On the relations among different measures of visible and informational persistence

Attentional distraction affects maintenance of information in visual sensory memory

Classical theoretical models suggest that visual short-term memory can be divided in two main memory systems: sensory memory, a short-lasting but high-capacity memory storage and working memory, a long-lasting but low-capacity memory store. Whilst, previous research has systematically shown a strong interplay between attentional mechanisms and working memory, less clear is the role of attention in sensory memory. In the present study we approach this issue by asking whether withdrawing attentional resources by a dual task (Experiment 1) or by presenting task irrelevant information during memory maintenance (Experiment 2 and 3) similarly or differently affect sensory and working memory. Overall, results showed that sensory memory content was undermined not only by a simultaneous high-demanding cognitive task but even when purely task-irrelevant and non-masking visual distractors were presented during maintenance. Our data provide support against theories that consider sensory memories as a case of visual awareness free of attention.

Perception of Happening: How the Brain Deals with the No-History Problem

In physics, the temporal dimension has units of infinitesimally brief duration. Given this, how is it possible to perceive things, such as motion, music, and vibrotactile stimulation, that involve extension across many units of time? To address this problem, it is proposed that there is what is termed an "information construct of happening" (ICOH), a simultaneous representation of recent, temporally differentiated perceptual information on the millisecond time scale. The main features of the ICOH are (i) time marking, semantic labeling of all information in the ICOH with ordinal temporal information and distance from what is informationally identified as the present moment, (ii) vector informational features that specify kind, direction, and rate of change for every feature in a percept, and (iii) connectives, information relating vector informational features at adjacent temporal locations in the ICOH. The ICOH integrates products of perceptual processing with recent historical information in sensory memory on the subsecond time scale. Perceptual information about happening in informational sensory memory is encoded in semantic form that preserves connected semantic trails of vector and timing information. The basic properties of the ICOH must be supported by a general and widespread timing mechanism that generates ordinal and interval timing information and it is suggested that state-dependent networks may suffice for that purpose. Happening, therefore, is perceived at a moment and is constituted by an information structure of connected recent historical information.

Adaptive event integration in the missing element task

Evidence for adaptive event integration has previously been provided using the Rapid Serial Visual Presentation (RSVP) task. However, it is not straightforward to generalize this finding to other types of tasks that measure temporal integration, because integration in such tasks is known to vary, depending on the method that is used. This variability has been seen as an indication that integration may result from more than a single type of perceptual persistence, and that different integration tasks may not tap into same type of persistence. Therefore, we investigated whether adaptive control of integration in the RSVP task can be replicated using another technique for measuring temporal integration, which may rely more on low-level mechanisms, namely the dot-array integration or Missing Element Task (MET). As in the RSVP studies, stimulus speed expectancy was presently manipulated. The results indicated that integration performance in the MET was not subject to adaptive control. We argue that this discrepancy with previous RSVP studies can most likely be attributed to a specific difference in the type of persistence underlying task performance. Temporal integration in the MET might rely mostly on visible persistence, while for the RSVP task integration relies more on informational persistence. The present findings suggest that, contrary to informational persistence, visible persistence may not be susceptible to adaptive control.

Shape specificity of neural persistence for the kinetic-depth effect matches perceptual adaptation but not sensory memory

When multistable displays-stimuli that are compatible with several comparably likely perceptual interpretations-are presented intermittently, the perceptual state at the stimulus onset shows a complex dependence on the duration of the preceding blank interval. Specifically, perception is maximally destabilized for interruptions that are approximately 500 ms long, but it is stabilized by the use of shorter or longer blank intervals. This nonmonotonic dependence of perceptual stability on the blank interval duration raises questions about a number of history effects that are involved and about their nature, including the underlying neural representations. One way to characterize history effects is by looking at their specificity to the change of display properties. Here we measured the shape specificity for perception of the kinetic-depth effect when interruptions were brief (50 ms). For this time interval, perception is thought to be stabilized by neural persistence, a lingering trace of the prior neural activity. We found that perceptual stability depended on the shapes of the objects presented both before and after the break, but not on the similarity between the objects. These results matched earlier reports of the shape specificity of neural adaptation (destabilizing aftereffect for blanks 200-800 ms long). However, our results were markedly different from the shape specificity of sensory memory of multistable perception (a stabilizing effect for blanks > 800-1,000 ms). We concluded that whereas neural persistence and adaptation both act on the same motion-selective neural representation, sensory memory depends on another, possibly partially overlapping, shape-selective neural ensemble.

