Pavlovian aversive and appetitive odor conditioning in humans: subjective, peripheral, and electrocortical changes

The effects of presynaptic guanosine-5'-O-(3-thio)triphosphate (GTPγS) on GABAergic inhibitory postsynaptic currents (IPSCs) were studied in cultured hippocampal neurons using whole-cell recordings. Inclusion of GTPγS (0.5–1 mM) in the presynaptic electrode reduced both the amplitude and paired-pulse depression of IPSCs, indicating that the probability of GABA-release had been reduced. Presynaptic GTPγS increased the depression of IPSCs by the GABA_B-receptor-agonist baclofen (10 μM), and the effect of baclofen was poorly reversible after washing. Stimulation of the GABAergic neuron at 80 Hz for 1 s was accompanied by tetanic depression of the IPSCs by 52±6% and was followed by post-tetanic potentiation (PTP), reaching a peak value of 71±21% and lasting about 100 s. IPSCs evoked after tetanic stimulation were depressed and PTP was absent when tetanic stimulation was applied within 3 min after starting injection of GTPγS into the presynaptic neuron. At longer times, basal release underlying a single IPSC was depressed. This affected the ratios recorded in response to tetanic stimulations such that tetanic depression was abolished, while PTP increased to 117±34%. In conclusion, GTPγS reduces the probability of GABA-release in both a use- and time-dependent manner, most likely through an inhibitory action on presynaptic Ca²⁺-influx through voltage-gated Ca²⁺ channels or an interaction with small GTP-binding proteins in the nerve terminals. Electronic Publication     

Fear acquisition requires awareness in trace but not delay conditioning

The present study explored fear acquisition in differential delay versus trace conditioning in regard to the potential role of the acquired contingency awareness. One of two neutral pictures (CS+) either coterminated with (delay group; n=32) or was followed by the aversive unconditioned stimulus (UCS) after CS offset (trace group; n=32), while startle blink and skin conductance responses (SCR) were measured. As expected, the acquisition of conditioned startle potentiation in delay conditioning was independent of contingency awareness. In contrast, fear-potentiated startle in trace conditioning was only observed for those participants who were aware of the CS-UCS contingencies. SCR conditioning was generally only obtained for aware participants. The present results suggest a more implicit learning process in delay fear conditioning, whereas the explicit acquisition of contingency awareness might be a prerequisite for trace fear conditioning.     

Fear-potentiated startle conditioning in humans: Explicit and contextual cue conditioning following paired versus unpaired training

Conditioned fear in response to explicit and contextual cues was examined using the startle reflex in three groups of participants over two sessions separated by 4–5 days. The conditioned stimulus (CS) was paired with an aversive unconditioned stimulus (US) (shock) during conditioning in the paired but not in the unpaired group. In the reaction time (RT) group, the US was a nonaversive visual signal for an RT task. In the paired group, the CS potentiated startle in the postconditioning phase. This conditioned response was fully retained over the retention interval. There was no substantial change in baseline startle (startle delivered in the absence of CS). By contrast, startle was not potentiated by the CS in the unpaired group, but baseline startle was increased from Session 1 to Session 2. In the RT group, startle was not affected by the CS, and baseline startle was reduced from Session 1 to Session 2. These results suggest that paired presentations of a CS and an aversive US result in conditioned fear in response to the CS but little contextual fear, whereas unpaired presentations of a CS and US leads to poor explicit cue conditioning but substantial contextual fear.     

Deficient fear conditioning and self‐reported psychopathy: The role of fearless dominance

The role of the two dimensions of psychopathy—dispositional fearlessness (theorized to reflect variations in reactivity of the brain's defensive system) and externalizing proneness (presumed to reflect variations in function of anterior regulatory systems)—in fear learning was examined in a sample of undergraduates assessed using the Psychopathic Personality Inventory‐Revised (PPI‐R) who participated in a differential aversive conditioning task. Only scores on self‐reported “fearless dominance,” irrespective of scores on “impulsive antisociality,” were related to diminished acquisition of physiological fear. Consistent with dual‐process accounts of psychopathy proposing divergent etiological pathways for the interpersonal/affective and the social deviance features of the disorder, our results lend support to the existence of a deficit in reactivity of the brain's defensive system underlying the fearlessness dimension of psychopathy.     

