The Influence of Masked Stimuli on Response Selection: Evidence from a Semantic Categorization Task.

Unconscious visual stimuli can be processed by human observers and influence their behaviour. A striking example is a phenomenon known as "free-choice priming," where masked "prime" stimuli-of which participants are unaware-modulate which of two response alternatives they are likely to choose. Recent efforts to uncover the mechanisms underlying this intriguing effect have revealed that free-choice priming can emerge even in the absence of automatized stimulus-response (S-R) associations between masked primes and specific motor responses, indicating that free choices can be influenced by a masked prime's meaning (Ocampo, 2015). It remains unknown, however, whether masked primes bias response selections because they are implicitly classified according to task instructions, or because spreading activation occurs within the prime's semantic network. To adjudicate between these two possibilities, participants in the present experiment categorised targets as either animals or people and selected which of two response alternatives they wanted to make following presentation of a free-choice target. Crucially, while implicit classifications could proceed during processing of both animal and person masked primes, only animal primes could trigger spreading activation within their semantic network. This manipulation modulated free-choice priming; only masked animal primes influenced response selections to free-choice targets. This result indicates that an automatic spreading activation mechanism might underlie a masked prime's ability to influence free-choice responses.

Evaluating automatic attentional capture by self-relevant information.

Our everyday decisions and memories are inadvertently influenced by self-relevant information. For example, we are faster and more accurate at making perceptual judgments about stimuli associated with ourselves, such as our own face or name, as compared with familiar non-self-relevant stimuli. Humphreys and Sui propose a "self-attention network" to account for these effects, wherein self-relevant stimuli automatically capture our attention and subsequently enhance the perceptual processing of self-relevant information. We propose that the masked priming paradigm and continuous flash suppression represent two ways to experimentally examine these controversial claims.

Unconscious manipulation of free choice by novel primes.

The extent to which non-conscious perception can influence behaviour has been a topic of considerable controversy in psychology for decades. Although a challenging task, convincing empirical demonstrations have emerged suggesting that non-consciously perceived 'prime' stimuli can influence motor responses to subsequent targets. Interestingly, recent studies have shown that the influence of masked primes is not restricted to target-elicited responses, but can also bias free-choices between alternative behaviours. The present experiment extends these findings by showing that free-choices could also be biased by novel primes that never appeared as targets and therefore could not trigger acquired stimulus-response (S-R) mappings. This new evidence suggests that free-choice behaviour can be influenced by non-consciously triggered semantic representations. Furthermore, the results reported here support accounts of masked priming that posit an automatic semantic categorisation of non-consciously perceived visual stimuli.

Direct evidence of cognitive control without perceptual awareness.

A central question within the domain of human cognition is whether or not the ability to replace a current action with a new one (i.e., cognitive control) depends on a conscious appreciation of the environmental change that necessitates the new behavior. Specifically, it is not yet known if non-consciously perceived stimuli can trigger the modification of a currently ongoing action. We show for the first time that individuals are able to use non-consciously perceived information to modify the course and outcome of an ongoing action. Participants were presented with a masked (i.e., subliminal) 'stop' or 'go-on' prime stimulus whilst performing a routine reach-to-touch action. Despite being invisible to participants, the stop primes produced more hesitations mid-flight and more movement reversals than the go-on primes. This new evidence directly establishes that cognitive control (i.e., the ability to modify a currently ongoing action) does not depend on a conscious appreciation of the environmental trigger.

The negative compatibility effect with relevant masks: a case for automatic motor inhibition.

For many years controversy has surrounded the so-called "negative compatibility effect" (NCE), a surprising phenomenon whereby responses to a target stimulus are delayed when the target is preceded by an unconscious, response-compatible prime. According to proponents of the "self-inhibition" hypothesis, the NCE occurs when a low-level self-inhibitory mechanism supresses early motor activations that are no longer supported by perceptual evidence. This account has been debated, however, by those who regard the NCE to be a stimulus-specific phenomenon that can be explained without recourse to a self-inhibitory mechanism. The present study used a novel reach-to-touch paradigm to test whether unconscious response priming would manifest as motor activation of the opposite-to-prime response (supporting mask-induced priming accounts), or motor inhibition of the primed response (supporting the notion of low-level self-inhibition). This paper presents new findings that show the emergence of positive and negative compatibility effects as they occur in stimulus processing time. In addition, evidence is provided suggesting that the NCE is not driven by the activation of the incorrect, "opposite-to-prime" response, but rather might reflect automatic motor inhibition.

Event coding and motor priming: how attentional modulation may influence binding across action properties.

