Experiments on the Fehrer-Raab effect and the 'Weather Station Model' of visual backward masking

Awareness and Stimulus-Driven Spatial Attention as Independent Processes

To investigate the relation between attention and awareness, we manipulated visibility/awareness and stimulus-driven attention capture among metacontrast-masked visual stimuli. By varying the time interval between target and mask, we manipulated target visibility measured as target discrimination accuracies (ACCs; Experiments 1 and 2) and as subjective awareness ratings (Experiment 3). To modulate stimulus-driven attention capture, we presented the masked target either as a color-singleton (the target stands out by its unique color among homogeneously colored non-singletons), as a non-singleton together with a distractor singleton elsewhere (an irrelevant distractor has a unique color, whereas the target is colored like the other stimuli) or without a singleton (no stimulus stands out; only in Experiment 1). As color singletons capture attention in a stimulus-driven way, we expected target visibility/discrimination performance to be best for target singletons and worst with distractor singletons. In Experiments 1 and 2, we confirmed that the masking interval and the singleton manipulation influenced ACCs in an independent way and that attention capture by the singletons, with facilitated performance in target-singleton compared to distractor-singleton conditions, was found regardless of the interval-induced (in-)visibility of the targets. In Experiment 1, we also confirmed that attention capture was the same among participants with worse and better visibility/discrimination performance. In Experiment 2, we confirmed attention capture by color singletons with better discrimination performance for probes presented at singleton position, compared to other positions. Finally, in Experiment 3, we found that attention capture by target singletons also increased target awareness and that this capture effect on subjective awareness was independent of the effect of the masking interval, too. Together, results provide new evidence that stimulus-driven attention and awareness operate independently from one another and that stimulus-driven attention capture can precede awareness.

Stuck on a Plateau? A Model-Based Approach to Fundamental Issues in Visual Temporal-Order Judgments

Humans are incapable of judging the temporal order of visual events at brief temporal separations with perfect accuracy. Their performance-which is of much interest in visual cognition and attention research-can be measured with the temporal-order judgment (TOJ) task, which typically produces S-shaped psychometric functions. Occasionally, researchers reported plateaus within these functions, and some theories predict such deviation from the basic S shape. However, the centers of the psychometric functions result from the weakest performance at the most difficult presentations and therefore fluctuate strongly, leaving the existence and exact shapes of plateaus unclear. This study set out to investigate whether plateaus disappear if the data accuracy is enhanced, or if we are "stuck on a plateau", or rather with it. For this purpose, highly accurate data were assessed by model-based analysis. The existence of plateaus is confidently confirmed and two plausible mechanisms derived from very different models are presented. Neither model, however, performs well in the presence of a strong attention manipulation, and model comparison remains unclear on the question of which of the models describes the data best. Nevertheless, the present study includes the highest accuracy in visual TOJ data and the most explicit models of plateaus in TOJ studied so far.

Keeping postdiction simple

Postdiction effects are phenomena in which a stimulus influences the appearance of events taking place before it. In metacontrast masking, for instance, a masking stimulus can render a target stimulus shown before the mask invisible. This and other postdiction effects have been considered incompatible with a simple explanation according to which (i) our perceptual experiences are delayed for only the time it takes for a distal stimulus to reach our sensory receptors and for our neural mechanisms to process it, and (ii) the order in which the processing of stimuli is completed corresponds with the apparent temporal order of stimuli. As a result, the theories that account for more than a single postdiction effect reject at least one of these theses. This paper presents a new framework for the timing of experiences-the non-linear latency difference view-in which the three most discussed postdiction effects-apparent motion, the flash-lag effect, and metacontrast masking-can be accounted for while simultaneously holding theses (i) and (ii). This view is grounded in the local reentrant processes, which are known to have a crucial role in perception. Accordingly, the non-linear latency difference view is both more parsimonious and more empirically plausible than the competing theories, all of which remain largely silent about the neural implementation of the mechanisms they postulate.

Priming of Visual Search Facilitates Attention Shifts: Evidence From Object-Substitution Masking

Priming of visual search strongly affects visual function, releasing items from crowding and during free-choice primed targets are chosen over unprimed ones. Two accounts of priming have been proposed: attentional facilitation of primed features and postperceptual episodic memory retrieval that involves mapping responses to visual events. Here, well-known masking effects were used to assess the two accounts. Object-substitution masking has been considered to reflect attentional processing: It does not occur when a target is precued and is strengthened when distractors are present. Conversely, metacontrast masking has been connected to lower level processing where attention exerts little effect. If priming facilitates attention shifts, it should mitigate object-substitution masking, while lower level masking might not be similarly influenced. Observers searched for an odd-colored target among distractors. Unpredictably (on 20% of trials), object-substitution masks or metacontrast masks appeared around the target. Object-substitution masking was strongly mitigated for primed target colors, while metacontrast masking was mostly unaffected. This argues against episodic retrieval accounts of priming, placing the priming locus firmly within the realm of attentional processing. The results suggest that priming of visual search facilitates attention shifts to the target, which allows better spatiotemporal resolution that overcomes object-substitution masking.

