The Emory group then used positron emission tomography (PET) to show that a region of the cerebral cortex associated with sight is engaged when humans attempt to distinguish orientation via delicate touch, as in the fingertip grating task (NeuroReport, 1997). But whether this region was truly necessary for tactile performance was unclear.
The current study aimed to answer this question. The researchers asked normally sighted volunteers to perform a series of tactile discrimination tasks designed to quantify tactile abilities, including the fingertip grating task. Subjects were asked to tell either the orientation (direction) of the grating (along or across the finger) or whether the grooves were wide or narrow (spacing task). The 14 subjects performed the tasks while researchers used harmless transcranial magnetic stimulation to transiently block the function of, first, various parts of the brain region associated with the sense of sight (the occipital cortex), and then, the brain region mediating the sense of touch (the somatosensory cortex).
When a key region of visual cortex (the parieto-occipital cortex) was blocked, subjects were significantly less successful in discriminating the orientation of the grating via touch, according to first author of the paper Andro Zangaladze, M.D., Ph.D., now a resident in neurology at Thomas Jefferson University Hospital in Philadelphia, who conducted the research while a fellow in Dr. Sathian's laboratory at Emory. However, performance on the spacing task was unaffected, implying that the effect was selective for orientation. In contrast, blocking somatosensory cortex interfered nonselectively with performance on both tasks.
"Together with the subjective reports of visual imagery in this task and
the associated parieto-occipital cortical a
Contact: Sarah Goodwin
Emory University Health Sciences Center