Central Fixations are Inadequately Controlled by Instructions Alone: Implications for Studying Cerebral Asymmetry

A fundamental concern when using visual presentations to study cerebral asymmetry is to ensure that stimuli are presented with the same degree of retinal eccentricity from a central fixation point in either visual field. However, a widely used procedure intended to control fixation location merely instructs participants to fixate appropriately without any other means of ensuring that central fixations actually occur. We assessed the validity of assuming that instructions alone ensure central fixation by using the traditional R VF advantage for words and either (a) only instructions to fixate centrally, or (b) an eye-tracking device that ensured central fixation on every trial. Experiments 1 and 2 found that when only instructions were given, the vast majority of fixations were not central, and more occurred to the right of centre than to the left. Moreover, the prevalence of non-central fixations was otherwise disguised by the finding that both fixation procedures produced similar R VF advantages in overt performance. The impact of typical non-central fixations on performance was revealed by systematically manipulating fixation location in Experiment 3, where deviations in fixation of only 0.25 from centre had a reliable impact on visual field effects. Implications of these findings for studies of cerebral asymmetry are discussed.     

Serial position functions for letter identification at brief and extended exposure durations

The properties of serial position functions for tachistoscopic report were investigated over a wide range of viewing times. Four-letter strings of random consonants were presented in varying display locations relative to the fixation point with the observers’ eye movements monitored to limit them to a single fixation for each display. Salient properties of the serial position curves include an overall central-peripheral gradient, higher performance at the ends than the interior of letter strings regardless of absolute location, and left-right asymmetry in the visual field, all of these being largely independent of viewing time. Errors reflecting loss of positional information are prominent even at extended viewing times, are more nearly symmetrical in the left and right visual fields than other types of errors, and, in contrast to item errors, occur less frequently in letter sequences that have high frequencies in English. Further, transposition errors exhibit a pronounced peripheral-to-central drift, possibly reflecting gradients of positional uncertainty. Such gradients may be implicated in the peripheral-central asymmetry of the lateral interference effects exerted by other letters on a target letter in a nonfoveal location.     

Effects of prime and target eccentricity on masked repetition priming

Three experiments examined the influence of briefly presented, pattern-masked prime stimuli on target word recognition at varying eccentricities. The prime was either the same word as the targets or a different word, and prime position varied horizontally from a central fixation point. The targets were either in the same location as the primes (Experiment 1A) or always centrally located (Experiments 1B and 2). In Experiment 1A, target word recognition showed a typical right visual field advantage, and priming effects diminished with increasing prime and target eccentricity. With centrally located targets, priming effects tended to be more constrained by prime location. After eye fixation location and prime visibility were controlled for (Experiment 2), a right visual field advantage for priming effects was also evident for central targets, suggesting an influence of endogenous attentional biases in masked repetition priming.     

Increasing the benefits of eye-tracking devices in divided visual field studies of cerebral asymmetry

A wide range of methods has been proposed for ensuring central fixation in divided visual field studies of cerebral asymmetry. We present some of the problems associated with ensuring central fixation and argue that objectively monitoring fixation location with an eye-tracking device can provide the only accurate and, therefore, acceptable technique. However, the mere use of this equipment does not overcome all of the problems associated with controlling fixation location, and previous studies have overlooked the importance of providing feedback on fixation accuracy and the role of visual attention. We propose a straightforward method of accommodating both these influences in studies using eyetracking devices to control fixation location.     

The spatial distribution of attention following an exogenous cue

Three target-discrimination experiments were conducted to explore the spatial distribution t)f covert visual attention following an exogenous cue. On each trial, a brief peripheral onset was followed by a target stimulus in an otherwise empty visual field at one of eight (Experiment 1) or one of four (Experiments 2 and 3) possible locations centered at the fixation point. The spatial relation between the cue and the target was manipulated. The main results were that (1) performance was better at the cued location than at another nearby location in the same visual quadrant; (2) performance was not affected by the major horizontal and vertical visual meridians; and (3) performance was affected by the spatial distance between the cued and target locations. Together, the results suggest that the spatial distribution of exogenously oriented attention cart be most easily integrated with a simple gradient model.     

