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Whereas acuity is the ability to resolve separate stimuli, hyperacuity involves discriminations that are finer than the normal acuity limit determined by the innervation density of touch receptors. Our study involves an analysis of tactile hyperacuity in normally sighted subjects. A compilation of neuroimaging data revealed the activation of several visual areas during a tactile task, suggesting that the function of these areas may not be restricted to visual processing. Psychophysical data revealed an average threshold across subjects within the hyperacuity range of the index fingerpad (< 1mm).
Our exploration of tactile hyperacuity was two-fold:
1. Functional magnetic resonance imaging (fMRI) - identify common areas of activation, between 2 subjects for the hyperacuity task.
2. Psychophysical data - establish thresholds for each subject and an average threshold across all subjects to confirm that subjects are performing within the hyperacuity range.
Image Acquisition
• Equipment: 3T Siemens Trio scanner, quadrature head coil.
• Anatomical Images: 3D MPRAGE, 176 (1mm) sagittal slices, TR 2300ms, TE 3.9ms, TI 1100ms, FA 8, in-plane resolution 1x1mm, in-plane matrix 256x256.
• Functional Images: 2D T2* single-shot EPI BOLD, 27(4mm) axial slices, TR 2000ms, TE 30ms, FA 90, in-plane resolution 3.4x3.4mm, in-plane matrix 64x64.
II. Experimental Paradigm
• Neuroimaging: n=2, 2 female, mean age 20, both right-handed.
• Stimuli: raised-outline of trapezoid and parallelogram made of aluminum, offset 0.25-1 mm towards the left or right of the center of a base-plate.
• Stimulus Delivery: MRI-compatible pneumatic stimulator, Presentation software package keeps duration and applied pressure of stimulus constant; subjects blindfolded; stimuli applied to immobilized right index fingerpad.
• Block Design: 12 active blocks (24 seconds) made up of 6 location experimental tasks and 6 shape control tasks alternating with rest periods (16-20 seconds) between each block; location task was a left vs. right discrimination; shape task was a parallelogram vs. trapezoid discrimination. Example given below:
-Psychophysical: n=9, 5 males, 4 females, mean age 20.8, all right-handed and neurologically normal; no block design, solely location task.
III. Image Processing and Analysis
• Software: BrainVoyager QX v1.6 (Brain Innovation, Maastricht, The Netherlands)
• EPI Images: realigned, preprocessed, coregistered and transformed into Talairach space.
• Statistical Analysis: individual and group analysis used the general linear model (GLM) with correction for multiple comparisons by the false discovery rate (FDR) approach; for group analysis, data was spatially smoothed to 4mm and baseline periods were z-normalized.
fMRI Data
• Accuracy: Average accuracy on all tasks was 91% correct or better.
• Haptic Activations: images below display activity during the tactile, hyperacuity location task.
o Somatosensory Cortical Areas: the postcentral sulcus (PCS), anterior intraparietal sulcus (aIPS), and posterior intraparietal sulcus (pIPS).
o Visual Cortical Areas: the lateral occipital complex (LOC), superior temporal sulcus (STS), and early visual areas around the calcarine sulcus in the medial occipital cortex (MOC).
o Other Areas: the cingulate sulcus, ventral pre-motor cortex (PMv), anterior insula, cerebellar vermis, and frontal eye field (FEF).
II. Psychophysical Data
• Threshold Determination: thresholds for each individual were calculated by finding the offset value that corresponded to a 75% accuracy for left vs. right discrimination; when necessary, threshold values were found by interpolation using the FORECAST function to predict the value; the average threshold across subjects was found to be 0.73 mm with a standard error of 0.08.
After analyzing the neuroimaging data, we found some expected and unexpected cortical activity during the tactile task. As expected, activation of somatosensory cortical areas was found in the aIPS, pIPS, and PCS. Unexpectedly, multiple visual cortical areas were activated as well including the visual form perception area (LOC,) an area that could include the primary visual cortex (MOC,) a visual area for eye movement (FEF,) and the STS. Some additional motor areas were also activated; however, this could be explained by movement by the left hand to elicit a discrimination response to the task. The psychophysical data confirmed that most subjects were able to perform the task within the hyperacuity range.
All of this suggests the following:
• The function of some visual areas may not be restricted to visual processing. Instead, the primary role of certain visual areas may be processing fine discriminations regardless of the sensory modality in which stimuli are introduced.
• Visual cortical activation may be the result of the use of visual imagery during top-down processing.
• Further research is needed in order to examine whether or not the magnitude of certain visual cortical activations predicts the hyperacuity threshold for individual subjects.
This work was supported by the Howard Hughes Medical Institute under Grant No. 52003727 and by the National Science Foundation under Grant No. BCS-0519417 to Dr. Krish Sathian. Assistance from the Sathian lab personnel was greatly appreciated.
Whereas acuity is the ability to distinguish separate stimuli, hyperacuity involves discriminations that are finer than the acuity limit. There have been few studies that address touch hyperacuity in normally sighted subjects using both psychophysical (one's response to physical stimuli) and brain imaging techniques. Our study involves an analysis of performance across 9 normally sighted subjects during a touch hyperacuity task, in which subjects were blindfolded while stimuli were applied to the immobilized right index fingerpad using a pneumatic stimulator. Stimuli were raised shapes, offset towards the left or right of a base-plate. The psychophysical aspect of the study established the thresholds for each subject, corresponding to 75% accuracy for left vs. right discrimination. The average threshold across subjects – 0.73 mm – was within the hyperacuity range (< 1mm) for the index fingerpad. The functional magnetic resonance imaging (fMRI) aspect of the study identified common areas of brain activation, between 2 subjects, for the hyperacuity task. As expected, activation of somatosensory brain areas was found; however, more interesting was the activation found in multiple visual brain areas. This suggests that the function of some visual areas may not be restricted to visual processing. Instead, the primary role of certain visual areas may be processing very fine discriminations . Further research could examine whether the magnitude of visual brain activations predicts the hyperacuity threshold for individual subjects.
Recording of responses during a tactile task; Averaging of performance to task; Statistical analysis involving standard deviation and standard error of the mean; Interpolation using the FORECAST function on Microsoft Excel; Behavioral observation; Functional magnetic resonance imaging; Operation of pneumatic stimulator; Operation of the Presentation Software Package, computer program that controls for duration and application of stimuli
tactile hyperacuity, tactile (or touch) perception, neuroimaging, psychophysical, neuroscience
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