The time course of visual afterimages: data and theory

Sequential viewing of two orthogonally related patterns produces an afterimage of the first pattern (Vidyasagar et al, 1999 Nature 399 422-423; Francis and Rothmayer, 2003 Perception and Psychophysics 65 508-522). We investigated how the timing between the first stimulus (a vertical bar grating) and the second stimulus (a horizontal bar grating) affected the visibility of the afterimage (a perceived vertical grating). As the duration from offset of the first stimulus increased, reports of afterimages decreased. Holding fixed the total time from offset of the first stimulus and increasing the duration from offset of the second stimulus while decreasing the time between the first and second stimuli, caused a decrease in afterimage reports. We interpret this finding in terms of Grossberg's BCS - FCS (boundary contour system--feature contour system) theory. In this theory, the afterimage percept is the result of color complement after-responses in the FCS system interacting with orientation after-responses in the BCS system. The two types of after-responses interact at a stage of neural filling-in to produce the afterimage percept. As the duration between the stimuli increases, the color after-responses weaken so that visible filling-in is less likely to occur. A similar effect occurs for the orientation after-responses but at a faster time scale. Simulations of the model match the experimental data.     

The levels of perceptual processing and the neural correlates of increasing subjective visibility

According to the levels-of-processing hypothesis, transitions from unconscious to conscious perception may depend on stimulus processing level, with more gradual changes for low-level stimuli and more dichotomous changes for high-level stimuli. In an event-related fMRI study we explored this hypothesis using a visual backward masking procedure. Task requirements manipulated level of processing. Participants reported the magnitude of the target digit in the high-level task, its color in the low-level task, and rated subjective visibility of stimuli using the Perceptual Awareness Scale. Intermediate stimulus visibility was reported more frequently in the low-level task, confirming prior behavioral results. Visible targets recruited insulo-fronto-parietal regions in both tasks. Task effects were observed in visual areas, with higher activity in the low-level task across all visibility levels. Thus, the influence of level of processing on conscious perception may be mediated by attentional modulation of activity in regions representing features of consciously experienced stimuli.     

Effects of stimulus probability and visual similarity on stimulus encoding

Visual similarity, stimulus probability, and stimulus contrast were manipulated in two memory-scanning experiments to determine how stimulus probability affects encoding. Two hypotheses were tested: The first, a featural facilitation hypothesis, localizes the effect of stimulus probability on feature extraction; the second claims that stimulus probability has its effect on stimulus recognition. In both experiments, visual similarity was found to slow encoding, particularly under low-contrast conditions. This effect was larger for low probability stimuli than for high probability stimuli and increased as the difference in probability between two visually similar stimuli increased. These results are inconsistent with the featural facilitation hypothesis, but can be explained in terms of differential priming of internal recognition responses for members of the stimulus set, such that more probable members are more easily recognized.     

Letter encoding in visual word identification: more evidence for a word-integration explanation for parafoveal preview effects in reading.

A letter string presented briefly in the parafovea facilitates naming a foveally presented word provided that the two stimuli are orthographically similar. The facilitation (called priming) is asymmetrical in that to obtain it, both letter strings must have the first letters in common. One possible explanation, a letter-integration hypothesis, proposes that readers only identify the letters at the beginning of the parafoveal stimulus, an action that facilitates processing the target. Another explanation, a word-integration hypothesis, postulates that all the letters of the parafoveal stimulus are identified and that the asymmetry occurs because the first letters of the parafoveal stimulus are weighted more heavily than the later ones. The two accounts differ in the way the position of the first letter is determined: The first postulates that readers know the side to identify first without reference to the stimulus; the second postulates that readers establish an order on the stimuli postcategorically. To distinguish the views, we presented English and Hebrew stimuli to bilingual readers. Readers could not anticipate the position of the first letters; hence, if the letter-integration explanation is correct, the asymmetry in the priming should be attenuated. Consistent with the word-integration explanation, however, priming occurred when the target shared the beginning letters with the prime in both languages.     

Immediate and short memory: Recall of simple auditory stimuli

Experiments were carried out on conditions affecting the successful recall of simple, non-verbalised auditory stimuli. The effectiveness of recall of a given stimulus was measured by the ability to assess the pitch of one stimulus as compared to another stimulus presented after an interval of given length. The results indicate that the storage in memory of a simple auditory stimulus is possible, even in the case of pairs of stimuli separated in time by more than 5 min. In all experiments (stimuli differing by 2 semitones or by one semitone, and equal in intensity, or differing by ± 25 dB) the curve of errors shows a sharp increase when the interval between the two stimuli is 80 sec. It is possible that this sudden deterioration in the effectiveness of recall is connected with same alteration of the mechanism of memory. It is postulated that this alteration is due to a “switch-over” from immediate memory to short-term memory. The analysis of the errors shows that in certain circumstances there is a tendency towards a marked preponderance of errors resulting from underestimation rather than from overestimation of the first stimulus. This preponderance is obtained when we have pairs of equal loudness and it is even more marked when the first stimulus is softer than the second, and it is decreases when the first stimulus is louder than the second. These results suggest that in differentiating pitches of stimuli (in the 700–2000 Hz band) presented one after the other at certain fixed intervals of time, we are to find a phenomenon analogous to the classic time error found in estimations of loudness.     

