Visibility of Brief Images: The Dual-Process Approach

If successive, brief visual images are exposed for recognition or for psychophysical ratings, various effects and phenomena of fast dynamics of conscious perception such as mutual masking, metacontrast, proactive enhancement of contrast, proactive speed-up of the latency of subjective visual experience, the Fröhlich Effect, the Tandem Effect, attentional facilitation by visuospatial precuing, and some others have been found. The theory proposed to deal with these phenomena proceeds from the assumption that two types of brain processes are necessary in order to consciously recognize visual stimuli: (1) fast, specific processes of encoding that allocate and reactivate the stimulus representation which is based on the activity of selected cortical neurons and (2) relatively slower processes of facilitation of the activity of this specific representation that are mediated by the excitatory modulation of the EPSPs of those selected cortical neurons by the ascending input from nonspecific thalamus. Theperceptual retouchconstruct is proposed in order to characterize and analyze the interaction of (1) and (2). The neurophysiological characteristics of this bifunctional system of afference help to put forward several predictions that are found to be consistent with the empirical regularities of the above-described perceptual-attentional phenomena. These data form a body of converging evidence that is consistent with the predictions of the perceptual retouch approach.     

Segregation by color and stereoscopic depth in three-dimensional visual space

Two experiments were conducted to investigate how color and stereoscopic depth information are used to segregate objects for visual search in three-dimensional (3-D) visual space. Eight observers were asked to indicate the alphanumeric category (letter or digit) of the target which had its unique color and unique depth plane. In Experiment 1, distractors sharing a common depth plane or a common color appeared in spatial contiguity in thexy plane. The results suggest that visual search for the target involves examination of kernels formed by homogeneous items sharing the same color and depth. In Experiment 2, thexy contiguity of distractors sharing a common color or a common depth plane was varied. The results showed that when target-distractor distinction becomes more difficult on one dimension, the other dimension becomes more important in performing visual search, as indicated by a larger effect on search time. This suggests that observers can make optimal use of the information available. Finally, color had a larger effect on search time than did stereoscopic depth. Overall, the results support models of visual processing which maintain that perceptual segregation and selective attention are determined by similarity among objects in 3-D visual space on both spatial and nonspatial stimulus dimensions.     

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.     

Tachistoscopic exposure and masking of real three-dimensional scenes

Although there are many well-known forms of visual cues specifying absolute and relative distance, little is known about how visual space perception develops at small temporal scales. How much time does the visual system require to extract the information in the various absolute and relative distance cues? In this article, we describe a system that may be used to address this issue by presenting brief exposures of real, three-dimensional scenes, followed by a masking stimulus. The system is composed of an electronic shutter (a liquid crystal smart window) for exposing the stimulus scene, and a liquid crystal projector coupled with an electromechanical shutter for presenting the masking stimulus. This system can be used in both full- and reduced-cue viewing conditions, under monocular and binocular viewing, and at distances limited only by the testing space. We describe a configuration that may be used for studying the microgenesis of visual space perception in the context of visually directed walking.     

More than meets the eye: Implicit perception in legally blind individuals

Legally blind participants (uncorrected vision of 20/200+) were able to identify a visual stimulus attribute (clock hand position) in the absence of consciously identifying its presence. Specifically, participants—with their corrective lenses removed—correctly guessed the hour-hand position above chance (8%) on a clockface shown on a computer screen. This occurred both when presented in a 1-clockface display (28%), as well as when shown a display containing 4 clockfaces (21%), in which only 1 face contained a hand. Even more striking, hand identification accuracy in the 4-clockface condition was comparable whether the clockface containing the hand was (21%) or was not (20%) correctly identified. That legally blind individuals are capable of identifying stimulus attributes without conscious awareness provides an additional vehicle for exploring implicit perception. Consistent with previous research, the visualsystem can apparently cope with degraded visual input through information available through a(n unconscious) secondary pathway via the superior colliculi.     

