This allowed us to configure the stimulus such that a peripheral cued location was placed either in the affected

region of visual space during SC inactivation or diametrically opposite it (see Fig. 1B and ‘Results’). We localised the cannula tip within the SC before injection, using several methods. First, we targeted a depth of 1.5–3 mm below the SC surface, corresponding to the intermediate and deep layers of this structure. Second, we recorded activity during saccades consistent with known responses in the SC, which allowed us to confirm both the depth in the SC and our placement within the SC retinotopic map. Third, we used electrical microstimulation to evoke saccades. The current needed ZD1839 mouse to evoke such EPZ015666 saccades (typically 10 μA) provided further evidence of depth in the SC, and the metrics of the evoked saccades indicated the position of our cannula within the retinotopic map. We also oriented the bevel in our injection cannula to aim it towards the caudal SC rather than the rostral SC, a strategy

similar to that described in Zenon & Krauzlis (2012). This allowed us to direct drug spread towards the peripheral SC as much as possible, in order to avoid inactivating the rostral SC, where the motor control of microsaccades might be more directly affected. We injected the entire 0.3–0.5-μL volume of muscimol into the SC slowly, over an interval of ~20–30 min (one pulse of solution every ~2 min until our entire volume was injected). Based on previous experience, this strategy helped to stabilise the behavioral effects of the injections and minimise tissue damage. We then took several measures to confirm that our injections affected the peripheral eccentricities that we were interested in. First, we estimated the extent of drug spread in the SC for

each injection by measuring the peak velocities of visually guided saccades (Lovejoy & Krauzlis, 2010; Zenon & Krauzlis, 2012), and estimating the regions of space for which these peak velocities were reduced relative to pre-injection levels. Examples of such analysis are shown in Fig. 2A for several about injections from each monkey, where each shaded region in the figure shows the area with reduced peak velocities (Lovejoy & Krauzlis, 2010). As can be seen, saccades smaller than ~3–4° in amplitude (often much larger) were not affected, suggesting that muscimol did not dramatically spread towards the rostral SC. Second, we performed several analyses to help confirm that our results in this study were not fully explained only by a rostral spread of muscimol towards the foveal representation in the SC. We did this by analysing the characteristics of microsaccades that occurred within 50 ms from cue onset in our main task of Fig. 1.