Tracking stimulus processing in Pavlovian pupillary conditioning

The aim of the present study was to evaluate whether the anticipatory pupillary dilation response is a useful indicator for the examination of complex differential conditioning problems like patterning. A human fear conditioning procedure with six groups (n=20 each) was used to examine conditioned stimulus (CS) processing when a compound stimulus was reinforced, but not its elements (positive patterning) or when the elements were reinforced, but not the compound (negative patterning), as well as modifications in which the compound was replaced by either a new compound or by a new element. We found evidence for conditioning within 2 s after CS onset. Group differences in differential conditioning indicated systematic differences in CS processing due to different discrimination task difficulties. This study showed that the pupil response is a suitable indicator for human Pavlovian conditioning with high time resolution within and between trials.     

Extinction in fear conditioning: Effects on startle modulation and evaluative self-reports

A basic feature of human evaluative conditioning is that the reported acquired valence of a previously neutral conditioned stimulus (CS) that has been paired with a valenced unconditioned stimulus (US), is resistant to extinction. The present study investigated whether startle modulation, sometimes presented as an index of acquired valence, reflected this basic feature. In a differential fear conditioning paradigm ( n = 38) with an electrocutaneous stimulus as the US, a strong extinction manipulation was conducted by removing the US-electrodes and by extended extinction trials. At the end of extinction, the results corroborated previous findings of evaluative conditioning in that the reported valence for CS+ was still more negative than for CS−, despite disappearance of the differential skin conductance responses. However, startle modulation did not show resistance-to-extinction. Results were discussed in terms of recent conceptualizations of extinction.     

Semantic and affective processing in psychopaths: an event-related potential (ERP) study.

We tested the hypothesis that psychopathy is associated with abnormal processing of semantic and affective verbal information. In Task 1, a lexical decision task, and in Task 2, a word identification task, participants responded faster to concrete than to abstract words. In Task 2, psychopaths made more errors identifying abstract words than concrete words. In Task 3, a word identification task, participants responded faster to positive than to negative words. In all three tasks, nonpsychopaths showed the expected event-related potential (ERP) differentiation between word stimuli, whereas psychopaths did not. In each task, the ERPs of the psychopaths included a large centrofrontal negative-going wave (N350); this wave was absent or very small in the nonpsychopaths. The interpretation and significance of these differences are discussed.     

Startle reflex modulation, affective ratings and autonomic reactivity in incarcerated Spanish psychopaths

Startle probe modulation during affective picture viewing was assessed in a Spanish prison population. As for North American inmates, psychopaths failed to display normal blink potentiation during unpleasant slides even though their evaluative judgments and autonomic reaction to affective stimuli paralleled those of other inmate and noninmate participants. The results suggest that diminished defense activation characterizes psychopaths despite cultural differences.     

Differential conditioning to facial emotional expressions: effects of hemispheric asymmetries and CS identification

Previous studies on aversive learning have suggested a right hemispheric advantage for eliciting autonomic reactions to a masked conditioned facial stimulus (CS) depicting anger. The present study investigated the effects of visual field (VF), stimulus awareness, and emotional valence of the CSs on indicators of conditioning (bilateral SCRs, HR) using a differential conditioning paradigm (N = 41). In Group 1, four different negatively valenced facial expressions (CS+) but not four positively valenced CS- were associated with an unconditioned stimulus (US, aversive vocalization, 97 dB, 3 s) during acquisition. Group 2 received a treatment reversal with positive CS+ associated with the US. In a repeated measures design, CSs were presented with or without awareness during extinction (two weeks interval, order counterbalanced). SOAs were adapted for each subject and condition prior to the experiment so that identification performance was approaching chance level. The results revealed that both negative and positive facial expressions could be aversively conditioned providing evidence for a generalization of learning in the valence dimension. During extinction, preattentive negative CS+ presented to the left VF showed a trend towards greater electrodermal and cardiac reactions. However, no such effect emerged under full awareness of the CSs. These results confirm and further specify the nature of hemispheric asymmetries in emotional associative learning.     

