Threshold variability in subliminal perception experiments: fixed threshold estimates reduce power to detect subliminal effects.

In studies of subliminal perception, a threshold for awareness is usually estimated for each S (e.g., the stimulus duration that yields chance discrimination performance). This article shows that if the S's true threshold varies randomly from trial to trial, estimation of a fixed threshold has important consequences. Specifically, the estimated threshold approaches the minimum of the distribution of true thresholds, and the power of an experiment to detect subliminal perception is reduced. Because there is much evidence that true thresholds vary across trials, threshold variability may have seriously weakened previous experiments that tested for subliminal perception. Fortunately, it is possible to estimate the maximum power loss resulting from threshold variability and therefore to design an experiment with acceptable power even in the presence of variability.     

Orientation discrimination across the visual field: size estimates near contrast threshold

Performance in detection and discrimination tasks can often be made equal across the visual field through appropriate stimulus scaling. The parameter E2 is used to characterize the rate at which stimulus dimensions (e.g., size or contrast) must increase in order to achieve foveal levels of performance. We calculated both size and contrast E2 values for orientation discrimination using a spatial scaling procedure that involves measuring combination size and contrast thresholds for stimuli with constant size-to-contrast ratios. E2 values for size scaling were 5.77 degrees and 5.92 degrees. These values are three to four times larger than those recovered previously using similar stimuli at contrasts well above detection threshold (Sally & Gurnsey, 2003). E2 values for contrast scaling were 324.2 degrees and 44.3 degrees, indicating that for large stimuli little contrast scaling (.3% to 2.3% increase) was required in order to equate performance in the fovea and the largest eccentricity (10 degrees). A similar pattern of results was found using a spatial scaling method that involves measuring contrast thresholds for target identification as a function of size across eccentricities. We conclude that the size scaling for orientation discrimination at near-threshold stimulus contrasts is much larger than that required at suprathreshold contrasts. This may arise, at least in part, from contrast-dependent changes in mechanisms that subserve task performance.     

Effect of stimulus duration on stereo and vernier displacement thresholds

Stereo and vernier thresholds were determined as a function of stimulus duration in both misalignment and displacement paradigms. All four thresholds increase as stimulus duration decreases. The increase is greatest for the stereo displacement threshold, less for the vernier displacement threshold, and least for the misalignment thresholds. The results are consistent with the hypothesis that depth signals are averaged over time.     

A Truncated-Probit Item Response Model for Estimating Psychophysical Thresholds

Human abilities in perceptual domains have conventionally been described with reference to a threshold that may be defined as the maximum amount of stimulation which leads to baseline performance. Traditional psychometric links, such as the probit, logit, and t, are incompatible with a threshold as there are no true scores corresponding to baseline performance. We introduce a truncated probit link for modeling thresholds and develop a two-parameter IRT model based on this link. The model is Bayesian and analysis is performed with MCMC sampling. Through simulation, we show that the model provides for accurate measurement of performance with thresholds. The model is applied to a digit-classification experiment in which digits are briefly flashed and then subsequently masked. Using parameter estimates from the model, individuals’ thresholds for flashed-digit discrimination is estimated.     

SUBLIMINAL GRADATION IN THE VISUAL FIELD

K ragh , U. Subliminal gradation in the visual field. Scand. J. Psychol , 1962, 3 , 165–170.—The difference of thresholds for correct recognition of a central figure and of a peripheral figure in the same picture, presented by the tachisto-scope, is referred to two factors: foveal vs . extrafoveal stimulation, and threat towards the central figure (in terms of 'perceptual defense'). Differences of size of the figures, again, do not effect 'subliminal gradation'. The analysis is considered relevant for the comparison between two tachistoscopic techniques: one in which one subliminal stimulus is exposed immediately before a liminal one, and the other with only one subliminal stimulus.     

Semantic priming without awareness: Some methodological considerations and replications

Previous studies of subliminal semantic priming effects are considered. The distinction between subjective and objective detection thresholds proposed by Cheesman and Merikle (1984) is accepted, and it is concluded that no convincing evidence has been adduced to support or deny the existence of semantic priming effects at or below objective threshold. New criteria for the assessment of objective thresholds are proposed, and four experiments are reported in each of which pattern-masked primes were presented 10% below objective threshold. The first three experiments presented primes, mask, and targets binocularly; the fourth used dichoptic presentation. Data were analysed using both conventional F ratios and the quasi F ratio advocated by Clark (1973). Results showed that significant subliminal semantic priming effects occurred in each experiment when conventional F ratios were used, and significant effects occurred in all but the first experiment when quasi F was used. Binocular and dichoptic presentation of stimuli gave semantic priming effects whose magnitudes did not differ significantly. It is concluded that semantic priming effects can be obtained below objective detection threshold. It is also concluded that replication with different participants and different stimuli is preferable to the use of quasi F ratios in investigations where verbal stimuli are used.     

