Intramodal and crossmodal pairing and anchoring in comparisons of successive stimuli

Two experiments were conducted to study effects of modality, temporal position, and their interaction on comparisons of successive stimuli. In Experiment 1, intramodal (tone–tone and line–line) and crossmodal (tone–line and line–tone) stimulus pairs, with two interstimulus intervals (ISIs), 400 and 2,000 ms, were presented. Participants indicated which stimulus was the “stronger.” Time-order errors (TOEs) were assessed using the D% measure and were found in all types of pairs. Variation in TOEs across conditions was well accounted for by changes in parameters (stimulus weights, reference levels) in an extended version of Hellström’s sensation weighting (SW) model. With an ISI of 2,000 ms, the first stimulus had a lower weight (less impact on the response) than did the second stimulus. More negative TOEs were found with the longer ISI in all pair types except tone–line. In Experiment 2, participants indicated which of two lines was the longer or which of two tones was the louder. An intra- or crossmodal anchor, or no anchor, was interpolated between the stimuli. Anchoring tended to reduce the weight of the first stimulus, suggesting interference with memory, and to yield negative TOEs. Intramodal anchors yielded reduced weights of both stimuli, most dramatically for tones, suggesting an additional effect of stimulus interference. Response times decreased with crossmodal anchors. For line–line pairs, strong negative TOEs were found. In both experiments, the variation in TOE across conditions was well accounted for by the SW model.     

Processing of haptic texture information over sequential exploration movements

Where textures are defined by repetitive small spatial structures, exploration covering a greater extent will lead to signal repetition. We investigated how sensory estimates derived from these signals are integrated. In Experiment 1, participants stroked with the index finger one to eight times across two virtual gratings. Half of the participants discriminated according to ridge amplitude, the other half according to ridge spatial period. In both tasks, just noticeable differences (JNDs) decreased with an increasing number of strokes. Those gains from additional exploration were more than three times smaller than predicted for optimal observers who have access to equally reliable, and therefore equally weighted, estimates for the entire exploration. We assume that the sequential nature of the exploration leads to memory decay of sensory estimates. Thus, participants compare an overall estimate of the first stimulus, which is affected by memory decay, to stroke-specific estimates during the exploration of the second stimulus. This was tested in Experiments 2 and 3. The spatial period of one stroke across either the first or second of two sequentially presented gratings was slightly discrepant from periods in all other strokes. This allowed calculating weights of stroke-specific estimates in the overall percept. As predicted, weights were approximately equal for all strokes in the first stimulus, while weights decreased during the exploration of the second stimulus. A quantitative Kalman filter model of our assumptions was consistent with the data. Hence, our results support an optimal integration model for sequential information given that memory decay affects comparison processes.     

Time error in lifted weights as affected by presentation order and judgment mode

In the method of constant stimulus differences, results obtained under two presentation orders (standard first and standard second) are commonly combined to eliminate time error. The present study shows that this technique is not adequate for controlling time error as the presentation orders have differential effects upon PSE. Using a two-handed technique, 96 subjects judged lifted weights on the comparative rating scale. The subjects lifted the standard before, with, or after the variable stimulus. They also either judged the standard in terms of the variable or the variable in terms of the standard (mode of judgment). Judgment mode, a previously uninvestigated confounding variable of presentation order was not an important factor in influencing the value of the PSE.     

Dissipation of retroactive interference in human infants

In 3 experiments with 85 human 3-month-olds, the authors asked whether retroactive interference with their memory of the original training stimulus is temporary or permanent. Infants learned to move a mobile by kicking and then were exposed to a different mobile (Experiment 1) or context (Experiment 2) immediately or 3 days afterward (Experiment 3). They were tested after increasing delays with the original stimulus, the exposed stimulus, or a completely novel stimulus. Retroactive interference was temporary and unrelated to the exposure delay. The data are consistent with a retrieval-based account of interference. Memory updating (i.e., responding to the interfering stimulus) was coincident with retroactive interference, suggesting that retroactive interference is an adaptive mechanism that facilitates memory updating within a narrow time window.     

Overcoming interference: an fMRI investigation of pattern separation in the medial temporal lobe

The medial temporal lobe (MTL) supports the formation and retrieval of long-term declarative memories, or memories for facts and everyday events. One challenge posed for this type of memory stems from the highly overlapping nature of common episodes. Within cognitive psychology, it is widely accepted that interference between information learned at different times is a major limitation on memory. In spite of several decades of intense research in the fields of interference theory and the neurobiological underpinnings of declarative memory, there is little direct evidence bearing on how the MTL resolves this interference to form accurate memories of everyday facts and events. Computational models of MTL function have proposed a mechanism in which the MTL, specifically the hippocampus, performs pattern separation, whereby overlapping representations are made less similar. However, there is little evidence bearing on how this process is carried out in the intact human MTL. Using high-resolution fMRI, we conducted a set of experiments that taxed behavioral pattern separation by using highly similar, interfering stimuli in a modified continuous recognition task. Regions within the parahippocampal gyrus demonstrated activity consistent with a "recall to reject" strategy. In contrast and critical to performing the task, activity within the hippocampus distinguished between correctly identified true stimulus repetitions, correctly rejected presentations of similar lure stimuli, and false alarms to similar lures. These data support the computational models' assertion that the hippocampus plays a key role in pattern separation.     

