A model for signal detection based on an adaptive filter

A model for the detection of brief stimuli based on a change detection algorithm and a random walk traversed by the residuals generated by an adaptive filter is proposed. A linear relationship between mean RT and response proportion measures obtained in a simulation of the model was consistent with data obtained in psychophysical discrimination tasks using human observers. In this way the role of the sensory system as a detector of change in ambient stimulation could be incorporated into a signal detection model.     

The perception of painful and nonpainful stimuli during voluntary motor activity in man

Previous studies have shown that voluntary movement diminishes the transmission of cutaneous afferent input through the dorsal column-medial lemniscal system, and also raises the threshold for detecting nonpainful, cutaneous stimuli (electrical shocks). Although there is some evidence that pain elicited by electrical stimulation is diminished during movement, no studies have tested the effect of movement on the perception of pain produced by natural stimulation. For this reason, we tested the effects of voluntary motor activity on the perception of noxious thermal stimuli in human volunteers. We first developed a motor paradigm in which the thermal stimulation could be applied to the immobile limb (isometric elbow flexion-extension). Both isometric and isotonic muscle contractions about the elbow increased the threshold for detecting weak cutaneous stimuli (electrical shocks) applied to the forearm, and to a lesser extent the detection of stimuli applied to the dorsum of the hand. Afterwards, noxious and innocuous heat stimuli were applied to the forearm during isometric contractions and at rest. Magnitude estimates for the intensity of the pain, as well as latency measures of the onset of pain, were recorded. We found no evidence that isometric motor activity diminished either the threshold for pain or the subjective intensity of the noxious and innocuous thermal stimuli. Thus, motor activity decreases the ability to detect weak low-threshold cutaneous inputs, but has no effect on the perception of warmth and heat pain.     

Comparing sensibility for touch,cold, and warmth in different skin areas

The primary objective of this pilot study was to assess if the magnitude estimation of suprathreshold brushing, warmth (40?°C), and cold (25?°C) stimuli of the skin over the dorsum of the hand and the dorsum of the foot are comparable to the perceived intensity for the same stimuli applied to the skin over any of the following areas: forehead, m. trapezius, m. deltoideus, thoracic back, and lumbar back, respectively. Thirty-two subjects aged 18–64 years were included. Participants were examined by two physicians on two different occasions, 1–58 days apart. Participants rated the magnitude of the perceived sensation of each stimulus on an 11-point numerical rating scale (NRS) 0–10, where 0 was anchored to “no sensation at all for touch/cold/warmth” and 10 anchored to “the most intense imaginable non-painful sensation of touch/cold/warmth”. The criterion for sensory equivalence for one modality was arbitrarily considered satisfactory if two regions had the same numerical rating ±1 point in at least 85% of the individuals. Based on the pre-study criteria for sensory equivalence applied in this study only one area was found to be equivalent to the foot skin for the percept of brushing, that is, the skin over the deltoid muscle and one area for the hand, that is, the skin over the forehead. We failed to find any area with equivalent sensitivity to the hand or the foot for the cold or warm stimuli.     

Cortical activation changes during repeated laser stimulation: a magnetoencephalographic study

Repeated warm laser stimuli produce a progressive increase of the sensation of warmth and heat and eventually that of a burning pain. The pain resulting from repetitive warm stimuli is mediated by summated C fibre responses. To shed more light on the cortical changes associated with pain during repeated subnoxious warm stimulation, we analysed magnetoencephalographic (MEG) evoked fields in eleven subjects during application of repetitive warm laser stimuli to the dorsum of the right hand. One set of stimuli encompassed 10 laser pulses occurring at 2.5 s intervals. Parameters of laser stimulation were optimised to elicit a pleasant warm sensation upon a single stimulus with a rise of skin temperature after repeated stimulation not exceeding the threshold of C mechano-heat fibres. Subjects reported a progressive increase of the intensity of heat and burning pain during repeated laser stimulation in spite of only mild (4.8°C) increase of skin temperature from the first stimulus to the tenth stimulus. The mean reaction time, evaluated in six subjects, was 1.33 s, confirming involvement of C fibres. The neuromagnetic fields were modelled by five equivalent source dipoles located in the occipital cortex, cerebellum, posterior cingulate cortex, and left and right operculo-insular cortex. The only component showing statistically significant changes during repetitive laser stimulation was the late component of the contralateral operculo-insular source peaking at 1.05 s after stimulus onset. The amplitude increases of the late component of the contralateral operculo-insular source dipole correlated with the subjects' numerical ratings of warmth and pain. Results point to a pivotal role of the contralateral operculo-insular region in processing of C-fibre mediated pain during repeated subnoxious laser stimulation.     

