Neural coding for tactile motion: Scanning speed or temporal frequency?

Humans effortlessly perceive the speed of an object moving across their fingers, but how the brain encodes this information, especially across the hierarchical stages in the primary somatosensory cortex, remains unclear. This study thus investigated coding schemes, including rate and temporal codes, for tactile motion speed in macaque S1 areas 3b, 1 and 2. Extracellular electrophysiology recorded single-unit activities when a rotating sinusoidal grating ball of a fixed spatial period (wavelength of 1, 2 or 4 mm) was presented on the finger pad at various speeds (20-320 mm/s). The results showed that the rate code was commonly employed to differentiate the stimulus scanning speed, spatial period and scanning direction across S1 regions. In contrast the temporal code was used to faithfully represent the stimulus temporal frequency, which was defined as the speed divided by the spatial period. Notably area 3b had a wider range of frequency responses than did areas 1 and 2. These findings demonstrate that S1 uses both rate and temporal codes to encode distinct aspects of tactile motion. Future research should investigate how temporal patterns in S1 neuronal activity are potentially transformed and utilized in downstream somatosensory areas to form tactile motion perception and guide perceptual decisions. KEY POINTS: Primate's primary somatosensory cortex (S1) employs a sophisticated 'division of labour' to encode tactile motion. Downstream areas encode speed using overall neuronal firing rate, whereas an earlier stage encodes texture using precise spike timing. This hierarchical strategy for disentangling sensory information could serve as a blueprint for restoring a naturalistic sense of touch.