Unsupervised categorization and category learning.

When people categorize a set of items in a certain way they often change their perceptions for these items so that they become more compatible with the learned categorization. In two experiments we examined whether such changes are extensive enough to change the unsupervised categorization for the items-that is, the categorization of the items that is considered more intuitive or natural without any learning. In Experiment 1 we directly employed an unsupervised categorization task; in Experiment 2 we collected similarity ratings for the items and inferred unsupervised categorizations using Pothos and Chater's (2002) model of unsupervised categorization. The unsupervised categorization for the items changed to resemble more the learned one when this was specified by the suppression of a stimulus dimension (both experiments), but less so when it was almost specified by the suppression of a stimulus dimension (Experiment 1, nonsignificant trend in Experiment 2). By contrast, no changes in the unsupervised categorization were observed when participants were taught a classification that was specified by a more fine tuning of the relative salience of the two dimensions.     

Task demands and representation in long-term repetition priming

The effects of task demands on the representation of letter strings in long-term repetition priming (LTRP) were explored in two lexical decision experiments. The stimuli in both experiments were words and nonwords, some presented horizontally and some vertically. The only difference between the two experiments was the response required by the participant. In Experiment 1, the participants pressed one of two buttons, indicating whether or not a given stimulus was a word. In Experiment 2, the participants pressed one of four buttons, indicating both the lexical status and the orientation of a given stimulus. The results were that in Experiment 1, LTRP was not disrupted by a change in stimulus orientation, whereas in Experiment 2 it was, suggesting that the nature of the stimulus representation utilized in LTRP is partially dependent on the demands of the task.     

Strength-based criterion shifts in recognition memory

In manipulations of stimulus strength between lists, a more lenient signal detection criterion is more frequently applied to a weak than to a strong stimulus class. However, with randomly intermixed weak and strong test probes, such acriterion shift often does not result. A procedure that has yieldeddelay-based within-list criterion shifts was applied to strength manipulations in recognition memory for categorized word lists. When participants made semantic ratings about each stimulus word, strength-based criterion shifts emerged regardless of whether words from pairs of categories were studied in separate blocks (Experiment 1) or in intermixed blocks (Experiment 2). In Experiment 3, the criterion shift persisted under the semantic-rating study task, but not under rote memorization. These findings suggest that continually adjusting the recognition decision criterion is cognitively feasible. They provide a technique for manipulating the criterion shift, and they identify competing theoretical accounts of these effects.     

Localizing the first position of a moving stimulus: The Fröhlich effect and an attention-shifting explanation

When subjects are asked to determine where a fast-moving stimulus enters a window, they typically do not localize the stimulus at the edge, but at some later position within that window (Fröhlich effect). We report five experiments that explored this illusion. An attentional account is tested, assuming that the entrance of the stimulus in the window initiates a focus shift toward it. While this shift is under way, the stimulus moves into the window. Because the first phenomenal (i.e., explicitly reportable) representation of the stimulus will not be available before the end of the focus shift, the stimulus is perceived at some later position. In Experiment 1, we established the Fröhlich effect and showed that its size depends on stimulus parameters such as movement speed and movement direction. In Experiments 2 and 3, we examined the influence of eye movements and tested whether the effect changed when the stimuli were presented within a structural background or when they started from different eccentricities. In Experiments 4 and 5, specific predictions from the attentional model were tested: In Experiment 4 we showed that the processing of the moving stimulus benefits from a preceding peripheral cue indicating the starting position of the subsequent movement, which induces a preliminary focus shift to the position where the moving stimulus would appear. As a consequence the Fröhlich effect was reduced. Using a detection task in Experiment 5, we showed that feature information about the moving stimulus is lost when it falls into the critical interval of the attention shift. In conclusion, the present attentional account shows that selection mechanisms are not exclusively space based; rather, they can establish a spatial representation that is also used for perceptual judgment—that is, selection mechanisms can bespace establishing as well.     

