Friederike Schlaghecken

Effects of masked stimuli on motor activation: behavioral and electrophysiological evidence

Three experiments investigated the influence of unperceived events on response activation. Masked primers were presented before a target. On compatible trials, primes and targets were identical; on incompatible trials, opposite responses were assigned to them. Forced-choice performance indicated that prime identification was prevented by the masking procedure, but overt performance and motor activation as mirrored by the lateralized readiness potential (LRP) were systematically influenced by the prime. The direction of these effects was unexpected: Performance costs for compatible and performance benefits for incompatible trials were obtained relative to a neutral trial condition. The LRP revealed a sequential pattern of motor activation. A partial activation of the response corresponding to the prime was followed by a reverse activation pattern. It is argued that these effects primarily reflect an inhibition of the response initially triggered by the prime.

Links between conscious awareness and response inhibition: Evidence from masked priming

Recent results from “subliminal priming” experiments have shown that masked prime stimuli which can not be consciously perceived can trigger response activation processes, but that these response activations can later be subject to inhibition. Links between conscious awareness and response inhibition were investigated by manipulating the visibility of masked prime stimuli, from clearly visible primes to prime stimuli that were inaccessible to conscious perception. Response inhibition was observed with unperceived prime stimuli, but not for suprathreshold primes. Correlations between individual prime identification thresholds and the onset of response inhibition indicate that the absence or presence of conscious awareness can predict whether or not response inhibition is elicited. These results demonstrate qualitative differences in the effects of conscious and unconscious information. It is argued that response facilitation produced by consciously available perceptual information can counteract automatic effects of self-inhibitory motor control circuits.

Motor activation with and without inhibition: Evidence for a threshold mechanism in motor control

Masked primes presented prior to a target can trigger response activation processes that may later be subject to inhibition. Evidence for response inhibition has previously been obtained with primes presented at fixation, but not with primes presented in the periphery of the visual field. It is argued that thiscentral—peripheral asymmetry reflects a threshold mechanism in motor control. Foveal masked primes give rise to stronger perceptual representations than do peripheral primes, resulting in stronger response activations. Strong response activations are actively inhibited, whereas weaker activations remain below a hypothetical inhibition threshold. Evidence in favor of this hypothesis is obtained in four experiments that manipulated the perceptual strength of foveal and peripheral primes. Experiments 1 and2 demonstrate that when the perceptual strength of peripheral primes is gradually increased by delaying mask onset, positive prime—target compatibility effects (reflecting the absence of response inhibition) turn into negative effects (indicating the presence of response inhibition). Experiments 3 and 4 demonstrate that when the perceptual strength of foveal primes is gradually decreased by degradation, negative compatibility effects turn into positive effects. The results are discussed in terms of a simple functional model of early motor control.

Explicit and implicit learning of event sequences: Evidence from event-related brain potentials

Event-related brain potentials (ERPs) were recorded during a serial reaction time (RT) task, where single deviant items seldom (Experiment 1) or frequently (Experiment 2) replaced 1 item of a repeatedly presented 10-item standard sequence. Acquisition of sequence knowledge was reflected in faster RTs for standard as compared with deviant items and in an enhanced negativity (N2 component) of the ERP for deviant items. Effects were larger for participants showing explicit knowledge in their verbal reports and in a recognition test. The lateralized readiness potential indicated that correct responses were activated with shorter latencies after training. For deviant items, participants with explicit knowledge showed an initial activation of the incorrect but expected response. These findings suggest that the acquisition of explicit and implicit knowledge is reflected in different electrophysiological correlates and that sequence learning may involve the anticipatory preparation of responses.

Masked prime stimuli can bias "free" choices between response alternatives.

