Comparison of Frequentist and Bayesian Statistics for Studying Unconscious Perception: Differences Between Null Awareness Dissociation and Relative Sensitivity Dissociation.

For unconscious perception research, Bayesian statistics are more appropriate for assessing null awareness of masked stimuli than traditional (frequentist) statistics. This assertion is based mostly upon the theoretical features of Bayesian statistics and modeling studies. To further assess the potential advantages, we compared frequentist and Bayesian statistical tests in a masked Stroop priming experiment in which the prime stimuli were presented at varying degrees of visibility. A novel contribution was to compare a null awareness dissociation approach (i.e., stimulus awareness = 0) to a relative sensitivity approach (indirect or priming effects > direct effects) for the same data. From a null awareness perspective, the frequentist t-tests for the Stroop effect (i.e., perception) for the briefest display conditions had non-significant outcomes. Similar Bayesian t-tests were inconclusive. In contrast, the relative sensitivity dissociation approach was more interpretable, with strong evidence against unconscious perception from a single Bayesian t test. For the longer display conditions, both statistical approaches suggested large conscious perception effects. We conclude that the utility of Bayesian statistics is highly dependent upon the type of dissociation approach, with a relative sensitivity approach being more straightforward to interpret than a null awareness approach.

Binary vs. continuous experimental designs for the study of unconscious perceptual processing.

Binary vs. continuous conceptualizations of consciousness may have an unstated influence on experimental designs in unconscious perception research. The binary approach aims to compare a conscious condition (e.g., supraliminal, no or weak stimulus masking) to an unconscious condition (e.g., subliminal, heavy stimulus masking). In contrast, continuous designs tend to vary stimulus energy along a near-threshold continuum to determine changes in perception as a function of stimulus energy (or duration). The present study compared two experimental designs, binary vs. continuous, for the influence of target-masked prime stimuli on a Stroop task. The display parameters were inspired by emotional Stroop studies reporting unconscious perception. Neither experiment produced strong evidence of unconscious perception, but the experiment with a continuous design was more informative. We thus recommend sampling a range of near-threshold display parameters to yield straight-forward, unambiguous interpretations.

Classic Stroop Color Words Produce No Stroop Effect When the Display Characteristics Are Based Upon Emotional Stroop Studies With Subliminal Presentations.

Two forms of the Stroop task have produced contradictory findings regarding unconscious perceptual processing. Emotional Stroop task studies with prime words presented at an objective threshold (i.e., subliminal) produce Stroop-like effects, but comparable studies conducted with classic Stroop stimuli do not produce Stroop effects. We tested the possibility that differences in the display appearance might explain this discrepancy. Color word prime stimuli from the traditional Stroop task were used with display characteristics based upon the emotional Stroop studies. There was a Stroop effect for the relatively long prime stimulus durations (59, 87, or 108 milliseconds) but not for the brief durations (18, 24, or 38 milliseconds). Accordingly, the discrepancy in research findings cannot be attributed to simple differences in display methodology. The failure to find strong evidence of unconscious perceptual processing is consistent with the negative findings from some emotional Stroop studies that use subliminal stimulus presentations.

Interaction of Perceived Frequency and Intensity in Fingertip Electrotactile Stimulation: Dissimilarity Ratings and Multidimensional Scaling.

Sensations elicited by electrical stimulation of touch are multidimensional, varying in perceived intensity and quality in response to changes in stimulus current or waveform timing. This paper manipulated both current and frequency, while volunteer participants estimated the dissimilarity of all non-identical pairs of 16 stimulus conditions. Multidimensional scaling analysis revealed that a model having two perceptual dimensions was adequate in representing the electrotactile (electrocutaneous) sensations. The two dimensions were identified as perceptual frequency and intensity, and were strongly correlated with the two stimulus variables, frequency and current, although not in a 1:1 correspondence. Perception of frequency differences increased monotonically with stimulus intensity, which is consistent with other human sensory systems, such as hearing and vision. Our results are consistent with previously-reported research using a different methodology and cutaneous locus. Congruence across different methods and laboratories suggests similar underlying perceptual mechanisms.

Awareness of "Invisible" Arrows in a Metacontrast Masking Paradigm.

