Interpretation of a Discontinuity in the Sense of Verticality at Large Body Tilt

Results of earlier spatial-orientation studies focusing on the sense of verticality have emphasized an intriguing paradox. Despite evidence that nearly veridical signals for gravicentric head orientation and egocentric visual stimulus orientation are available, roll-tilted subjects err in the direction of the long body axis when adjusting a visual line to vertical in darkness (Aubert effect). This has led to the suggestion that a central egocentric bias signal with fixed strength and direction acts to pull the perceived vertical to the subjects' zenith (M-model). In the present study, the subjective visual vertical (SVV) was tested in six human subjects, across the entire 360° range. For comparison, body-tilt estimates from four subjects where collected in a separate series of experiments. For absolute tilts up to ∼135°, SVV responses showed a gradually increasing Aubert effect that could not be attributed to errors in perceived body tilt but was nicely in line with the M-model. At larger absolute tilts, SVV errors abruptly reversed sign, now showing a pattern concordant with errors in body-tilt estimates but incompatible with the M-model. These results suggest that, in the normal working range, the perception of external space and the perception of body posture are based on different processing of body-tilt signals. Beyond this range, both spatial-orientation tasks seem to rely mainly on a common tilt signal.

Near-field seismic displacement and tilt associated with the explosive activity of Stromboli

Broadband seismic recordings in the near-field of Strombolian explosions, at 500 m distance, show pronounced effects of tilt. The tilt signal is predominant in the horizontal components beyond about 50 s period while it is negligible in the vertical component. The waveform of the tilt signal at the seismometer output is a double time integral of the waveform due to ground displacement. Since the waveform of the displacement is known from the vertical component, the waveform of the tilt signal in the horizontal seismogram can be reconstructed and both contributions can be separated from each other with a linear regression. We have analyzed data recorded in the summit region of Stromboli in 1995 and 1996. The regional tilt can be determined from the differential vertical displacement between instruments a few tens of meters apart. Local tilts determined with individual instruments scatter around the regional value, most probably due to local strain-tilt-coupling. Mogi's (1958) formulae for a pressure source in a homogeneous halfspace are used to interpret the results. The source displaces a volume of several tens of cubic meters of the surrounding rock before the explosive discharge; typical volumes were 25 m3 in July 1995 and 60 m3 in September 1996.