Hemispheric symmetry in duration of visible persistence

Nine right-handed subjects performed a visual task requiring perceptual integration of a pattern whose parts were displayed sequentially in time. Correct performance on the task depended critically on the simultaneous visibility of all parts of the pattern; duration of visible persistence could therefore be gauged by varying the duration of the temporal interval between successive portions of the display. The pattern was displayed either foveally or parafoveally in either visual field. Analysis of overall performance and of distribution of errors at each temporal interval revealed more accurate performance for foveal displays but no hemispheric asymmetries in duration of visible persistence. These and other results reported in the literature are interpreted in terms of Moscovitch’s (1979) information-processing model of hemispheric functioning.     

Visible persistence and form correspondence in Ternus apparent motion.

Visual stimuli remain visible for some time after their physical offset (visible persistence). Visible persistence has been hypothesized to play an important role in determining the pattern of correspondence matching in the Ternus apparent-motion display. In this display, one or more elements reappears in overlapping locations at different times, whereas another element appears alternately to the right or the left of these elements. Usually either the elements are perceived to move coherently as a group (group motion), or one element may be perceived to hop over one or more other elements (element motion). According to the visible-persistence account of the perceptual organization of the Ternus display, element motion is seen when the temporal gap between elements in overlapping locations is small enough to be bridged by visible persistence; if it is not, group motion is seen. We conducted four experiments to test this visible-persistence account. In Experiments 1 and 2, a form correspondence cue (line length) was introduced to bias the visual system toward the element-motion interpretation, while visible persistence was either reduced or eliminated. The element-motion percept dominated despite the elimination of visible persistence. In Experiments 3 and 4, we found that Ternus elements presented without interruption, and thus presumably persisting over time, can be perceived in group motion. Together, the results indicate that visible persistence is neither necessary nor sufficient to account for the pattern of correspondence matches in the Ternus display.     

Visible persistence and form correspondence in Ternus apparent motion

Visual stimuli remain visible for some time after their physical offset (visible persistence) Visible persistence has been hypothesized to play an important role in determining the pattern of correspondence matching in the Ternus apparent-motion display. In this display, one or more elements reappears in overlapping locations at different times, whereas another element appears alternately to the right or the left of these elements. Usually either the elements are perceived to move coherently as a group (group motion), or one element may be perceived to hop over one or more other elements (element motion). According to the visible-persistence account of the perceptual organization of the Ternus display, element motion is seen when the temporal gap between elements in overlapping locations is small enough to be bridged by visible persistence; if it is not, group motion is seen. We conducted four experiments to test this visible-persistence account. In Experiments 1 and 2, a form correspondence cue (line length) was introduced to bias the visual system toward the element-motion interpretation, while visible persistence was either reduced or eliminated. The element-motion percept dominated despite the elimination of visible persistence. In Experiments 3 and 4, we found that Ternus elements presented without interruption, and thus presumably persisting over time, can be perceived in group motion. Together, the results indicate that visible persistence is neither necessary nor sufficient to account for the pattern of correspondence matches in the Ternus display.     

Suppression of visible persistence

Five experiments were conducted to examine duration of visible persistence in sequences of stimuli. The basic display consisted of a point that stepped around a circular path on the face of an oscilloscope. Observers estimated the number of points seen simultaneously. Results were compared with a control condition in which the points were plotted in random order rather than sequentially. It was found that visible persistence of a point is suppressed if other points are shown nearby and after an appropriate delay. The degree of suppression depended on the spatial proximity of successive points. It was also found that both duration of visible persistence and degree of suppression increase with eccentricity in the visual field. The results are discussed in terms of two independent processes, persistence and suppression, that operate in a hierarchically antithetical relation.     