Blocking and nonsimultaneous compounds: Comparison of responding during compound conditioning and testing

Three blocking experiments were run using a conditional emotional response procedure with rats as subjects. In each experiment, following initial conditioning to a light stimulus, blocking rats were conditioned to a compound in which a short-duration (30-second) noise was superimposed on the terminal portion of the longer-duration light. In Experiment 1, the light was always three minutes in duration; in Experiment 2, the light was switched from a constant (three-minute) to a variable (0.5–5.0 minute) duration at the start of compound conditioning; in Experiment 3, the light was variable in duration throughout conditioning. Control rats received the same compound conditioning experiences as the blocking rats but did not receive prior conditioning to the light by itself. These temporal manipulations had strong and systematic influence on the rats’ pattern of responding during compound conditioning. Most notably, experience with a constantduration light resulted in both blocking and compound animals showing little conditional suppression to the early conditional stimulus portion of light by itself, followed by strong conditional suppression to the terminal 30 seconds of light plus noise. When testing was done with the noise in isolation subsequent to compound conditioning, however, an equally strong blocking effect was obtained across all the temporal manipulations. In all cases, the blocking animals showed much less conditional suppression to the noise than did the compound control animals. This lack of correspondence between the rats’ responding during compound conditioning and their response to the noise by itself is theoretically puzzling. The confounding effect of inhibition of delay may provide at least a partial explanation of this pattern of results.     

Object memory and change detection: dissociation as a function of visual and conceptual similarity

People often fail to detect a change between two visual scenes, a phenomenon referred to as change blindness. This study investigates how a post-change object's similarity to the pre-change object influences memory of the pre-change object and affects change detection. The results of Experiment 1 showed that similarity lowered detection sensitivity but did not affect the speed of identifying the pre-change object, suggesting that similarity between the pre- and post-change objects does not degrade the pre-change representation. Identification speed for the pre-change object was faster than naming the new object regardless of detection accuracy. Similarity also decreased detection sensitivity in Experiment 2 but improved the recognition of the pre-change object under both correct detection and detection failure. The similarity effect on recognition was greatly reduced when 20% of each pre-change stimulus was masked by random dots in Experiment 3. Together the results suggest that the level of pre-change representation under detection failure is equivalent to the level under correct detection and that the pre-change representation is almost complete. Similarity lowers detection sensitivity but improves explicit access in recognition. Dissociation arises between recognition and change detection as the two judgments rely on the match-to-mismatch signal and mismatch-to-match signal, respectively.     

Measuring the search image: expectation,detection, and recognition in pigeon visual search

Pigeons searched computer screens for a grating target that varied in spatial frequency and orientation. In Experiment 1, a pretrial cue signaled that a particular target would appear. The cue speeded response to this primed target and to targets of the same orientation and similar spatial frequency to the primed target. However, the cue slowed search for targets differing from the primed target in orientation. In Experiment 2, pecks to a single target were reinforced. Responding generalized somewhat to targets of spatial frequency similar to the primed target and to targets of either orientation. Thus, stimulus aspects that affect detection may differ from those that affect recognition; if a common representation serves both, it functions differently in the 2 cases.     

The influence of visual symmetry on the encoding of objects

Two experiments examined how symmetry affects the visual encoding of simple flat objects (lamellae). Experiment 1 showed that subjects encode a lamella's shape as both a facsimile and its enantiomorph. In Experiment 2 the errors made when responding to symmetrical and asymmetrical stimuli showed that there were three factors affecting the reproduction of a symmetrical stimulus. These are, in order of decreasing influence, (i) the orientation of the axis of symmetry, (ii) the presence of overall symmetry (irrespective of orientation), and (iii) identity of the orientation of the elements of the response in relation to those of the stimulus. They also show that symmetry is likely to be reproduced even where the reproduction is not of the same shape as the original stimulus. The relevance of these results to representation of objects is discussed.     

The integration of orientation information in the motion correspondence problem

We examine how differently oriented components contribute to the discrimination of motion direction along a horizontal axis. Stimuli were two-frame random-dot kinematograms that were narrowband filtered in spatial frequency. On each trial, subjects had to state whether motion was to the left or the right. For each stimulus condition, Dmax (the largest displacement supporting 80% correct direction discrimination performance) was measured. In experiment 1, Dmax was measured for orientationally narrowband stimuli as a function of their mean orientation. Dmax was found to increase as the orientation of the stimuli became closer to the axis of motion. Experiment 2 used isotropic stimuli in which some orientation bands contained a coherent motion signal, and some contained only noise. When the noise band started at vertical orientations and increased until only horizontal orientations contained a coherent motion signal, Dmax increased slightly. This suggests that near-vertical orientations interfere with motion perception at large displacements when they contain a coherent motion signal. When the noise band started at horizontal and increased until only vertical orientations contained the motion signal, Dmax decreased steadily. This implies that Dmax depends at least partly on the most horizontal motion signal in the stimulus. These results were contrasted with two models. In the first, the visual system utilises the most informative orientations (nearest horizontal). In the second, all available orientations are used equally. Results supported an intermediate interpretation, in which all orientations are used but more informative ones are weighted more heavily.     

