The role of feedback in manual tracking of visual targets

The role of visual feedback in manual tracking was investigated in 24 subje cts who tracked 5-, 10-, and 40-mm/diameter targets, moving on a screen at 18 to 25 mm/sec., along various paths, by moving an unseen handle over a di gitizing tablet. A cursor indicating instantaneous handle position was visi ble at all times on half the trials and hidden within a circle coaxial with the target but double its diameter in the other half. The handle had to be within the instantaneous target's digitizer-defined boundaries for the lat ter to keep moving. All tracking movements were segmented into small moveme nt steps. A tendency to outrun the target was seen, indicating predictive c ontrol. Absence of visual feedback had negligible effect on movement veloci ty. Movement direction appeared to involve open-loop programming but improv ed significantly when subjects could see the cursor. Occasional corrective movements occurred only when visual feedback was given. Otherwise, a large positional error accumulates despite reasonable ability to control tracking direction.