Evidence for visual persistence during saccadic eye movements

Summary 1. The persistence of visual perception was investigated under conditions of visual fixation as well as eye movement. The Ss' task was to discriminate brief double light impulses; their responses were recorded as a function of the duration of the interstimulus interval. Based on these data the critical interstimulus interval was calculated, which yielded equal response frequencies for the perception of one or two stimuli upon presentation of double light pulses.2. In the condition of visual fixation the two stimuli could not be discriminated until the mean value of interstimulus interval exceeded 73 msec. In the condition with eye movements, when the first stimulus was presented in the parafoveal region of the retina before the beginning of the saccade and the second stimulus in the foveal region just after termination of the eye movement, this duration was shown to be statistically of the same magnitude (76 msec).3. Possible alternative interpretations of this latter result, e.g., that it could be explained in terms of masking or saccadic suppression rather than visual persistence was discussed; it was attempted to invalidate such explanations by means of three control experiments.4. The main result, the persistence of visual perception during voluntary eye movements, was discussed in relation to the problem of spatial and temporal stability of visual perception.I thank Prof. Dr. H.W. Wendt for support in correcting the English translation.     

The effect of an after-coming random pattern on the perception of brief visual stimuli

The perception of briefly exposed visual forms is shown to be masked by an after- coming random pattern stimulus of approximately equal intensity. This effect occurs only under certain well defined conditions; it is limited by the minimum stimulus exposure time in excess of threshold which overcomes masking (critical stimulus duration) as well as by the minimum interval between presentation of the two stimuli which permits evasion of the masking action (critical interval). Over the range of stimulus duration in which masking occurs, critical interval varied with stimulus duration in such a way that the interval multiplied by the stimulus duration equals a constant. Critical stimulus duration and critical interval at threshold are shown to vary little under a variety of conditions. The effect of the random pattern stimulus is limited to the part of the visual field to which it is presented.     

An attentional mechanism in the analysis of spatial frequency

Sinusoidal gratings of various spatial frequencies were used as masking stimuli in a detection task and a vernier acuity task. The test stimuli were 1 cycle/deg square-wave gratings. The spatial frequency of the most effective mask was 1 cycle/deg for the detection task but 3 cycles/deg for the vernier acuity task. The different masking functions for the two tasks show that the visual system analyzes the square-wave stimulus into its various spatial-frequency components. Since the test stimulus was the same for both tasks, the different masking functions may be the result of an attentional mechanism that weighs the importance of the output from various spatial-frequency analyzers. Whether the information from a particular spatial-frequency analyzer is attended or not depends upon the task the visual system must perform.     

The levels of perceptual processing and the neural correlates of increasing subjective visibility

According to the levels-of-processing hypothesis, transitions from unconscious to conscious perception may depend on stimulus processing level, with more gradual changes for low-level stimuli and more dichotomous changes for high-level stimuli. In an event-related fMRI study we explored this hypothesis using a visual backward masking procedure. Task requirements manipulated level of processing. Participants reported the magnitude of the target digit in the high-level task, its color in the low-level task, and rated subjective visibility of stimuli using the Perceptual Awareness Scale. Intermediate stimulus visibility was reported more frequently in the low-level task, confirming prior behavioral results. Visible targets recruited insulo-fronto-parietal regions in both tasks. Task effects were observed in visual areas, with higher activity in the low-level task across all visibility levels. Thus, the influence of level of processing on conscious perception may be mediated by attentional modulation of activity in regions representing features of consciously experienced stimuli.     

Latency facilitation in temporal-order judgments: time course of facilitation as a function of judgment type

The paper is concerned with two models of early visual processing which predict that priming of a visual mask by a preceding masked stimulus speeds up conscious perception of the mask (perceptual latency priming). One model ascribes this speed-up to facilitation by visuo-spatial attention [Scharlau, I., & Neumann, O. (2003a). Perceptual latency priming by masked and unmasked stimuli: Evidence for an attentional explanation. Psychological Research 67, 184-197], the other attributes it to nonspecific upgrading mediated by retino-thalamic and thalamo-cortical pathways [Bachmann, T. (1994). Psychophysiology of visual masking: The fine structure of conscious experience. Commack, NY: Nova Science Publishers]. The models make different predictions about the time course of perceptual latency priming. Four experiments test these predictions. The results provide more support for the attentional than for the upgrading model. The experiments further demonstrate that testing latency facilitation with temporal-order judgments may induce a methodological problem resulting in fairly low estimates. A method which provides a more exhaustive measure is suggested and tested.     

Does prior knowledge facilitate the detection of visual targets in random noise?