Comparison of the orthonasal and retronasal detection thresholds for carbon dioxide in humans

Several studies have investigated the orthonasal detection threshold for carbon dioxide (CO(2)) in humans. The aim of current study was to investigate whether 24 healthy young subjects exhibited differences of CO(2) detection thresholds during orthonasal or retronasal stimulation. As nasal mucosa is believed to desensitize to CO(2) concentrations at or below 4% (v/v) during expiration, the second aim of the study was to explore the influence during nasal versus oral breathing on the detection thresholds. CO(2) stimuli of varying concentrations and a duration of 1000 ms were applied with an air-dilution olfactometer in either the anterior nasal cavity or the nasopharynx during nasal respectively oral breathing. In these 4 conditions, the mean CO(2) detection thresholds using the staircase forced-choice procedure were between 3.9% and 5.3% (v/v). Statistical analysis revealed a significant difference between orthonasal and retronasal stimulation. The CO(2) detection threshold was lower in retronasal stimulation. The nasopharyngeal mucosa is more sensitive to perithreshold CO(2) stimuli than the nasal mucosa. The breathing route had no influence on the detection thresholds. The results of this study indicate that the natural contact of the nasal mucosa with approximately 4% (v/v) CO(2) during nasal expiration does not influence CO(2) detection thresholds.     

Comparative psychophysical characteristics of cutaneous CO2 laser and contact heat stimulation

Psychophysical visual analog scaling can be used to reveal critical determinants of the neural processing underlying non-painful and painful heat sensations produced by radiant and contact heat stimulation. This study determined the stimulus-response (S-R) functions of cutaneous non-painful and painful heat stimuli delivered by an infra-red CO2 laser or by a contact thermode in a series of experiments in healthy volunteers. In experiment 1 ( n = 12), with the rating scale anchored at pain threshold, the S-R curve for brief (60 ms) laser pulse stimulation with a beam diameter of 10 mm was a negatively accelerating function. Transformation of laser stimulus intensity (W) into temperatures ( C) did not change the form of the S-R curve. In experiment 2 ( n=9), using the same laser stimulus parameters as in experiment 1, but without an anchored rating scale, the form of the S-R relationship did not change. In experiment 3 ( n =9), increases of the laser pulse duration up to 5 s and the beam diameter up to 18 mm produced linear S-R curves. In contrast, in experiment 4 ( n =21), the S-R curve for cutaneous contact heat stimuli applied for 5 s with an 18 mm diameter probe was best described by a positively accelerating power function with an exponent greater than 2.0. These experiments have (1) characterized the S-R functions for different parameters of infra-red laser stimulation of the skin, and (2) have shown that the form of the S-R function for innocuous and noxious heat sensation is influenced strongly by the physical conditions of heat stimulus application, including mechanical contact with the skin.     

Comparison of cortical activation patterns by somatosensory stimulation on the palm and dorsum of the hand

Abstract

Objectives: Little is known about differences of cortical activation according to body location. We attempted to compare brain activation patterns by somatosensory stimulation on the palm and dorsum of the hand, using functional magnetic resonance imaging (fMRI).

Method: We recruited 15 healthy right-handed volunteers for this study. fMRI was performed during touch stimulation using a rubber brush on an area of the same size on the palm or dorsum of the hand. Regions of interest (ROIs) were drawn at the primary sensory–motor cortex (SM1), posterior parietal cortex, and secondary somatosensory cortex.

Results: Group analysis of fMRI data indicated that touch stimulation on the palm resulted in production of more activated voxels in the contralateral SM1 and posterior parietal cortex than on the dorsum of the hand. The most activated ROI was found to be the contralateral SM1 by stimulation of the palm or dorsum, and the number of activated voxels (5875) of SM1 by palm stimulation was more than 2 times that (2282) of dorsum stimulation. The peak activated value in the SM1 by palm stimulation (16.43) was also higher than that of the dorsum (5.52).