When we perceive an action it is internally transformed into a motor representation akin to the execution of that same action. Motor priming studies show that action observation facilitates the execution of physically similar actions, but interferes with the performance of dissimilar actions. Some evidence suggests, however, that once a specific motor plan is formed, perceiving an action with partially overlapping features (e.g. a congruent grip-type but and incongruent end-goal) can result in interference. In two experiments we investigate how modulating attention towards observed actions influences the binding that occurs between action features, and therefore the amount of partial-overlap interference to participants' performance. In the first study we directed attention towards a salient action feature (the grip-type). We found that perceiving partially overlapping (i.e. partially congruent) actions slowed participants' responses compared with observation of completely congruent or incongruent actions. In the second experiment attentional resources were taxed through the use of a secondary task. This resulted in the elimination of the partial-overlap interference effect. We discuss results in relation to feature binding and event codes.

How frontoparietal brain regions mediate imitative and complementary actions: an FMRI study.

The 'mirror neuron system' (MNS), located within inferior parietal lobe (IPL) and inferior frontal gyrus (IFG), creates an internal motor representation of the actions we see and has been implicated in imitation. Recently, the MNS has been implicated in non-identical responses: when the actions we must execute do not match those that we observe. However, in such conflicting situations non action-specific cognitive control networks also located in frontoparietal regions may be involved. In the present functional magnetic resonance imaging (fMRI) study participants made both similar and dissimilar actions within two action contexts: imitative and complementary. We aimed to determine whether activity within IPL/IFG depends on (i) responding under an imitative versus complementary context (ii) responding with similar versus dissimilar responses, and (iii) observing hand actions versus symbolic arrow cue stimuli. Activity within rIPL/rIFG regions was largest during observation of hand actions compared with arrow cues. Specifically, rIPL/rIFG were recruited only during the imitative context, when participants responded with similar actions. When responding to symbolic arrow cues, rIPL/rIFG activity increased during dissimilar responses, reflecting increased demands placed on general cognitive control mechanisms. These results suggest a specific role of rIPL/rIFG during imitation of hand actions, and also a general role of frontoparietal areas in mediating dissimilar responses to both hand actions and symbolic stimuli. We discuss our findings in relation to recent work that has examined the role of frontoparietal brain structures in joint-actions and inter-actor cooperation. We conclude that the specific brain regions identified here to show increased activation during action observation conditions are likely to form part of a mechanism specifically involved in matching observed actions directly with internal motor plans. Conversely, observation of arrow cues recruited part of a wider cognitive control network involved in the rapid remapping of stimulus-response associations.

Interpreting actions: the goal behind mirror neuron function.

Crucial to our everyday social functioning is an ability to interpret the behaviors of others. This process involves a rapid understanding of what a given action is not only in a physical sense (e.g., a precision grip around the stem of a wine glass) but also in a semantic sense (e.g., an invitation to "cheers"). The functional properties of fronto-parietal mirror neurons (MNs), which respond to both observed and executed actions, have been a topic of much debate in the cognitive neuroscience literature. The controversy surrounds the role of the "mirror neuron system" in action understanding: do MNs allow us to comprehend others' actions by allowing us to internally represent their behaviors or do they simply activate a direct motor representation of the perceived act without recourse to its meaning? This review outlines evidence from both human and primate literatures, indicating the importance of end-goals in action representations within the motor system and their predominance in influencing action plans. We integrate this evidence with recent views regarding the complex and dynamic nature of the mirror neuron system and its ability to respond to broad motor outcomes.

Placing actions in context: motor facilitation following observation of identical and non-identical manual acts.

It has been argued that through a process of internal 'simulation', we automatically map observed actions directly onto our motor system to facilitate imitation. Instead, here we show that observed actions influence the kinematic parameters of manual responses in a dynamic, context-dependent fashion. Participants observed object-directed hand actions in imitative and complementary action contexts, and performed identical (same grip-type) and non-identical (opposite grip-type) responses to a similar object, respectively. In imitative contexts, identical actions were performed optimally. In complementary contexts, however, non-identical actions were enhanced relative to identical actions. A further experiment using arrow cues instead of hand actions confirmed that these results were specific to action observation. Our findings demonstrate that action context plays a critical role in determining the relationship between action observation and execution. Crucially, this relationship is not fixed, but depends on an agent's goal when observing others act.

Facilitation of responses to degraded targets by non-degraded distractors.

It is well-established that distractors interfere with goal-directed responses. Our recent findings indicate that the presence of corners in degraded line drawings of distractor stimuli modulates response times and accuracy to non-degraded targets (Kritikos and Pavlis 2007, Experimental Brain Research 183 159-170). In the present study we asked whether non-degraded distractors may facilitate responses to degraded targets (corners or line segments missing). We presented targets at fixation and accompanied by identical, category-congruent, or category-incongruent distractors. Participants responded to two object categories (musical instruments and tools) consisting of four line drawings. Corners-missing targets in particular were associated with greater interference from distractors than non-degraded targets. This interference was modulated when distractor locations were endogenously or exogenously cued. Findings are discussed in the context of additional processing of object features that are crucial to action.