Neurophysiology. Our non-invasive sensory testing is made possible through neurophysiological studies. Using traditional electrophysiological recordings and optical intrinsic signal imaging, labs across the globe have been able to investigate the underpinnings of the primate somatosensory cortex. As a result, we have been able to create sensory testing paradigms that can be used on human subjects in a clinical setting. Without this research, it would be impossible to draw conclusions as to the underlying neurophysiology of human perception. Consequently, we are able to use mathematical analysis on a patient's sensory testing response to determine whether or not the cortex is functioning properly. Additionally, though much lower in resolution than animal neurophysiological studies, we are currently conducting validation studies with human imaging methods (fMRI, MRS, EEG and MEG).
How do we know what the results of perceptual metrics means in terms of what is happening in the brain when the skin is stimulated? For more than 30 years, our lab has conducted investigations into what happens in the brain when different types and/or different combinations of stimulation are delivered to the skin. These in vivo studies were done at very high resolution in the non-human primate.
For example, stimulating two adjacent sites on the skin will result in two adjacent areas in the cortex having significant activity. At above left is an optical image that has two pronounced and distinct sites of activation. At above right is a direct comparison of optical imaging data from monkeys compared with perceptual data from humans (Francisco et al, 2008)
Recordings from electrodes indicate times of occurrence when individual neurons become activated – note the difference between the recordings obtained from the periphery (at left) vs. from the cortex (at right). These differences indicate the amount of information processing that goes on in the CNS.
In vitro studies were performed in the rat. At left is a slice of brain tissue that has been stimulated under two different conditions – the first without a drug, the second with. This drug simulates what happens in some neurological conditions.
Validation with human imagng studies. Currently, we are working with several different collaborators who are conducting either fMRI, MRS and/or MEG imaging studies to investigate
Pilot study with fMRI. A subset of the autism population that was studied via sensory testing participated in a parallel functional Magnetic Resonance Imaging (fMRI) study in which the brain was imaged in response to different types of stimulus conditions. Comparisons of the responses in the brain to these conditions with the newly developed sensory metrics demonstrate a high degree of correlation (figure at left; R2=0.80). Additionally, the fMRI metric obtained categorized the subjects into the same 2 groups that were detected via sensory metrics. Traditional sensory metrics (such as threshold testing), on the other hand, failed to yield such a correlation with the fMRI metric and did not show a distinct segregation of the two populations (R2 of 0.12) The high correlation between fMRI and the sensory metric could actually be higher with a larger data sample and if the fMRI data could be obtained at higher resolution. It is important to keep in mind that there are currently no non-invasive means for evaluating activity in the brain that are as sensitive to subtle changes in sensory experience as sensory percept is. In other words, this pilot study demonstrated good correlation between a crude measure of brain activity (fcMRI) vs. a highly refined measure of brain activity (sensory percept).