Effects of similarity and experience on discrimination learning: A nonassociative connectionist model of perceptual learning.

This article describes a novel connectionist model of perceptual learning (PL) that provides a mechanism for nonassociative differentiation (J. J. Gibson & E. J. Gibson, 1955). The model begins with the assumption that 2 processes—1 that decreases associability and 1 that increases discriminability—operate during preexposure (S. Channell & G. Hall, 1981). In contrast to other models (e.g., I. P. L. McLaren, H. Kaye, & N. J. Mackintosh, 1989), in the current model the mechanisms for these processes are compatible with a configural model of associative learning. A set of simulations demonstrates that the present model can account for critical PL phenomena such as exposure learning and effects of similarity on discrimination. It is also shown that the model can explain the paradoxical result that preexposure to stimuli can either facilitate or impair subsequent discrimination learning. Predictions made by the model are discussed in relation to extant theories of PL. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Detection, discrimination and sensation of visceral stimuli

Afferent visceral signals serve mostly regulatory functions and produce vague, diffuse body sensations, which are not well reflected in experimental paradigms requiring distinct and localized 'visceral perceptions'. A series of studies is summarized which compared behavioral discrimination of gastrointestinal distension stimuli with subjective sensation ratings in relation to basic perceptual operations such as detection, localization, graduation, and identification. Results showed that detection of gastrointestinal stimuli is possible without the subject's awareness (discrimination without reportable sensation). This process does not depend on stimulus intensity at volumes below those sufficient for subjective sensation. The latter is necessary, however, for intensity discrimination (graduation). Contrary to exteroception (e.g. vision), conscious subjective sensation is also required for stimulus localization. These differences in processing could not be explained by simple differences in response criteria. An intensity-dependent two-process model of gastrointestinal interoception is derived which differs from standard models of somatosensory information processing.     

A model for the early stages of motion processing based on spatial and temporal edge detection by X-cells

A model for the early stages of motion processing in the visual cortex is presented. The 'building block' for this model is the 'rebound response', which is the neuronal response evoked when a sufficient inhibitory stimulus is turned off. This response enables detection of temporal changes when the stimulus involves spatial changes. The model suggests that adjacent subunits in primary cortical cells have different weight functions for rebound responses, and thus a synergistic type of response is evoked in the preferred direction, which is predicted for both light and dark stimuli. Predictions of the model for different stimuli and receptive field structures are discussed. It appears to be more economical than previous motion models.     

Red-green and achromatic temporal filters: a ratio model predicts contrast-dependent speed perception

We simultaneously measured detection and identification performance by using isoluminant red-green (RG) and achromatic flickering stimuli and fitted these data with a modified line-element model that does not make high-threshold assumptions. The modeling shows that detection and identification data are adequately described by postulating only two underlying temporal filters each for RG and achromatic vision, even when more than two threshold classifications are evident. We use a spatial frequency of 1.5 cycles per degree (c/deg) and compare the derived temporal impulse response functions with those obtained previously with the use of 0.25 c/deg stimuli under otherwise identical conditions [J. Opt. Soc. Am. A 13, 1969 (1996)]. We find that at 1.5 c/deg the luminance impulse response functions peak later and integrate out to longer times compared with those measured at 0.25 c/deg. For RG stimuli, although their relative overall sensitivities change, the impulse response functions are similar across spatial frequency, indicating a constancy of chromatic temporal properties across spatial scales. In a second experiment, we measured RG and achromatic flicker discrimination over a wide range of suprathreshold contrasts. These data suggest a common nonlinear contrast response function operating after initial temporal filtering. Using a ratio model of speed perception in which both RG and achromatic filters are combined at a common motion site, we can predict (1) the perceived slowing of RG stimuli compared with the perceived drift of achromatic drifting stimuli, (2) the contrast dependency of speed perception for RG and achromatic drifting stimuli, and (3) how this dependency changes with base speed. Thus we conclude that there is no need to postulate separate mechanisms for fast and slow motion [Nature (London) 367, 268 (1994)], since a unified ratio model can explain both RG and achromatic contrast-speed dependency.     

Pragmatics of measuring recognition memory: Applications to dementia and amnesia.