The detectability of forms embedded within random visual noise has been found to be predictable from the autocorrelation transform of the stimulus pattern (Uttal, 1975). A basic assumption in the autocorrelation theory of form detection is that detectability is determined by the organization of the stimulus pattern, irrespective of the observer’s prior knowledge or expectations about the characteristics of the form. This assumption was tested by determining the effect of the size of the set of alternative target forms on performance of a forced-choice detection task. The targets were composed of dots in a straight line, appearing in one of a specified set of 2, 4, or 8 alternative positions in a pattern of randomly distributed masking dots. Detection accuracy was found to decrease as set size increased, but this decrease was close to what was predicted on the assumption that the random background was independently confusable with the target at each of the alternative positions. Thus, prior knowledge of the set of alternative targets appeared to have no effect on thevisual process, but only on thedecision process by virtue of the features that were relevant criteria for deciding which of the two patterns on each trial was most likely to contain the target. This result is consistent with the autocorrelation theory. This experiment may illustrate how the decision process has influenced the performance in many other experiments that have been assumed to demonstrate an effect of prior knowledge on perception.     

Size (mostly) doesn’t matter: the role of set size in object substitution masking

Conscious detection and discrimination of a visual target stimulus can be prevented by the presentation a spatially nonoverlapping, but temporally trailing, visual masking stimulus. This phenomenon, known as object substitution masking (OSM), has long been associated with spatial attention, with diffuse attention seemingly being key for the effect to be observed. Recently, this hypothesis has been questioned. We sought to provide a definitive test of the involvement of spatial attention in OSM by using an eight-alternative forced choice task under a range of mask durations, set sizes, and target/distractor spatial configurations. The results provide very little evidence that set size, and thus the distribution of spatial attention, interacts with masking magnitude. These findings have implications for understanding the mechanisms underlying OSM and the relationship between consciousness and attention.     

What's in a mask? Information masking with forward and backward visual masks

Three experiments tested how the physical format and information content of forward and backward masks affected the extent of visual pattern masking. This involved using different types of forward and backward masks with target discrimination measured by percentage correct in the first experiment (with a fixed target duration) and by an adaptive threshold procedure in the last two. The rationale behind the manipulation of the content of the masks stemmed from masking theories emphasizing attentional and/or conceptual factors rather than visual ones. Experiment 1 used word masks and showed that masking was reduced (a masking reduction effect) when the forward and backward masks were the same word (although in different case) compared to when the masks were different words. Experiment 2 tested the extent to which a reduction in masking might occur due to the physical similarity between the forward and backward masks by comparing the effect of the same content of the masks in the same versus different case. The result showed a significant reduction in masking for same content masks but no significant effect of case. The last experiment examined whether the reduction in masking effect would be observed with nonword masks—that is, having no high-level representation. No reduction in masking was found from same compared to different nonword masks (Experiment 3). These results support the view that the conscious perception of a rapidly displayed target stimulus is in part determined by high-level perceptual/cognitive factors concerned with masking stimulus grouping and attention.     

What's in a mask? Information masking with forward and backward visual masks

Three experiments tested how the physical format and information content of forward and backward masks affected the extent of visual pattern masking. This involved using different types of forward and backward masks with target discrimination measured by percentage correct in the first experiment (with a fixed target duration) and by an adaptive threshold procedure in the last two. The rationale behind the manipulation of the content of the masks stemmed from masking theories emphasizing attentional and/or conceptual factors rather than visual ones. Experiment 1 used word masks and showed that masking was reduced (a masking reduction effect) when the forward and backward masks were the same word (although in different case) compared to when the masks were different words. Experiment 2 tested the extent to which a reduction in masking might occur due to the physical similarity between the forward and backward masks by comparing the effect of the same content of the masks in the same versus different case. The result showed a significant reduction in masking for same content masks but no significant effect of case. The last experiment examined whether the reduction in masking effect would be observed with nonword masks--that is, having no high-level representation. No reduction in masking was found from same compared to different nonword masks (Experiment 3). These results support the view that the conscious perception of a rapidly displayed target stimulus is in part determined by high-level perceptual/cognitive factors concerned with masking stimulus grouping and attention.     

What's new in visual masking?

A brief display that is clearly visible when shown alone can be rendered invisible by the subsequent presentation of a second visual stimulus. Several recently described backward masking effects are not predicted by current theories of visual masking, including masking by four small dots that surround (but do not touch) a target object and masking by a surrounding object that remains on display after the target object has been turned off. A crucial factor in both of these effects is attention: almost no masking occurs if attention can be rapidly focused on the target, whereas powerful masking ensues if attention directed at the target is delayed. A new theory of visual masking, inspired by developments in neuroscience, can account for these effects, as well as more traditional masking effects. In addition, the new theory sheds light on related research, such as the attentional blink, inattentional blindness and change blindness.     