Conditioned suppression, punishment, and aversion

Three experiments were conducted to assess the aversive properties of a visual stimulus in the presence of which one group of birds received response-contingent shock (discriminated punishment) while a yoked group of birds received non-contingent shocks (conditioned suppression). In Experiment 1, presentation of the visual stimulus contingent on key pecking reduced the response rate (conditioned punishment effect) for birds under the conditioned suppression procedure but did not reduce the response rate of birds under the discriminative punishment procedure. Non-contingent shocks also produced greater suppression of responding maintained by positive reinforcement in the presence of a visual stimulus than did response-contingent shocks. In Experiment 2, a greater shock intensity (2 mA) was used. All the differences between the two groups found in Experiment 1 were also found in Experiment 2. Experiment 3 demonstrated that response-contingent shock did not result in a conditioned punishment effect even when positive reinforcers were unavailable during the discriminative punishment schedule. The exteroceptive stimulus that was paired with shock in the conditioned suppression procedure acquired the ability to punish behavior. The exteroceptive stimulus in the discriminative punishment schedule did not acquire this ability.     

Verbal representation in task order control: An examination with transition and task cues in random task switching

Recent task-switching studies in which a predictable task sequence has been used have indicated that verbal representation contributes to the control of task order information. The present study focused on the role of verbal representation in sequential task decisions, which are an important part of task order control, and examined the effects of articulatory suppression in a random-task-cuing paradigm with two different types of cues presented just before the presentation of a stimulus: a transition cue and a task cue. The former cue provided information only about switching or repeating the task, whereas the latter was associated directly with the identity of the task (i.e., indicating a parity or a magnitude task). In Experiment 1, in which transition cues guided task sequences, articulatory suppression impaired performance in both repetition and switch trials, thereby increasing the mixing costs. In Experiment 2, in which a task cue, rather than a transition cue, was presented to examine the influence of a cue-decoding process, articulatory suppression had no specific effect on task performance. Experiment 3, in which the transition cue and the task cue were randomly presented in the same block to equalize the memory load and task strategy for the two types of cues, confirmed that articulatory suppression significantly increased the mixing costs only in transition cue trials. The results from the three experiments indicated that the use of verbal representation is effective in sequential task decision—that is, in selecting a task set on the basis of transient task order information in both repetition and switch trials.     

Experiments on the Fehrer–Raab effect and the ‘Weather Station Model’ of visual backward masking

The Fehrer–Raab effect (simple reaction time is unaffected by metacontrast masking of the test stimulus) seems to imply that a stimulus can trigger a voluntary reaction without reaching a conscious representation. However, it is also possible that the mask triggers the reaction, and that the masked test stimulus causes a focussing of attention from which processing of the mask profits, thus reaching conscious representation earlier. This is predicted by the Weather Station Model of visual masking. Three experiments tested this explanation. Experiment 1 showed that the masked test stimulus caused a temporal shift of the mask. Experiment 2 showed that the reaction in the Fehrer–Raab effect was not exclusively triggered by a conscious representation of the test stimulus: the mask was involved in evoking the reaction. Experiment 3 again revealed a temporal shift of the mask. However, the shift was only about half as large as the Fehrer–Raab effect. The psychometric functions suggested that the observers used two different cues for their temporal order judgments. The results cast doubts on whether judged temporal order yields a direct estimate of the time of conscious perception. Some methodological alternatives are discussed.     

First impressions are lasting impressions: A primacy effect in memory for repetitions

Two experiments demonstrated that the encoding of a repeated object is biased toward the attributes of its first presentation. In Experiment 1, subjects saw objects five times each, but either the first presentation or the fifth presentation was the mirror reverse of the standard orientation seen on the other four trials. When recognition was tested with both orientations simultaneously, subjects reported seeing only the single mirror-reverse orientation more often if it was the first presentation than when it was the fifth presentation, and seeing only the standard orientation more often if it was presentations 1–4 than when it was presentations 2–5. A second experiment demonstrated that this primacy effect generalized to size changes. This pattern of results is consistent with the hypothesis that top-down biases affect what subjects learn: The first representation established for a stimulus is likely to influence the encoding of subsequent repetitions.     

