Somatosensory Cues Research Articles

Pre-conditioned place preference treatment of chloral hydrate interrupts the rewarding effect of morphine

The medical use of morphine as a pain killer is hindered by its side effects including dependence and further addiction. As the prototypical μ receptor agonist, morphine's rewarding effect can be measured by conditioned place preference (CPP) paradigms in animals. Chloral hydrate is a clinical sedative. Using a morphine CPP paradigm that mainly contains somatosensory cues, we found that pre-CPP treatment in rats using chloral hydrate for 6 consecutive days could disrupt the establishment of CPP in a U shape. Chloral hydrate had no effect on the body weight of rats. Our results indicate that prior treatment with chloral hydrate can interrupt the rewarding effect of morphine.

Sensory substitution in bilateral vestibular a-reflexic patients.

Patients with bilateral vestibular loss have balance problems in darkness, but maintain spatial orientation rather effectively in the light. It has been suggested that these patients compensate for vestibular cues by relying on extravestibular signals, including visual and somatosensory cues, and integrating them with internal beliefs. How this integration comes about is unknown, but recent literature suggests the healthy brain remaps the various signals into a task-dependent reference frame, thereby weighting them according to their reliability. In this paper, we examined this account in six patients with bilateral vestibular a-reflexia, and compared them to six age-matched healthy controls. Subjects had to report the orientation of their body relative to a reference orientation or the orientation of a flashed luminous line relative to the gravitational vertical, by means of a two-alternative-forced-choice response. We tested both groups psychometrically in upright position (0°) and 90° sideways roll tilt. Perception of body tilt was unbiased in both patients and controls. Response variability, which was larger for 90° tilt, did not differ between groups, indicating that body somatosensory cues have tilt-dependent uncertainty. Perception of the visual vertical was unbiased when upright, but showed systematic undercompensation at 90° tilt. Variability, which was larger for 90° tilt than upright, did not differ between patients and controls. Our results suggest that extravestibular signals substitute for vestibular input in patients’ perception of spatial orientation. This is in line with the current status of rehabilitation programs in acute vestibular patients, targeting at recognizing body somatosensory signals as a reliable replacement for vestibular loss.

Motion analysis of real-time somatosensory cue on freezing of gait during turning in people with Parkinson's disease

Background: Freezing of gait (FOG) has a substantial impact on balance and walking in people with Parkinson’s disease (PD). Previous studies have shown that insufficient weight shift to unload the swinging leg may contribute to gait discontinuation. Purpose: This study aimed to examine the immediate effect of a real-time somatosensory cue to facilitate lateral weight shift during FOG in people with PD. Methods:Wedesigned awearable cue devicewhich composedof a sensor part and stimulator part. The sensor part is an inertial sensor placed at shank, to detect FOG episodes with the embedded time-frequency analysis algorithm. The stimulator part is a vibrator pad placed below the lateral malleolus, to facilitate lateral weight shift during FOG. Eighteen idiopathic people with PD (Age: 68.4± 4.2 year) having FOG were recruited, and took this study during their medication OFFperiod. Participants conducted the timedUp-and-Go test with the stimulator part turned on (cued trials) and off (nocue trials). Participants were videotaped and the clips were reviewed by two physical therapists to identify the freezing episodes. Weight shift was evaluated by the inclination angle of the joint line of center of mass and center of pressure. FOG severity was evaluated by FOG number and FOG duration during turning. Turning performance was evaluated by turning time and turning steps. Results: The sensor part showed moderate-to-high accuracy in detecting FOG episodes with respect to therapists’ visual inspection (sensitivity: 0.78; specificity: 0.88). Compared with no-cue trials, participants in the cued trials had greater frontal inclination angle during FOG (cued: 8.17± 0.98 vs. no-cue: 7.26± 0.53, degree). In addition, the cued trials had decreased FOG duration (cued: 2.12 vs. nocue: 3.62, second) and turning time (cued: 5.49± 2.48 vs. no-cue: 8.67± 3.70, second) than the no-cue trials, despite no difference in turning steps (cued: 10.6± 4.2 vs. no-cue: 12.2± 5.0, steps) and FOG number (cued: 0.8 vs. no-cue: 1.1, FOG per turning). Conclusion(s): Our results showed that the real-time somatosensory cue could help gait reinitiation by facilitating lateral weight shift during FOG. The shortened FOGduration decreased the turning time in people with PD. Implications: A wearable device with inertial sensor is capable of detecting FOG episodes and controlling realtime somatosensory cue to facilitate weight shift. This could provide a potential intervention for FOG during daily activities in people with PD.

