Mirror-touch synaesthesia: the role of shared ... - UCL Discovery

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Chapter 3
Two coloured caps remain in the tray, which represent the two end points of the
colour sequence (e.g. a purple and pink cap). The participant is given 2 minutes to
arrange the remaining caps into an ordered colour series from one hue to another (e.g.
purple through violet through pink). The correct order for the hues can be identified
by the experimenter from the numeric coding on the underside of each cap. A
deviation score is calculated by considering how far each colour cap deviates from the
correct location in the sequence. For example, in a correct ordering such as 4–5–6;
colour number “5” has a score of 2 because it is 1 unit from 4 and 1 unit from 6. An
incorrect ordering such as 2–5–9 would yield a score of 7 for colour “5” because it is
3 units from “2” and 4 units from “9”. The error score is the difference between the
actual score obtained and the expected score based on flawless ordering. The same
procedure was used for each of the four trays and the order of trays was randomized
across participants.
In order to investigate tactile discrimination, the Gratings Orientation Test
(GOT) was used to measure tactile acuity on the index finger tip. The GOT is a well
established method for measuring the spatial resolution of touch. It consists of a
series of square wave gratings with varying ridge widths (0.35mm – 3mm; Van Boven
and Johnsen, 1994). Each grating is applied to the finger tip in one of two orthogonal
orientations (across or along the axis of the finger tip). The task is to report the
orientation of the probe. Typically as ridge width decreases, accuracy decreases. The
GOT is thought to reflect cortical representations of the finger tip in SI (Van Boven
and Johnsen, 1994) and in this vein provides a method for accurate threshold
estimates of sensory function.
Six spatial grating probes (0.35mm, 0.5mm, 0.75mm, 1mm, 1.25mm, 1.5mm)
were used to investigate each participant’s tactile sensory threshold. Using a blocked