Activity of the Cerebral Cortex during Intraoral Form-discrimination Task

Objectives: The importance of somatosensory information arising from intraoral mechanoreceptors in intraoral sensory discrimination (e.g. detecting position, consistency, size and shape of an object in the oral cavity) should be stressed, since sensory discrimination in the oral cavity are performed without visual information by nature. Nonetheless, there is little information about how the somatosensory information is processed within the cerebral cortex during intraoral sensory discrimination tasks. For this, many studies showed that visual cortical areas are active during tactile discrimination such as Braille reading and blind discrimination of Mah-Jong tiles both in blind and normally sighted humans.

Methods: The present study was conducted in 10 normally sighted adults (23-62 years old). Non-invasive 44 channel near-infrared spectroscopy (NIRS) was used to investigate if visual cortical areas as well as prefrontal area are activated during blind discrimination of a form of rice biscuits with varied forms 1) in the oral cavity (intraoral-form discrimination task) or 2) using hands (hand-form discrimination task) by assessing total ([total-Hb]), oxygenated ([oxy-Hb]) and deoxygenated ([deoxy-Hb]) hemoglobin concentration changes.

Results: Among 10 subjects tested, increase in [total-Hb] and [oxy-Hb] was observed in all subjects for visual cortical areas and in 8 subjects for prefrontal area during intraoral-form discrimination task. On the other hand, such task-related increase in [total-Hb] and [oxy-Hb] was observed in 7 subjects for visual cortical areas and in 2 subjects for prefrontal area during hand-form discrimination task.

Conclusions: Visual cortical areas as well as prefrontal area may play important roles in intraoral somatosensory discrimination. In addition, somatosensory processing patterns within the cerebral cortex may be variable among individuals.