About set-shifting

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About the task

 

Presents two objects. One is correct and one is incorrect. If the subject touches the correct one, it is rewarded; if it touches the wrong one, it is punished. The discrimination varies in difficulty. The subject learns over several trials which stimuli are correct and which are incorrect; then we confuse it by changing the rules.

 

Stimuli can be simple (when they vary along only one dimension, such as shape, colour, intensity, or location), or compound (when they vary along more than one dimension).

 

Dimensional shifting in humans

 

In human testing, typical dimensions might be colour, shape, and number. A famous example of this kind of test is the Wisconsin Card Sorting Test (Grant & Berg, 1948, J Exp Psychol 38: 404), in which subjects must sort cards with a variable number of coloured shapes. They must discover the sorting rule solely by reinforcement from the experimenter. The rule is then changed: for example, they may be required to switch from "red correct, blue wrong" to "blue correct, red wrong" - a reversal. Alternatively, they may be given a new set of colours to learn - an intradimensional shift. Again, they may also be required to switch from "blue correct, red wrong" to "circles correct, squares wrong" - an extradimensional shift.

 

Dimensional shifting in non-human primates

 

In primate testing, two typical dimensions are "blue shapes" and "white squiggly lines". [See Dias R, Robbins TW, Roberts AC (1997). Dissociable forms of inhibitory control within prefrontal cortex with an analog of the Wisconsin Card Sort Test: restriction to novel situations and independence from "on-line" processing. Journal of Neuroscience 17: 9285-9297.] They use a large number of exemplars in each dimension (shapes and lines).

 

Basic test sequence in the general case

 

Assume there are two dimensions, A and B. There is a large catalogue of objects. Each object is assigned a value for each dimension (or is stated not to possess that dimension). For example, if the dimensions are colour and shape, then each object would be assigned a value for colour (e.g. "red") and shape (e.g. "triangle"). If there were a third dimension, of "superimposed wiggly line", a red triangle stimulus might have a value of "nonexistent" for this third dimension.

 

OK. For the sake of the following general prototype, keep in mind the primate tests in which the dimensions are typically "background shape" (values e.g. square, circle, triangle...) and "superimposed wiggly line" (similarly, with a variety of values).

 

•1. Simple discrimination. Displayed objects possess dimension A but not dimension B. Within dimension A, value 1 is designated correct and value 2 is designated incorrect. In other words, we reward subjects if A()=1 and punish them if A()=2. (We can run this discrimination for as long as we can find target objects such that A(object)=1 and distractor objects such that A(object)=2, as long as in both cases B(object)=undefined.)
•2. Simple reversal. We carry on except that we now designate A()=1 as incorrect and A()=2 as correct. The subject has to reverse its responses to a previously-learned discrimination.
•3. Compound discrimination. We now introduce a second dimension, B, but ignore its value. Now objects are selected for presentation on the basis that ((A(object)=1 or A(object)=2) and B(object) != undefined). The symbol "!=" means "does not equal". The subject is rewarded if it selects an object such that A(object)=2 and are punished if they select an object such that A(object)=1. This continues the "A" discrimination from the previous, reversal stage.
•4. Reversal of compound discrimination. We reverse the discrimination along dimension A and continue to ignore B. The trials are just the same as the previous stage, but A()=1 is correct and A()=2 is incorrect once more.
•5. Intradimensional shift. We introduce new values of dimension A; dimension B is still ignored. Objects are selected such that ((A(object)=3 or A(object)=4) and B(object) != undefined). Subjects are rewarded if A(object)=3 and punished if A(object)=4.
•6. Extradimensional shift. We switch our attention to dimension B and ignore A. Objects are selected such that ((B(object)=1 or B(object)=2) and A(object) != undefined). Subjects are rewarded if B(response)=1 and punished if B(response)=2.

