Spatial-frequency-dependent visible persistence and specific reading disability

Visible persistence duration for sine-wave gratings was measured in 9-year-old normal and specific-reading-disabled children. Experiment 1 investigated the influence of stimulus duration on visible persistence. The results demonstrated a Reading Group X Spatial Frequency interaction with disabled readers showing a smaller increase in persistence duration with increasing spatial frequency than controls. This interaction was greatest with stimulus durations longer than 80 msec. In Experiments 2a and 2b persistence was measured across a range of contrasts from .1 to .5. The stimulus durations employed were 100 msec in Experiment 2a and 300 msec in Experiment 2b. In both experiments, increasing contrast decreased persistence duration at 2 and 12 cycles per degree (c/deg) for the control group. In the specific-reading-disabled group, however, contrast had little effect on the persistence of 2-c/deg gratings in either experiment. In Experiment 2a the persistence of the 12-c/deg grating decreased with increasing contrast for both groups. In Experiment 2b contrast had significantly less effect on persistence duration in the specific-reading-disabled group, however, contrast had little effect on the persistence of 2-c/deg ratings in either experiment. In Experiment 2b contrast had significantly less effect on persistence duration in the specific-reading-disabled group than in the control group at 12 c/deg. Consequently, contrast had less effect on persistence in specific-reading-disabled children than in normal readers, especially at durations longer than the "critical duration" for each spatial frequency. Experiment 3 extended this finding to gratings with spatial frequencies of 4 and 8 c/deg. These results indicate a difference between normal and specific-reading-disabled children in cortical visible persistence. Two scores of visual processing were derived from the above experiments. On these scores the reading-disabled children were divided into Visual Disabled Readers (approximately 70%--eight subjects) and Nonvisual Disabled Readers (approximately 30%--four subjects). The percentages of disabled readers in each category remained constant when the sample size was increased to 61 normal and disabled readers.     

Temporal integration and contrast sensitivity in foveal and peripheral vision

Spatial contrast sensitivity functions and temporal integration functions for gratings with dark surrounds were measured at various eccentricities in photopic vision. Contrast sensitivity decreased with increasing eccentricity at all exposure durations and spatial frequencies tested. The decrease was faster at high than at low spatial frequencies, but similar at different exposure durations. When cortically similar stimulus conditions were produced at different eccentricities by M-scaling, contrast sensitivity became independent of visual field location at all exposure durations tested. The results support the view that in photopic vision spatiotemporal information processing is qualitatively similar across the visual field, and that quantitative differences result from retino-topical differences in ganglion cell sampling. For gratings of constant retinal area temporal integration (improvement of contrast sensitivity with increasing exposure duration) was more extensive at high than at low retinal spatial frequencies but independent of cortical spatial frequency and eccentricity. For M-scaled gratings temporal integration was more extensive at high than at low cortical spatial frequencies but independent of retinal spatial frequency and eccentricity. The results suggest that the primary determinant of temporal integration is not spatial frequency but grating value that is calculated as AF2 square cycles (cycle2), where A is grating area and F spatial frequency.     

Visible persistence of moving objects

A single line was presented in a succession of orientations, each orientation separated by a fixed angle and by a fixed interval of time, and subjects reported the number of successive lines that appeared to rotate together. The perceived number of rotating lines increased linearly with the rate of stimulus presentation, with a slope that was proportional to the spatial separation. The linear functions obtained in this first experiment predicted the results of a second experiment in which subjects adjusted the spatial and temporal variables to a discrimination threshold for seeing two rotating lines. If the slope of the linear functions is considered to be an estimate of the duration of visible persistence, then these results suggest that the visible persistence of a briefly presented stimulus increases with the distance separating that stimulus from other stimuli.     

