VISUAL ACUITY TASK
The rationale of the visual water task is to use an animal’s ability to associate a sine-wave grating with escape from water, as an index of its acuity.
The basic apparatus consists of a trapezoidal-shaped pool with two computer-controlled monitors placed side-by-side at one end. The pool is filled with water to a depth of 15 cm. Following release, animals choose to swim on the side of the pool displaying the grating in order to find the hidden platform and escape from the water. Visual stimuli are displayed on two computer monitors facing the pool. In the training phase, animals are conditioned to distinguish between very low spatial frequency sine-wave grating (0.05 cpd) and homogeneous gray. For the testing phase we use a method of limits procedure to minimize the number of incorrect responses by the animals: small incremental changes in the spatial frequency of the stimulus are made between successive blocks of trials until the ability of animals to distinguish a grating from gray falls to chance. A preliminary threshold is attained for mice when they fail to achieve 70% accuracy at a spatial frequency. The highest spatial frequency achieved consistently is recorded as the acuity threshold. A final threshold estimate is generated in 2 or 3 days.
VIRTUAL OPTOMOTOR SYSTEM
Excitingly, non-invasive, low-level training has recently been reported to yield significant perceptual benefits that transfer to higher level tasks in human patients (Polat et al 2004; Mandavilli 2006). Practicing an optomotor tracking task (below) has similarly been found to enhance visual acuity in adult mice (Douglas et al, 2005; Prusky et al, 2006).
A virtual cylinder comprising a vertical sine wave grating is projected in two-dimensional coordinate space on computer monitors arranged in a quadrangle around a testing arena located inside a sound proof box. A video camera is positioned directly above the animal.
Visual cortical plasticity is observed rapidly in distinct portions of the visual field (Prusky et al., J. Neurosci. 2006). When a rotating (12 deg/sec) grating perceptible to the mouse is projected on the cylinder wall, the mouse tracks the grating with reflexive head movements in concert with the rotation. A computer system automatically scores if animals track the cylinder. When measuring grating acuity, we first project a homogeneous gray stimulus on the cylinder, followed by a low-spatial-frequency (0.05 cyc/deg) sine wave grating (100% contrast) of the same mean luminance and moving in one direction. The animal is assessed for tracking behavior for a few seconds, and then the gray stimulus is restored. The short testing epochs reduces the possibility of the mouse’s adapting to the stimulus and establishes that each animal is capable of tracking when a salient stimulus is present. Using either a staircase or method-of-limits procedure, we then systematically increases the spatial frequency of the grating until the animal no longer responds. A threshold for each direction of rotation is assessed this way, and the highest spatial frequency tracked in either direction is recorded as the threshold. A contrast-sensitivity function is assessed by using the general procedures just described. The differences include that testing at a spatial frequency begin with a grating of 100% contrast, which is then systematically reduced until the contrast threshold is identified. In addition, a contrast threshold is identified at six spatial frequencies between 0.02 and 1.2 cyc/deg.
The mice are generally tested during the first few hours of their daylight cycle (12-hour light-dark), normally for 5 to 30 minutes at a time.