不同框架下的视觉定位

本实验探讨了对置于正方形、等边三角形和圆中的刺激点所进行的１点定位、２点定位、３点定位,并比较了在定位有框架和定位无框架时的情况。结果发现：（１）在刺激呈现时间为１００ｍｓ和３００ｍｓ时．在３种定位任务中,当定位有框架时被试定位的绝对误差均小于定位无框架的。（２）在两种呈现时间下被试在３种定位任务中的绝对误差均处于同一水平,表现出现了视觉定位中的结构效应,并且几个不同的框架没有不同的作用。     

An integration of color and motion information in visual scene analyses

To analyze complex scenes efficiently, the human visual system performs perceptual groupings based on various features (e.g., color and motion) of the visual elements in a scene. Although previous studies demonstrated that such groupings can be based on a single feature (e.g., either color or motion information), here we show that the visual system also performs scene analyses based on a combination of two features. We presented subjects with a mixture of red and green dots moving in various directions. Although the pairings between color and motion information were variable across the dots (e.g., one red dot moved upward while another moved rightward), subjects' perceptions of the color-motion pairings were significantly biased when the randomly paired dots were flanked by additional dots with consistent color-motion pairings. These results indicate that the visual system resolves local ambiguities in color-motion pairings using unambiguous pairings in surrounds, demonstrating a new type of scene analysis based on the combination of two featural cues.     

Using temporal order to identify spatial reference frames

A new method for identifying spatial reference frames is described and applied to the study of spatial span. In this method, a spatial sequence is displayed in relation to two reference frames that move relative to each other, so that different temporal orders are described in each reference frame. As applied to spatial span, a sequence of dots was displayed on a rectangular template that moved relative to the computer screen and the observer. Three experiments showed that the choice of reference frame is influenced by prior conditions, the size of the template, and the presence of an alternative stationary reference frame. Recall was impaired (1) when the template moved, suggesting that movement interference occurred, and (2) with increasing template size, but only if the template was used as the reference frame.     

Discrimination of intentional and random motion paths by pigeons

Twelve pigeons (Columba livia) were trained on a go/no-go schedule to discriminate between two kinds of movement patterns of dots, which to human observers appear to be "intentional" and "non-intentional" movements. In experiment 1, the intentional motion stimulus contained one dot (a "wolf") that moved systematically towards another dot as though stalking it, and three distractors ("sheep"). The non-intentional motion stimulus consisted of four distractors but no stalker. Birds showed some improvement of discrimination as the sessions progressed, but high levels of discrimination were not reached. In experiment 2, the same birds were tested with different stimuli. The same parameters were used but the number of intentionally moving dots in the intentional motion stimulus was altered, so that three wolves stalked one sheep. Despite the enhanced difference of movement patterns, the birds did not show any further improvement in discrimination. However, birds for which the non-intentional stimulus was associated with reward showed a decline in discrimination. These results indicated that pigeons can discriminate between stimuli that do and do not contain an element that human observer see as moving intentionally. However, as no feature-positive effect was found in experiment 1, it is assumed that pigeons did not perceive or discriminate these stimuli on the basis that the intentional stimuli contained a feature that the non-intentional stimuli lacked, though the convergence seen in experiment 2 may have been an effective feature for the pigeons. Pigeons seem to be able to recognise some form of multiple simultaneously goal-directed motions, compared to random motions, as a distinctive feature, but do not seem to use simple "intentional" motion paths of two geometrical figures, embedded in random motions, as a feature whose presence or absence differentiates motion displays. Electronic Publication     

Detection of moving local density differences in dynamic random patterns by human observers

The way in which movement enhances target visibility has been investigated by measuring the detectability of the direction of motion of a dot pattern added to a background of dynamic visual noise. When the positions of all the dots were changed randomly from frame to frame, so that there was no dot configuration to define the target area (experiments 1 and 2), the threshold density difference necessary was for direction of motion detection less than 3 dots/frame (between 20% and 50% density difference). The spatial displacement (S) at which optimal detection occurs increased when a target elongated in the direction of motion was used. If S was either larger or smaller than its optimal value, thresholds rose progressively. The rise in threshold when S was smaller than 0.25 deg (the width of the target area) decreased when the target dots had a fixed spatial arrangement (experiment 3). It is suggested that in both fixed and random target configurations there is a grouping of dots with similar trajectories via a global directionally-selective process. The strength of the overall motion signal is greater in the fixed-dot configuration because each target dot has associated with it a vector precisely aligned in the direction of the target motion.     

Visual stream Segregation

A visual analog of auditory stream segregation occurs when a dot moving in discrete, jumps, alternates between positions on two regular trajectories. At slow speeds, one dot in irregular motion is seen. At higher speeds, two dots are seen, each moving in a regular trajectory.     

