AN ANATOMICAL SUBSTRATE FOR THE SPATIOTEMPORAL TRANSFORMATION

The purpose of the present experiments was to test the hypothesis that the metrics of saccades caused by the activation of distinct collicul ar sites depend on the strength of their projections onto the burst ge nerators. This study of morphofunctional correlations was limited to t he horizontal components of saccades. We evoked saccades by stimulatio n of the deeper layers of the superior colliculus (SC) in alert, head- fixed cats. We used standard stimulus trains of 350 msec duration, 200 Hz pulse rate, and intensity set at two times saccade threshold in al l experiments. Evoked saccades were analyzed quantitatively to determi ne the amplitude of the horizontal component of their ''characteristic vectors''. This parameter is independent of eye position and was used as the physiological, saccade-related metric of the stimulation sites . Anatomical connections arising from these sites were visualized afte r anterograde transport of biocytin injected through a micropipette ad joining the stimulation electrode. The stimulation and injection sites were, therefore, practically identical. We counted boutons deployed i n regions of the paramedian pontine reticular formation reported to co ntain long-lead and medium-lead burst neurons of the horizontal burst generator. Regression analysis of the normalized bouton counts reveale d a significant positive correlation with the size of the horizontal c omponent of the characteristic vectors. This data supports a frequent modelling assumption that the spatiotemporal transformation in the sac cadic system relies on the graded strength of anatomical projections o f distinct SC sites onto the burst generators.