Expectancy and intermittency

When a human subject responds to the second of two closely succeeding stimuli, his reaction time to the second stimulus tends to increase sharply as the interstimulus interval decreases. Controversy has centred on the issue of whether this increase is mainly due to the effects of the first stimulus in producing some kind of block in the central analysing systems or whether it is mainly due to the temporal uncertainty of the second signal, as determined by the distribution of interstimulus intervals used.

By substituting for the first stimulus a spontaneous response on the part of the subject and holding the distribution of interstimulus intervals constant, it is shown that the delays in responding to the succeeding signal are eliminated, even at intervals as short as 50 millisec. This is interpreted as evidence in support of the intermittency hypothesis and as a clear indication that the increase in reaction times normally observed is not a result of the distribution of interstimulus intervals.     

Sub-optimal choice in pigeons does not depend on avoidance of the stimulus associated with the absence of reinforcement

When pigeons are given a choice between two alternatives, one leading to a stimulus 20% of the time that always signals reinforcement (S+) or another stimulus 80% of the time that signals no reinforcement (S−), and the other alternative leading to one of two stimuli each signaling reinforcement 50% of the time, they show a strong preference for the first alternative. This preference occurs in spite of the fact that, overall, the second alternative provides two and a half times more reinforcement than the first. In the present experiment we tested the hypothesis that the S− is a less effective conditioned inhibitor because as soon as it is recognized, the pigeon may orient away from it, whereas it does not orient away from the other signals. To test this hypothesis, for Group HLS−, we made the S− more salient and less avoidable by using a ceiling mounted houselight. To control for a possible aversion to the houselight we included Group HLS+, a group for which the houselight served as the S+. And the preferences of both groups were compared to those of a standard no houselight group. The pigeons in all three groups showed a strong preference for the lower probability of reinforcement alternative. Thus, reduced peripheral orienting during presentation of the S− stimulus was not likely responsible for pigeons’ sub-optimal choice.     

The line-motion illusion can be reversed by motion signals after the line disappears

In the line-motion illusion, a briefly flashed line appears to propagate from the locus of attention, despite being physically presented on the screen all at once. It has been proposed that the illusion reflects low-level visual information processing that occurs faster at the locus of attention (Hikosaka et al 1993 Vision Research 33 1219-1240; Perception 22 517-526). Such an explanation implicitly embeds the assumption that speeding or slowing of neural signals will map directly onto perceptual timing. This 'online' hypothesis presupposes that signals which arrive first are perceived first. However, other evidence suggests that events in a window of time after the disappearance of a visual stimulus can influence the brain's interpretation of that stimulus (Eagleman and Sejnowski 2000 Science 287 2036-2038; 289 1107a; 290 1051a; 2002 Trends in Neuroscience 25 293). If the online hypothesis were true, we should expect that events occurring after the flashing of the line would not change the illusion. Consistent with our hypothesis that awareness is an a posteriori reconstruction, we demonstrate that the perceived direction of illusory line-motion can be reversed by manipulating stimuli after the line has disappeared.     

How personal standards perfectionism and evaluative concerns perfectionism affect the error positivity and post-error behavior with varying stimulus visibility

Error detection is required in order to correct or avoid imperfect behavior. Although error detection is beneficial for some people, for others it might be disturbing. We investigated Gaudreau and Thompson’s (Personality and Individual Differences, 48, 532?537, 2010) model, which combines personal standards perfectionism (PSP) and evaluative concerns perfectionism (ECP). In our electrophysiological study, 43 participants performed a combination of a modified Simon task, an error awareness paradigm, and a masking task with a variation of stimulus onset asynchrony (SOA; 33, 67, and 100 ms). Interestingly, relative to low-ECP participants, high-ECP participants showed a better post-error accuracy (despite a worse classification accuracy) in the high-visibility SOA 100 condition than in the two low-visibility conditions (SOA 33 and SOA 67). Regarding the electrophysiological results, first, we found a positive correlation between ECP and the amplitude of the error positivity (Pe) under conditions of low stimulus visibility. Second, under the condition of high stimulus visibility, we observed a higher Pe amplitude for high-ECP–low-PSP participants than for high-ECP–high-PSP participants. These findings are discussed within the framework of the error-processing avoidance hypothesis of perfectionism (Stahl, Acharki, Kresimon, Völler, & Gibbons, International Journal of Psychophysiology, 97, 153–162, 2015).     