Two forms of persistence in visual information processing

Iconic memory, which was initially regarded as a unitary phenomenon, has since been subdivided into several components. In the present work we examined the joint effects of two such components (visible persistence and the visual analog representation) on performance in a partial report task. The display consisted of 15 alphabetic characters arranged around the perimeter of an imaginary circle on the face of an oscilloscope. The observer named the character singled out by a bar-probe. Two factors were varied: exposure duration of the array (10, 50, 100, 150, 200, 300, 400 or 500 ms) and duration of blank period (interstimulus interval, ISI) between the termination of the array and the onset of the probe (0, 50, 100, 150, or 200 ms). Performance was progressively impaired as both exposure duration and ISI were increased. The results were explained in terms of a probabilistic combinatorial model in which the timecourses of visible persistence and of the visual analog representation are regarded as time-locked to the onset and to the end of stimulation, respectively. The impairing effect of exposure duration was attributed to the relatively high spatial demands of the task that could be met optimally by information in visible persistence (which declines as a function of exposure duration), but less adequately by information in the visual analog representation. A second experiment, employing a task with lesser spatial demands, confirmed this interpretation.     

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.     

Dominance of global visual properties at birth

Six experiments are reported that were aimed at demonstrating the presence in newborns of a perceptual dominance of global over local visual information in hierarchical patterns, similar to that observed in adults (D. Navon, 1977, 1981). The first four experiments showed that, even though both levels of visual information were detectable by the newborn (Experiments 1A and 1B), global cues enjoyed some advantage over local cues (Experiments 2 and 3). Experiments 4A and 4B demonstrated that the global bias was strictly dependent on the low spatial frequency content of the stimuli and vanished after selective removal of low spatial frequencies. The results are interpreted as suggesting parallels between newborns' visual processing and processing later in development.     

The perceived duration of gratings.

Since visible persistence of grating patterns increases with spatial frequency, it is often inferred that the perceived duration of a grating is also longer at higher spatial frequencies. However, other work has demonstrated that the perceived onset of a grating is also delayed at higher spatial frequencies. Thus it is impossible to infer the subjective duration from the results of visible persistence studies alone. In order to estimate perceived duration in the present study, reaction times (RTs) to grating onsets and offsets were measured for a range of spatial frequencies. The results indicate that although the perceived duration (ie the difference between offset and onset RTs) was consistently longer than the physical duration, the estimates of perceived duration did not vary with changes in spatial frequency. Differences between the present results and earlier findings are interpreted in the context of the different methods used to measure perceived offset.     

Information persistence in the integration of partial cues for object recognition

A great many studies have shown that the perceptual effects of very brief visual stimuli can persist beyond the duration of the stimulus itself. These effects include sustained perception of the stimulus even though it is no longer present and the integration of information across an interstimulus interval. These two forms of sustained activity can be characterized as visible persistence and information persistence. Iconic memory protocols and a number of discrimination tasks have demonstrated the existence of information persistence that can last up to several hundred milliseconds, but there is little evidence that the cues needed for identification of objects can be transferred across intervals in this range. In the present experiments, a minimal transient discrete cue protocol was used to demonstrate that shape cues, these being provided by subsets of dots that mark the outer boundary of nameable objects, can be integrated over several hundred milliseconds and that the duration is a function of ambient room illumination. The experiments further evaluated whether this information persistence is mediated by visible persistence. Although both perceptual effects have durations that are an inverse function of room illumination, the ability to integrate partial shape cues was not determined by the duration of visible persistence.     

The duration of a brief event in the mind's eye.

A new illusion of perceived duration associated with focused spatial attention is reported. Brief flashes in attended locations were perceived to last longer than the same flashes in unattended locations. That illusion was shown to be completely independent of another illusion concerning the perceived onset of a flash, ruling out the possibility that the effect on perceived duration is derivative of a comparison between perceived onset and offset. The illusion also occurred when the event duration was composed of a temporal gap rather than a brief flash, ruling out low-level visible persistence as a basis for the illusion. Taken together, the results point to cortical connections from higher brain centers' both speeding and prolonging the visual signals occurring in lower sensory regions. Those temporal consequences could easily subserve many of the perceptual benefits ascribed to attention for spatial and intensive properties.     

