Embodied Precision: Intranasal Oxytocin Modulates Multisensory Integration.

Multisensory integration processes are fundamental to our sense of self as embodied beings. Bodily illusions, such as the rubber hand illusion (RHI) and the size-weight illusion (SWI), allow us to investigate how the brain resolves conflicting multisensory evidence during perceptual inference in relation to different facets of body representation. In the RHI, synchronous tactile stimulation of a participant's hidden hand and a visible rubber hand creates illusory body ownership; in the SWI, the perceived size of the body can modulate the estimated weight of external objects. According to Bayesian models, such illusions arise as an attempt to explain the causes of multisensory perception and may reflect the attenuation of somatosensory precision, which is required to resolve perceptual hypotheses about conflicting multisensory input. Recent hypotheses propose that the precision of sensorimotor representations is determined by modulators of synaptic gain, like dopamine, acetylcholine, and oxytocin. However, these neuromodulatory hypotheses have not been tested in the context of embodied multisensory integration. The present, double-blind, placebo-controlled, crossover study ( n = 41 healthy volunteers) aimed to investigate the effect of intranasal oxytocin (IN-OT) on multisensory integration processes, tested by means of the RHI and the SWI. Results showed that IN-OT enhanced the subjective feeling of ownership in the RHI, only when synchronous tactile stimulation was involved. Furthermore, IN-OT increased an embodied version of the SWI (quantified as estimation error during a weight estimation task). These findings suggest that oxytocin might modulate processes of visuotactile multisensory integration by increasing the precision of top-down signals against bottom-up sensory input.

Dopamine modulation affects the performance of parkinsonian patients in a precision motor task measured by an antropomorphic device.

Several parameters related to the timing, grip force and load force involved in a precision grasping task were studied in patients with Parkinson's disease (PD) at different moments of medication and healthy controls, using a sensorized anthropomorphic device which was totally adapted to the human hand. The aim of this work was to carry out an accurate study of the reach-load-grip-hold-place-release subtasks to identify any physical motor impairment, its relation to medication and Parkinsonian strategies. Twenty seven patients in ON and OFF-like medication moments, and twenty seven age-matched controls took part in the experiment, which consisted of using the index finger and the thumb to perform a precision motor task involving different experimental objects. Visual cues were used as distracting elements. Results showed several motor parameters impaired in OFF-like medication moment but attenuated in ON state, suggesting a medication effect on the performance of the task.

Overestimation of force during matching of externally generated forces.

If a weight is applied to a finger and the subject asked to produce the same force, the subject generates a force larger than the weight. That is, subjects overestimate the force applied by an external target when matching it. Details of this force overestimation are not well understood. We show that subjects overestimate small target weights, but not larger ones. Furthermore we show for the first time that the force overestimation consists of two components. The first component is a constant. The second component depends on the precise magnitude of the weight and is only present when subjects hold the target weight against gravity. We suggest that the two components are generated in different phases of the force-matching task, are due to different processes, and must have an influence on all proprioceptive judgements of force.

Narcosis has additive rather than interactive effects on discrimination reaction time.

A central feature of the impairment in performance produced by inert gas narcosis, which poses a threat to divers breathing compressed air, is a slowing of reaction time (RT). To investigate the locus of this slowing, the effects of 35% nitrous oxide on Crossman's confusion function were determined using line-length and weight discrimination tasks, with accuracy held constant. For both tasks narcosis slowed RT by increasing the intercept rather than the slope of Crossman's function. These results are interpreted in terms of additive factors method logic as being consistent with the predictions of the slowed processing model that has been proposed to account for the effects of narcosis on human performance.

Alterations in perceived heaviness during digital anaesthesia.

1. It was shown by Gandevia & McCloskey (1977) that anaesthesia of the thumb causes weights lifted by thumb flexion to feel heavier, and weights lifted by thumb extension to feel lighter. This was confirmed by Marsden, Rothwell & Traub (1979). Gandevia & McCloskey explained the effects in terms of altered motor command (and so, altered 'sense of muscular force, or heaviness') required when sources of peripheral reflex facilitation or inhibition were removed by anaesthesia. Marsden et al. proposed, instead, that co-contraction of antagonists altered in anaesthesia. They said that increases in heaviness occurred for flexion because anaesthesia was associated with increased activity in the thumb extensor and, similarly, that decreases in heaviness occurred for extension because anaesthesia reduced activity in the thumb flexor. 2. The apparent heaviness of weights lifted by flexion of the thumb is not systematically altered by paralysis of the extensor of the thumb (radial nerve block at the elbow). Heaviness is increased by local anaesthetization of the thumb (digital nerve block). If paralysis of the thumb extensor is induced after digital nerve block, no further alteration in apparent heaviness occurs. 3. If extensor co-contraction is encouraged for stabilization of the wrist, as by having subjects perform an isometric contraction with the index-finger while lifting weights by thumb flexion, the apparent heaviness of the weights is increased. If co-contraction of the extensors is prevented by radial nerve block the manoeuvre of simultaneous contraction of the index finger then fails to alter the heaviness of weights lifted by thumb flexion. 4. The middle finger can be postured in such a way that active extension but not active flexion of its distal joint is impossible. When co-contraction of the extensor cannot act at the distal joint, anaesthetization of the middle finger causes an increase in apparent heaviness of weights lifted by flexion of this joint. 5. The effects of digital anaesthesia on the apparent heaviness of objects lifted by digital flexion are not caused by co-contraction of the extensors of the digits, as proposed by Marsden et al. Where extensor co-contraction occurs unrelated to the digital flexion task, as in stabilization of the wrist during simultaneous flexion of more than one digit, it may affect the apparent heaviness of objects borne by individual digits.