Temporal integration and attentional selection of color and contrast target pairs in rapid serial visual presentation

Performance in a dual target rapid serial visual presentation task was investigated, dependent on whether the color or the contrast of the targets was the same or different. Both identification accuracy on the second target, as a measure of temporal attention, and the frequency of temporal integration were measured. When targets had a different color (red or blue), overall identification accuracy of the second target and identification accuracy of the second target at Lag 1 were both higher than when targets had the same color. At the same time, increased temporal integration of the targets at Lag 1 was observed in the different color condition, even though actual (non-integrated) single targets never consisted of multiple colors. When the color pairs were made more similar, so that they all fell within the range of a single nominal hue (blue), these effects were not observed. Different findings were obtained when contrast was manipulated. Identification accuracy of the second target was higher in the same contrast condition than in the different contrast condition. Higher identification accuracy of both targets was furthermore observed when they were presented with high contrast, while target contrast did not influence temporal integration at all. Temporal attention and integration were thus influenced differently by target contrast pairing than by (categorical) color pairing. Categorically different color pairs, or more generally, categorical feature pairs, may thus afford a reduction in temporal competition between successive targets that eventually enhances attention and integration.

Between one event and two: The locus of the effect of stimulus contrast on temporal integration

The effects of relative stimulus contrast on temporal integration were investigated in a missing element task. Integration frequency was strongly modulated when the contrast of either the first or the second stimulus display was reduced. When the contrast of the first display was low, integration was enhanced, while it was reduced when the contrast of the second display was low. To reveal the processing phases implicated in these modulations of integration, the amplitude of ERP components was examined. N1 component amplitude was increased when the second display was low contrast, matching a full contrast condition. At the N2pc component, amplitude was strongly suppressed only in the former condition. P3 amplitude was also lowest when contrast on the second display was low but with successively increasing amplitudes observed for the other conditions, largely matching the pattern observed in behavioral performance. Taken together, contrast effects on temporal integration seem to originate from increased discriminative processing of the first stimulus display in particular (N1), which is consequently followed by an impairment in attentional processing (N2pc) and working memory consolidation (P3) of the missing element location.

Visual perception is often characterized in space, but in a dynamic world, time is equally important. The perceptual system tends to process relatively long‐lasting events in which information is accumulated through temporal integration. However, which processes contribute to adaptive event integration is not yet well understood. The present study tracks how stimulus contrast alters integration. Contrast first modulates the amplitude of the N1 ERP component, reflecting increased segregation, followed by modulations of the N2pc and P3 components, which reflect consequent difficulties with attending to and consolidating the event. These findings show how the perceptual system achieves adaptive event integration.

Ghost imaging with the human eye

Computational ghost imaging relies on the decomposition of an image into patterns that are summed together with weights that measure the overlap of each pattern with the scene being imaged. These tasks rely on a computer. Here we demonstrate that the computational integration can be performed directly with the human eye. This builds upon the known persistence time of the human eye and we use our ghost imaging approach as an alternative to evaluate the temporal response of the eye. We verify that the image persistence time is of order 20 ms, followed by a further 20 ms exponential decay. These persistence times are consistent with previous studies but can now potentially be extended to include a more precise characterisation of visual stimuli and provide a new experimental tool for the study of visual perception.

Visual percepts modify iconic memory in humans

Our visual system briefly retains a trace of a stimulus after it disappears. This phenomenon is known as iconic memory and its contents are thought to be temporally integrated with subsequent visual inputs to produce a single composite representation. However, there is little consensus on the temporal integration between iconic memory and subsequent visual inputs. Here, we show that iconic memory revises its contents depending upon the configuration of the newly produced single representation with particular temporal characteristics. The Poggendorff illusion, in which two collinear line segments are perceived as non-collinear by an intervening rectangle, was observed when the rectangle was presented during a period spanning from 50 ms before to 200 ms after the presentation of the line segments. The illusion was most prominent when the rectangle was presented approximately 100 to 150 ms after the line segments. Furthermore, the illusion was observed at the center of a moving object, but only when the line segments were presented before the rectangle. These results indicate that the contents of iconic memory are susceptible to the modulatory influence of subsequent visual inputs before being translated into conscious perception in a time-locked manner both in retinotopic and non-retinotopic, object-centered frames of reference.

