Across the population, the mean false-positive response rate to
catch trials was 3.2%, ranging from 0% to 53%. In 65% of visual
field tests no false-positive responses were made to catch trials. In
14% of tests, the false-positive response rate was higher than 10%,
and in 1.5% of tests, the false-positive response rate was greater
than 25%. False-positive response rates did not vary with the
patient’s age (Spearman’s R = 0.326, P = 0.51). At locations classified as normal by the
test, the probability of detecting the 5-dB suprathreshold stimulus
(number of stimuli seen/presented) was 96%, independent of
eccentricity out to the test limit of 25°.
RT distributions varied greatly among patients. Across the sample of
patients, individual median RTs ranged from 316 to 908 msec with a
group mean of 451 msec
(Fig. 1) . Individual interquartile ranges of RTs extended from 41 to 422 msec,
with a group mean of 108 msec. Interquartile ranges were related to
median RTs (Pearson
r = 0.74,
P <
0.001). Median RTs decreased slightly with age (9 msec per decade, 95%
CI 4–13 msec per decade;
P < 0.001), but the
correlation was poor (Pearson
r = 0.18). Median RTs
from the right and left eyes of individual patients were highly related
(Pearson
r = 0.72,
P < 0.001). Tests
of the left eye, which were always performed last, yielded responses
that were, on average, 12 msec faster (
P < 0.001,
paired
t-test) and somewhat less variable (mean difference
of individual interquartile range 9 msec,
P < 0.001,
paired
t-test) than those of the right eyes. The mean
false-positive rate of the left eye results (2.8%) was slightly lower
than that of the right eye results (3.4%,
P = 0.103,
paired
t-test).
Figure 2 gives an example of RTs from a patient with a high false-positive rate.
The RT distributions were usually positively skewed. A square-root
transformation was applied to reduce this skew and to separate the main
body of data from early or late outliers. To compensate for the large
between-subject variability in average latency and dispersion, the
square-rooted RTs (of stimulus as well as catch trial responses) were
standardized to
z-scores using the mean and SD of the
stimulus RTs from each visual field test
(Fig. 3) . The ratio between the proportions of false-positive and stimulus
responses (i.e., the relative likelihood of a false-positive response)
increased dramatically with positive or negative deviations from zero
on the
z-scale. Less than 5% of stimulus responses had
latencies outside the interval of ±2
z, compared with more
than 60% of false-positive responses.
If responses were classified as suspect false-positives simply on the
basis of
z-scored RTs, the proportion of such responses
would be approximately constant throughout the population of patients.
With a cutoff value of ±2
z, for example, the proportion of
responses classified as suspect would always be approximately 5%. This
approach has poor specificity for the majority of patients who make no
false-positive errors and poor sensitivity if the false-positive rate
is greater than 5%. Better performance is achieved with an algorithm
that determines, iteratively, which values deviate most from the mean
of the remaining square-rooted sample. These values are successively
removed if their exclusion reduces the sample variance by an amount
larger than a predetermined criterion
(
V crit).
\[V_{\mathrm{crit}}{=}V_{\mathrm{pre}}-V_{\mathrm{post}}\]
where
V crit is the criterion
value,
V pre is the sample variance
before and
V post the sample variance
after removal of most deviating square-rooted RTs. The response window
is then defined by the smallest and largest squared values of the
trimmed sample.
Figure 4 shows the performance of this algorithm at different values of
V crit. With a criterion value of 0.5,
more than 70% of false-positive responses to catch trials, but less
than 3% of stimulus responses, occur outside the RT window. In 35% of
tests, no stimulus responses were detected outside the RT window
defined by this criterion. In fewer than 5% of tests was the
proportion of stimulus responses outside the RT window greater than
10%.