Summation versus suppression in metacontrast masking: On the potential pitfalls of using metacontrast masking to assess perceptual-motor dissociation

A briefly flashed target stimulus can become "invisible" when immediately followed by a mask-a phenomenon known as backward masking, which constitutes a major tool in the cognitive sciences. One form of backward masking is termed metacontrast masking. It is generally assumed that in metacontrast masking, the mask suppresses activity on which the conscious perception of the target relies. This assumption biases conclusions when masking is used as a tool-for example, to study the independence between perceptual detection and motor reaction. This is because other models can account for reduced perceptual performance without requiring suppression mechanisms. In this study, we used signal detection theory to test the suppression model against an alternative view of metacontrast masking, referred to as the summation model. This model claims that target- and mask-related activations fuse and that the difficulty in detecting the target results from the difficulty to discriminate this fused response from the response produced by the mask alone. Our data support this alternative view. This study is not a thorough investigation of metacontrast masking. Instead, we wanted to point out that when a different model is used to account for the reduced perceptual performance in metacontrast masking, there is no need to postulate a dissociation between perceptual and motor responses to account for the data. Metacontrast masking, as implemented in the Fehrer-Raab situation, therefore is not a valid method to assess perceptual-motor dissociations.

Attention and the speed of information processing: posterior entry for unattended stimuli instead of prior entry for attended stimuli

Why are nearly simultaneous stimuli frequently perceived in reversed order? The origin of errors in temporal judgments is a question older than experimental psychology itself. One of the earliest suspects is attention. According to the concept of prior entry, attention accelerates attended stimuli; thus they have "prior entry" to perceptive processing stages, including consciousness. Although latency advantages for attended stimuli have been revealed in psychophysical studies many times, these measures (e.g. temporal order judgments, simultaneity judgments) cannot test the prior-entry hypothesis completely. Since they assess latency differences between an attended and an unattended stimulus, they cannot distinguish between faster processing of attended stimuli and slower processing of unattended stimuli. Therefore, we present a novel paradigm providing separate estimates for processing advantages respectively disadvantages of attended and unattended stimuli. We found that deceleration of unattended stimuli contributes more strongly to the prior-entry illusion than acceleration of attended stimuli. Thus, in the temporal domain, attention fulfills its selective function primarily by deceleration of unattended stimuli. That means it is actually posterior entry, not prior entry which accounts for the largest part of the effect.

Two dimensions of visibility revealed by multidimensional scaling of metacontrast

An increasing number of studies use subjective reports of visibility, so as to delineate the domain of perceptual awareness. It is generally assumed that degrees of visibility can be ordered on a single unidimensional scale. Here, I put this assumption to test with metacontrast, one of the most studied visual masking paradigms. By means of multidimensional scaling, I show that even though metacontrast stimuli only differ along the dimension of time, the perceptual space they generate unfolds in three dimensions: time and two kinds of visibilities, that are confounded when projected onto a unitary visibility scale. I argue that metacontrast creates multidimensional complex percepts, a property that may run counter to its use as a simple modulator of visibility. More broadly the results cast doubt on the use of visibility scales that ignore the qualities of the percepts.

Temporal-order judgment of visual and auditory stimuli: modulations in situations with and without stimulus discrimination

Temporal-order judgment (TOJ) tasks are an important paradigm to investigate processing times of information in different modalities. There are a lot of studies on how temporal order decisions can be influenced by stimuli characteristics. However, so far it has not been investigated whether the addition of a choice reaction time (RT) task has an influence on TOJ. Moreover, it is not known when during processing the decision about the temporal order of two stimuli is made. We investigated the first of these two questions by comparing a regular TOJ task with a dual task (DT). In both tasks, we manipulated different processing stages to investigate whether the manipulations have an influence on TOJ and to determine thereby the time of processing at which the decision about temporal order is made. The results show that the addition of a choice RT task does have an influence on the TOJ, but the influence seems to be linked to the kind of manipulation of the processing stages that is used. The results of the manipulations indicate that the temporal order decision in the DT paradigm is made after perceptual processing of the stimuli.

