Response selection,sensitivity, and taste-test performance

Tasters selected the odd stimulus from among sets of three samples of party dip. Two samples came from one batch, and one sample came from another batch. The physicochemical difference between the batches consisted of the presence or absence of added salt. Two different tests of discriminability were undertaken by the same subjects with the same stimuli: the triangle test and the three-alternative forced-choice (3-AFC) method. Although different numbers of correct selections were obtained in the two tasks, an index of discriminability,d’, had the same value when the data were analyzed in accordance with the Thurstone-Ura and signal-detection models, respectively. The average data support Frijters’s (1979b) contention that different models of the discrimination process are appropriate to the results of the triangular and the 3-AFC procedures. Further analysis of the data revealed that discrimination was poorer for trios containing one physicochemically weak stimulus and two stronger stimuli than it was for trios containing one stronger stimulus and two weak stimuli. A two-signal 3-AFC task was undertaken by some subjects, andd’ estimates from this task were lower than expected on the basis of performance in the other tasks.     

An olfactory investigation of the compatibility of oddity instructions with the design of a 3-AFC signal detection task

There are two principal differences between the Triangular Method and the 3-Alternative Forced-Choice (3-AFC) procedure originating from the theory of signal detection. These are the instructions given and the types of three-stimulus sets (‘triangles’) presented to the subject.In the Triangular Method the subjects are instructed to select the odd stimulus from each triangle presented, whilst in the 3-AFC task the subject must either select the strongest or identify the weakest stimulus.The triangles presented in the Triangular Method sometimes contain two physically weak stimuli plus one stronger, and at other times two strong stimuli plus one weaker. In contrast, in a 3-AFC experiment all the triangles comprise either two weak plus one stronger, or two strong plus one weaker stimulus.Previous research has shown that the parameter d' is invariant with the two procedures, if the appropriate models are used for conversion of proportions of correct responses into corresponding values of d'.The present experiments provide evidence that the Triangular Method instructions (to select the odd stimulus) used in combination with the 3-AFC task design (using three types of triangles) does not result in biased estimates of d'. In addition, it was shown that even after a large number of presentations the value of d' remained stable, which supports the notion of stability of the decision rule implied by the oddity instructions.     

A multivariate model for discrimination methods

We describe a multivariate model for a certain class of discrimination methods in this paper and discuss a multivariate Euclidean model for a particular method, the triangular method. The methods of interest involve the selection or grouping of stimuli drawn from two stimulus sets on the basis of attributes invoked by the subject. These methods are commonly used for estimation and hypothesis testing concerning possible differences between foods, beverages, odorants, tastants and visual stimuli.Mathematical formulation of the bivariate model for the triangular method is provided as well as extensive Monte Carlo results for up to 10-dimensional cases. The effect of correlation structure and variance inequality are discussed. Results from these methods (as probability of a correct response) are not monotonically related to the distance between the means of the stimulus sets from which the stimuli are drawn but depend in a particular way on dimensionality, correlation structure, and the relative orientation of the momentary sensory values in a multidimensional space. The importance of these results to the validity of these methods as currently employed is discussed and the possibility of developing a new approach to multidimensional scaling on the basis of this new theory is considered.     

Cross-modality matching functions generated by magnitude estimation

It is possible to generate cross-modality matching functions by having subjects make magnitude estimates of sets of stimuli appropriate to different modalities. The sets are interspersed among each other in the same test session and judged on a common absolute scale of sensory magnitude. An appropriate statistical device locates stimulus levels that appear, on the average, to match. The method is fast, efficient, circumvents the need for continuous stimulus adjustment, and holds promise for the study of the individual as well as the average psychophysical function. To illustrate its potential uses, advantages, and limitations, we used the method to generate cross-modality matching functions relating loudness and brightness. Compared to the scales of loudness and brightness generated by the magnitude estimations of the same stimuli, the matching functions (1) conform better to power functions, (2) may show less variation in slope (exponent), and (3) show far less variation in absolute magnitude (position).     

Stimulus control and compounding with ambient odor as a discriminative stimulus on a free-operant baseline

Previous experiments have demonstrated that the simultaneous presentation of independently established discriminative stimuli can control rates of operant responding substantially higher than the rates occasioned by the individual stimuli. This "additive summation" phenomenon has been shown with a variety of different reinforcers (e.g., food, water, shock avoidance, cocaine, and heroin). Discriminative stimuli previously used in such studies have been limited to the visual and auditory sensory modalities. The present experiment sought to (1) establish stimulus control on a free-operant baseline with an ambient olfactory discriminative stimulus, (2) compare olfactory control to that produced with an auditory discriminative stimulus, and (3) determine whether compounding independently established olfactory and auditory discriminative stimuli produces additive summation. Rats lever pressed for food on a variable-interval schedule in the presence of either a tone or an odor, with comparable control developed to each stimulus. In the absence of these stimuli responding was not reinforced. During stimulus compounding tests, the tone-plus-odor compound occasioned more than double the responses occasioned by either the tone or odor presented individually. Thus, the current study (1) established stimulus control with an ambient olfactory discriminative stimulus in a traditional free-operant setting and (2) extended the generality of stimulus-compounding effects by demonstrating additive summation when olfactory and auditory discriminative stimuli were presented simultaneously.     