Aversive Pavlovian conditioning in psychopaths: peripheral and central correlates

Differential aversive Pavlovian conditioning with a foul odor as unconditioned stimulus (US) and neutral faces as conditioned stimuli (CS) was compared between 9 noncriminal psychopaths as defined by the Hare Psychopathy Checklist Revised and 12 healthy controls. Event-related potentials (ERP), heart rate, skin conductance response, corrugator EMG, and startle response potentiation as well as valence, arousal, and contingency of the CS were assessed. Whereas the healthy controls (HC) showed significant CS +/CS- differentiation, the psychopaths (PP) failed to exhibit a conditioned response although unconditioned responses were comparable between the groups. N100, P200, and P300 to the CSs revealed that psychopaths were not deficient in information processing and showed even better anticipatory responding than the HC group indicated by the terminal contingent negative variation (tCNV), that lacked, however, CS+ and CS- differentiation. These data indicate a deficit in association formation in psychopaths that may be related to deficient interaction of limbic-subcortical and cortical structures.     

Similarity and discrimination in human Pavlovian conditioning

We report three Pavlovian eyelid conditioning experiments with humans, designed to experimentally decide between elemental and configural learning theories. We used two different designs originally proposed by Redhead and Pearce (1995). In Experiments 1 and 2, three stimulus elements, A, B, and C, were presented in all possible combinations. All patterns were reinforced except for pattern ABC (A/B/C+, AB/AC/BC+, ABC-). According to elemental learning theories, response proportions on A/B/C+ trials should be smaller than on AB/AC/BC+ trials, whereas configural learning theory makes the opposite prediction. The results confirmed neither prediction. In Experiment 3, the A/B/C+, AB/AC/BC+, and ABC- trials were interspersed by D/E/F-, DE/DF/EF-, DEF+ trials. Again, neither prediction was confirmed. We suggest a modification of configural learning theory as a possible explanation of our results.     

Affect, attention, or anticipatory arousal? Human blink startle modulation in forward and backward affective conditioning

Affect modulates the blink startle reflex in the picture-viewing paradigm, however, the process responsible for reflex modulation during conditional stimuli (CSs) that have acquired valence through affective conditioning remains unclear. In Experiment 1, neutral shapes (CSs) and valenced or neutral pictures (USs) were paired in a forward (CS-->US) manner. Pleasantness ratings supported affective learning of positive and negative valence. Post-acquisition, blink reflexes were larger during the pleasant and unpleasant CSs than during the neutral CS. Rather than affect, attention or anticipatory arousal were suggested as sources of startle modulation. Experiment 2 confirmed that affective learning in the picture-picture paradigm was not affected by whether the CS preceded the US. Pleasantness ratings and affective priming revealed similar extents of affective learning following forward, backward or simultaneous pairings of CSs and USs. Experiment 3 utilized a backward conditioning procedure (US-->CS) to minimize effects of US anticipation. Again, blink reflexes were larger during CSs paired with valenced USs regardless of US valence implicating attention rather than anticipatory arousal or affect as the process modulating startle in this paradigm.     

