Neural substrates of human fear generalization: A 7T-fMRI investigation.

Fear generalization - the tendency to interpret ambiguous stimuli as threatening due to perceptual similarity to a learned threat - is an adaptive process. Overgeneralization, however, is maladaptive and has been implicated in a number of anxiety disorders. Neuroimaging research has indicated several regions sensitive to effects of generalization, including regions involved in fear excitation (e.g., amygdala, insula) and inhibition (e.g., ventromedial prefrontal cortex). Research has suggested several other small brain regions may play an important role in this process (e.g., hippocampal subfields, bed nucleus of the stria terminalis [BNST], habenula), but, to date, these regions have not been examined during fear generalization due to limited spatial resolution of standard human neuroimaging. To this end, we utilized the high spatial resolution of 7T fMRI to characterize the neural circuits involved in threat discrimination and generalization. Additionally, we examined potential modulating effects of trait anxiety and intolerance of uncertainty on neural activation during threat generalization. In a sample of 31 healthy undergraduate students, significant positive generalization effects (i.e., greater activation for stimuli with increasing perceptual similarity to a learned threat cue) were observed in the visual cortex, thalamus, habenula and BNST, while negative generalization effects were observed in the dentate gyrus, CA1, and CA3. Associations with individual differences were underpowered, though preliminary findings suggested greater generalization in the insula and primary somatosensory cortex may be correlated with self-reported anxiety. Overall, findings largely support previous neuroimaging work on fear generalization and provide additional insight into the contributions of several previously unexplored brain regions.

Eyeblink conditioning with a noise burst as unconditioned stimulus / Condicionamiento de la respuesta de parpadeo con una explosión de ruido como estímulo incondicionado

BACKGROUND: The aim of this study was to determine if a white noise burst could be used as an effective unconditioned stimulus (US) to produce differential conditioning of eyeblink responses that were recorded as EMG activity of the orbicularis oculi. METHOD: Two fear-relevant stimuli served as conditioned stimuli (CS). An angry woman's face (CS+) was consistently followed by a white noise burst (US) with 100 dB intensity and 100 milliseconds in duration. A fearful face of the same woman (CS-) was not followed by the US. CS duration was 500 milliseconds (ms) for 18 participants (long interval group), and 250 ms for 19 participants (short interval group). The US was presented in both groups immediately after terminating CS+. RESULTS: The results showed acquisition of differential conditioning in the long interval group, but not in the short interval group. CONCLUSIONS: These results suggest that a white noise burst as US could be used in one single experimental procedure which was capable of simultaneously producing conditioning in neural, autonomic and somatomotor response systems

ANTECEDENTES: el objetivo de este trabajo era determinar si una ráfaga de ruido blanco podía ser utilizada como un eficaz estímulo incondicionado (EI) para producir condicionamiento de la respuesta de parpadeo, que era registrada como actividad electromiográfi ca (EMG) del músculo orbicularis oculi. MÉTODO: dos estímulos relevantes de miedo servían como estímulos condicionados (EC). Un rostro enfadado de mujer (EC+) se presentaba siempre seguido por una explosión de ruido blanco (EI) con una intensidad de 100 dBs y 100 milisegundos de duración, mientras que otro rostro de la misma mujer con expresión de miedo (EC-) nunca iba seguido del EI. La duración del EC era de 500 milisegundos (ms) para 18 participantes (grupo de intervalo largo) y de 250 ms para otros 19 participantes (grupo de intervalo corto). El EI era presentado en los dos grupos inmediatamente después de la terminación del EC+. RESULTADOS: los resultados mostraron adquisición de condicionamiento diferencial en el grupo de intervalo largo, pero no en el grupo de intervalo corto. CONCLUSIONES: estos resultados sugieren que es posible utilizar una explosión de ruido blanco como EI en un único procedimiento experimental, que sería capaz de producir simultáneamente condicionamiento de respuestas del sistema nervioso central, autónomo y somático

Linking perceptual learning with identical stimuli to imagery perceptual learning.

