Sequential Item Presentation and Testing
The memory scanning experiment stm is modeled after Sternberg's
item-recognition paradigm. A small set of
memory items is presented to the subject sequentially. After the items have
been shown and a short time interval has passed a single probe item is
presented and the subject has to decide as fast as possible whether the
probe item was a member of the set of memory items presented earlier or not.
The response time and the decision are recorded. The number of memory items
usually is less than ten and the subject is instructed to decide fast but
make as few errors as possible.
Ensemble and Memory Items
The complete ensemble of items is defined as an array of
ensemble = ["Berlin", "London", "Paris", "Rom", "Budapest",
"Prag", "Moskau", "Sofia", "Chicago", "Montreal",
"Belgrad", "Madrid", "Kairo", "Beirut", "Wien",
"Genf", "Peking", "Saigon", "Havanna", "Tokio"]
The strings may also be single character items like the more
conventional sequence of digits or letters usually used in memory scanning
experiments. PXL also has character sets that contain symbols and thus
allow them to be used as memory items (see the demo program hfonts and
Chapter graphics for a complete list of PXL's graphic fonts).
Usually the complete set of items in ensemble also defines the complete
sampling set for the memory items. If memory set item sampling should be
restricted to a subset of ensemble this subset may be defined by an
integer offset into the ensemble array and an effective ensemble size
value. The offset value for the effective sampling subset is defined by
ensoffset and the effective ensemble size is defined by ensize.
Offset values default to 0 and ensize defaults to the complete
length of ensemble.
The memory set is defined by the variable memoryset. If the flag Mis not set, then the memory set will be sampled automatically from the
ensemble. Memory set size is defined by memsetsize if automatic
memory set sampling is used or implicitly if the memory sets are given in
the parameter file. Using memory sets from the parameter file is switched on
by setting the flag M. Note that memory set item sampling is restricted
to the effective ensemble as defined by ensoffset and ensize.
Also note that protocol and parameter file memory item index values always
are relative to the start of the effective ensemble not to the start of the
array ensemble itself. Thus if a nonzero value for ensoffset is
specified, then the memory item index values have to be relative to the
start of the effective ensemble subset in ensemble.
The font used to draw the memory items is defined by the variable
itemfont and the character size is defined by itemsize. Memory
items are always written in a white color with luminance given by
itemlum. The flag C controls whether the single items are printed
centered or flush left which may be better for readability if the items are
rather long strings. In a fixed-sets procedure the memory set display can be
suppressed completely by setting the flag N. In this case it might be a
good idea to incorporate the memory set into the instructions file (see
Chapter files Section instruct for an introduction how to
activate PXL's instruction file mechanism).
Probe Item Selection
The probe item is drawn with colors and luminance defined by probecolx,
probecoly, and probelum. If these define a color which is very
similar to the background then legibility of the probe item will be low.
The probe item selection is controlled by several flags and parameters.
\beginenumerate If the flag F is on, the probe item is determined by the actual
value of the variable probeindex in the parameter file, which gives the
index of the probe item in the string array ensemble.
If the flag F is off, then the probe item is selected at random
either from the memory item set or from its complement, the non-memory item
If the flag O is on then the selection set for the probe item
is determined by the value of the experimental variable memprobe.
\beginenumerate If memprobe is 1, then the
probe item is sampled from the memory
If memprobe is 0, then the probe item is sampled from the
non-memory item set.
\endenumerate If the flag O is off, then the set from which the probe
item is sampled is determined by a random decision.
The probability of drawing the probe
from the memory item set is defined by the parameter memprobechance.
The complement of memprobechance is the probability of drawing the
probe item from the non-memory item set.
\endenumerateIn any case, if the flag F is off, the probe item is sampled such that
each item in the set where it is sampled from has the same probability to be
\endenumerateA subset of ensemble may be selected for probe item sampling by setting
the variable probeoffset to a nonzero value. This variable specifies the
start of the subset in ensemble that should be used for probe item
sampling. The size of this subset is given by ensize. Note however,
that probe item index values always are relative to the effective subset of
ensemble that is used.
Some experiments may want to use probe items that are translations from
ensemble items. In this case the flag T should be set in order to
select the probe items from the array trensemble of translated ensemble
items. The array trensemble has to have the same number of elements as
ensemble such that correspondence of items is via their index. If
T is on then the index probeindex refers to the array
Probe item translation may also be achieved by using the mechanism of
ensoffset, probeoffset, and ensize. This mechanism is more
flexible than the flag T since it is possible to use different values
for these variables in each trial, while the flag T is effective for the
Cue Item Selection
The flag U makes visual cueing possible. If U is set, then the
item with index cueindex (referring to ensemble) will not be
written in the same color as the other memory items, but will be written
with cuecolx, cuecoly, and cuelum as its color and luminance.
