The cerebral balance of power: confrontation or cooperation?

Two visual search experiments were carried out using as stimuli large letters made of small identical letters presented in right, or left, or central visual fields. Considering the spatial frequency contents of the stimuli as the critical variable, Experiment 1 showed that a left-field superiority could be obtained whenever a decision had to be made on a large (low frequency) letter alone, and a right-field advantage emerged when a small (high frequency) letter had to be processed. Experiment 2 showed that the two levels of structure of the stimulus were not encoded at the same rate and that at very brief exposure, only the large letter could be accurately identified. This was accompanied by a left-field superiority, whether or not the stimulus contained the target. These results are interpreted as revealing a differential sensitivity of the hemispheres to the spatial frequency contents of a visual image, the right hemisphere being more adept at processing early-available low frequencies and the left hemisphere operating more efficiently on later-available low frequencies. From these and other experiments reviewed, it is suggested that (a) cerebral lateralization of cognitive functions results from differences in sensorimotor resolution capacities of the hemispheres; (b) both hemispheres can process verbal and visuospatial information, analytically and holistically; (c) respective hemispheric competence is a function of the level of sensorimotor resolution required for processing the information available.     

A New Pharmacological Tool to Investigate Integration Processes

Recent work is presented to suggest that lorazepam (a benzodiazepine), enhancing the inhibitory effect of GABAA, might be a useful tool to investigate visuo-perceptual integration processes and to link experimental psychology and electrophysiology. Previous results, obtained with drawings and letters, suggested that lorazepam enhances the processing of line-ends. These results are generalized to simpler stimuli. In Experiment 1, subjects had to detect the presence of corners in fragmented squares. In Experiment2, they had to discriminate between continuous and discontinuous stimuli. In both experiments, elements were either collinear or parallel. The results suggested that lorazepam improved detection of discontinuities and impaired integration for collinear elements. These results sustain the hypothesis that lorazepam enhances responses to line-ends. They are related to studies suggesting that line-ends are coded by end-stopped cells and that GABA is involved in the generation of end-stopped cell responses.     

Beyond the forest and the trees: local and global interference in hierarchical visual stimuli containing three levels

Our visual world can be thought of as organised in a hierarchical manner. Studies on hierarchical letter stimuli (a large letter composed of smaller letters) suggest that processing of a visual scene is global to local, a phenomenon known as the global-precedence effect. Elaborating on this global-to-local hypothesis we tested whether global interference will increase with increasing level of globality. For this, we used three-level hierarchical letter stimuli with a global, middle, and local level. When attending to the local level of the stimulus, only the middle level showed an interference effect, whereas the global level did not interfere at all. We argue that, considering the perceptual and attentional contributions to this effect, the hypothesis of global-to-local processing of a visual scene may only hold within a limited spatial attentional window.     

The effects of lorazepam on visual integration processes: How useful for neuroscientists?

The effects of psychotropic drugs can be seen as a reversible lesion of the central nervous system. We studied the effect of lorazepam, a benzodiazepine facilitating the fixation of GABA on the GABA_A receptor. Evidence will be reviewed that lorazepam affects integration processes, using static fragmented pictures or compound stimuli. It will be shown how these effects can be dissociated from other peripheral or central effects of the drug. As a whole, the results suggest that lorazepam improves the detection of discontinuities between collinear elements, but impairs the detection of discontinuities between two parallel line-ends. Recent results will be presented that support the hypothesis that lorazepam does not affect integration processes themselves but modulates the mechanisms involved in the processing of discontinuities, i.e., the processing of line-ends and/or the production of virtual lines orthogonal to the line-ends. An understanding of the effects of the drug might give insights into the way these signals are used and modulated by the visual system.     

Selective attention and interhemispheric response competition in the split-brain

The interfering effect of an unattended stimulus on processing of an attended item was studied in a single split-brain participant (LB) and in normal controls. Pairs of letters were presented to the left visual field (LVF), right visual field (RVF), or bilaterally. Participants attended to the rightmost letter while attempting to ignore the leftmost letter. Responses associated with the attended and to-be-ignored letters could be compatible or incompatible. Manual response latencies were generally slower on Response Incompatible compared to Response Compatible trials. Notably, LB displayed this effect on Bilateral trials, where target and distractor were presented to opposite visual fields. LB was unable to perform a same-different matching task with bilateral letter stimuli, but was able to name bilateral letters accurately. Hence, in the bilateral condition, the ability to cross-compare letters was dissociated from attentional interference and from letter naming. Implications of these findings are discussed.     