Software for the generation and display of random-dot cinematograms on the Macintosh computer

The software packageDotMovie 1.3.8 was designed to create and display dynamic random-dot cinematograms for studies of motion perception on Macintosh II microcomputers. In each frame of the cinematogram, some dots move in random uncorrelated directions, while other dots move with constant speed in one direction. As the relative number of dots moving in a correlated direction increases, the observer sees flow in the direction of the movement. The software described facilitates the creation of random-dot cinematograms, their presentation, and collection of data from subjects’ responses.     

Interocular Delay Produces Depth in Subjectively Moving Noise Patterns

Brief apparent motion sequences were introduced into a dynamic visual dot display by spatially shifting selected dots between successive frames. This causes the display to look as if it is drifting continuously in one direction. When such a display is observed with an interocular delay the drifting dots appear to be displaced in depth, even though there is no conventional retinal disparity in the display. We found that the magnitude of this depth shift increased with the duration of the apparent motion sequences. With sequences of five or more frames duration the depth effect was very similar to that which would have been predicted with a continuously moving target. With briefer sequences the size of the depth effect decreased rapidly. We suggest that apparent motion cascades form the basis of Tyler's dynamic visual noise stereophenomenon, and we question his “random spatial disparity” hypothesis.     

Matching cue size and task properties in exogenous attention

Exogenous attention is an involuntary, reflexive orienting response that results in enhanced processing at the attended location. The standard view is that this enhancement generalizes across visual properties of a stimulus. We test whether the size of an exogenous cue sets the attentional field and whether this leads to different effects on stimuli with different visual properties. In a dual task with a random-dot kinematogram (RDK) in each quadrant of the screen, participants discriminated the direction of moving dots in one RDK and localized one red dot. Precues were uninformative and consisted of either a large or a small luminance-change frame. The motion discrimination task showed attentional effects following both large and small exogenous cues. The red dot probe localization task showed attentional effects following a small cue, but not a large cue. Two additional experiments showed that the different effects on localization were not due to reduced spatial uncertainty or suppression of RDK dots in the surround. These results indicate that the effects of exogenous attention depend on the size of the cue and the properties of the task, suggesting the involvement of receptive fields with different sizes in different tasks. These attentional effects are likely to be driven by bottom-up mechanisms in early visual areas.     

Systematic angular biases in the representation of visual space

Representing spatial information is one of our most foundational abilities. Yet in the present work we find that even the simplest possible spatial tasks reveal surprising, systematic misrepresentations of space—such as biases wherein objects are perceived and remembered as being nearer to the centers of their surrounding quadrants. We employed both a placement task (in which observers see two differently sized shapes, one of which has a dot in it, and then must place a second dot in the other shape so that their relative locations are equated) and a matching task (in which observers see two dots, each inside a separate shape, and must simply report whether their relative locations are matched). Some of the resulting biases were shape specific. For example, when dots appeared in a triangle during the placement task, the dots placed by observers were biased away from certain parts of the symmetry axes. But other systematic biases were not shape specific, and seemed instead to reflect differences in the grain of resolution for different regions of space. For example, with both a circle and even a shapeless configuration (with only a central landmark) in the matching task, observers were better at discriminating angular differences (when a dot changed positions around the circle, as opposed to inward/outward changes) in cardinal versus oblique sectors. These data reveal a powerful angular spatial bias, and highlight how the resolution of spatial representation differs for different regions and dimensions of space itself.

     

Enumeration of dots: An eye movement analysis

The present study reports the measurement of response latencies and the recording of eye movements in a task in which adults had to enumerate dots in figures that differed in number of dots (n^d = 19–23) and grouping of dots. The functional relationship between latencies per dot and mean group size was in agreement with earlier findings (van Oeffelen & Vos, 1982). Temporal information from eye movement data indicated that the relative contribution of fixation durations to overall latency was far larger than the contribution of saccades, which superseded the contribution from eyeblinks. Spatial information in the form of eye movement trajectories indicated that, in general, there occurred one or two fixations at the starting position. From this position onward, eye movements were directed toward areas of dots rather than to each dot in particular. Scanning behavior was sometimes reiterative, in the sense that groups of dots were visited more than once. The results are discussed with respect to the nature of strategies employed during a dot-enumeration task.     

