Fig. 2. Dot matrix evaluation of the cutaneus trunci muscle (CTM) reflex. (A) An overlay drawing of superimposed video images of a guinea pig. Black dots are permanent markers tattooed onto the shaved back and pink dots show the positions of the markers at the peak of skin contraction, captured by stop- frame video analysis (see Materials and methods). The exact point of tactile stimulation producing these CTM contractions is shown by the position of the monofilament probe used to stimulate the skin. During the period of testing, a boundary line was drawn on the back of the animal with a marker (highlighted in green) to reveal the total CTM receptive field. Stimulation within the circumscribed area produced skin contractions; stimulation outside the area did not. The actual video image source of the drawing is shown in C. The box in A is magnified in B, showing that the direction of skin contraction is generally towards the probe (arrow); the distance moved by a marker during contraction is also shown (hatched lines). This distance (2 mm) divided by the time required to produce it (0.12 s) gives the velocity of skin contraction (16.7 mm s^–1).