Startle reflex inhibition in the rat: its persistence after extended repetition of the inhibitory stimulus

Startle reflexes to intense sound bursts are inhibited by weak stimuli that briefly precede their elicitation. In three experiments the startle stimulus (a 110-dB SPL tone burst) was presented 100 ms after the final link in a train of stimuli, the length of the train varying from 1 to 1,000, its repetition rate varying from 1 per s to 10 per s, and its constituents being 40 dB or 50 dB white noise bursts of 25 ms duration. Inhibition was invariant across train length and repetition rate. In a final experiment the startle stimulus was presented a variable interval after the final link, from 40 ms to 1280 ms, with 1 or 100 noise bursts (50 dB) in the train. Inhibition developed more rapidly following the last member of the 100-stimulus train, suggestive of a "priming" or sensitization effect of stimulus repetition, but its overall strength and subsequent rate of decay were not different in the two conditions. The general persistence of inhibition following these extended series of stimuli reveals that reflex inhibition must be the outcome of a fixed and obligatory process associated with sensory input.     

Effects of pattern, spatial frequency, number, and rate of stimulus presentation on the accuracy of detection.

Brief trains of pulsed stimuli were used to assess whether magnocellular or parvocellular visual pathways could be differentiated perceptually. Trains of either one to four sine-wave, square-wave, or checkerboard gratings were presented at three temporal and two spatial frequencies to six observers. The task of the observer was to report the perceived number of stimuli (gratings) in a train. The difference between actual number and perceived number of gratings was recorded as an error score. It was found that neither the pattern nor the spatial frequency of the gratings significantly affected perceptual accuracy. On the other hand, the number of gratings in a train and the interstimulus interval between gratings produced significant differences. Perceptual accuracy was greater when lower numbers of gratings in a train were presented with longer interstimulus intervals. The observers typically reported fewer stimuli than were presented. The source of the discrepancy is discussed in terms of a light adaptive process initiated in the retina.     

Simultaneous auditory discrimination.

Stimuli in many visual stimulus control studies typically are presented simultaneously; in contrast the stimuli in auditory discrimination studies are presented successively. Many everyday auditory stimuli that control responding occur simultaneously. This suggests that simultaneous auditory discriminations should be readily acquired. The purpose of the present experiment was to train rats in a simultaneous auditory discrimination. The apparatus consisted of a cage with two response levers mounted on one wall and a speaker mounted adjacent to each lever. A feeder was mounted on the opposite wall. In a go-right/go-left procedure, two stimuli were presented on each trial, a wide-band noise burst through one speaker and a 2-kHz complex signal through the other. The stimuli alternated randomly from side to side across trials, and the stimulus correlated with reinforcement for presses varied across subjects. The rats acquired the discrimination in 400 to 700 trials, and no response position preference developed during acquisition. The ease with which the simultaneous discrimination was acquired suggests that procedures, such as matching to sample, that require simultaneous presentation of stimuli can be used with auditory stimuli in animals having poor vision.     

Better discrimination of small changes in commonly encountered than in less commonly encountered auditory stimuli

Results from 3 auditory tasks revealed that small changes made in stimuli commonly encountered in everyday life are more easily discriminated than are the same changes made in stimuli not as commonly encountered. The tasks required discrimination of a frequency difference in 1 tone of 6-tone chords or nonchords, discrimination of a duration difference in 1 note of common tunes or nontunes, and discrimination of the deletion of a band of frequencies from speech sounds played forward or backward. Different crews of college-aged listeners served in the different tasks. If future research shows this difference in discriminability to be a general property of commonly encountered stimuli--attributable to a difference in the way they are processed in the nervous system--then discrimination tests of this sort could become useful for assessing whether stimuli have made the transition from one form of processing to the other.     