Are conditioned stimuli determinants of oriented exploratory behavior in the rat?

To test Bindra and Palfai's hypothesis about enhancing and inhibitory motivational effects of a conditioned stimulus on exploratory behavior, rats in a first experiment were submitted to pairings of a tone with (a) water, (b) intermittent deliveries of water after a period of constant pairing. In a control condition, the tone was presented alone. The tone was subsequently presented during oriented exploration of a novel stimulus, in the conditioning context. There was no difference in exploratory performance between conditions. In a second experiment with another kind of stimulus change and a larger number of training trials, exploration did not increase when occurring during CS presentation. The lack of facilitation which is possibly the product of competition between exploration and magazine-directed responses, can also be interpreted as indicating that the effects of a conditioned stimulus are reinforcer specific and that the diffuse activity it generates in a situation different from the training one is not equivalent, as concerns the processes involved, to oriented exploratory behavior.     

Perceptual isolation effects: Short-term visual storage vs cognitive-perceptual structure

Two hypotheses concerning the effects of perceptual isolation were examined. Three experimental groups were defined by the interval between the first and second stimulus presentations within a recognition-discrimination problem. Ninety Ss were exposed to 12 recognition-discrimination problems for each of three physical dimensions of random forms, both before and after 1/2 h of perceptual isolation. Analyses of the number of correct responses indicated support for the hypothesis that isolation increases the duration of the short-term visual storage, but not for the hypothesis that isolation produces change in the hierarchy of physical dimensions to which Ss selectively attend.     

Multiple bottlenecks in information processing? An electrophysiological examination

When two stimuli are to be processed in rapid succession, reaction time (RT) to the second stimulus is delayed. The slowing of RT has been attributed to a single processing bottleneck at response selection (RS) or to a central bottleneck following the initiation of the first response. The hypothesis of a response initiation bottleneck is mainly based on reports of underadditive interactions between stimulus onset asynchrony (SOA) and the number of stimulus—response alternatives (simple vs. two-choice response). The present study tested the hypothesis of a response initiation bottleneck by recording the lateralized readiness potential (LRP), a brain wave, emerging during or immediately following RS. The LRP findings were consistent with a central bottleneck but did not support the late bottleneck hypothesis. Instead, the LRP provided direct evidence that the underadditive interaction of number of alternatives and SOA is due to an increase of response anticipations in the simple response condition.     

Stimulus imagery and the effect of response categorization on recall in retroactive inhibition

This study deals with the interaction between characteristics of stimulus-response encoding and the effect of response categorization on recall in an A-B, A-C paradigm. After a discussion of the results of previous experiments by the present author a ‘restricted-constraint hypothesis’ is formulated. It states that category information will be used if the stimulus provides for an amount of constraint that is not sufficient for direct retrieval but still enough for identifying the response after the list as a whole has been retrieved. In the case of a high amount of constraint however, direct retrieval of the response is possible and category cues are bypassed. This hypothesis is tested by varying the Imagery-value of the noun-stimulus. A first experiment is inconclusive since the categorization effect fails to materialize. The second experiment comes out as predicted.     

Uncertainty reduction, conditioned reinforcement, and observing

In a concurrent-chains procedure, pigeons chose between equivalent mixed and multiple fixed-interval schedules of reinforcement. In the first experiment, preference for the multiple schedule was higher when the probability of the shorter fixed interval was less than .50 than for complementary points, an outcome consistent with the delay-reduction hypothesis of conditioned reinforcement and observing, but inconsistent with the uncertainty-reduction hypothesis which requires symmetrical preferences with a maximum when the two intervals are equiprobable. A second experiment assessed preference for equivalent mixed and multiple schedules when each choice outcome resulted in two reinforcements, one on the longer and one on the shorter fixed interval. The order of the two fixed intervals was determined probabilistically. Pigeons again preferred multiple to mixed schedules, although multiple-schedule preference did not vary systematically with the likelihood of the shorter fixed interval occurring first. The results from these choice procedures are consistent with those from the observing-response literature in suggesting that the strength of a stimulus cannot be well described as a function of the degree of uncertainty reduction the stimulus provides about reinforcement.     

