Visual control of locomotion

Abstract.— How is locomotion controlled? What information is necessary and how is it used? It is first of all argued that the classical twofold division of information into ex-teroceptive and proprioceptive is inadequate and confusing, that three fundamental types of information need to be distinguished and that the information is used in a continual process of formulating locomotor programs, monitoring their execution and adjusting them. The paper goes on to show (1) how steering could be controlled on the basis of the “locomotor flow line” in the optic array, which specifies the potential future course, whether curved or straight, and (2) how stopping for an obstacle could be controlled simply on the basis of the information in the optic array about the time-to-collision and its rate of change. The problems inherent in pedestrian locomotion of controlling footing and balance are discussed and an investigation of visual locomotor programming in the long jump reported.     

Warning: Attending to a mask may be hazardous to your perception

Object substitution is a type of backward masking that occurs when a mask appears during visual search for a target. We tested the hypothesis that object substitution is an overwriting process triggered by attentional selection of the mask. Impeding attentional selection of a mask by embedding it in an array of distractors eliminated object substitution. Similarly, object substitution did not occur when the mask appeared in advance of the target and, therefore, could not capture attention during search for the target. However, masking was reinstated when the mask was revealed from background contours at the moment of target onset and could therefore capture attention during search. These observations demonstrate that attentional selection of the mask is a necessary step in this type of masking and suggest that object substitution is active overwriting of unattended information triggered by selection of other visual information at a nearby location.     

Are different affordances subserved by different neural pathways?

This paper questions whether affordances are allied exclusively to dorsal stream processing within the visual system, or whether in fact different affordances are subserved by functionally independent neural pathways. Using case study evidence from patients with various visual pathologies, I argue that affordances can be categorised into type based upon their respective neurological underpinning. Such categorisation has implications for the extent to which affordances are consciously perceived or non-consciously 'picked up' within the optic array, as well as whether they indicate merely potentials for action or provide necessary information in the actualisation of behaviour.     

Balancing awareness: Vestibular signals modulate visual consciousness in the absence of awareness

The processing of visual and vestibular information is crucial for perceiving self-motion. Visual cues, such as optic flow, have been shown to induce and alter vestibular percepts, yet the role of vestibular information in shaping visual awareness remains unclear. Here we investigated if vestibular signals influence the access to awareness of invisible visual signals. Using natural vestibular stimulation (passive yaw rotations) on a vestibular self-motion platform, and optic flow masked through continuous flash suppression (CFS) we tested if congruent visual–vestibular information would break interocular suppression more rapidly than incongruent information. We found that when the unseen optic flow was congruent with the vestibular signals perceptual suppression as quantified with the CFS paradigm was broken more rapidly than when it was incongruent. We argue that vestibular signals impact the formation of visual awareness through enhanced access to awareness for congruent multisensory stimulation.     

Attentional capture by irrelevant color singletons

Four experiments investigated attentional capture by a color singleton in visual search for a nonsingleton target. B. S. Gibson and E. M. Kelsey (1998) found that a color singleton in a precuing array facilitated target discrimination at that location if the same color also signaled the target array onset. The authors found similar cuing effects regardless of whether the singleton color matched the target array and regardless of whether subjects could anticipate the singleton or target-array color. In Experiment 4, a color singleton captured attention when it appeared in the precuing array but not when it appeared in the target array. The results indicate that attentional control settings for displaywide attributes are imprecise: Although subjects may anticipate a specific color, they cannot avoid attentional capture by other irrelevant colors. In addition, the effect of irrelevant singletons depends on whether a target is simultaneously present in the array.     

Visual and nonvisual information disambiguate surfaces specified by motion parallax.

Motion parallax has been shown to be an effective and unambiguous source of information about the structure of three-dimensional (3-D) surfaces, both when an observer makes lateral movements with respect to a stationary surface and when the surface translates with respect to a stationary observer (Rogers & Graham, 1979). When the same pattern of relative motions among parts of the simulated surface is presented to a stationary observer on an unmoving monitor, the perceived corrugations are unstable with respect to the direction of the peaks and troughs. The lack of ambiguity in the original demonstrations could be due to the presence of (1) non-visual information (proprioceptive and vestibular signals) produced when the observer moves or tracks a moving surface, and/or (2) additional optic flow information available in the whole array. To distinguish between these two possibilities, we measured perceived ambiguity in simulated 3-D surfaces in situations where either nonvisual information or one of four kinds of visual information was present. Both visual and nonvisual information were effective in disambiguating the direction of depth within the simulated surface. Real perspective shape transformations affecting the elements of the display were most effective in disambiguating the display.     