What You See Is What You Remember: Visual Chunking by Temporal Integration Enhances Working Memory

Human memory benefits from information clustering, which can be accomplished by chunking. Chunking typically relies on expertise and strategy, and it is unknown whether perceptual clustering over time, through temporal integration, can also enhance working memory. The current study examined the attentional and working memory costs of temporal integration of successive target stimulus pairs embedded in rapid serial visual presentation. ERPs were measured as a function of behavioral reports: One target, two separate targets, or two targets reported as a single integrated target. N2pc amplitude, reflecting attentional processing, depended on the actual number of successive targets. The memory-related CDA and P3 components instead depended on the perceived number of targets irrespective of their actual succession. The report of two separate targets was associated with elevated amplitude, whereas integrated as well as actual single targets exhibited lower amplitude. Temporal integration thus provided an efficient means of processing sensory input, offloading working memory so that the features of two targets were consolidated and maintained at a cost similar to that of a single target.

Distortions of temporal integration and perceived order caused by the interplay between stimulus contrast and duration

Stimulus contrast and duration effects on visual temporal integration and order judgment were examined in a unified paradigm. Stimulus onset asynchrony was governed by the duration of the first stimulus in Experiment 1, and by the interstimulus interval in Experiment 2. In Experiment 1, integration and order uncertainty increased when a low contrast stimulus followed a high contrast stimulus, but only when the second stimulus was 20 or 30ms. At 10ms duration of the second stimulus, integration and uncertainty decreased. Temporal order judgments at all durations of the second stimulus were better for a low contrast stimulus following a high contrast one. By contrast, in Experiment 2, a low contrast stimulus following a high contrast stimulus consistently produced higher integration rates, order uncertainty, and lower order accuracy. Contrast and duration thus interacted, breaking correspondence between integration and order perception. The results are interpreted in a tentative conceptual framework.

Extended temporal integration in rapid serial visual presentation: Attentional control at Lag 1 and beyond

In the perception of target stimuli in rapid serial visual presentations, the process of temporal integration plays an important role when two targets are presented in direct succession (at Lag 1), causing them to be perceived as a singular episodic event. This has been associated with increased reversals of target order report and elevated task performance in classic paradigms. Yet, most current models of temporal attention do not incorporate a mechanism of temporal integration and it is currently an open question whether temporal integration is a factor in attentional processing: It might be an independent process, perhaps little more than a sensory sampling rate parameter, isolated to Lag 1, where it leaves the attentional dynamics otherwise unaffected. In the present study, these boundary conditions were tested. Temporal target integration was observed across sequences of three targets spanning an interval of 240ms. Integration rates furthermore depended strongly on bottom-up attentional filtering, and to a lesser degree on top-down control. The results support the idea that temporal integration is an adaptive process that is part of, or at least interacts with, the attentional system. Implications for current models of temporal attention are discussed.

Transformation priming helps to disambiguate sudden changes of sensory inputs

Retinal input is riddled with abrupt transients due to self-motion, changes in illumination, object-motion, etc. Our visual system must correctly interpret each of these changes to keep visual perception consistent and sensitive. This poses an enormous challenge, as many transients are highly ambiguous in that they are consistent with many alternative physical transformations. Here we investigated inter-trial effects in three situations with sudden and ambiguous transients, each presenting two alternative appearances (rotation-reversing structure-from-motion, polarity-reversing shape-from-shading, and streaming-bouncing object collisions). In every situation, we observed priming of transformations as the outcome perceived in earlier trials tended to repeat in subsequent trials and this repetition was contingent on perceptual experience. The observed priming was specific to transformations and did not originate in priming of perceptual states preceding a transient. Moreover, transformation priming was independent of attention and specific to low level stimulus attributes. In summary, we show how "transformation priors" and experience-driven updating of such priors helps to disambiguate sudden changes of sensory inputs. We discuss how dynamic transformation priors can be instantiated as "transition energies" in an "energy landscape" model of the visual perception.

