High-fidelity perceptual long-term memory revisited--and confirmed

Experiments on short-term perceptual memory for elemental visual attributes, such as contrast, motion, orientation, and spatial frequency, have relied on a delayed discrimination technique in which the subject compares two stimuli presented at different points in time and memory is indexed by discrimination thresholds measured for the different time intervals between reference and test. In a variant of this procedure, used in experiments on long-term memory, the presentation of a single reference is followed by a memory test that combines two-alternative forced-choice decisions with the method of constant stimuli. With this procedure, it is impossible to distinguish the effects of criterion-setting processes and memory on performance, but this confound can be eliminated by testing many subjects and having each subject make a single decision. The resulting "group thresholds" are stable across time intervals of 24 hr, confirming previous findings of high-fidelity storage in the long-term memory range.     

Haptic detection of sine-wave gratings

We studied human haptic perception of sine-wave gratings. In the first experiment we measured the dependence of amplitude detection thresholds on the number of cycles and on the wavelength of the gratings. In haptic perception of sine-wave gratings, the results are in agreement with neural summation. The rate at which detection thresholds decrease with increasing number of cycles is much higher than can be accounted for by probability summation alone. Further, neural summation mechanisms describe the detection thresholds accurately over the whole spatial range probed in the experiment, that is wavelengths from 14 mm up to 225 mm. Earlier, we found a power-law dependence of thresholds on the spatial width of Gaussian profiles (Louw et al, 2000 Experimental Brain Research 132 369-374). The current results extend these findings; the power-law dependence holds not only for Gaussian profiles, but also for a broad range of sine-wave gratings with the number of cycles varying between 1 and 8. Haptic perception involves tactual scanning combined with an active, dynamic exploration of the environment. We measured characteristics of the velocity and force with which stimuli were scanned while performing a psychophysical task. One particularly surprising finding was that, without being instructed, participants maintained an almost constant scanning velocity during each 45-min session. A constant velocity in successive trials of the experiment might facilitate or even be necessary for discrimination. Further, a large systematic dependence of velocity on scanning length was found. An eightfold increase in scanning length resulted in about a fourfold increase in scanning velocity. A second experiment was conducted to study the influence of scanning velocity on psychophysical detection thresholds. This was done by systematically imposing specific scanning velocities to the participants while the thresholds were measured. The main result of the second experiment was that psychophysical detection thresholds are constant over a relatively broad range of scanning velocities.     

Aging and the perception of speed

Two experiments were conducted to explore the potential effects of aging upon the perception and discrimination of speed. In the first experiment, speed difference thresholds were obtained for younger and older observers for a variety of standard speeds ranging from slow to fast. The second experiment was designed to evaluate the observers' ability to discriminate differences in the speed of moving patterns in the presence of significant amounts of noise (the noise was manipulated by limiting the lifetimes of individual moving stimulus elements). The results of both experiments revealed a significant deterioration in the ability of the older observers to perceive or detect differences in speed. While the presence of noise was found to affect the observers' discrimination performance, it affected both younger and older observers' thresholds in a proportionally equivalent manner-the older observers were no more affected by noise than the younger observers.     

Auditory discrimination of anisochrony: influence of the tempo and musical backgrounds of listeners

This study explored the influence of several factors, physical and human, on anisochrony's thresholds measured with an adaptive two alternative forced choice paradigm. The effect of the number and duration of sounds on anisochrony discrimination was tested in the first experiment as well as potential interactions between each of these factors and tempo. In the second experiment, the tempo or the inter onset interval (IOI) was varied systematically between 80 and 1000 ms. The results showed that just noticeable differences increase linearly and proportionally with IOI in accordance with Weber's law except for quickest tempo (IOI of 80 ms). The third experiment investigated the role of musical training on anisochrony thresholds obtained for different IOI. It focused on differential effects of musical experiences by comparing non-musicians, instrumentalists, and percussionists thresholds. The results of the present study replicated the findings of previous experiments regarding the adequacy of Weber's law for slow rhythm and provided evidence for its departure for fast tempos. Moreover, thresholds from percussionists seem distinguishable from the ones of other listeners by their highest sensitivity to temporal shifts suggesting therefore the necessity to control the nature of musical experiences. The results are discussed according to current models of time perception.     

