Readout from iconic memory and selective spatial attention involve similar neural processes

Iconic memory and spatial attention are often considered separately, but they may have functional similarities. Here we provide functional magnetic resonance imaging evidence for some common underlying neural effects. Subjects judged three visual stimuli in one hemifield of a bilateral array comprising six stimuli. The relevant hemifield for partial report was indicated by an auditory cue, administered either before the visual array (precue, spatial attention) or shortly after the array (postcue, iconic memory). Pre- and postcues led to similar activity modulations in lateral occipital cortex contralateral to the cued side. This finding indicates that readout from iconic memory can have some neural effects similar to those of spatial attention. We also found common bilateral activation of a fronto-parietal network for postcue and precue trials. These neuroimaging data suggest that some common neural mechanisms underlie selective spatial attention and readout from iconic memory. Some differences were also found; compared with precues, postcues led to higher activity in the right middle frontal gyrus.     

Voluntary visual attention and phenomenal line length

In 1956, Fraisse, et al. reported subjects judged that lines were longer when voluntary attention was focused on the lines than when attention was distracted from the lines. In the many attempts to repeat these results, none has ascertained whether attention on reported line length was a phenomenal effect. In the present study, 46 subjects were shown as stimuli pairs of horizontal or vertical briefly flashed lines with a fixation cross placed equidistant between the lines but far from each one. A change in color of one arm of the cross was used as a cue to focus subjects' voluntary attention on one line. Analysis showed attention increased the judged length of attended lines. Since this effect of attention also occurred when subjects were absolutely certain they saw the stimulus lines differed in length, this effect indicates that attention increased the phenomenal length of the attended lines. This lengthening was quite small: it involved a maximum mean increase of about .15 in the probability of the comparative response that the attended line was longer. This effect occurred in the horizontal dimension and was almost absent in the vertical dimension. In agreement with data indicating that flashed lines expand phenomenally by activating motion detectors and that focused attention makes neural motion responses increase in amplitude, the present results suggest that focused attention makes attended lines look longer because it makes these lines expand phenomenally more rapidly.     

Impact of spatial grouping on mean size estimation

People represent summary statistics of visual scenes, but it is not fully clear whether such summary statistics are extracted automatically. To determine whether different levels of summary representation (i.e., at the perceptual-group or the entire-display level) may be formed differently, in two experiments we investigated how people extracted summary statistics for displays consisting of spatially segregated groups. Participants were asked to report the mean sizes of either entire sets or perceptual groups in precue and postcue conditions. There was no precueing advantage in the mean size estimations of entire sets. However, when these precues identified target perceptual groups, participants reported the perceptual-group means more accurately than when postcues were used. In the postcue condition, participants were biased toward the entire-set mean even when they were probed to report the perceptual-group mean. There was also greater bias toward the entire-set mean for more erroneous perceptual-group summaries. These findings suggest that ensemble representations are extracted more efficiently for the whole than for the perceptual parts and that ensemble perception is not a uniform process across perceptual groups and entire sets.     

Information tradeoffs in complex stimulus structure: Local and global levels in naturalistic scenes

An information tradeoff is an increased processing or utilization of information from one stimulus source at the expense of processing or utilization of information from a different source. An experiment was conducted to determine whether information tradeoffs occurred when subjects attended selectively to one of two different structural levels of naturalistic scenes. The subjects’ attentional focus was directed to either the global or local structure of a scene (i.e., the scene or an object in the scene, respectively) either before or after presentation of a scene. They then had to use the information obtained from a 100-msec exposure of the scene to choose between two forced-choice alternatives that described one of the levels. The nature of the alternatives was such that both alternatives adequately characterized one of the structural levels on the basis of physical and semantic relations within the scene. Results showed that the subjects were significantly slower and less accurate when their attentional focus and the forced-choice alternatives were at different levels of stimulus structure than when they were at the same level, providingevidence of an information tradeoffwhen different types of information from a scene were used. When processinginformation from a particular structural level, information fromtheother level either was less available or was not used efficiently. Furthermore, the information tradeoffs were more severe in the precue than in the postcue condition, indicating differences in the efficiency of the selectivity process. The results are interpreted with respect to the role of selective attention in processing complex stimuli such as naturalistic scenes.     

Information tradeoffs in complex stimulus structure: local and global levels in naturalistic scenes.