Visible Persistence of Single-Transient Random Dot Patterns: Spatial Parameters Affect the Duration of Fading Percepts

Visible persistence refers to the continuation of visual perception after the physical termination of a stimulus. We studied an extreme case of visible persistence by presenting two matrices of randomly distributed black and white pixels in succession. On the transition from one matrix to the second, the luminance polarity of all pixels within a disk- or annulus-shaped area reversed, physically creating a single second-order transient signal. This transient signal produces the percept of a disk or an annulus with an abrupt onset and a gradual offset. To study the nature of this fading percept we varied spatial parameters, such as the inner and the outer diameter of annuli (Experiment I) and the radius and eccentricity of disks (Experiment III), and measured the duration of visible persistence by having subjects adjust the synchrony of the onset of a reference stimulus with the onset or the offset of the fading percept. We validated this method by comparing two modalities of the reference stimuli (Experiment I) and by comparing the judgments of fading percepts with the judgments of stimuli that actually fade in luminance contrast (Experiment II). The results show that (i) irrespective of the reference modality, participants are able to precisely judge the on- and the offsets of the fading percepts, (ii) auditory reference stimuli lead to higher visible persistence durations than visual ones, (iii) visible persistence duration increases with the thickness of annuli and the diameter of disks, but decreases with the diameter of annuli, irrespective of stimulus eccentricity. These effects cannot be explained by stimulus energy, which suggests that more complex processing mechanisms are involved. Seemingly contradictory effects of disk and annulus diameter can be unified by assuming an abstract filling-in mechanism that speeds up with the strength of the edge signal and takes more time the larger the stimulus area is.

Iconic memory is not a case of attention-free awareness

Whether or not awareness entails attention is a much debated question. Since iconic memory has been generally assumed to be attention-free, it has been considered an important piece of evidence that it does not (Koch & Tsuchiya, 2007). Therefore the question of the role of attention in iconic memory matters. Recent evidence (Persuh, Genzer, & Melara, 2012), suggests that iconic memory does depend on attention. Because of the centrality of iconic memory to this debate, we looked again at the role of attention in iconic memory using a standard whole versus partial report task of letters in a 3×2 matrix. We manipulated attention to the array by coupling it with a second task that was either easy or hard and by manipulating the probability of which task was to be performed on any given trial. When attention was maximally diverted from the matrix, participants were able to report less than a single item, confirming the prior results and supporting the conclusion that iconic memory entails attention. It is not an instance of attention-free awareness.

Limits to the usability of iconic memory

Human vision briefly retains a trace of a stimulus after it disappears. This trace—iconic memory—is often believed to be a surrogate for the original stimulus, a representational structure that can be used as if the original stimulus were still present. To investigate its nature, a flicker-search paradigm was developed that relied upon a full scan (rather than partial report) of its contents. Results show that for visual search it can indeed act as a surrogate, with little cost for alternating between visible and iconic representations. However, the duration over which it can be used depends on the type of task: some tasks can use iconic memory for at least 240 ms, others for only about 190 ms, while others for no more than about 120 ms. The existence of these different limits suggests that iconic memory may have multiple layers, each corresponding to a particular level of the visual hierarchy. In this view, the inability to use a layer of iconic memory may reflect an inability to maintain feedback connections to the corresponding representation.

Spatial resolution in visual memory

Representations in visual short-term memory are considered to contain relatively elaborated information on object structure. Conversely, representations in earlier stages of the visual hierarchy are thought to be dominated by a sensory-based, feed-forward buildup of information. In four experiments, we compared the spatial resolution of different object properties between two points in time along the processing hierarchy in visual short-term memory. Subjects were asked either to estimate the distance between objects or to estimate the size of one of the objects' features under two experimental conditions, of either a short or a long delay period between the presentation of the target stimulus and the probe. When different objects were referred to, similar spatial resolution was found for the two delay periods, suggesting that initial processing stages are sensitive to object-based properties. Conversely, superior resolution was found for the short, as compared with the long, delay when features were referred to. These findings suggest that initial representations in visual memory are hybrid in that they allow fine-grained resolution for object features alongside normal visual sensitivity to the segregation between objects. The findings are also discussed in reference to the distinction made in earlier studies between visual short-term memory and iconic memory.