复合字母刺激心理旋转加工中的整体优先效应

采用复合字母材料结合正镜像判断任务,首次揭示了心理旋转加工中的整体优先效应。实验一延长复合字母材料的呈现时间,检验心理旋转实验常用的正/镜像判断任务中的视知觉整体优先效应;实验二将实验一的复合字母材料旋转一定角度,考察心理旋转加工中的整体优先效应。结果发现：（1）延长复合字母呈现时间后的字母正镜像判断任务中,被试判断大字母和判断小字母的时间没有显著差异,并且大小字母是否一致对大字母判断和小字母判断的影响无显著差异;（2）在排除了视知觉的整体优先效应存在的情况下,发现心理旋转加工中存在明显的整体优先效应,大字母（整体）旋转条件下的反应时显著短于小字母（局部）旋转条件下的反应时;（3）旋转角度一致性对大字母旋转和小字母旋转条件下的反应时均无显著影响,表明心理旋转的整体优先效应模式可能有别于视知觉加工的整体优先效应模式     

Can the spotlight of attention be shaped like a doughnut? Evidence from steady-state visual evoked potentials

Visual attention enables observers to extract and process high-priority information in the visual field. Controversy remains as to whether or not observers can ignore information that falls within the spatial beam of attention. We used an objective physiological measure, the steady-state visual evoked potential (SSVEP), to investigate this question. A stream of flickering small, uppercase letters was embedded in the center of a stream of large, uppercase letters. A unitary beam would result in no difference of the SSVEP amplitude elicited by the small letter stream when it was attended versus ignored (i.e., when subjects attended the large letter stream). Contrary to this prediction, SSVEP amplitude increased by almost 100% when the small letter stream was attended compared with when it was ignored. The results support the notion that the attentional spotlight can be formed like a doughnut, processing central information differentially depending on whether it is attended or ignored.     

The flanker compatibility effect as a function of visual angle, attentional focus, visual transients, and perceptual load: a search for boundary conditions

When subjects must respond to a relevant center letter and ignore irrelevant flanking letters, the identities of the flankers produce a response compatibility effect, indicating that they are processed semantically at least to some extent. Because this effect decreases as the separation between target and flankers increases, the effect appears to result from imperfect early selection (attenuation). In the present experiments, several features of the focused attention paradigm were examined, in order to determine whether they might produce the flanker compatibility effect by interfering with the operation of an early selective mechanism. Specifically, the effect might be produced because the paradigm requires subjects to (1) attend exclusively to stimuli within a very small visual angle, (2) maintain a long-term attentional focus on a constant display location, (3) focus attention on an empty display location, (4) exclude onset-transient flankers from semantic processing, or (5) ignore some of the few stimuli in an impoverished visual field. The results indicate that none of these task features is required for semantic processing of unattended stimuli to occur. In fact, visual angle is the only one of the task features that clearly has a strong influence on the size of the flanker compatibility effect. The invariance of the flanker compatibility effect across these conditions suggests that the mechanism for early selection rarely, if ever, completely excludes unattended stimuli from semantic analysis. In addition, it shows that selective mechanisms are relatively insensitive to several factors that might be expected to influence them, thereby supporting the view that spatial separation has a special status for visual selective attention.     

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. The results are interpreted in terms of a model in which parallel encoding processes are capable of automatically analyzing information from several regions of the visual field simultaneously, but fail to accurately encode location information. The resulting representation is thus insufficient to distinguish one token from another because two tokens of a given type differ only in location. However, with serial attentional processing multiple tokens can be kept distinct, pointing to yet another limit on the ability to process visual information in parallel.     

Temporal limits of selection and memory encoding: A comparison of whole versus partial report in rapid serial visual presentation

People often fail to recall the second of two visual targets presented within 500 ms in rapid serial visual presentation (RSVP). This effect is called the attentional blink. One explanation of the attentional blink is that processes involved in encoding the first target into memory are slow and capacity limited. Here, however, we show that the attentional blink should be ascribed to attentional selection, not consolidation of the first target. Rapid sequences of six letters were presented, and observers had to report either all the letters (whole-report condition) or a subset of the letters (partial-report condition). Selection in partial report was based on color (e.g., report the two red letters) or identity (i.e., report all letters from a particular letter onward). In both cases, recall of letters presented shortly after the first selected letter was impaired, whereas recall of the corresponding letters was relatively accurate with whole report.     

Temporal and spatial characteristics of selective encoding from visual displays

The time required for Ss to voice a target letter in a visual display was studied as a function of the spatial proximity of two kinds of noise elements (letters or disks) to the target and as a function of when the noise elements were presented following the onset of the target letter. The results were not consistent with a focusing model of attention or selective encoding. Instead, there appears to be a small area in the visual field (about 1 deg of angle) in which all stimuli are processed in detail.     