Observing responses maintained by conditional discriminative stimuli.

In a conditional discrimination procedure, pigeons' observing responses were analyzed to examine whether two color stimuli (blue or red), conditionally related to whether each of two line stimuli (vertical or horizontal) accompanied reinforcement or nonreinforcement, functioned as conditioned reinforcers. If a variable-interval (VI) 10-s requirement was fulfilled, an observing response produced onset of a color stimulus. A little later, a line stimulus was presented independently of responding, added to the color stimulus to form a compound stimulus. If 55 s elapsed with a response not having occurred either through 55 s or after the variable-interval 10-s had timed out, one of the color-line compound stimuli was presented independently of responding. To control for sensory reinforcement effects and for earlier entrance to the later link, a simple discrimination procedure also was conducted in which reinforcement was not correlated with the color stimuli but with the line stimuli only. As in the conditional discrimination, the observing response also could produce earlier presentation of blue or red. The observing response occurred more frequently during the conditional discrimination than during the simple discrimination. The results were related to different theoretical accounts of conditioned reinforcement, particularly the information hypothesis.     

Time perception with and without a concurrent nontemporal task

Prospective time estimates were obtained from human subjects for stimulus durations ranging from 2 to 23 sec. Presence and absence of a concurrent nontemporal task was manipulated within subjects in three experiments. In addition, location of the task within temporal reproduction trials and psychophysical method were varied between groups in Experiments 2 and 3, respectively. For long-duration stimuli, the results of all three experiments conformed to results in the literature, showing a decrease in perceived duration under concurrent task conditions, in accord with attentional resource allocation models of timing. The effects of task location and psychophysical method on time estimates were also compatible with this analysis. However, psychophysical functions obtained under task conditions were fit well by power functions, an outcome that would not be anticipated on the basis of attention theory. The slopes of the functions under no-task conditions were steeper than those under task conditions. The data support the perceptual hypothesis that different sources of sensory input mediate timing under task and no-task conditions.     

Directed attention prolongs the perceived duration of a brief stimulus

Stelmach, Herdman, and McNeil (1994) suggested recently that the perceived duration for attended stimuli is shorter than that for unattended ones. In contrast, the attenuation hypothesis (Thomas & Weaver, 1975) suggests the reverse relation between directed attention and perceived duration. We conducted six experiments to test the validity of the two contradictory hypotheses. In all the experiments, attention was directed to one of two possible stimulus sources. Experiments 1 and 2 employed stimulus durations from 70 to 270 msec. A stimulus appeared in either the visual or the auditory modality. Stimuli in the attended modality were rated as longer than stimuli in the unattended modality. Experiment 3 replicated this finding using a different psychophysical procedure. Experiments 4-6 showed that the finding applies not only to stimuli from different sensory modalities but also to stimuli appearing at different locations within the visual field. The results of all six experiments support the assumption that directed attention prolongs the perceived duration of a stimulus.     

Identity matching-to-sample with olfactory stimuli in rats

Identity matching-to-sample has been difficult to demonstrate in rats, but most studies have used visual stimuli. There is evidence that rats can acquire complex forms of olfactory stimulus control, and the present study explored the possibility that identity matching might be facilitated in rats if olfactory stimuli were used. Four rats were trained on an identity match-to-sample procedure with odorants mixed in cups of sand as stimuli. Digging in the sample cup produced two comparison cups, and digging in the comparison cup that contained the same scent as the sample was reinforced. When criterion accuracy levels were reached, novel stimuli were added to the baseline training regimen. All 4 rats reached terminal performance of above 90% correct matching with more than 20 different baseline stimuli and matched novel stimulus combinations with above-chance accuracy; 3 of the 4 rats matched novel stimuli at levels significantly above chance. Accurate matching performance was demonstrated both with 2- and 3-comparison procedures. These results suggest that generalized matching-to-sample can be observed in rats when olfactory stimuli are used and, furthermore, that multiple-exemplar training may be important for its emergence.     