Coordinate frame for symmetry detection and object recognition

Can subjects voluntarily set an internal coordinate frame in such a way as to facilitate the detection of symmetry about an arbitrary axis? If so, is this internal coordinate frame the same as that involved in determining perceived top and bottom in object recognition and shape perception? Subjects were required to determine whether dot patterns were symmetric. Cuing the subjects in advance about the orientation of the axis of symmetry produced a substantial speedup in performance (Experiments 1 and 3) and an increase in accuracy with brief displays (Experiment 2). The effects appeared roughly additive, with an overall advantage for vertical symmetry; thus, the vertical axis effect is not due to a tendency to prepare for the vertical axis. The cuing advantage was found to depend upon the subject's knowing in advance the spatial location as well as orientation of the frame of reference (Experiment 4). The fifth experiment provided evidence that the frame of reference responsible for these effects is the same as the one that determines shape perception: Subjects viewed displays containing a letter (at an unpredictable orientation) and a dot pattern, rapidly naming the letter and then determining whether the dots were symmetric about a prespecified axis. When the top-bottom axis of the letter was oriented the same way as the axis of symmetry for the dots, symmetry judgments were significantly more accurate. Thus, the results suggest a single frame of reference for both types of judgment. The General Discussion proposes a theory of how visual symmetry may be computed, which might account for these phenomena and also characterize their relation to "mental rotation" effects.     

Simultaneous encoding of direction at a local and global scale

Human observers can simultaneously encode direction information at two different scales, one local (an individual dot) and one global (the coherent motion of a field of dots distrisbuted over a 10°-diameter display). We assessed whether encoding global motion would preclude the encoding of a local trajectory component and vice versa. In the present experiments, a large number (100–150) of dots were randomly assigned directions in each frame from a uniform distribution of directions spanning a range of 160° to create global motion in a single direction (Williams & Sekuler, 1984). Amidst these background dots, 1 dot moved in a consistent direction (trajectory) for the duration of the display. The direction of this “trajectory dot” was similar to the mean direction of the distribution of directions determining the movement of the background dots. Direction discrimination for both the global motion and the trajectory was measured, using the method of constant stimuli, under precued and postcued partial report conditions. A low- or high-frequency 85-msec tone signaled which motion the subject was to judge. In the precue condition, the tone was presented 200 msecbefore the onset of the stimulus, whereas in the postcue condition, the tone was presented immediatelyafter the offset of the stimulus. Direction discrimination thresholds for both global and local motion in the postcued condition were not significantly different from those obtained in the precued condition. These results suggest that direction information for both global and local motion is encoded simultaneously and that the observer has access to either motion signal after the presentation of a stimulus.     

Left-right visual field asymmetry in bistable motion perception

Twelve observers viewed two alternating frames, each consisting of three rectangular bars which were displaced laterally by one cycle in one frame with respect to the other. At long interframe intervals (IFIs) observers perceived a group of three elements moving as a whole (group movement), whereas with IFIs shorter than 40-60 ms the overlapping elements in each frame appeared stationary while the third element appeared to move from one end of the display to the other (end-to-end movement). The percentage of group movement responses in central viewing was compared to those obtained for stimulus presentation in the left and right visual fields (4 deg eccentricity), for opposite horizontal directions of motion. All ten right-handed subjects showed a left-field advantage in sensitivity to group movement. The two left-handed subjects showed a similar advantage in sensitivity with right-field presentation. The effects of monocular vision, hand used in the task, spatial frequency, and contrast on visual field asymmetry were all investigated in two right-handed subjects. None of these factors affected the left-right asymmetry.     

Strength theories of disjunctive visual detection

Zero, 1, 2, or 3 black dots are tachistoscopically presented on a white field. There are two alternative tasks: (a) to decide on the presence of each of the left, middle, and right dots (multiple detection) or (b) to decide whether any of the dots was present (disjunctive detection), The results indicate that in disjunctive detection, Ss do not add together thestrengths of the three dot positions and compare this sum to a criterion. Rather they combine theirdecisions about each dot, responding “yes” to the array, if and only if they decide “yes” to any one dot. Strength distributions appear to be invariant with respect to irrelevant stimuli. Invariance with respect to report order holds approximately. However, dots reported on first are slightly more detectable. This suggests a successive scanning process, whose rate is independent of whether a stimulus is present or absent at the position scanned.     

Evidence for an attentional component of the perceptual misalignment between moving and flashing stimuli