Languages for Gestalts of line patterns

Any formal model of visual Gestalt perception requires a language for representing possible perceptual structures of visual stimuli, as well as a decision criterion that selects the actually perceived structure of a stimulus among its possible alternatives. This paper discusses an existing model of visual Gestalt perception that is based on Structural Information Theory. We investigate two factors that determine the representational power of this model: the domain of visual stimuli that can be analyzed, and the class of perceptual structures that can be generated for these stimuli. We show that the representational power of the existing model of Structural Information Theory is limited, and that some of the generated structures are perceptually inadequate. We argue that these limitations do not imply the implausibility of the underlying ideas of Structural Information Theory and introduce alternative models based on the same ideas. For each of these models, the domain of visual stimuli that can be analyzed properly is formally defined. We show that the models are conservative modifications of the original model of Structural Information Theory: for cases that are adequately analyzed in the original model of Structural Information Theory, they yield the same results.     

Visual cortical evoked potentials under conditions of sequential blanking

Two separate experiments were conducted to study the visual evoked potential (VEP) correlates of sequential blanking, a phenomenon in which up to one-half of a discrete train of stimulus inputs are not perceived for certain orders of input. A PDP-7 digital computer was used to present stimuli (from 2 to 5 letter Xs) on a CRT display in both experiments. In Experiment 1 there were four basic conditions designed and counterbalanced to indicate the nature of the VEP when stimuli were blanked and when Ss reported all stimuli. All of the stimuli were of equal intensity. The main finding was that although the eight Ss did not perceive and report blanked stimuli, they did respond to them physiologically as indicated by the VEP. The implications of these findings were discussed in relation to recent studies of visual masking and metacontrast in which VEP was recorded. In Experiment 2 the normally blanked and the normally blanking stimuli were alternately increased in intensity to determine the effect on sequential blanking and the VEP. Six Ss were tested under three basic counterbalanced conditions. It was found that sequential blanking could be reliably overcome by increasing the intensity of the normally blanked stimuli. In addition, when the normally blanking stimuli were of greater intensity than the blanked stimuli, not only did perceptual suppression occur, but the evidence indicated that there was no VEP to the first of the two blanked stimuli.     

Can visual cognitive neuroscience learn anything from the philosophy of language? Ambiguity and the topology of neural network models of multistable perception

The Necker cube and the productive class of related stimuli involving multiple depth interpretations driven by corner-like line junctions are often taken to be ambiguous. This idea is normally taken to be as little in need of defense as the claim that the Necker cube gives rise to multiple distinct percepts. In the philosophy of language, it is taken to be a substantive question whether a stimulus that affords multiple interpretations is a case of ambiguity. If we take into account what have been identified as hallmark features of ambiguity and look at the empirical record, it appears that the Necker cube and related stimuli are not ambiguous. I argue that this raises problems for extant models of multistable perception in cognitive neuroscience insofar as they are purported to apply to these stimuli. Helpfully, similar considerations also yield reasons to suggest that the relevant models are well motivated for other instances of multistable perception. However, a different breed of model seems to be required for the Necker cube and related stimuli. I end with a sketch how one may go about designing such a model relying on oscillatory patters in neural firing. I suggest that distinctions normally confined to the philosophy of language are important for the study of perception, a perspective with a growing number of adherents.     

A behavioral frequency discrimination paradigm for use in adult primates

A psychophysical apparatus and procedure that can be used to define the performance and estimate the thresholds of frequency discrimination in adult primates is described. This protocol has been used to train adult owl monkeys to discriminate differences in the frequencies of successively presented sinusoidal tactile stimuli delivered to a restricted skin surface of a single digit, and to train a second set of adult owl monkeys to discriminate. differences in frequencies between paired auditory tonal stimuli. The thresholds estimated with this procedure are comparable to those measured in other primates. The advantages of using this procedure for combined behavioral and physiological studies are discussed.     

Constrained multidimensional unfolding of confusion matrices: Goal point and slide vector models

Two constrained multidimensional unfolding models, the goal point and slide vector models, are proposed for analyzing confusion matrices. In both models, the row and column stimuli are expressed as two sets of points in a low-dimensional space, where the difference vector connecting a column point to the corresponding row point indicates the change in the stimulus representation through a cognitive process. The difference vector is constrained by the hypothesis that the trend in the representational change is invariant across stimuli: the goal point model constrains all difference vectors to point toward a single point, and the slide vector model constrains all difference vectors to be parallel to each other. In both models the coordinates of points are estimated by the maximum-likelihood method. Examples illustrate that the two models allow us to examine hypotheses about invariant trends in representational changes and to grasp such trends from the resulting configurations.     