Assessments of changes in the effective salience of stimulus elements as a result of stimulus preexposure

Rats received exposure to 3 flavor compounds, AX and BX, presented in alternation, and CX, presented on a separate block of trials. The hypothesis that this treatment would leave B effectively more salient than C was tested in 3 ways. Experiment 1 showed that the unconditioned response evoked by B was stronger than that evoked by C. Experiment 2 showed that B was more effective than C when used as a reinforcer in a sensory preconditioning procedure. Experiment 3 showed that B was learned about more readily than C as a conditioned stimulus in flavor aversion conditioning. Alternating preexposure to 2 similar stimuli may protect their distinctive features from the loss of salience normally produced by nonreinforced exposure to a stimulus.     

The delay-reduction hypothesis of conditioned reinforcement and punishment: Observing behavior

Pigeons responded in an observing-response procedure in which three fixed-interval components alternated. Pecking one response key produced food reinforcement according to a mixed schedule. Pecking the second (observing) key occasionally replaced the mixed-schedule stimulus with the stimulus correlated with the fixed-interval component then in effect. In Experiment 1, observing was best maintained by stimuli correlated with a reduction in mean time to reinforcement. That finding was consistent with the conditioned-reinforcement hypothesis of observing behavior. However, low rates of observing were also maintained by stimuli not representing delay reduction. Experiment 2 assessed the role of sensory reinforcement. It showed that response rate was higher when maintained by stimuli uncorrelated with reinforcement delay than when the stimuli were correlated with a delay increase. This latter result supports a symmetrical version of the conditioned-reinforcement hypothesis that requires suppression by stimuli correlated with an increase in time to reinforcement. The results were inconsistent with hypotheses stressing the reinforcing potency of uncertainty reduction.     

Comparing and unifying slope estimates across psychometric function models

The psychometric function relating stimulus intensity to response probability generally presents itself as a monotonically increasing sigmoid profile. Two summary parameters of the function are particularly important as measures of perceptual performance: the threshold parameter, which defines the location of the function over the stimulus axis (abscissa), and the slope parameter, which defines the (local) rate at which response probability increases with increasing stimulus intensity. In practice, the psychometric function may be modeled by a variety of mathematical structures, and the resulting algebraic expression describing the slope parameter may vary considerably between different functions fitted to the same experimental data. This variation often restricts comparisons between studies that select different functions and compromises the general interpretation of slope values. This article reviews the general characteristics of psychometric function models, discusses three strategies for resolving the issue of slope value differences, and presents mathematical expressions for implementing each strategy.     

Separate but interacting recognition memory systems for different senses: the role of the rat perirhinal cortex

Two different models (convergent and parallel) potentially describe how recognition memory, the ability to detect the re-occurrence of a stimulus, is organized across different senses. To contrast these two models, rats with or without perirhinal cortex lesions were compared across various conditions that controlled available information from specific sensory modalities. Intact rats not only showed visual, tactile, and olfactory recognition, but also overcame changes in the types of sensory information available between object sampling and subsequent object recognition, e.g., between sampling in the light and recognition in the dark, or vice versa. Perirhinal lesions severely impaired object recognition whenever visual cues were available, but spared olfactory recognition and tactile-based object recognition when tested in the dark. The perirhinal lesions also blocked the ability to recognize an object sampled in the light and then tested for recognition in the dark, or vice versa. The findings reveal parallel recognition systems for different senses reliant on distinct brain areas, e.g., perirhinal cortex for vision, but also show that: (1) recognition memory for multisensory stimuli involves competition between sensory systems and (2) perirhinal cortex lesions produce a bias to rely on vision, despite the presence of intact recognition memory systems serving other senses.     

An inexpensive and automated method for presenting olfactory or tactile stimuli to rats in a two-choice discrimination task

An inexpensive and automated method for presentation of olfactory or tactile stimuli in a two-choice task for rats was implemented with the use of a computer-controlled bidirectional motor. The motor rotated a disk that presented two stimuli of different texture for tactile discrimination, or different odor for olfactory discrimination. Because the solid olfactory stimuli were placed outside the chamber in metal pods with a mesh at front for odor sampling, "washout" of odors between trials was not necessary. To avoid differential auditory cues from motor rotation, the stimuli were arranged such that on each trial the motor always rotated exactly one quarter revolution (in 1 s), left or right, to present the next stimulus at trial start. To illustrate the use of the equipment, 2 rats were trained on tactile discrimination and 2 rats on olfactory discrimination. The rats sampled the stimulus on the disk through a port on the back wall by sniffing at it (olfactory) or touching it (tactile). The task was a go-left/go-right discrimination with the stimulus on the disk being discriminative for which lever provided reinforcement. The rats reached a stable level above 90% correct after 21 and 32 training sessions for tactile and olfactory discrimination, respectively. The article outlines how the equipment was constructed from low-cost components. Inputs from and outputs to the equipment were implemented through the parallel port of a personal computer without the use of a commercial interface board. The method of automated and low-cost presentation of olfactory or tactile stimuli should be of use for a variety of experimental situations such as matching-to-sample and cross-modal discrimination.     