Perceptual learning is usually thought to be exclusively driven by the stimuli presented during training (and the underlying synaptic learning rules). In some way, we are slaves of our visual experiences. However, learning can occur even when no stimuli are presented at all. For example, Gabor contrast detection improves when only a blank screen is presented and observers are asked to imagine Gabor patches. Likewise, performance improves when observers are asked to imagine the nonexisting central line of a bisection stimulus to be offset either to the right or left. Hence, performance can improve without stimulus presentation. As shown in the auditory domain, performance can also improve when the very same stimulus is presented in all learning trials and observers were asked to discriminate differences which do not exist (observers were not told about the set up). Classic models of perceptual learning cannot handle these situations since they need proper stimulus presentation, i.e., variance in the stimuli, such as a left versus right offset in the bisection stimulus. Here, we show that perceptual learning with identical stimuli occurs in the visual domain, too. Second, we linked the two paradigms by telling observers that only the very same bisection stimulus was presented in all trials and asked them to imagine the central line to be offset either to the left or right. As in imagery learning, performance improved.

El uso de sonidos como estímulos en el condicionamiento clásico electrodérmico humano / The use of sounds as stimuli in human electrodermal classical conditioning

Tradicionalmente, en el estudio del condicionamiento clásico humano se emplearon descargas eléctricas y ruidos intensos como estímulos incondicionados aversivos. Sin embargo, el uso de este tipo de estímulos plantea una serie de problemas de carácter ético y metodológico. Consecuentemente, en trabajos recientes se comprobó que podía obtenerse condicionamiento sustituyendo el habitual ruido blanco por un sonido cuya aversividad se derivaba más de su contenido semántico que de su intensidad. Sin embargo, los estímulos empleados en estos trabajos plantean, a su vez, una serie de problemas relacionados tanto con la complejidad técnica requerida para su grabación y posterior edición como con la replicación de los resultados. En este estudio se pretende superar dichos problemas utilizando sonidos del IADS ("International Affective Digitized Sounds") como estímulos condicionados e incondicionados en un procedimiento de condicionamiento clásico electrodérmico. Los resultados mostraron una rápida disminución en la amplitud de las respuestas que se interpretó en términos de una habituación de la reacción de orientación. Se comentan las limitaciones encontradas y se propone la posibilidad de utilizar la metodología empleada en este estudio en futuros trabajos (AU)

Traditionally, human classical conditioning studies used electric shocks and loud noises as aversive unconditioned stimuli. However, the use of such stimuli raises a number of methodological and ethical issues. Consequently, in recent works it was found that conditioning could be obtained by replacing the usual white noise with a sound whose aversivity could be derived from its semantic content rather than its intensity. However, the stimuli used in these studies raise, in turn, a number of problems related to both technical complexity required for recording and further editing as difficulties in replication of the results. The present work suggests the possibility of overcoming these problems using sounds selected from the IADS (International Affective Digitized Sounds) as conditioned and unconditioned stimuli on electrodermal classical conditioning procedure. The results showed a rapid decrease in the amplitude of the responses that was interpreted in terms of a habituation of the orienting reaction. We discuss the limitations found, and propose the possibility of using the methodological approach employed in this study in future work (AU)

Lack of generalization of auditory learning in typically developing children.

PURPOSE: To understand the components of auditory learning in typically developing children by assessing generalization across stimuli, across modalities (i.e., hearing, vision), and to higher level language tasks. METHOD: Eighty-six 8- to 10-year-old typically developing children were quasi-randomly assigned to 4 groups. Three of the groups received twelve 30-min training sessions on multiple standards using either an auditory frequency discrimination task (AFD group), auditory phonetic discrimination task (PD group), or visual frequency discrimination task (VFD group) over 4 weeks. The 4th group, which was the no-intervention control (NI) group, did not receive any training. Thresholds on all tasks (AFD, PD, and VFD) were assessed immediately before and after training, along with performance on a battery of language assessments. RESULTS: Relative to the other groups, both the AFD group and the PD group, but not the VFD group, showed significant learning on the stimuli upon which they were trained. However, in those instances where learning was observed, it did not generalize to the nontrained stimuli or to the language assessments. CONCLUSIONS: Nonspeech (AFD) or speech (PD) discrimination training can lead to auditory learning in typically developing children of this age range. However, this learning does not always generalize across stimuli or tasks, across modalities, or to higher level measures of language ability.

