Fragmented Saccades: Inappropriate Operation of the Fixation System?

2. Fragmented Saccades: Inappropriate Operation of the Fixation System? Edward Keller1 and Kyoung-Min Lee1,2 1Smith-Kettlewell Eye Research Institute San Francisco 2Seoul National University, Korea

3. Saccades are normally stereotyped, relatively straight, single movements that end on a new target of interest. They are intermixed with longer periods of stable fixation Thus, normal operation of the saccadic system involves the seamless alternation of movement and fixation, but under some circumstances this carefully orchestrated interplay is interrrupted.

4. We have recently found two different situations in which • saccades become fragmented: • Late-onset Tay-Sachs disease (LOTS) • Tasks that involve saccades generated in response to difficult decision processes among multiple choices of response

5. Lots patient (Rucker et al., 2004)

6. LOTS patient Monkey stim. OPNs

7. Signals in the brainstem immediate Pre-motor network

8. Fragmented movements when saccades are generated in a task which requires extensive cognitive processing

9. Typical choice response task Reaction times in choice response tasks follow Hick’s Law, RT = T*log2(NA)

10. Targets 8NA 2NA Masks Cue Saccade Saccade Time Saccadic choice response task

11. 900 90 % 800 700 600 choice response time (ms) 500 400 10 % 300 200 1 2 4 8 Number of alternatives A 50 number of alternatives 1 2 4 8 40 30 # trials in 50-msec bin 20 10 0 100 300 500 700 900 1100 Choice response time (ms)

12. Fragmented two-step saccade Normal single-step saccade 5 5 HEP 0 5 0 5 -5 -5 VEP (deg) 5 VEP (deg) 0 0 5 VEP 0 0 -5 -5 -5 0 5 -5 -5 0 5 -5 Time (ms) HEP (deg) HEP (deg) Time (ms)

14. SCFN n=43 Number of neurons Time w/r to saccade end (Everling et al. 1998)

16. Signals in the brainstem immediate Pre-motor network

17. SCFN

28. FEF Sommer et al.