We included 1 additional participant with a rare large‐fiber sensory neuropathy, consisting of a complete loss of large‐myelinated Aβ fibers below the neck, but sparing thinly myelinated Aδ and unmyelinated C fibers. 6 , 7 This 61‐year‐old man developed an acute sensory neuronopathy about 40 years ago, thought to follow viral diarrhea, leaving him without any sense of touch or proprioception below the neck (C3). His clinical characteristics and electrophysiological findings have been described in several single‐case studies. 6 - 9

Twenty‐six healthy volunteers participated in the study, after having given written informed consent. Sixteen volunteers took part in Experiment 1 (9 females; mean age ± standard deviation [SD] = 23 ± 2.8 years), and 10 volunteers participated in Experiment 2 (6 females; mean age ± SD = 22.9 ± 3.3 years). The study was conducted in accordance with the principles of the Declaration of Helsinki, and was approved by the local ethics committee.

Noxious radiant‐heat stimuli were generated by 2 identical infrared Nd:YAP lasers with a wavelength of 1.34μm (Electronic Engineering, Florence, Italy). The laser pulses were transmitted through optic fibers, and focused by lenses to a spot with a diameter of 1.3mm (approximately 1.3mm 2 ; 4‐millisecond duration). He‐Ne lasers pointed to the area to be stimulated. The laser energy (0.35–0.46J/mm 2 ) was adjusted in each subject and stimulated district to: (1) elicit a clear pinprick sensation, reflecting Aδ‐fiber activation 10 ; (2) achieve a mean pain intensity rating of 3 (0 = no pinprick pain, 1 = pinprick pain threshold, 10 = worst pinprick pain imaginable); and (3) match the perceived pain intensity across body territories. We allowed pain ratings to vary by ±1 score between individuals, and ±0.5 within individuals, across the explored body regions.

The participant with Aβ‐fiber loss did not perceive any stimulus delivered with the von Frey hair (range = 0.008–300g) on the hand dorsum, palm, and fingertip, confirming that he was totally devoid of tactile sensitivity. 7 , 12

To assess 2PD for touch, we manually delivered somatosensory stimuli using von Frey filaments (diameter = 0.4mm, weight = 1g) mounted on an electronic vernier caliper. Stimulus duration was 1 second. In all healthy participants, these stimuli elicited a clear tactile percept, which was never described as painful and was comparable in perceived intensity. 11

Procedure

In a first experiment in 16 healthy volunteers (Experiment 1), we measured 2PD using simultaneous stimuli. We randomly delivered either single stimuli (25% of trials) or 2 simultaneous stimuli (75% of trials). Participants reported whether they felt 1 or 2 stimuli. Importantly, we varied the intensity of the single nociceptive stimuli, so that some of them had a much higher intensity than the intensity of the 2 simultaneous stimuli. Therefore, the participant could not use the perceived intensity of the laser pulses as a cue to resolve the spatial task.

In a second experiment in 10 healthy volunteers (Experiment 2), we measured 2PD using successive stimuli. Each trial involved 2 stimuli, separated by an interval of 3 seconds. The first stimulus was located either more proximally or more distally than the second stimulus, with equal probability of occurrence. The task was to judge whether the second stimulus was proximal or distal relative to the first one.

In both experiments, somatosensory stimuli were delivered to 11 body regions, in separate sessions over the course of a week, at similar day times. Each session involved either tactile or nociceptive stimulation exclusively, and tested 3 or 4 randomly selected body regions in separate blocks. Throughout each session, participants lay on a bench, wearing a blindfold.

The explored body regions (see Fig 1) included: (1) the first trigeminal division on the forehead; (2) the dorsal aspect of the shoulder, about 3 to 5cm laterally to the C5 and C6 vertebral spinous processes; (3) the volar surface of forearm; (4) the hand dorsum; (5) the hand palm; (6) the volar surface of the index and middle fingertips; (7) the lower back, about 3 to 5cm laterally to the T10 and T11 vertebral spinous processes; (8) the midportion of the anterior shaft of the thigh; (9) the midcalf; (10) the foot dorsum; and (11) the inner side of the foot sole. When 2 stimuli were delivered, they were aligned along the proximal–distal axis of the body region studied, whereas the single stimulus was randomly delivered within the same skin area. The sole of the foot was not tested in Subject 1 in Experiment 1, and the forehead was not tested in Subject 2 in Experiment 2.

To measure discrimination thresholds, we used the method of limits with interleaved ascending and descending staircases (see Fig 1). In ascending staircases, the initial distance was 0.2cm. In descending staircases, the initial distance was the maximal achievable for the explored body territory. The distance between the 2 stimuli was initially adjusted in steps of 3cm, and then progressively reduced until the minimal distance at which the stimuli were correctly discriminated on 3 consecutive stimulations was reached.13 This distance was defined as the spatial discrimination threshold.

To avoid fatigue or sensitization of skin receptors, the stimulus locations were slightly varied from trial to trial, and the same spot was never stimulated twice within 1 minute. Threshold measurements were alternated on homologous regions of the right and left side of the body part studied, so that the same side was never stimulated in 2 consecutive blocks. In each participant, the 2PD threshold of a given body region was the average between 2 thresholds obtained on 1 side and 1 threshold obtained on the opposite side. The order of stimulated sides was balanced across participants.

Spatial discrimination in the participant with complete Aβ‐fiber loss was assessed using pairs of successive stimuli, on 3 hand regions in a single session: the hand dorsum, palm, and the volar surface of the tips of the index and middle fingers. The spatial discrimination threshold was defined with the method of limits described above, using 3 staircases per body region.