A common strategy in unconscious perception research is to use either pattern masking or metacontrast masking to render prime stimuli "invisible" to consciousness. However, several recent studies have questioned whether the identities of prime stimuli (typically arrows or diamonds and squares) in metacontrast masking studies are impossible to consciously perceive. In a series of studies, we concurrently related prime awareness, target response time priming, and prime identification across 3 prime-mask stimulus onset asynchronies (27, 40, and 67 ms). We found that increases in prime awareness ratings were accompanied by better prime identification performance. Significant prime identification in the 27-ms condition was obtained only at the highest awareness rating; for the other 2 stimulus onset asynchronies most awareness ratings were associated with above-chance prime identification. The priming effects obtained in these paradigms occur, to some degree, when participants are likely to be aware of the prime stimuli. Our results, collectively, suggest that metacontrast masking of primes does not necessarily preclude their awareness. Priming effects may depend on at least partial awareness of the prime stimuli.

Highly correlated stimuli do not necessarily facilitate the measurement of unconscious perception: exclusion failure is hard to find in forced-choice tasks.

Persaud and McLeod (2008) report that unconscious perception is easier to measure with forced-choice exclusion tasks when the stimuli are highly similar, such as choosing between the letters 'h' and 'b'. The high degree of stimulus similarity may decrease conscious awareness of the target stimuli while leaving unconscious cognition intact. The present experiments used forced-choice exclusion tasks (i.e., choosing the opposite of a masked target stimulus) with the aim of replicating these findings. No evidence of relevant perception - either conscious or unconscious - was obtained with short duration targets. The forced-choice exclusion task was correctly performed at longer target durations (25 ms and higher), which suggests conscious perception of the target stimuli. We conclude that increasing stimulus similarity does not reliably produce exclusion failure effects and does not appear to facilitate the measurement of unconscious cognition.

A comparison of signal detection theory to the objective threshold/strategic model of unconscious perception.

A key problem in unconscious perception research is ruling out the possibility that weak conscious awareness of stimuli might explain the results. In the present study, signal detection theory was compared with the objective threshold/strategic model as explanations of results for detection and identification sensitivity in a commonly used unconscious perception task. In the task, 64 undergraduate participants detected and identified one of four briefly displayed, visually masked letters. Identification was significantly above baseline (i.e., proportion correct > .25) at the highest detection confidence rating. This result is most consistent with signal detection theory's continuum of sensory states and serves as a possible index of conscious perception. However, there was limited support for the other model in the form of a predicted "looker's inhibition" effect, which produced identification performance that was significantly below baseline. One additional result, an interaction between the target stimulus and type of mask, raised concerns for the generality of unconscious perception effects.

The relationship between the objective identification threshold and priming effects does not provide a definitive boundary between conscious and unconscious perceptual processes.

The Objective Threshold/Strategic Model (OT/S) proposes that strong, qualitative inferences of unconscious perception can be made if the relationship between perceptual sensitivity (typically priming effects) and stimulus visibility is nonlinear and nonmonotonic. The model proposes a nadir in priming effects at the objective identification threshold (identification d'=0). These predictions were tested with masked semantic priming and repetition priming of a lexical decision task. The visibility of the prime stimuli was systematically varied above and below the objective identification threshold. The obtained relationship between prime visibility and priming facilitation was nonlinear, but the results failed to confirm a nadir in priming effects at the objective identification threshold. We conclude that the objective identification threshold does not necessarily indicate the point where presumably unconscious priming effects might be inhibited by conscious cognitive processes.

Exclusion failure does not demonstrate unconscious perception.

Elevations in exclusion error rates (i.e., responding with the target stimulus despite instructions to the contrary) in experiments with masked, briefly presented stimuli have been attributed to unconscious perception. The present studies tested the validity of exclusion methods for studying unconscious perception. Experiment 1 replicated Merikle, Joordens, and Stolz (1995; Experiment 1) by showing more exclusion errors (exclusion failure) for masked word stimuli in a stem completion task. However, this experiment did not replicate the finding of fewer exclusion errors (exclusion success) at long stimulus durations. Experiments 2 and 3 showed that exclusion errors are accompanied by significant discrimination sensitivity to the target stimulus, which suggests conscious perception of the target stimulus. Experiments 3 and 4 demonstrated that the exclusion errors obtained from stem completion tasks depend in part on the exclusion method. Altogether, elevated exclusion failure does not provide an unambiguous demonstration of unconscious perception.

