Aerial single target acuity of harbor seals (Phoca vitulina) for stationary and moving targets of varying contrast.

Harbor seals (Phoca vitulina) need to detect single objects for example when orienting to landmarks or hunting prey. The detection of single objects, described by the single target acuity (STA), cannot be deduced from formerly determined grating acuity (GA) as different mechanisms underlie STA and GA. Thus, we assessed STA for stationary and moving single targets with varying contrast in two harbor seals in a first approach in air. In a two-alternative-forced-choice discrimination task, the seals had to indicate whether the single target was presented in a left or right stimulus field on a monitor. The STA for full-contrast stationary targets was determined as 0.27 deg of visual angle for both experimental animals. Contrary to our expectations, neither adding motion nor reducing contrast had a strong impact on STA. Additionally, we also determined GA in the two harbor seals (1.2 and 1.1 cycles/deg or 0.42 and 0.45 deg for a single stripe of the grating at threshold) to be slightly inferior to STA. Our results are in good correspondence with contrast sensitivity and allow calculating viewing distances in the context of for example visual orientation.