Unconscious processing of color and form in metacontrast masking

Three experiments employed a metacontrast masking procedure to examine the extent and nature of priming effects from visual stimuli not consciously perceived. The results showed effects of unconscious stimuli on subsequent target responses that (1) were more consistent, reliable, and not subject to strategic control, as compared with consciously perceived stimuli (Experiment 1); (2) produced both facilitation and interference of subsequent processing (Experiment 2); and (3) did not influence indirect response-related levels of processing (Experiment 3). These results demonstrate that color and form attributes of unconscious stimuli are sufficiently registered within the visual system to influence behavior, and that some of these unconscious effects occur at early levels of stimulus encoding, prior to higher level perceptual and response-related processes.     

Recovery from object substitution masking induced by transient suppression of visual motion processing: a repetitive transcranial magnetic stimulation study

Object substitution masking is a form of visual backward masking in which a briefly presented target is rendered invisible by a lingering mask that is too sparse to produce lower image-level interference. Recent studies suggested the importance of an updating process in a higher object-level representation, which should rely on the processing of visual motion, in this masking. Repetitive transcranial magnetic stimulation (rTMS) was used to investigate whether functional suppression of motion processing would selectively reduce substitution masking. rTMS-induced transient functional disruption of cortical area V5/MT+, which is important for motion analysis, or V1, which is reciprocally connected with V5/MT+, produced recovery from masking, whereas sham stimulation did not. Furthermore, masking remained undiminished following rTMS over the region 2 cm posterior to V5/MT+, ruling out nonspecific effects of real stimulation and confirming regional specificity of the rTMS effect. The results suggest that object continuity via the normal function of the visual motion processing system might in part contribute to this masking. The relation of these findings to the reentrant processing view of object substitution masking and other visual phenomena is discussed.     

A mathematical theory of optical illusions and figural aftereffects

If it is assumed that spurious enhancement of receptive field excitations near the intersection of image lines on the retina contributes to the cortical determination of the geometry of two-dimensional figures, an equation based on the least-squares fit of data points to a straight line-can be obtained to represent theapparent line. Such a fit serves as anextreemum on the precision with which a data set can be represented by a straight line. The disparity between theapparent line and the actual line that occurs in the case of peripheral (and to a lesser degree in more central regions of the retina) vision is sufficient to produce the perceptual errors that occur in the Poggendorff, Hering, and Mueller-Lyer illusions. The magnitude of the Poggendorff illusion as a function of the line angle is derived and experimentally tested. Blakemore, Carpenter, and Georgeson’s (1970) experimental data on angle perception are shown to fit this same function. Theapparent curve is derived for the Hering illusion. The Mueller-Lyer illusion is found to be a variation of the Poggendorff illusion. The equations are further developed and used to derive Pollack’s (1958) experimental results on figural aftereffects. The results involve onlyone experimentally determined coefficient that can be evaluated, within the limits of experimental error, in terms of physiological data. The use of these concepts provides a foundation for the abstract modeling of the initial phases of the central nervous system data reduction processes, including receptive field structure, that is consistent with the physiological limitations of the retina as a source of visual data, as well as with the findings of Hubel and Wiesel (1962).     

Where perception meets memory: A review of repetition priming in visual search tasks

What we have recently seen and attended to strongly influences how we subsequently allocate visual attention. A clear example is how repeated presentation of an object’s features or location in visual search tasks facilitates subsequent detection or identification of that item, a phenomenon known as priming. Here, we review a large body of results from priming studies that suggest that a short-term implicit memory system guides our attention to recently viewed items. The nature of this memory system and the processing level at which visual priming occurs are still debated. Priming might be due to activity modulations of low-level areas coding simple stimulus characteristics or to higher level episodic memory representations of whole objects or visual scenes. Indeed, recent evidence indicates that only minor changes to the stimuli used in priming studies may alter the processing level at which priming occurs. We also review recent behavioral, neuropsychological, and neurophysiological evidence that indicates that the priming patterns are reflected in activity modulations at multiple sites along the visual pathways. We furthermore suggest that studies of priming in visual search may potentially shed important light on the nature of cortical visual representations. Our conclusion is that priming occurs at many different levels of the perceptual hierarchy, reflecting activity modulations ranging from lower to higher levels, depending on the stimulus, task, and context—in fact, the neural loci that are involved in the analysis of the stimuli for which priming effects are seen.