Conclusion: We found that stimulation of the palm resulted in more cortical activation in the contralateral SM1 than stimulation of the dorsum. Our results suggested that the palm of the hand might have larger somatotopy of somatosensory representation for touch in the cerebral cortex than the dorsum of the hand. Our results would be useful as a rehabilitation strategy when more or less somatosensory stimulation of the hand is necessary.     

The effects of cross-modal manipulations of attention on the detection of vibrotactile stimuli in humans

Although it is well known that attention to a visual or auditory stimulus can enhance its perception, less is known concerning the effects of attention on the perception of natural tactile stimuli. The present study was conducted to examine the magnitude of the effect of cross-modal manipulations of attention in human subjects on the detection of weak, low-frequency vibrotactile stimuli delivered to the glabrous skin of the finger pad of the right index finger via an Optacon. Three suprathreshold vibrotactile arrays (40 Hz), varying in the number of activated pegs and hence the area of skin stimulated, were used. Subjects were trained to detect the occurrence of vibrotactile or visual stimuli and to respond by pressing a foot pedal as quickly as possible thereafter. Two instructional lights were used to cue the subjects as to which stimulus modality they should attend, in three experimental conditions. In the first cue condition, the forthcoming stimulus modality was indicated by the illumination of its associated light. In the second cue condition, both instructional lights were illuminated, and the subjects were asked to divide their attention equally between the two modalities. In the third cue condition, the stimulus modality was falsely indicated by the illumination of the cue not associated with the stimulus to be presented. Reaction times (RTs) were calculated for each trial. For each modality, tactile and visual, the RTs varied significantly with the cue condition, with the mean RT changing in a graded manner across the experimental conditions (being shortest for the correctly cued condition, intermediate for the neutrally cued condition, and longest for the incorrectly cued condition.(ABSTRACT TRUNCATED AT 250 WORDS)     

Mechanical stimulation in wheat triggers age- and dose-dependent alterations in growth,development and grain characteristics

Background and AimsWheat crops are exposed to a range of mechanical stimulations in their natural environment, yet we know very little about their response to such conditions. The aim of this study was to better understand the effect of mechanical stimulation on wheat growth and development, stem mechanical properties and grain measures. We focused on the following questions: (1) Does plant age affect the response to mechanical stimulation? (2) Is there a minimum threshold for the perception of mechanical stimuli? (3) Is the effect of manual brushing different to natural wind stimulation?MethodsFor age– and dose–response experiments, wheat plants were grown under controlled glasshouse conditions with brushing treatments applied using a purpose-built rig. The results of the controlled experiments are compared with those from an outside experiment where wheat plants were exposed to natural wind, with or without additional brushing. Detailed phenotypic measurements were conducted and treatment effects on grain characteristics were determined using micro-computed tomography imaging.Key ResultsTwo-week-old wheat plants were particularly sensitive to mechanical stimulation by controlled brushing treatments. Amongst others, plants exhibited a large reduction in height and grain yield, and an increase in tillers, above-ground biomass and stiffness of stem segments. Plants responded significantly to doses as small as one daily brushstroke. Outdoor experiments by and large confirmed results from controlled environment experiments.ConclusionsThe morphological and developmental response to mechanical brushing treatment, in relation to vegetative above-ground biomass and grain yield, is dependent on plant age as well as the dose of the treatments. This study shows that mechanical stimulation of wheat impacts on a multitude of agriculturally relevant traits and provides a much needed advancement of our understanding of wheat thigmomorphogenesis and the potential applications of mechanical conditioning to control relevant traits.     

Effects of passive and active movement on vibrotactile detection thresholds of the Pacinian channel and forward masking