Stimulus control: the sought or unsought influence of the objects we tend to

Does the mere presence of the things we have tended to influence our actions systematically, in ways that escape our awareness? For example, while entering a tool shed, does perceiving objects that we once tended to (e.g., tools, musical instruments) influence how we then execute a simple action (e.g., flicking the shed’s light switch)? Ancient traditions (e.g., feng shui) and contemporary approaches to action production (e.g., continuous flow and cascade models) hypothesize that the answer is yes. Although relevant to several fields (e.g., motor cognition, social cognition), for various reasons this hypothesis cannot be tested by traditional choiceresponse time interference paradigms, which involve more complex processes than our tool shed scenario. Using new paradigms that resemble detection tasks, three studies demonstrated that ‘very incidental’ actionrelated distracters systematically interfere with simple, repeated actions that involve minimal response selection and decision-making processes. In Study 2, incidental musical notation interfered more with the simple actions of expert sight-readers than with the same actions of non-musicians. A similar pattern of effects was obtained with a fully experimental design. The implications for theories of action production, environmentally-driven automaticity, and social cognition are discussed(AU)

Two pathways to stimulus encoding in category learning?

Category learning theorists tacitly assume that stimuli are encoded by a single pathway. Motivated by theories of object recognition, we evaluated a dual-pathway account of stimulus encoding. The part-based pathway establishes mappings between sensory input and symbols that encode discrete stimulus features, whereas the image-based pathway applies holistic templates to sensory input. Our experiments used rule-plus-exception structures, in which one exception item in each category violates a salient regularity and must be distinguished from other items. In Experiment 1, we found discrete representations to be crucial for recognition of exceptions following brief training. Experiments 2 and 3 involved multisession training regimens designed to encourage either part- or image-based encoding. We found that both pathways are able to support exception encoding, but have unique characteristics. We speculate that one advantage of the part-based pathway is the ability to generalize across domains, whereas the image-based pathway provides faster and more effortless recognition.

Functional MRI reveals the existence of modality and coordination-dependent timing networks.

Growing evidence suggests that interval timing in humans is supported by distributed brain networks. Recently, we demonstrated that the specific network recruited for the performance of rhythmic timing is not static but is influenced by the coordination pattern employed during interval acquisition. Here we expand on this previous work to investigate the role of stimulus modality and coordination pattern in determining the brain areas recruited for performance of a self-paced rhythmic timing task. Subjects were paced with either a visual or an auditory metronome in either a synchronized (on the beat) or syncopated (off the beat) coordination pattern. The pacing stimulus was then removed and subjects continued to move based on the required interval. When compared with networks recruited for auditory pacing and continuation, the visual-specific activity was observed in the classic dorsal visual stream that included bilateral MT/V5, bilateral superior parietal lobe, and right ventral premotor cortex. Activity in these regions was present not only during pacing, when visual information is used to guide motor behavior, but also during continuation, when visual information specifying the temporal interval was no longer present. These results suggest a role for modality-specific areas in processing and representing temporal information. The cognitive demands imposed by syncopated coordination resulted in increased activity in a broad network that included supplementary motor area, lateral pre-motor cortex, bilateral insula, and cerebellum. This coordination-dependent activity persisted during the subsequent continuation period, when stimuli were removed and no coordination constraints were imposed. Taken together, the present results provide additional evidence that time and timing are served by a context-dependent distributed network rooted in basic sensorimotor processes.

CS-specific gamma, theta, and alpha EEG activity detected in stimulus generalization following induction of behavioral memory by stimulation of the nucleus basalis.

Tone paired with stimulation of the nucleus basalis (NB) induces behavioral memory that is specific to the frequency of the conditioned stimulus (CS), assessed by cardiac and respiration behavior during post-training stimulus generalization testing. This paper focuses on CS-specific spectral and temporal features of conditioned EEG activation. Adult male Sprague-Dawley rats, chronically implanted with a stimulating electrode in the NB and a recording electrode in the ipsilateral auditory cortex, received either tone (6kHz, 70dB, 2s) paired with co-terminating stimulation of the nucleus basalis (0.2s, 100Hz, 80-105 microA, ITI approximately 45s) or unpaired presentation of the stimuli (approximately 200 trials/day for approximately 14 days). CS-specificity was tested 24h post-training by presenting test tones to obtain generalization gradients for the EEG, heart rate, and respiration. Behavioral memory was evident in cardiac and respiratory responses that were maximal to the CS frequency of 6kHz. FFT analyses of tone-elicited changes of power in the delta, theta, alpha, beta1, beta2, and gamma bands in the paired group revealed that conditioned EEG activation (shift from lower to higher frequencies) was differentially spectrally and temporally specific: theta, and alpha to a lesser extent, decreased selectively to 6kHz during and for several seconds following tone presentation while gamma power increased transiently during and after 6kHz. Delta exhibited no CS-specificity and the beta bands showed transient specificity only after several seconds. The unpaired group exhibited neither CS-specific behavioral nor EEG effects. Thus, stimulus generalization tests reveal that conditioned EEG activation is not unitary but rather reflects CS-specificity, with band-selective markers for specific, associative neural processes in learning and memory.

