C.R.T. spot-follower device for eye-movement measurements

Smooth periodic eye movements can entrain perceptual alternation

The reversal signals and eye movements of three subjects looking at a Necker cube that moved sinusoidally in the subject's field of view were recorded. The aim of the experiment was to provide evidence of possible synchronisation of perceptual alternation with smooth periodic eye movements. The occurrence of synchronisation was demonstrated for a proper choice of the eye oscillation period by both a sharp reduction in the fluctuations of the reversal time and a change in the mean value of this parameter. Such variations resulted in a close match of the mean reversal time to a multiple of the eye oscillation period. Further, inversions of the direction of eye movements elicited pattern reversals in a systematic way, characteristic for each subject. The described phenomenon, which is a typical example of nonlinear-resonant behaviour, stresses a new important aspect of the complex interaction pattern relating pursuit eye movements to visual perception.

Perceptual ambiguity and stability of reversible figures

In this work, the results of two experiments on ambiguous patterns are reported, which have been obtained by presenting a series of stimuli designed, in both cases, to reduce gradually the ambiguity of the patterns. Such reduction has been performed by respectively increasing or decreasing the amount of graphic details in the experiments. Data of both experiments show a lengthening of mean reversal time. The increase in the stability of one percept can be regarded as associated with the increasing difficulties encountered by an observer in organizing and restating the alternative "hypothesis" through the perceptual mechanisms. The loss of balance in the durations of percepts is discussed in terms of their different informational contents. Finally, in Exp. 1 an analysis is made to evaluate to what extent an addition of perceptual cues, designed to reinforce a three-dimensional interpretation of a pattern, can influence its figure-ground alternation.

Limit cycle oscillations of the human eye

The altered feedback technique is very suited to display nonlinearities of the human smooth pursuit system. In fact, when the gain of the retinal feedback path is raised, for the horizontal channel, above its normal unitary negative value, a threshold is met beyond which sustained horizontal self-excited smooth oscillations of the eye can be observed, which point out the existence of a stable limit cycle. Furthermore, the characterizing features of both the transient and steady state show a well defined dependence on the total feedback factor K. In particular, the analytical dependence on K of the amplitude and frequency of limit cycle oscillations can be derived. Implications of the experiment with respect to the mathematical modelling of the system are discussed.