Coarse-to-fine encoding of spatial frequency information into visual short-term memory for faces but impartial decay

Face perception studies investigated how spatial frequencies (SF) are extracted from retinal display while forming a perceptual representation, or their selective use during task-imposed categorization. Here we focused on the order of encoding low-spatial frequencies (LSF) and high-spatial frequencies (HSF) from perceptual representations into visual short-term memory (VSTM). We also investigated whether different SF-ranges decay from VSTM at different rates during a study-test stimulus-onset asynchrony. An old/new VSTM paradigm was used in which two broadband faces formed the positive set and the probes preserved either low or high SF ranges. Exposure time of 500 ms was sufficient to encode both HSF and LSF in the perceptual representation (experiment 1). Nevertheless, when the positive-set was exposed for 500 ms, LSF-probes were better recognized in VSTM compared with HSF-probes; this effect vanished at 800-ms exposure time (experiment 2). Backward masking the positive set exposed for 800 ms re-established the LSF-probes advantage (experiment 3). The speed of decay up to 10 seconds was similar for LSF- and HSF-probes (experiment 4). These results indicate that LSF are extracted and consolidated into VSTM faster than HSF, supporting a coarse-to-fine order, while the decay from VSTM is not governed by SF.

Temporal Integration in Visual Word Recognition

When two displays are presented in close temporal succession at the same location, how does the brain assign them to one versus two conscious percepts? We investigate this issue using a novel reading paradigm in which the odd and even letters of a string are presented alternatively at a variable rate. The results reveal a window of temporal integration during reading, with a nonlinear boundary around ∼80 msec of presentation duration. Below this limit, the oscillating stimulus is easily fused into a single percept, with all characteristics of normal reading. Above this limit, reading times are severely slowed and suffer from a word-length effect. ERPs indicate that, even at the fastest frequency, the oscillating stimulus elicits synchronous oscillations in posterior visual cortices, while late ERP components sensitive to lexical status vanish beyond the fusion threshold. Thus, the fusion/segregation dilemma is not resolved by retinal or subcortical filtering, but at cortical level by at most 300 msec. The results argue against theories of visual word recognition and letter binding that rely on temporal synchrony or other fine temporal codes.

Consciousness, accessibility, and the mesh between psychology and neuroscience

How can we disentangle the neural basis of phenomenal consciousness from the neural machinery of the cognitive access that underlies reports of phenomenal consciousness? We see the problem in stark form if we ask how we can tell whether representations inside a Fodorian module are phenomenally conscious. The methodology would seem straightforward: Find the neural natural kinds that are the basis of phenomenal consciousness in clear cases--when subjects are completely confident and we have no reason to doubt their authority--and look to see whether those neural natural kinds exist within Fodorian modules. But a puzzle arises: Do we include the machinery underlying reportability within the neural natural kinds of the clear cases? If the answer is "Yes," then there can be no phenomenally conscious representations in Fodorian modules. But how can we know if the answer is "Yes"? The suggested methodology requires an answer to the question it was supposed to answer! This target article argues for an abstract solution to the problem and exhibits a source of empirical data that is relevant, data that show that in a certain sense phenomenal consciousness overflows cognitive accessibility. I argue that we can find a neural realizer of this overflow if we assume that the neural basis of phenomenal consciousness does not include the neural basis of cognitive accessibility and that this assumption is justified (other things being equal) by the explanations it allows.

Do we see more than we can access?

One of Block's conclusions, motivated by partial-report superiority experiments, is that there is phenomenally conscious information that is not cognitively accessible. We argue that this conclusion is not supported by the data.

Overflow, access, and attention

In this response to 32 commentators, I start by clarifying the overflow argument. I explain why the distinction between generic and specific phenomenology is important and why we are justified in acknowledging specific phenomenology in the overflow experiments. Other issues discussed are the relations among report, cognitive access, and attention; panpsychic disaster; the mesh between psychology and neuroscience; and whether consciousness exists.