Abstract

We investigated the gating effect of passive and active movement on the vibrotactile detection thresholds of the Pacinian (P) psychophysical channel and forward masking. Previous work on gating mostly used electrocutaneous stimulation and did not allow focusing on tactile submodalities. Ten healthy adults participated in our study. Passive movement was achieved by swinging a platform, on which the participant’s stimulated hand was attached, manually by a trained operator. The root-mean-square value of the movement speed was kept in a narrow range (slow: 10–20?cm/s, fast: 50–60?cm/s). Active movement was performed by the participant him-/herself using the same apparatus. The tactile stimuli consisted of 250-Hz sinusoidal mechanical vibrations, which were generated by a shaker mounted on the movement platform and applied to the middle fingertip. In the forward-masking experiments, a high-level masking stimulus preceded the test stimulus. Each movement condition was tested separately in a two-interval forced-choice detection task. Both passive and active movement caused a robust gating effect, that is, elevation of thresholds, in the fast speed range. Statistically significant change of thresholds was not found in slow movement conditions. Passive movement yielded higher thresholds than those measured during active movement, but this could not be confirmed statistically. On the other hand, the effect of forward masking was approximately constant as the movement condition varied. These results imply that gating depends on both peripheral and central factors in the P channel. Active movement may have some facilitatory role and produce less gating. Additionally, the results support the hypothesis regarding a critical speed for gating, which may be relevant for daily situations involving vibrations transmitted through grasped objects and for manual exploration.     

Theta Burst Stimulation Applied over Primary Motor and Somatosensory Cortices Produces Analgesia Unrelated to the Changes in Nociceptive Event-Related Potentials

Continuous theta burst stimulation (cTBS) applied over the primary motor cortex (M1) can alleviate pain although the neural basis of this effect remains largely unknown. Besides, the primary somatosensory cortex (S1) is thought to play a pivotal role in the sensori-discriminative aspects of pain perception but the analgesic effect of cTBS applied over S1 remains controversial. To investigate cTBS-induced analgesia we characterized, in two separate experiments, the effect of cTBS applied either over M1 or S1 on the event-related brain potentials (ERPs) and perception elicited by nociceptive (CO₂ laser stimulation) and non-nociceptive (transcutaneous electrical stimulation) somatosensory stimuli. All stimuli were delivered to the ipsilateral and contralateral hand. We found that both cTBS applied over M1 and cTBS applied over S1 significantly reduced the percept elicited by nociceptive stimuli delivered to the contralateral hand as compared to similar stimulation of the ipsilateral hand. In contrast, cTBS did not modulate the perception of non-nociceptive stimuli. Surprisingly, this side-dependent analgesic effect of cTBS was not reflected in the amplitude modulation of nociceptive ERPs. Indeed, both nociceptive (N160, N240 and P360 waves) and late-latency non-nociceptive (N140 and P200 waves) ERPs elicited by stimulation of the contralateral and ipsilateral hands were similarly reduced after cTBS, suggesting an unspecific effect, possibly due to habituation or reduced alertness. In conclusion, cTBS applied over M1 and S1 reduces similarly the perception of nociceptive inputs originating from the contralateral hand, but this analgesic effect is not reflected in the magnitude of nociceptive ERPs.     

Reaction times to painless and painful CO2 and argon laser stimulation

The introduction of lasers in pain research has made it possible to activate the nociceptive system without activating mechanosensitive afferents. In the present study the reaction times to painless and painful laser stimuli were studied to investigate if the reaction time to experimental pain is reproduceable. CO2 and argon lasers were used for stimulation, and the influence of stimulus (intensity and duration) and skin parameters (temperature, thickness, and reflectance) on reaction time were investigated. When these parameters were controlled the reaction times to painful CO2 and argon laser stimulation were within the same range (350-450 ms), and the intra-individual variability minimal (6.9%). The reaction time was used to estimate peripheral conduction velocity (10 m.s-1) for the activated fibre population when distinct pain was perceived. Determination of reaction times to non-painful and painful stimuli may be suitable ways to assess the functioning of thermal and nociceptive pathways.     

Cutaneous mechanical sensory stimulation increases extracellular acetylcholine release in cerebral cortex in anesthetized rats.

The effects of noxious and innocuous mechanical stimulation of various segmental skin areas (face, forelimb and paw, back, hindlimb and paw) on extracellular acetylcholine release in the cerebral cortex in the parietal lobe were examined in halothane-anesthetized rats, by means of the microdialysis technique. Pinching of a forepaw or hindpaw and brushing of a hindlimb for 10 min produced significant increases in extracellular acetylcholine release, whereas pinching of the face or back and brushing of the face, forelimb or back produced no significant changes. These results demonstrate that cutaneous sensory stimulation can regulate extracellular acetylcholine release in the cerebral cortex, and that the efficacy of the stimulation on cortical acetylcholine release is dependent on the sensory modality and the region of skin stimulated.     