If a pair of dots, diametrically opposed to each other, is flashed in perfect alignment with another pair of dots rotating about the visual fixation point, most observers perceive the rotating dots as being ahead of the flashing dots (flash-lag effect). This psychophysical effect was first interpreted as the result of a perceptual extrapolation of the position of the moving dots. Also, it has been conceived as the result of differential visual latencies between flashing and moving stimuli, arising from purely sensory factors and/or expressing the contribution of attentional mechanisms as well. In a series of two experiments, we had observers judge the relative position between rotating and static dots at the moment a temporal marker was presented in the visual field. In experiment 1 we manipulated the nature of the temporal marker used to prompt the alignment judgment. This resulted in three main findings: (i) the flash-lag effect was observed to depend on the visual eccentricity of the flashing dots; (ii) the magnitude of the flash-lag effect was not dependent on the offset of the flashing dot; and (iii) the moving stimulus, when suddenly turned off, was perceived as lagging behind its disappearance location. Taken altogether, these results suggest that neither visible persistence nor motion extrapolation can account for the perceptual flash-lag phenomenon. The participation of attentional mechanisms was investigated in experiment 2, where the magnitude of the flash-lag effect was measured under both higher and lower predictability of the location of the flashing dot. Since the magnitude of the flash-lag effect significantly increased with decreasing predictability, we conclude that the observer's attentional set can modulate the differential latencies determining this perceptual effect. The flash-lag phenomenon can thus be conceived as arising from differential visual latencies which are determined not only by the physical attributes of the stimulus, such as its luminance or eccentricity, but also by attentional mechanisms influencing the delays involved in the perceptual processing.     

Dependence, independence, and emergence of word features

The degree to which a letter is perceived to be an e rather than a c is a function of the central, horizontal bar, which can be made to be more or less present in any given stimulus. By this means, word-form stimuli were produced through the independent, continuous manipulation of e versus c and of r versus h in two word frames (wat-- and --ase) yielding stimuli that ranged between water and watch in the former instance and between erase and chase in the latter. The identification probabilities for these stimuli were well accounted for in each case by a version of a fuzzy propositional model that is based on the assumption of independent features, indicating that the manipulated features within each word frame were not perceptually interdependent. However, the probabilities differed systematically between the two stimulus matrices; this means that there must be some other kind of higher order effect involved in identification. The hypothesis that the interaction was due to an emergent word envelope feature was rejected on the basis of the model analysis.     

Sensitivity to shearing and compressive motion in random dots

The sensitivity of the visual system to motion of differentially moving random dots was measured. Two kinds of one-dimensional motion were compared: standing-wave patterns where dot movement amplitude varied as a sinusoidal function of position along the axis of dot movement (longitudinal or compressional waves) and patterns of motion where dot movement amplitude varied as a sinusoidal function orthogonal to the axis of motion (transverse or shearing waves). Spatial frequency, temporal frequency, and orientation of the motion were varied. The major finding was a much larger threshold rise for shear than for compression when motion spatial frequency increased beyond 1 cycle deg-1. Control experiments ruled out the extraneous cues of local luminance or local dot density. No conspicuous low spatial-frequency rise in thresholds for any type of differential motion was seen at the lowest spatial frequencies tested, and no difference was seen between horizontal and vertical motion. The results suggest that at the motion threshold spatial integration is greatest in a direction orthogonal to the direction of motion, a view consistent with elongated receptive fields most sensitive to motion orthogonal to their major axis.     

Intrasensory attention: kinaesthetic versus cutaneous inputs

Blindfolded participants felt pairs of raised-line drawings simultaneously, one with each index finger. The stimuli presented at each fingertip were 180 degrees rotations of each other (eg 6 and 9). One finger moved (either actively or passively), and this in turn caused movement of a matched raised line underneath the stationary finger on the other hand, in a yoked manner. Thus, a 6 at the moving finger would be felt as a 9 on the stationary finger. On all trials there was a raised line moving underneath the stationary fully passive finger. For the moving finger, a raised line was present on only half of the trials. When a raised line could be felt at the moving fingertip, the shape followed by this finger was more often reported than was the shape present at the other (stationary) fingertip. However, when no line was present under the moving finger (ie when movement became the major cue for shape), subjects reported experiencing the shape moved under the stationary fingertip. Results are interpreted as an indication that cutaneous information can be more 'attention-getting' than kinaesthetic information, and are considered to support the modality-appropriateness theory.     

The pigeon's discrimination of movement patterns (Lissajous figures) and contour-dependent rotational invariance

The ability of pigeons to discriminate complex motion patterns was investigated with the aid of moving Lissajous figures. The pigeons successfully learned to differentiate two successively presented cyclic trajectories of a single moving dot. This suggests that they can recognize a movement Gestalt when information about shape is minimal. They also quickly learned a new discrimination between moving-outline stimuli with repetitively changing contour patterns. Contrasting results were obtained when the dot or outline stimuli were axis-rotated through 90 degrees. Rotational invariance of pattern discrimination was clearly demonstrated only when moving contours were visible. Nevertheless, pigeons could discriminate the axis-orientation of a moving-dot or moving-outline pattern when trained to do so. Discrimination did not seem to depend on single parameters of motion but rather on the recognition of a temporally integrated movement Gestalt. The visual system of pigeons, as well as that of humans, may be well adapted to recognize the types of oscillatory movements that represent components of the motor behaviour shown by many living organisms.