The influence of stimulus parameters on the reported location of electrocutaneous stimuli

The body schema is the unconscious awareness of our body, which is fed by
various sensory modalities. Various disorders have been hypothesized to be
reflected in this awareness. When people report the location of a cutaneous
stimulus they refer to their body schema, therefore studying the reported
locations of stimuli on the skin may provide information about this schema.
Although tactile localization has been studied repeatedly, many factors which
may influence spatial perception of touch remain unidentified, which impedes
interpretation of the results. Localization data is known to have both
systematic and stochastic errors when comparing it to the actual stimulus
sites. The systematic component differs between subjects. It is at present
unknown whether the systematic component is a trait of a subject or whether it
changes when repeating the same measurement at another time. Also, the effect
of stimulus strength and duration on the stochastic and systematic components
is unkown. Another unknown is whether the spatial perception of touch and
nociception are the same. Finally, the study of spatial perception would
benefit from a critical evaluation about the statistical methods used for
analyzing the data.

In this study we address the influence of stimulus strength, modality and
duration on reported locations of cutaneous stimuli. In addition, we assess the
reproducibility of these reports and test whether we can improve the analysis
of localizations by identifying clusters in this data. All stimuli will be
applied using electric stimulation on the skin.

The study consists of 5 series of experiments which use electric stimuli of
tactile afferents: 1. Reproducibility and clustering study consisting of a
pilot phase (5 subjects, 2 experiments each) and final series (25 subjects, 2
experiments each). 2. Influence of stimulus strength on tactile localization
(45 subjects). 3. Effect of stimulus frequency and duration on tactile
localization with a pilot stage (5 subjects) and final stage (40 subjects). 4.
The effect of gaze direction on clustering in tactile localization with a pilot
stage (max 5 subjects) and final stage (15 subjects). Experiment series 5 also
includes nociceptive electric stimuli and will compare the localization of
tactile and nociceptive stimuli (15 subjects).

Each experiment will last approximately 1.5-2 hours depending on the
experiment. The first hour consists of preparations; the remainder is taken up
by the main experiment. In 4/5 experiments all stimuli feel as a dull tap. In
the tactile/nociceptive comparison experiment half of the stimuli will feel as
a light pinprick. There are no risks involved in participating in these
experiments. The needle electrodes used in experiment series 5 can cause mild
skin irritation which disappears within half an hour after removal of the
electrodes. These electrodes are sterilizable.