Effective use of the blank comparison procedure in simple discrimination by infant capuchin monkeys: A methodological note

In studies of simple and conditional discrimination, procedures are needed to measure those aspects of stimuli that control behavior. The blank comparison procedure is one such procedure. It was designed explicitly for assessing S+ and S- functions when discriminative stimuli are presented simultaneously. In this procedure, a neutral stimulus serves sometimes as S+ and sometimes as S-. Its discriminative function is defined in relation to other stimuli in the display. The present study aimed to prepare 2 infant female capuchin monkeys for the effective use of the blank comparison procedure in a simple discrimination task. First, simple discrimination training was applied up to a stable accuracy criterion of ≥90%. This training was followed by the replacement of S+ and then of S- stimuli with new stimuli. Ultimately, trials with the blank comparison were introduced. Following this sequence, both monkeys immediately displayed highly accurate blank-comparison performances without the need for stimulus control shaping or other preparatory discrimination training. Thus, this procedure sequence may be an efficient, effective method for establishing blank-comparison baselines for experimental analyses of S+/S- discriminative functions and perhaps for other applications in teaching simple and conditional discrimination performances to this species and others.     

Magnitude estimation of short electrocutaneous pulses

Summary A comparison was made of electrocutaneous magnitude estimation data across two experiments with contextual differences not involving stimulus parameters, such as number and range of stimuli and relative position of the standard in the stimulus range. The data were fitted by 2-parameter linear, log-linear and power functions. When the data are fitted by either linear or log-linear equations, both intercept and slope parameters are significantly affected by the different contextual factors. When the data are fitted by a power function, however, only the intercept is altered; the slope remains invariant despite contextual changes introduced in the second experiment.Although the empirically derived psychophysical power law has been applied to magnitude estimation data for all other sensory modalities, its application to electrocutaneous stimuli has been less successful.     

The Effects of Selective Cholinergic Basal Forebrain Lesions and Aging upon Expectancy in the Rat

The effects of selective cholinergic cell loss within the basal forebrain (BF) were determined using a task that requires shifting of attention between two visual stimuli. Discriminability between two stimuli and response bias were determined in young and old F-344 rats given BF injections of IgG-192 saporin (100 ng). The lesion reduced ChAT activity in the frontal and parietal cortices, hippocampus, and olfactory bulbs. The lesion did not significantly alter Na⁺/K⁺-ATPase activity in cortex, hippocampus, or olfactory bulbs, or endogenous levels of neuropeptide Y and neurokinin B within the BF. The BF lesions impaired both stimulus discriminability and response bias in young and old rats. The BF lesions had a significantly greater effect upon stimulus discriminability and response bias in aged rats, compared to young rats, only when the stimulus duration was very brief, i.e., when the task was most difficult to solve. At longer stimulus durations, aging and lesions showed no interaction. The results suggest that the selective loss of cholinergic cells in the BF, but not normal aging, impairs the ability to discriminate between independent sensory stimuli. The loss of these cells confers a response bias in simple operant tasks involving motor responses to reward-related visual stimuli.     

The effect of stimulus delivery technique on perceived intensity functions for taste stimuli: Implications for fMRI studies

Classic psychophysical studies have provided significant information on the psychophysical functions for taste stimuli. With the advent of fMRI, studies are being conducted that provide insight into central processing of gustation in humans. However, fMRI experiments impose physical limitations on stimulus delivery. In the present study, we compared psychophysical functions relating perceived intensity to concentration, derived from previous studies that used the traditional sip-and-spit and dorsal flow delivery techniques, to psychophysical functions generated in this study using a simulated stimulus delivery technique (SSDT). The SSDT delivered minute quantities of taste stimuli to the dorsal surface of the tongue, just as in an fMRI scanner. As was hypothesized, the results indicated that slopes of intensity functions were dependent on the type of stimulus delivery technique. The SSDT resulted in slopes that were more similar to those generated by dorsal flow than by sip-andspit stimulus delivery techniques, suggesting the importance of considering the influence of stimulus delivery on psychophysical response in designing and interpreting experiments.     

Anchor effects on the discrimination of pitch

Two experiments considered the effects of introducing an extreme stimulus (anchor) upon the differential perception of tonal stimuli. In the first experiment, in which Ss rated series stimuli from 1,000 to 2,000 Hz or from 2,000 to 3,000 Hz, the presence of a 750-Hz anchor apparently disrupted discrimination. A second experiment involved testing for a difference threshold (method of limits) in which the standard was a tone of 1,500 Hz. The uncertainty interval (Iu) was larger when the interval between paired stimulus presentations was filled with a 750-Hz tone, again suggesting that discrimination is impaired by the introduction of a low-frequency anchor. Results are discussed in terms of theories relating range extent and discriminability.