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Fig. 1. The machine. (A) A cross section of the machine. The rat is haltered (see
in more detail in B) on top of an air-cushioned (ain, air inlet) polystyrene
spherical shell of 50 cm diameter and mass about 400 g. The haltering consists
of a soft leather harness fastened by a hook and loop ribbon to three aluminum
plates with axial joints. The haltering allows the animal to lift and lower
its body, to rotate it about the yaw axis (the body orientation is measured by
the angular incremental encoder, aie) and to tilt it somewhat. The animal
rests with its full weight on top of the sphere. Any rotation of the sphere
about the yaw axis is prevented by two wheels (w), which are supported by
horizontal axes with tip bearings and which touch the sphere on its equator
under 90°. The air cup is tilted by about 7° and lets the sphere swim
against the wheels (w). The sphere can easily be rotated around any horizontal
axis. Any rotation of the sphere is monitored by two motion detectors
(HDNS2000, Agilent), which `look' on two 7 mm x 7 mm squares under
90° on the equator of the sphere. On the sphere surface a fine statistical
pattern of black points is sputtered with an air brush. The signals from the
motion detectors are fed to an incremental counter board implemented into the
PC that controls the VR image generation. The image of the VE is projected
from a DMD beamer (b) via two plane mirrors (p) from vertically below
onto an angular amplification mirror (aam) (black lines indicate the limits of
the illuminating light bundle). The aam is a polished rotational symmetric
aluminum surface with a vertical rotation axis, which widens the elevation of
the incoming light bundle by a constant factor of 39 over its whole surface
(red light bundle). From the aam the light is projected onto a toroidal screen
made of 24 segments of white elastic cotton. The torus acts as a horopter. The
rat sees any object on the screen under the same angle as it is projected from
the aam (blue light bundle). The screen covers the visual field of the rat
from 20° below until 60° above the horizon and 360° of azimuth
(green light bundle).
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