Voluntary eyeblinks disrupt iconic memory

In the present research, we investigated whether eyeblinks interfere with cognitive processing. In Experiment 1, the participants performed a partial-report iconic memory task in which a letter array was presented for 106 msec, followed 50, 150, or 750 msec later by a tone that cued recall of onerow of the array. At a cue delay of 50 msec between array offset and cue onset, letter report accuracy was lower when the participants blinked following array presentation than under no-blink conditions; the participants made more mislocation errors under blink conditions. This result suggests that blinking interferes with the binding of object identity and object position in iconic memory. Experiment 2 demonstrated that interference due to blinks was not due merely to changes in light intensity. Experiments 3 and 4 demonstrated that other motor responses did not interfere with iconic memory. We propose a new phenomenon, cognitive blink suppression, in which blinking inhibits cognitive processing. This phenomenon may be due to neural interference. Blinks reduce activation in area V1, which may interfere with the representation of information in iconic memory.

Visual temporal window of integration as revealed by the visual mismatch negativity event-related potential to stimulus omissions

We studied whether, similarly to the auditory modality, short-period temporal integration processes occur in vision. Event-related potentials (ERP) were recorded for occasional stimulus omissions from sequences of patterned visual stimuli. A posterior negative component emerged only when the constant stimulus onset asynchrony (SOA) was shorter than 150 ms. This upper limit is comparable with the duration of the temporal window of integration observed in the auditory modality (including experiments studying the effects of stimulus omissions). Parameters of the posterior negativity were highly similar irrespective of whether the stimuli were task-relevant or not (Experiment 1). Thus, we identified this potential as the visual mismatch negativity (vMMN) component, which reflects task-independent detection of violating regularities of the stimulation. vMMN was followed by an anterior positivity (the P3a), indicating attentional shifts induced by the stimulus omissions. In Experiment 2, a posterior negativity similar to that observed in Experiment 1 emerged after the termination of short trains of stimuli, again only when the SOA was shorter than 150 ms. These results support the notion of a temporal integration window in the visual modality, the duration of which is between 150 and 180 ms.

The effect of target contrast on the attentional blink

In this set of five rapid serial visual presentation experiments, observers identified one or two target letters that were embedded in a stream of distractors. Target contrasts were varied, and their effects on the attentional blink (AB) were examined. Target identification improved when its contrast was increased. But whereas an increase in the first target's (T1) contrast facilitated its identification, the recovery of the second target (T2) was paradoxically hampered (Experiments 2 and 5). Similarly, identification of the target suffered when the preceding singleton's contrast was increased (Experiment 1). The AB was eliminated by inserting a blank after a low-contrast, but not a high-contrast, T1 (Experiment 5). Increasing T2's contrast attenuated the blink (Experiment 3) and compensated the larger AB caused by a high-contrast T1 (Experiment 4). In all, these results showed that attention continued to be engaged as long as the target's contrast prolonged its perceptibility. When the high-contrast target was T1, a larger AB was produced; when it was T2, there was protection from substitution masking.

How Many Dimensions Underlie Judgments of Learning and Recall? Evidence From State-Trace Methodology

The authors used state-trace methodology to investigate whether a single dimension (e.g., strength) is sufficient to account for recall and judgments of learning (JOLs) or whether multiple dimensions (e.g., intrinsic and extrinsic factors) are needed. The authors separately manipulated the independent variables of intrinsic and extrinsic cues, determining their state traces for recall and JOLs. In contrast to the supposition that intrinsic cues have similar effects on both recall and JOLs whereas extrinsic cues affect JOLs less strongly than recall (i.e., 2 dimensions underlying recall and JOLs), the authors found repeated support for the sufficiency of a single dimension for both recall and JOLs (not only immediate JOLs but also delayed JOLs) across a variety of intrinsic and extrinsic cues.