Not fixating at the line of text comes at a cost

Previous research on eye guidance in reading has investigated systematic tendencies with respect to horizontal fixation locations on letters within words and the relationship between fixation location in a word and the duration of the fixation. The present study investigates where readers place their eyes vertically on the line of text and how vertical fixation location is related to fixation duration. Analyses were based on a large corpus of eye movement recordings from single-sentence reading. The vertical preferred viewing location was found to be within the vertical extent of the font, but fixations beyond the vertical boundaries of the text also frequently occurred. Analyzing fixation duration as a function of vertical fixation location revealed a vertical optimal viewing position (vOVP) effect: Fixations were shortest when placed optimally on the line of text, and fixation duration gradually increased for fixations that fell above or below the line of text. The vOVP effect can be explained by the limits of visual resolution along the vertical meridian. It is concluded that vertical and horizontal landing positions in single-sentence reading are associated with differences in fixation durations in opposite ways.     

Temporal attentional capture: Effects of irrelevant singletons on rapid serial visual search

The presence of a unique yet irrelevantsingleton in visual search or spatial-cuing tasks is typically associated with performance costs, suggesting that singletons tend to capture attention. However, since singletons have always been spatially separated from targets in previous experiments, it remains unclear whether an irrelevant visual singleton that occurs at the same spatial location as the target but at a different point in time can produce temporal capture of attention. Here, we asked participants to search visual sequences at fixation for targets defined by size (larger or smaller than the nontargets). The presence (vs. absence) of a color singleton lengthened response times on the size discrimination task, suggesting that irrelevant singletons can lead to a temporal attentional capture.     

视网膜离心率影响空间注意梯度分布

采用同步判断(Simultaneous judgment, SJ)的研究范式探讨空间注意梯度是否受到视网膜离心率的影响。实验中给被试呈现两个目标刺激, 调节它们的呈现距离和间隔时间, 让被试进行不限时的同步判断, 计算出各种距离上的主观等同点(Point of Subjective Simultaneous, PSS)变化情况。实验中对比了注视点和注意点重合与分离的两种状态。其中注视点和注意点分离的状态是指使用反向掩蔽后的线索刺激将注意点内隐地引导到外周。通过控制似动等影响因素, 发现在注意点和注视点分离的状态下存在距离效应—— 也就是随着距离的增长主观等同点变化越大, 但是在重合的状态下则没有。结果表明分离状态下存在传统意义上的注意梯度效应, 而重合时, 注意易化梯度受到一个抑制梯度的影响。这说明, 注意梯度分布受到视网膜离心率的影响, 而影响方式符合注意点为中心的易化梯度场和注视点为中心的抑制梯度场的双梯度场理论。     

Visual search for object orientation can be modulated by canonical orientation

The authors studied the influence of canonical orientation on visual search for object orientation. Displays consisted of pictures of animals whose axis of elongation was either vertical or tilted in their canonical orientation. Target orientation could be either congruent or incongruent with the object's canonical orientation. In Experiment 1, vertical canonical targets were detected faster when they were tilted (incongruent) than when they were vertical (congruent). This search asymmetry was reversed for tilted canonical targets. The effect of canonical orientation was partially preserved when objects were high-pass filtered, but it was eliminated when they were low-pass filtered, rendering them as unfamiliar shapes (Experiment 2). The effect of canonical orientation was also eliminated by inverting the objects (Experiment 3) and in a patient with visual agnosia (Experiment 4). These results indicate that orientation search with familiar objects can be modulated by canonical orientation, and they indicate a top-down influence on orientation processing.     

On varying the span of visual attention: Evidence for two modes of spatial attention

Three experiments investigated whether subjects could selectively attend to a target item presented in close spatial proximity to a distractor element. Typically, the display consisted of two curved lines, one a target and the other a distractor. The task was to judge the direction of curvature of the target. When subjects attended to a target away from fixation, performance was affected by the presence of a distractor within an area of 1° around the target. In contrast, the distractor did not influence target processing when subjects fixated on the target location. Two modes of visual attention are proposed. When a target is located away from fixation, a “wide” attentional span is employed. With the present stimuli, this led to the detailed processing of items within 1° of the attended position. However, when targets are at fixation a “narrow” span can be adopted, with the result that there is differential processing of attended items even within the formerly critical area.     

Stimulus discrimination following covert attentional orienting to an exogenous cue.