Direct measures of short-term visual storage

Two experiments, involving seven conditions, explored the use of direct measures of visual persistence. In each, the subject was asked to judge if an intermittent stimulus appeared perceptually continuous, or whether it completely faded before the next presentation occurred. The first experiment showed that visual persistence was set at approximately 250 msec. for a recycling presentation of a circle in a tachistoscope; in another task employing a moving opaque slit passing back and forth over a circle, persistence times averaged 50 msec. longer. Reducing luminance by 2 log units increased persistence only slightly, though removing the adapting field increased it by over 100 msec. The second experiment, using the repeating circle, varied the duration of the stimulus, and compared monoptic with dichoptic presentations. Visual persistence was found to be independent of stimulus duration over a range of 4 to 200 msec., where all durations were above recognition threshold for the stimulus. Persistence was unaffected whether the stimulus was repeatedly presented in the same eye or alternated between eyes, strongly suggesting that the storage is central. Finally, a re-analysis of Dodwell and Engel's paper on stereopsis suggests that their effects can be adequately explained by visual persistence of the asynchronous stereo pairs, rather than a more complex fusion model. All of these results strongly support the use of visual persistence as a direct measure of short-term visual storage.     

Critical importance of exposure duration for affective discrimination of stimuli that are not recognized

Previous research has found that repeated exposure to briefly presented visual stimuli can increase the positive affect for the stimuli without enhancing their recognition. Subjects could discriminate target and distractor shapes by affective preference in the absence of recognition memory. This study examined this phenomenon as a function of stimulus exposure duration. Over exposure durations of 0, 2, 8, 12, 24, and 48 ms, the functions for affect and recognition judgments exhibited different temporal dynamics. Target selection by affect was possible at very brief exposures and was influenced little by increasing durations; target selection by recognition required longer stimulus exposures and improved with increasing durations. Affective discrimination of stimuli that are not recognized is a reliable phenomenon, but it occurs only within a narrow band of time. This parametric study has specified the relationship between exposure duration and affect and recognition judgments and has located that temporal window.     

Reinforcement duration and the peak shift in post-discrimination gradients

Pigeons were trained to key-peck for food, first with single-stimulus training and then with successive discrimination (multiple schedule) training. In the multiple schedule, two different wavelengths were each correlated with equally frequent variable-interval reinforcement but different durations (6 sec vs. 2 sec) of access to grain. For some birds, the different durations of feeding cycle were cued by different intensities of the food hopper light. For some of these “cued” birds, single-stimulus training had been carried out with 6-sec feedings and when multiple-schedule training was introduced, the novel stimulus was correlated with 2-sec feedings. For the others, 2-sec feedings were originally used, and the novel stimulus was then present during the 6-sec reinforcement duration. The cueing procedure enhanced discrimination performance, and was necessary for the consistent production of a peak shift. In addition, the condition in which original training had been carried out with 6-sec feedings, and thus reinforcement duration was reduced in the presence of the novel stimulus, led to the best performance.     

Effects of spatial-frequency specific adaptation and target duration on visual persistence

Two experiments investigated the properties of visual persistence as functions of spatial frequency, stimulus duration, and pattern-specific adaptation. In Experiment 1, increasing the duration of high spatial-frequency gratings from 50 to 500 msec decreased the duration of visual persistence produced by that grating to an asymptotic level. However, low-frequency gratings produced a constant estimate of visual persistence independent of presentation time. Also, spatial-frequency specific adaptation reduced the persistence of the high-frequency gratings to this asymptotic level, but the lower frequency persistence estimates already at this level were unaffected (Experiment 2). These findings are related to possible temporal properties of the sustained and transient visual systems.     

The time course of configural change detection for novel 3-D objects

The present study investigated the time course of visual information processing that is responsible for successful object change detection involving the configuration and shape of 3-D novel object parts. Using a one-shot change detection task, we manipulated stimulus and interstimulus mask durations (40—500 msec). Experiments 1A and 1B showed no change detection advantage for configuration at very short (40-msec) stimulus durations, but the configural advantage did emerge with durations between 80 and 160 msec. In Experiment 2, we showed that, at shorter stimulus durations, the number of parts changing was the best predictor of change detection performance. Finally, in Experiment 3, with a stimulus duration of 160 msec, configuration change detection was found to be highly accurate for each of the mask durations tested, suggesting a fast processing speed for this kind of change information. However, switch and shape change detection reached peak levels of accuracy only when mask durations were increased to 160 and 320 msec, respectively. We conclude that, with very short stimulus exposures, successful object change detection depends primarily on quantitative measures of change. However, with longer stimulus exposures, the qualitative nature of the change becomes progressively more important, resulting in the well-known configural advantage for change detection.     

