Contrast discrimination under temporally varying contrast conditions

Psychophysical contrast discrimination of a 0.8-cpd vertical grating was tested using a paradigm that alternated test and masking gratings at 8 Hz. Masking contrasts were lower than, equal to, or higher than the test contrasts. Six test contrasts were combined factorially with six masking contrasts to generate a series of six contrast increment threshold versus test contrast curves (tvc curves). A particularly simple relationship existed between these curves. The curves could be brought into alignment by shifting them diagonally by the ratio of their masking contrasts. It is shown that this behavior is predicted by a model in which contrast gain is set by the average of the test and masking contrasts coupled with a simple model of contrast discrimination. Contrast gain control integrates contrast over a period of at least 125 msec, and contrast discrimination is a function of this time-averaged contrast.     

The precision of velocity discrimination across spatial frequency

The precision of velocity coding for moving stimuli of different spatial frequencies was assessed by measuring velocity discrimination thresholds for a 1-c/deg grating paired with a grating whose spatial frequency ranged from 0.25 to 4 c/deg and for grating pairs of the same spatial frequency (0.25, 1, and 4 c/deg). The gratings always moved upward, with velocities ranging from 0.5 to 16 deg/sec, Velocity discrimination was as precise for stimuli that varied in spatial frequency by: ±2 octaves (0.25 vs. 1 c/deg and 4 vs. 1 c/deg) as for stimuli of the same spatial frequency, for specific ranges of velocity that depended on the spatial and, therefore, the temporal frequencies of the stimuli. Compared with a 1-c/deg grating, the perceived velocity of 4-c/deg gratings was about 1.3 times faster and that of 0.25-c/deg gratings was about 1.3 times slower. Although these perceived velocity biases imply variation of velocity-signal processing among spatial frequency channels, the discrimination results indicate that the motion-sensing system can compare signals across different spatial frequency channels to make fine velocity discrimination within appropriate temporal frequency limits.     

Scaling extrafoveal detection of distortion in a face and grating

Peripheral performance involving simple visual tasks and stimuli can be equated with foveal performance by spatial scaling, whilst more complex tasks and stimuli seem to need additional scaling of image contrast. We therefore determined whether the contrast manipulation needed to compensate for eccentricity-dependent performance changes is due to an increase in stimulus or task difficulty. We measured contrast sensitivities to determine foveal and peripheral ability to discriminate between an original and a distorted version of a polar-circular sinusoidal grating and a face image. Contrast sensitivities as a function of image size were spatially scaleable across eccentricities for both the face and grating. Furthermore, irrespective of stimulus, performance could be scaled with the same individual E2 value. Thus task simplicity overrides the nature of the stimulus in determining scaling requirements, suggesting that it is the complexity of the task, not of the stimulus, that makes contrast scaling necessary in complex tasks.     

On utrocular discrimination

Observers with good stereoacuity judged which eye received sine-wave grating patterns in a two-category forced-choice procedure. Large individual differences were found, but for most observers reliable discrimination was achieved at low spatial frequencies. No observer could perform the task above chance levels at high spatial frequencies. Discrimination was unaffected by retinal location, grating orientation, grating contrast, stimulus duration, or practice with feedback. Among observers who could perform the task, the following results were obtained: (1) Introduction of high spatial frequency components did not interfere with performance so long as a low spatial frequency component was present. (2) When gratings of low equal spatial frequency were presented to both eyes simultaneously at different contrast levels, observers could identify which eye received the higher contrast. (3) At low spatial frequencies, observers could distinguish monocular from binocular presentation. (4) Temporal frequency variations (counterphase flicker) influenced performance for some observers. Binocular summation and interocular transfer were unaffected by the spatial frequency variations which modulate utrocular discrimination. A new procedure for measuring stereopsis was developed which made possible comparison of utrocular discrimination with stereopsis at specific spatial frequencies. Stereopsis appeared mildly affected by spatial frequency.     

Tactile learning is task specific but transfers between fingers

Practice-related improvement in visual perception is highly specific for properties of the stimulus used in training. We explored the specificity of such perceptual learning in the human tactile system, using gratings consisting of alternating ridges and grooves. Practice effects on grating discrimination showed limited transfer between grating sets defined by spatial variation in either groove width or ridge width, consistent with partially overlapping neural representations of these two spatial parameters. In contrast, substantial interdigital transfer of practice effects occurred for discrimination of gratings varying in either spatial parameter and also for spatial acuity-dependent discrimination of grating orientation. We conclude that tactile learning, although quite as task specific as in other sensory systems, generalizes with considerable facility across fingers, unlike visual learning, which is highly location specific.     