The effects of change in lead stimulus modality on the modulation of acoustic blink startle.

In two experiments we investigated the effect of generalized orienting induced by changing the modality of the lead stimulus on the modulation of blink reflexes elicited by acoustic stimuli. In Experiment 1 (n = 32), participants were presented with acoustic or visual change stimuli after habituation training with tactile lead stimuli. In Experiment 2 (n = 64), modality of the lead stimulus (acoustic vs. visual) was crossed with experimental condition (change vs. no change). Lead stimulus change resulted in increased electrodermal orienting in both experiments. Blink latency shortening and blink magnitude facilitation increased from habituation to change trials regardless of whether the change stimulus was presented in the same or in a different modality as the reflex-eliciting stimulus. These results are not consistent with modality-specific accounts of attentional startle modulation.

(+)Amphetamine-stimulus generalization to an herbal ephedrine product.

We have previously demonstrated that a (+)amphetamine stimulus generalizes both to (-)ephedrine and caffeine. Using rats trained to discriminate intraperitoneal (IP) administration of 1.0 mg/kg of (+)amphetamine (ED(50) = 0.4 mg/kg) from saline vehicle in a standard two-lever drug discrimination procedure, the present investigation shows that the (+)amphetamine stimulus generalizes to (+)amphetamine (ED(50) = 1.0 mg/kg) when administered via the intragastric (IG) route, and that (+)amphetamine appears about 2. 5-fold less potent when administered via the IG route compared to the IP route. Likewise, (-)ephedrine (ED(50) = 10.8 mg/kg) and caffeine (ED(50) = 32.9 mg/kg) are also 2.5-fold less potent when administered via the IG route compared to their potency when administered via the IP route. The (+)amphetamine stimulus also generalizes to an IG-administered herbal preparation (i.e., Herbal XTC; the herbal preparation possesses an approximate potency roughly comparable to what might have been expected on the basis of its reported ephedrine and/or caffeine content. These results demonstrate, for the first time, that an ephedrine-containing herbal preparation can produce a (+)amphetamine-like effect in animals.

P3a, perceptual distinctiveness, and stimulus modality.

A three-stimulus oddball paradigm (target, standard, nontarget) was employed in which subjects responded to an infrequent target, when its discrimination from the frequent standard was difficult. In separate auditory and visual modality conditions, the stimulus characteristics of an infrequent nontarget were manipulated such that its perceptual distinctiveness from the target was varied systematically. For both the low and high distinctiveness conditions, target stimulus P300 amplitude was larger than the nontarget only at the parietal electrode. In contrast, nontarget P3a amplitude was larger and earlier than the target P300 over the frontal/central electrode sites. The distinctiveness manipulation between the target and nontarget produced larger P3a component profiles for the auditory compared to visual stimuli. The results support previous findings that target/standard stimulus context determines P3a generation for both auditory and visual stimulus modalities and suggest that the distinctiveness of the eliciting stimulus contributes to P3a amplitude. Theoretical implications are discussed.

Stimulus intensity modulates associative and nonassociative responding in preweanling rats.

The purpose of the present study was to determine whether preweanling rats respond differentially to the intensity and energy source of a stimulus. Previous studies have suggested that infants process compound stimuli based on net stimulus intensity regardless of the energy source of the compound's elements, but more direct tests have been needed of the infant's response to the stimulus attributes of intensity and energy source. This response was tested for auditory and visual stimuli that had been equated (Experiment 1) in terms of perceived intensity (low and high). Intensity level and energy source of the stimuli were varied independently within nonassociative (Experiment 2) and associative (Experiment 3) procedures. The overall results indicate that stimuli of a low-perceived intensity were processed in terms of their intensity, whereas high-intensity stimuli were processed on the basis of energy source. These results are relevant to contemporary issues of cognitive development in humans and animals.

Cocaine--stimulus generalization to MDA optical isomers: a reevaluation.