Further evidence for two components in visual persistence.

The phenomenal durations of tachistoscopically presented letter arrays at five levels of redundancy were measured using a subtractive reaction-time technique. Phenomenal duration was inversely related to redundancy only when report of the letters was required. The duration of functional availability of stimulus information for stimuli at the five levels of redundancy was determined in a backward masking paradigm. The measurements of the duration of functional availability and phenomenal presence were convergent, indicating that one process is responsible for both the phenomenal and functional duration of a persisting representation of a visual stimulus. The results are interpreted as providing further evidence for two components in visual persistence: a physical component whose duration is unrelated to stimulus parameters and an informational component whose duration is inversely related to the efficiency of encoding stimulus information.     

Duration of visible persistence in relation to stimulus complexity

Visible persistence refers to the phenomenal impression that a stimulus is still present after its offset. A dispute exists whether visible persistence is due to temporal sluggishness in the visual pathway (neural hypothesis) or whether it is a byproduct of information-extraction processes under cognitive control (process hypothesis). This was investigated by manipulating stimulus complexity in five temporal integration experiments and one recognition memory experiment. According to the process hypothesis, complex stimuli should persist longer than simple stimuli because they require more information extraction. This prediction was not confirmed; in all six experiments, complexity was found to have no reliable effect on the duration of visible persistence. By contrast, and in accordance with earlier findings, complexity was shown to have a significant effect on a short-lived, nonvisible form of memory known as schematic persistence. This pattern of results supports two major conclusions: First, that the effects of complexity reported in earlier research were probably on schematic—rather than visible—persistence; and second, that visible persistence must be regarded as a residual neural trace of an extinguished stimulus, rather than as a byproduct of information-extraction processes.     

The interaction of stimulus and reinforcer control in complex temporal discrimination.

Six pigeons were trained in a discrete-trials signal-detection procedure to discriminate between a fixed-duration stimulus (5 s or 20 s) and a set of variable durations ranging from 2.5 s to 57.5 s in steps of 5 s. For each fixed-duration stimulus, the ratio of reinforcer frequencies contingent upon reporting the fixed versus the variable stimulus was systematically manipulated. Detection performance was well controlled by both the stimulus value and the reinforcer ratio. Both the discriminability between the fixed duration and the set of variable durations, and the discriminability between the fixed duration and each of the variable durations, were independent of the reinforcer-frequency ratio when discriminability was measured as log d. The sensitivity of response bias to reinforcement-ratio changes was independent of the value of the fixed duration, but was not independent of the discriminability of the variable durations from the fixed durations. Under current models, discriminability measures in complex temporal discrimination may be independent of biasing manipulations, but bias measures are not independent of stimulus values.     

Multidimensional encoding of visual form

The encoding of stimulus dimensions of visual form in the human S was investigated under conditions of threshold exposure durations. Three stimulus dhnensions defined on a spatial grating were investigated: spatial line frequency, grating orientation, and orientation of a transverse break in the lines of grating. Results support the conclusion that, within the general category of visual form, different primary stimulus dimensions such as spatial frequency and orientation may be encoded simultaneously, whereas, when the two stimulus components for report are defined on the same dimension (orientation), overall performance is consistent with the predictions of a single channel model.     