A preferential looking clinical acuity test: Improvements in implemented microcomputer control

A manually operated preferential looking grating acuity test was modified to include a micro-computer for controlling both stimulus presentation and data acquisition, storage, and analysis. The computerized test requires only one observer-operator and minimizes several testing biases and errors of the manual test. A technique was developed to keep the observer-operator masked during the generation of a transformed staircase. The monocular test time of about 8 min, and the success rate—81% for 224 children aged 2–39 months—compare favorably with our experience with the manual two-operator test. For 39 pairs of age-matched children without visual pathology, grating acuities were 0.6 octaves better with the computerized test. The computerized version was used to measure acuity in 120 children who had, according to clinical examination, poorer vision in one eye than in the other. The suspect eyes showed an average of 0.8 octaves poorer grating acuity. The improvements of computerization were realized without loss of the advantages of the manual test.     

Attentional effects on concurrent psychophysical discriminations: Investigations of a sample-size model

In two experiments, a concurrent discrimination paradigm was used to study the effects of visual attention on psychophysical judgments and the consistency of these effects with a sample-size model in which attention influences the variance of the internal representation used to make psychophysical judgments. Two pairs of lines were presented simultaneously—one on each side of fixation—and subjects had to indicate for each pair separately whether or not the lines had the same length. Attention was manipulated by instructing subjects to pay 100%, 75%, 50%, 25%, or 0% of their attention to the discrimination on one side, with the complementary amount of attention to the other side. In the first experiment, the relationship between attention and discrimination accuracy was consistent with the sample-size model both when attentional allocation varied from trial to trial and when it varied between blocks, and the relationship held over more widely varying attentional allocations than had previously been studied. In addition, discriminations were more accurate overall with varied than with blocked attentional allocation, suggesting that the two types of allocation do not merely differ in the degree to which attention is focused. The second experiment examined the effects of attentional allocation and stimulus variance, the latter being manipulated by randomly incrementing or decrementing line lengths. These manipulations had additive effects on total Thurstonian variance, and a version of the samplesize model gave an excellent quantitative fit to the obtained results. Besides supporting the samplesize model, the results of Experiment 2 suggest that criterion variance is at least as large as sensory variance and that criterion but not sensory variance increases with stimulus variance.     

Two types of behavioral contrast in discrimination learning

Two groups of pigeons received daily discrimination training at two values on a line-tilt continuum. S+ (VI 1) and S- (EXT) intervals alternated, and a 30-sec criterion of no responding to S- was required before S+ returned. Rates of responding to S+ showed two separate contrast effects: at an intermediate stage of training a high peak rate appeared which declined, later in training, to a stable level still in excess of the VI baseline rate. The peak rate was correlated with the total number of responses to S-, while the final rate was not; suggesting that the peak rate and final rate may not be functions of the same variable. These results were compared with performance on a red-green discrimination where the two stages were not so clear. A line-tilt discrimination was repeated with fixed length S- intervals terminated by TO, and showed the same contrast magnitude in the final rate without any peak. The peak rate was interpreted as an effect of the ;punishment' contingency where responding to S- prolongs S- for 30 sec, while the final rate was taken to be analogous to previous demonstrations of contrast.     

Differential aversive learning enhances orientation discrimination

A number of recent studies have documented rapid changes in behavioural sensory acuity induced by aversive learning in the olfactory and auditory modalities. The effect of aversive learning on the discrimination of low-level features in the visual system of humans remains unclear. Here, we used a psychophysical staircase procedure to estimate discrimination thresholds for oriented grating stimuli, before and after differential aversive learning. We discovered that when a target grating orientation was conditioned with an aversive loud noise, it subsequently led to an improvement of discrimination acuity in nearly all subjects. However, no such change was observed in a control group conditioned to an orientation shifted by ±90° from the target. Our findings cannot be explained by contextual learning or sensitisation factors. The results converge with those reported in the olfactory modality and provide further evidence that early sensory systems can be rapidly modified by recently experienced reinforcement histories.     

Local contrast and Pavlovian induction

Two experiments examined the effects of number and similarity of stimuli on local contrast. In the first experiment, local contrast effects differed in magnitude as a function of the similarity among stimuli; greater positive local contrast appeared when stimuli were less similar, though this effect sometimes reversed for very dissimilar stimuli. In the second experiment, both positive and negative local contrast appeared transiently during the course of training a discrimination including two quite dissimilar stimuli. When two new stimuli were added, both effects reappeared in several cases. The effects remained when the discrimination was rendered more difficult by substituting a new stimulus very similar to one of the original pair. These and other data suggest that local contrast depends on the same factors that produce Pavlovian induction; in the absence of an alternative account, Pavlov's interpretation may be useful in suggesting further research that will help identify the mechanisms involved in both classical and operant discrimination learning.     

