Lid and eye saccades were recorded simultaneously by means of
the electromagnetic search coil technique.
14 16 18 19 Lid
saccades were recorded with handmade search coils. Every such coil
consisted of 50 turns of insulated copper wire (diameter, 0.05 mm). A
typical coil had an outer diameter of approximately 4 mm, weighed 15
mg, and was less than 0.5 mm thick. To reduce spurious induction, the
leads of the coil were tightly twisted together. The coils were fixed
on the lower part of the eyelid, just above the eyelid margin and right
above the center of the pupil with a piece of adhesive tape (diameter,
6.5 mm). Once the coils were attached, the subjects hardly noticed
them. Eye saccades were recorded with commercially available search
coils (Skalar, Delft, The Netherlands).
The field frequency used was 20 kHz. The recordings were amplified to a±
5-V range, low-pass filtered at 120 Hz (−3 dB), digitized with
12-bit precision, and sampled at a frequency of 250 Hz. The recordings
were stored on disc for off-line analysis. Signal noise level was less
than 1.8 minutes of arc. Both the recording equipment and the search
coils were calibrated objectively before each recording session, with
the coils mounted on a calibration device. Any misalignment of the
coils on the eyes, determined when the subjects monocularly fixed a lit
LED in the primary position of gaze, was later adjusted by software.
The accuracy of the calibration procedure was better than 0.5%.
One search coil was attached to the upper eyelid, and a scleral search
coil was placed on the ipsilateral eye under topical anesthesia (0.4%
oxybuprocaine; Novesine; Chauvin, Düsseldorf, Germany). Subjects
were then seated, with their heads centered in a cubic coil frame in
which an alternating horizontal and vertical electromagnetic field was
generated. Head movements were restricted by a chin rest and forehead
support. The subjects faced a stimulus screen, containing red LEDs at a
viewing distance of 1 m. The LEDs were positioned symmetrically
around the straight-ahead position along the midvertical meridian,
10°, 20°, 30°, and 40° apart. The subjects were asked to shift
their gazes from one lit LED to the other after hearing an
electronically generated tone, at a pace of 1 per second. The four
target ranges were tested in a randomized sequence. Each trial lasted
16 seconds, in which, typically, eight saccades in each saccadic
direction were made. Before each trial, subjects were allowed to
practice briefly.