Visible persistence is reduced by fixed-trajectory motion but not by random motion.

Despite the sluggish temporal response of the human visual system, moving objects appear clear and without blur, which suggests that visible persistence is reduced when objects move. It has been argued that spatiotemporal proximity alone can account for this modulation of visible persistence and that activation of a motion mechanism per se is not necessary. Experiments are reported which demonstrate that there is a motion-specific influence on visible persistence. Specifically, points moving in constant directions, or fixed trajectories, show less persistence than points moving with the same spatial and temporal displacements but taking random walks, randomly changing direction each frame. Subjects estimated the number of points present in the display for these two types of motion conditions. Under conditions chosen to produce 'good' apparent motion, ie small temporal and spatial increments, the apparent number of points for the fixed-trajectory condition was significantly lower than the apparent number in the random-walk condition. The traditional explanation of the suppression of persistence based on the spatiotemporal proximity of objects cannot account for these results. The enhanced suppression of persistence observed for a target moving in a consistent direction depends upon the activation of a directionally tuned motion mechanism extended over space and time.     

Visual dissociation of digitized photographs

Visual dissociation occurs when a visual marker from one display in a rapidly presented sequence (e.g., 9 item/sec) is perceived as having occurred in a temporally adjacent display. Three experiments that evaluated the application of computer graphics technology to study this phenomenon with digitized color photographs are reported. The primary concern was that visible phosphor persistence might artificially increase the frequency of integration errors. In Experiment 1, visible phosphor persistence was assessed using a multiple-repetition shutter test to determine which stimulus conditions did not yield reportable persistence. On the basis of these results, visual dissociation performance when the same color photographs were presented using mechanical 16-mm projection (as in previous research) and when they were presented on a computer monitor were compared in Experiments 2 and 3. The results supported computer application, in that computer presentation yielded the same pattern of errors and accuracy levels as did research using mechanical projection.     

Iconic memory and visible persistence

There are three senses in which a visual stimulus may be said to persist psychologically for some time after its physical offset. First, neural activity in the visual system evoked by the stimulus may continue after stimulus offset (“neural persistence”). Second, the stimulus may continue to be visible for some time after its offset (“visible persistence”). Finally, information about visual properties of the stimulus may continue to be available to an observer for some time after stimulus offset (“informational persistence”). These three forms of visual persistence are widely assumed to reflect a single underlying process: a decaying visual trace that (1) consists of afteractivity in the visual system, (2) is visible, and (3) is the source of visual information in experiments on decaying visual memory. It is argued here that this assumption is incorrect. Studies of visible persistence are reviewed; seven different techniques that have been used for investigating visible persistence are identified, and it is pointed out that numerous studies using a variety of techniques have demonstrated two fundamental properties of visible persistence: theinverse duration effect (the longer a stimulus lasts, the shorter is its persistence after stimulus offset) and theinverse intensity effect (the more intense the stimulus, the briefer its persistence). Only when stimuli are so intense as to produce afterimages do these two effects fail to occur. Work on neural persistences is briefly reviewed; such persistences exist at the photoreceptor level and at various stages in the visual pathways. It is proposed that visible persistence depends upon both of these types of neural persistence; furthermore, there must be an additional neural locus, since a purely stereoscopic (and hence cortical) form of visible persistence exists. It is argued that informational persistence is defined by the use of the partial report methods introduced by Averbach and Coriell (1961) and Sperling (1960), and the term “iconic memory” is used to describe this form of persistence. Several studies of the effects of stimulus duration and stimulus intensity upon the duration of iconic memory have been carried out. Their results demonstrate that the duration of iconic memory is not inversely related to stimulus duration or stimulus intensity. It follows that informational persistence or iconic memory cannot be identified with visible persistence, since they have fundamentally different properties. One implication of this claim that one cannot investigate iconic memory by tasks that require the subject to make phenomenological judgments about the duration of a visual display. In other words, the so-called “direct methods” for studying iconic memory do not provide information about iconic memory. Another implication is that iconic memory is not intimately tied to processes going on in the visual system (as visible persistence is); provided a stimulus is adequately legible, its physical parameters have little influence upon its iconic memory. The paper concludes by pointing out that there exists an alternative to the usual view of iconic memory as a precategorical sensory buffer. According to this alternative, iconic memory is post-categorical, occurring subsequent to stimulus identification. Here, stimulus identification is considered to be a rapid automatic process which does not require buffer storage, but which provides no information about episodic properties of a visual stimulus. Information about these physical stimulus properties must, in some way, be temporarily attached to a representation of the stimulus in semantic memory; and it is this temporarily attached physical information which constitutes iconic memory.     

