Understanding Forced Choice and Multiple Choice Tasks in Cognitive Psychology

Hello and welcome to the course basics of experimental design for cognitive psychology. I am Dr. Arkwarma from the

department of cognitive science at ID Kpur. This is week four of the course where we are talking about experimental

designs and we're basically discussing elements of an experiment. Basically what are the things that go when you are

setting up an experiment and we basically discussing that if you remember in the previous lecture we

talked about the general considerations that play a very important part when an experimentter has to decide which task

to choose for answering the research question for testing the hypothesis and so on. We also talk talked about uh the

task typically used and we discussed in detail uh the free description and the rating scale uh kinds of tasks. All

right. So in the current lecture what we'll do is we'll extend the same discussion and we'll talk about two

different kinds of task which are the force choice task and the special multiplechoice task in in some detail.

Okay, I'm not going into extreme details and all the nitty-g gritties of these tasks but I'm giving you an overview of

what are the things necessary or what kind of tasks are used for force choice or special force choice tasks. Now the

force choice tasks are basically uh ones that measure you know limited number of measure basically a participant's

response in terms of a limited number of uh potential answers from the participants about a given stimulus.

Okay. So uh remember we are talking about rating scale where you had to within a uh particular bounded range

from 1 to five basically tell uh choose either one or five for the stimula. So which of the simil is more symmetric or

beautiful and so on. In first choice task there is a limited number of options. So it constrains the kinds of

responses that the respondent or the participant can actually give. All right. Again these are still about the

re relevant dimensions of the stimulus which the participant has to respond but within a given range. So that is

basically why it is called a forced choice task. The choice is forced. It is constraint. All right. So it is broadly

at its core a discrimination task because it shows how well the participants can perceive a specific

difference between the stimula at hand. So it could be two stimula, five stimula, 10 stimula. But the idea is

that we expect the participants to distinguish between stimula, discriminate between stimula along a

particular specific dimension of interest. Say for example, which of these three pictures uh you know is is

most symmetric or which of the two uh letter strings are a word or a non-word something like that. Now if constructed

carefully if you pay a lot of attention if you construct these carefully this task can also be used to measure

recognition times and accuracy recognition performance or sometimes even full psychometric functions. All

right. So going forward in a force choice task participants are required to choose from a limited number of

explicitly listed alternatives. The alternatives typically describe some aspect of the simul such as color. So

which of the two is brighter in color something like that. Which of the two uh shapes presented is a rectangle or a

triangle. Uh location pending style anything that you want to ask but you have to basically give a limited number

of options about a relevant dimension of the stimulus about which the participant decides. All right. Now the alternatives

are almost always mutually exclusive. So this is something uh which is important in the in this force choice task that if

one alternative is true typically the others cannot be. So for example if you are given a letter string and you have

been asked to decide in a lexical decision task which of the two is a word. Now the thing is if you say that

this particular stimulus is a word then this stimulus cannot be a non-word. All right. Or if say for example which of

the two stimulus is symmetric or not symmetric or even one stimulus let's not talk about comparison uh given stimulus

tell whether this stimulus is symmetric or not symmetric. Now you have a stimulus you have two options symmetric

not symmetric. If you choose symmetric then obviously it is not not symmetric. If you choose uh not symmetric then

obviously you cannot come back and say no it was symmetric. Okay. So the options are listed and constrainted and

these options are mutually exclusive. They are mutually uh you know uh not possible at the same time. Also these

options are mutually exhaustive. What does that mean? It basically covers all other possible options. All right. So

once you cover this, it basically covers all the other dimensions. The other one is not even going to be considered. Uh

and these are specifically tailored to the experiment and therefore they require prior knowledge about the

expected results. All right. So we we'll see some examples as we go ahead. We'll discuss some of the force choice tasks

that are available and that are used widely by experimental researchers in psychology. Now uh force choice tasks

can be designed to either give a qualitative or a quantitative measurement of about how distinct or

discriminable several stimula are from each other. Okay. So you can have say for example two or five or 10 different

stimula uh and you can basically ask participants to discriminate between them. You can basically sometimes ask

for a qualitative judgment. Say for example, which of the uh stimuli presented here are more aesthetically

pleasing and so on. Or you can just ask on a single dimension just like I was saying symmetric, asymmetric, word,

non-word, tool, non-tool, object, anything like that. Okay. The results range from a simple threshold that say

for example, which of the two came first? So if you're doing let's say a temporal order judgment task, you can

ask which of the two stimula flashed first on the screen. So you can basically uh get a simple threshold how

much of a change is needed for you to be able to distinguish the two stimuli from each other or sometimes just detection

and both of these can be plotted on a full psychometric function and this is basically why uh these tasks are very

popularly used the four choice tasks are very popularly used in psychopysics and experimental psychology experiments.