Visible persistence following a brief increment in stimulus luminance

In previous studies, it has been demonstrated that visible persistence--the period for which the perceived duration of a stimulus exceeds its physical duration--can be extended by briefly incrementing the luminance of the stimulus immediately prior to offset. Using a two-component pattern integration task, we show that this effect is not an artifact of change in the total luminous flux within the stimulus. Visible persistence was unaffected by overall luminance of the stimulus. It was also time-locked to the luminance increment. Visible persistence is seen to result from a process that is initiated by stimulus onset and that can be either wholly or partially reinitiated by the onset of the luminance increment. The duration of this process (which determines the duration of stimulus visibility) can be modified in a graded fashion by stimulus events that occur after its initiation. We outline a single-process inhibitory feedback model of the persistence mechanism that accounts for the present findings.     

Olfactory blocking and odorant similarity in the honeybee

Blocking occurs when previous training with a stimulus A reduces (blocks) subsequent learning about a stimulus B, when A and B are trained in compound. The question of whether blocking exists in olfactory conditioning of proboscis extension reflex (PER) in honeybees is under debate. The last published accounts on blocking in honeybees state that blocking occurs when odors A and B are similar (the "similarity hypothesis"). We have tested this hypothesis using four odors (1-octanol, 1-nonanol, eugenol, and limonene) chosen on the basis of their chemical and physiological similarity (experiment 1). We established a generalization matrix that measured perceptual similarity. Bees in the "block group" were first trained with an odor A and, in the second phase, with the mixture AB. Bees in the "novel group" (control group) were first trained with an odor N and, in the second phase, with the mixture AB. After conditioning, bees in both groups were tested for their response to B. We assayed all 24 possible combinations for the four odors standing for A, B, and N. We found blocking in four cases, augmentation in two cases, and no difference in 18 cases; odor similarity could not account for these results. We also repeated the experiments with those six odor combinations that gave rise to the similarity hypothesis (experiment 2: 1-hexanol, 1-octanol, geraniol) and found augmentation in one and no effect in five cases. Thus, blocking is not a consistent phenomenon, nor does it depend on odor similarity.     

A new look at the relationship between perceptual and motor responses

A visual stimulus may affect a motor response although its visibility is prevented by a mask. This implies that the sensorimotor system is more susceptible to stimulation than the perceptual system. We report data that are contrary to this intuition. Experiments where both the observer's perceptual state related to the presence/absence of a masked stimulus and the motor behaviour elicited by the same stimulus were jointly assessed on a trial-by-trial basis show that masked visual stimulation at constant visibility (d′) has two types of effect on the motor system. When the physical energy of the masked stimulus is weak, it affects the motor response only if it exceeds the observer's perceptual response criterion. It is only when the physical energy of the masked stimulus is relatively strong that its impact on the motor response is independent of the state of the perceptual system. This indicates that reflex, “nonconscious” behaviour has a high energy threshold.     

Exposure effects and associative learning

The contributions of initial stimulus affect and of associative learning to the effects of repeated stimulus exposures were examined in two experiments. Stimuli that were initially positive and stimuli that were initially negative were presented for different number of times, and subjects rated these stimuli afterward on a number of affective dimensions. In all cases, except when negative affect was associatively paired with every stimulus exposure, affective responses became increasingly more positive with increasing exposures. The results were taken to indicate that the exposure effect can overcome an initially negative stimulus affect when the conditions of the mere exposure hypothesis are satisfied. Initial stimulus affect and associative learning of affect were shown to be independent factors, the first influencing the intercept of the exposure function, the second its slope.     

Invisible Stimuli,Implicit Thresholds: Why Invisibility Judgments Cannot be Interpreted in Isolation

Some studies of unconscious cognition rely on judgments of participants stating that they have “not seen” the critical stimulus (e.g., in a masked-priming experiment). Trials in which participants gave invisibility judgments are then treated as those where the critical stimulus was “subliminal” or “unconscious,” as opposed to trials with higher visibility ratings. Sometimes, only these trials are further analyzed, for instance, for unconscious priming effects. Here I argue that this practice requires implicit assumptions about subjective measures of awareness incompatible with basic models of categorization under uncertainty (e.g., modern signal-detection and threshold theories). Most importantly, it ignores the potential effects of response bias. Instead of taking invisibility judgments literally, they would better be employed in parametric experiments where stimulus visibility is manipulated systematically, not accidentally. This would allow studying qualitative and double dissociations between measures of awareness and of stimulus processing per se.     

负相容效应理论与研究

负相容效应指在掩蔽的箭头启动项后呈现箭头目标项时,对与启动项方向不一致的目标项反应更快。综述了负相容效应的主要理论及研究,包括自我抑制理论和知觉交互作用理论等。讨论了目前负相容效应研究面临的困难与挑战,包括实验控制条件的限制、掩蔽项与启动项特征的相似性以及应用认知神经研究方法进行研究的困难,并从理论更新与整合、神经基础、生态学意义视角等几方面对未来的研究做出展望。