OFFSET DYNAMICS OF HUMAN SMOOTH-PURSUIT EYE-MOVEMENTS - EFFECTS OF TARGET PRESENCE AND SUBJECT ATTENTION

Subjects made smooth pursuit eye movements with a target moving horizo ntally at 15 deg/sec, At a specified location the target either: (1) s uddenly vanished; or (2) jumped to the fovea with target retinal veloc ity and feedback becoming 0 (target stabilized at the fovea), In each type of trial, the subjects either: ''looked'' at the target, ''pushed '' the target, or ''passively'' gazed, When the target vanished, eye v elocity decreased exponentially with a short time-constant (tau approx imate to 0.10 sec), regardless of whether the subjects were ''looking, '' ''pushing'' or ''passively'' gazing. However, some subjects while ' 'pushing'' (using an imaginary target) did generate low velocity smoot h movement (1-2.5 deg/sec) late in the offset, When the target was sta bilized at the fovea, eye velocity also decreased, but with a relative ly long time-constant (tau = 0.4-0,8 sec), The time-constant was the s ame with both ''looking,'' and ''pushing'', but was shorter for some s ubjects with ''passive'' gazing (tau = 0.1-0.5 sec), These findings sh ow that smooth pursuit offset is influenced by the presence of a targe t, but is relatively independent of attentional mode, All of the pursu it offset responses can be simulated using a model of the pursuit syst em with target velocity and position inputs, and an internal positive feedback loop enabled by target presence, (C) 1997 Elsevier Science Lt d.