Line Löken

Coding of pleasant touch by unmyelinated afferents in humans

Pleasant touch sensations may begin with neural coding in the periphery by specific afferents. We found that during soft brush stroking, low-threshold unmyelinated mechanoreceptors (C-tactile), but not myelinated afferents, responded most vigorously at intermediate brushing velocities (1−10 cm s−1), which were perceived by subjects as being the most pleasant. Our results indicate that C-tactile afferents constitute a privileged peripheral pathway for pleasant tactile stimulation that is likely to signal affiliative social body contact.

The skin as a social organ

In general, social neuroscience research tends to focus on visual and auditory channels as routes for social information. However, because the skin is the site of events and processes crucial to the way we think about, feel about, and interact with one another, touch can mediate social perceptions in various ways. This review situates cutaneous perception within a social neuroscience framework by discussing evidence for considering touch (and to some extent pain) as a channel for social information. Social information conveys features of individuals or their interactions that have potential bearing on future interactions, and attendant mental and emotional states. Here, we discuss evidence for an affective dimension of touch and explore its wider implications for the exchange of social information. We consider three important roles for this affective dimension of the cutaneous senses in the transmission and processing of social information: first, through affiliative behavior and communication; second, via affective processing in skin–brain pathways; and third, as a basis for intersubjective representation.

Somatotopic organization of gentle touch processing in the posterior insular cortex.

A network of thin (C and Aδ) afferents relays various signals related to the physiological condition of the body, including sensations of gentle touch, pain, and temperature changes. Such afferents project to the insular cortex, where a somatotopic organization of responses to noxious and cooling stimuli was recently observed. To explore the possibility of a corresponding body-map topography in relation to gentle touch mediated through C tactile (CT) fibers, we applied soft brush stimuli to the right forearm and thigh of a patient (GL) lacking Aβ afferents, and six healthy subjects during functional magnetic resonance imaging (fMRI). For improved fMRI analysis, we used a highly sensitive multivariate voxel clustering approach. A somatotopic organization of the left (contralateral) posterior insular cortex was consistently demonstrated in all subjects, including GL, with forearm projecting anterior to thigh stimulation. Also, despite denying any sense of touch in daily life, GL correctly localized 97% of the stimuli to the forearm or thigh in a forced-choice paradigm. The consistency in activation patterns across GL and the healthy subjects suggests that the identified organization reflects the central projection of CT fibers. Moreover, substantial similarities of the presently observed insular activation with that described for noxious and cooling stimuli solidify the hypothesized sensory-affective role of the CT system in the maintenance of physical well-being as part of a thin-afferent homeostatic network.

Physiological and Behavioral Responses Reveal 9-Month-Old Infants’ Sensitivity to Pleasant Touch

Caregiving touch has been shown to be essential for the growth and development of human infants. However, the physiological and behavioral mechanisms that underpin infants’ sensitivity to pleasant touch are still poorly understood. In human adults, a subclass of unmyelinated peripheral nerve fibers has been shown to respond preferentially to medium-velocity soft brushing. It has been theorized that this privileged pathway for pleasant touch is used for close affiliative interactions with conspecific individuals, especially between caregivers and infants. To test whether human infants are sensitive to pleasant touch, we examined arousal (heart rate) and attentional engagement (gaze shifts and duration of looks) to varying velocities of brushing (slow, medium, and fast) in 9-month-old infants. Our results provide physiological and behavioral evidence that sensitivity to pleasant touch emerges early in development and therefore plays an important role in regulating human social interactions.