眼跳的双相调节理论的行为证据

为了研究眼跳的双相调节理论是否适用于人类的视觉系统,本研究测量了人类被试对分别呈现在三种眼跳时间段（基线、眼跳抑制和眼跳增强）内的光栅的朝向辨别准确率。研究发现：相对于光栅呈现在基线时间段内,被试对呈现在抑制（或增强）时间段内的光栅的朝向辨别准确率显著地更低（或更高）（实验1）;另外,只有使用低或中等空间频率光栅作为测试刺激时,才有这种双相调节作用（实验2）。这些结果表明：人类的视觉系统在眼跳过程中存在双相调节机制,并且这种双相调节机制具有刺激选择性。     

Binocular contrast interactions in two-frame motion discrimination

Previous studies have shown that two-frame motion detection thresholds are elevated if one frame's contrast is raised, despite the increase in average contrast--the "contrast paradox". In this study, we investigated if such contrast interactions occurred at a monocular or binocular site of visual processing. Two-frame motion direction discrimination thresholds were measured for motion frames that were presented binocularly, dichoptically or interocularly. Thresholds for each presentation condition were measured for motion frames that comprised either matched or unmatched contrasts. The results showed that contrast mechanisms producing the contrast paradox combine contrast signals from both eyes prior to motion computation. Furthermore, the results are consistent with the existence of monocular and binocular contrast gain control mechanisms that coexist either as combined or independent systems.     

再认条件下元监测判断的影响因素

探讨了再认条件下元监测判断的影响因素。以中文双字词对作为实验材料 ,安排在不同预先呈现时间、有无进行有意识记 ,以及不同识记遍数等条件下进行元监测判断。用计算机随机呈现项目 ,再认之前先让被试做元监测判断 ,判断是否是记过的项目的把握程度 ,再对有意识记过的项目做迫选再认 ,作为标准测验 ,以验证元监测判断的准确性。结果是预先呈现时间长、有意识记过的和识记遍数多的项目 ,对其再认的反应时较短 ,相应的元监测判断等级也较高 ;对只看而未有意记过的项目的判断是呈现时间长的判断等级反而低于呈现时间短的。此结果表明 ,再认条件下 ,元监测判断的等级的高低与识记项目的激活程度有关     

Visual perception of surface curvature: psychophysics of curvature detection induced by motion parallax

The continuous approach to optic-flow processing shows that the curvature of a moving surface is related to a second spatial derivative of the velocity field, the spin variation (Droulez & Cornilleau-Pérès, 1989). With this approach as a theoretical framework, visual sensitivity to the curvature of a cylinder in motion was measured using a task of discrimination between cylindrical and planar patches. The results confirm the predictions suggested by the theory: (1) Sensitivity to curvature was always greater when the cylinder axis and the frontal translation were parallel than when they were orthogonal. The ratio of curvature detection thresholds in the two cases was between 1.3 and 2.5; the value predicted from the spin variation theory is about 2. (2) Sensitivity to curvature increased strongly with the velocity of the motion but was only weakly affected by its amplitude and the duration of viewing for the range of values used in our experiments.     

The minimum temporal thresholds for motion detection of grating patterns

The minimum temporal thresholds for absolute motion detection were measured for sinusoidal grating patterns in foveal vision. Test patterns of relatively low temporal frequencies and low velocities were examined. The thresholds clearly decreased with test velocities rather than with test temporal frequencies. Modified velocity-time reciprocity was observed (i.e. the relationship between test velocity and temporal thresholds was described by a simple equation including two constants which indicate temporal and spatial limits). The temporal constant was about 35 ms and the spatial constant was about 1 min of arc. These constants are thought to provide the basic constraints on motion detection.     

Direction-of-motion discrimination is facilitated by visible motion smear

Recent evidence indicates that motion smear can provide useful information for the detection and discrimination of motion. Further, it has been shown that the perception of motion smear depends critically on the density of dots in a random-dot (RD) stimulus. Therefore, in the present experiments, the contribution of perceived motion smear to direction-of-motion discrimination was evaluated using RD targets of different densities. Thresholds for direction-of-motion discrimination and the extent of perceived motion smear were determined for RD stimuli with densities of 1, 2, and 10 dots/deg(2), presented for 200 msec at a velocity of 4, 8, or 12 deg/sec. To evaluate the contribution of information about orientation from motion smear, thresholds for orientation discrimination were measured using parallel lines with the same length as the extent of perceived smear. Despite the opportunity for increased summation as RD density increases, our results indicate that direction-of-motion discrimination worsens. Because perception of motion smear is reduced with an increase in RD density, our results are consistent with a facilitation of direction-of-motion discrimination by visible motion smear.     

