Several electrophysiological findings support the hypothesis that the variability in saccadic endpoints finds its source, at least in part, in the SC. These reveal that saccades evoked by focal electrical stimulation of the deeper SC are, just as visually guided saccades, variable, mainly in amplitude, but also in direction, and gradually more as the stimulation site is more caudal (van Opstal et al.,
1990; see also Schiller & Stryker,
1972). Importantly, electrical saccades are also preceded by activity of a population of neurons, presumably as a result of the lateral spread of excitatory activity (McIlwain,
1982), combined with longer-range inhibition (e.g., Munoz & Istvan,
1998; see also van Opstal & van Gisbergen,
1989a; for a review, see Isa & Hall,
2009). Although this is consistent with our assumption that the variability in saccadic endpoints may reflect location jitter in SC point images (see also van Opstal & van Gisbergen,
1989b), it may suggest in addition that location jitter results, at least partly, from variations in the strength, the extent, and/or the distribution of lateral interactions. There is yet no direct evidence for this rather speculative hypothesis. Still, a few findings suggest that lateral interactions could possibly yield variability in the location of point images, which is both greater near the SC borders and overall rostrally biased, as would be expected based on the here-observed properties of landing-position distributions in SC space. First, excitatory, intercollicular, connections, which are likely involved in the generation of small-amplitude and vertical saccades, tend to be denser near the rostral one-fourth of the SC and the vertical-meridian representation (Behan & Kime,
1996; Takahashi, Sugiuchi, Izawa, & Shinoda,
2005; see also Infante & Leiva,
1986; Olivier, Porter, & May,
1998; Olivier, Corvisier, Pauluis, & Hardy,
2000; Paré & Guitton,
1994). Moreover, intracollicular inhibition in the intermediate SC seems to be mostly directed away from the rostral pole (Meredith & Ramoa,
1998; Munoz & Istvan,
1998; but see Bayguinov, Ghitani, Jackson, & Basso,
2015; Phongphanphanee, Marino, Kaneda, Yanagawa, Munoz, & Isa,
2014).