Two experiments are reported in which, by means of a pointing task, we
studied the stimulus-position effect, i.e. the inverted U-shape form
of the reaction-time function in relation to stimulus position in task
s in which stimuli and/or responses are arranged in a horizontal array
. The response consisted of aiming the index finger from a central sta
rting point at a target area on a screen. Reaction time was the main d
ependent variable. The spatial relation between the position of the im
perative signal and the position of the response was manipulated by va
rying the spatial S-R compatibility and physical distance that separat
ed the positions of stimulus and response. The stimulus-position effec
t was shown to depend on the compatibility of the S-R relation (Exp. 1
). In Exp. 2 it was found that the modulation of the stimulus-position
effect by spatial compatibility disappeared completely when the dista
nce between the positions of stimulus and response was reduced. None o
f the experiments revealed that the stimulus position effect depended
on signal discriminability, which renders an interpretation of this ef
fect in terms of perceptual processes unlikely. We argue that the atte
ntional model of spatial coding provides the most reasonable explanati
on of the obtained reaction-time patterns.