Fear-related pictures deteriorate the performance of university students with high fear of snakes or spiders

Despite research regarding emotional processing, it is still unclear whether fear-evoking stimuli are processed when they are irrelevant and when attention is oriented elsewhere. In this study, 63 healthy university students with high fear from snakes or spiders participated in two different experiments. In an emotional modification of the spatial cueing task, 31 subjects (5 males) were asked to detect a target letter while ignoring a neutral or fear-related distracting picture. The distribution of attention was independently manipulated by a spatial cue that preceded the appearance of the picture and the target letter. In an emotional modification of the cognitive load paradigm, 32 subjects (4 males) were asked to discriminate between two target letters, while ignoring a central neutral or fear-related picture, and additional 1, 3, or 5 distracting letters that created a varied attentional load. Fear-related pictures interfered with the performance of highly fearful participants, even when the pictures were presented outside the focus of attention and when the task taxed attentional resources. We suggest that highly fearful individuals process fear-related information automatically, either inattentively or with prioritized attention capture over competing items, leading to deteriorated cognitive performance. Different results were shown in healthy individuals while processing negative--but not phobic--pictures, suggesting that emotional processing depends on the fear value of the stimulus for a specific observer.     

Mechanisms of imagery-perception interaction

A number of different research findings have shown that mental imagery can affect the perceptual processing of stimuli. The present research was aimed at characterizing the representations and processes underlying imagery-perception interactions. In four experiments, subjects mentally projected images of letters into the visual field, and either detected or detected and localized point threshold stimuli that fell on or off the image. Stimuli falling on the image were detected more often than stimuli falling off the image, consistent with the hypothesis that the representations at the interface between imagery and perception have an array format. When the facilitation was analyzed in terms of signal detection theory, it was found to consist only of criterion lowering, and not of enhanced sensitivity. The local criterion-lowering effect of imaged letters was then compared with the effect of perceiving a letter and attending to a letter. Perceiving a letter had no discernible effect on stimulus detection, whereas attending to the letter caused the same local criterion lowering, without sensitivity changes, as imaging the letter. This is consistent with the claims of Neisser (1976) and others that imagery is an attentional state.     

Detecting distortions of peripherally presented letter stimuli under crowded conditions

When visual features in the periphery are close together they become difficult to recognize: something is present but it is unclear what. This is called “crowding”. Here we investigated sensitivity to features in highly familiar shapes (letters) by applying spatial distortions. In Experiment 1, observers detected which of four peripherally presented (8 deg of retinal eccentricity) target letters was distorted (spatial 4AFC). The letters were presented either isolated or surrounded by four undistorted flanking letters, and distorted with one of two types of distortion at a range of distortion frequencies and amplitudes. The bandpass noise distortion (“BPN”) technique causes spatial distortions in Cartesian space, whereas radial frequency distortion (“RF”) causes shifts in polar coordinates. Detecting distortions in target letters was more difficult in the presence of flanking letters, consistent with the effect of crowding. The BPN distortion type showed evidence of tuning, with sensitivity to distortions peaking at approximately 6.5 c/deg for unflanked letters. The presence of flanking letters causes this peak to rise to approximately 8.5 c/deg. In contrast to the tuning observed for BPN distortions, RF distortion sensitivity increased as the radial frequency of distortion increased. In a series of follow-up experiments, we found that sensitivity to distortions is reduced when flanking letters were also distorted, that this held when observers were required to report which target letter was undistorted, and that this held when flanker distortions were always detectable. The perception of geometric distortions in letter stimuli is impaired by visual crowding.     

Spatial frequencies and the cerebral hemispheres: contrast sensitivity, visible persistence, and letter classification

The hypothesis that the two cerebral hemispheres are specialized for processing different visual spatial frequencies was investigated in three experiments. No differences between the left and right visual fields were found for: (1) contrast-sensitivity functions measured binocularly with vertical gratings ranging from 0.5 to 12 cycles per degree (cpd); (2) visible persistence durations for 1- and 10-cpd gratings measured with a stimulus alternation method; and (3) accuracy (d') and reaction times to correctly identify digitally filtered letters as targets (L or H) or nontargets (T or F). One significant difference, however, was found: In Experiment 3, a higher decision criterion (beta) was used when filtered letters were identified in the right visual field than when they were identified in the left. The letters were filtered with annular, 1-octave band-pass filters with center spatial frequencies of 1, 2, 4, 8, and 16 cpd. Combining four center frequencies with three letter sizes (0.5 degrees, 1 degree, and 2 degrees high) made some stimuli equivalent in distal spatial frequency (cycles per object) and some equivalent in proximal spatial frequency (cycles per degree). The effective stimulus in the third experiment seemed to be proximal spatial frequency (cycles per degree) not distal (cycles per object). We conclude that each cerebral hemisphere processes visual spatial frequency information with equal accuracy but that different decision rules are used.     