无意识信息加工中的“完型”——无意识捆绑假说的新证据

采用连续闪烁范式(Continuous Flash Suppression,CFS)考察了无意识状态下完型能否发生.研究通过2(线条完整性)×2(意义破坏)×2(熟悉性)的被试内实验设计考察了8种条件下图片的突破抑制时间(Suppression Time,ST).结果发现:线条完整和意义未破坏的图片能更快地克服噪音图片的抑制进入意识;熟悉性不影响突破抑制时间.重要的是,不完整的图片也可以出现意义破坏效应.这表明即使在被抑制的状态下,不完整图形的信息也能够传达至与完型相关的高级视觉区域形成客体表征.研究为无意识捆绑假说(The Unconscious Binding Hypothesis)提供了新的实验证据.     

Training and testing determinants of short-term associative suppression of phototaxic behavior in Hermissenda

Hermissenda crassicornis shows both short- and long-term retention of conditioning following light-rotation pairings. Previous research has shown that prolonged training (50 trials per session, three consecutive daily sessions) produces a suppression of phototaxis lasting for days. This long-term retention reflects associative learning processes, with little or no contribution of nonassociative learning. In contrast, both associative learning and nonassociative behavioral modification contribute to short-term retention following a single session of five pairing trials. In this paper, we describe important associative and nonassociative determinants of short-term changes in phototaxis. In Experiment 1, animals received successive hourly tests for phototaxis in either a horizontal or a vertical orientation. Repeated testing resulted in decreased phototaxis which was especially pronounced for animals tested horizontally. Experiments 2-4 demonstrated that both repeated handling of animals and repeated periods of dark adaptation prior to each phototaxic test contributed to the development of phototaxic suppression with repeated testing. These nonassociative influences on phototaxis interacted with the gravitational orientation employed during behavioral testing, being most pronounced for testing in the horizontal orientation. An implication of these findings is that attempts to demonstrate short-term pairing-specific suppression will be most successful when nonassociative contributions are minimized (by testing animals vertically). Experiment 5 tested this prediction. We also tested the influence of training light intensity and the stimulus specificity of conditioned suppression. Animals received either five paired or five random presentations of light and rotation. Training light intensity was either moderate or bright. Following training, animals were tested for either suppression of phototaxis or suppression of negative geotaxis, using either a horizontal or a vertical testing orientation. Consistent with previous results, horizontally tested animals exhibited pronounced nonassociative suppression following training. The use of a bright training light also produced nonassociative suppression. However, when trained with a light of moderate intensity and tested vertically, Hermissenda showed associative suppression of phototaxis (significant paired--random difference) but not geotaxis (no paired--random difference). In a final experiment we observed that the longer the period of dark adaptation (prior to testing) the longer the phototaxic latency.(ABSTRACT TRUNCATED AT 400 WORDS)     

Stimulus specificity in avoidance learning

The present experiments address the issue of stimulus specificity in fear and avoidance learning. First, it was established that light stimuli are effective warning signals (WS) in shuttle avoidance. Then light stimuli were shown to produce conditioned suppression in the conditioned emotional response situation comparable to that produced by noise conditioned stimuli. Finally, the effectiveness of noise onset and noise offset as feedback signals was tested. This was assessed under conditions of immediate and delayed termination of a light WS. Delayed termination of a light WS interfered with avoidance learning and the introduction of noise offset as a feedback signal enhanced it. The only demonstration of stimulus specificity was the failure of noise onset to function as a feedback signal.     

Relative durations of conditioned stimulus and intertrial interval in conditioned suppression.