Cross-Modal Calibration of Vestibular Afference for Human Balance.

To determine how the vestibular sense controls balance, we used instantaneous head angular velocity to drive a galvanic vestibular stimulus so that afference would signal that head movement was faster or slower than actual. In effect, this changed vestibular afferent gain. This increased sway 4-fold when subjects (N = 8) stood without vision. However, after a 240 s conditioning period with stable balance achieved through reliable visual or somatosensory cues, sway returned to normal. An equivalent galvanic stimulus unrelated to sway (not driven by head motion) was equally destabilising but in this situation the conditioning period of stable balance did not reduce sway. Reflex muscle responses evoked by an independent, higher bandwidth vestibular stimulus were initially reduced in amplitude by the galvanic stimulus but returned to normal levels after the conditioning period, contrary to predictions that they would decrease after adaptation to increased sensory gain and increase after adaptation to decreased sensory gain. We conclude that an erroneous vestibular signal of head motion during standing has profound effects on balance control. If it is unrelated to current head motion, the CNS has no immediate mechanism of ignoring the vestibular signal to reduce its influence on destabilising balance. This result is inconsistent with sensory reweighting based on disturbances. The increase in sway with increased sensory gain is also inconsistent with a simple feedback model of vestibular reflex action. Thus, we propose that recalibration of a forward sensory model best explains the reinterpretation of an altered reafferent signal of head motion during stable balance.

Effect of ankle weight on blind navigation.

This study tested the hypotheses that loading the ankle with a 2.3 kg weight would modify deviation (unilateral loading) and distance (unilateral and bilateral loading) during three blind navigation tasks. Ankle loading increased the distance traveled while navigating toward a previously seen target at an 8 m distance and reduced the undetected fore-aft displacement while stepping in place for 100 steps. Unilateral ankle loading had no effect on deviation during these tasks, nor in walking back and forth on an imaginary straight line. The results suggest that somatosensory cues associated with ankle loading and the increased effort to walk and step interacted with motor and cognitive functions involved in blind navigation and influenced the control of anterior-posterior body displacement.

Memory delay and haptic feedback influence the dissociation of tactile cues for perception and action

The somatosensory processing model (SPM) asserts that dissociable cortical processing streams mediate tactile perceptions and actions via relative and absolute cues, respectively (Dijkerman and de Haan, 2007). Accordingly, we sought to determine whether the introduction of a memory delay and/or physically touching a target object (i.e., haptic feedback) differentially influences the cues supporting tactile perceptions and actions. Participants used their right hand to manually estimate (i.e., perceptual task) or grasp (i.e., action task) differently sized objects placed on the palm of their left limb in conditions wherein the target object was available for the duration of the response (i.e., closed-loop condition), or was removed prior to response cuing (i.e., memory-guided condition). As well, trials were performed in conditions wherein the physical object was available (i.e., haptic feedback) or unavailable (i.e., no haptic feedback) to touch. Notably, we computed just-noticeable-difference (JND) scores to determine whether the aforementioned tasks and conditions adhered to – or violated - the relative properties of Weber's law. JNDs for manual estimations adhered to Weber's law across each condition – a finding supporting the SPM's contention that an immutable and relative percept supports tactile perceptions. In turn, JNDs for grasping violated Weber's law only when haptic feedback was available. Such a finding indicates that haptic feedback supports the absolute calibration between a tactile defined object and the required motor output. What is more, our study highlights that multiple somatosensory cues (i.e., tactile and haptic) support goal-directed grasping.

An MRI-compatible caloric stimulation device for the investigation of human vestibular cortex.

Self-motion perception involves the integration of vestibular, visual, somatosensory and other sensory cues. The neural responses to caloric vestibular stimulation (CVS) in humans have been investigated with functional magnetic resonance imaging (fMRI). We developed an fMRI-compatible, bithermal caloric stimulation device for repeated CVS. Tempered water is pumped via a closed-loop tube-system to one or both ear canals. Water temperature transmits to the surface of the ear canal via a small glass-pod. For our purposes we used hot (47-49°C), cold (5-7.5°C), or warm for baseline (30-32.5°C). The pods are integrated in the MRI ear protection and connected to water influx and efflux tubes. With our device we can apply multiple vestibular stimulation and baseline trials consecutively. Control measurements indicate that the applied temperatures are stable across trials. MRI-signal differences due to water flow and water temperature are restricted to the area surrounding the pod and are unlikely to intrude into brain tissue. Vestibular stimulation with our device elicits caloric nystagmus when no central fixation is presented. We validated our system by conducting a CVS experiment during fMRI-scanning. Participants indicated the presence or absence of a self-motion sensation. Periods of self motion yielded activation in the cortical vestibular network including putative human parieto-insular vestibular cortex (PIVC). Our closed-loop device eliminates many problems associated with caloric stimulation during fMRI. Our device allows researchers to explore neural responses to CVS and those evoked by combined sensory stimulation.