Visual and nonvisual information disambiguate surfaces specified by motion parallax

Motion parallax has been shown to be an effective and unamhiguous:source of information about the structure of three-dimensional (3-D) surfaces, both when an observer makes lateral movementswith respect to a stationary surface and when the surface translates with respect to a stationary observer (Rogers & Graham, 1979). When the same pattern of relative motions among parts of the simulated surface is presented to a stationary observer on an unmoving monitor, the perceived corrugations are unstable with respect to the direction of the peaks and troughs. The lack of ambiguity in the original demonstrations could be due to the presence of (1) non-visual information (proprioceptive and vestibular signals) produced when the observer moves or tracks a moving surface, andlor (2) additional optic flow information available in the whole array. To distinguish between these two possibilities, we measured perceived ambiguity in simulated 3-D surfaces in situations where either nonvisual information or one of four kinds of visual information was present. Both visual and nonvisual information were effective in disambiguating the direction of depth within the simulated surface. Real perspective shape transformations affecting the elements of the display were most effective in disambiguating the display.     

The contributions of static visual cues, nonvisual cues, and optic flow in distance estimation

By systematically varying cue availability in the stimulus and response phases of a series of same-modality and cross-modality distance matching tasks, we examined the contributions of static visual information, idiothetic information, and optic flow information. The experiment was conducted in a large-scale, open, outdoor environment. Subjects were presented with information about a distance and were then required to turn 180 before producing a distance estimate. Distance encoding and responding occurred via: (i) visually perceived target distance, or (ii) traversed distance through either blindfolded locomotion or during sighted locomotion. The results demonstrated that subjects performed with similar accuracy across all conditions. In conditions in which the stimulus and the response were delivered in the same mode, when visual information was absent, constant error was minimal; whereas, when visual information was present, overestimation was observed. In conditions in which the stimulus and response modes differed, a consistent error pattern was observed. By systematically comparing complementary conditions, we found that the availability of visual information during locomotion (particularly optic flow) led to an 'under-perception' of movement relative to conditions in which visual information was absent during locomotion.     

Affordance-Based Control of Visually Guided Action

The purpose of this essay is to compare and contrast existing theoretical approaches to understanding the visual guidance of action and to introduce a new approach. The focus is on tasks, such as steering, braking, and intercepting, that are (more or less) continuously guided on the basis of visual information. The prominent approach, information-based control, captures important aspects of behavior but is incompatible with the theory of affordances, a core principle of the ecological approach. Information-based control also fails to capture how actors behave in ways that take the limits of their action capabilities into account. I attempt to resolve these problems by introducing a new approach, affordance-based control, which asserts that a primary function of vision is to allow actors to see the world in terms of what they can and cannot do. Affordance-based control captures the tight coupling between information in optic flow and movement that is characteristic of visually guided action but also provides a parsimonious explanation of how actors take into account the dynamic properties of their body and the environment.     

Escaping capture: bilingualism modulates distraction from working memory

We ask whether bilingualism aids cognitive control over the inadvertent guidance of visual attention from working memory and from bottom-up cueing. We compare highly-proficient Catalan-Spanish bilinguals with Spanish monolinguals in three visual search conditions. In the working memory (WM) condition, attention was driven in a top-down fashion by irrelevant objects held in WM. In the Identify condition, attention was driven in a bottom-up fashion by visual priming. In the Singleton condition, attention was driven in a bottom-up fashion by including a unique distracting object in the search array. The results showed that bilinguals were overall faster than monolinguals in the three conditions, replicating previous findings that bilinguals can be more efficient than monolinguals in the deployment of attention. Interestingly, bilinguals were less captured by irrelevant information held in WM but were equally affected by visual priming and unique singletons in the search displays. These observations suggest that bilingualism aids top-down WM-mediated guidance of attention, facilitating processes that keep separate representations in WM from representations that guide visual attention. In contrast, bottom-up attentional capture by salient yet unrelated input operates similarly in bilinguals and monolinguals.     