Some of the typical questions that you can ask uh you know uh using these uh four choice tasks are I'm just going to

tell them uh but you'll see that broadly these questions are uh you know on the qualitative side but you can also have

quantitative questions asked as well. Now for example which style of paintings do people prefer this one or that one

something like that or how easy are the following expressions to recognize you know uh you know hard or easy

something like so you'll give a list of choices 1 2 3 4 5 1 and two or say for example 1 2 3 4 5 6 7 8 9 10 but you're

basically asking the participant to make a decision along a particular dimension of your stimulus.

Now let's look at some of the variants of the force choice task. All right. Now one of the very popular ones is the n

alternative force choice task. Here uh as the name suggests n alternatives are given not two not three not four any

number can be given. Okay not necessarily two or three or four but you can actually have a large number of

choices as well. Now n alternatives are given and the participant must choose one of them for each stimulus. Okay. The

participant is free to choose the same alternative for different stimula as well. Say for example in a lexical

decision task there are only two choices word and non-word and there are let's say 500 stimula. So obviously uh you

know some of them uh will be for a lot of them uh word option will be chosen for a lot of them non-word option will

be chosen. So across different stimuli you can choose the same option for different stimula as well. Although the

stimula is selected such that each alternative will be chosen equally often, it is not really a requirement.

Typically when you're doing force choice task, you try and make sure that both options occur uh relatively equally. So

for example, if there is uh two alternative force choice task, word non-word, symmetric, non-ymmetric and so

on. You'll try and uh arrange your stimulus presentation as such that there will be uh let us say uh you know 50% of

the time it will be uh you know word 50% of the time it will be non-word but it is not really a requirement. You can

actually have filler words as well and other things uh you know other options emerging as well. All right. Now the

alternatives might be sometimes absolute descriptions. So look at the face and tell whether it is happy or a sad or it

can be relative to some standard say for example faster than this, slower than this, earlier than this, later than this

something like that. Okay. Now in a special variant of this task multiple stimula can be shown simultaneously and

the participant would be required to pick one of them. So there are also you know tasks where people are asked to

make a choice. Say for example among the five faces present here which of the faces is the happiest. So participant

sort of moves their curs moves their uh you know cursor on the screen and selects one and says this one seems to

be the happiest. Now there are other smaller variations of this. The main one was this in

alternative force choice task but there are other variations of this as well. For example n plus1 alternative force

choice task is basically the same as the previous one but there's one more alternative here where you can basically

uh you refuse to make a choice. So which of the two is faster? A B neither something like that. All right. Now

there is N + N interval force choice task. Now interval force choice task are a special variant of the force choice

task where n stimula shown sequentially. So 2 3 4 n is that number and the participants are required to choose one

interval based uh you know criterion. For example, which of the following two stimula is the happiest or which of the

following three stimula is the fastest. So you basically are in some sense uh saying that okay this one is the

reference then is this the fastest is that and this is the choice is based not on absolute values but on intervals. So

that is the idea. Again the n+1 interval non-force choice task is just a variation of this one where you also

have where you basically have an option to refuse making a choice. Say one alternative might be none of the above.

You know in MCQs we use this a lot. Uh choose a b c d or uh e will be none of the above. Something like that. Now

let's sort of attend to uh you know the details of this task as well. For example, you uh are an experimentter and

you've posed this question to a participant. Do you like this painting or not? Now, this question would require

the participant to make a specific forced choice. For instance, the participant wants to say I like this

painting. Now, the second response becomes mutually exclusive already uh with the first response. You cannot say