Division of labor between the hemispheres for complex but not simple tasks: an implemented connectionist model

When stimuli have to be matched in a complex task (such as whether 2 letters have the same name), then performance is better when stimuli are presented across the hemispheres of the brain, whereas for simpler tasks (such as whether 2 letters have the same shape), better performance is achieved when stimuli are presented unilaterally. The authors show that this bilateral distribution advantage effect emerged spontaneously in a neural network model learning to solve simple and complex tasks with separate input layers and separate, but interconnected, resources in a hidden layer. The authors show that relating computational models to behavioral and imaging data proves fruitful for understanding hemispheric processing and generating testable hypotheses.     

Sequence effects in the categorization of tones varying in frequency

In contrast to exemplar and decision-bound categorization models, the memory and contrast models described here do not assume that long-term representations of stimulus magnitudes are available. Instead, stimuli are assumed to be categorized using only their differences from a few recent stimuli. To test this alternative, the authors examined sequential effects in a binary categorization of 10 tones varying in frequency. Stimuli up to 2 trials back in the sequence had a significant effect on the response to the current stimulus. The effects of previous stimuli interacted with one another. A memory and contrast model, according to which only ordinal information about the differences between the current stimulus and recent preceding stimuli is used, best accounted for these data.     

Effects of stimulus order on discrimination processes in comparative and equality judgements: Data and models

In typical discrimination experiments, participants are presented with a constant standard and a variable comparison stimulus and their task is to judge which of these two stimuli is larger (comparative judgement). In these experiments, discrimination sensitivity depends on the temporal order of these stimuli (Type B effect) and is usually higher when the standard precedes rather than follows the comparison. Here, we outline how two models of stimulus discrimination can account for the Type B effect, namely the weighted difference model (or basic Sensation Weighting model) and the Internal Reference Model. For both models, the predicted psychometric functions for comparative judgements as well as for equality judgements, in which participants indicate whether they perceived the two stimuli to be equal or not equal, are derived and it is shown that the models also predict a Type B effect for equality judgements. In the empirical part, the models' predictions are evaluated. To this end, participants performed a duration discrimination task with comparative judgements and with equality judgements. In line with the models' predictions, a Type B effect was observed for both judgement types. In addition, a time-order error, as indicated by shifts of the psychometric functions, and differences in response times were observed only for the equality judgement. Since both models entail distinct additional predictions, it seems worthwhile for future research to unite the two models into one conceptual framework.     

A Propositional Model of Implicit Evaluation

Implicit evaluation can be defined as the automatic effect of stimuli on evaluative responses. A major advantage of this definition is that it is neutral with regard to the mental processes and representations that mediate implicit evaluation. Whereas many existing models postulate that implicit evaluation is mediated by the automatic spreading of activation along associations in memory, it is also possible to entertain the idea that implicit evaluation is due to the automatic formation or activation of propositions. In line with such a propositional model of implicit evaluation, evidence suggests that implicit evaluation (a) can be based on instructions and inferences, (b) is sensitive to information about how stimuli are related and (c) can reflect several propositions that differ only with regard to how stimuli are related. Although it might be difficult to differentiate between propositional models on the one hand and association–activation or dual process models on the other hand, merely considering the idea that implicit evaluation might be mediated by propositions offers a new perspective on existing findings and leads to novel predictions about the conditions under which implicit evaluation occurs.     

Testing Evidence Accrual Models by Manipulating Stimulus Onset

Many models of response time assume that subjects accrue stimulus "evidence" samples in time (e.g., random walk models, counter models). In this paper, the concept of one stimulus dominating another is used to construct a test of the whole class of evidence accrual models. For an example of dominance, consider stimuli that are presented either virtually instantaneously (stepped) or in a gradually increasing manner (ramped). Ramped stimuli are presented such that the ramped portion precedes the stepped onset of stepped stimuli. In this case ramped stimuli dominate stepped stimuli. In this paper the class of evidence accrual models is formalized. It is shown that under appropriate assumptions evidence accrual models do predict more accurate responses to dominating stimuli. However, this result does not hold for response latencies. There are anomalous cases where an evidence accrual model (the accumulator model of Vickers (1970, Ergonomics 13, 37-58)) predicts slower mean correct response latencies to dominating stimuli. It is shown through extensive computer simulation that these anomalous cases occur only when response criteria are so asymmetric that there are exceedingly extreme response biases. For experiments where response biases are not exceedingly extreme, random walk and accumulator models predict more accurate and quicker correct responses to dominating stimuli. In sum, manipulating the time course of stimuli in accordance with the concept of dominance can provide empirical tests of the class of evidence accrual models. Copyright 2001 Academic Press.     