This article has two purposes. The first is to describe four theoretical models of yes-no recognition memory and present their associated measures of discrimination and response bias. These models are then applied to a set of data from normal subjects to determine which pairs of discrimination and bias indices show independence between discrimination and bias. The second purpose is to use the indices from the acceptable models to characterize recognition memory deficits in dementia and amnesia. Young normal subjects, Alzheimer's disease patients, and parkinsonian dementia patients were tested with picture recognition tasks with repeated study–test trials. Huntington's disease patients, mixed etiology amnesics, and age-matched normals were tested by Butters, Wolfe, Martone, Granholm, and Cermak (1985) using the same paradigm with word stimuli. Three major points are emphasized. First, any index of recognition memory performance assumes an underlying model. Second, even acceptable models can lead to different conclusions about patterns of learning and forgetting. Third, efforts to characterize and ameliorate abnormal memory should address both discrimination and bias deficits. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

An intracellular study of the contrast-dependence of neuronal activity in cat visual cortex

Extracellular recordings indicate that mechanisms that control contrast gain of neuronal discharge are found in the retina, thalamus and cortex. In addition, the cortex is able to adapt its contrast response function to match the average local contrast. Here we examine the neuronal mechanism of contrast adaptation by direct intracellular recordings in vivo. Both simple (n = 3) and complex cells (n = 4) show contrast adaptation during intracellular recording. For simple cells, that the amplitude of fluctuations in membrane potential induced by a drifting grating stimulus follows a contrast response relation similar to lateral geniculate relay cells, and does not reflect the high gain and adaptive properties seen in the action potential discharge of the neurons. We found no evidence of significant shunting inhibition that could explain these results. In complex cells there was no change in the mean membrane potential for different contrast stimuli or different states of adaptation, despite marked changes in discharge rate. We use a simplified electronic model to discuss the central features of our results and to explain the disparity between the contrast response functions of the membrane potential and action potential discharge in simple cells.     

A model for the responses of low-frequency auditory-nerve fibers in cat

A computational model was developed for the responses of low-frequency auditory-nerve (AN) fibers in cat. The goal was to produce realistic temporal response properties and average discharge rates in response to simple and complex stimuli. Temporal and average-rate properties of AN responses change as a function of sound-pressure level due to nonlinearities in the auditory periphery. The input stage of the AN model is a narrow-band filter that simulates the mechanical tuning of the basilar membrane. The parameters of this filter vary continuously as a function of stimulus level via a feedback mechanism, simulating the compressive nonlinearity associated with the mechanics of the basilar membrane. A memoryless, saturating nonlinearity and two low-pass filters simulate transduction and membrane properties of the inner hair cell (IHC). A diffusion model for the IHC-AN synapse introduces adaptation. Finally, a nonhomogeneous Poisson process, modified by absolute and relative refractoriness, provides the output discharge times. Responses to several different stimuli are presented. These responses illustrate nonlinear temporal response properties that cannot be achieved with linear models for AN fibers.     

Combining information: Probability summation and probability averaging in detection and discrimination.

Detection may improve if a stimulus offers 2 cues rather than 1. This is sometimes attributed to probability summation of independent detections, which provides an especially simple model for sensory information combination. However, this model assumes a strong bias toward the positive response, which is not appropriate for discrimination. The probability summation model is here extended to apply to discrimination and to allow different degrees of summation, ranging from complete through partial to probability averaging, and the use of this model is illustrated for the method of constant stimuli. It allows performance based on independent decisions to be distinguished from performance (e.g., integration) that is better than summation can explain. Models for the discrimination of complex stimuli may provide a tool for studying the development of expertise in areas where this involves a perceptual component, such as clinical judgment. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The "heart" of categorical speech discrimination in young infants

Categorical discrimination of place of articulation in the stimuli [dae] and [gae] was investigated in three- to four-month-old infants using a 20/20 cardiac orienting response paradigm. In this procedure infants were presented with 20 tokens of one syllable followed immediately by 20 tokens of a change stimulus. Each infant received a between-category shift, a within-category shift, and a no-shift control condition, with the order of conditions completely counterbalanced across subjects. Discrimination was indexed by an orienting response to the onset of the second 20 stimuli. The data were analyzed for evidence of categorical discrimination both within subjects (each condition pooled across orders of presentation) and between subjects, as is typical of infant-categorical discrimination studies. Both sets of results revealed that infants discriminate the cues for place of articulation categorically. These findings demonstrate the generality of the infant categorical discrimination observed with the nonnutritive operant sucking paradigm and indicate the usefulness of the heart rate paradigm employed in the present study for the collection of within-subject speech discrimination data across a wide-range of subject populations.     

