Response priming

In 1962, Fehrer and Raab reported experiments where participants were required to press a single key as quickly as possible upon presentation of a visual target stimulus. The visibility of the target was strongly reduced by so-called metacontrast masking (see below). The authors found that the response times were independent of the subjective visibility of the target, i.e., responses to well-visible targets were just as fast as those to nearly invisible targets (Fehrer-Raab effect). The term response priming was first employed by Rosenbaum and Kornblum with respect to an experimental paradigm where different aspects of motor responses were primed by visual stimuli. The modern procedure of response priming was developed in the 1980s and 1990s by Peter Wolff, Werner Klotz, Ulrich Ansorge, and Odmar Neumann at the University of Bielefeld, Germany. The paradigm was developed further in the 1990s by a research team led by Dirk Vorberg at the University of Braunschweig, Germany. Typical time course of a trial in a response priming paradigm. Here, the participant responds as quickly as possible to the shape of the target stimulus (diamond or square) by pressing the assigned response key. Shortly before, a prime is presented (also a diamond or square) that influences the response to the target. The time interval between prime onset and target onset is called the "stimulus-onset asynchrony" (SOA). In many response priming experiments, the target also serves to visually mask the prime. Therefore, a second task is often employed where participants are asked to identify the masked prime. b) Prime and target are consistent when assigned to the same response, and inconsistent when assigned to different responses. c) The visibility of the prime can be strongly influenced by masking effects from the target. In all response priming paradigms, participants have to respond to a specific target stimulus. In a simple experiment, this could be one of two geometric stimuli, each of which is assigned to one of two response keys (e.g., diamond - left key; square - right key). The experiment consists of a large number of trials where the participant presses the left key upon appearance of a diamond, and the right key upon appearance of a square, as quickly and correctly as possible. In each trial, the target is preceded by a prime that is also a diamond or square, and therefore able to elicit the same motor responses as the target (Fig. 1a). If prime and target are linked to the same response (diamond preceded by diamond, square preceded by square), they are called "consistent" (also "congruent", "compatible"); if they are linked to different motor responses (diamond preceded by square, square preceded by diamond), they are called "inconsistent" (also "incongruent", "incompatible"; Fig. 1b). The time interval between onset of the prime and onset of the target is called "stimulus onset asynchrony" (SOA). Typically, SOAs up to 100 milliseconds (ms) are employed. Priming effects occur when the prime influences the motor response to the target: consistent primes speed responses to the target, whereas inconsistent primes slow responses (Fig. 2). Priming effects in response times are calculated by taking the difference between response times in inconsistent and consistent trials. Apart from their effects on response speed, primes can greatly affect the rate of response errors (i.e., erroneous responses to the target): consistent primes rarely lead to errors, whereas error rates can become very high for inconsistent primes. In response times as well as error rates, priming effects typically increase with SOA, leading to the typical response priming patterns in Fig. 2. from primed pointing responses, from measurements of response force, and from simulation studies, The time course of the response priming effect described so far only holds for SOAs up to about 100 ms. For longer SOAs, the priming effect can increase further. Under some circumstances, however, a reversal of the effect can be observed where inconsistent primes lead to faster responses to the target than do consistent primes. This effect is known as the "negative compatibility effect". ==Masked priming==

Response priming can be employed to investigate phenomena of perception without awareness. Here, the visibility of the prime can be systematically reduced or even abolished by means of a masking stimulus. This can be accomplished by presenting the masking stimulus shortly before or after the prime. The prime's visibility can be assessed by different measures, such as forced-choice discrimination tasks, stimulus detection judgments, brightness judgments, and others. A phenomenon named metacontrast masking can be produced when the prime is followed by a mask enclosing the prime's shape such that both stimuli share adjacent contours. For instance, a prime can be masked by a larger annulus whose inner contours exactly fit the prime's shape. In many response priming experiments, the target serves the additional purpose of masking the prime (Fig. 1). Metacontrast is a form of visual backward masking where the visibility of the prime is reduced by a stimulus following the prime. In Fig. 3, some typical time-courses of visual masking effects are shown as a function of the prime-target SOA, in a response-priming experiment where the target itself serves as a masking stimulus (Fig. 1a, c). Here, the measure of prime visibility could be the discrimination performance of a participant trying to guess the shape of the prime (diamond or square) in each trial. Without masking, performance would be nearly perfect; the participant would have little difficulty classifying the prime correctly as a square or a diamond in every trial. In contrast, if masking was complete, discrimination performance would be at chance level (Fig. 3, left panel). In many experiments, however, the time-course of masking is less extreme (Fig. 3, right panel). Most stimulus conditions lead to so-called "type-A masking", where the degree of masking is highest at short SOAs and then diminishes, so that the prime becomes easier to discriminate for increasing SOAs. Under some circumstances, however, "type-B masking" can be obtained, where the degree of masking is highest at intermediate SOAs but where the prime becomes easier to discriminate at shorter or longer SOAs. Type-B masking can occur with metacontrast masking but critically depends on stimulus properties of primes and targets. ==Independence of response priming from visual awareness==