It has already been demonstrated that the psychoactive agent 1-(3,4-methylenedioxyphenyl)-2-aminopropane (MDA) produces effects that are both hallucinogen-like and amphetamine or stimulant-like in animals. Hallucinogenic activity is associated primarily with the R(-)-isomer of MDA whereas stimulant activity is primarily associated with the S(+)-isomer. Because a previous report indicates that S(+)MDA fails to substitute for cocaine in rats trained to discriminate cocaine from vehicle, and because these findings are inconsistent with the purported stimulant nature of S(+)MDA, we reinvestigated the effect of both MDA isomers in rats. In this investigation, S(+)MDA doses of 1.25 and 1.5 mg/kg were found to produce > 80% cocaine-appropriate responding in rats trained to discriminate 8 mg/kg of cocaine from saline. However, consistent with a previous report, R(-)MDA resulted only in partial generalization. These new results support the hypothesis that the optical isomers of MDA produce distinguishable stimulus effects in rats and that S(+)MDA is the more stimulant isomer of MDA.

Initial characterization of PMMA as a discriminative stimulus.

The phenylisopropylamine PMMA or N-methyl-1-(4-methoxyphenyl)-2-aminopropane, a structural hybrid of paramethoxyamphetamine (PMA) and methamphetamine, has been previously shown to unexpectedly lack amphetamine-like or hallucinogen-like stimulus properties in animals. For example, in tests of stimulus generalization, neither a (+)amphetamine stimulus nor a DOM stimulus generalized to PMMA. It has also been shown, however, that stimulus generalization does occur in animals trained to discriminate the designer drug MDMA ("Ecstasy" or N-methyl-1-(3,4-methylenedioxyphenyl)-2-aminopropane) from vehicle. In order to further characterize this unique agent, we trained a group of six Sprague-Dawley rats to discriminate 1.25 mg/kg of PMMA (ED50 = 0.44 mg/kg) from saline vehicle. The PMMA stimulus failed to generalize to the phenylisopropylamine stimulant (+)amphetamine, or to the phenylisopropylamine hallucinogen DOM. Stimulus generalization occurred to (+/-)MDMA (ED50 = 1.32 mg/kg) and S(+)MDMA (ED50 = 0.48 mg/kg). Partial generalization occurred with R(+)MDMA, PMA, 3.4-DMA, and fenfluramine. The PMMA stimulus also generalized to the alpha-ethyl homolog of PMMA (EH/PMMA, ED50 = 1.29 mg/kg). Taken together, the results of these studies suggest that PMMA is an MDMA-like agent that lacks the amphetamine-like stimulant character of MDMA. These findings support our previous suggestion that PMMA be considered the structural parent of the MDMA-like family of designer drugs.

Desflurane, enflurane, isoflurane and ether produce ethanol-like discriminative stimulus effects in mice.

In the present studies, drug discrimination procedures were used to compare the discriminative stimulus effects of ethanol (ETOH) and several volatile anesthetics. Male albino mice were trained to discriminate between IP injections of ETOH (1.25 g/kg) and saline in a two-lever operant task in which responding was under the control of a fixed-ratio 20 (FR20) schedule of food presentation. Stimulus generalization was examined after 20-min inhalation exposures to desflurane (4,000-32,000 ppm), enflurane (3,000-12,000 ppm), isoflurane (1,000-8,000 ppm) and ether (4,000-32,000 ppm). Concentration-related increases in ETOH-lever responding were observed for all four volatile anesthetics. For enflurane and ether, maximal levels of > 85% ETOH-lever responding were obtained at one or more concentrations. For desflurane and isoflurane, the maximal mean percentages of ETOH-lever responding were somewhat lower, but 6 out of 7 mice showed full substitution with desflurane and 5 out of 7 for isoflurane. The shared discriminative properties of these compounds with ETOH suggest that these anesthetics may share some of ETOH's pharmacological properties. These results are similar to previous research results showing ETOH-like discriminative stimulus effects in mice with other anesthetics and abused volatile inhalants (i.e. halothane, toluene and 1.1,1-trichloroethane) and may reflect the CNS-depressant drug-like effects of inhaled anesthetics and abused solvents.

Cross-generalization of an EtOH "hangover" cue to endogenously and exogenously induced stimuli.