Exclusion failure does not demonstrate unconscious perception II: evidence from a forced-choice exclusion task.

Systematic failure to perform exclusion (making a response that opposes the participant's natural inclinations) for briefly displayed, masked words has been interpreted as evidence of unconscious perception. The present study required participants to make a forced-choice exclusion after viewing masked word targets. The forced-choice exclusion task was properly performed in all experiments, in contrast to previous studies that have utilized stem-completion as a dependent variable. The exclusion failure effects interpreted as unconscious perception in earlier studies appear to be caused by an insensitive dependent variable (stem-completion) rather than unconscious perception.

Polarity effect in electrovibration for tactile display.

Electrovibration is the tactile sensation of an alternating potential between the human body and a smooth conducing surface when the skin slides over the surface and where the current is too small to stimulate sensory nerves directly. It has been proposed as a high-density tactile display method, for example to display pictographic information to persons who are blind. Previous models for the electrovibration transduction mechanism are based on a parallel-plate capacitor in which the electrostatic force is insensitive to polarity. We present experimental data showing that electrovibratory perceptual sensitivity to positive pulses is less than that for negative or biphasic pulses and propose that this disparity may be due to the asymmetric electrical properties of human skin. We furthermore propose using negative pulses for insulated tactile displays based on electrovibration because their sensory thresholds were found to be more stable than for waveforms incorporating positive pulses.

Unconscious perception or not? An evaluation of detection and discrimination as indicators of awareness.

Most investigations of unconscious perception use a dissociation design in which an awareness variable (e.g., detection) is compared with a perceptual processing variable (e.g., identification). Unconscious perception is inferred when the awareness variable lacks sensitivity to the stimulus but evidence of perceptual processing is still obtained. In two studies we examined the relationship between word identification and detection (Study 1) or discrimination (words vs. nonwords; Study 2) with a variety of techniques. In both studies, dissociations suggestive of unconscious perception occurred when the data were examined with subjective threshold approaches, but these differences disappeared when the variables were compared with techniques derived from signal detection theory (SDT). These results do not support unconscious perception in subjective threshold paradigms. In addition, detection appears to be the most sensitive and appropriate task for assessing stimulus awareness, provided that several SDT assumptions are met.

Valid distinctions between conscious and unconscious perception?

In this commentary, we discuss the strengths and limitations of Snodgrass, Bernat, and Shevrin's (2004) theory of unconscious perception. Our commentary centers on the value of signal detection theory (SDT) to understanding the unconscious perception controversy, a value that Snodgrass et al. for the most part agree with (i.e., that most approaches to studying unconscious perception are invalid because they have confounded the criterion for detection with the criterion for awareness). However, we believe that their model relies on a somewhat restricted application of SDT. We discuss how SDTcan be better applied to provide the necessary tests to validate their model.

Pattern identification as a function of stimulation current on a fingertip-scanned electrotactile display.

Two studies were conducted to determine the effect of stimulation current on pattern perception on a 49-point fingertip-scanned electrotactile (electrocutaneous) display. Performance increased monotonically from near chance levels at the lowest sub-threshold current levels tested to approximately 90% at the highest comfortable current levels. This suggests the existence of a tradeoff between spatial performance and usable "gray scale" range in electrotactile presentation of graphical information.

Pattern identification and perceived stimulus quality as a function of stimulation waveform on a fingertip-scanned electrotactile display.

The effect of stimulation waveform on pattern perception was investigated on a 49-point fingertip-scanned electrotactile (electrocutaneous) display. Waveform variables burst frequency (F), number of pulses per burst (NPB), and pulse repetition rate (PRR) were varied in a factorial design. Contrast reduction was used to limit performance of perceiving a 1-tactor gap defined within a 3 x 3 tactor outline square. All three variables accounted for significant variations in performance with higher levels of F and NPB and lower levels of PRR, leading to better performance. In addition, we collected qualitative data on each waveform, and the qualitative differences were related to performance (e.g., waveforms perceived as having a more localized sensation were correlated with better pattern identification performance than those waveforms perceived as more broad). We also investigated the effect of stimulation contrast on pattern perception.