A developmental comparison of trace and delay eyeblink conditioning in rats using matching interstimulus intervals

The effects of conditioning paradigm and interstimulus interval (ISI) were evaluated in this study of the development of associative learning in rats. The acquisition of classical eyeblink conditioning (EBC) was examined in two paradigms (trace vs. delay), with three different ISIs (short, medium, or long) at two ages (postnatal Days 21-23 or 29-31). These data provide the first parametric analysis of ISI in developing animals trained with trace EBC procedures. Further, by comparing trace and delay EBC, it was determined that when ISI is held constant, acquisition is similar for both conditioning paradigms regardless of developmental age. This suggests a similar ontogeny of associative processes for delay and trace EBC that relies on common neural substrates. However, conditioned response timing (onset and peak latencies) was affected by age and paradigm, suggesting that different neural mechanisms may play a role in timing delay versus trace conditioned responses.     

Nonconscious associative learning: Pavlovian conditioning of skin conductance responses to masked fear-relevant facial stimuli

We examined the possibility of nonconscious associative learning in a context of skin conductance conditioning, using emotional facial expressions as stimuli. In the first experiment, subjects were conditioned to a backwardly masked angry face that was followed by electric shock, with a masked happy face as the nonreinforced stimulus. In spite of the effectively masked conditioned stimuli, differential conditioned skin conductance responses were observed in a subsequent nonmasked extinction phase. This effect could not be attributed to differential sensitization or pseudo-conditioning. In the second experiment, the differential responding during extinction was replicated with angry but not with happy faces as conditioned stimuli. It was concluded that with fear-relevant facial expressions as the conditioned stimulus, associative learning was possible even in conditions where the subjects remained unaware of the conditioned stimulus and its relationship to the unconditioned stimulus.     

Ocular artifacts in EEG and event-related potentials I: Scalp topography

Summary The ocular artifacts that contaminate the EEG derive from the potential difference between the cornea and the fundus of the eye. This corneofundal or corneoretinal potential can be considered as an equivalent dipole with its positive pole directed toward the cornea. The cornea shows a steady DC potential of approximately +13 mV relative to the forehead. Blink potentials are caused by the eyelids sliding down over the positively charged cornea. The artifacts from eye-movements result from changes in orientation of the corneo-fundal potential. The scalp-distribution of the ocular artifacts can be described in terms of propagation factors — the fraction of the EOG signal at periocular electrodes that is recorded at a particular scalp location. These factors vary with the location of the scalp electrode. Propagation factors for blinks and upward eye-movements are significantly different.This research was supported by a grant from the Medical Research Council of Canada (MA5465). Adrian Kellett helped with technical aspects of the recording.     

The use of an unpleasant sound as an unconditional stimulus in a human aversive Pavlovian conditioning procedure

Ethical considerations can limit the use of traditional unconditional stimuli (US), such as electric shock and loud tones, when used in a human aversive Pavlovian conditioning procedure. The risk of the US causing pain or excessive anxiety is a particular concern when testing sensitive populations such as children, the elderly, and those with psychological or neurodevelopmental disorders. Two experiments used a differential conditioning procedure to determine whether an unpleasant sound (metal scraping on slate) could support the acquisition and extinction of conditioned responses to the same extent as either electric shock or a 100 dB(A) tone US. Experiment 1 (N=48) demonstrated equivalent or superior conditioning effects for the signal-based learning measures of US expectancy, skin conductance responses, and heart rate. Experiment 2 (N=57) yielded similar outcomes in the affective-based learning measures of startle blink modulation and pleasantness ratings. The results support the use of an unpleasant sound as a US in human Pavlovian conditioning experiments.     