Establishing stimulus equivalences among retarded adolescents.

Two separate sets of stimuli were established as stimulus classes in a match-to-sample task by using each stimulus as both a sample and choice stimulus for every other member within that set. After classes were established, the subjects were taught to select one member of each class when an associated auditory stimulus was presented. After this training, three of six subjects were able to select the remaining members of each stimulus class in response to the appropriate auditory stimuli without direct training. The other three subjects also demonstrated transfer after additional members of each stimulus class were brought under the control of the auditory stimuli through direct training.     

Serial recall of colors: Two models of memory for serial order applied to continuous visual stimuli

This study investigated the effects of serial position and temporal distinctiveness on serial recall of simple visual stimuli. Participants observed lists of five colors presented at varying, unpredictably ordered interitem intervals, and their task was to reproduce the colors in their order of presentation by selecting colors on a continuous-response scale. To control for the possibility of verbal labeling, articulatory suppression was required in one of two experimental sessions. The predictions were derived through simulation from two computational models of serial recall: SIMPLE represents the class of temporal-distinctiveness models, whereas SOB-CS represents event-based models. According to temporal-distinctiveness models, items that are temporally isolated within a list are recalled more accurately than items that are temporally crowded. In contrast, event-based models assume that the time intervals between items do not affect recall performance per se, although free time following an item can improve memory for that item because of extended time for the encoding. The experimental and the simulated data were fit to an interference measurement model to measure the tendency to confuse items with other items nearby on the list—the locality constraint—in people as well as in the models. The continuous-reproduction performance showed a pronounced primacy effect with no recency, as well as some evidence for transpositions obeying the locality constraint. Though not entirely conclusive, this evidence favors event-based models over a role for temporal distinctiveness. There was also a strong detrimental effect of articulatory suppression, suggesting that verbal codes can be used to support serial-order memory of simple visual stimuli.     

Evaluative conditioning of high-novelty stimuli does not seem to be based on an automatic form of associative learning

Evaluative conditioning (EC) refers to the change in valence of initially neutral stimuli (conditioned stimuli, or CSs) as a result of their pairing with positive or negative stimuli (unconditioned stimuli, or USs). EC is critical to dual-attitudes models as it is often presented as an evaluative effect that results from a purely automatic form of associative learning. Although evidence suggests that EC does not occur without contingency awareness, Ruys, and Stapel (2009) recently argued that contingency awareness is unnecessary for high-novelty stimuli. Researchers may thus be tempted to conclude that EC rests on an automatic form of associative learning, at least for CSs associated with little prior evaluative knowledge. Taking issue with this claim, the present study reveals that EC of high-novelty stimuli is dependent on attentional resources. The role of contingency awareness in EC of high-novelty stimuli is also discussed.     

Newborn temperament and Sudden Infant Death Syndrome: A comparison of victims and their cotwins

Temperament assessed in the newborn period was compared for four twin infants who subsequently died of Sudden Infant Death Syndrome (SIDS) and their surviving cotwins. The assessment focused on irritability, resistance to soothing, reactivity, reinforcement value, activity while awake, and activity during sleep. No differences were observed between cotwins on a combined irritability rating. SIDS victims, however, were less irritable than their siblings in response to aversive stimuli. When irritable, the SIDS infants were more difficult to soothe than their cotwins. While awake, SIDS infants were less active than their cotwins, but during sleep, no activity differences were observed. The SIDS infants were more reactive to orienting stimuli, and neither group was found to be more or less rewarding to the examiner. It was concluded that this study has helped to define behavioral variables that might be characteristic of infants whose subsequent deaths are attributed to SIDS.