An information tradeoff is an increased processing or utilization of information from one stimulus source at the expense of processing or utilization of information from a different source. An experiment was conducted to determine whether information tradeoffs occurred when subjects attended selectively to one of two different structural levels of naturalistic scenes. The subjects' attentional focus was directed to either the global or local structure of a scene (i.e., the scene or an object in the scene, respectively) either before or after presentation of a scene. They then had to use the information obtained from a 100-msec exposure of the scene to choose between two forced-choice alternatives that described one of the levels. The nature of the alternatives was such that both alternatives adequately characterized one of the structural levels on the basis of physical and semantic relations within the scene. Results showed that the subjects were significantly slower and less accurate when their attentional focus and the forced-choice alternatives were at different levels of stimulus structure than when they were at the same level, providing evidence of an information tradeoff when different types of information from a scene were used. When processing information from a particular structural level, information from the other level either was less available or was not used efficiently. Furthermore, the information tradeoffs were more severe in the precue than in the postcue condition, indicating differences in the efficiency of the selectivity process. The results are interpreted with respect to the role of selective attention in processing complex stimuli such as naturalistic scenes.     

Selected parameters of response organization in mildly mentally retarded children

The purpose of this study was to investigate the effect of response organization on reaction time (RT) and movement time (MT) of mildly mentally retarded children. Two groups of 30 subjects each were formed: a retarded group 9 yr. of age and a normal group matched for chronological age. Subjects were randomly assigned to one of three precued conditions in which they were told that the forthcoming response would be performed with the right or left hand (precue hand), be to the right or left side (precue direction), or cross or not cross the body midline (precue midline). The retarded group performed significantly more slowly than the normal group on both RT and MT. Both groups, however, were able to utilize precued hand information as opposed to other precued variables, indicating that knowing which hand to use is important when organizing responses.     

Mechanisms underlying spatial coding in a multiple-item Simon task

Choice responses are faster when target position and response side correspond than when they do not, even if target position is response-irrelevant. This "Simon effect" has also been observed in case of multi-item arrays. Generally, it is assumed that an automatically generated spatial response code is responsible for the effect. The referential-coding account assumes that this code is directly related to the target, although the moment of production of the code is not fully clear. The attention-shift account assumes that the code is directly related to the direction of the most recent attentional shift. An experiment was performed in which left or right target locations were indicated by arrows occurring before (precue), simultaneous with (simcue), or after (postcue) six-element arrays. Overt responses and EEG potentials were recorded. The Simon effect was present in all conditions, and decreased when responses were slower. No relation was found between amplitude of posterior lateralized components and the magnitude of the Simon effect. A posterior contralateral negativity was also found after presenting the arrays in the precue condition, which might reflect the reorienting of attention toward the target position. The results are more favorable to the referential-coding account although this account becomes very similar to the attention-shift hypothesis as the moment of formation of the spatial response code is related to effective target onset rather than to stimulus onset.     

On the spatial extent of attention in object-based visual selection

A new test was devised to avoid previous confounds in measures of object-based limits on divided visual attention. The distinction between objects was manipulated across a wide spatial extent. Target elements appeared on the same object only when far apart, and appeared close only when on different objects, so that object effects could not be reduced to spatial effects, nor vice versa. Subjects judged whether two odd elements within a display of two dashed lines were the same or different. They performed better when the target elements were far apart on a common line rather than on two distinct lines, even though the latter arrangement was more likely. Thus, nonstrategic object-based limits on divided attention can arise even across large distances. However, when subjects were precued to expect targets in a narrow region of the display, the object effect was eliminated, implying that object-based selection may only operate within spatially attended regions.     

A developmental study of attention to auditory and visual signals

Three experiments tested for developmental changes in attention to simple auditory and visual signals. Subject pressed a single button in response to the onset (Experiment 1) or offset (Experiment 2) of either a tone or a light. During one block of trials subjects knew which stimulus would come on or go off on each trial (precue condition) whereas during the other block of trials no precue was provided. In both experiments subjects as young as 4 years old responded more rapidly with precues, indicating that they were able to allocate their attention to the indicated modality. Experiment 3 utilized a choice reaction paradigm (in which subjects pressed different buttons in response to the onset of the light and the tone) in order to examine their attention allocation when no precues were provided. It was found that the adults and 7-year-olds tended to allocate their attention to vision rather than audition when no precue was provided. The results with the 4-year-olds were not entirely consistent, but suggested a similar biasing of attention to vision on their part as well.     