Control of responding during stimuli that precede transitions in reinforcement frequency

Pigeons' responses were reinforced on a variant of a mixed variable-interval extinction schedule of reinforcement in which the transition to the higher reinforcement rate was signaled by a trace stimulus projected on the response key prior to the onset of the component correlated with food delivery. In the first of two experiments, the duration of the trace stimulus preceding the component correlated with food delivery was varied from 1.5 to 50.0 s and in the second experiment, the reinforcement frequency in the same component was varied from 10 to 60 reinforcers per hour. Pigeons pecked at the trace stimulus preceding the onset of the component correlated with food delivery even though responding was not reinforced in its presence and only one of the changes in reinforcement rate (i.e., from extinction to reinforcement) was signaled. The rate of pecking during the trace stimulus was a function of its duration but not of the reinforcement frequency in the following component. Higher rates generally occurred at the shorter trace-stimulus durations. Component responding following the offset of the trace stimulus was under discriminative control of the trace stimulus whether or not responding occurred in the presence of the trace stimulus.     

The effect of simultaneous and sequential presentation of stimulus dimensions on absolute judgment accuracy

The effects of spatial stimulus repetition, sequential stimulus repetition, spatially separated dimensional redundancy, and sequentially presented dimensional redundancy on absolute judgment accuracy of hue and brightness were compared. Two exposure durations, 0.1 and 2.0 sec, were used. While spatial repetition did not improve accuracy for either dimension, the sequential repetition of brightness produced a small increase in accuracy. The spatial presentation of correlated values of both dimensions increased accuracy only at the 2.0-sec duration. The sequential presentation of both dimensions increased accuracy, but only at the 2.0-sec duration was this gain substantial and greater than that provided by the sequential repetition of brightness alone.     

Responding under chained and tandem fixed-ratio schedules

The role of stimuli in chained fixed-ratio schedules of reinforcement was examined. At various ratio values, responding on schedules consisting of three or five equal components, with a different colored light in each component (“block counter”) was compared with responding on tandem or simple fixed-ratio schedules having the same color present throughout the entire ratio. At all ratio values except the smallest, the chain stimuli resulted in longer pauses after reinforcement. The magnitude of this effect became greater as the size of the ratio was increased. Post-reinforcement pause durations were longer under five-component schedules than under three-component schedules. Running rates in the first component were lower on the chained schedules than on the tandem schedules; on both kinds of schedule, rates were lower in the first component than in the rest of the ratio. When the sequence of stimuli was reversed, the duration of the post-reinforcement pause dropped markedly and the running rate in the initial component increased, but these effects gradually disappeared after the first reversal session. When the final chain stimulus was substituted for the first component stimulus but continued to appear in the final chain component as well, the pause duration dropped and remained at this lower level during subsequent sessions.     

Effects of response rate, reinforcement frequency, and the duration of a stimulus preceding response-independent food

Food-reinforced key pecking in the pigeon was maintained under a four-component multiple schedule. In two components, responding was maintained at high rates under a random-ratio schedule. In the other two components, responding was maintained at low rates under a schedule that specified a minimum interresponse time. For both high and low response rates, one of the schedule components was associated with a high reinforcement frequency and the other components with a lower reinforcement frequency. During performance under these schedules, a stimulus terminated by access to response-independent food was periodically presented. The duration of this pre-food stimulus was 5, 30, 60, or 120 sec. Changes in rate of key pecking during the pre-food stimulus were systematically related to baseline response rate and the duration of the stimulus. Both high and low response rates were increased during the 5-sec stimulus. At longer stimulus durations, low response rates were unaffected and high response rates were decreased during the stimulus. For two of three pigeons, high response rates maintained under a lower frequency of reinforcement tended to be decreased more than high response rates maintained under a higher reinforcement frequency. In general, the magnitude of decrease in high response rates was inversely related to the duration of the pre-food stimulus.     

