Superior sensitivity for tactile stimuli oriented proximally-distally on the finger: Implications for mixed class 1 and class 2 anisotropies

Inferior performance for obliquely oriented stimuli is often observed on higher level visual and somatosensory tasks and also on tests of low-level visual sensory ability. This study demonstrated an anisotropy of low-level somatosensory performance. Sensitivity to gratings on the finger pad was highest for gratings oriented proximally-distally, intermediate for oblique gratings, and lowest for medial-lateral gratings. This pattern supports a model proposing that detection threshold is determined by the number of neurons tuned to a stimulus (A. Anzai, A. Bearse, Jr., R. D. Freeman, & D. Cai, 1995). A classification of somatosensory and visual anisotropies is proposed in which orientation biases are classified as being attributable to either anisotropic sensory filtering (Class 1) or anisotropic higher level processing (Class 2). It was concluded that a given instance of anisotropic visual or somatosensory performance may stem from low-level sensory factors, high-level factors, or a mixture of the two, depending on the task demands.