I do not like this painting and I like this painting for the same painting. All right? So this basically becomes an

example of the simple two alternative force choice task or the two AF task versions of which are widely used in a

lot of experimental and cognitive psychology experiments. Okay. Now there are a couple of points that I would like

uh us to sort of pay attention here. So I'll just talk about them. Uh for example, there are two alternatives here

and the participant must choose one and only one of them for the stimulus. So that is an absolute necessity and this

is basically why uh you know uh it's very aminable uh for example when you want to measure reaction time or

accuracy. So you want to basically give 500 words and ask people to make judgments about which of the two is a

word or a nonword or which of these uh or whether the presented stimula is a word or a non-word or the presented

stimula is a phase or a non-face is a happy expression sad expression. You can do a bunch of the these things and the

dependent variable a very likely dependent variable could be reaction time or accuracy but also qualitative

measurements can also be taken. You can basically ask why do you like this painting and you can have follow-ups and

so on. Now as there is a single painting in consideration for the most part in this 2FC task the results provide

information solely about this painting. Now the problem here is if the experimentter asks several people the

same question about this uh painting then you'll start to get some idea about this specific painting because you're

presenting one stimulus and you're asking about this. So they would be uh you know it'll help you average out what

typically people feel about this painting. Do lot of people like this painting is the average likability of

this painting let's say on a scale of 1 to five above five uh you know uh above three or below three and something like

that. All right. Now if the experimenters were to add more paintings and ask the participants to decide for

each painting whether or not they like it the task itself will not really change but the type of information that

you can obtain can uh can diverse. So you can basically have more participants single stimula you can have more stimula

as well as more participants as well. And uh it depends on what is it that you are seeking from the task. You can

sometimes ask to make comparative decisions also. So you can just basically try and summit or say for

example collate the decision of different participants about this single painting as well. Okay. So these two

kinds of information can be gotten from this task obtained from this task. Third is uh I'm sure this is uh already

clear from the first point is that the second alternative is the pure negation of the first. So the two alternatives

are mutually exclusive. It is not possible for both to be true at the same time. If you don't like this painting,

you don't like this painting. If you cannot say, "Oh, I like this painting as well." If this is if a given letter

string is a word, then it is a word. It is not a non-word. All right? So, these two things are mutually exclusive. And

this is typically how these tasks are structured. Also, the task provides two

alternatives. Since the task provides two alternatives for a single stimulus, an experiment could just as easily use

uh the stimula as the alternatives and provide a single category. For example, you could show two paintings and ask

them which do you like more, the first one or the second one, the earlier one or the later one. And in that sense, it

the first one can act as a baseline for the other one or the second one can act as a baseline for the previous one. And

you can make these comparisons. So given that you are comparing these and you have given two options, you can play

with these kinds of uh you know uh things as well. Another interesting thing is that it is you know uh broadly

a discrimination task. Okay. So if the research question deals with whether people can discriminate between two

stimula, the clearest way would be to show the stimula either sequentially you show one and then you show the other or

you show both of them together and ask them to choose one of them based on the relative dimension. So uh which of the

two is more symmetrical or say for example you present one first and then the second one and say whether this is

more symmetrical than the previous one or not. If the results are tied then it can be concluded that the participants

are not able to discriminate between the two. Hence you have to probably play with the variable a little bit more.

Maybe make the difference larger so that the participant can you know uh detect whether one is more symmetrical than the

other. Now there are some general guidelines which we can keep in mind when we are

designing these task and I'm just mentioning because the two alternative force choice task is something that a

lot of experimenters use. So first is keeping non-mutually exhaustive uh alternatives. For example, the

alternatives do not necessarily have to be mutually exhaustive. There is however a very real chance that the results, you

know, if you don't create non-mutually alter exhaustive alternatives, then your results might be less clear. For

example, uh you know, if the alternatives are not mutually exhaustive, let's say you're presenting

a facial expression and asking if it is a happy or sad, you know, in the two alternative post-choice task, as long as

the expressions are more towards the happy side or more towards the sad side, the discrimination can be made and you

can make this decision. However, and till that point, the participant will be able to perform the task. But say for

example, you're presenting an expression that is pleasantly surprised. So your stimulus have to be in in such a way if

you're presenting presently surprised then the interpretation of results will become less clearer because either the

participant will uh you know chalk it towards more happy or sometimes by mistake chalk it towards more sad and

that will create uh a bit of ambiguity about what the participants are actually doing. So uh when you're presenting

these alternatives when you're presenting the stimulus it is extremely important that the stimula are sort of

in in some sense uh very different on the on the you know relevant dimension also asymmetry in the kind of responses

that are presented. So participants usually assume that alternatives will occur equally often. So as I said in a

lexical decision task typically what you would assume what you would assume is that 50% of the time non-words will come

and 50% of the time uh words will come and the the order shall be randomized. Okay. Now if one of the alternatives

does not appear uh in the stimulus set at all participants will still choose it on some trials simply because they think

that oh it must be there as well. Some words might be nonwords as well. Suppose you're presenting 500 words. All 500 of

them are words. Then sometimes what might happen if the participant is not very sure. Let's say they are not very

proficient in the language of testing. They might start indicating some of the actual words as non-words as well

because they think okay it's a word non-word decision task and the question that is being asked is is this a word.