Comparison of visual and olfactory reversal learning in pigeons

Pigeons trained to discriminate between either two differentodors or two different colored lights acquired their discriminationsat the same rate. When the discrimination problems were reversedwithin a modality, however, the birds using visual cues acquirednew discriminations more rapidly than in original learning (positivetransfer), whereas the birds using olfactory cues acquired thediscrimination reversal less rapidly. On subsequent reversals,pigeons in the visual task condition developed a successivediscrimination reversal set with repeated reversals of the stimuli,while those in the olfactory condition did not. In a secondexperiment designed to assess the acquisition of redundant cues,birds received additional training with visual and olfactorycues in compound as discriminative stimuli, and were then testedwith only visual or olfactory cues. Birds previously trainedwith odor attended to visual cues in the compound, whereas birdspreviously trained to discriminate between lights did not attendto odor cues until they were presented alone. These resultsdemonstrate that the selection of stimuli may play a crucialrole in the performance of successive discrimination reversalsand suggest that, in contrast to rats, birds selectively attendto visual over olfactory cues in discrimination learning.     

Delayed perceptual awareness in rapid perceptual decisions

The flourishing of studies on the neural correlates of decision-making calls for an appraisal of the relation between perceptual decisions and conscious perception. By exploiting the long integration time of noisy motion stimuli, and by forcing human observers to make difficult speeded decisions--sometimes a blind guess--about stimulus direction, we traced the temporal buildup of motion discrimination capability and perceptual awareness, as assessed trial by trial through direct rating. We found that both increased gradually with motion coherence and viewing time, but discrimination was systematically leading awareness, reaching a plateau much earlier. Sensitivity and criterion changes contributed jointly to the slow buildup of perceptual awareness. It made no difference whether motion discrimination was accomplished by saccades or verbal responses. These findings suggest that perceptual awareness emerges on the top of a developing or even mature perceptual decision. We argue that the middle temporal (MT) cortical region does not confer us the full phenomenic depth of motion perception, although it may represent a precursor stage in building our subjective sense of visual motion.     

Hypnotizability,Hypnosis and Prepulse Inhibition of the Startle Reflex in Healthy Women: An ERP Analysis

A working model of the neurophysiology of hypnosis suggests that highly hypnotizable individuals (HHs) have more effective frontal attentional systems implementing control, monitoring performance, and inhibiting unwanted stimuli from conscious awareness, than low hypnotizable individuals (LHs). Recent studies, using prepulse inhibition (PPI) of the auditory startle reflex (ASR), suggest that HHs, in the waking condition, may show reduced sensory gating although they may selectively attend and disattend different stimuli. Using a within subject design and a strict subject selection procedure, in waking and hypnosis conditions we tested whether HHs compared to LHs showed a significantly lower inhibition of the ASR and startle-related brain activity in both time and intracerebral source localization domains. HHs, as compared to LH participants, exhibited (a) longer latency of the eyeblink startle reflex, (b) reduced N100 responses to startle stimuli, and (c) higher PPI of eyeblink startle and of the P200 and P300 waves. Hypnosis yielded smaller N100 waves to startle stimuli and greater PPI of this component than in the waking condition. sLORETA analysis revealed that, for the N100 (107 msec) elicited during startle trials, HHs had a smaller activation in the left parietal lobe (BA2/40) than LHs. Auditory pulses of pulse-with prepulse trials in HHs yielded less activity of the P300 (280 msec) wave than LHs, in the cingulate and posterior cingulate gyrus (BA23/31). The present results, on the whole, are in the opposite direction to PPI findings on hypnotizability previously reported in the literature. These results provide support to the neuropsychophysiological model that HHs have more effective sensory integration and gating (or filtering) of irrelevant stimuli than LHs.     

Dopamine Precursor Depletion Influences Pain Affect Rather than Pain Sensation

Pain is a multidimensional experience, which includes sensory, cognitive, and affective aspects. Converging lines of evidence indicate that dopaminergic neurotransmission plays an important role in human pain perception. However, the precise effects of dopamine on different aspects of pain perception remain to be elucidated. To address this question, we experimentally decreased dopaminergic neurotransmission in 22 healthy human subjects using Acute Phenylalanine and Tyrosine Depletion (APTD). During APTD and a control condition we applied brief painful laser stimuli to the hand, assessed different aspects of pain perception, and recorded electroencephalographic responses. APTD-induced decreases of cerebral dopaminergic activity did not influence sensory aspects of pain perception. In contrast, APTD yielded increases of pain unpleasantness. The increases of unpleasantness ratings positively correlated with effectiveness of APTD. Our finding of an influence of dopaminergic neurotransmission on affective but not sensory aspects of phasic pain suggests that analgesic effects of dopamine might be mediated by indirect effects on pain affect rather than by direct effects on ascending nociceptive signals. These findings contribute to our understanding of the complex relationship between dopamine and pain perception, which may play a role in various clinical pain states.     