Five experiments explored exogenous covert visual-attentional orienting following a brief peripheral cue. On each trial an attentional cue was followed by a stimulus in an empty field at 1 of 8 locations on an imaginary circle centered on the fixation point. The cued area size and the cue-target spatial relation were manipulated. Accuracy and response time were affected by the exogenous cue validity. Attention was allocated to a specific location in a visual quadrant: A target at an uncued location in a quadrant was not facilitated as much as target at the cued location, and a target in a different quadrant was inhibited in relation to a neutral condition. Cuing 2 locations in a quadrant was not as facilitative for targets at the cued locations or as inhibitive for targets at other locations compared with cuing a single location in a quadrant. Results are discussed in the context of several extent models of covert visual-spatial attention.     

Saccades to targets in three-dimensional space: dependence of saccadic latency on target location.

The latency of saccadic movements to targets appearing at various positions in three-dimensional visual space was measured in four experiments. The first experiment confirmed that latencies of saccades to visual targets are greater in the lower visual field and showed that the increase is not influenced by the vertical starting position of the eye in the orbit, nor by a time gap between the fixation offset and the target onset. A hypothesis that this visual field difference was caused by a link between downward saccades and convergence movements was tested by recording saccade latencies when the targets were in a different depth plane from that of the original fixation. We did not find any direct support for the vergence involvement hypothesis, although the lower/upper visual field effect was shown to decrease consistently in monocular viewing. It was also shown that saccades to targets positioned in a different depth plane have longer latencies. In a final experiment, the visual field effect was shown to depend on the egocentric rather than the gravitational vertical.     

Saccades to targets in three-dimensional space: Dependence of saccadic latency on target location

The latency of saccadic movements to targets appearing at various positions in three-dimensional visual space was measured in four experiments. The first experiment confirmed that latencies of saccades to visual targets are greater in the lower visual field and showed that the increase is not influenced by the vertical starting position of the eye in the orbit, nor by a time gap between the fixation offset and the target onset. A hypothesis that this visual field difference was caused by a link between downward saccades and convergence movements was tested by recording saccade latencies when the targets were in a different depth plane from that of the original fixation. We did not find any direct support for the vergence involvement hypothesis, although the lower/upper visual field effect was shown to decrease consistently in monocular viewing. It was also shown that saccades to targets positioned in a different depth plane have longer latencies. In a final experiment, the visual field effect was shown to depend on the egocentric rather than the gravitational vertical.     

The whereabouts of visual attention: Involuntary attentional bias toward the default gaze direction

This study proposed and verified a new hypothesis on the relationship between gaze direction and visual attention: attentional bias by default gaze direction based on eye-head coordination. We conducted a target identification task in which visual stimuli appeared briefly to the left and right of a fixation cross. In Experiment 1, the direction of the participant’s head (aligned with the body) was manipulated to the left, front, or right relative to a central fixation point. In Experiment 2, head direction was manipulated to the left, front, or right relative to the body direction. This manipulation was based on results showing that bias of eye position distribution was highly correlated with head direction. In both experiments, accuracy was greater when the target appeared at a position where the eyes would potentially be directed. Consequently, eye–head coordination influences visual attention. That is, attention can be automatically biased toward the location where the eyes tend to be directed.     

Visual laterality and the acuity gradient: potential artifacts and control procedures

As visual stimuli are moved away from the fixation point toward either side, perceptibility falls off rapidly. This acuity gradient can influence visual half-field differences if the two sides (or ends) of a stimulus do not provide equal amounts of task-relevant information. In general, performance should be better in the visual field with the more informative side of the stimulus closer to the fixation point. Kirsner and Schwartz (1986, Brain and Cognition, 5, 354-361) discuss implications for experiments using words. The present article considers the assumptions involved in an explanation of visual half-field differences in terms of the visual acuity gradient and offers guidelines for the design of experiments using letters, faces (and other nonverbal stimuli), and words. Special attention is given to the interpretation of Visual Field X Task Variable interactions and to the use of words with letters arranged vertically. It is concluded that, with sufficient care in their design and interpretation, visual laterality experiments are an important converging operation for the study of cerebral hemisphere asymmetry.     

Looking for two targets at the same time: One search or two?

A considerable amount of evidence suggests that, under conditions of high discriminability, subjects are able to process multiple elements in a visual display simultaneously when searching for a single target among distractors. Relatively little emphasis, however, has been placed on the question of whether subjects can search for and detect multiple targets simultaneously. This latter question is the focus of the present report. In two experiments, we compare performance in single-target and multiple-target detection tasks in order to investigate whether or not multiple targets can be detected simultaneously. In Experiment 1, subjects searched for one or two targets that were defined by color. In Experiment 2, subjects searched for a color and/or a letter target. When the two targets were presented in the same location (e.g., a red X when Target 1 was red and Target 2 was an X), they seemed to be detected simultaneously. Implications for object-based processing of visual information are discussed.     