Stimuli presented below the threshold of awareness can systematically influence choice responses determined by the instructed stimulus-response (S-R) mapping (task set). In this study, we investigated whether such stimuli will also bias a free choice between two response alternatives under conditions in which this choice subjectively appears to be internally generated and free. Participants had to respond to targets preceded by masked arrow primes. Left-pointing and right-pointing arrow targets required left or right responses, whereas randomly interspersed “free-choice” targets indicated that the participants were free to choose either response. Although masked primes could not be consciously discriminated, they systematically affected not only performance to arrow targets, but also the free choice between response alternatives. This demonstrates that apparently “free” choices are not immune to nonconsciously triggered biases. However, in blocks in which no specific S-R mapping was imposed, masked primes did not affect free-choice performance, indicating that these effects are not automatic but are determined by currently active task sets.

A central-peripheral asymmetry in masked priming.

Masked primes presented prior to a target result in behavioral benefits on incompatible trials (in which the prime and the target are mapped onto opposite responses) when they appear at fixation, but in behavioral benefits on compatible trials (in which the prime and the target are mapped onto the same response) when appearing peripherally. In Experiment 1, the time course of thiscentral-peripheral asymmetry (CPA) was investigated. For central primes, compatible-trial benefits at short stimulus onset asynchronies (SOAs) turned into incompatible-trial benefits at longer SOAs. For peripheral primes, compatible-trial benefits at short SOAs increased in size with longer SOAs. Experiment 2 showed that these effects also occur when primes and targets are physically dissimilar, ruling out an interpretation in terms of the perceptual properties of the stimulus material. In Experiments 3 and 4, the question weis investigated as to whether the CPA is related to visual-spatial attention and/or retinal eccentricity per se. The results indicate that the CPA is independent of attentional factors but strongly related to the physiological inhomogeneity of the retina. It is argued that central and peripheral primes trigger an initial motor activation, which is inhibited only if primes are presented at retinal locations of sufficiently high perceptual sensitivity. The results are discussed in terms of an activation threshold model.

Channel selection and classification of electroencephalogram signals

OBJECTIVE An electroencephalogram-based (EEG-based) brain-computer-interface (BCI) provides a new communication channel between the human brain and a computer. Amongst the various available techniques, artificial neural networks (ANNs) are well established in BCI research and have numerous successful applications. However, one of the drawbacks of conventional ANNs is the lack of an explicit input optimization mechanism. In addition, results of ANN learning are usually not easily interpretable. In this paper, we have applied an ANN-based method, the genetic neural mathematic method (GNMM), to two EEG channel selection and classification problems, aiming to address the issues above. METHODS AND MATERIALS Pre-processing steps include: least-square (LS) approximation to determine the overall signal increase/decrease rate; locally weighted polynomial regression (Loess) and fast Fourier transform (FFT) to smooth the signals to determine the signal strength and variations. The GNMM method consists of three successive steps: (1) a genetic algorithm-based (GA-based) input selection process; (2) multi-layer perceptron-based (MLP-based) modelling; and (3) rule extraction based upon successful training. The fitness function used in the GA is the training error when an MLP is trained for a limited number of epochs. By averaging the appearance of a particular channel in the winning chromosome over several runs, we were able to minimize the error due to randomness and to obtain an energy distribution around the scalp. In the second step, a threshold was used to select a subset of channels to be fed into an MLP, which performed modelling with a large number of iterations, thus fine-tuning the input/output relationship. Upon successful training, neurons in the input layer are divided into four sub-spaces to produce if-then rules (step 3). Two datasets were used as case studies to perform three classifications. The first data were electrocorticography (ECoG) recordings that have been used in the BCI competition III. The data belonged to two categories, imagined movements of either a finger or the tongue. The data were recorded using an 8 × 8 ECoG platinum electrode grid at a sampling rate of 1000 Hz for a total of 378 trials. The second dataset consisted of a 32-channel, 256 Hz EEG recording of 960 trials where participants had to execute a left- or right-hand button-press in response to left- or right-pointing arrow stimuli. The data were used to classify correct/incorrect responses and left/right hand movements. RESULTS For the first dataset, 100 samples were reserved for testing, and those remaining were for training and validation with a ratio of 90%:10% using K-fold cross-validation. Using the top 10 channels selected by GNMM, we achieved a classification accuracy of 0.80 ± 0.04 for the testing dataset, which compares favourably with results reported in the literature. For the second case, we performed multi-time-windows pre-processing over a single trial. By selecting 6 channels out of 32, we were able to achieve a classification accuracy of about 0.86 for the response correctness classification and 0.82 for the actual responding hand classification, respectively. Furthermore, 139 regression rules were identified after training was completed. CONCLUSIONS We demonstrate that GNMM is able to perform effective channel selections/reductions, which not only reduces the difficulty of data collection, but also greatly improves the generalization of the classifier. An important step that affects the effectiveness of GNMM is the pre-processing method. In this paper, we also highlight the importance of choosing an appropriate time window position.

