PDF《The Role of炉he燨toliths》

15 compensatoryrotation,orcounterroll,oftheeyeiselicited.Incontrast,whenperformed during spaceflight, in the absence of the gravity bias to the otolith organs, the same manoeuvre elicits no counterroll. Comparative recordings of eye and head position for a head tilt to the left, under one-g and zero-g conditions, are shown in Fig.?2.3. Under one-g test conditions, active head-to-shoulder tilt, or rotation, stimulates not only the semicircular canals but also the otolith organs due to the reorientation to gravity. The combined canal- and otolith-mediated oculomotor response mani- fests as a volley of torsional nystagmus beats combined with a tonic ocular counterroll (OCR) (Fig.?2.3 upper panel). In microgravity (Fig.?2.3 lower panel), in the absence of gravity, only the transitory canal-mediated torsional nystagmus response remains.1

1 ?Under both one-g and zero-g conditions, the volley of nystagmus beats commences consistently with an anticompensatory saccade. This has been observed in previous ground-based studies (Pansell et?al. 2003) and can be likened to the case of the horizontal anticompensatory saccade found to initiate the eye movement response to active and passive yaw rotation of the head (Melvill Jones 1964;

Henriksson et?al. 1974;

Barnes 1979). This is understood to be related to the intention of directing visual attention, and a neuronal substrate has recently been proposed (Roy and Cullen 2002). Whether a similar perhaps more rudimentary mechanism exists for the control of torsional eye position remains to be determined. 30°

0 G

0 G 30°

0 G 0.5 G 1G 1G One-g 5° Ocular torsion 5° Ocular torsion

30 s Zero-g Zero-g

30 s 60° 60° Head tilt Head tilt One-g Fig. 2.3? Upper panels: Under one-g Earthbound conditions, a head tilt to the left directly elicits a rapid counterclockwise eye movement to the left, followed by a transient burst of nystagmus beats and a static ocular torsion to the right. Lower panels: In microgravity a similar counterclockwise head tilt elicits a similar burst of counterclockwise nystagmus beats but no change in static ocular torsion occurs (from Clarke and Kornilova 2007) 2.1? Which Functions Do the Otoliths Fulfil?

16 2.2? Early Spaceflight-Related Studies of?Otolith Responses The effects of exposure to microgravity on the otolith-mediated oculomotor responses have been investigated in a number of ways in previous studies (Yakovleva et?al. 1982;

Vogel and Kass 1986;

Reschke and Parker 1987;

Diamond and Markham 1988;

Wetzig et?al. 1990;

Merfeld 1996;

Clarke and Kornilova 2007). In this con- text, the so-called tilt-translation hypothesis was proposed after the observation was made that immediately postflight pure roll stimulation in the dark was perceived by the astronauts as translatory self-motion with only a small angular component (Reschke and Parker 1987);

i.e. the CNS adapts to prolonged microgravity by rein- terpreting all otolith signals to be an indication of linear translation. With regard to the functioning of the otolith organs, the asymmetry hypothesis (Yegorov and Samarin 1970;

von Baumgarten and Thumler 1979) proposed that differences in weight between the right and left otolith apparatus of the inner ear are appropriately compensated on Earth, but when exposed to novel gravitational states, these com- pensatory stratagems become ineffective, leading to unstable vestibular responses. Testing this hypothesis, Diamond and Markham (1998) measured examined increases in OCR disconjugacy, which they interpreted as an indicator for such oto- lith asymmetry. This increased fluctuation of torsional eye position in their zero-? g recordings could also be an indication of a stabilising, or inhibitive, role of the otolith information. All of these previous studies lacked the methodology for unilateral stimulation to each of the otolith organs, the utricle and saccule. A unique approach to applying linear acceleration independently to the left or the right otolith organ was introduced a number of years ago (Wetzig et?al. 1990). Rather than accelerate the whole body with simultaneous stimulation of both vestibular labyrinths, this approach employs unilater........