Learning to discriminate simple sinusoidal gratings is task specific

Learning transfer effects of a modified spatial frequency discrimination task with simple sinusoidal gratings on various untrained test tasks, testing performance in relative position, local width and global size discrimination, have been investigated. Six subjects were exposed to grating stimuli of varying spatial frequency and size but constant relative position while they had to respond only to spatial frequency. In the course of 7 consecutive days all subjects showed significant reduction of spatial frequency discrimination thresholds. Comparison of discrimination thresholds for the untrained test tasks, taken before and after the learning epoch, reveals complete learning transfer to spatial frequency discrimination with gratings of constant size and variable relative position, but complete lack of transfer when grating spatial frequency is shifted about one octave lower. Further, there is improvement of relative position discrimination and width discrimination of single luminance bars, which is not very specific for the learnt spatial frequency. Although size variation was part of the learning procedure, discrimination of global stimulus size did not improve. Generally, the observed scheme of learning transfer reveals that there is learning only for stimulus attributes that are behaviorally relevant in the learning task. The differential scheme of improvement and code usage in the test tasks strongly indicates involvement of higher stages capable of independent access to different coding domains, as well as attentionally guided attribute selection and suppression. Supported by other recent findings, it is suggested that discrimination learning can be understood as a higher level process of gradual refinement of code selection out of a rich code base provided by lower level stages.     

Belief congruence and racial discrimination: Review of the evidence and critical evaluation

A review of the literature pertaining to Rokeach, Smith, and Evans' (1960) belief congruence theory provided a context for discussion of some methodological and theoretical issues relating to conceptualization of the dependent variables, beliefcontent, belief discrepancy, meaningfulness of the race—belief comparison, attitude—belief feedback loops, attitude structure, and the relation between self and ideal similarity. The literature was judged supportive of a weak version of belief congruence theory which states that in those contexts in which social pressure is nonexistent or ineffective, belief is more important than race as a determinant of racial or ethnic discrimination. Evidence for a strong version of belief congruence theory (which states that in those contexts in which social pressure is nonexistent, or ineffective, belief is the only determinant of racial or ethnic discrimination) and was judged much more problematic.     

Directionally selective channels mediate the discrimination of moving aperiodic stimuli

Adaptation procedures were used to demonstrate that directionally selective channels mediate the discrimination of aperiodic stimuli moving at a common speed. Subjects were adapted to a grating that drifted to either the left or the right. They then determined the orientation of the gap in a 'C' that was also moving to either the left or the right. Performance in identifying the gap orientation was most impaired when the C moved in the same direction as the adapting grating.     

Contrast and spatial frequency components in visual preferences of newborns

A preferential-looking procedure was used to investigate newborns' responses to square-wave gratings varying in spatial frequency and contrast. A preliminary study confirmed that the gratings used in the experiment were suprathreshold. In the experiment newborns' preference for a grating of 0.1 cycle deg-1 within the peak contrast sensitivity range was examined. Reduction in the contrast of this grating led to a transfer of the preference to a high-contrast grating of the same space-averaged luminance with a spatial frequency outside this range (0.42 cycle deg-1). The findings are discussed with reference to the role of the contrast sensitivity function in pattern preferences of newborns: it is suggested that contrast and spatial frequency interact in determining pattern preferences.     

Cueing attention by relative motion in the periphery of the visual field

Sudden changes of visual stimulation attract attention. The observer's body motion generates retinal-flow field patterns containing information about his/her own speed and trajectory and relative motion of other objects. We investigated the effectiveness of relative motion as an attentional cue and compared it with conventional cueing by appearance of a frame in the far periphery of the visual field. In a group of ten subjects, contrast thresholds for the perception of static Gabor grating orientation [four alternative non-forced-choice (4ANFC)] task were determined at 20 degrees, 30 degrees, 40 degrees, and 60 degrees eccentricity. Subsequently, near-threshold discrimination performance of Gabor pattern orientation without versus with a ring-shaped cue was measured at the same positions. The same Gabor patterns were then presented embedded in a random-dot flow field, and uncued discrimination performance was compared with performance after presentation of a relative-motion cue (RMC), ie a small random-dot field with motion in the opposite direction of the flow field. Both the conventional ring cue and the RMC induced significantly increased discrimination performance at all test locations. With the parameters chosen for this study, the RMC was slightly less effective than the conventional cue, but its effects were somewhat more pronounced in the far periphery of the visual field. Thus, relative motion is a powerful cue to attract attention to peripheral visual objects and improves performance as effectively as a conventional ring cue. The findings have practical relevance for everyday life, in particular for tasks like driving and navigation.     

