The Limits of Touch: Spatial acuity for frequency-resolved air-borne ultrasound vibrotactile stimuli

Spatial acuity is a fundamental property of any sensory system. In the case of the somatosensory system, the two-point discrimination (2PD) test has long been used to investigate the spatial resolution of tactile perception. The somatosensory system comprises multiple mechanoreceptive channels, each tuned to specific vibrotactile frequencies. In particular, the rapidly adapting channel (RA) responds to low-frequency vibration and is thought to have high spatial acuity. The Pacinian channel (PC) responds to high-frequency vibration and is thought to convey little or no spatial information. However, the mechanical stimulations used in most 2PD tests make it difficult to disentangle the contribution of each mechanoreceptive channel to spatial tactile perception. Here we developed a novel 2PD test based on ultrasound stimulation to deliver frequency-resolved vibrotactile stimuli designed to preferentially activate specific tactile channels. Across four experiments, we systematically investigated the spatial resolution of the two main vibrotactile channels. Contrary to the textbook view of poor spatial resolution for PC-like stimuli, we found that high-frequency vibration produced surprisingly good spatial acuity. This effect remained after controlling for differences between the channels in stimulus detectability and perceived intensity. Laser doppler vibrometry experiments confirmed that the acuity of the PC channel was not simply an artifact of the skins resonance to high-frequency mechanical stimulation. Thus, PC receptors may transmit substantial spatial information, despite their sparse distribution, deep location, and large receptive fields.