Perceptual orientational asymmetries: A comparison of visual and haptic space

Stimulus orientation discrimination was investigated in visual and haptic modalities under conditions of simultaneous matching and memory. Discrimination of vertical and horizontal was significantly more accurate than discrimination of oblique stimulus orientations (45°, 135°, 225°, and 315°) for both modalities; haptic errors, however, were significantly greater at each orientation. While subjects were reliably more accurate in visually matching oblique stimulus orientations to a standard than producing them from memory, for the haptic modality, differences between memory and matching conditions were less consistent across the orientations sampled.     

Power Function Forgetting Curves as an Emergent Property of Biologically Plausible Neural Network Models

Empirical forgetting curve data have been shown to follow a power function. In contrast, many connectionist models predict either an exponential decay or flat forgetting curves. This paper simulates power function forgetting curves in a Hopfield network modified to incorporate the more biologically realistic assumptions of bounded weights and a distribution of learning rates. The modified model produces power function forgetting curves. The bounded weights introduce exponential decay for individual weights, and a power function forgetting curve when summing exponential decays with different learning rates. Because these assumptions are biologically reasonable, power function forgetting curves may be an emergent property of biological networks. The results fit empirical data and indicate that forgetting curves restrict possible implementation of models of memory.     

P300 stimulus sequence effects in children and adults

Immediate memory capability for 12 children (8 years) and 12 adults (21 years) was assessed electrophysiologically by using P300 event-related brain potential (ERP) stimulus sequences. These were derived from an auditory discrimination paradigm in which participants detected target stimuli in a series of target (T) and standard (S) tones that were varied by randomly presenting one of four sequence patterns (SSSS, TTTT, TTTS, SSST). Short-term memory capability was assessed behaviorally by recall performance of a 20-word list. Children and adults showed virtually identical P300 amplitude sequence patterns, such that for both groups component size increased systematically as the discrepancy between standard and target stimuli increased across sequence patterns. P300 latency evinced similar, albeit weaker, stimulus sequence effects, with children having longer component peak latencies. Memory recall performance was substantially weaker for the children than for adult participants, especially for recency effects. The findings suggest that immediate memory for stimulus sequences is fully developed in young children, although long-term memory is not.     

A feature-segmentation model of short-term visual memory

A feature-segmentation model of short-term visual memory (STVM) for contours is proposed. Memory of the First stimulus is maintained until the second stimulus is observed. Three processes interact to determine the relationship between stimulus and response: feature encoding, memory, and decision. Basic assumptions of the model are twofold: (i) the STVM system divides a contour into convex parts at regions of concavity; and (ii) the value of each convex part represented in STVM is an independent Gaussian random variable. Simulation showed that the five-parameter fits give a good account of the effects of the four experimental variables. The model provides evidence that: (i) contours are successfully encoded within 0.5 s exposure, regardless of pattern complexity; (ii) memory noise increases as a linear function of retention interval; (iii) the capacity of STVM, defined by pattern complexity (the degree that a pattern can be handled for several seconds with little loss), is about 4 convex parts; and (iv) the confusability contributing to the decision process is a primary factor in deteriorating recognition of complex figures. It is concluded that visually presented patterns can be retained in STVM with considerable precision for prolonged periods of time, though some loss of precision is inevitable.     

Subjective ratios and differences in perceived heaviness

In previous studies, judgments of ratios and differences in subjective magnitude have yielded similar orders, consistent with a hypothesis that a single perceived relation underlies both judgment tasks. In the present research, 15 subjects estimated heaviness differences between 28 pairs of eight weights and each of 8 groups of 10 subjects evaluated heaviness ratios of eight variable stimuli with respect to a different standard stimulus. Presenting stimuli that were equally spaced on a cube-root scale of weight enhanced expected ordinal discrepancies between ratio and difference estimates, and employing independent groups for each standard stimulus in ratio estimation eliminated a possible bias due to varying standards within the presentation sequence. Differences in orders of ratio and difference estimates together with differences in scales obtained from non-metric analyses in terms of a difference model indicated that the judgments were based on two perceived relations that are ordinally consistent with arithmetic operations of ratios and differences. A ratio scale of heaviness was derived from the combined orders of subjective ratios and differences.     

