EFFECTS OF STIMULUS-DURATION AND TASK EXPERIENCE ON GAZE SHIFT

The effects of stimulus duration and prior task experience on gaze shi ft were studied to determine the spatial characteristics of gaze shift as affected by prior experience. Thirty-six subjects (with normal or corrected-to-normal vision) participated in two sessions of a task tha t required viewing two consecutively presented letters at either 15 de grees or 50 degrees eccentricity and deciding whether they were the sa me or different. In the first session (SS1) a letter was presented for either 500 (N = 18) or 1000 ms (N = 18), followed by a second letter. In session 2 (SS2), the groups were divided with half of the group pe rforming the task under the same conditions as they did in SS1, while the other half was switched to the other first letter duration. Head a nd eye movements were recorded using a photoelectric transducer and el ectrooculogram (EOG), respectively. Few head movements occurred with t argets presented at 15 degrees eccentricity. Approximately 80% of gaze shifts to the 50 degrees stimuli included a head movement component. DURATION had a significant effect on gaze shift. Head movement amplitu de (HMA) increased, while saccade amplitude decreased with an increase in DURATION. On the average, the proportion of gaze shift accomplishe d via head movement was 24% if DURATION was 500 ms, and 37% when DURAT ION was extended to 1000 ms. SS1 task experience affected HMA in SS2. When DURATIONs differed between SS1 and SS2, HMA in SS2 were drawn tow ard the level of HMA in SS1. A positive and significant correlation wa s observed between head movement amplitude and head movement duration. The results suggest that both stimulus availability and prior experie nce affect head movement amplitude in gaze shift.