Minimum Expected Entropy Staircase
The staircase gives suggestions for which probe value to test next,
choosing the probe that will provide the most information (based on the
principle of minimum entropy = maximally unambiguous probability
distribution). Probes are chosen from a set of possible probe values
provided at staircase init, and their use is evaluated based on the
expected amount of information gain given a space of PSE and slope values
to test over.
See MinExpEntStairDemo for an example and the comments in the method
functions below for use of the different staircase methods.
By default, a psychometric function ranging from 0% to 100% is used, as
is suitable for discrimination experiments with a standard in the middle
of the possible stimulus parameter range. For other paradigms, such as
n-AFC detection tasks, one can set the guessrate input during staircase
init to 1/num_alternatives, e.g. .5 when doing a 2IFC detection task.
This guess rate is thus not the rate at which participants guess instead
of do your task (thats the lapse rate), it the minimum rate of correct
responses as determined by your design. NB: below discussion is based on
the default psychometric function with the full range, but all points are
equally valid for a scaled psychometric function.
It is recommended to have the staircase determine the optimal next probe
based on only a random subset of the response history (see the
‘toggle_use_resp_subset’ and ‘toggle_use_resp_subset_prop’ methods). This
makes its operation more robust for response errors and also avoids probe
oscillations when the fit estimate is converging.
When we are close to convergence, probes will tend to be near the 25% and
75% points. If a probe is 25% and you answer ‘1’ (pedestal faster, which
is likely, because it’s near the correct 25% point), then for the next
trial the peak in expected entropy reduction will generally be the 75%
point, and vice versa. This can lead to undesirable probe sequences where
the correct response alternates 0,1,0,1,0,1. If you choose a random
subset, this will completely eliminate the problem. If the staircase has
converged to where there are two almost equal expected entropy minima,
then small variations due to the selection of subsets will randomly vary
which minimum emerges as lowest.
This strategy does not significantly affect optimal operation of the
staircase. Lots of probe values provide useful information. Therefore, it
is not crucial to have a highly accurate estimate of likelihoods, so
relatively few trials are sufficient (less than are needed to for final
estimates of PSE and DL). Throwing out trials for the staircase
computation yields robustness without much cost.
Another option would be to load a non-uniform prior on the space of
possible location/mean/PSE and dispersion/slope parameters (known as mu
and sigma respectively for a cumulative Gaussian - see the loadprior
method). Probe sampling will then stay reasonable in early trials even if
there were a couple bad responses. But this strategy is not as robust as
using a random subset – bad trials will continue to have an effect
In absence of anything to base the optimal probe value on, the first
probe is chosen randomly from the set of possible probes. When a prior
was loaded, a likelihood distribution is available based on which the
optional probe value can be computed. If for any other reason choosing
the next probe based on the measure of minimum expected entropy fails,
the staircase will fall back on the same random probe sampling strategy.
There is an option to set the first probe value to be tested (see the
set_first_value method), which, for the first trial only, will overrule
both of the above probe choice strategies. This can be useful if you want
to be sure that the first trial is an easy one so the participant knows
what to expect.
Another measure for robustness is to choose a small lapse rate. If lapse
would be zero and a response error is made by the observer, immediately a
whole range of mean-slope combinations becomes impossible. If lapse rate
is non-zero, these would still have a non-zero probability and the
staircase can rebound. Therefore a lapse rate of 5% or even more
depending on task difficulty is always recommended. NB: in the default
discrimination setup of the staircase (guessrate is not specified or set
to 0), half of the lapse rate is taken off the bottom of the psychometric
function and half is taken off the top. So if the lapse rate is 0.05, the
psychometric function will range from 0.025 to 0.975. In the setup for a
n-AFC detection experiment when the psychometric function has a lower
bound of 1/num_alternatives, the lapse rate is taken off the top. So when
the guess_rate is set to .5 (2AFC) and the pase rate is set to .05, the
psychometric function will range from 0.05 to 0.95.
Note that the staircase does not support a 0 lapse rate in the first
place as it works with log-probability and we get in trouble if we would
take the log of a 0 probability. Any lapse rate lower than 1e-10 will be
adjusted to 1e-10 upon calling the init method.
If the staircase gets stuck at one of the bounds of the probe set, check
that the sign of the slope space matches the expected sign of the
response. E.g., lets look at an experiment in which you are doing 2IFC
task in which the observer is asked to report which interval contained
the faster motion. If the observer choses the test over the pedestal
interval the response is 1, if the observer chosen the pedestal to be
faster, the response is 0. All slopes in the set would in this case be
positive as the low end of the probe space (slow speeds) is associated
with response 0 and the high end with response 1. If we however asked the
observer to indicate the slower interval, the slopes in our slope set
would not match the task, and the staircase would get stuck at one of the
probe bounds. In this case, the lower end of the probe space is
associated with the response 1 and the higher end with the response
0–we’d thus have a negative slope for the fitted cumulative probability
The staircase currently only supports logistic and cumulative Gaussian
(default) psychometric functions (see the set_psychometric_func method),
but others could easily be implemented. Changes should be needed only to
the private fit_a_point method near the bottom of this mfile, providing
that the function is characterized by two parameters (which do not
necessarily have to be PSE and slope, though that is the terminology
Should you implement such a function, please do send me your code to
dcnieho @at@ gmail.com.
The above discussion assumes that response inputs to the process_resp
method are either 0 or 1 (see note above about their meaning) though in
practice anything larger than 0 is treated as 1 and anything lower than
0, including 0, is treated as 0. the staircase can thus easily be
integrated with programs that use a 1, -1 response scheme.
For actual offline fitting of your data, you would probably want to use a
dedicated toolbox such as Prins, N & Kingdom, F. A. A. (2009) Palamedes:
Matlab routines for analyzing psychophysical data.
http://www.palamedestoolbox.org. instead of using the function parameters
or PSE and DL returned from staircase methods get_fit and get_PSE_DL.
Also note that while the staircase runs far more robust when a small
lapse rate is assumed, it is common to either fit the psychometric
function without a lapse rate, or otherwise with the lapse rate as a free
parameter (possibily varying only over subjects, but not over conditions
within each subject).
Based on the Minimum expected entropy staircase method developed by:
Saunders JA & Backus BT (2006). Perception of surface slant from
oriented textures. Journal of Vision 6(9), article 3
Discussions of conceptually similar staircases can be found in:
Kontsevich LL & Tyler CW (1999). Bayesian adaptive estimation of
psychometric slope and threshold. Vision Res 39(16), pp. 2729-37
Lesmes LA, Lu ZL, Baek J & Albright TD (2010). Bayesian adaptive
estimation of the contrast sensitivity function: The quick CSF method.
Journal of Vision 10(3), article 17