Timing of response differentiation in human motor cortex during a speeded Go/No-Go task

We explored the brain's ability to quickly prevent a pre-potent but unwanted motor response. To address this, transcranial magnetic stimulation was delivered over the motor cortex (hand representation) to probe excitability changes immediately after somatosensory cues prompted subjects to either move as fast as possible or withhold movement. Our results showed a difference in motor cortical excitability 90ms post-stimulus contingent on cues to either promote or prevent movement. We suggest that our study design emphasizing response speed coupled with well-defined early probes allowed us to extend upon similar past investigations into the timing of response inhibition.

Temperature systematically modifies neural activity for sweet taste.

Temperature can modify neural and behavioral responses to taste stimuli that elicit "sweetness," a perception linked to intake of calorie-laden foods. However, the role of temperature in the neural representation of sweet taste is poorly understood. Here we made electrophysiological recordings from gustatory neurons in the medulla of inbred mice to study how adjustments in taste solution temperature to cool (18°C), ambient (22°C), and warm (30°C and 37°C) values changed the magnitude and latency of gustatory activity to sucrose (0, 0.05, 0.1, 0.17, 0.31, and 0.56 M). Analysis of 22 sucrose-best neurons revealed that temperature markedly influenced responses to sucrose, which, across concentrations, were largest when solutions were warmed to 30°C. However, reducing solution temperature from warm to ambient to cool progressively steepened the slope of the sucrose concentration-response function computed across cells (P < 0.05), indicating that mean activity to sucrose increased more rapidly with concentration steps under cooling than with warming. Thus the slope of the sucrose concentration-response function shows an inverse relation with temperature. Temperature also influenced latency to the first spike of the sucrose response. Across neurons, latencies were shorter when sucrose solutions were warmed and longer, by hundreds of milliseconds, when solutions were cooled (P < 0.05), indicating that temperature is also a temporal parameter of sucrose activity. Our findings reveal that temperature systematically modifies the timing of gustatory activity to sucrose in the mammalian brain and how this activity changes with concentration. Results further highlight how oral somatosensory cues function as physiological modulators of gustatory processing.

Vestibular modulation of muscle sympathetic nerve activity by the utricle during sub-perceptual sinusoidal linear acceleration in humans

We assessed the capacity for the vestibular utricle to modulate muscle sympathetic nerve activity (MSNA) during sinusoidal linear acceleration at amplitudes extending from imperceptible to clearly perceptible. Subjects (n = 16) were seated in a sealed room, eliminating visual cues, mounted on a linear motor that could deliver peak sinusoidal accelerations of 30 mG in the antero-posterior direction. Subjects sat on a padded chair with their neck and head supported vertically, thereby minimizing somatosensory cues, facing the direction of motion in the anterior direction. Each block of sinusoidal motion was applied at a time unknown to subjects and in a random order of amplitudes (1.25, 2.5, 5, 10, 20 and 30 mG), at a constant frequency of 0.2 Hz. MSNA was recorded via tungsten microelectrodes inserted into muscle fascicles of the common peroneal nerve. Subjects used a linear potentiometer aligned to the axis of motion to indicate any perceived movement, which was compared with the accelerometer signal of actual room movement. On average, 67% correct detection of movement did not occur until 6.5 mG, with correct knowledge of the direction of movement at ~10 mG. Cross-correlation analysis revealed potent sinusoidal modulation of MSNA even at accelerations subjects could not perceive (1.25-5 mG). The modulation index showed a positive linear increase with acceleration amplitude, such that the modulation was significantly higher (25.3 ± 3.7%) at 30 mG than at 1.25 mG (15.5 ± 1.2%). We conclude that selective activation of the vestibular utricle causes a pronounced modulation of MSNA, even at levels well below perceptual threshold, and provides further evidence in support of the importance of vestibulosympathetic reflexes in human cardiovascular control.