The cueindex is taken from the parameter file. However, if it is
negative, then the cued item is selected at random from the memory set. The
value of cueindex in the data file will reflect the cued item in this
A major part of the experimental variables in stm control the time
course of a single trial. Here is an overview on the timing parameters
within a single trial:
An experimental session starts with a greeting message defined by
bosmsg which is visible for bostm milliseconds. As usual the message
is visible until a key is pressed if bostm is negative and the message
is suppressed if bostm is zero. In a fixed-sets procedure the greeting
message is followed by the presentation of the memory set provided the flag
N is not set. The first trial starts immediately after the item
presentation or after the greeting if no items are shown.
The message eosmsg shown for eostm milliseconds finishes a
- Each trial starts with an intdsptime millisecond pause.
- If the varied-set procedure is used, then comes a
message defined by dspmsg which is visible for dsptm milliseconds.
If the actual value of dspmsg is negative, the program waits until one
of the response keys is pressed. After the display of dspmsg there is a
wait of aftermsg milliseconds and then starts the sequential
presentation of the memory set items. Each item is shown for
ontime milliseconds and between successive items is a time interval
of offtime milliseconds.
- Now follows an empty time interval of attwait milliseconds in
both procedure types. If Rehearsal is controlled then the rehearsal control
procedure is run during the attwait interval.
- At the end of this interval there is a warning signal which is on for
- Now the program checks for an early response. If the subject has
already pushed a response button, the trial is stopped and
the warning message earlymsg appears instead of a feedback.
- Finally comes the probe item. It is visible until there is a response.
- The response time is limited by timeout. If there is no response
within timeout milliseconds after the probe display, the
warning message tmoutmsg appears instead of a feedback.
- If the response was in time, there is a feedback message defined by
corrmsg for correct responses and by errmsg for false responses.
Early responses have earlymsg and late responses have tmoutmsg as
a feedback. The feedback message is visible for feedbacktime
- This finishes a trial.
- aftermsg (int)
- In a varied-sets procedure there has to be a waiting time
after the instruction message which is shown immediately before
the new memory items are displayed. aftermsg gives the length of this
- attduration (int)
- Duration of the attention signal. The probe
item is shown immediately after the attention mark has been switched off.
- attwait (int)
- After the last memory item has been shown there is some
time to wait for the attention signal. attwait
ms after the last memory item has disappeared the attention signal starts.
- attmsg (string)
- The attention signal is an acoustic beep in case of optic
signal presentatoins and a text string in case of acoustic test item
presentation (flag Y).
- bglum (float)
- Background screen luminance.
- countmark (string)
- Symmetric count down marker. This marker indicates the
subject how many items are in the list of memory items.
- countprefix (string)
- Prefix item for count down. This character is presented
immediately before the memory item list starts.
- cuecolx , cuecoly , cuelum (float)
- Color and luminance coordinates of the
optically cued item in the memory set. Cueing is activated by setting the
- cueindex (int)
- Index of the cued item in the memory set. The index refers
to the position of the cued item in the ensemble array. If the index value
cueindex is negative, then stm will randomly select one of the
memory set items to be cued. Ithis item's index is then stored in
cueindex. Cueing only makes sense in a varied-sets procedure.
- earlymsg (string)
- Warning message for early responses.
- ensemble (string)
- This is the set of possible stimuli.
It has to be an array of strings. Using a string array here is highly
flexible since one can set the strings to single characters or digits as
done for most memory scanning experiments or use numerals or words for the
analysis of categorization effects.
- ensize (int)
- Usually the ensemble size is inferred automatically
from the actual size of the array ensemble. In some cases one may want
to specify a different value by setting the variable ensize explicitly
in the parameter file. In case ensoffset is used to define a subset of
ensemble for item selection, then ensize must necessarily be set.
- ensoffset (int)
- This is an offset index value into the array
ensemble which specifies the subset of ensemble that should
actually be used for memory item selection. The subset size has to be given
by ensize. The default value for ensoffset is zero. Note that the
probe item subset of ensemble has its own offset value given by
- errorlimit (int)
- Stop the experimental session if there are more errors in a
block than this limit. This parameter is used by experiments which determine
- flags (string)
- Show an attention signal immediately before the probe item.
- Get single character response instead of binary yes/no responses.
- Print strings centered. Otherwise they
are printed flush left at the horizontal position defined by xpos.
- Use probe index values as they are defined in the parameter file.
Otherwise the probe items are determined automatically.
- Get multiple character string response. Input is finished by the
RETURN-key. The response time in rtime is the time until the
response is finished.
- Controls rehearsing. Setting this flag will introduce a control
task during the time interval between memory item presentation and probe
presentation in order to prevent the subject from rehearsing the memory set
items. The control task is to count backwards at a given rate starting from
a 3-digit number presented at the beginning of the rehearsal interval.
- Do not give any response echo.
- If this flag is set then the program expects that the trials in
the parameter file contain the memory set item indices. If the flag is off
then the memory sets are selected automatically and at random from
ensemble. Taking the memory set from the parameter file allows complete
control of the memory set.
The memory set item
index values are given in the parameter file by setting the variable
memoryset to a string that contains the index values separated by a
minus sign: "3-7-0-15" is an example of such a string selecting the
items that have index values 3, 7, 0, and 15 in the array ensemble as
memory set items.
