Measuring Touch

Aside from looking at GABA using MRS, I am interested in tactile processing. But tactile processing is a very broad term and the somatosensory cortex encompasses the processing of touch, temperature, pain and proprioception. My interest lies in touch processing, but even there, different types of receptors process different types of touch, e.g. high frequency or low frequency. There is also haptic perception, more "higher level" touch, consisting of actively exploring objects or surfaces.

In my mind, to be able to investigate touch and it's neuronal correlates, it is best to keep it incredibly simple. So I investigate very basic aspects of touch, single digits, simple stimulation, and modulation of relatively "basic" concepts such as touch detection, touch amplitude, and touch frequency.

Investigating behavioural aspects of touch (e.g. sensitivity) can be done using psychophysics. Generally, certain types of tasks or certain algorithms are used to find someone's threshold, e.g. for detection a stimulus or for being able to feel a difference between two stimuli. Often, these findings can be related to what we know of how these stimuli are processed in the brain.

In 2012-2013, we developed a battery of tactile tasks (paper) that can be applied to children within a reasonable amount of time (40 min). In this battery, several aspects of touch are measured to obtain a lot of information about different tactile aspects. One reason for using the tasks that we use (see below) is that they, to some degree, provide information about the inhibitory system as is shown by invasive/animal studies.

Fundamentals

Tactile sensitivity is difficult to measure, and in disorders it has classically been approached via questionnaires and rating scales. While informative, these metrics provide little information as to the sensory nature of the tactile function and often include emotional as well as sensory aspects. Psychophysics allos for the controlled and objective assessment of tactile sensitivity and tasks are often linked to physiological mechanisms.

In the tasks we use, we typically use a 2 forced choice paradigm. In each trial, participants are presented with two choices and they have to choose one of these two choices (without interference of an examiner, and the same for each and every participant). For instance, I present a weak stimulus on your middle finger and a strong stimulus on your index finger and ask you which of the two fingers received the strongest stimulus.

To measure the smallest difference in stimulus intensity that you can detect, I make the difference smaller in the next trial, so I present a weak stimulus, and a stronger-stimulus-slightly-weaker-than-in-the-previous-trial. When I make this difference smaller each time you have it correct, but slightly bigger again every time you have it wrong, after a while the difference is at a level where you can sometimes feel the difference and sometimes can't. This would be your intensity discrimination threshold. There are different ways of doing this (see below)

EXAMPLE: Each circle is a trial, the y-axis indicates the measure you're interested in, e.g. intensity difference. The first 3 trials are correct (blue circle) and so the difference decreases. The answer in the 4th trial is wrong and so the difference increases. the next couple of trials are correct again and so the difference decreases. Now in this case, for the second half of the experiment, participants have to have two in a row correct in order to decrease the difference, but one wrong in order to increase it again. The average across the last few trials can be taken as the threshold.

There are different ways of doing such a task and the above is a mere example. The step size can be constant, or can be adaptive, the threshold can be 50% correct or 75% correct. An entirely different way of doing this is via the method of constant stimuli, where a predefined set of stimuli are presented, pseudo-randomly and repetitively, and the percentage detection curve is fitted to give a threshold.

NOTE: In our pediatric studies, we do relatively few trials because children have a limited attention span. This is not-optimal, and so rather than doing the most thorough test we can do, we test a lot of children to see group differences.