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 (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Discrimination of changes in the slopes of the amplitude spectra of natural images: band-limited contrast and psychometric functions

Thresholds were measured for discriminating changes in the slopes of the amplitude spectra of stimuli derived from photographs of natural scenes and from random-luminance patterns. The variety and magnitudes of the thresholds could be explained by a model based on the discrimination of the changes in band-limited local contrast. Different spatial scales of local contrast (or different spatial-frequency bands of about 1 octave) were implicated for different reference spectral slopes; the model implicated a lower frequency-band for stimuli with shallower amplitude spectra. The implications of the model were tested experimentally by using stimuli in which the spectra were changed within restricted spatial-frequency bands. When the amplitude spectra of the test and reference stimuli differed only within the implicated frequency bands, thresholds were affected little. However, when the test and reference spectra differed at all frequencies except those in the implicated bands, thresholds were elevated markedly. The forms of the psychometric functions for the discrimination task were entirely compatible with the hypothesis that the task relies upon the ability to discriminate changes of contrast. The Weibull functions fitted to the data had slope parameters (beta) in the range 1 to 3, compatible with discrimination of low (but suprathreshold) contrasts.     

Neural dynamics of motion processing and speed discrimination

A neural network model of visual motion perception and speed discrimination is presented. The model shows how a distributed population code of speed tuning, that realizes a size-speed correlation, can be derived from the simplest mechanisms whereby activations of multiple spatially short-range filters of different size are transformed into speed-turned cell responses. These mechanisms use transient cell responses to moving stimuli, output thresholds that covary with filter size, and competition. These mechanisms are proposed to occur in the V1-->MT cortical processing stream. The model reproduces empirically derived speed discrimination curves and simulates data showing how visual speed perception and discrimination can be affected by stimulus contrast, duration, dot density and spatial frequency. Model motion mechanisms are analogous to mechanisms that have been used to model 3-D form and figure-ground perception. The model forms the front end of a larger motion processing system that has been used to simulate how global motion capture occurs, and how spatial attention is drawn to moving forms. It provides a computational foundation for an emerging neural theory of 3-D form and motion perception.     

Odor processing in the frog olfactory system

In the frog, unitary electrophysiological recordings have been extensively used to investigate odor processing along the olfactory pathways. From the responses of primary second-order neurons, neuroreceptor and mitral cells, odor stimuli could be classified in qualitative groups, revealing that neuronal discriminative mechanisms are partly based on the structure of odor molecule. In the olfactory bulb, thanks both to the anatomical convergence of primary afferences and intrinsic network properties, mitral cells have been demonstrated to gain in odor discrimination and detection power abilities. GABAergic bulbar interneurons were found to be involved in the control of mitral cell excitability, adjusting response thresholds and duration and promoting a progressive increase of burst discharges with stimulus concentration. Otherwise, dopamine was observed to shunt off mitral cell spontaneous activity without altering their odor responsivity properties. Dopamine was demonstrated to act through D2 receptors. Matching anatomical and electrophysiological data, D2 receptors are assumed to be localized on mitral cells. The frog olfactory cortex neurons, silent at rest, could be segregated in two functional groups basing on their odor response properties. The first group shared most intensity coding properties with mitral cells while showing a lower discriminative power, similar to that of neuroreceptor cells. By contrast, the second group provided only minimal intensity coding and, basing on its high discrimination power, was assumed to be mainly devoted to odor discrimination. Thus, along the olfactory pathways, intensity and quality odor parameters which are simultaneously encoded by a neuroreceptor or mitral cell, become specified by two distinct populations in the cortex.     

Comparing elemental and configural associative theories in human causal learning: A case for attention.

