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Abstract
Stimuli presented in a non-attended location are responded to much slower than stimuli
presented in an attended one. The hypotheses proposed to explain this effect make
reference to covert movement of attention, hemifield inhibition, or attentional gradients.
The experiment reported here was aimed at discriminating among these hypotheses. Subjects
were cued to attend to one of four possible stimulus locations, which were arranged
either horizontally or vertically, above, below, to the right or left of a fixation
point. The instructions were to respond manually as fast as possible to the occurrence
of a visual stimulus, regardless of whether it occurred in a cued or in a non-cued
location. In 70% of the cued trials the stimulus was presented in the cued location
and in 30% in one of the non-cued locations. In addition there were trials in which
a non-directional cue instructed the subject to pay attention to all four locations.
The results showed that the correct orienting of attention yielded a small but significant
benefit; the incorrect orienting of attention yielded a large and significant cost;
the cost tended to increase as a function of the distance between the attended location
and the location that was actually stimulated; and an additional cost was incurred
when the stimulated and attended locations were on opposite sides of the vertical
or horizontal meridian. We concluded that neither the hypothesis postulating hemifield
inhibition nor that postulating movement of attention with a constant time can explain
the data. The hypothesis of an attention gradient and that of attention movements
with a constant speed are tenable in principle, but they fail to account for the effect
of crossing the horizontal and vertical meridians. A hypothesis is proposed that postulates
a strict link between covert orienting of attention and programming explicit ocular
movements. Attention is oriented to a given point when the oculomotor programme for
moving the eyes to this point is ready to be executed. Attentional cost is the time
required to erase one ocular program and prepare the next one.