Twenty male Sprague-Dawley rats were trained in a two-choice food-reinforced drug discrimination task (10 min sessions) using the state-dependent interoceptive stimulus attributes of ethanol's (EtOH) delayed or rebound effects (EDE) versus "normal" basal homeostasis. Cross-generalization tests were conducted with 0.18 mg/kg naloxone injected after three days of three injections per day of either SAL or 10 mg/kg morphine. Naloxone failed to generalize to the EDE-state after chronic saline; however, the precipitated morphine withdrawal state produced complete generalization to the EDE training cue. Daily tests were conducted after 8 h photoperiod phase-shifts. An 8 h phase-advance, equivalent to a west-to-east intercontinental night-time flight in humans, produced a biphasic, graded, increase in EDE-appropriate responding, which peaked on the second day after the phase-advance and recovered by the fourth day. The 8 h phase-delays failed to engender significant EDE-appropriate responding. These data provide evidence for the subjective similarity between EtOH hangover, opiate withdrawal states, and the physiological disruption induced by circadian phase-advances.

Generalization among related complex odorant mixtures and their components: analysis of olfactory perception in the spiny lobster.

We investigated the processing of odorant mixtures containing two to seven components by the spiny lobster Panulirus argus. The chemicals tested were food-related compounds that are attractive to spiny lobsters, and include adenosine-5'-monophosphate, betaine, L-cysteine, L-glutamate, DL-succinate, taurine, and ammonium. Components were tested at concentrations that produced search behavioral responses of equal magnitude in unconditioned animals. Responses of unconditioned animals to mixtures and their components reveal hypoadditivity, in which the response to a mixture is less than the sum of the responses to that mixture's components. Aversive conditioning coupled with generalization testing was used to evaluate generalization and hence perceptual similarity between related mixtures. Animals were conditioned to either an individual odorant, a four-compound mixture, or a seven-compound mixture, followed by generalization testing with submixtures or larger mixtures containing the conditioned stimulus. Animals tended not to generalize, but significant generalization between a more simple conditioned stimulus and more complex mixtures containing that conditioned stimulus occurred in 2 of 11 cases, and significant generalization between a conditioned mixture and its submixtures was observed in 4 of 9 cases. Both the number and chemical identity of components of mixtures may contribute to the degree of generalization between mixtures. Overshadowing, in which the ability to learn about a chemical is affected by simultaneous presentation of other chemicals, occurred in two of three cases. We discuss implications of these findings with respect to elemental and configural processing of odorant mixtures in the spiny lobster, possible neural mechanisms responsible for these results, and the potential utility of generalization and overshadowing to the spiny lobster's natural behavior.

The formation of a complex skill in Black Sea dolphins under free choice conditions.

A free choice situation, characterized by the fact that any sequence of actions of dolphins on manipulators set up in a pen was reinforced, providing that they pressed on three specific levers in a required order, independent of the number of "superfluous" reactions before and between the required actions, was created in the experiment. No sequence of actions leading to reinforcement was formed in the dolphin in the first series of experiments using eight above-water levers. In the second series, using eight under-water manipulators, both experimental dolphins regularly acquired reinforcement. Stereotypical trajectories of movements about the pen, which included swimming passed the levers and sequential actions on them, were formed in these dolphins. The formation of a minimal chain of motoric reactions did not take place, although presses on some of the "superfluous" levers did stop. During the accomplishment of the task, the phenomenon of efferent generalization was manifested in two ways in the dolphins in both series of experiments: the animals acted in different ways on the very same manipulator and performed presses on various levers.

Pavlovian conditioning in human skilled motor behavior.

The role of Pavlovian contingencies in human skilled motor behavior was investigated in three experiments by means of a new conditioning preparation. In Experiment 1 the present method was shown to be appropriate for the study of associative learning. Subjects who experienced a standard delay configuration performed significantly more conditioned responses than subjects who received either backward conditioning or random pairings. Stimulus generalization was shown to be slight in two additional groups. Subsequent experiments examined conditioning with multiple conditioned stimuli (CSs). In particular, in Experiment 2 some reciprocal overshadowing was demonstrated when two conditional stimuli (tone and vibration) were compounded. Experiment 3 investigated blocking. Blocking was less than expected, however. Subjects' perceptions of the stimuli and reaction time data suggest that a certain proportion had shifted their attention to the added element of the CS compound. Results are discussed in relation to other studies on Pavlovian learning in humans and animals, which are concerned with "stimulus selection."