Abstract
fMRI studies that investigate somatotopic tactile representations in the human cortex typically use either block or phase-encoded stimulation designs. Event-related (ER) designs allow for more flexible and unpredictable stimulation sequences than the other methods, but they are less efficient. Here we compared an efficiency-optimized fast ER design (2.8s average intertrial interval, ITI) to a conventional slow ER design (8s average ITI) for mapping voxelwise fingertip tactile tuning properties in the sensorimotor cortex of 6 participants at 7 Tesla. The fast ER design yielded more reliable responses compared to the slow ER design, but with otherwise similar tuning properties. Concatenating the fast and slow ER data, we demonstrate in each individual brain the existence of two separate somatotopically-organized representations of the fingertips, one in the primary somatosensory cortex (S1) on the post-central gyrus, and the other shared across the motor and pre-motor cortices on the pre-central gyrus. In both S1 and motor representations, fingertip selectivity decreased progressively, from narrowly-tuned Brodmann areas 3b and 4a respectively, towards associative parietal and frontal regions that responded equally to all fingertips, suggesting increasing information integration along these two pathways. In addition, fingertip selectivity in S1 decreased from the cortical representation of the thumb to that of the pinky.
Significance Statement Sensory and motor cortices in the human brain contain map-like representations of the body in which adjacent brain regions respond to adjacent body parts. The properties of these somatotopic maps provide important insight into how tactile and motor information is processed by the brain. Here, we describe an efficient mapping method using functional magnetic resonance imaging (fMRI) to measure somatotopic maps and their tuning properties. We used a fast event-related sequence to map the five fingers of the left hand in six human participants, and show that this method is superior to a conventional, slower event-related design. Furthermore, we confirm previously-identified tuning properties of fingertip representations in somatosensory cortex, and reveal a hitherto unknown fingertip map in the motor cortex.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
Conflict of Interest Disclosure: The authors declare that they have no conflicts of interest.
Funding Statement: This work was supported by a BBSRC grant (BB/G008906/1) and by the University Research Board of the American University of Beirut. The funding sources had no involvement in the study design, collection, analysis and interpretation of data, writing of the manuscript or decision to submit the article for publication.
Title, Abstract, Significance statement and Introduction updated; Figure 4 updated; Added Supplemental Figure 4-2