The effects of working memory load on
negative priming in an N-back task
Ewald Neumann Brain-Inspired Cognitive Systems (BICS) July, 2010
de Fockert, Rees, Frith, & Lavie (2001). The Role of Working Memory in Visual Selective Attention. Science, 291
500 ms
1500 ms
.
Example of a high memory load
trial sequence involving a
Congruent and an Incongruent
attention display. Correct
answer to memory item is 4.
Exp. 1 N = 10
Interference
850 ms
825
2 to 4 attention
displays are
presented for
500 ms, each
followed by a
1250 ms blank
response
interval.
775
Incong
675
625
3000 ms
Cong
725
Low
Load
High
Load
de Fockert, Rees, Frith, & Lavie (2001). The Role of Working Memory in Visual Selective Attention. Science, 291
.
500 ms
1500 ms
Exp. 2 N = 6
Interference
850 ms
825
775
2 to 4 attention
displays are
presented for
500 ms, each
followed by a
1250 ms blank
response
interval.
Cong
725
Incong
675
625
3000 ms
Low
Load
High
Load
de Fockert, Rees, Frith, & Lavie (2001). The Role of Working
Memory in Visual Selective Attention. Science, vol. 291.
Summary of results and conclusions
• RTs in the selective attention task revealed that
interference from incongruent (versus congruent)
distractor faces was significantly greater with
high working memory load than with low working
memory load.
• High memory load leads to greater intrusion of
irrelevant distractors, because of difficulty in prioritizing
targets from distractors.
• Low memory load enables more successful filtering,
or blocking out of irrelevant distractors, thus minimizing
intrusion from distractors.
Why is it important to eliminate Congruent stimuli in
Stroop-like conflict tasks?
Because it is known that having congruent stimuli
in Stroop-like conflict tasks induces subjects to
maintain less attentional selectivity solely to the
target.
That is why participants consistently show greater
amounts of interference in Incongruent
trials as a function of increasing the proportion of
Congruent trials in such conflict tasks (e.g.,
Lindsay & Jacoby, 1994).
Why is it important to eliminate Congruent stimuli in
Stroop-like conflict tasks (cont’d)?
Since one of the stated objectives in using the present paradigm
was to investigate selective attention, rather than more diffuse or
divided attention, it is important to try to induce maximal
attentional selectivity.
Including Congruent stimuli in the task could be counterproductive in that regard.
In addition, assessing “interference” by contrasting Congruent vs.
Incongruent conditions is another potential problem in their study.
By doing so, it is not possible to disentangle the interference cost
from incongruent stimuli from the potential benefit in processing
due to congruent stimuli.
To illustrate, consider their results from Experiment 1 again:
de Fockert, Rees, Frith, & Lavie (2001). The Role of Working Memory in Visual Selective Attention. Science, 291
500 ms
1500 ms
850 ms
.
To assess “interference” more
purely, the Incongruent condition
should be contrasted with a neutral
Control condition, not a Congruent
condition.
Exp. 1 N = 10
Interference
825
2 to 4 attention
displays are
presented for
500 ms, each
followed by a
1250 ms blank
response
interval.
775
Incong
675
625
3000 ms
Cong
725
Low
Load
High
Load
Why is it important to have an “ignored repetition”
(negative priming) condition in Stroop-like conflict
tasks?
In some instances negative priming effects can
provide a more sensitive behavioral index, than
concurrent interference effects, regarding the
depth to which distractors have been
processed.
Why is it important to have an “ignored repetition”
(negative priming) condition in Stroop-like conflict tasks?
Name the color of the “odd man out” letter task
Problems with inferences from null results
XXXXX
XXXX
vs
G R E E N (no Stroop interference)
BLUE
(significant negative priming)
The Stroop color-naming effect has often been taken as evidence for the automaticity of
word processing. However, Besner et al. (1997) reported that coloring a single letter
instead of the whole word eliminated the Stroop effect. From this finding, they concluded
that word processing could not be purely automatic, since it can be prevented.
Mari-Beffa et al. (2000) asked whether the elimination of the Stroop effect is sufficient
evidence for concluding that the word is not processed. Combining Besner et al.'s
manipulation with a negative-priming procedure, she found intact negative priming from
the prime color word in the absence of a Stroop effect. This result clearly indicates that
the meaning of the prime word was processed. These findings highlight the importance
of using converging methods to evaluate lack of processing.
Main goal of the present study:
Test implications of the working memory load theory
in a new experiment designed to eliminate some of the
potential flaws I just outlined.
Method - regarding memory load: The N-back task is used
extensively as a way to manipulate working memory (WM)
load. Typically, in the N-back task participants are presented
with a stream of stimuli, and the task is to decide for each
stimulus if it matches the one N items before.
The WM processing load can be varied systematically by
manipulating the value of N. WM load is deemed to be low in
a 1-back task and high in a 2-back task.
Method - regarding selective attention: Because this study is
designed to investigate how WM and visual selective attention
interact, this N-back task also involved selective attention.
Sample 1-back task: Respond “same” if the
immediately preceding picture is
of the same object, otherwise
respond “different.”
Red Picture Targets
(ignore green letters)
Schematic illustration
of stimulus configuration
Sample 2-back task: Respond “same” only if a
picture is of the same object
that was 2 pictures back, otherwise
respond “different.”
Red Picture Targets
(ignore green letters)
Sample experimental trials
Ignored Repetition
Control
Response
Same
(bird then bird)
Insertion here of an
unrelated picture
creates a 2-back trial.
Different
(bird then finger)
Different
(ashtray then bird)
Probe Displays
Prime Displays
1-back sample stream of stimuli
2-back sample stream of stimuli
Sample experimental trials
Ignored Repetition
Control
Response
Same
(bird then bird)
Insertion here of an
unrelated picture
creates a 2-back trial.
Different
(bird then finger)
Different
(ashtray then bird)
Probe Displays
Prime Displays
Results
Mean RT and % Error for Control vs. Ignored Repetition (IR)
conditions as a function of Low vs. High Working Memory Load
Negative Priming (RT)
875
825
Negative Priming % Errors
3.5
3
Control
Control
IR
2.5
IR
2
775
1.5
1
725
0.5
0
675
1-back
Low
Load
2-back
High
Load
1-back
Low
Load
2-back
High
Load
Results summary and implications:
RTs and Errors much greater in 2-back than 1-back task suggests
WM load manipulation was successful.
RTs significantly longer in Ignored Repetition (IR) condition than
Control condition, and Errors occurred significantly more often in the
IR than Control condition, which indicates that ignoring a picture’s
name can impede response to the picture on the following trial. This
negative priming effect was produced in both the 1- and 2-back
versions of the task.
For RTs there was no interaction between the N- back and priming
conditions; however, the error rate analysis for this interaction was
significant. In this case, it appears that greater WM demands in the
2-back task did yield an exaggerated difference in the error rates
between the IR and Control conditions, compared to the 1-back task.
This provides some support for the idea that participants may be
better able to block or filter out distractors under low WM load.
Summary and Conclusions regarding how WM and
selective attention interact:
At least according to the error rates, the present findings
could be interpreted to suggest that the working memory
system is modulating attention and affecting processing
within the brain’s recognition system.
High WM load might reduce the efficiency of early
selection and increase the effects of irrelevant stimuli.
In contrast, Low WM load might enable relatively
increased efficiency of early selection so that participants
are better able to block or filter out the irrelevant
distractors under low memory load conditions, as
proposed by de Fockert et al. (2001).
End