An Operant Intra-/Extra-dimensional Set-shift Task for Mice

The overall goal of this experiment is to create an automated task that is used to assess the executive functions in mice and also mimics the original human neuropsychological tests. This method can help answer key questions in the cognitive neuroscience field. For example, how selective, cognitive, or pharmacological treatment can impact higher order cognitive functions relevant to psychiatric disorders.

The main advantages of this technique are that it is completely automated. It avoids any source of subjectivity. It is very flexible, and we obtain a number of results that are very selective.

The implication of this technique extend toward the diagnosis and the treatment of executive dysfunctions in psychiatric disorders because it allows large drug and genetic screenings that are relevant to cognitive symptoms in psychiatric disorders. Visual demonstration of this method is critical as the correct setup of this task is crucial for a successful outcome of the experiment. Prepare the mice for the experiment by acclimating them to single housing and slightly food depriving them prior to testing as described in the accompanying text protocol.

Also prior to testing, ensure that each chamber of the apparatus has two nose poke holes equipped with infrared photo beams and between them, a food magazine with photo beams where a pellet dispenser delivers the food reinforcement. Next, prepare changeable stimuli that vary in three different perceptual dimensions. A list of suggested stimuli can be found in Table One of the accompanying text protocol.

Choose five pairs of exemplars for each dimension. To deliver olfactory stimuli into the nose poke holes, use a dilution olfactometer. Set up one olfactometer to control two nose poke holes, then load it by filling the scent bottles with odor scents diluted one to twenty in mineral oil.

Connect the air pump to the inlet of the olfactometer and the vacuum to the outlets mounted on the nose poke device. Then, connect the outlet for odor delivery to each nose poke inlet and adjust the flow rate to 1.5 liters per minute. For visual stimuli, place light emitting diodes on top of each nose poke hole and secure them in the metal panel of the chamber.

Connect each diode to the output interface. For tactile stimuli, place changeable floor textures on a sliding support under a three by three centimeter opening in front of each nose poke hole. Mount the different textures so that they can be moved using frames underneath the floor.

Prior to testing, be sure to follow the training sessions as described in the accompanying text protocols to habituate the mice to the testing chamber and procedure. At each stage during the actual test, measure the time it takes the mouse to make a choice in each presentation, as well as how long it takes them to make eight correct choices out of ten consecutive trials to complete each stage. Stop the daily session after 40 minutes, or if a mouse fails to make any response for five consecutive minutes.

Terminate the session and continue the next day, with the mouse on the same stage where it left off. For the simple discrimination test, introduce the mice to an odor, light, or texture that is relevant in all the tasks until the extra-dimensional shift. In this example, two odors, vanilla and lavender, are presented, and vanilla is selected as the correct response.

For the compound discrimination, introduce a second dimension, such as light, which does not indicate where the correct response is located. Make the light status irrelevant to the outcome and maintain vanilla as the correct response. Test this discrimination so that vanilla is paired with both the blue light and the yellow light in subsequent tests.

For the reversal of the compound discrimination, have the animals learn that the previously correct stimulus is now incorrect. For this example, select lavender as the correct response, with vanilla now being the incorrect choice. Again, pair the correct scent with both the blue light and yellow light in subsequent tests.

After that, select an intra-dimensional shift, in which a total change in design occurs. Use strawberry and cinnamon as the odors, and change to orange and white lights. Ensure that the testing subjects follow the same relevant dimension in order to find the correct response, which is strawberry.

Pair the scent with both the orange and white lights in subsequent tests. Then have the mice perform the reversal of the intra-dimensional shift, and switch the correct scent to cinnamon. Again, pair the scent with both the orange and white lights.

For an intra-dimensional shift, use the same conditions as before, but change the exemplar pairs with the new odors and lights. Now, perform the extra-dimensional shift by introducing textures as a new relevant dimension. Add coarse sand paper to the opening under one of the nose poke holes and fine sand paper under the second opening.

At the same time, present them with two new odors, such as lemon and apricot. Set the hole with the corresponding coarse sand paper as the correct response. Pair the coarse sandpaper with both the lemon and apricot scents in subsequent tests.

Finally, reverse the correct response so the fine sandpaper indicates the position of the hole with the correct response. Pair the fine sandpaper with both the lemon and apricot scents in subsequent tests. For the two dimensional protocol, repeat the stuck-in-set procedure up to the extra-dimensional shift.

Instead of switching to texture and odor at this point, switch the relevant dimension so that light is relevant and odor is irrelevant. Set up the new extra-dimensional shift so that the lights are now red and green, with the green light now acting as the correct response. The odor is now the irrelevant dimension.

Test the green light paired with both odors in subsequent stages. For the reversal, use the same odors but switch so that the red light is now the correct response. Pair the red light with both odors in subsequent stages.

Once a mouse finishes with the session, wipe down the setup with 70%ethanol to prepare it for the next mouse. Always perform the tests in the same order but change up the stimuli in various tests so that each of the chosen stimuli are equally represented in the study. Also, make sure to equally counterbalance the perceptual dimensions used within and between each experimental group.

Visual, tactile, and odor discriminations in this novel apparatus required a similar amount of time and a similar number of trials in order to reach the criterion, suggesting that animals are able to perform simple discriminations regardless of the dimension presented. When using the stuck-in-set protocol, analysis of the mice's performance reveal that discrimination effect in that mice needed more trials and more time to solve the extra-dimensional shift stage compared to the compound discrimination, the intra-dimensional shift, the intra-dimensional shift 2, and the extra-dimensional shift reversal stages. This was also true in the two-dimension protocol.

Once mastered, this experiment can be done in 10 to 12 days including all the preparation and the training session.