The effects of the relative durations of the conditional stimulus and the intertrial interval on bar pressing during a conditioned-suppression procedure were examined as a function of two additional variables--type of operant baseline schedule and rate of shock presentation. In Experiment 1, response suppression was compared across components of a multiple fixed-ratio, random-ratio, fixed-interval, random-interval schedule, at relative conditioned-stimulus/intertrial-interval durations of 1/1, 1/4, and 1/9. In Experiment 2, relative conditioned-stimulus/intertrial-interval duration (1/5, 3/3, or 5/1) was manipulated across groups, while shock frequency (2, 6, or 10 shocks/hr) was manipulated within groups. In both experiments, suppression during the signal was virtually complete at all relative durations. Responding was also suppressed during the intertrial interval, but that suppression varied as a function of experimental manipulations. In Experiment 1, intertrial-interval response rates were higher when relative signal duration was 1/9 than when it was 1/1, although both relative signal duration and shock frequency, which covaried, could have contributed to the difference. In Experiment 2, the patterning of response rates between successive shocks was affected by relative duration, absolute rates during the intertrial interval varied as a function of shock frequency, and differences between suppression during the signal and suppression during the intertrial interval were affected by both relative duration and shock frequency. The data support an analysis based upon relationships between shock-correlated and intertrial-interval stimuli and, as assessed by the relative-delay-to-reinforcement metric, are comparable to results that have been reported from experiments using similar manipulations under the autoshaping paradigm.     

What happens to an individual visual working memory representation when it is interrupted?

This study tested the hypothesis that even the simplest cognitive tasks require the storage of information in working memory (WM), distorting any information that was previously stored in WM. Experiment 1 tested this hypothesis by requiring observers to perform a simple letter discrimination task while they were holding a single orientation in WM. We predicted that performing the task on the interposed letter stimulus would cause the orientation memory to become less precise and more categorical compared to when the letter was absent or when it was present but could be ignored. This prediction was confirmed. Experiment 2 tested the modality specificity of this effect by replacing the visual letter discrimination task with an auditory pitch discrimination task. Unlike the interposed visual stimulus, the interposed auditory stimulus produced little or no disruption of WM, consistent with the use of modality‐specific representations. Thus, performing a simple visual discrimination task, but not a simple auditory discrimination task, distorts information about a single feature being maintained in visual WM. We suggest that the interposed task eliminates information stored within the focus of attention, leaving behind a WM representation outside the focus of attention that is relatively imprecise and categorical.     

Hemispheric asymmetries for temporal information processing: transient detection versus sustained monitoring

This study investigated functional differences in the processing of visual temporal information between the left and right hemispheres (LH and RH). Participants indicated whether or not a checkerboard pattern contained a temporal gap lasting between 10 and 40 ms. When the stimulus contained a temporal signal (i.e. a gap), responses were more accurate for the right visual field-left hemisphere (RVF-LH) than for the left visual field-right hemisphere (LVF-RH). This RVF-LH advantage was larger for the shorter gap durations (Experiments 1 and 2), suggesting that the LH has finer temporal resolution than the RH, and is efficient for transient detection. In contrast, for noise trials (i.e. trial without temporal signals), there was a LVF-RH advantage. This LVF-RH advantage was observed when the entire stimulus duration was long (240 ms, Experiment 1), but was eliminated when the duration was short (120 ms, Experiment 2). In Experiment 3, where the gap was placed toward the end of the stimulus presentation, a LVF-RH advantage was found for noise trials whereas the RVF-LH advantage was eliminated for signal trials. It is likely that participants needed to monitor the stimulus for a longer period of time when the gap was absent (i.e. noise trials) or was placed toward the end of the presentation. The RH may therefore be more efficient in the sustained monitoring of visual temporal information whereas the LH is more efficient for transient detection.     

Effect of classical conditioning on an internal clock

Six experiments with rats used a psychophysical choice procedure to study the internal clock used to discriminate duration. They investigated if the clock is sensitive to the signal value (associative strength) of a stimulus. The experiments involved two types of trials. On choice trials, a stimulus lasted a short (e.g., 3 s) or long (e.g., 12 s) duration; then the rats chose between two levers. The rewarded choice depended on the duration of the stimulus. On conditioning trials, the stimulus used on choice trials was presented, but it ended without food (extinction trials) or with food (pairing trials) regardless of what the rat did. The main stimulus minus accuracy with the long stimulus. Experiment 1 showed that extinction trials increased short bias relative to training without conditioning trials or to training with pairing trials. The rest of the experiments tested explanations of these results. The same results were found when extinction trials were the same duration as the short stimulus (Experiment 2), when extinction trials were a random duration (Experiment 5), and when the signal value of the conditioned stimulus was changed in another way (Experiment 6). The effect of conditioning trials was modality specific (Experiments 3 and 4). Of the explanations considered, the best one--the only one not contradicted at least once--is that changing the signal value of a stimulus changes how the clock times the stimulus. Reducing signal value reduces the measured duration.