Unilateral otolith centrifugation by head tilt

To test for otolith asymmetries, several studies described horizontal translation of the body and head en bloc during fast vertical axis rotation. This stimulus causes one otolithic organ to rotate on-axis, and the other to experience centripetal acceleration. To test a new, more simple method of unilateral stimulation with head tilt and the body remaining on axis. During stationary and during 360 deg/s rotation, 12 healthy blindfolded subjects had their heads tilted 30 degrees sideways, positioning one otolithic organ on the axis of rotation after the other. The haptic subjective vertical (SV) was recorded several times by means of a manually adjustable rod. It was found that during stationary the SV tilted about 4 degrees on average in the direction of the head. During rotation, the SV tilted about 9 degrees on average. We therefore estimate the effect of eccentric otolith rotation to be 5 degrees on average. Tilt of the subjective vertical induced by head tilt during on-axis body rotation can provide a relatively uncomplicated alternative to test unilateral otolithic function as compared to body and head translation during rotation. Moreover, unlike eccentric rotation of the entire body, somatosensory cues are minimized by keeping the body fixed on axis and by subtracting the effect of head tilt per se.

Stabilometric assessment of context dependent balance recovery in persons with multiple sclerosis: a randomized controlled study.

BackgroundBalance control relies on accurate perception of visual, somatosensory and vestibular cues. Sensory flow is impaired in Multiple Sclerosis (MS) and little is known about the ability of the sensory systems to adapt after neurological lesions reducing sensory impairment. The aims of the present study were to verify whether:1. Balance rehabilitation administered in a challenging sensory conditions would improve stability in upright posture.2. The improvement in a treated sensory condition would transfer to a non treated sensory condition.MethodsFifty three persons with MS, median (min-max) EDSS score of 5 (2.5-6.5), participated in a RCT and were randomly assigned to two groups. The Experimental group received balance rehabilitation aimed at improving motor and sensory strategies. The Control group received rehabilitation treatment which did not include training of sensory strategies. Persons with MS were blindly assessed by means of a stabilometric platform with eyes open, eyes closed and dome, on both firm surface and foam. Anterior-posterior and medio-lateral sway, velocity of sway and the length of Center of Pressure (CoP) trajectory were calculated in the six sensory conditions.ResultsExperimental group showed statistically significant improvement (P < 0.05) in stability in upright posture in eyes closed condition on firm surface, and in eyes open, closed, and dome conditions on foam. No differences were observed between groups in the eyes open condition on firm surface nor in the sensory condition not addressed during the treatment.ConclusionsAfter rehabilitation people with MS can recover from sensory impairments thus improving upright balance. Further, the improvement seems to be context-dependent and present just in the treated sensory conditions.Trial registrationClinicalTrials NCT02131285

Impaired force control in writer's cramp showing a bilateral deficit in sensorimotor integration

Abnormal cortical processing of sensory inputs has been found bilaterally in writer's cramp (WC). This study tested the hypothesis that patients with WC have an impaired ability to adjust grip forces according to visual and somatosensory cues in both hands. A unimanual visuomotor force-tracking task and a bimanual sense of effort force-matching task were performed by WC patients and healthy controls. In visuomotor tracking, WC patients showed increased error, greater variability, and longer release duration than controls. In the force-matching task, patients underestimated, whereas controls overestimated, the force applied in the other hand. Visuomotor tracking and force matching were equally impaired in both the symptomatic and nonsymptomatic hand in WC patients. This study provides evidence of bilaterally impaired grip-force control in WC, when using visual or sense of effort cues. This suggests a generalized subclinical deficit in sensorimotor integration in WC.

Somatosensory conditioning of sexual arousal and copulatory behavior in the male rat: A model of fetish development

Previous studies have established that neutral olfactory cues associated with sexual reward or inhibition become conditioned stimuli that direct male or female rats toward or away from potential sex partners bearing the cue. Here we examined the ability of a somatosensory cue to exert stimulus control over sexual arousal and copulation in male rats. In the first experiment, two groups of sexually naïve male rats had their first copulatory experiences with receptive females in bilevel chambers with or without a rodent jacket. On a final copulatory test, half of the rats in each group were tested with the jacket on or off. Rats trained and tested without the jacket copulated normally, as did rats trained and tested with the jacket on, and rats trained without the jacket but tested with the jacket on. However, significantly fewer rats trained with the jacket on but tested with the jacket off copulated to ejaculation, and those that did made significantly fewer anticipatory level changes and had fewer significantly longer mount, intromission, and ejaculatory latencies, and fewer ejaculations. In the second experiment, one group of sexually naïve males was given differential conditioning trials in bilevel chambers to associate the jacket on with sexual reward (copulation to ejaculation with a sexually receptive female) and the jacket off with sexual inhibition (thwarted copulatory attempts with a sexually nonreceptive female). A second group had the opposite order of association. A final test with receptive females was made for all males with the jacket on. Males trained to associate the jacket with excitation displayed normal copulation whereas males trained to associate the jacket with inhibition displayed significantly fewer anticipatory level changes and ejaculations, and had significantly longer ejaculation latencies. Thus, somatosensory cues can signal sexual excitation or inhibition in male rats depending on the conditioning history.

