The spatial values of retinal coordinates are "recalibrated" to the "egocentric" coordinates during and after a saccade within a fraction of a second. We measured the time constant of this retinal coordinate transformation by means of an afterimage technique: our ten subjects performed "auditory" horizontal saccades in total darkness (0.2-4.5 saccades/sec). At a saccade frequency below 1 saccade/sec, the subjects observed saccadic displacement of the foveal afterimage, but the afterimage seemed to arrive at its final position more slowly than the center of gaze (state 1). At saccade frequencies above 1.5 saccades/sec, the perceived amplitude of afterimage displacement decreased with increased saccade frequency (state 2). Above 2 saccades/sec all subjects perceived two stationary afterimages simultaneously at the saccadic end-position (state 3). A further increase in saccade frequency reduced the distance between the two afterimages till only one stationary afterimage was seen in a mid-position between the two auditory targets at a saccade frequency above 3.2-3.5 saccades/sec (state 4). Saccade amplitude remained constant within the frequency range between 0.2 and 4.5 saccades/sec. A one-step or two-step linear model was applied to simulate the experimental data, indicating that the spatial coordinates shift more slowly than the saccadic eye movements.

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In order to uncover encoder properties of primary muscle spindle afferent fibers, time coupling (phase-locking) of action potentials on cyclic muscle stretch was studied by means of pseudo-random noise. In cats Ia action potentials were recorded from dorsal root filaments and the gastrocnemius muscles of one hind leg were stretched. The stimulus time course was a determined sequence of randomly varying muscle length which could be applied repeatedly (sequence duration 0.6 or 20 s). The noise amplitude sigma (standard deviation of displacements) was varied between 5 and 300 micron, the upper cut-off frequency of noise fc was varied between 20 and 100 Hz. The responses to the consecutive pseudo-random noise cycles were displayed as raster diagrams and cycle histograms. Phase-locking characterized the responses at all noise amplitudes outside the near threshold range (sigma greater than 10 micron). The higher sigma and fc, the stronger was the phase-locking of impulses on the stretch. When sigma and fc were selected to achieve high mean stretch velocities of about 500 mm/s, phase-locking was as precise as 0.15 ms, measured as the variability of spike occurrences with respect to stretch. The rasters obtained with low noise amplitudes (less than 40 micron) showed a loose phase-locking and this gave insight into underlying mechanisms: The elicitation of action potentials caused by dynamic stretch can be prevented by a post-spike depression of excitability. This disfacilitation was very effective in counteracting weak stretch components within the random sequence and less effective or even missing when relatively strong stretch components could force the spike elicitation.(ABSTRACT TRUNCATED AT 250 WORDS)

Experiments were conducted in anaesthetized and spinalized cats to measure the extent to which the non-linear response of Ia afferent fibers to sinusoidal muscle stretch as expressed by the peristimulus-time-histograms, PSTHs, can be transformed into a linear one by means of the superposition of random stretch ("mechanical noise"). The gastrocnemius muscles of one hind leg were stretched and the response to sinewave muscle stretch (amplitudes between 0.01 and 4.0 mm, frequencies between 0.1 and 20 Hz) were investigated while band-limited mechanical noise was superimposed on the sinewave stretch. The random stretch upper cut-off frequency was varied between 60 and 300 Hz; the displacements were normally distributed. The noise amplitude sigma, i.e. the standard deviation of the displacement distributions, was varied systematically between 0.002 and 0.4 mm. Mechanical noise was very effective in raising the mean discharge rate. Added to the sinusoidal stretch it prevented the cessation of firing during the release phase of the stretch cycle, or at least reduced the duration of discharge pauses, i.e., a linearization occurred. In general, the larger the noise amplitude, the more the amplitude of the fundamental harmonic component was attenuated and the phase lead reduced. Apart from this rule the particular combination of superimposing small noise (sigma less than 0.02 mm) on small sinewave stretch (A less than 0.02 mm) could enhance the depth of sinusoidal modulation of cycle histograms (compared with responses to pure sinusoids). Linearizing the sinewave response by additional noise allowed the estimation of frequency response characteristics in the otherwise non-linear range of amplitudes (sinewave amplitude 0.5-1.0 mm).(ABSTRACT TRUNCATED AT 250 WORDS)