Magnitudes of Spatial Orienting and the Foreperiod Effect

Finally, we assessed whether the magnitudes of spatial orienting (uncued RT-
cued RT) and the foreperiod effect (the average RT at 106 ms—the average RT at
931 ms) varied systematically with changes in target probabilities.

The analysis of orienting magnitudes, illustrated in Figures  4A  –  D  ,
supported the RT data indicating larger overall inhibition with lengthening of
cue-target time [−3.8 vs. −22.7 ms; _F_ (1, 42)  = 29.16, _p_ < 0.001, see
Figures  4A  –  D  ] with no difference between the magnitudes of orienting
with changes in _within_ trial target probability (−13.4 vs. −13.2 ms; _F_ <
1; Figures  4A  ,  C  vs. Figures  4B  ,  D  ). The early inhibitory effect
was present only when the targets occurred frequently overall, i.e., under
high tonic alertness, as shown in Figures  4A  ,  B  [across trials target
probability × cue-target interval; _F_ (1, 42)  = 8.08, _p_ < 0.01],
indicating once again that the overall high probability of target presence
_across_ trials influences the time course of reflexive orienting. Finally,
increased overall magnitude of inhibition depended on an interaction between
processes engaged by _across_ and _within_ trial target probabilities emerging
only when tonic alertness was high _and_ voluntary temporal preparation was
induced by the task sequence [across trials target probability × within trial
target probability; _F_ (1, 42)  = 5.77, _p_ < 0.05; see Figure  4A  ].

FIGURE 4

**Figure 4. Magnitudes of orienting and the foreperiod effect.** Magnitudes of
orienting (uncued RT—cued RT) and the foreperiod effect (average RT at the 106
ms cue-target interval—average RT at the 931 ms cue-target interval) are shown
for each of the four experimental conditions: **(A)** high _across_ trials
target probability (i.e., 94% Target) and increasing _within_ trial target
probability; **(B)** high _across_ trials target probability (i.e., 94%
Target) and equal _within_ trial target probability; **(C)** low _across_
trials target probability (i.e., 75% Target) and increasing _within_ trial
target probability; and **(D)** low _across_ trials target probability (i.e.,
75% Target) and equal _within_ trial target probability. Error bars represent
the standard error of the difference between the means.

The magnitude of the foreperiod effect was reduced when targets appeared less
often overall [14.5 vs. −14.6 ms; across trials target probability; _F_ (1,
42)  = 15.56, _p_ < 0.001; Figures  4A  ,  B  vs. Figures  4C  ,  D  ] and
when the probability of target occurrence was equated _within_ trials [16.5
vs. −16.9 ms; within trial target probability; _F_ (1, 42)  = 58.53, _p_ <
0.001; Figures  4A  ,  C  vs. Figures  4B  ,  D  ]. The most pronounced
decline in the magnitude of the foreperiod effect was found when both target
probabilities were reduced, as shown in Figure  4D  . This strongly suggests
that both the probability of target presence _across_ trials, affecting tonic
alertness, and the probability of target presence _within_ trials, affecting
voluntary temporal preparation, modulate the foreperiod effect.

Taken together, our results indicate that a typical sequence of events in the
Posner cuing task, in which the targets appear frequently overall and the
probability of target occurrence within a trial increases with lengthening of
cue-target time, leads to interactions between tonic alertness and voluntary
temporal preparation. This in turn critically modulates both the time course
and the magnitude of the measured attentional effects as well as the magnitude
of the foreperiod effect.