Neurological Basis for Eye Movements of the Blind

When normal subjects fix their eyes upon a stationary target, their gaze is not perfectly still, due to small movements that prevent visual fading. Visual loss is known to cause greater instability of gaze, but reported comparisons with normal subjects using reliable measurement techniques are few. We measured binocular gaze using the magnetic search coil technique during attempted fixation (monocular or binocular viewing) of 4 individuals with childhood-onset of monocular visual loss, 2 individuals with late-onset monocular visual loss due to age-related macular degeneration, 2 individuals with bilateral visual loss, and 20 healthy control subjects. We also measured saccades to visual or somatosensory cues. We tested the hypothesis that gaze instability following visual impairment is caused by loss of inputs that normally optimize the performance of the neural network (integrator), which ensures both monocular and conjugate gaze stability. During binocular viewing, patients with early-onset monocular loss of vision showed greater instability of vertical gaze in the eye with visual loss and, to a lesser extent, in the normal eye, compared with control subjects. These vertical eye drifts were much more disjunctive than upward saccades. In individuals with late monocular visual loss, gaze stability was more similar to control subjects. Bilateral visual loss caused eye drifts that were larger than following monocular visual loss or in control subjects. Accurate saccades could be made to somatosensory cues by an individual with acquired blindness, but voluntary saccades were absent in an individual with congenital blindness. We conclude that the neural gaze-stabilizing network, which contains neurons with both binocular and monocular discharge preferences, is under adaptive visual control. Whereas monocular visual loss causes disjunctive gaze instability, binocular blindness causes both disjunctive and conjugate gaze instability (drifts and nystagmus). Inputs that bypass this neural network, such as projections to motoneurons for upward saccades, remain conjugate.

When Standing on a Moving Support, Cutaneous Inputs Provide Sufficient Information to Plan the Anticipatory Postural Adjustments for Gait Initiation

Gait initiation is preceded by initial postural adjustments whose goal is to set up the condition required for the execution of the focal stepping movement. For instance, the step is preceded by a shift of the body’s center of mass towards the stance foot unloading the stepping leg. This displacement is produced by exerting forces on the ground (i.e., thrust) while the body is still motionless. The purpose of this study was to identify whether the mere cutaneous inputs from the feet soles evoked by a lateral translation of the support could be used to scale the initial postural adjustments. Participants stood with their eyes closed on a force platform that could be moved laterally with a low acceleration (between 0.14 m/s2 and 0.30 m/s2) to reach a constant velocity of 0.02 m/s. This translation resulted in a change in the somatosensory cues from the feet soles without modifying vestibular inputs. Participants were instructed to produce a step with the right foot as soon as they felt the platform start to move (on either side) or heard an auditory cue. In the latter case, the platform stayed stationary. We found that the thrust duration was lengthened when the platform moved towards the supporting foot. In this condition, the cutaneous stimulation provided information related to a body shift towards the stepping leg. This increased thrust duration likely helped overcoming the non-functional body shift perceived towards the stepping leg. This result highlights the accuracy with which the actual standing position can be determined from foot sole cutaneous cues in the absence of visual and vestibular or proprioceptive inputs.

Subjective posture in tridimensional space

The internal representation of the body is intimately related to postural orientation. This assists us to correctly identify our position relative to the earth-vertical. The postural orientation about the pitch-Y axis is by default forward tilted while there is no such default about the roll-X axis. We hypothetized that the internal representation of body position would be different between the Y and X axes. We designed a study to assess the accuracy of self-driven whole-body orienting to the horizontal and the vertical about the X (roll) and Y (pitch) axes, with healthy seated subjects in complete darkness. Consistent with our hypothesis, the results showed that when trying to reach the horizontal with the X-axis, subjects remained on the same tilted direction as the initial posture. On the return way to the vertical, the subjects did not cross this (vertical) line. Whereas when reaching the horizontal with the Y-axis, there was an undershoot on the supine direction, and an overshoot on the prone direction: this discrepancy could be due to somatosensory cues to subjects in seated posture. The relevant mechanisms related to the internal representation of the body and graviceptive systems underlying the construction of an head-foot line are discussed.

