Studies in human sensory perception provide baseline data for both determining the significance of deviations from normative values in subjects with neurological deficits and a basis for understanding fundamental mechanisms of central information processing. These metrics are compared both with metrics obtained from subjects with compromised neurological systems as well as data obtained from high resolution neurophysiological experiments in non-human primates.. The diagnostic system is novel in that the protocols are relatively fast, the stimulator is portable and it makes collecting data from large numbers of diverse subject populations (some with compromised central nervous systems) pragmatic. Studies of such diverse groups could lead to novel insights about the perceptual changes that occur with systemic alterations of cerebral cortical function.
Since the design of our measures of perception are cortically rather than perceptually based, we often refer to perceptual metrics as Cortical Metrics.
First order measures: Sensory thresholds. Tactile detection thresholds are typically collected by clinicians because sensitivity to a tactile stimulus generally increases with a neurological alteration. However, threshold measures are most useful if baselines are recorded – i.e., if a subject’s threshold is acquired before something impacts his/her neurological health, then comparisons can be directly made. Without baseline measures, there can be a great deal of variability from one subject to another, primarily due to skin physiology (e.g., the callused hands of a manual laborer could result in higher than normal detection thresholds.
Tactile detection thresholds across age spectrum. Note increase in threshold with increasing age.
Second order measures: Discrimination tasks Second order measures are perceptual metrics whose values are influenced predominantly by interactions between or within cortical areas. Such metrics are not influenced significantly by skin sensitivity. To test the idea that comparison of two adjacent stimuli is relatively stable in healthy populations, an amplitude discrimination task in which both stimuli were delivered simultaneously to adjacent finger tips (D2 and D3) was performed on healthy subjects across a wide age spectrum. This measure was effectively constant across all age groups (see Figure 5). It should be noted that, in order to maximize signal to noise ratio, we conduct this amplitude discrimination task at supra-threshold levels. This allows us to deliver the same size stimuli to all subjects and thus, although threshold variation (discounting for subjects with very significant peripheral neuropathy) is in the range of 10-30 microns, all subjects are able to effectively compare stimuli that are 100 microns or greater.
Average DLs recorded across the age spectrum. Note the absence of change with age as was the case in sensory thresholds.
In some populations, deviations from normative values do occur in amplitude discriminative capacity, and we interpret this as the result of the subject having difficulty in being able to discriminate between two adjacent cortical areas. In this metric,values above control values are indicators of poor information processing capacity.
Third order measures: Adaptation metrics Third order measures are perceptual metrics that reflect integration of information across time and how that added dimension influences cortical-cortical interactions, normally measured with 2nd order metrics. Repetitive stimulation results in temporally defined changes of cortical activity, the most prominent of which is a reduction in cortical response with extended stimulus duration. Randomly applying a conditioning stimulus to one of the two skin sites before an amplitude discrimination task significantly alters a subject’s ability to determine the actual difference between the two stimuli, and the impact that the conditioning stimulus has is duration dependent (between 0.2 and 2 secs; Tannan et al, 2007). This finding suggests that the method could be viewed as a reliable indicator of the influence of adapting stimuli on central nervous system response, as changes in the peripheral response are not significantly changed at these short stimulus durations. Simply stated, the reason that subjects get worse with conditioning stimulation to one of the stimulus sites is because the subsequent stimulus, which is used for comparison, now feels smaller than it really is. This creates an illusory effect which appears to be relatively constant across healthy populations regardless of age (see Figure at left)
Change in adaptation metric across the age spectrum.
When this measure is examined across a number of subject populations, we do see significant deviations from control values. To summarize, the chart in the figure below demonstrates that this centrally mediated measure deviates from the control values for subjects in a number of neurological categories.
Neuroadaptation Metrics: Summary of adaptation metrics obtained from several different subject populations. Note that the amount that several subject populations adapt is much less than that of controls. The two exceptions are provoked VVS (peripherally mediated) and post-treatment of alcohol subjects (measures obtained after 12 weeks of sobriety). Thus, the impact of changes in centrally mediated mechanisms can be detected using a relatively fast vibrotactile methodology. Note that DXM refers to a post-ingestion of 60mg of dextromethorphan in control subjects. Data from control, autism and DXM subjects has been previously reported (Tannan et al, 2007b, 2008; Folger et al, 2008).
Multi-parametric analysis: Putting all the metrics together. The above described metrics are exemplary of the type perceptual metrics that we typically obtain from subjects/patients. In the plot below, 5 such metrics are used to derive a PCA plot in order to characterize a patient population.