Variations in apparent spatial frequency with stimulus orientation: L Incidence of the effect in the general population

The human visual system is anisotropic not only for stimulus detection and discrimination but also for stimulus appearance. The apparent length of a stationary stimulus and the apparent velocity of a moving stimulus vary with orientation. These variations are predictable from a model assuming a compression of the horizontal spatial meridian relative to the vertical meridian. In this paper, the apparent spatial frequency of a suprathreshold grating is shown to depend on the grating orientation. Specifically, horizontal gratings appear coarser than vertical gratings of equivalent spatial frequency. Additional data collected from the same group of observers show that horizontal lines appear shorter than vertical lines of the same extent. The presence of the spatial frequency and length illusions, although both predictable from the compression model, were not found to be correlated within observers. Implications of these results for other data in the literature are discussed.     

Behavioral contrast and the automaintained key peck

In two experiments behavioral contrast was demonstrated during discrimination training in a positive automaintenance procedure. During the baseline condition in each experiment, a key was transilluminated for eight seconds by one of two colors (CS) following a variable intertrial interval signaled by a dark key. Keylight transillumination terminated with a response-independent food presentation. In the first experiment, food was eliminated during one CS for up to fifty sessions. After reinstatement of food following each CS, the discrimination was reversed. Six of the eight subjects showed positive behavioral contrast, i.e., response rates increased during the CS associated with food as they decreased during the CS associated with no food. The effect was replicated in Experiment II, but it did not occur when both the food and its associated CS were eliminated. These results were comparable to those obtained with operant discrimination training procedures (behavioral contrast) and with Pavlovian discrimination training. The results suggest that additivity theories of behavioral contrast may be insufficient to account for these data.     

What masks utrocular discrimination

Utrocular discrimination, the ability to judge which eye has received monocular stimulation, was studied under conditions designed to reduce the salience of the cue mediating this discrimination. In one series of experiments, these conditions involved reducing the likelihood of motor command signals, overwhelming the motor command centers, and triggering motor command signals for both eyes. Those results indicate that utrocular discrimination is not mediated by ocular movements or command signals. In a second series of experiments, a visual masking paradigrn was used. A monocular test grating appeared superimposed upon a background of binocular masking noise, which was presented either simultaneously with the target or at some interval preceding or following the target (asynchronous masking). When presented simultaneously, the binocular masking noise interfered with utrocular performance, but only when the noise contained spatial frequencies near the test frequency. Masking functions obtained under conditions of asynchronous masking indicated that the time course of the forward and backward masking changed with spatial frequency. Finally, a sustained masking pattern that was ineffective in reducing utrocular performance could be made effective by pulsing the mask simultaneously with the target. Taken together, these results implicate a transient response to the target onset as a critical factor in generating the cue for successful utrocular discrimination. This conclusion may account for the fact that utrocular discrimination falls off for normal observers as the spatial frequency of the target grating increases above 4 cycles/deg.     

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。     

Local contrast in natural images: normalisation and coding efficiency

The visual system employs a gain control mechanism in the cortical coding of contrast whereby the response of each cell is normalised by the integrated activity of neighbouring cells. While restricted in space, the normalisation pool is broadly tuned for spatial frequency and orientation, so that a cell's response is adapted by stimuli which fall outside its 'classical' receptive field. Various functions have been attributed to divisive gain control: in this paper we consider whether this output nonlinearity serves to increase the information carrying capacity of the neural code. 46 natural scenes were analysed with the use of oriented, frequency-tuned filters whose bandwidths were chosen to match those of mammalian striate cortical cells. The images were logarithmically transformed so that the filters responded to a luminance ratio or contrast. In the first study, the response of each filter was calibrated relative to its response to a grating stimulus, and local image contrast was expressed in terms of the familiar Michelson metric. We found that the distribution of contrasts in natural images is highly kurtotic, peaking at low values and having a long exponential tail. There is considerable variability in local contrast, both within and between images. In the second study we compared the distribution of response activity before and after implementing contrast normalisation, and noted two major changes. Response variability, both within and between scenes, is reduced by normalisation, and the entropy of the response distribution is increased after normalisation, indicating a more efficient transfer of information.