The control of saccade trajectories: direction of curvature depends on prior knowledge of target location and saccade latency

Recent reports have shown that saccades can deviate either toward or away from distractors. However, the specific conditions responsible for the change in initial saccade direction are not known. One possibility, examined here, is that the direction of curvature (toward or away from distractors) reflects preparatory tuning of the oculomotor system when the location of the target and distractor are known in advance. This was investigated by examining saccade trajectories under predictable and unpredictable target conditions. In Experiment 1, the targets and the distractors appeared unpredictably, whereas in Experiment 2 an arrow cue presented at fixation indicated the location of the forthcoming target prior to stimulus onset. Saccades were made to targets on the horizontal, vertical, and principal oblique axis, and distractors appeared simultaneously at an adjacent location (a separation of +/- 45 degrees of visual angle). On average, saccade trajectories curved toward distractors when target locations were unpredictable and curved away from distractors when target locations were known in advance. There was no overall difference in mean saccade latencies between the two experiments. The magnitude of the distractor modulation of saccade trajectory (either toward or away from) was comparable across the different saccade directions (horizontal, vertical, and oblique). These results are interpreted in terms of the time course of competitive interactions operating in the neural structures involved in the suppression of distractors and the selection of a saccade target. A relatively slow mechanism that inhibits movements to distractors produces curvature away from the distractor. This mechanism has more time to operate when target location is predictable, increasing the likelihood that the saccade trajectory will deviate away from the distractor.     

Indications for two attentional gradients in endogenous visual-spatial attention

Four experiments were designed to investigate the meridian effect on attentional orienting (characterized by a larger cost when the cue and target fall in different visual fields relative to when they fall in the same fields) when employing an endogenous cue. In Experiment 1 a circular display of eight boxes was employed and no meridian effect was found. In Experiment 2 three displays were compared: Semicircular, vertical, and horizontal. No meridian effect was found in the semicircular display. However, in the two linear displays (vertical, horizontal) there was a significant meridian effect. Experiments 3 and 4 were designed to investigate an attentional gradient hypothesis. In all experiments a distance effect was found. Nevertheless, the distance effect was not directly related to the actual distance between the target positions. The results suggest the existence of two attentional gradients that develop simultaneously in the visual field: an excitation gradient centred at the cued location (the strongest excitation) and an inhibition gradient, centred at the fixation point (the strongest inhibition).     

Effects of location and form cuing on the allocation of attention in the visual field

Two experiments were conducted which tested the effect of two different types of cues on the allocation of attention in the visual field. Subjects responded to a target appearing either 6.5 degrees to the right or to the left of fixation in a field filled with small randomly positioned line segments. The target form was either a circle or diamond in which a horizontal or vertical line segment was positioned. Subjects made a speeded discrimination response on the basis of the orientation of this line segment. In the first experiment, there was always one target at each trial. In the second experiment, both circle and diamond were presented left or right of fixation simultaneously whereby the line segment only appeared in one of the forms. In different conditions, the most likely target location (left or right) and form (circle or diamond) were cued. Control conditions served as a baseline for determining costs and benefits. Reaction time measures showed that the validity of the location cue resulted in both costs and benefits, whereas the validity of the form cue had no such effects. The results cannot be reconciled with the claim of zoom lens theories that spatial attention can switch between different modes of operations. They are also at odds with theories that claim that attention has no spatial locus. The results are consistent with a simple spotlight theory in which spatial attention involves selecting a particular restricted area of the visual field for which the perceptual efficiency is enhanced.     

Recall of letters from the left and right visual half-fields under unilater and bilateral presentation.

Bilateral presentation in the visual half-field greatly increases superiority of the right visual half-field in tachistoscopic recognition of words when fixation is controlled using a center digit. Two experiments explored left-right asymmetry with bilateral presentation on a visual half-field short-term memory task, with fixation controlled by a sequence of letters at fixation. A total of 40 subjects served in the two experiments, which compared recall under unilateral versus bilateral presentation to the visual half-field. Bilateral presentation increased over-all recall from the last serial position but did not alter asymmetry of the visual half-field. As in previous experiments, the superiority of the right visual half-field was greatest from the initial serial positions. It was concluded that asymmetry of the visual half-field on this recall task with controlled fixation depends primarily on masking and short-term memory but is independent of unilateral-bilateral presentation.