Uh must be that some of these stimula that are being presented to us are non-words as well and maybe I'm not able

to def detect it. So I will make these choice on some of the trials at least. All right.

So as the frequency with which the different alternatives occur in the uh you know stimulus uh set and as as this

frequency changes some participants will try and guess uh the relative frequency of this in the stimulus set. One of the

reasons that uh you know is done is that you use typically randomized uh you know uh distribution of the stimula. So uh

trial across trials uh you randomize the presentation and randomize the frequency in in some frequency is broadly said the

best one is 50/50 but in some task you will see that people used uh 70% and 30% and uh 30%. So people sort of also have

manipulated the probability of uh you know presenting the two different kinds of stimul and sometimes that becomes a

critical dimension. how are people uh with you know being able to uh recognize that given dimension or not. So if that

is uh relevant to your research question obviously do that otherwise it is always a good idea to take care of this

asymmetry a little bit order of presentation. Uh so another important issue that is closely related

to this asymmetry between choices is also the order of the correct answers. Now it has been shown that participants

have a very clear expectation about random distributions and are willing to accept short sequences of identical

stimula but not long. As I was telling if there if you're presented a lexical decision task yeah there are 500 words

to be presented. If there are 500 words to be presented you will your participant will accept uh will

basically expect that uh around half of them will be words half of them will be non-words and something like that. Now

the thing is uh if the participant is making the same choice if the similar are coming like that that you present a

word then another a word then another word then another word and so on. If it goes on for three trials, maybe five

trials, maybe even 10 trials, the participant might still say, "Okay, this is fine." But say for example, if it is

a long sequence, you present all the words, let's say 40 of them in the same sequence and then the normal start

appearing. then the participant will sort of start having concerns about uh you know what is happening here and they

might basically become uh you know start responding erratically uh on the task and that sort of disturbs uh you know

the overall uh the kind of responses you get and the analysis of the task as well. So the alternatives in the 2 AFC

task must be presented in a manner that uh you know uh both options occur uh symmetrically and broadly in a random

order. All right. Now another interesting thing when you create these two alternatives for this force choice

task uh is basically this uh issue related to category level. Okay. So the experiments experimenters could present

say for example several pictures of animals and ask participants to indicate if the animal let's say is a dog or a

Siamese cat. Now the thing is uh dog is a general level category you know say for example you know animal mammal dog

and so on. Siamese cat is a very specific subcategory within the cat. Now if you create you know options like this

where the options are one is at a particular level of hierarchical uh you know organization and the other is at a

particular other level then this kind of comparison becomes tricky and it basically creates problems for the

participants to respond. So the idea is whenever you are creating two options it is always a good idea to keep the two

options at the same uh semantic level or the same hierarchical level of organization so that the participants

can actually make a you know better decision. It does not become a problem for them because typically dog and simat

they will not be able to uh compare also chance level. So for example uh any forceh choice task typically we are

saying two alternative force choice task. Now in a two alternative force choice task what is the performance

going to be? There is something called as a chance level. Say for example 50%. Given that there are 500 word stimuli

50% of them are words 50% of them are nonwords. If you're checking for them there is something like a child chance

level which is basically the percent at which uh you know the participant is going to choose uh any of the given

alternative. So chance basically reflects the percent of which uh percent of the time that a given alternative

would be chosen if the participant had no knowledge of the alternatives or the stimula would uh you know or the stimula

and basically we are choosing the alternatives blindly see it's a two alternative task I two alternative task

either the word is there either a nonword is there so even if I'm blindly sort of uh saying uh some of the trials

I'll say word some of the trials I'll say nonward and I'll sort of get through the task nicely.