Timing the “magical number seven”: Presentation rate and regularity affect verbal working memory performance

The informative value of time and temporal structure often remains neglected in cognitive assessments. However, next to information about stimulus identity we can exploit temporal ordering principles, such as regularity, periodicity, or grouping to generate predictions about the timing of future events. Such predictions may improve cognitive performance by optimising adaptation to dynamic stimuli. Here, we investigated the influence of temporal structure on verbal working memory by assessing immediate recall performance for aurally presented digit sequences (forward digit span) as a function of standard (1000 ms stimulus-onset-asynchronies, SOAs), short (700 ms), long (1300 ms) and mixed (700–1300 ms) stimulus timing during the presentation phase. Participant's digit spans were lower for short and mixed SOA presentation relative to standard SOAs. This confirms an impact of temporal structure on the classic “magical number seven,” suggesting that working memory performance can in part be regulated through the systematic application of temporal ordering principles.     

The frequency principle for comparative judgments)

This experiment supplements a recetlf study of the effects of an interpolated weight upon memory for a standard (Parducci, Marshall, & Degner, 1966). The earlier data suggested that the stimulus-averaging approach as represented by the theory of adaptation level should be modified to incorporate the principle that Ss use alternative categories with equal frequency. The present data support this modification by demonstrating that even when the stimulus mean is held constant, judgments are affected by the relative frequencies of the comparison stimuli.     

Choice in the time-left procedure and in concurrent chains with a time-left terminal link.

In two experiments, rats chose between a standard fixed-duration food-associated stimulus and a stimulus whose duration was the time remaining to reinforcement in an elapsing comparison interval. In Experiment 1, 4 rats responded in a time-left procedure wherein a single initial-link variable-interval schedule set up two potential terminal links simultaneously. As time elapsed in the initial-link schedule, the choice was between a standard fixed-interval 30-s terminal link and a time-left terminal link whose programmed interval requirement equaled 90 s minus the elapsed time in the initial link. Rats generally responded more on the lever with the shortest programmed terminal-link duration, but the temporal parameters of the procedure were found to vary with response distributions. Contrary to previous reports, therefore, time-left data were well predicted by choice models that make no assumptions about animal timing. In Experiment 2, 8 rats responded on a concurrent-chains schedule with independent variable-interval initial links and a time-left terminal link in one of the choice schedules. On the time-left lever, the programmed terminal-link delay equaled 90 s minus the elapsed time in the time-left initial link. On the standard lever, terminal-link responses were reinforced according to a variable-interval schedule whose average value varied over four conditions. Relative time-left initial-link responses increased in the elapsing time-left initial-link schedule as the time-left terminal link became shorter relative to the standard terminal link. Scalar expectancy theory failed to predict the resultant data, but a modified version of the delay-reduction model made good predictions. An analysis of the elaboration of scalar expectancy theory for variable delays demonstrated that the model is poorly formulated for arithmetically distributed delays.     

The dynamics of perceptual learning: an incremental reweighting model

The mechanisms of perceptual learning are analyzed theoretically, probed in an orientation-discrimination experiment involving a novel nonstationary context manipulation, and instantiated in a detailed computational model. Two hypotheses are examined: modification of early cortical representations versus task-specific selective reweighting. Representation modification seems neither functionally necessary nor implied by the available psychophysical and physiological evidence. Computer simulations and mathematical analyses demonstrate the functional and empirical adequacy of selective reweighting as a perceptual learning mechanism. The stimulus images are processed by standard orientation- and frequency-tuned representational units, divisively normalized. Learning occurs only in the "read-out" connections to a decision unit; the stimulus representations never change. An incremental Hebbian rule tracks the task-dependent predictive value of each unit, thereby improving the signal-to-noise ratio of their weighted combination. Each abrupt change in the environmental statistics induces a switch cost in the learning curves as the system temporarily works with suboptimal weights.     

Measuring and modeling attentional dwell time

Attentional dwell time (AD) defines our inability to perceive spatially separate events when they occur in rapid succession. In the standard AD paradigm, subjects should identify two target stimuli presented briefly at different peripheral locations with a varied stimulus onset asynchrony (SOA). The AD effect is seen as a long-lasting impediment in reporting the second target, culminating at SOAs of 200?C500?ms. Here, we present the first quantitative computational model of the effect??a theory of temporal visual attention. The model is based on the neural theory of visual attention (Bundesen, Habekost, & Kyllingsb?k, Psychological Review, 112, 291?C328 2005) and introduces the novel assumption that a stimulus retained in visual short-term memory takes up visual processing-resources used to encode stimuli into memory. Resources are thus locked and cannot process subsequent stimuli until the stimulus in memory has been recoded, which explains the long-lasting AD effect. The model is used to explain results from two experiments providing detailed individual data from both a standard AD paradigm and an extension with varied exposure duration of the target stimuli. Finally, we discuss new predictions by the model.     