The functional role of attention for spatial coding in the Simon effect

Summary Two experiments investigated relative spatial coding in the Simon effect. It was hypothesized that relative spatial coding is carried out with reference to the position of the focus of visual attention. The spatial code for an imperative stimulus presented exactly at the position of focal attention should be neutral on the horizontal plane, and therefore no Simon effect should be observed. However, when the imperative stimulus is presented to the left or to the right of the current position of focal attention, the spatial code should not be neutral, thus producing a Simon effect. In both experiments, focal attention was manipulated either by a peripherally presented onset precue (Experiment 1) or by a centrally presented symbolic precue (Experiment 2). Results showed that the Simon effect was substantially reduced in both experiments when a valid precue preceded the imperative stimulus just in time to conclude refocusing of attention to the position of the imperative stimulus before it was presented. However, conditions with neutral precues yielded a normally sized Simon effect. In both experiments, the Simon effect decreased as the SOA grew when the precue was valid. At least for the Simon effect, the results can be interpreted as evidence that relative spatial coding is functionally related to the position of the focus of attention.     

Modulations of the processing of line discontinuities under selective attention conditions?

We examined whether the processing of discontinuities involved in figure-ground segmentation, like line ends, can be modulated under selective attention conditions. Subjects decided whether a gap in collinear or parallel lines was located to the right or left. Two stimuli were displayed in immediate succession. When the gaps were on the same side, reaction times (RTs) for the second stimulus increased when collinear lines followed parallel lines, or the reverse, but only when the two stimuli shared the same orientation and location. The effect did not depend on the global form of the stimuli or on the relative orientation of the gaps. A frame drawn around collinear elements affected the results, suggesting a crucial role of the "amodal" orthogonal lines produced when line ends are aligned. Including several gaps in the first stimulus also eliminated RT variations. By contrast, RT variations remained stable across several experimental blocks and were significant for interstimulus intervals from 50 to 600 msec between the two stimuli. These results are interpreted in terms of a modulation of the processing of line ends or the production of amodal lines, arising when attention is selectively drawn to a gap.     

Automatic Versus Directed Attention with Single-element and Multiple-element Precues

Four experiments were conducted to determine whether peripheral singleelement precues (SEPs) and multiple-element precues (MEPs) would elicit similar attention effects on target identification. For an MEP, characters were presented near each possible target location; however, one that differed from the others by either colour or luminance specified the target location. At short precue-target intervals (SOAs), target identification accuracy rose more quickly with SEPs than with MEPs; at long SOAs, performance declined with SEPs, but not with MEPs. Furthermore, when validity of the precue was manipulated, an SEP attracted attention automatically even if the precue was irrelevantas to target location, whereas an irrelevant precue did not benefit performance with MEPs. Accuracy atlong SOAs with MEPs was more similarto thatproduced by a precue at fixation than to that produced by an SEP. Thus, MEPs do not elicit attention in the same manner as SEPs.     

Attention effects of abrupt-onset precues with central, single-element, and multiple-element precues

Endogenous and exogenous processes of attention have been inferred with different types of precues used in allocation of attention to a target location. In the present research, a comparison was made between the typical peripheral single-element precue (SEP), a central precue, and a multiple-element precue (MEP) in order to further understanding of the processes involved in allocation of attention. Two precues were used on each trial in these experiments. An abrupt-onset precue appeared with an SEP, an MEP, or a central precue and was followed 50 or 300 ms later by a screen containing a target and two distractor characters. The abrupt-onset precue and the other precue each could be valid or invalid in indicating the location of the target, as in the study by Juola, Koshino, and Warner (1995). Response times to the targets showed that validity effects of the abrupt-onset precue and the MEP or central precue were additive, whereas those of the abrupt-onset precue and the SEP were interactive. These data suggest that, like a central precue, an MEP is an endogenous precue that guides conscious control of attention and has its attentional effects at a different processing level from an SEP, which is an exogenous precue and may compete for attentional resources with an abrupt-onset precue.     

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.     

Attentional cueing effect in the identification of words and pseudowords of different length.

Three experiments studied the influence of spatial attention on familiar and unfamiliar letter string identification. Siéroff and Posner's (1988) cueing procedure was used: A cue indicated in advance either the beginning (left) or the end (right) of a foveally presented letter string that participants were instructed to read aloud. Results showed that the precue had a stronger influence on pseudoword than on word identification. Similar results were obtained when participants were instructed to report the identity of the cue or not. For pseudowords, a cueing effect was obtained regardless of length (6, 8, and 10 letters), whereas only 10-letter words showed such an effect, though to a lesser degree than pseudowords of the same length. However, results showed that shorter words were also influenced by the cue location when the exposure duration was reduced. Results are compatible with an early role of spatial attention in letter string processing, but they also suggest that the lexical status of a letter string can directly influence the distribution of attention before the identification process is completely achieved. Although orienting of spatial attention seems heavily involved in a pseudoword identification, some spatial attention mechanism could also take place in the case of familiar words. The results are discussed within two theoretical frameworks concerning the involvement of spatial attention in word identification: The "replacement" theory and the "redistribution" theory.     