Subliminal understanding of negation: Unconscious control by subliminal processing of word pairs

A series of five experiments investigated the extent of subliminal processing of negation. Participants were presented with a subliminal instruction to either pick or not pick an accompanying noun, followed by a choice of two nouns. By employing subjective measures to determine individual thresholds of subliminal priming, the results of these studies indicated that participants were able to identify the correct noun of the pair – even when the correct noun was specified by negation. Furthermore, using a grey-scale contrast method of masking, Experiment 5 confirmed that these priming effects were evidenced in the absence of partial awareness, and without the effect being attributed to the retrieval of stimulus–response links established during conscious rehearsal.     

The reliability of an extrapolated measure of an aversive threshold of pain from radiant stimulation

The relationship between the logarithm of pain threshold latency (sec) and the logarithm of the stimulus intensity (mcal/cm²/sec) has been shown to be linear, and the extrapolated x-intercept value of the linear equation could be a useful measure in pain research. In order to assess the reliability of the x-intercept, pain latency thresholds, were obtained from 23 Ss by utilizing the D’Amour-Smith modification of the dolorimetric technique and by testing under a variety of conditions at each of four stimulus intensities. The empirically determined log t (sec) values were shown to be slightly, but significantly, higher when stimulation was applied to either the dominant limb or to the legs rather than to the nondominant limb or to the arms. The computed slope and x-intercept values did not differ between limbs or between dominant and nondominant sides. None of the three measures changed significantly between the two sessions. The x-intercept value was interpreted as an index of a threshold of aversion and its meaning and applicability were discussed.     

Interaction of prime and target in the subliminal affective priming effect

It has been found that an emotional stimulus such as a facial expression presented subliminally can affect subsequent information processing and behavior, usually by shifting evaluation of a subsequent stimulus to a valence congruent with the previous stimulus. This phenomenon is called subliminal affective priming. The present study was conducted to replicate and expand previous findings by investigating interaction of primes and targets in the affective priming effect. Two conditions were used. Prime (subliminal presentation 35 msec.) of an angry face of a woman and a No Prime control condition. Just after presentation of the prime, an ambiguous angry face or an emotionally neutral face was presented above the threshold of awareness (500 msec.). 12 female undergraduate women judged categories of facial expressions (Anger, Neutral, or Happiness) for the target faces. Analysis indicated that the Anger primes significantly facilitated judgment of anger for the ambiguous angry faces; however, the priming effect of the Anger primes was not observed for neutral faces. Consequently, the present finding suggested that a subliminal affective priming effect should be more prominent when affective valence of primes and targets is congruent.     

On determining pain thresholds using the limiting method

A model developed by Cane (1956) for the study of relative thresholds is modified to apply to absolute thresholds. It is assumed that the fluctuation in the subject's sensation of a stimulus is uniformly distributed, and the model is used to estimate the threshold from data presented by several authors.     

The effect of warning interval on the electric phosphene and auditory thresholds

Using different warning signals and threshold stimuli, the thresholds, as determined by a method of limits, were found to rise monotonically as the interval between warning signal and threshold stimulus increased from I to 9 sec. It was found that the variability of the threshold did not increase as the threshold increased. Similar results were obtained for phosphene and auditory thresholds and with visual and auditory warnings; therefore the effect was considered to be central. Motokawa's finding of a minimum in the phosphene threshold 2 sec. after a flash of white light was not repeated. The rise in threshold was not obtained when the warning intervals were randomized and so seemed to depend on the use of fixed warning intervals. A model was developed relating threshold level to accuracy of anticipation of the end of the warning interval.     

Musical duplex perception: perception of figurally good chords with subliminal distinguishing tones.

In a variant of duplex perception with speech, phoneme perception is maintained when distinguishing components are presented below intensities required for separate detection, forming the basis for the claim that a phonetic module takes precedence over nonspeech processing. This finding is replicated with music chords (C major and minor) created by mixing a piano fifth with a sinusoidal distinguishing tone (E or E flat). Individual threshold intensities for detecting E or E flat in the context of the fixed piano tones are established. Chord discrimination thresholds defined by distinguishing tone intensity were determined. Experiment 2 verified masked detection thresholds and subliminal chord identification for experienced musicians. Accurate chord perception was maintained at distinguishing tone intensities nearly 20 dB below the threshold for separate detection. Speech and music findings are argued to demonstrate general perceptual principles.     

Rhesus monkey (Macaca mulatta) cool sensitivity measured by a signal detection method

Two rhesus monkeys were trained to detect cool stimuli (decrements in skin temperature from the adapted temperature) presented to the palm of the left hand after the skin had been preadapted to a 33°C temperature. The procedure was analogous to the signal detection “yes-no” method used with human observers. Receiver operating characteristic curves were generated by varying the a priori probability of stimulus oecurrence. The proportion of hits and of false alarms increased with increases in the a priori probability of stimulation. The points of isosensitivity of both subjects for cool intensities of 0.3°C, but not at greater intensities, yielded straight lines with slopes of approximately one when plotted on normal-normal coordinates. The values of the d’_s detectability index decreased from 3.52 to 0.75 for one subject and from 2.81 1to 1.28 for the other as the stimulus intensity was decreased from 0.8° to 0.15° cooling from the adapting temperature. Cool thresholds for each subject of 0.19° and 0.12°C were computed from classical psychometric functions when threshold was defined as 50% hits at a 50% probability of stimulus occurrence. These thresholds are comparable to published reports of the cool threshold for the rhesus.     