Spatial frequencies and the cerebral hemispheres: contrast sensitivity, visible persistence, and letter classification

The hypothesis that the two cerebral hemispheres are specialized for processing different visual spatial frequencies was investigated in three experiments. No differences between the left and right visual fields were found for: (1) contrast-sensitivity functions measured binocularly with vertical gratings ranging from 0.5 to 12 cycles per degree (cpd); (2) visible persistence durations for 1- and 10-cpd gratings measured with a stimulus alternation method; and (3) accuracy (d') and reaction times to correctly identify digitally filtered letters as targets (L or H) or nontargets (T or F). One significant difference, however, was found: In Experiment 3, a higher decision criterion (beta) was used when filtered letters were identified in the right visual field than when they were identified in the left. The letters were filtered with annular, 1-octave band-pass filters with center spatial frequencies of 1, 2, 4, 8, and 16 cpd. Combining four center frequencies with three letter sizes (0.5 degrees, 1 degree, and 2 degrees high) made some stimuli equivalent in distal spatial frequency (cycles per object) and some equivalent in proximal spatial frequency (cycles per degree). The effective stimulus in the third experiment seemed to be proximal spatial frequency (cycles per degree) not distal (cycles per object). We conclude that each cerebral hemisphere processes visual spatial frequency information with equal accuracy but that different decision rules are used.     

Asymmetrical Perception of Changing Intensity in Short Tonal Stimuli: Duration of Stimulus

A number of reports have suggested that changing intensity in short tonal stimuli is asymmetrically perceived. In particular, steady stimuli may be heard as growing louder; stimuli must decrease in intensity to be heard as steady in loudness. The influence of stimulus duration on this perceptual asymmetry was examined. Three participants heard diotic tonal stimuli of eight durations between 0.8 s and 2.5 s. Each stimulus increased, decreased, or remained steady in intensity; initial intensity was 40 dB SPL (sound pressure level relative to 0.0002 dynes/cm²), and carrier frequency was 1 kHz. Participants made forced binary responses of “growing louder” or “growing softer” to each stimulus. For each duration, that value of intensity change eliciting equal numbers of both responses was determined. The results indicated a pronounced perceptual asymmetry for 0.8-s stimuli, which diminished for longer stimuli; changing intensity in 2.5-s stimuli was perceived symmetrically. Additionally, sensitivity to changing intensity improved as stimulus duration increased, suggesting that responses may be based in part on the difference in intensity between the beginning and end of the stimulus. Possible ramifications of the asymmetry reside in (a) the percussive nature of many natural sounds and (b) selective responding to approaching sound sources.     

Cortical dynamics of lateral inhibition: Visual persistence and ISI

Psychophysical studies show that increasing the interstimulus interval (ISI) between two stimuli decreases persistence of the first stimulus. While some researchers account for these results with interactions of transient and sustained inhibition, this paper describes an alternative explanation. In a neural-network model of boundary detection called the boundary contour system, persistence is the result of feedback-generated reverberations. Mechanisms to control these reverberations include lateral inhibition, which computer simulations show allows persistence in the network to qualitatively match the psychophysical data. Additional simulations predict that increasing the duration of the second stimulus should cause persistence to increase with ISI. The model links psychophysical data on visual persistence with computational requirements of spatial vision and properties of cells in visual cortex.     

Similarity between Fourier transforms of objects predicts their experimental confusions

Two sets of stimuli were presented tachistoscopically to 4 subjects. On each trial, a single stimulus was presented, and the subject was required to identify the stimulus by verbal response. An exposure duration was chosen such that the subject's identification performance fell within a range from faultless identification to chance guessing. The object-identification data of each subject obtained for all stimulus exposures were pooled to form an object confusion matrix. A model of visual processing based on two-dimensional spatial frequency content (Fourier transforms) was used to predict confusions among stimulus pairs. The model properties that appear to be the most essential are those that allow it (1) to account for the obvious dependence of the Fourier transform on the choice of an origin point; and (2) choose the point of origin for each object separately, irrespective of other objects of the set. The point of origin of the reference frame, in which Fourier transforms are performed, is chosen so as to minimize the low-frequency phase component for each object. A high correlation (up to .96) between confusion matrices and model interobject distances was attained. The results demonstrate that such a distance measure gives a good prediction of object confusability.