Mental measurement of line length: the role of the standard

Reaction times were examined for magnitude estimates of line length. In the first two experiments, reaction times increased linearly with judged length. This result is consistent with the hypothesis the judgments are made by laying off a mental image of the standard along the line to be judged. The slope of the function relating judged length to reaction time was not affected by the length of the standard line, suggesting that the rate at which the image of the standard is laid off is not a function of the length of the standard. Reaction time also increased linearly with judged length when subjects judged line length when the standard of 1 in. was suggested but not provided as well as when no standard was suggested. The hypothesized laying-off process was compared to other cognitive manipulations, such as mental rotation and size scaling. Equivalence of judgments based on the representation of the standard in perceptual memory and in imagination is discussed.     

ON THE PROBLEM OF SELECTION IN ICONIC MEMORY

von W right J. M. On the problem of selection in iconic memory. Scad J. Psychol ., 1972, 1 3 , 159–171.—Conditions determining the selection of items in iconic memory were analysed. The accuracy of partial reports and the strategies adopted by the subjects varied regularly as a function of the mode of report as well as of the subject's uncertainty about the type of response he would be asked to make. By varying these factors, the apparently conflicting results of previous studies could be fairly well replicated. In optimal conditions, selection by simple physical characteristics was efficient whereas selection by class (letterslnumbers) remained difficult. Similar results were obtained when precue and postcue conditions were compared. The role of output interference and of the subjects' encoding and response strategies are discussed.     

Interference from related items in object identification

Interference between related items in the identification of objects was examined using a postcue procedure. Pairs of objects were presented as differently colored line drawings followed by a color cue to indicate which object to name. Naming latencies were longer when both objects were from the same superordinate category than when they were unrelated. This interference effect was replicated when subjects were cued to report the color of a drawing rather than its name. Interference was greatly reduced when more distinctive attributes were used to distinguish members of a pair, both when the task required naming an object and when it required report of an object's attribute. These results challenge accounts of interference in the postcue paradigm that are based on competitive interactions in the activation of phonological representations by semantics and instead implicate object-attribute integration in memory as the source of interference.     

A framework for understanding the allocation of attention in location-precued discrimination

The effects of attention on visual perception are assessed in the location-precuing paradigm. First, we present a review of some current metaphors for attention and relevant data. Then, a framework is suggested that provides an interpretation of the temporal sequence of external and assumed internal processes within a location-cuing trial. Cases when a precue correctly indicates the target location (valid trials) are compared to cases when the precue directs attention to the wrong location (invalid trials) with the cue location either at fixation or peripheral to the target location. Several specific hypotheses are suggested; these concern decrements in performance on invalid trials and effects of the location of a precue. For the most part, these hypotheses are supported by data in the literature and in some new studies. A gradient-filter metaphor for attention, which includes a synthesis of ideas from the gradient model and the attention gate model, is more consistent with the data than is a spotlight metaphor.     

Crossover by Line Length and Spatial Location

It is well known that line length has a systematic influence on line bisection error in neglect. Most patients with neglect misbisect long lines on the same side of true center as their brain lesion but then cross over on short lines, misbisecting them on the opposite side (i.e., crossover by line length). What is less recognized is that the spatial location of lines relative to the viewer can similarly induce a crossover effect when one considers line bisection error scores that have been averaged across individual line lengths. Patients with right hemisphere injury and neglect classically make averaged line bisection errors that fall right of true center on lines located either at midline or to the left of the viewer; however, we observed that the averaged line bisection error can fall left of true center when lines are located to the right of the viewer (i.e., crossover by spatial location). We hypothesized that crossover by both line length and spatial location stem from systematic errors in magnitude estimation, i.e., perceived line length. We tested predictions based on this hypothesis by examining how the crossover effect by line length is altered by the spatial location of lines along a horizontal axis relative to the viewer. Participants included patients with unilateral lesions of the right and left cerebral hemispheres and age-appropriate normal subjects. All groups demonstrated a crossover effect by line length at the midline location but the effect was altered by placing lines to the right and left of the viewer. In particular, patients with right hemisphere injury and neglect crossed-over across a broader range of line lengths when the lines were located to the right of the viewer rather than at either midline or left of the viewer. It is proposed that mental representations of stimulus magnitude are altered in neglect, in addition to mental representations of space, and that traditional accounts of neglect can be enhanced by including the psychophysical concept of magnitude estimation.