Basically, what is happening here is that if my performance is at chance level, it basically reflects that I do

not know and I'm not evaluating the critical dimension of the stimuli. That is the meaning part of the lexical

status part of it. So chance level performances are typically frowned upon and you don't try you know and include

them in your data analysis because in two FC tasks this is around 50%. So that is extremely low accuracy and it

basically uh is not enough uh information to tell you that the participants were actually making

decision based on the critical dimension that you have manipulated in your simul. So that is why in most 2FC uh tasks you

will see that chance level uh you know participants that are performed at chance level are actually excluded from

the data analysis. Uh there is a slight sort of a theoretical point here that whether it

is a identification task or a discrimination task. Now uh a lot of times uh people incorrectly assume that

these uh 2FC tasks are identification tasks. uh for example if the uh experimenters are showing a series of

expression to the participants and they are asking them to choose a name for each one of the let's say 10 possible

alternatives and if the participants are able to choose uh the correct uh name you would say that they identified the

expression that is all right but it is also possible that the choice was made not on the basis of identification but

by discrimination between other different alternatives. All right. So that is one of the reason why broadly it

has to be considered as a discrimination task because what you're doing is let's say coming back to this lexical decision

example word non-word. Now some stimula uh it is possible that the participants are regarding some stimula as words

because they are sufficiently different just on the statistical properties from the things that he or she has earlier

regarded as non-words. Okay. It is not necessary that the participant knows the meaning of this word and is basically

making the judgment based on that aspect but is just making this discrimination between non-word stimula and word

stimula and on the basis of that making these further decisions. So one of the things uh that has to be considered is

that whether this 2FC task is a identification or a discrimination task and typically it seems that it has to be

treated you know as a discrimination task. Last two points. So clarity of alternatives. Obviously experimenters

are encouraged to present the alternatives in clear and amigos terms uh so that the participants can actually

understand what are the two options. So for example when uh you are doing let's say a lexical decision task and you are

basically asking your participants to make these decisions. You would want to be sure that the participants understand

what a word is and what a non-word is. What are you know say for example if you're telling something uh this is

symmetrical choose between uh you know symmetrical and unsymmetrical response you have to tell what do you mean by

symmetry you'll have to say for example uh you know vertical symmetry vertical mirror symmetry something you'll have to

describe and show the participants that is why it's always a good idea to uh have some practice trials to have some

uh you know uh pilots where you are sort of sure that the participants are actually understanding what is going on.

Okay. Also uncertaintity because a force choice task does not actually measure any physical property. It basically uh

you know uh measures a participant's uh judgment or belief and thus the participant's response typically

includes a bit of a measure of uh uncertaintity as well. Uh which by the way if you see nowadays people are sort

of very conscious of this and some of the data analysis maneuvers are actually accounting for this uncertainty. But

we'll we'll talk about this at some later point. Okay. Now two alternative force choice

task. This is what we have been discussing. There are two uh typically this task has been used in psychopysics

and experimental psychology. It is very useful in determining say for example uh distinguishing between simulation.

Sometimes on the basis of what is called an absolute threshold. Sometimes on the basis of what is called a difference

threshold. For example, what is the absolute threshold? I'll talk about this in when we talking about psychophysical

experiments. Absolute threshold is the minimal amount of uh dimension that is needed for people to be able to detect

that dimension. Okay. So uh imagine you are sort of uh you know uh uh turning the knob of a music system. Uh the point

at which the music becomes barely audible detectable to the participant is that absolute threshold. And there are

psychometric uh functions that you can plot with this. And you can basically uh do this uh across the dimension several

times with the same participant and then also across participants to determine typically this is done with the same

participant to determine the absolute threshold for a given stimulus on a given dimension for a particular

participant. Okay. The other kind of uh decision that these task are used for is for determining the uh difference

threshold. What is the difference threshold? It is basically also referred to as the just noticeable difference. So

here basically what you have is you have two uh uh stimula and you basically want to vary the stimula along a dimension so

that they can just notice a change. Sometimes you know you can do uh this with a single single stimulus also

you're changing the the contrast. So and you ask the question whether the uh stimulus is the same as before or not.

So the point at which uh along a dimension the stimula begins to change and that change is detectable. Okay. So

J&Ds are measured by typically asking participants to discriminate between two stimula along the same dimension. Which

of the two is heavier? Which of the two is brighter, more symmetric? And you can sort of have a number of questions with

that. Now uh there are obviously two you know primary requirements of uh the two AFC tasks that are selected. Firstly

that the two alternatives cannot be similarly initially true. We've talked about this and secondly they need to

cover the manipulated space as well. So uh let's say what is the option space in a lexical decision task word non-word.

So obviously jointly they should exhaust there should not be a third option available for the stimuli. That's the

idea. And this is a very useful task. It is being uh used to measure a wide variety of things. uh depending on uh

the categories chosen the questions asked the stimula used the manner in which the stimula are presented and so

on. So also different kinds of participant populations as well. So this is something of a very important uh you

know uh version of the four choice task that the experimenters and psychopysics uh people have used in in great detail.

