4. 2. Experiment 2: retention over time
An authentication mechanism is only useful if authentication can still be accurately performed at some time after the
password is memorized. In Experiment 2, we confirmed that
sequence-specific knowledge acquired by users was retained
over prolonged periods of time. Although skill learning generally persists over time, a SISL-based test had never been
conducted with a substantial delay and a sufficient number
of participants.
In Experiment 2, participants agreed to perform the SISL
task over two sessions. In the first session, participants
completed a training sequence with the same structure as
the one in Experiment 1. The training was immediately followed by the same SISL test to assess sequence knowledge
before the delay. A group of 32 participants returned to the
online applet after 1 week to perform a retention test and
recognition assessment for the trained sequence. A separate
group of 80 participants returned after a 2-week delay for
the retention and recognition tests. For the 1-week group,
the test session consisted of a 540-trial implicit sequence
learning assessment. For the 2-week group, the test session
was doubled in length to additionally evaluate whether a
longer test provided better sensitivity to individual sequence
knowledge. For both groups, the initial speed of the test on
the delay session was set to match the speed with which the
participants had been performing the task at the end of the
training session. A short warm-up block of 180 trials was
used to adjust this initial speed so that participants were
performing at around the target 70% correct at the beginning of the retention test.
Figure 4 shows gradual learning of the trained sequence
during the first session for both groups as in Experiment 1.
Implicit sequence knowledge at both immediate and
delayed tests is shown in Figure 5. On all five assessments,
participants exhibited reliable sequence learning as a
group, ts > 4. 3, ps < 0.01. On the 1-week delay test, 15 of 32
participants exhibited individually reliable sequence knowl-
edge. However, for the 2-week delay group, 49 of 80 partici-
pants exhibited reliable sequence knowledge, reflecting the
increased sensitivity in the longer assessment test used.
Future research will examine both increased training time
and 70.6% correct for the untrained sequence. The differ-
ence of 8.6% correct (SEa 2.4%) indicated reliably better per-
formance for the trained sequence by a paired-sample t-test
versus zero, t( 34) = 3. 55, p < 0.01.
Group-level differences in performance are commonly seen on tests of implicit learning, but being able
to reliably assess individual learning is necessary for an
authentication method. On an individual participant
basis, performance on the trained sequence could be
discriminated from that on the untrained sequence on
the 540 test trials (by chi-squared analysis at p < 0.05) in
25 of 35 cases. For authentication purposes, the individual reliability of the assessment will need to be further
improved by longer training to establish the implicitly
learned sequence. However, the ability to identify learning in a large fraction of individuals with relatively short
training is a feature of the SISL task not seen in most tests
of implicit learning.
7 Traditionally, measures of implicit
learning rely on assessing performance within groups of
individuals, without the ability to identify learning at the
individual level.
9
Explicit recognition test. After the training and test
blocks, participants were presented with five different animated sequences and asked how familiar each looked on a
scale of 0 to 10. Of the five sequences, one was the trained
sequence and the other four were randomly selected foils.
This test assessed explicit recognition memory for the
trained sequence.
On the recognition test, participants rated the trained
sequence as familiar at an average of 6. 5 (SE 0.4) on the
0– 10 scale and rated novel untrained sequences at 5. 15
(SE 0.3). The modestly higher recognition of the trained
sequence was reliable across the group, t( 34) = 3. 69, p <
0.01, but did not correlate with SISL performance (r =
0.13), indicating that it did not contribute to the implicit
performance. Slightly higher recognition of the trained
sequence is often seen in implicit learning experiments
as healthy participants find some parts of the training
sequence familiar after practice. It is worth noting that
implicit memory does not transform into explicit knowledge, even with repeated use, and the structure and length
of the training and test sequences specifically aim to
reduce the possibility that explicit knowledge is accumulated over time.
The general small difference in recognition ratings
( 5. 15 vs. 6. 5) indicates that participants would not be able
to recall the 30-item sequence, meaning that they could
not consciously produce the training information (e.g., to
compromise the security of the authentication method).
We discuss the reconstruction question further in our
third experiment.
a SE is short hand for Standard Error. In other words, if the percent correct measurements for trained and untrained sequences followed the
same normal distribution, the t-value calculated with N = 35 samples (and
thus N − 1 = 34 degrees of freedom) should be near zero—less than the t-
value obtained here of 3. 55, which represents a 99% probability that the
difference is significant. The t-test is a standard statistical method used
to confirm that the manipulated variable (here, sequence type) affects the
measured variable (percent correct).
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Training Block (540 trials)
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2 weeks
Figure 4. Across training participants gradually begin to express
knowledge of the repeating sequence by exhibiting a performance
advantage for the trained sequence compared to randomly
interspersed noise segments. Learning performance was similar
across both groups and similar to Experiment 1, as expected.