Discrimination of time intervals presented in sequences: spatial effects with multiple auditory sources

This article discusses two experiments on the discrimination of time intervals presented in sequences marked by brief auditory signals. Participants had to indicate whether the last interval in a series of three intervals marked by four auditory signals was shorter or longer than the previous intervals. Three base durations were under investigation: 75, 150, and 225 ms. In Experiment 1, sounds were presented through headphones, from a single-speaker in front of the participants or by four equally spaced speakers. In all three presentation modes, the highest different threshold was obtained in the lower base duration condition (75 ms), thus indicating an impairment of temporal processing when sounds are presented too rapidly. The results also indicate the presence, in each presentation mode, of a 'time-shrinking effect' (i.e., with the last interval being perceived as briefer than the preceding ones) at 75 ms, but not at 225 ms. Lastly, using different sound sources to mark time did not significantly impair discrimination. In Experiment 2, three signals were presented from the same source, and the last signal was presented at one of two locations, either close or far. The perceived duration was not influenced by the location of the fourth signal when the participant knew before each trial where the sounds would be delivered. However, when the participant was uncertain as to its location, more space between markers resulted in longer perceived duration, a finding that applies only at 150 and 225 ms. Moreover, the perceived duration was affected by the direction of the sequences (left-right vs. right-left).     

Rapid discrimination of McCollough effects.

The McCollough effect is a colour aftereffect that is contingent on pattern orientation. Three experiments were conducted to establish whether such aftereffect colours could serve as a basis for discrimination in several rapid discrimination tasks. In the first experiment it was investigated whether aftereffect colours could act like a simple 'feature' in a visual search task involving a difficult orientation discrimination. Without McCollough adaptation, the time taken to detect a 'target' among 'distractors' increased substantially as the number of distractors increased. With adaptation, detection time was essentially independent of the number of distractors, indicating that the nature of the task changed from a difficult orientation discrimination to a simple discrimination based on differences in aftereffect colours. The second and third experiments employed a difficult four-alternative forced-choice procedure in which subjects were required to discriminate a monochromatic patch of square-wave grating oriented at 45 degrees from three others oriented at 135 degrees (and vice versa). The gratings were presented very briefly (67-333 ms) followed by a 500 ms mask. Subjects performed the task with and without McCollough adaptation. Performance was strikingly better after adaptation: colour aftereffects could be used to make the discrimination even at exposure durations as short as 67 ms. The third experiment demonstrated that this enhanced performance was indeed due to perceived colour differences (rather than a possible contrast difference). The results of the three experiments are discussed in relation to proposals about the locus of the McCollough effect.     

数目及其呈现方式在不同范围内对时间知觉的影响

采用时间比较法,考察数目及其呈现方式对时间知觉的影响。任务中,首先呈现标准时距,此时变化刺激的数目或呈现方式,然后呈现比较时距,被试的任务是判断比较时距比标准时距长还是短。实验1和实验2分别采用1秒内和1秒以上的时间,结果发现： 1秒内的时间加工中,刺激一个一个连续呈现时,小数引起标准时距的低估,大数导致标准时距的高估,而同时呈现时,刺激的数目对知觉到的时间无影响;数目及其呈现方式影响计时的准确性,而对计时的变异性和敏感性无影响。时间在1秒以上,同时呈现和连续呈现条件下,刺激的数目影响知觉到的时间;计时的     

Extent-of-motion thresholds under subject-relative and object-relative conditions

The experiment was designed to discover the threshold extent of motion at medium speeds amounting to 41, 82, and 164 min./sec., and to compare the perception of motion arising from subject-relative displacement with the perception of motion arising from object-relative displacement. Extent thresholds were found while velocity was kept constant. Different groups of ten Ss were used for each displacement velocity, and for each S the extent threshold was twice obtained by the method of constant stimuli, once under subjectrelative and once under object-relative displacement conditions. Sensitivity to brief displacements of a continuously visible target was high; average thresholds ranged from 1.0 to 4.4 min. under the various conditions employed. The thresholds were higher for subject-relative conditions and the slower displacement velocities and lower for objectrelative conditions and faster displacements.     

