Using Looming Visual Stimuli to Evaluate Mouse Vision

Abstract

The visual system in the central nervous system processes diverse visual signals. Although the overall structure has been characterized from the retina through the lateral geniculate nucleus to the visual cortex, the system is complex. Cellular and molecular studies have been conducted to elucidate the mechanisms underpinning visual processing and, by extension, disease mechanisms. These studies may contribute to the development of artificial visual systems. To validate the results of these studies, behavioral vision testing is necessary. Here, we show that the looming stimulation experiment is a reliable mouse vision test that requires a relatively simple setup. The looming experiment was conducted in a large enclosure with a shelter in one corner and a computer monitor located on the ceiling. A CCD camera positioned next to the computer monitor served to observe mouse behavior. A mouse was placed in the enclosure for 10 minutes and allowed to acclimate to and explore the surroundings. Then, the monitor projected a program-derived looming stimulus 10 times. The mouse responded to the stimuli either by freezing or by fleeing to the hiding place. The mouse's behavior before and after the looming stimuli was recorded, and the video was analyzed using motion tracking software. The velocity of the mouse movement significantly changed after the looming stimuli. In contrast, no reaction was observed in blind mice. Our results demonstrate that the simple looming experiment is a reliable test of mouse vision.

Figure 1:. Experimental system.

(A) Schematic of the looming stimuli enclosure. A computer monitor (21”) covers the ceiling. There is an opaque dome in one corner of the enclosure in which a mouse may take refuge. A video monitor with a wide-angle lens captures the mouse behavior. (B) Overall view of our entire setup. The secondary monitor duplicates the image showing on the stimulus display. (C) Diagram of the looming stimulus. The looming stimulus begins at 2° (1.15cm) and holds at this size for 250 ms. It then expands to 50° (30.8cm) over the course of 250 ms and remains 50° for an additional 500 ms. This 1s sequence then repeats 9 more times prior to terminating. (D) Diagram of stimulus calculations. The height of the cage dictates the necessary start and end size (in centimeters) of the stimulus in order to produce a stimulus that expands from 2° to 50° when directly above the mouse.

Figure 2:. Analysis calculations.

Calculations to correct skew from wide-angle lens. Due to the placement of the camera, the floor of the arena appears as a trapezoid instead of a rectangle (left). Therefore, the X and Y coordinates of the mouse must be corrected to accurately analyze mouse position (mid). Using the geometry of congruent triangles, it is possible to find how much the x-coordinate must shift in order to correctly represent the mouse’s movement in the 3-dimensional space (right).

Figure 3:. Representative responses to looming stimuli.

(A) An example of a mouse’s movement tracked within the arena. A red circle shows the dome where the mouse fled to and stayed until the looming disappeared. 1 = mouse position starting point when video capturing started. 2 = movement prior to stimulus onset when the mouse explored the arena. 3 = looming stimulus started. The mouse dashed to the dome (shown by a red dashed line). 4 = the mouse stayed in the dome until and after termination of the stimulus. (B) Velocity changes as a function of time for this mouse. The dotted line indicates when the looming stimulation began. Stimulus duration is indicated by the yellow background. The full 10 second cycle is not shown here since the mice remained stationary in the refuge for the entire stimulus duration. (C) Distance from the dome over time for the same mouse in (A) and (B). (D) and (E) The velocity and distance to the dome for a mouse that exhibited the freeze reaction (freezing duration shown by the red double sided arrow) prior to flight. The velocity was reduced compared to the control (before looming). The distance to the dome did not change during this period.