视觉搜索中基于工作记忆内容的注意捕获与抑制

注意是对有关信息的选择及对无关信息的抑制, 工作记忆是对有关信息的保持以及对无关信息的抑制, 两者在一定程度上都体现了选择性地指向与当前任务有关的刺激并同时抑制无关刺激。在系统地回顾已有研究的基础上结合新近的ERP研究, 从视觉搜索中基于工作记忆内容的注意捕获与注意抑制两个角度综述了视觉工作记忆与视觉选择性注意的相关研究。研究者们主要从两方面对该领域的问题进行了探讨：(1)视觉工作记忆内容对视觉选择性注意的导向作用; (2)视觉工作记忆内容对视觉选择性注意产生导向作用的条件。文章对以往的研究进行了总结, 指出了无论保持在视觉工作记忆的内容是捕获注意还是受到抑制, 都存在着自上而下和自下而上两种加工机制。     

The time course of attentional and oculomotor capture reveals a common cause

Eye movements are often misdirected toward a distractor when it appears abruptly, an effect known as oculomotor capture. Fundamental differences between eye movements and attention have led to questions about the relationship of oculomotor capture to the more general effect of sudden onsets on performance, known as attentional capture. This study explores that issue by examining the time course of eye movements and manual localization responses to targets in the presence of sudden-onset distractors. The results demonstrate that for both response types, the proportion of trials on which responses are erroneously directed to sudden onsets reflects the quality of information about the visual display at a given point in time. Oculomotor capture appears to be a specific instance of a more general attentional capture effect. Differences and similarities between the two types of capture can be explained by the critical idea that the quality of information about a visual display changes over time and that different response systems tend to access this information at different moments in time.     

Path integration from optic flow and body senses in a homing task

We examined the roles of information from optic flow and body senses (eg vestibular and proprioceptive information) for path integration, using a triangle completion task in a virtual environment. In two experiments, the contribution of optic flow was isolated by using a joystick control. Five circular arenas were used for testing: (B) both floor and wall texture; (F) floor texture only, reducing information for rotation; (W) wall texture only, reducing information for translation; (N) a no texture control condition, and (P) an array of posts. The results indicate that humans can use optic flow for path integration and are differentially influenced by rotational and translational flow. In a third experiment, participants actively walked in arenas B, F, and N, so body senses were also available. Performance shifted from a pattern of underturning to overturning and exhibited decreased variability, similar responses with and without optic flow, and no attrition. The results indicate that path integration can be performed by integrating optic flow, but when information from body senses is available it appears to dominate.     

Why is information displaced from visual working memory during visual search?

Research has shown that performing visual search while maintaining representations in visual working memory displaces up to one object's worth of information from memory. This memory displacement has previously been attributed to a nonspecific disruption of the memory representation by the mere presentation of the visual search array, and the goal of the present study was to determine whether it instead reflects the use of visual working memory in the actual search process. The first hypothesis tested was that working memory displacement occurs because observers preemptively discard about an object's worth of information from visual working memory in anticipation of performing visual search. Second, we tested the hypothesis that on target absent trials no information is displaced from visual working memory because no target is entered into memory when search is completed. Finally, we tested whether visual working memory displacement is due to the need to select a response to the search array. The findings rule out these alternative explanations. The present study supports the hypothesis that change-detection performance is impaired when a search array appears during the retention interval due to nonspecific disruption or masking.     

Attentional capture by globally defined objects.

The abrupt appearance of a new perceptual object in the visual field typically captures visual attention. However, if attention is focused in advance on a different location, onsets can fail to capture attention (Yantis & Jonides, 1990). In the present experiments, we investigated the extent to which the deployment of attention to the local level of a hierarchical scene may be affected by the abrupt appearance of a new object at the global level. Participants searched for a semi-disk target in an array of randomly oriented segmented disks ("pacmen"). On half the trials, a subset of the segmented disks induced a subjective square. On these critical trials, participants were significantly slower to respond to the presence of a local target even though the local features of the display were qualitatively identical across all conditions. This slowing was absent when outline pacmen were used (which do not induce subjective figures) and when the subjective square was perceptually old. When the participants' task was defined at the global level of the display, a new local element failed to capture attention, suggesting an asymmetry in the ability of objects at different levels of a hierarchical scene to capture attention. In a control experiment, a new local element captured attention, however, when the participants' task was defined at the local level, indicating that the local item was in principle capable of capturing attention. It is argued that global objects capture attention because they convey important information about the environment that is not available at the local level.     

