VISUALLY INDUCED CROSS-AXIS POSTSACCADIC EYE DRIFT

1. It has been previously shown that, if a visual pattern is transient ly moved just after every saccade, it is possible to induce horizontal , postsaccadic, ocular drift after horizontal saccades that persists i n the dark. In this study we show that horizontal ocular drift can als o be created after vertical saccades. Five human subjects viewed binoc ularly the interior of a full-field hemisphere filled with a random-do t pattern. They were encouraged to make frequent vertical saccades. Du ring training, eye movements were recorded by the electrooculogram. A computer detected the end of every saccade and immediately moved the p attern to the left after up saccades and right after down saccades. Th e motion was exponential, its amplitude was 25% of the vertical compon ent of the antecedent saccade, its time constant was 50 ms. Before and after 2-3 h of training, movements of both eyes were measured by the eye-coil/magnetic-field method while subjects were instructed to make vertical saccades in the dark, in the presence of the movable adapting pattern, and between stationary targets for calibration. 2. After tra ining (approximately 20,000 saccades) all subjects developed a zero-la tency, exponential ocular drift to the left after up saccades and to t he right after down saccades. The amplitude of the horizontal drift, e xpressed as a percentage of the vertical component of the preceding sa ccade, was 2.7% in the dark. This rose to 10.2% in the presence of the movable adapting stimulus. The latter rise is not due to visual follo wing systems but to a zero-latency increase in initial drift velocity. 3. The horizontal drifts were usually unequal between the two eyes, i ndicating the presence of disconjugate movements. We measured intrasac cadic disconjugate horizontal movements of all subjects. In agreement with studies by others of saccades in the light, we measured a diverge nce during up saccades (1.3-degrees) and a convergence for down (0.4-d egrees), but in this case for spontaneous saccades in the dark. After training, these values increased for saccades in the dark but decrease d in the light in the presence of the adapting stimulus. These changes were largely idiosyncratic and statistically significant in only a fe w subjects. 4. The cross-axis postsaccadic drifts were separated into their conjugate and disconjugate components. The disconjugate componen ts were small and idiosyncratic, and the means were small for saccades in the dark. The only consistent trend was in the presence of the ada pting stimulus where up saccades were often followed by convergence. T he presence of these vergence components do not interfere with our con clusion that this paradigm demonstrated cross-axis plasticity in posts accadic drift. 5. The induced drift was specific to the stimulus patte rn. The horizontal induced drift became smaller for oblique saccades, decreased as their vertical components became smaller, and disappeared for horizontal saccades. There was no induced vertical drift after ho rizontal saccades. 6. We suggest a hypothesis of crossed innervation. During a vertical saccade, horizontal burst neurons are known to be bi laterally coactivated by a signal presumably from vertical burst neuro ns. Normally, these activities cancel each other to produce no net hor izontal movement. Thus the lack of a horizontal component would seem t o be not passive (no signal at all) but active and determined by a bal ance between opposing forces. The same argument can be made for the st ep of innervation. Plastic modification of synapses between the horizo ntal burst neurons and the horizontal neural integrator could therefor e create, during a vertical saccade, a step of innervation to horizont al motoneurons in the absence of a pulse. This hypothesis, with the us e of only demonstrable neural pathways, shows how one might create a s tep of innervation without a pulse and thus a horizontal postsaccadic drift without a horizontal saccade.