Subliminal or not? Comparing null-hypothesis and Bayesian methods for testing subliminal priming

A difficulty for reports of subliminal priming is demonstrating that participants who actually perceived the prime are not driving the priming effects. There are two conventional methods for testing this. One is to test whether a direct measure of stimulus perception is not significantly above chance on a group level. The other is to use regression to test if an indirect measure of stimulus processing is significantly above zero when the direct measure is at chance. Here we simulated samples in which we assumed that only participants who perceived the primes were primed by it. Conventional analyses applied to these samples had a very large error rate of falsely supporting subliminal priming. Calculating a Bayes factor for the samples very seldom falsely supported subliminal priming. We conclude that conventional tests are not reliable diagnostics of subliminal priming. Instead, we recommend that experimenters calculate a Bayes factor when investigating subliminal priming.     

Frequency selectivity for hearing-impaired and broadband-noise-masked normal listeners

When evaluating frequency selectivity, it is difficult to determine if the deviation from normal performance observed for hearing-impaired listeners reflects abnormal cochlear function or normal, level-dependent changes in frequency selectivity. This experiment was designed to investigate the dependence of frequency-selectivity measures on threshold and signal level. Auditory-filter characteristics, critical ratios, forward-masked psychophysical tuning curves, and narrowband-noise masking patterns were obtained from normal-hearing listeners in quiet and in the presence of broadband noise at five levels. These findings were then compared to previously obtained frequency-selectivity measurements for subjects with mild-to-moderate sensorineural hearing loss whose absolute thresholds are comparable to the normal-hearing listeners' masked thresholds. The results suggest that frequency selectivity deterioriates as threshold increases for all subjects. However, the poor frequency selectivity exhibited by hearing-impaired listeners may not be explained entirely by the effects of threshold and signal level. Nevertheless, because frequency selectivity is poorer in the normal auditory system at higher stimulus levels, the deviation from normal performance observed for hearing-impaired listeners may not be as large as previously suggested.     

阈下启动信息加工的心理机制

无意识如何影响人类的认知？或者说,无意识知觉在什么水平对人类的认知产生作用。由于方法和标准的限制,对上述问题的争论依然存在。当前对无意识知觉的研究主要集中在阈下启动的加工机制上,即启动刺激如何有效引发被试对目标刺激的加工。研究者提出知觉加工,中枢加工,反应加工以及行为触发等理论来解释阈下启动的心理加工机制。     

SUBLIMINAL EXPOSURE TO DEATH-RELATED STIMULI INCREASES DEFENSE OF THE CULTURAL WORLDVIEW

Abstract— Three experiments reported here provide empirical support for the hypothesis derived from terror management theory that unconscious concerns about death motivate allegiance to cultural beliefs, Study 1, contrasted exposure to a subliminal death-related stimulus, a standard mortality-salience treatment, and a neutral subliminal stimulus, and found that both the subliminal and the standard reminder of mortality led to more favorable evaluations of people who praised subjects' cultural worldview and more unfavorable evaluations of those who challenged it Study 2, replicated this finding by comparing the effects of exposure to subliminal death stimuli and subliminal pain stimuli. Study 3, contrasted subliminal death stimuli, supraliminal death stimuli, and subliminal pain stimuli and found that only subliminal death stimuli produced these effects.     

Visual long-term memory for spatial frequency?

It has been suggested that a visual long-term memory based on a sensory representation of the stimulus accounts for discrimination performance when the reference and the test stimuli are separated in time. Decision processes involved in setting response criteria, however, may also contribute to discrimination performance. In the present study, it is shown that under proper control, spatial frequency discrimination thresholds from a group of observers, each performing on a single trial, are significantly higher for a 2-h than for a 5-sec retention interval, whereas thresholds from individual observers performing in repeated trials with a 2-h retention interval are considerably lower. The results suggest that discrimination performance may depend on the retention of task-relevant information, such as a response criterion, rather than on visual memory of the stimulus. It is concluded that it is risky to postulate ahigh-fidelity long-term visual memory for spatial frequency on the basis of psychophysical group discrimination thresholds.     

Recognising what you like: Examining the relation between the mere-exposure effect and recognition

The perceptual fluency/attributional model of the mere-exposure effect proposed by R. F. Bornstein and P. D'Agostino (1992) predicts that when recognition of a previously presented stimulus is above chance, feelings of fluency associated with that stimulus are discounted and thus the amount of fluency (mis)attributed to liking is reduced. This correction process results in smaller mere-exposure effects for supraliminal stimuli than for “subliminal” stimuli because when recognition is below chance participants are unaware of the source of fluency and they do not engage in correction. We tested this prediction in three experiments by presenting photographs of faces (Experiments 1 and 2) and polygons (Experiment 3) at varying exposure frequencies for 40 ms and 400 ms durations. Contrary to the prediction of the model, a significant mere-exposure effect was only found when recognition performance was at its highest level. Furthermore, across the three experiments liking and recognition were positively correlated.