Linear theory, dimensional theory, and the face-inversion effect

We contrast 2 theories within whose context problems are conceptualized and data interpreted. By traditional linear theory, a dependent variable is the sum of main-effect and interaction terms. By dimensional theory, independent variables yield values on internal dimensions that in turn determine performance. We frame our arguments within an investigation of the face-inversion effect--the greater processing disadvantage of inverting faces compared with non-faces. We report data from 3 simulations and 3 experiments wherein faces or non-faces are studied upright or inverted in a recognition procedure. The simulations demonstrate that (a) critical conclusions depend on which theory is used to interpret data and (b) dimensional theory is the more flexible and consistent in identifying underlying psychological structures, because dimensional theory subsumes linear theory as a special case. The experiments demonstrate that by dimensional theory, there is no face-inversion effect for unfamiliar faces but a clear face-inversion effect for celebrity faces.

Why is it difficult to see in the fog? How stimulus contrast affects visual perception and visual memory

Processing visually degraded stimuli is a common experience. We struggle to find house keys on dim front porches, to decipher slides projected in overly bright seminar rooms, and to read 10th-generation photocopies. In this research, we focus specifically on stimuli that are degraded via reduction of stimulus contrast and address two questions. First, why is it difficult to process low-contrast, as compared with high-contrast, stimuli? Second, is the effect of contrast fundamental in that its effect is independent of the stimulus being processed and the reason for processing the stimulus? We formally address and answer these questions within the context of a series of nested theories, each providing a successively stronger definition of what it means for contrast to affect perception and memory. To evaluate the theories, we carried out six experiments. Experiments 1 and 2 involved simple stimuli (randomly generated forms and digit strings), whereas Experiments 3-6 involved naturalistic pictures (faces, houses, and cityscapes). The stimuli were presented at two contrast levels and at varying exposure durations. The data from all the experiments allow the conclusion that some function of stimulus contrast combines multiplicatively with stimulus duration at a stage prior to that at which the nature of the stimulus and the reason for processing it are determined, and it is the result of this multiplicative combination that determines eventual memory performance. We describe a stronger version of this theory--the sensory response, information acquisition theory--which has at its core, the strong Bloch's-law-like assumption of a fundamental visual system response that is proportional to the product of stimulus contrast and stimulus duration. This theory was, as it has been in the past, highly successful in accounting for memory for simple stimuli shown at short (i.e., shorter than an eye fixation) durations. However, it was less successful in accounting for data from short-duration naturalistic pictures and was entirely unsuccessful in accounting for data from naturalistic pictures shown at longer durations. We discuss (1) processing differences between short- and long-duration stimuli, (2) processing differences between simple stimuli, such as digits, and complex stimuli, such as pictures, (3) processing differences between biluminant stimuli (such as line drawings with only two luminance levels) and multiluminant stimuli (such as grayscale pictures with multiple luminance levels), and (4) Bloch's law and a proposed generalization of the concept of metamers.

The locus of spatial attention during the temporal integration of visual memories and visual percepts

Temporal integration is a process by which two serially presented visual stimuli are mentally integrated to form a composite representation. In the present research, we explored how spatial selective attention is used during the delay separating stimuli, in order to determine the contents of spatial working memory in this task. A two-task situation was created. On the primary task, two dot arrays were serially presented within a grid, leaving one space empty, which subjects identified. On the secondary task, instead of the second array, a discrimination probe was presented. Integration accuracy increased through delays of 1,500 msec, revealing an estimate of the time required to form an optimal memory trace for integration. Once the memory trace was formed (but not before), response time to the probe was faster if it was presented in a location previously occupied by a dot from Array 1. This indicates that during the delay separating the arrays, the subjects assigned spatial attention to the locations occupied by the first array and actively maintained the leading array in working memory. Implications for theories of visual processing and memory are discussed.

Temporal integration between visual images and visual percepts

Using a temporal integration task, subjects in 5 experiments were expected to combine information from temporally separated visual presentations. Evidence from these experiments indicated that perceptual information can be integrated with previously generated and currently maintained visual images to form a representation that contains information from each source. Properties and limitations of this integration process were also explored, including the time required to generated the image, the speed at which percepts were integrated with images, and the capacity of the representation. Implications for theories of visual processing and memory are discussed.