Locus of inhibition in the masked priming of response alternatives.

Abstract Masked prime stimuli presented immediately before target stimuli in a choice reaction task give rise to behavioral costs when the primes and the target stimuli are mapped to the same response and result in benefits when they are mapped to opposite responses. Researchers assume that this negative compatibility effect reflects inhibitory processes in the control of perceptuomotor links. The authors investigated whether the inhibition operates at the level of abstract central codes or at effector-specific motor stages. In 2 experiments (N = 8 participants in each), left or right hand or foot responses were required to target stimuli that were preceded by masked arrow primes mapped to the same response side as the target stimuli in compatible trials and to the opposite response side in incompatible trials; the primes were irrelevant in neutral trials. In Experiment 1, when the masked primes determined both response side and modality, there was no transfer of negative compatibility effects across response modalities. That finding is inconsistent with a central abstract locus of inhibition and suggests that inhibition operates at effector-specific motor stages. In Experiment 2, primes conveyed only response side information but left response modality uncertain, and negative compatibility effects were elicited for both hand and foot responses, suggesting that partially informative masked primes can trigger a parallel activation and subsequent inhibition of response processes within separate effector systems.

Sub-processes of working memory in the N-back task: An investigation using ERPs

OBJECTIVE The N-back task is frequently used in working memory studies. N-parameters allow experimental psychologists to analyze the sub-processes of N-back tasks in addition to general processing. However, previous imaging studies have not closely scrutinized these sub-processes. In the current study, three sub-processes in the N-back task were proposed using a logical task analysis: matching, replacement and shift. Domain-specific lateralization in spatial and verbal working memory was investigated in terms of this model. METHODS This model was tested with two ERP experiments during N-back tasks, one conceptual (top-down) and one data-driven (bottom-up). RESULTS Domain-specific lateralization was observed as predicted in the shift sub-process of the conceptual task and in the replacement sub-process of the data-driven task. Match-specific lateralization was also found. CONCLUSIONS The results support our three-sub-process model of the N-back task and our hypothesis that replacement is a data-driven process with a posterior locus whereas shift is a more conceptual process with a more frontal locus. SIGNIFICANCE The proposed model correctly predicted ERP patterns in conceptual and data-driven N-back tasks and is potentially useful in understanding the neurophysiologic basis of N-back task performance. The similarity between match- and domain-specific lateralization in N-back tasks raised several issues for further investigation.

Active Masks and Active Inhibition: A Comment on Lleras and Enns (2004) and on Verleger, Jaskowski, Aydemir, van der Lubbe, and Groen (2004)

Verleger, Jaskowski, Aydemir, van der Lubbe, and Groen and Lleras and Enns have argued that negative compatibility effects (NCEs) obtained with masked primes do not reflect self-inhibition processes in motor control. Instead, NCEs are assumed to reflect activation of the response opposite to the prime, triggered by the presence of prime/targetlike features in the mask. Thus, no NCEs should be elicited when masks do not contain such task-relevant features. In Experiments 1 and 3, the authors demonstrate that NCEs can be obtained when masks contain only irrelevant features. Experiment 2 demonstrates that positive compatibility effects (PCEs) will occur with such masks when masked primes are presented peripherally. These results are inconsistent with the mask-induced activation accounts but are in line with the self-inhibition hypothesis of the NCE. Although perceptual interactions and mask-induced motor activations may contribute to NCEs under certain conditions, they cannot provide a full explanation for these effects.