Now I briefly just touch upon the specialized multiplechoice task as well. Now in essence the multiple choice tasks

are still basically discrimination tasks which basically measure how well a difference can be perceived among se

several simile again on the same on a a specific dimension and these t uh these type of tasks are just derives

derivatives of the force choice task and were designed to address slightly uh you know special questions but they have

been uh become standardized techniques now and people use them a lot. For example, what kind of questions do we

ask? Uh, how long do specific higher cognitive processes take? How well can people search for a specific target

given noise? How well and quickly can people learn and respond with a new rule? How does the perceptual system uh

select what gets further processing or not? Things like that. So these kinds of questions can broadly be uh addressed uh

you know and this is obviously not exhaustive uh list but this can be broadly addressed within the special

multiple choice tasks. uh I'll just discuss a few variants for example the go no go task it is very uh

popular task in terms of measuring executive functions inhibitions and so on so this if the stimula presented uh

meet certain criteria so uh you can ask the participant to that if the stimula presented meet certain criteria you

press the button if it does not you do not press the button say for example if the stimulus presented uh is uh you know

uh I don't know is a symmetrical shape press the key if it is not symmetrical don't press the key you can have a go-go

kind of task like that. Uh there are also lot of versions of this go task uh presented in uh you know language

switching research, task switching research and so on. There's also a match to sample task. It's a slight variation

on this. Here basically what you do is you present a standard stimulus and uh the standard stimulus is then presented

with another simile which is called the comparison stimulus and you compare the standard and the comparison stimulus on

a particular dimension and the participant is typically asked to choose the comparison simulus that most closely

matches the sample. Okay. So I remember we did this figure matching task in one of our experiments. Uh basically what

you do is you start with the reaction time experiment. You basically have uh blank screen fixation cross uh figure

presented at the center. Then two figures uh there's this uh arrow on the middle and the there's a figure on the

left and the right side. The task of the participant is to check which of the figures matches the previous one. All

right, it's just a simple figure matching task. It is asking the people to sort of uh you know uh broadly match

the two shapes and what we are measuring is reaction time and accuracy. But this is also an example of what you call a

match to sample task. Another uh very popular task in cognitive psychology research especially

with attention researchers is the visual search task. Uh here typically a specific target let's say a letter T can

be presented simultaneously with a series of other items which are called distractors because you know they're

making the job of finding this T more difficult. uh these distractors uh are generally similar to the target along uh

some dimension of interest. Say for example uh you can have uh participants searching for a T between a visual space

that has uh lots of L's and I's and because L I and T are very similar to each other it sort of makes finding the

T slightly more difficult. Now again uh this variation between the target and distractor is systematic so that you

know exactly what is making uh searching for this difficult or easy. Okay. Uh so experimenters also by the way they can

vary the number of distractors across trials systematically and the participant is required to uh indicate

whether the target was present among the distractors or not. uh typically what you are measuring here is uh the uh you

know how much the addition of the target increases the amount of time it takes to find the target. So reaction times

accuracy etc are very typical dependent measures for this kind of task. Also there is this rapid serial v visual

presentation task which is very common for a lot of experiments for example the attention blink experiment and uh some

others. uh it basically involves presenting a large number of stimula across uh you know very rapid sequence.

So 1 2 3 4 5 say for example a typical attential blink task uh around 15 16 stimula come very fast uh you know

rapidly followed by each other with a uh they are each presented for typically around 70 70 to 80 millconds. uh the

inter trial interval is around 15 20 milliseconds. one comes and the other comes and the other comes and so on and

you see a series of stimuli among which you have been told that you look for let's say a black colored O or a white

colored X and these become your targets in this rapid serial visual presentation which the participant has to decide and

typically what uh you know the experimenters are looking for is either reaction time or accuracy uh for

detecting a given target in this rapid serial visual presentation of stimula Okay. So this technique is uh is been

utilized in the general go no go task as well as the attention blink tasks. So I think this was a broad sort of an

overview of the kinds of tasks uh we are using. Obviously, this is neither an exhaustive list, a complete list or I've

not even touched upon all the factors, but I just wanted to give you an overview of the kind of experimental

tasks that are available and uh how they're used to uh address specific research questions and test uh different

kinds of hypothesis. I'll continue this discussion on the elements of uh an experiment in the next lecture as well.

Thank you.