Extent-of-motion thresholds under subject-relative and object-relative conditions

The experiment was designed to discover the threshold extent of motion at medium speeds amounting to 41, 82, and 164 min./sec., and to compare the perception of motion arising from subject-relative displacement with the perception of motion arising from object-relative displacement. Extent thresholds were found while velocity was kept constant. Different groups of ten Ss were used for each displacement velocity, and for each S the extent threshold was twice obtained by the method of constant stimuli, once under subjectrelative and once under object-relative displacement conditions. Sensitivity to brief displacements of a continuously visible target was high; average thresholds ranged from 1.0 to 4.4 min. under the various conditions employed. The thresholds were higher for subject-relative conditions and the slower displacement velocities and lower for objectrelative conditions and faster displacements.     

Serial search for targets defined by divergence or deformation of optic flow.

The optic flow field can be described in terms of the local differential measures, divergence, deformation, and rotation, which are informative about observer motion and the 3-D structure of the environment. Does an explicit representation of these measures exist in human visual processing in the form of a feature map? Triesman's criteria were used to investigate this; ie is there 'pop-out' for a target defined as different in local divergence or deformation from surrounding elements, or is a serial search necessary? The stimulus arrays contained 3, 5, or 9 square or rectangular elements, which each underwent repeated cycles of expansion, contraction, or deformation. The time required to detect a target undergoing the opposite transformation increased steeply with the number of elements, implying very slow serial search. (The mean time was 210 ms per element for divergence targets and 542 ms per element for deformation). The process was clearly still serial when the density and number of elements was increased up to 48 in an array 2.16 deg x 2.16 deg. In contrast, a single line element undergoing the opposite direction of translation motion to the rest of the display did show pop-out. It is concluded that no parallel processes seem to exist which are sensitive to the spatial uniformity of divergence and of deformation of optic flow. These differential properties may be derived as conjunctions of signals from a primary process which extracts local velocity. This result contrasts with our findings for targets defined by stereo disparity gradient, which show parallel processing in analogous experiments.     

Amplitude and spatial-period discrimination in sinusoidal gratings by dynamic touch

We measured discrimination thresholds for sinusoidal gratings using active dynamic touch. In the first experiment we measured the thresholds for amplitude discrimination as a function of amplitude and spatial period. Thresholds ranged between 10.8%, and 15.8% of the standard amplitude. We showed that amplitude differences as small as 2 microm can be detected. We found that Weber fractions for amplitude discrimination are constant over a range of amplitudes, but are influenced by the spatial period of the grating; discrimination improved with increasing spatial period. In the second experiment we determined the thresholds for spatial-period discrimination. We used the same design as in the first experiment. Weber fractions ranged from 6.4% to 11.8%. Amplitude was found to have no effect on the Weber fractions for spatial-period discrimination. However, the spatial period had an effect on the Weber fractions: larger spatial periods yielded lower Weber fractions.     

Working memory dynamics bias the generation of beliefs: The influence of data presentation rate on hypothesis generation

Although temporal dynamics are inherent aspects of diagnostic tasks, few studies have investigated how various aspects of time course influence hypothesis generation. An experiment is reported that demonstrates that working memory dynamics operating during serial data acquisition bias hypothesis generation. The presentation rate (and order) of a sequence of serially presented symptoms was manipulated to be either fast (180 ms per symptom) or slow (1,500 ms per symptom) in a simulated medical diagnosis task. When the presentation rate was slow, participants chose the disease hypothesis consistent with the symptoms appearing later in the sequence. When the presentation rate was fast, however, participants chose the disease hypothesis consistent with the symptoms appearing earlier in the sequence, therefore representing a novel primacy effect. We predicted and account for this effect through competitive working memory dynamics governing information acquisition and the contribution of maintained information to the retrieval of hypotheses from long-term memory.     

Effects of retinal eccentricity and acuity on global-motion processing

The present study assessed direction discrimination with moving random-dot cinematograms at retinal eccentricities of 0, 8, 22, and 40?deg. In addition, Landolt-C acuity was assessed at these eccentricities to determine whether changes in motion discrimination performance covaried with acuity in the retinal periphery. The results of the experiment indicated that discrimination thresholds increased with retinal eccentricity and directional variance (noise), independent of acuity. Psychophysical modeling indicated that the results for eccentricity and noise could be explained by an increase in channel bandwidth and an increase in internal multiplicative noise.