Similarity effects in backward recognition masking

Auditory backward recognition masking refers to the ability of a masking sound to terminate further perceptual resolution of a test sound presented slightly earlier in time. The present experiments were conducted to determine whether mask/test tone similarity effects in backward recognition masking could be reliably demonstrated. Although similarity effects were found in Experiments 1 and 2, only about 60% of the subjects demonstrated these effects. Experiment 3 was designed to isolate which stage of information processing is responsible for similarity effects. It was hypothesized that similarity effects are due to mask interference with the synthesized auditory memory of the test tone rather than to selective overwriting of a preperceptual auditory store: previous research has shown that interference in synthesized auditory memory depends on the similarity of the interfering stimulus to the items held in memory. By independently varying the backward masking interval and the interfering effect of the mask on the test tone memory, it was possible to demonstrate that similarity effects are indeed caused by mask interference in synthesized memory. The implications of these results are considered in the framework of auditory and visual masking.     

Using pavlovian higher-order conditioning paradigms to investigate the neural substrates of emotional learning and memory

In first-order Pavlovian conditioning, learning is acquired by pairing a conditioned stimulus (CS) with an intrinsically motivating unconditioned stimulus (US; e.g., food or shock). In higher-order Pavlovian conditioning (sensory preconditioning and second-order conditioning), the CS is paired with a stimulus that has motivational value that is acquired rather than intrinsic. This review describes some of the ways higher-order conditioning paradigms can be used to elucidate substrates of learning and memory, primarily focusing on fear conditioning. First-order conditioning, second-order conditioning, and sensory preconditioning allow for the controlled demonstration of three distinct forms of memory, the neural substrates of which can thus be analyzed. Higher-order conditioning phenomena allow one to distinguish more precisely between processes involved in transmission of sensory or motor information and processes involved in the plasticity underlying learning. Finally, higher-order conditioning paradigms may also allow one to distinguish between processes involved in behavioral expression of memory retrieval versus processes involved in memory retrieval itself.     

A within-subjects analysis of the relationship between memory span and processing rate in short-term memory

A within-subjects analysis of the relationship between memory span and processing rate for four stimulus classes in short-term memory indicated that while the two factors are correlated when averaged data is considered, as shown by Cavanagh (Psychological Review, 1972, 79, 525–530), the strength of the relationship for individual subjects varies from zero to unity. The intersubject correlations for each stimulus class were not consistent with the view that a unified model may be advanced to account for performance in recall and recognition tasks in shortterm memory.     

The long-term retention of time: A developmental study

Two experiments investigated the age-related changes in long-term retention of duration and their effects on time judgement. Children aged 3, 5, and 8 years old were given a temporal bisection task with or without a 15-min interfering task (Experiment 1), or a retention delay lasting for 0 min, 15 min, or 24 hr (Experiment 2) between the presentation of the standard durations and the comparison stimulus durations. An interfering task and the increase of the retention delay significantly decreased the time sensitivity in the 3- and the 5-year-olds, and to a greater extent in the younger children, but had no effect in the 8-year-olds. This decrease in time sensitivity with the interfering task or the retention delay might be due to an increase in the variability of the remembered duration.     

Effects of fimbria lesions on trace two-way active avoidance acquisition and retention in rats

The fimbria-fornix (FF) is the main subcortical input to the hippocampus. It has been shown that FF lesions facilitate performance on a standard-delay two-way active avoidance task (AA2), thought to involve implicit memory. The hippocampal region is required for explicit or relational memory. It has been proposed that the hippocampus and related structures might associate events that are separated in space or time and detect elements shared in common by such discontiguous episodes. Therefore, FF lesions would be expected to impair performance on a trace paradigm, which introduces an interval between the CS (conditioned stimulus) and the US (unconditioned stimulus) and is generally considered a model of explicit memory. We predicted that FF lesions would impair memory in a trace AA2 procedure, while the same lesions would facilitate memory in a standard delay version of the task. To test this hypothesis, two experiments were carried out in 102 male Wistar rats. The first experiment characterized the trace paradigm using this kind of conditioning and demonstrated that control rats were able to acquire and retrieve (24 h and 11 days postacquisition) the association between the CS (tone) and the US (electric foot shock) when a trace interval (5, 10, or 20 s) was interposed between both stimuli. In the second experiment, we investigated the effects of FF electrolytic lesions on the same task using delay and trace (10-s trace interval) paradigms. Surprisingly, FF lesions facilitated the acquisition and the 24-h retention of the AA2 not only on the standard delay paradigm, but also with the trace paradigm. We suggest that facilitative effects could be a result of impairment in contextual learning.