Prefrontal control of the tactile sense

Sense of touch provides information about textures, objects and physical forces acting on our body. Tactile information also guides the motor system by providing feedback of our internally generated movements. The aim of this thesis was to dissect the neural mechanisms underlying different levels of tactile processing in healthy subjects. In the first study of the thesis, the influence of temporal properties of stimulation on tactile spatial discrimination ability was studied. The study revealed that the ability to discriminate adjacent tactile stimuli as spatially distinct was influenced by temporal features of stimulation. The prefrontal cortex, a brain area involved in many complex cognitive abilities such as working memory, attention and introspection, has been suggested to modulate the functions of primary cortical areas such as the primary somatosensory cortex. However, in the prefrontal areas, there is considerable interindividual variability in the topography of neural structure and function that provides a challenge to studies addressing control of primary sensory areas by the prefrontal cortex. In the second study of the thesis, the challenge of interindividual variability was addressed by determining the individual neural pathways between the primary somatosensory cortex and the prefrontal cortex. First, the somatosensory representation area of the right hand thenar skin was functionally determined by blocking the sensation with navigated transcranial magnetic stimulation. The somatosensory cortical site in which the magnetic stimulation blocked the tactile stimuli, served as a seed point for the probabilistic tractography between the primary somatosensory cortex and two prefrontal areas of interest: the middle frontal gyrus and the superior frontal gyrus. These two tractography-informed prefrontal areas were probed with transcranial magnetic stimulation pulses which were delivered during a tactile temporal discrimination task. The results showed that the transcranial magnetic stimulation of the middle frontal gyrus, but not that of the superior frontal gyrus, reduced the temporal discrimination ability when compared to sham stimulation. Moreover, this impairment effect was temporally and spatially specific: the effect was dependent on the transcranial magnetic stimulation delay and the location of tactile test stimuli. In conclusion, the second study of the thesis suggested that the prefrontal cortex contains neural substrates for fine control of tactile discrimination. Metacognition, “knowing about knowing”, refers to the ability of humans to monitor and control one’s own cognitive processes. Lesion and brain imaging studies suggest that the prefrontal cortex is important for metacognition, but there are only few transcranial magnetic stimulation studies using state-of-art, bias-free measures of metacognitive ability. In the third study of the thesis, the metacognitive accuracy of tactile working memory was explored with a similar kind of tractography-based transcranial magnetic stimulation approach as in the second study. Two distinct prefrontal areas, the middle frontal gyrus and the superior frontal gyrus, were stimulated while the subjects performed tactile spatial and temporal working memory tasks. Subjective evaluations of confidence were used to calculate the metacognitive accuracy in each stimulation site and working memory task condition. The results showed that transcranial magnetic stimulation of the superior frontal gyrus, but not that of the middle frontal gyrus, enhanced the metacognitive accuracy of the temporal working memory task. When the superior frontal gyrus was stimulated, the subjects were able to better match the incorrectly performed test trials with lower confidence. In the spatial working memory task in which the tactile stimuli were identical to the temporal working memory task, the stimulation of either the middle frontal or superior frontal gyrus had no effect on the metacognitive accuracy. The results of the third study indicate that the prefrontal cortex might contain specific structural correlates for the metacognitive ability.