Experiments show that the time-course of the response priming effect (increasing effects with increasing SOA) is independent of the degree and time-course of masking. Klotz and Neumann (1999) demonstrated response priming effects under complete masking of the prime. This finding could be confirmed in many further experiments revealing numerous dissociations between masking and priming. The independence of priming and visual awareness clearly contradicts the traditional notion that effects of unconscious perception merely reflect some residual processing ability under very unfavorable viewing conditions, something that remains after a prime stimulus has been degraded so severely that conscious awareness of it has decreased below some "threshold". This conception has often led to sharp criticism of research on unconscious or "subliminal" perception, but it is probably wrong on a basic level. Instead, motor activation by masked primes is obviously independent of processes of backward masking, provided that visibility is controlled only by the masking stimulus while the prime stimulus remains unchanged. In other words: for a short time and under suitable experimental conditions, visually masked (invisible) stimuli can influence motor responses just as effectively as visible ones. ==Variants==

Given that the researcher is aware of the most influential experimental variables, the response priming method can be employed in a number of experimental variants and can contribute to the exploration of a multitude of research questions in the field of cognitive psychology. The most prevalent form of response priming employs a prime and target at the same monitor position, so that the target also serves to visually mask the prime (often by means of metacontrast). In many experiments, there are two different targets preceded by two different primes at the same monitor positions. (indeed, the Eriksen effect may be a special case of response priming). Response-priming effects have been demonstrated for a large number of stimuli and discrimination tasks, including geometric stimuli, natural images (animals vs. objects), letters, or so-called readiness potentials which reflect the degree of motor activation in the brain's motor cortex and can be measured by electro-encephalographic methods. Brain imaging methods like functional magnetic resonance imaging (fMRI) have been employed as well. ==Theories==

In the part that follows, three theories that explain the regular, positive response priming effects will be described. A review of theories of the negative compatibility effect can be found in Sumner (2007). The theory assumes that at the outset of a response priming experiment, participants acquire rules of stimulus-response assignment, which quickly become automatized. Following this practice phase, the motor response can be prepared so far that only a single critical stimulus feature (e.g., diamond vs. square) is still needed to specify the response. This incoming stimulus feature then defines the last missing action parameter (e.g., left vs. right keypress). Responses are elicited quickly and directly, without the need for a conscious representation of the eliciting stimulus. Response priming is explained by assuming that the prime's features elicit exactly the same parameter specification processes that are supposed to be elicited by the target stimulus. In parallel to the response elicitation process, a conscious representation of primes and targets emerges, which can be subject to visual masking processes. However, the conscious representation of the stimuli doesn't play any role for the motor processes in the current experimental trial. Action trigger account The action trigger account was developed by Wilfried Kunde, Andrea Kiesel, and Joachim Hoffmann at the University of Würzburg, Germany. This account assumes that responses to unconscious primes are neither elicited by semantic analysis of the primes nor by pre-established stimulus-response mappings. Instead, it is assumed that the prime fits a pre-existing action release condition, eliciting the assigned response like a key opening a lock. This happens in two consecutive steps. In the first step, action triggers are held active in working memory that fit the respective task and are able to elicit a specific motor response. Action triggers are established in the instruction and practice phase of the experiment. In the second step, called online stimulus processing, an upcoming stimulus is compared to the action release conditions. If the stimulus fits the trigger conditions, the action triggers automatically execute the response. As an example, the participant's task might be to indicate whether a visually presented number is smaller or larger than five, Again, the conscious representation of the stimulus plays no role for motor activation; however, it can lead to a strategic adjustment of response criteria in later trials (e.g., by choosing to respond more slowly to avoid errors). In sum, this theory can be viewed as expanding on the concept of direct parameter specification by focusing on the exact conditions that lead to response priming. Rapid-chase theory and Pieter Roelfsema from the University of Amsterdam have proposed that this wave starts as a pure feedforward process (feedforward sweep): A cell first reached by the wavefront has to pass on its activity before being able to integrate feedback from other cells. Lamme and Roelfsema assume that this kind of feedforward processing is not sufficient to generate visual awareness of the stimulus: For this, neuronal feedback and recurrent processing loops are required that link widespread neuronal networks. According to rapid-chase theory, response priming effects are independent of visual awareness because they are carried by rapid feedforward processes whereas the emergence of a conscious representation of the stimuli is dependent on slower, recurrent processes. The most important prediction of rapid-chase theory is that the feedforward sweeps of prime and target signals should occur in strict sequence. This strict succession should be observable in the time-course of the motor response, and there should be an early phase where the response is controlled exclusively by the prime and is independent of all properties of the actual target stimulus. One way to check these predictions is to examine the time-course of primed pointing responses. It has been shown that these pointing responses start at a fixed time after presentation of the prime (not the actual target) and start to proceed in the direction specified by the prime. If prime and target are inconsistent, the target is often able to reverse the pointing direction "on the fly", directing the response into the correct direction. However, the longer the SOA, the longer the time where the finger is moving in the direction of the misleading prime. Schmidt, Niehaus, and Nagel (2006) could show that the earliest phase of primed pointing movements exclusively depends on properties of the prime (e.g., the color contrast of red vs. green primes), but is independent of all properties of the target (its time of occurrence, its color contrast, and its ability to mask the prime). These findings could be confirmed with different methods and different types of stimuli. Because rapid-chase theory views response priming as a feedforward process, it maintains that priming effects occur before recurrent and feedback activity take part in stimulus processing. The theory therefore leads to the controversial thesis that response priming effects are a measure of preconscious processing of visual stimuli, which may be qualitatively different from the way those stimuli are finally represented in visual awareness. == See also ==