In two causal learning experiments with human participants, the authors compared various associative theories that assumed either elemental (unique cue, modified unique cue, replaced elements model, and Harris' model) or configural processing of stimuli (Pearce's theory and a modification of it). The authors used modified patterning problems initially suggested by Redhead and Pearce (1995). Predictions for all theories were generated by computer simulations. Both configural theories and the unique cue approach failed to account for the observations. The replaced elements model was able to account for part of the data, but only if the replacement parameters could vary across discrimination problems. The Harris model and the modified unique cue approach, assuming that the salience of stimuli decreases with an increasing number of stimuli in a compound, successfully accounted for all of our data. This success implies that attentional factors should be explicitly taken into account in associative learning theory. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Effects of flicker adaptation and temporal gain control on the flicker ERG

The flicker electroretinogram (ERG) to stimuli varying in temporal frequency and modulation depth was recorded to investigate retinal gain control. With increasing modulation of a sinusoidal flickering stimulus, the flicker ERG shows an amplitude compression and a phase retardation (of the fundamental component) at 16 Hz, an amplitude expansion and a phase advance around 40-48 Hz, and an approximately linear response at 72 Hz. With sum-of-two-sinusoids stimuli, the second stimulus enhances the fundamental response to a 40 or 48 Hz test stimulus at low modulations, and reduces the variation in phase with modulation. This interaction depends primarily on the amplitude of the response to the second stimulus, but not its frequency. With temporally alternating stimuli, a similar but smaller interaction effect is measured. The results suggest that there is an active nonlinear gain control mechanism in the outer retina and this gain control works by adjusting the phase delay of the retinal response. The phase control mechanism is set by the amplitude of the outer retinal response integrated over time.     

Impairments in negative patterning, but not simple discrimination learning, in rats with 192 IgG-saporin lesions of the nucleus basalis magnocellularis.

Rats with 192 IgG-saporin lesions of the nucleus basalis magnocellularis (NBM) and sham-operated rats were trained in either a simple discrimination paradigm assessing simple association learning or a negative patterning paradigm assessing configural association learning. In the simple discrimination task, rats were reinforced for responding to a light but were not reinforced for responding to a tone. In the negative patterning discrimination task, rats were reinforced for responding to either a light or a tone presented alone but were not reinforced for responding to both stimuli presented simultaneously. Simple discrimination learning was not affected, whereas acquisition of negative patterning was impaired by NBM lesions. Impaired configural association learning may reflect a loss in the ability of rats with NBM lesions to attend to multiple sensory stimuli or to cope with conflicting response strategies. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

A prototype effect and categorization of artificial polymorphous stimuli in pigeons

In a go-no-go discrimination task, pigeons were trained to discriminate artificial polymorphous stimuli differing along 3 six-valued features. Exemplars of each category were generated by systematic transformations of the features from a single stimulus, a base pattern (prototype). They were then tested for transfer to novel stimuli including the base patterns. The most pronounced discrimination occurred between the base patterns rather than between the extreme positive and negative stimuli. A distance-from-prototype rule and an additive integration of feature utilities or of feature frequencies are suggested to account for the prototype effect. In Experiment 2, pigeons were trained on a 2-key choice discrimination task, with the base patterns as conditional stimuli. Subsequent testing with distorted stimuli suggested that a distance-from-prototype model best explains the present findings.     

An attentional model of dimensional contrast.

In this article, the authors introduce a model of dimensional stimulus control designed to explain dimensional contrast effects. The model suggests that dimensional contrast is the result of the nonuniform allocation of limited attentional resources during discrimination training. Attention in the model is conceived as a gradient that extends throughout a spatial representation of stimuli. The authors examine the results of 5 experiments to assess the quality of fit of the model and its theoretical implications. Variations in training procedures, such as changing presentation probability of stimuli, changing the distribution of training stimuli, and changing the relative difficulty of discrimination, can all be accounted for by differences in the allocation of attentional resources. The good fit of the model indicates that attentional limitations may play an important role in stimulus control phenomena such as dimensional contrast. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Source discrimination, item detection, and multinomial models of source monitoring.

Source monitoring refers to the discrimination of the origin of information. Multinomial models of source monitoring (W. H. Batchelder & D. M. Riefer, 1990) are theories of the decision processes involved in source monitoring that provide separate parameters for source discrimination, item detection, and response biases. Three multinomial models of source monitoring based on different models of decision in a simple detection paradigm (one-high-threshold, low-threshold, and two-high-threshold models) were subjected to empirical tests. With a 3 (distractor similarity)?×?3 (source similarity) factorial design, the effect of difficulty of item detection and source discrimination on corresponding model parameters was examined. Only the source-monitoring model that is based on a two-high-threshold model of item recognition provides an accurate analysis of the data. Consequences for the use of multinomial models in the study of source monitoring are discussed. (PsycINFO Database Record (c) 2010 APA, all rights reserved)