five experiments demonstrate that in briefly presented displays, subjects have difficulty distinguishing repeated instances of a letter or digit (multiple tokens of the same type). When subjects were asked to estimate the numerosity of a display, reports were lower for displays containing repeated letters, for example, DDDD, than for displays containing distinct letters, for example, NRVT. This homogeneity effect depends on the common visual form of adjacent letters. A distinct homogeneity effect, one that depends on the repetition of abstract letter identities, was also found: When subjects were asked to report the number of As and Es in a display, performance was poorer on displays containing two instances of a target letter, one appearing in uppercase and the other in lowercase, than on displays containing one of each target letter. This effect must be due to the repetition of identities, because visual form is not repeated in these mixed-case displays. Further experiments showed that this effect was not influenced by the context surrounding the target letters, and that it can be tied to limitations in attentional processing. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Canadian studies of visual information-processing: 1970–1980.

Reviews Canadian research on reading and word identification and examines W. Herons's (see record 1959-00387-001) scanning theory that accounts for visual hemifield differences in terms of a serial left-to-right attentional mechanism operating on the stimulus trace. Heron's model, and subsequent work by M. P. Bryden (see record 1967-06490-001), introduced an information-processing approach to the study of perception in Canada and raised questions about the nature of attention, visual persistence, and letter and word recognition. These topics have continued to be a focus for research during the past decade. It is concluded that current results suggest at least 3 attentional processes, 3 buffer stores, and the use of extensive orthographic knowledge in processing letters and words. (French abstract) (2? p ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Measuring and modeling the trajectory of visual spatial attention.

In a novel choice attention-gating paradigm, observers monitor a stream of 3x3 letter arrays until a tonal cue directs them to report 1 row. Analyses of the particular arrays from which reported letters are chosen and of the joint probabilities of reporting pairs of letters are used to derive a theory of attention dynamics. An attention window opens 0.15 s following a cue to attend to a location, remains open (minimally) 0.2 s, and admits information simultaneously from all the newly attended locations. The window dynamics are independent of the distance moved. The theory accounts for about 90% of the variance from the over 400 data points obtained from each of the observers in the 3 experiments reported here. With minor elaborations, it applies to all the principal paradigms used to study the dynamics of visual spatial attention. (PsycINFO Database Record (c) 2011 APA, all rights reserved)     

Span of apprehension in schizophrenic patients as a function of distractor masking and laterality.

Compared 20 schizophrenic patients, 10 depressed control patients, and 20 normal control Ss in a forced-choice, target-detection method for assessing the span of apprehension. The detection task required the S to report which of 2 target letters was presented among 7 other (distractor) letters. Performance accuracy was examined as a function of target location and whether the distractor letters were masked after their presentation. The backward masking of the distractors improved target-detection accuracy of both control groups but reduced accuracy of the schizophrenic group. In addition, schizophrenics performed particularly poorly on targets located in the left half or lower half of the display. Results suggest that response to the masking of distractors may be a new index of attentional shortcoming in schizophrenia. Various theoretical explanations for the target location findings are also discussed. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Disentangling the adult attention-deficit hyperactivity disorder endophenotype: Parametric measurement of attention.

Attention deficit hyperactivity disorder (ADHD) persists frequently into adulthood. The decomposition of endophenotypes by means of experimental neuro-cognitive assessment has the potential to improve diagnostic assessment, evaluation of treatment response, and disentanglement of genetic and environmental influences. We assessed four parameters of attentional capacity and selectivity derived from simple psychophysical tasks (verbal report of briefly presented letter displays) and based on a “theory of visual attention.” These parameters are mathematically independent, quantitative measures, and previous studies have shown that they are highly sensitive for subtle attention deficits. Potential reductions of attentional capacity, that is, of perceptual processing speed and working memory storage capacity, were assessed with a whole report paradigm. Furthermore, possible pathologies of attentional selectivity, that is, selection of task-relevant information and bias in the spatial distribution of attention, were measured with a partial report paradigm. A group of 30 unmedicated adult ADHD patients and a group of 30 demographically matched healthy controls were tested. ADHD patients showed significant reductions of working memory storage capacity of a moderate to large effect size. Perceptual processing speed, task-based, and spatial selection were unaffected. The results imply a working memory deficit as an important source of behavioral impairments. The theory of visual attention parameter working memory storage capacity might constitute a quantifiable and testable endophenotype of ADHD. (PsycINFO Database Record (c) 2011 APA, all rights reserved)     

Organizational and spatial dynamics of attentional focusing in hierarchically structured objects.

Is the focusing of visual attention object-based, space-based, both, or neither? Attentional focusing latencies in hierarchically structured compound-letter objects were examined, orthogonally manipulating global size (larger vs. smaller) and organizational complexity (two-level structure vs. three-level structure). In a dynamic focusing task, participants successively identified the global and local letters in the same trial. Overall response latencies were generally longer for larger versus smaller global objects and for three-level versus two-level object structure, indicating that attentional focusing time increases both with the magnitude of change in attentional aperture size and with the number of traversed levels of object structure. Additional experiments showed that this pattern is unique to tasks that require dynamic attentional focusing. Taken together, the results support a hierarchical object-based-spatial model of attentional focusing. (PsycINFO Database Record (c) 2011 APA, all rights reserved)     

Attention, spatial representation, and visual neglect: Simulating emergent attention and spatial memory in the selective attention for identification model (SAIM).

The selective attention for identification model (SAIM) is presented. This uses a spatial window to select visual information for recognition, binding parts to objects and generating translation-invariant recognition. The model provides a qualitative account of both normal and disordered attention. Simulations of normal attention demonstrate 2-object costs and effects of object familiarity on selection, global precedence, spatial cueing, and inhibition of return. When lesioned, SAIM demonstrated either view- or object-centered neglect or spatial extinction, depending on the type and extent of lesion. The model provides a framework to unify (a) object- and space-based theories of normal selection, (b) dissociations within the syndrome of unilateral neglect, and (c) attentional and representational accounts of neglect. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Implications of visual search deficits for cognitive theory.

Neuropsychological evidence was presented and found to be consistent with Treisman's Feature Integration Theory (FIT). Several studies were reported using groups of patients with unilateral visual neglect, stable patients with known lesion loci, and a single patient with bilateral parietal-occipital lesions. Collectively, the data support a distinction between feature and conjunction search. They also support arguments for the primary role of spatial representation in forming correct conjunctions between features and for individuating objects. Nevertheless, the data do call for some refinements in FIT. The spatial maps involved in feature integration for serial visual search and for the experience of an accurately conjoined object appear to be relatively late. Features are likely to be integrated in the nervous system at sub-threshold levels in early vision (e.g., vl or v2), but it requires supra-threshold spatial knowledge to successfully conjoin features in visual experience (Wojciulik, Robertson & Kanwisher, 1994). The evidence as a whole, however, is consistent with FIT in that spatial information is necessary for controlled visual search and for the accurate conjunction of features in multi-item arrays. In addition, it suggests that explicit access to relatively late spatial information is necessary to correctly conjoin features in the experiences percept. (PsycINFO Database Record (c) 2010 APA, all rights reserved)