Why does this effect occur? In 
order to attend to and focus on 
the object in the color required 
by the instructions, the 
perception of the other, 
overlapping object must be 
suppressed or inhibited. If such 
inhibition carries over to the 
next trial, it will interfere 
with the processing necessary 
to consciously perceive that 
object and thus will increase 
the time to respond. However, 
the perception of the object on 
the trial when it is suppressed 
is not a conscious one nor is 
the suppression on the next 
trial in any way conscious.

    Based on Tipper’s research, 
Anne Treisman and Brett De 
Schepper at the University of 
California at Berkeley extended 
the findings and uncovered 
some rather amazing facts. 
Instead of using overlapping 
familiar figures, they used the 
kinds of novel figure that 
Gutman and I had used, which 
necessitated a change in 
procedure. Since novel figures 
have no names, the 
investigators used what is 
referred to as a Same-Different 
matching task on each trial. On 
one side was a pair of 
overlapping figures, one green 
and one red, and the subjects 
were to say as quickly as 
possible whether the attended 
figure on one side was the same 
as or different than a white 
figure on the other side. On 
negative successive priming 
trials, the irrelevant colored 
shape on one trial would 
become the relevantly colored 
shape on the next. The 
researchers found that it 
required more time to respond 
on that second trial"same" or 
"different" if the now 
relevantly colored shape was 
the one that was irrelevant on 
the preceding trial. The 
surprising new finding, in 
addition to the fact that 
negative priming occurred for 
unfamiliar shapes, was that the 
effect lasted for long periods of 
time. If the irrelevant shape 
only appeared much later in 
the series, after many other 
intervening trials with many 
different figures, it still had a 
negative, delaying effect on 
reaction time. In fact, the 
unattended and unconsciously 
processed shapes exerted an 
effect even after many days or 
weeks intervened.