Speech production under real-time simulation of cochlear implant acoustic feedback

Although previous research on simulation of cochlear implant (CI) processing with normal-hearing listeners relied on offline transformation of pre-recorded acoustic signals, the advent of new simulation technology using a Portable Real-Time Vocoder (PRTV) enables subjects to experience CI simulation not only of interlocutors’ speech during face-to-face interaction, but also of their own speech acoustics. This paper explores the effects of this novel acoustic feedback manipulation on subjects’ speech production. Nine normal-hearing speakers were recorded producing 114 isolated English words and 24 sentences during three time epochs: once under normal conditions, once immediately after being fitted with the PRTV, and again after experiencing a short, natural conversational interaction through the real-time (8-channel, noise-vocoded) CI simulation. Acoustic-phonetic analysis of subjects’ speech revealed substantial, segment-specific shifts in vowel acoustics, alteration of sibilant frication frequencies, and evidence of increased cognitive load (e.g. slower speaking rates) during speech production under conditions of vocoded acoustic feedback. Speakers also appeared to alter their articulatory strategies, spending more time on production of consonants relative to vowels, possibly reflecting a flexible exploitation of reliable somatosensory versus aural feedback cues.

Effect of tetrabenazine on computerized dynamic posturography in Huntington disease patients

BackgroundImpairment in computerized dynamic posturography scores has been documented in Huntington disease patients. Tetrabenazine is approved to treat chorea in Huntington disease, but its effect on posturography scores, and balance in general, is unknown. Materials and methodsWe designed a study to test computerized dynamic posturography performance while taking tetrabenazine and after stopping tetrabenazine for at least three days. Results10 Huntington disease patients were studied both ON and OFF tetrabenazine. The composite score was statistically different between ON and OFF conditions and both conditions were significantly worse than reference scores. There was no significant difference between ON and OFF trials in the number of falls. A significant improvement on Sensory Orientation Test conditions 3 (sway-referenced vision) and 5 (sway-referenced motion of the support surface and eyes closed) was seen while ON tetrabenazine. Strategy scores 1–3 were also significantly different while ON tetrabenazine. ConclusionThese findings suggest that tetrabenazine aided patients in gating out of abnormal visual cues when other sensory modalities were available, as well as in gating out abnormal kinesthetic cues when visual cues were not available. It could not help with gating out of simultaneous abnormal visual and somatosensory cues. Thus, tetrabenazine can improve postural stability when one sensory modality is irrelevant, but this effect is not sustained when multiple abnormal sensory modalities are present. This is the first study supporting the use of any medicine to treat balance problems in Huntington disease.

Who, What, Where, When (and Maybe Even Why)? How the Experience of Sexual Reward Connects Sexual Desire, Preference, and Performance

Although sexual behavior is controlled by hormonal and neurochemical actions in the brain, sexual experience induces a degree of plasticity that allows animals to form instrumental and Pavlovian associations that predict sexual outcomes, thereby directing the strength of sexual responding. This review describes how experience with sexual reward strengthens the development of sexual behavior and induces sexually-conditioned place and partner preferences in rats. In both male and female rats, early sexual experience with partners scented with a neutral or even noxious odor induces a preference for scented partners in subsequent choice tests. Those preferences can also be induced by injections of morphine or oxytocin paired with a male rat's first exposure to scented females, indicating that pharmacological activation of opioid or oxytocin receptors can "stand in" for the sexual reward-related neurochemical processes normally activated by sexual stimulation. Conversely, conditioned place or partner preferences can be blocked by the opioid receptor antagonist naloxone. A somatosensory cue (a rodent jacket) paired with sexual reward comes to elicit sexual arousal in male rats, such that paired rats with the jacket off show dramatic copulatory deficits. We propose that endogenous opioid activation forms the basis of sexual reward, which also sensitizes hypothalamic and mesolimbic dopamine systems in the presence of cues that predict sexual reward. Those systems act to focus attention on, and activate goal-directed behavior toward, reward-related stimuli. Thus, a critical period exists during an individual's early sexual experience that creates a "love map" or Gestalt of features, movements, feelings, and interpersonal interactions associated with sexual reward.