Attentional capture by globally defined objects

The abrupt appearance of a new perceptual object in the visual field typically captures visual attention. However, if attention is focused in advance on a different location, onsets can fail to capture attention (Yantis & Jonides, 1990). In the present experiments, we investigated the extent to which the deployment of attention to the local level of a hierarchical scene may be affected by the abrupt appearance of a new object at the global level. Participants searched for a semi-disk target in an array of randomly oriented segmented disks (“pacmen”). On half the trials, a subset of the segmented disks induced a subjective square. On these critical trials, participants were significantly slower to respond to the presence of a local target even though the local features of the display were qualitatively identical across all conditions. This slowing was absent when outline pacmen were used (which do not induce subjective figures) and when the subjective square was perceptually old. When the participants’ task was defined at the global level of the display, a new local element failed to capture attention, suggesting an asymmetry in the ability of objects at different levels of a hierarchical scene to capture attention. In a control experiment, a new local element captured attention, however, when the participants’ task was defined at the local level, indicating that the local item was in principle capable of capturing attention. It is argued that global objects capture attention because they convey important information about the environment that is not available at the local level.     

工作记忆负荷、形状干扰对聋人与听力正常人注意捕获影响的眼动研究

通过记录和分析被试的行为和眼动指标,本研究试图探讨不同工作记忆负荷下,形状干扰对聋人与听力正常人不同视野位置上刺激信息捕获注意的异同。研究结果表明：（1）形状干扰刺激虽然没有影响被试视觉加工的认知绩效,但被试的眼动模式会由于工作记忆负荷的改变而改变,高工作记忆负荷时,有形状干扰的注视次数高于无形状干扰。（2）工作记忆负荷、视野位置影响被试视觉加工任务的完成,被试在不同视野位置的加工过程表现出不一致的眼动模式。（3）聋人视觉加工在高工作记忆负荷时存在一定的劣势,但其对不同视野位置的信息加工及形状干扰所产生的注意捕获效应与听力正常人一致。     

The roles of beaconing and dead reckoning in human virtual navigation

Beaconing is a process in which the distance between a visual landmark and current position is reduced in order to return to a location. In contrast, dead reckoning is a process in which vestibular, kinesthetic and/or optic flow cues are utilized to update speed of movement, elapsed time of movement, and direction of movement to return to a location. In the present experiment, human participants engaged in a virtual triangle-completion task devoid of vestibular cues in which they navigated two legs of an outbound path before homing (returning to the origin). However, our task was modified such that a landmark array was placed at the origin, allowing both beaconing and dead reckoning to be utilized to home. During the second leg of the outbound path, we manipulated aspects of the landmark array to determine the extent to which beaconing and dead reckoning influenced homing. Specifically, during probe trials, the landmark array either remained the same, shifted 3 or 6 meters east/west, or north/south, or was removed. We analyzed homing performance to determine the influence of landmark shifts on search behavior. Results suggested that participants were able to home with equal accuracy in both the presence and the absence of the landmark array. Array shifts that put beaconing and dead reckoning in conflict resulted in an averaging of the two sources of information for the determination of heading but a mixture of strategies for path length.     

The distorted room illusion, equivalent configurations, and the specificity of static optic arrays

The distorted room illusion (DRI) and the attendant argument for perceptual ambiguity is critically analyzed from a Gibsonian/ecological point of view. The notions of multiple specification, conflicting information, and perceptual skill are invoked in showing how the ecological approach can accommodate illusion effects that may remain under mobile binocular viewing conditions. Static optic arrays are shown not to be ambiguous. So-called equivalent configurations are found to be analytic artifacts, appearing when the problem of information is treated in geometrical terms without regard for constraints due to physical and ecological regularities. The relative importance of motion-based and motion-independent information is discussed.