 

What's ignored? By whom? A caveat. When a dimension B (or anything else) is present and being ignored, we would be wise to ensure that p(response is correct) is not dependent on the value of this dimension. Otherwise, our subject may well learn to discriminate on the dimension we (and it) are supposed to be ignoring. (Other dimensions like location or presentation order are important to consider here.) A dimension can be made irrelevant by holding the value of the dimension constant (e.g. always presenting stimuli in a single location, always making the background shape purple, etc.), or by randomizing it (e.g. always randomizing the locations of pairs of objects presented). If a dimension is randomized rather than held constant, the subject may attempt to learn the discrimination based on this dimension but cannot succeed.

 

Therefore, we need a library of objects with at least one object in each of the following categories:

 

       A=1; B=undefined

       A=2; B=undefined

       A=1; B=anything except "undefined", constant or randomized

       A=2; B=anything except "undefined", constant or randomized

       A=3; B=anything except "undefined", constant or randomized

       A=4; B=anything except "undefined", constant or randomized

       A=anything except "undefined", constant or randomized; B=1

       A=anything except "undefined", constant or randomized; B=2

 

Having lots of objects in each of these categories may be a good thing behaviourally, as it may lead the subject to extract the general features of that dimension/value (for example, if A(1) is equivalent to colour(red), then having lots of red objects may lead the subject to learn that "red" is correct, and not "red triangle"). This is the approach taken by the primate tasks, but not by tasks used for rats (or, now, pigs).

 

Dimensional shifting in rats

 

Birrell & Brown (2000) implemented a set-shifting task in rats (Birrell JM & Brown VJ, 2000. Medial frontal cortex mediates perceptual attentional set shifting in the rat. Journal of Neuroscience 20: 4320-4). Rats dug in two bowls for food. The bowls has dimensions of (A) odour; (B) filling medium; (C) surface texture. They adopted a policy of changing all stimuli at times of ID or ED shifting (a "total change design", p4321, which is required for accurate interpretation of the difference between reversal learning and ED shifts; see p4323). Their test sequence was as follows (+ indicates correct stimuli, - incorrect, bold indicates the correct part of the stimulus):

 

Simple discrimination

A1(+), A2(-)

Compound discrimination

A1/B1(+), A2/B2(-)

A1/B2(+), A2/B1(-)

Reversal

A2/B1(+), A1/B2(-)

A2/B2(+), A1/B1(-)

ID shift

A3/B3(+), A4/B4(-)

A3/B4(+), A4/B3(-)

Reversal

A4/B3(+), A3/B4(-)

A4/B4(+), A3/B3(-)

ED shift

B5/A5(+), B6/A6(-)

B5/A6(+), B6/A5(-)

Reversal

B6/A5(+), B5/A6(-)

B6/A6(+), B5/A5(-)

 

This illustrates the general test sequence nicely.

 

The sequence used by RatBat

 

Much the same:

 

Simple discrimination

A1(+), A2(-)

Reversal

A2(+), A1(-)

Compound discrimination

A1/B1(+), A2/B2(-)

A1/B2(+), A2/B1(-)

Reversal

A2/B1(+), A1/B2(-)

A2/B2(+), A1/B1(-)

ID shift

A3/B3(+), A4/B4(-)

A3/B4(+), A4/B3(-)

Reversal (optional)

A4/B3(+), A3/B4(-)

A4/B4(+), A3/B3(-)

ED shift

B5/A5(+), B6/A6(-)

B5/A6(+), B6/A5(-)

Reversal (optional)

B6/A5(+), B5/A6(-)

B6/A6(+), B5/A5(-)

 

How long shall we test for? The usual measure on this task is trials (or errors) to criterion. Birrell & Bowman (2000) used a criterion of 6 consecutive correct correct responses. That seems reasonable (though make the number configurable).

 

Left/right position should be chosen randomly for each trial. Order of presentation of the two alternative pairs (e.g. which to present of A1/B1-A2/B2 or A1/B2-A2/B1) should be randomized in pairs of trials.