Top-down contingent feature-specific orienting with and without awareness of the visual input

In the present article, the role of endogenous feature-specific orienting for conscious and unconscious vision is reviewed. We start with an overview of orienting. We proceed with a review of masking research, and the definition of the criteria of experimental protocols that demonstrate endogenous and exogenous orienting, respectively. Against this background of criteria, we assess studies of unconscious orienting and come to the conclusion that so far studies of unconscious orienting demonstrated endogenous feature-specific orienting. The review closes with a discussion of the role of unconscious orienting in action control.

Relationships among Visual Awareness, Reaction Time, and Lateralized Readiness Potential in a Simple Reaction Time Task under the Backward Masking Paradigm

The goal of the present study was to examine whether a backward masking paradigm, in which a prime and a mask stimuli were consecutively presented with a short stimulus onset asynchrony affected the time needed for either the perceptual or motor stages of processing and the simple reaction times. The times needed for the perceptual and motor stages were evaluated by measuring the stimulus-locked and response-locked lateralized readiness potentials. The results showed that the onset of the stimulus-locked lateralized readiness potentials under the backward masking paradigm took place earlier than it did under the condition of a mask stimulus presented alone, whereas the onset of the response-locked lateralized readiness potentials did not significantly differ under different stimulus conditions. These results suggested that the participants responded to the masked prime stimulus despite being unaware of the prime stimulus. This may have been mediated by facilitation of the perceptual rather than motor stages.

Binding binding: Departure points for a different version of the perceptual retouch theory

In the perceptual retouch theory, masking and related microgenetic phenomena were explained as a result of interaction between specific cortical representational systems and the non-specific sub-cortical modulation system. Masking appears as deprivation of sufficient modulation of the consciousness mechanism suffered by the target-specific signals because of the temporal delay of non-specific modulation (necessary for conscious representation), which explicates the later-coming mask information instead of the already decayed target information. The core of the model envisaged relative magnitudes of EPSPs of single cortical cells driven by target and mask signals at the moment when the nonspecific, presynaptic, excitatory input arrives from the thalamus. In the light of the current evidence about the importance of synchronised activity of specific and non-specific systems in generating consciousness, the retouch theory requires perhaps a different view. This article presents some premises for modification of the retouch theory, where instead of the cumulative presynaptic spike activities and EPSPs of single cells, the oscillatory activity in the gamma range of the participating systems is considered and shown to be consistent with the basic ideas of the retouch theory. In this conceptualisation, O-binding refers to specific encoding which is based on gamma-band synchronised oscillations in the activity of specific cortical sensory modules that represent features and objects; C-binding refers to the gamma-band oscillations in the activity of the non-specific thalamic systems, which is necessary for the O-binding based data to become consciously experienced.

Electrophysiological activation by masked primes: Independence of prime-related and target-related activities

Visual stimuli that are made invisible by metacontrast masking (primes) have a marked influence on behavioral and psychophysiological measures such as reaction time (RT) and the lateralized readiness potential (LRP). 4 experiments are reported that shed light on the effects that masked primes have on the LRP. Participants had a go-nogo task in which the prime was associated with 1 of 2 responses even if the target required participants to refrain from responding. To analyze the electrophysiological responses, we computed the LRP and applied an averaging method separating the activation due to the prime and the target. The results demonstrated that (a) masked primes activate responses even in a nogo situation, (b) this prime-related activation is independent of masking, (c) and is also independent of whether prime and target require the same responses (congruent condition) or different responses (incongruent condition).

Sensorimotor supremacy: Investigating conscious and unconscious vision by masked priming

According to the sensorimotor supremacy hypothesis, conscious perception draws on motor action. In the present report, we will sketch two lines of potential development in the field of masking research based on the sensorimotor supremacy hypothesis. In the first part of the report, evidence is reviewed that masked, invisible stimuli can affect motor responses, attention shifts, and semantic processes. After the review of the corresponding evidence - so-called masked priming effects - an approach based on the sensorimotor supremacy hypothesis is detailed as to how the question of a unitary mechanism of unconscious vision can be pursued by masked priming studies. In the second part of the report, different models and theories of backward masking and masked priming are reviewed. Types of models based on the sensorimotor hypothesis are discussed that can take into account ways in which sensorimotor processes (reflected in masked priming effects) can affect conscious vision under backward masking conditions.

Perceptual latency priming: A measure of attentional facilitation

The present paper reviews recent research on perceptual latency priming (PLP). PLP is the relative latency advantage-earlier perception-of a visual stimulus that is preceded by another, masked stimulus at its location. The first stimulus attracts attention which accelerates perception of the second stimulus. This facilitation arises even if the first stimulus is visually backward-masked by the second one. The paper summarises research on temporal and spatial properties of PLP and the question whether intentions mediate shifts of attention to external events. Possible sources of PLP besides visuo-spatial attention are discussed. Finally, I give a review of feedforward and reentrant models of PLP and compare them to the empirical evidence.