Evidence for cortical coding of saccade velocity in memory-guided saccades in patients with CADASIL

Abstract

Saccadic eye movements were elicited in ten patients with CADASIL (an autosomal dominant cerebral angiopathy) and ten age-matched controls under six different conditions: 1) the gap paradigm, which tests visually guided, reflexive saccades; 2) the overlap paradigm; 3 and 4) the anti-gap and anti-overlap paradigms, which are thought to test especially the function of the frontal cortex; 5) the memory paradigm, which tests internally guided, intentional saccades; and 6) the sequential memory-guided saccades. Eye movements were recorded by means of DC electro-oculography, and the saccade latency, gain, duration, and peak eye velocity were estimated. The amplitude-to-duration as well as the amplitude-to-peak velocity ratios were calculated for both groups and conditions 1 and 5. There were no significant differences in the gains of the primary saccades in the gap, overlap, and memory paradigms. In general, the CADASIL patients showed a tendency toward longer saccade latencies and a larger number of unwanted saccades in the antitask saccade and memory saccade paradigms (suppression errors). Our main finding was that the patients had a highly significant reduction in peak velocity and a corresponding prolongation of the duration of the memory-guided saccades compared to the controls. In contrast, the duration and the peak velocity of the visually guided saccades did not differ between controls and patients. Thus, we observed a specific change in the dynamic properties of memory-guided saccades in CADASIL patients. The observed pattern of saccade abnormalities in the CADASIL patients does not allow localization to a specific cortical side; however, it might reflect the presence of a mostly frontal type of lesion. This finding contradicts the view that information passed from the cortex to the brainstem codes only the saccade vector and not saccade dynamics.