The optomotor response and the Y-maze are behavioral tests that are useful in measuring visual function and cognitive function respectively in a wide variety of disease models. Advantages of these tests include sensitivity, speed of testing, the use of innate responses such that training isn't needed, and the ability to perform the tests on awake non-anesthetized animals. OMR and Y-maze can be used along with other tests to identify temporal appearance of retinal and brain dysfunction in diseases like diabetes with the goal being earlier diagnosis.
Demonstrating the Y-maze will be Stephen Phillips, a research technician from my laboratory. Demonstrating the optomotor response or OMR procedure will be Amber Douglass, a research technician in the Allen and Viola Labs. To set up the OMR apparatus, first select an appropriate-sized platform for the experimental animal species to be used and open the OMR software.
Zoom in or out with the video camera as needed so that the platform and its surroundings are visible and click the asterisk and rotating stripes icons so that both the green asterisk and green rotating stripes disappear from the live feed. Click the compass icon so that a green circle and two perpendicular lines appear and stretch the green circle so that it aligns perfectly with the black circle on the platform. When the OMR is completely aligned, re-click the compass, asterisk, and rotating stripes icons.
Note that the green stripes will rotate in the same direction as the stripes in the drum. Under the testing tab, click the psychophysics tab. To measure spatial frequency, under threshold, select frequency.
Under testing, click presets and select the default settings for the experimental animal being used. Click the blanking tab and check the blank on tracking box to pause the stripes and blank out the computer screens in the drum whenever the mouse is right-clicked. Then click the results tab to open the window in which the results of the test will be displayed.
To assess spatial frequency, place the rodent onto the circular platform in the center of a virtual reality chamber comprised of four computer monitors showing vertical sine wave gradings circling the chamber at a velocity of 12 degrees per second. The video camera positioned at the top of the chamber should project the rodent's behavior live on the computer monitor. Look for the presence or absence of reflexive actions by the rodent's head as the gradings move in a clockwise or counterclockwise direction.
Make sure that illustrated bars are visible in the program as they will show the direction of the grading movement. Watch for the rodent's head to move in the same direction as the gradings. When there is a smooth pursuit, not erratic bursts of head motion, count this movement as tracking.
Click on yes or no as appropriate. Spatial frequency will start at 0.042 cycle or degree and adjust with each yes and no click to become easier or more difficult. As the rodent is tested, make sure to keep the asterisk positioned over the rodent's head and watch for the system to say done when the rodent's spatial frequency threshold is reached.
Note that the yes and no buttons will no longer be able to be clicked. Then open the results tab to view the spatial frequency for the left, right, and combined eyes. To measure contrast sensitivity, in the testing and psychophysics tabs, select contrast single.
Open the stimulus and gradings tabs and enter the appropriate value into the spatial frequency box to start gradings with the spatial frequency constant at the peak of the contrast sensitivity curve. Begin the contrast at 100%and look for the same reflexive head movements as observed during spatial frequency testing. Note that the contrast will decrease as the testing progresses until the rodent no longer has reflexive head movements in response to the stimulus.
When the contrast sensitivity threshold has been reached, open the results tab to view the contrast sensitivity values for the left, right, and combined eyes. To conduct a Y-maze analysis, first label the initial arm of the Y-maze as B and the other two arms as A and C and place the rodent in the B arm near the center of the maze. Immediately start the timer and allow the rodent to explore the Y-maze for eight minutes.
Sit several feet away from the maze, keeping it in sight and avoiding making any noise while taking recordings and noting any observations. Record the starting location as B.Each time the rodent makes an entry into a new arm, record the new location of the rodent. If the rodent remains in the same spot for more than 60 seconds and does not appear to show exploratory behavior, move the rodent toward the center of the Y-maze and continue the trial.
After at the end of each trial, remove any feces and clean the maze with sanitizing solution. Calculate the exploratory behavior as the total number of entries made over the eight minutes of the trial. The spatial cognition is calculated as the number of successful alternations divided by the total number of entries.
Here, the use of the OMR to assess spatial frequency is illustrated in naive controlled Brown Norway and Long-Evans rats. Brown Norway rats typically show a higher baseline spatial frequency than Long-Evans rats. Additionally, an aging effect on spatial frequency is observed in the Long-Evans rats.
Using the OMR to assess contrast sensitivity reveals significant deficits in contrast sensitivity in type one diabetic rats. Exercise treatment reduces these deficits, demonstrating that the OMR is useful both for detecting and tracking retinal deficits over time with and without intervention. Using a Y-maze to assess cognitive function and exploratory behavior in a type two model of diabetes shows significant deficits in spatial cognition as measured by spontaneous alternation and exploratory behavior as measured by number of entries in Goto-Kakizaki rats compared with Wistar control animals beginning at seven weeks of age.
Learning how to identify a tracking head movement in step 3.4 is the most difficult part of OMR. Learning what a smooth pursuit movement looks like is critical. The OMR and Y-maze paved the way for IM brain researchers to quickly and reliably measure visual and cognitive function without training and with minimal stress to the animals.