Discriminating time intervals presented in sequences marked by visual signals

This article presents the results of three experiments on the discrimination of time intervals presented in sequences marked by brief visual signals. In Experiment 1A (continuous condition), the participants had to indicate whether, in a series of 2-4 intervals marked by 3-5 visual signals, the last interval was shorter or longer than the previous one(s). In Experiment 1B (discontinuous condition), the participants indicated whether, in a presentation of two series of 1-3 intervals, with each series being marked by 2-4 signals, the intervals of the second sequence were shorter or longer than those of the first. Whenever one, two, or three standard intervals were presented, the difference threshold was as high at 150 msec as it was at 300 msec with the continuous method but increased monotonically from 150 to 900 msec with the discontinuous method. With both methods, the increase was well described by Weber's law--the Weber fraction was roughly constant--between 600 and 900 msec (Experiment 2), whereas between 900 and 1,200 msec (Experiment 3), the Weber fraction increased.

From physical time to the first and second moments of psychological time

After examination of the status of time in experimental psychology and a review of related major texts, 2 opposite approaches are presented in which time is either unified or fragmented. Unified time perception views, usually guided by Weber's law, are embodied in various models. After a brief review of old models and a description of the major contemporary models of time perception, views on fragmented time perception are presented as challenges for any unified time view. Fragmentation of psychological time emerges from (a) disruptions of the Weber function, which are caused by the types of interval presentation, by extensive practice, and by counting explicitly or not; and (b) modulations of time sensitivity and perceived duration by attention and interval structures. Weber's law is a useful guide for studying psychological time, but it is also reasonable to assume that more than one so-called central timekeeper could contribute to perceiving time.

Accounts of the confidence-accuracy relation in recognition memory

Confidence and accuracy, while often considered to tap the same memory representation, are often found to be only weakly correlated (e.g., Bothwell, Deffenbacher, & Brigham, 1987; Deffenbacher, 1980). There are at least two possible (nonexclusive) reasons for this weak relation. First, it may be simply due to noise of one sort or another; that is, it may come about because of both within- and between-subjects statistical variations that are partially uncorrelated for confidence measures on the one hand and accuracy measures on the other. Second, confidence and accuracy may be uncorrelated because they are based, at least in part, on different memory representations that are affected in different ways by different independent variables. We propose a general theory that is designed to encompass both of these possibilities and, within the context of this theory, we evaluate effects of four variables--degree of rehearsal, study duration, study luminance, and test luminance--in three face recognition experiments. In conjunction with our theory, the results allow us to begin to identify the circumstances under which confidence and accuracy are based on the same versus different sources of information in memory. The results demonstrate the conditions under which subjects are quite poor at monitoring their memory performance, and are used to extend cue utilization theories to the domain of face recognition.

A front end to a theory of picture recognition

We describe the results of four picture-recognition memory experiments in which we systematically manipulated four variables: stimulus duration, stimulus contrast, the duration of a blank gap between successive presentations of the same stimulus, and the presence or absence of a noise mask that immediately followed stimulus offset. The patterns of obtained data confirmed a simple extension of a theory previously used to account for digit recall data. This theory consists of a low-pass linear-filter front end that generates a sensory response from the physical stimulus, followed by an information-sampling process whose instantaneous sampling rate is based in part on the sensory response magnitude. The data confirm both qualitative and quantitative theoretical predictions, some of which were previously untestable in digit recall tasks because of ceiling effects that were not present in our picture-recognition tasks. We describe the role of our theory within the broader family of picture-memory theories, and we briefly discuss our theory's unification of two salient facets of visual behavior: information acquisition on the one hand, and phenomenological appearance on the other hand.

Temporal inhibition in character identification

Models of information processing tasks such as character identification often do not consider the nature of the initial sensory representation from which task-relevant information is extracted. An important component of this representation is temporal inhibition, in which the response to a stimulus may inhibit, or in some cases facilitate, processing of subsequent stimuli. Three experiments demonstrate the existence of temporal inhibitory processes in information processing tasks such as character identification and digit recall. An existing information processing model is extended to account for these effects, based in part on models from the detection literature. These experiments also discriminate between candidate neural mechanisms of the temporal inhibition. Implications for the transient deficit theory of dyslexia are discussed.