Combining attributes in rapid serial visual presentation tasks

A series of three exploratory experiments sought to compare and contrast two broad classes of theory pertaining to visual information pick-up. One theoretical approach, exemplified by Keele and Neill (1978) and by Treisman (1977) emphasizes the relatively passive, parallel development of information codes, with subsequent attentional integration of contiguous codes into a percept. Another theoretical approach, exemplified by Broadbent (1977a) emphasizes the relatively active, sequential, recursive interrogation of a visual display in which the pick-up of each piece of information guides the perceptual system in its quest for more information. The manner in which separable attributes of visual information are picked up and combined was studied by manipulating the roles of different attributes in a rapid serial visual presentation (RSVP) paradigm. Observers were asked to report the name of a letter in a specified target colour; to report the colour of a specified target letter; to report the colour of a specified target numeral embedded in a list of letters; to report the colour of an unspecified numeral embedded in a list of letters; and to perform the latter two tasks without knowing in which of two alternating spatial positions the target might appear. Examination of the patterns of mis-combinations of attributes from target and non-target stimuli yields evidence both for and against each theoretical position, and suggests that the two might better be considered as modes of processing available to the perceptual system, rather than as intrinsic properties of that system.     

Symbolic control of attention: Tracking its temporal dynamics

Three experiments examined the temporal dynamics of the impact of symbols with task-irrelevant spatial meanings on attentional control. In Experiments 1 and 2, participants were color-cued to report the first letter they saw in the left or right of two parallel letter streams. The cue appeared in the shape of an arrow pointing to the target stream (compatible) or to the other stream (incompatible). Incompatible arrows delayed letter selection; that is, participants reported later-appearing letters and made more errors. In Experiment 3, the target stream was cued in advance, and yet, incompatible symbols delayed target selection. These findings suggest that the irrelevant meaning of symbolic stimuli can still penetrate and bias attentional top-down control, an observation that challenges available control theories.     

Visual orienting with central and peripheral precues: Deconfounding the contributions of cue eccentricity,cue discrimination and spatial correspondence

Five experiments examined effects of spatial precues on visual attention. Precues consisted of letters (X, T, O) presented bilaterally at central and peripheral locations.In the spatial correspondence conditions targets tended (p= .8) to appear on the same side as one of the letters. In the spatial translation conditions the bilateral letters were identical; one letter (e.g., X) indicated that the target would probably (p= .8) appear on the left, and the other (e.g., T) indicated that the target would probably appear on the right. In the spatial correspondence conditions (Experiments 1, 3A, 4, and 5) response latencies were quicker on valid than invalid trials, even at very brief (0 ms, 33 ms, 66 ms, 100 ms) stimulus onset asynchronies (SOAs) between cue and target onset. Also,the dynamic pattern of cost and benefit resembled that seen in previous studies with unilateral peripheral precues. In the spatial translation conditions (Experiments 2 and 3B), a clear advantage for valid trials was only apparent at somewhat longer SOAs (150 ms, 300 ms, 500 ms). Results show that the need to discriminate between cue stimuli, and the visual eccentricity of cues, are not critical factors mediating attentional effects of spatial precues. However, the presence or absence of spatial correspondence between the location of a cue stimulus and the location of the target was critically important.     

Visual search under conditions of very rapid sequential Input rates

The purposes of the present experiment were to provide information on rate of information processing in visual perception and to determine the degree to which the “sequential blanking” effect found by Mayzner. Tresselt, and Cohen (1966) constituted a limitation on rapid sequential input rates. A 10-channel tachisto scope was employed that permitted controlled durations of each of the 10 channels and the 9 inter channel intervals. The S’s task was one of visual search or detection in which he searched for a target letter among noise letters. A temporal interval forced-choice procedure was used. In addition to varying the rate at which letters were sequentially presented, various irregular temporal spatial orders of presentation of the letter sequences were employed and the direction and orientation of the display in the visual field was varied as was also the spacing between adjacent stimuli. No evidence of “sequential blanking” was found either in terms of the detection criterion or in the Ss’ phenomenal reports. Detection performance was as good at a rate of 2 1/2 msec per letter as it was at a rate of 30 msec per letter.