Pleasantness of touch in human glabrous and hairy skin: Order effects on affective ratings

The tactile sense comprises pathways for both discriminative and affective touch. Low threshold unmyelinated mechanoafferents (C tactile, CT) in the human hairy skin have recently been linked to pleasant touch sensation. Here, we investigated how perception of the hedonic aspect of tactile stimulation differs between the hairy skin of the arm, and the glabrous skin of the palm, which is not innervated by CT afferents. Three groups of naïve, healthy subjects (total n=28) rated pleasantness on a visual analogue scale (VAS) when we stroked with a soft brush with speeds from 0.1 to 30cm/s on the palm or forearm. We used two different experimental approaches: in experiments 1 and 2, stimuli were delivered successively on the palm and arm (or arm and palm) in temporally separate sequential blocks. In experiment 3, stimuli were delivered alternately on arm and palm. We found that the order of stimulus presentation, palm/arm or arm/palm, has an effect on pleasantness ratings of gentle brush stroking with varying velocity. Notably, the perception of pleasantness for palm stimulation was affected by previous stimulation of the arm, but not vice versa. Thus, assessment of valence of touch may be influenced by affective reactions elicited by activation of the CT afferent pathway.

Disturbances in affective touch in hereditary sensory & autonomic neuropathy type III.

Hereditary sensory and autonomic neuropathy type III (HSAN III, Riley–Day syndrome, Familial Dysautomia) is characterised by elevated thermal thresholds and an indifference to pain. Using microelectrode recordings we recently showed that these patients possess no functional stretch-sensitive mechanoreceptors in their muscles (muscle spindles), a feature that may explain their lack of stretch reflexes and ataxic gait, yet patients have apparently normal low-threshold cutaneous mechanoreceptors. The density of C-fibres in the skin is markedly reduced in patients with HSAN III, but it is not known whether the C-tactile afferents, a distinct type of low-threshold C fibre present in hairy skin that is sensitive to gentle stroking and has been implicated in the coding of pleasant touch are specifically affected in HSAN III patients. We addressed the relationship between C-tactile afferent function and pleasant touch perception in 15 patients with HSAN III and 15 age-matched control subjects. A soft make-up brush was used to apply stroking stimuli to the forearm and lateral aspect of the leg at five velocities: 0.3, 1, 3, 10 and 30 cm/s. As demonstrated previously, the control subjects rated the slowest and highest velocities as less pleasant than those applied at 1–10 cm/s, which fits with the optimal velocities for exciting C-tactile afferents. Conversely, for the patients, ratings of pleasantness did not fit the profile for C-tactile afferents. Patients either rated the higher velocities as more pleasant than the slow velocities, with the slowest velocities being rated unpleasant, or rated all velocities equally pleasant. We interpret this to reflect absent or reduced C-tactile afferent density in the skin of patients with HSAN III, who are likely using tactile cues (i.e. myelinated afferents) to rate pleasantness of stroking or are attributing pleasantness to this type of stimulus irrespective of velocity.

Tactile direction discrimination and vibration detection in diabetic neuropathy

Löken LS, Lundblad LC, Elam M, Olausson HW. Tactile direction discrimination and vibration detection in diabetic neuropathy. Acta Neurol Scand: 2010: 121: 302–308.
© 2009 The Authors Journal compilation

FC12.1 Quantification of mild diabetic neuropathy

world’ topography (characterized by strong local clustering in combination with short path lengths) are known to facilitate synchronization, and possibly seizure generation. Objective: We tested the hypothesis that real brain networks during seizures display small-world features, using intracerebral recordings of mesial temporal lobe seizures. Methods: We used the synchronization likelihood (SL) as a measure of functional correlation in the intracerebral EEG recordings of 7 patients with mesial temporal lobe epilepsy (MTLE). The signals were analyzed in different frequency bands. After correction for the differences in synchronization in the periods, the cluster coefficient C and the path length L were computed for 5 periods of interest. These periods were: interictal, before-, duringand after rapid discharges (resp. BRD, DRD and ARD) (in which the periods DRD and ARD are ictal and BRD is seen as preictal) and postictal. Results: The neuronal network changed during seizure activity, with an increase of C most prominent in the lower frequencies (1–13 Hz) at the end and after the seizure and shortening of L at the end of the seizure (4–8 Hz), expanding after the seizure to 4–13 Hz. Conclusions: During MTLE seizures the neuronal network changes towards a small-world configuration compared to the interictal network, even after correcting for changes in synchronization strength. The interictal network has characteristics of a random network. Analysis of neuronal networks during seizures may provide insight into seizure genesis and development.