Note that no ensemble item index may be used more than once in any memory
set. If replications are needed then the ensemble may to contain multible
copies of items.
- Do not display memory set items at all.
- Controls where the probe item comes from if it is not given in the
parameter file as requested by the flag F. If the flag O is on
then the probe item is selected according to the state of the experimental
variable memprobe. If memprobe is 1, then the probe item is
sampled from the memory set, otherwise from the non-memory set items.
If the flag O is off then the probe item is sampled in a two stage
random process. The first step is to decide whether the probe item is
seleceted from the memory set. This happens with probability
memprobechance. In all other cases the probe item is selected from the
complement set in ensemble. The second step is to select an item from
the set decided upon in the first step.
- Present probe items.
No probe items are presented if the flag is off. This flag may be used to
implement a recall procedure.
Controls the response direction of the response keys. If the
flag is set then the left key means "yes" and the right key means
"no". Clearing the flag reverses this order.
- Use a response cue in order to enforce a certain trade off between
time and precision in a subject's response.
- Activate probe item translation. If this flag is set then the probe
item index values probeindex refer to the string array trensemble
instead of the array ensemble where the memory items come from. This
makes it possible to use "translated" probe items.
- Activates cueing for one of the memory set items. If this flag is on
the memory set item whose index in ensemble is cueindex is drawn
with the color defined by cuecolx, cuecoly, cuelum instead of
the normal color white with luminance itemlum which is used for the
other memory set items. A randomized cue item selection is done if
cueindex is negative. The data file always contains the index of the
actually cued item.
Indicates the varied mapping procedure. If this flag is on
then a new memory set is displayed on each trial. If the flag is off then
the fixed mapping procedure is used and the memory set is selected and
displayed only once at the start of a session.
- Use acoustic stimulus presentation. Stimulus items are names
of audio wave files.
- maskstring (string)
- Set of mask characters. The mask ist a superposition of
all characters in maskstring. Fonts that contain a special masking
character may set maskstring to that single character.
- memoryset (string)
- This variable is used to record the memory
set in a data file. The memory set items are selected at random from the
ensemble array of strings. The variable memoryset contains a list
of indices into the ensemble array indicating which of the items was a
member of the memory set. The memory set, however, may also be given
explicitly in the parameter file.
- memprobe (int)
- This variable will be set to 1 in the data file
protocol if the probe item is selected from memory set. It indicates
positive trials. The value zero indicates negative trials.
- memprobechance (float)
- Gives the probability for the probe item
being selected from the memory set instead of its complement set. This is
identical to the probability of a positive trial.
- memsetsize (int)
- The number of elements in the memory set.
- offtime , ontime (int)
- The memory set items are displayed sequentially. Each
item is shown for ontime milliseconds and there is a blank intervall of
offtime milliseconds between two item presentations. Also note that
there is an offtime ms empty interval before the first item is shown.
offtime has to be long enough for the hardware to put the item on the
screen in case of optic presentations. In case of acoustic presentations
ontime has to be long enough to present the item.
- probecolx , probecoly , probelum (float)
- Probe item character color and
- probeindex (int)
- The index of the probe stimulus in the array
ensemble of possible items. This index is relative to probeoffset
such that a zero probeindex points to the item at probeoffset.
- probeoffset (int)
- The index offset into the array ensemble
that specifies where the probe item subset starts. The subset size is
specified by ensize. The default value for probeoffset is zero
such that the probe item set is equal to the ensemble.
Probe item translation may be created by setting probeoffset different
from ensoffset such that different item sets are used for memory sets
and probe items. Note, however, that memory and probe item index values
(memoryset, probeindex) are relative to the start of the effective
ensemble in the array ensemble. Thus the true probe item index in
ensemble is probeoffset + probeindex.
- probepos (int)
- Serial position of probe item in the memory list.
- probeypos (int)
- Vertical screen position of probe item.
- rcuedelay (int)
- Response cue delay time. This is the time between appearance
of the probe item and the response cue.
- rcuetime (int)
- Response cue duration.
- rhtick (int)
- Rehearsal timer tick.
- trensemble (string)
- This is the set of possible probe
items in an experiment where the probe should not be selected from the array
- ensemble (float)
- . The variable trensemble must be an array of the same
size as ensemble and corresponding index positions must be item
translation pairs. A response for a probe item from trensemble is
defined to be correct if the item at the corresponding index in
ensemble was a member of the memory set. Translation is activated by
the flag T. Note that there is a second way to activate translation by
using the offset parameters ensoffset and probeoffset.
- wavedir (string)
- Directory where to find the audio files for acoustic item
- xpos , ypos (int)
- Horizontal and vertical position of the memory and probe item
presentations on the screen.
The first example is a simple
varied-sets demonstration procedure for the Sternberg
(1966) type of memory scanning experiments.
Parameter file stern.x from directory \pxl\app\stm
The Peterson & Peterson Paradigm
The next file is a replication of the
Brown (1958) and Peterson & Peterson\NPeterson,
L.R. (1959) paradigm.
Parameter file petpet.x from directory \pxl\app\stm
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