Selective attention and Pavlovian conditioning

The present results show that the common practice of using self-indexing conditioned stimuli (CSs) in research on Pavlovian conditioning is a major source of experimental bias. The typical stimulus used is either a light flash or a sound pulse in a light/sound-shielded chamber. Under these conditions the onset characteristics of the CS signal totally predominate over the durational characteristic, i.e. the pattern information. Thus a visual pattern presented as a CS in a dark chamber is confounded with a brightness change from darkness to light. In the first experiment, animals were conditioned with a brightness CS using a self-indexing signal paradigm. When tested for specificity of the conditioning, they showed complete transfer of learning to either a visual pattern or even an auditory CS. These findings indicated that the traditional conditioning paradigm is biased towards non-specific sensory learning. The second experiment showed that specific sensory conditioning is critically dependent on selective attention mechanisms. When the onset characteristics of the CS signal were de-emphasized by the use of equal energy background illumination in the intertribal interval (ITI) during conditioning, the animals were not able to feature extract either the onset or the durational component of the CS signal from the ITI background despite prolonged training. It was only by starting with conditioning that was initially anchored to the CS onset characteristics that a perceptual fade-in procedure would bias attention to feature extract the durational characteristics of the CS. Thus conditioning occurred only when the rabbit’s attention was directed to detection of the gratings display without any associated changes in visual albedo. Perhaps the most important finding of the present experiments is that the use of self-indexing CS signals in Pavlovian conditioning inevitably introduces non-specific sensory processing involving multiple sensory input pathways in the conditioning. This inherent uncertainty of the sensory input pathways presents a problem for clarifying the role of sensory pathways in the neural mechanisms of NM conditioning. In addition, the use of self-indexing CSs inevitably leads to an underestimation of the role of forebrain mechanisms in Pavlovian conditioning.     

Effects of stimulus modality and task condition on blink startle modification and on electrodermal responses

Participants in Experiments 1 and 2 performed a discrimination and counting task to assess the effect of lead stimulus modality on attentional modification of the acoustic startle reflex. Modality of the discrimination stimuli was changed across subjects. Electrodermal responses were larger during task-relevant stimuli than during task-irrelevant stimuli in all conditions. Larger blink magnitude facilitation was found during auditory and visual task-relevant stimuli, but not for tactile stimuli. Experiment 3 used acoustic, visual, and tactile conditioned stimuli (CSs) in differential conditioning with an aversive unconditioned stimulus (US). Startle magnitude facilitation and electrodermal responses were larger during a CS that preceded the US than during a CS that was presented alone regardless of lead stimulus modality. Although not unequivocal, the present data pose problems for attentional accounts of blink modification that emphasize the importance of lead stimulus modality.     

The effect of emotional and attentional processes on blink startle modulation and on electrodermal responses

Emotional accounts of startle modulation predict that startle is facilitated if elicited during aversive foreground stimuli. Attentional accounts hold that startle is enhanced if startle-eliciting stimulus and foreground stimulus are in the same modality. Visual and acoustic foreground stimuli and acoustic startle probes were employed in aversive differential conditioning and in a stimulus discrimination task. Differential conditioning was evident in electrodermal responses and blink latency shortening in both modalities, but effects on magnitude facilitation were found only for visual stimuli. In the discrimination task, skin conductance responses, blink latency shortening, and blink magnitude facilitation were larger during to-be-attended stimuli regardless of stimulus modality. The present results support the notion that attention and emotion can affect blink startle modulation during foreground stimuli.     

Classical conditioning in oddball paradigm: A comparison between aversive and name conditioning

The nature of cortical plasticity in learning is one of the most intriguing questions of modern cognitive neuroscience. Classical conditioning (as a typical case of associative learning) and electroencephalography together provide a good framework for expanding our knowledge about fast learning‐related cortical changes. In our experiment, we employed a novel paradigm in which classical conditioning was combined with passive oddball. Nineteen subjects participated in the first experiment (aversive conditioning with painful shock as unconditioned stimulus (US) and neutral tones as conditioned stimulus (CS)), and 22 subjects in the second experiment (with a subject’s own name as US). We used event‐related potentials (ERPs) and time‐frequency analyses to explore the CS‐US interaction. We found a learning‐induced increment of P3a in the first experiment and the late positive potential (LPP) in both experiments. These effects may be related to increased attentional and emotional significance of conditioned stimuli. We showed that the LPP and P3a effects, earlier found only in visual paradigms, generalize to the auditory sensory system. We also observed suppression of the low beta activity to CS+ in aversive conditioning over the hemisphere contralateral to expected electrical shocks, presumably indicating preparation of the somatosensory system to the expected nociceptive US.