The phenomenology of spatial integration: data and models

A briefly presented visual stimulus followed by darkness seems to persist beyond its physical offset. We are concerned here with the relation between two characteristics of this visible persistence: first, its phenomenological resemblance to the stimulus that spawned it and second, its usefulness as a basis for integrating visual stimuli that are separated in time. We describe two experiments using a task in which two halves of a visual stimulus were presented successively and observers reported how complete the stimulus appeared to be. Stimuli appeared less complete with increases in both the duration of the interval intervening between presentation of the two halves and the duration of the initially presented stimulus half. This data pattern is similar to that obtained in tasks in which spatial integration of two temporally disparate stimuli is necessary for correct responding. On the basis of this similarity, we argue that phenomenological appearance and ability to integrate stimuli over time are two facets of the same perceptual events. We describe a formal model to account for these and other data.     

Apparent duration and spatial structure

In three experiments, we investigated the relative perceived duration of a full bandwidth iniage and a set of high- and lowpass filtered images of a scene, briefly presented on a visual display unit. In Experiment 1, the various images were compared with each other, using a paired comparison method. All images were presented for 40 msec, and observers were asked to judge which of each pair of images had the longest duration. The results showed that images containing a wide spatial frequency bandwidth were judged to be of longer duration than were images of a narrower bandwidth, regardless of whether the latter were high- or lowpass filtered. In Experiment 2, a 40-msec presentation of each of the images was compared with a presentation of a probe that was 20,40, 60, or 80 msec in duration. Observers again judged which of each pair of images had the longest duration. The results were very similar to those of Experiment 1, with wide bandwidth images being judged to be of longer duration than were narrow bandwidth images. In Experiment 3, instead of comparing the various filtered versions of the image with each other, we attempted to obtain a direct measure of perceived duration by comparing a flashing LED to a 40-msec flash of a subset of the images used in the previous experiments. The observers’ task was to adjust the duration of the LED flash to match the perceived duration of each image. The results confirmed the results of the previous experiments, again indicating that wide bandwidth images are perceived to have longer phenomenal durations than narrow bandwidth images are perceived to have. These results could be predicted from previous research in the literature on the effects of spatial frequency on perceptual lag but not from research on visual persistence. It is argued that the effects described here can probably be explained best by postulating a link between perceived duration and the integration of separately processed spatial frequency information.     

Cortical dynamics of visual persistence and temporal integration

In a temporal integration experiment, subjects must integrate two visual stimuli, presented at separate times, to perform an identification task. Many researchers have assumed that the persistence of the leading stimulus determines the ability to integrate the leading and trailing stimuli. However, recent studies of temporal integration have challenged that hypothesis by demonstrating that several theories of persistence are incompatible with data on temporal integration. This paper shows that an account of visual persistence given by a neural network model of preattentive vision, called the boundary contour system, explains data on temporal integration. Computer simulations of the model explain why temporal integration becomes more difficult when the display elements are separated by longer interstimulus intervals or are of longer duration or of higher luminance, or are spatially closer together. The model suggests that different mechanisms underlie the inverse duration effects for leading and for trailing elements. The model further predicts interactions of spatial separation, duration, and luminance of the trailing display.     