Hysteresis in Audiovisual Synchrony Perception

The effect of stimulation history on the perception of a current event can yield two opposite effects, namely: adaptation or hysteresis. The perception of the current event thus goes in the opposite or in the same direction as prior stimulation, respectively. In audiovisual (AV) synchrony perception, adaptation effects have primarily been reported. Here, we tested if perceptual hysteresis could also be observed over adaptation in AV timing perception by varying different experimental conditions. Participants were asked to judge the synchrony of the last (test) stimulus of an AV sequence with either constant or gradually changing AV intervals (constant and dynamic condition, respectively). The onset timing of the test stimulus could be cued or not (prospective vs. retrospective condition, respectively). We observed hysteretic effects for AV synchrony judgments in the retrospective condition that were independent of the constant or dynamic nature of the adapted stimuli; these effects disappeared in the prospective condition. The present findings suggest that knowing when to estimate a stimulus property has a crucial impact on perceptual simultaneity judgments. Our results extend beyond AV timing perception, and have strong implications regarding the comparative study of hysteresis and adaptation phenomena.     

Thermal detection thresholds of Aδ- and C-fibre afferents activated by brief CO2 laser pulses applied onto the human hairy skin

Brief high-power laser pulses applied onto the hairy skin of the distal end of a limb generate a double sensation related to the activation of Aδ- and C-fibres, referred to as first and second pain. However, neurophysiological and behavioural responses related to the activation of C-fibres can be studied reliably only if the concomitant activation of Aδ-fibres is avoided. Here, using a novel CO(2) laser stimulator able to deliver constant-temperature heat pulses through a feedback regulation of laser power by an online measurement of skin temperature at target site, combined with an adaptive staircase algorithm using reaction-time to distinguish between responses triggered by Aδ- and C-fibre input, we show that it is possible to estimate robustly and independently the thermal detection thresholds of Aδ-fibres (46.9±1.7°C) and C-fibres (39.8±1.7°C). Furthermore, we show that both thresholds are dependent on the skin temperature preceding and/or surrounding the test stimulus, indicating that the Aδ- and C-fibre afferents triggering the behavioural responses to brief laser pulses behave, at least partially, as detectors of a change in skin temperature rather than as pure level detectors. Most importantly, our results show that the difference in threshold between Aδ- and C-fibre afferents activated by brief laser pulses can be exploited to activate C-fibres selectively and reliably, provided that the rise in skin temperature generated by the laser stimulator is well-controlled. Our approach could constitute a tool to explore, in humans, the physiological and pathophysiological mechanisms involved in processing C- and Aδ-fibre input, respectively.     

Corticospinal Excitability Preceding the Grasping of Emotion-Laden Stimuli

Evolutionary theories posit that emotions prime organisms for action. This study examined whether corticospinal excitability (CSE) is modulated by the emotional valence of a to-be-grasped stimulus. CSE was estimated based on the amplitude of motor evoked potentials (MEPs) elicited using transcranial magnetic stimulation (TMS) and recorded on the first dorsal interosseous (FDI) muscle. Participants were instructed to grasp (ACTION condition) or just look at (NO-ACTION condition) unpleasant, pleasant and neutral stimuli. TMS pulses were applied randomly at 500 or 250 ms before a go signal. MEP amplitudes were normalized within condition by computing a ratio for the emotion-laden stimuli by reference to the neutral stimuli. A divergent valence effect was observed in the ACTION condition, where the CSE ratio was higher during the preparation to grasp unpleasant compared to pleasant stimuli. In addition, the CSE ratio was lower for pleasant stimuli during the ACTION condition compared to the NO-ACTION condition. Altogether, these results indicate that motor preparation is selectively modulated by the valence of the stimulus to be grasped. The lower CSE for pleasant stimuli may result from the need to refrain from executing an imminent action.