Effects of display luminance, stimulus meaningfulness, and probe duration on visible and schematic persistence

Partial report performance is conceived of as consisting of two independent sources of information: visible persistence and schematic persistence. Following the work of Di Lollo and Dixon (1988), these two sources of information were separated experimentally by varying interstimulus interval and stimulus onset asynchrony independently. In the present experiment we asked whether visible persistence and schematic persistence would be affected by display luminance, stimulus meaningfulness, or probe duration. The results were fit accurately by an extension of the Di Lollo and Dixon independent-decay model in which display luminance affected visible persistence but stimulus meaningfulness and probe duration affected neither form of persistence directly.     

Spatial frequency content of visual imagery

Three experiments employing the McCollough paradigm were conducted to determine the spatial-frequency content of visual imagery. In Experiment 1, large and reliable pattern-contingent color aftereffects were obtained after adaptation to visual imagery. The direction of the aftereffects indicated that subjects were adapting to higher spatial frequencies in their imagery. These results contrast with the data of Experiment 2, which demonstrate that color aftereffects obtained with adaptation to physically present stimuli are mediated by the fundamental spatial frequency components. The magnitude of the imagery-induced aftereffects in Experiment 1 equaled the magnitude of the externally induced aftereffects obtained in Experiment 2 with the same subjects. By blurring the to-be-imaged patterns (Experiment 3), the fundamental Fourier components became the salient perceptual features of the stimuli, and the direction of the imagery-induced aftereffects was reversed from that of Experiment 1, indicating that the spatial frequency content of the imagery had changed from higher to lower frequencies. Under normal viewing conditions, subjects use the higher spatial frequencies associated with the perceptually salient edges of stimuli to construct their images. The results of Experiments 1 and 3 are discussed in light of a current controversy over the nature of information representation in imagery, and it is concluded that support has been obtained for the analog model of visual imagery.     

Effects of head restraint on signal detectability in simultaneous and successive vigilance tasks

Dittmar, Warm, and Dember (1985) suggested that visual parallax may lead to declines in perceptual sensitivity over time in spatial vigilance tasks involving comparative judgments. The present study tested this possibility by restraining subjects' head movements during a 1-hr vigil in which comparative (simultaneous task) or absolute (successive task) judgments of line length were necessary for signal detection. Under free-viewing conditions, perceptual sensitivity declined over time with both types of tasks. Head restraint eliminated the sensitivity decrement in both cases. The results highlight signal quality as a crucial determinant of perceptual decrements in sustained attention.     

Multielement visual tracking: attention and perceptual organization.

Two types of theories have been advanced to account for how attention is allocated in performing goal-directed visual tasks. According to location-based theories, visual attention is allocated to spatial locations in the image; according to object-based theories, attention is allocated to perceptual objects. Evidence for the latter view comes from experiments demonstrating the importance of perceptual grouping in selective-attention tasks. This article provides further evidence concerning the importance of perceptual organization in attending to objects. In seven experiments, observers tracked multiple randomly moving visual elements under a variety of conditions. Ten elements moved continuously about the display for several seconds; one to five of them were designated as targets before movement initiation. At the end of movement, one element was highlighted, and subjects indicated whether or not it was a target. The ease with which the elements in the target set could be perceptually grouped was systematically manipulated. In Experiments 1-3, factors that influenced the initial formation of a perceptual group were manipulated; this affected performance, but only early in practice. In Experiments 4-7, factors that influenced the maintenance of a perceptual group during motion were manipulated; this affected performance throughout practice. The results suggest that observers spontaneously grouped the target elements and directed attention toward this coherent but nonrigid virtual object. This supports object-based theories of attention and demonstrates that perceptual grouping, which is usually conceived of as a purely stimulus-driven process, can also be governed by goal-directed mechanisms.