Often times prey objects will suddenly appear from behind a rock or plant or disappear behind the same. This means that the frog visual system must deal with partial stimulus information. Such a situation was tested by David Ingle using the apparatus shown in figure 4. They oriented best towards 4 degree wide slits but snapped best at 15 degree wide slits. Frogs and toads also tend to snap more at disappearing worms than at worms which are just appearing as shown in figure 6.
Frogs even tend to snap at the last minute to a shrinking worm just before it disappears.
Despite orienting towards the head of white worms toad will snap at the middle of the worm. Yet this middle is determined relative to the endpoints of the worm as shown in figure 7. Yet in a dark environment toads seem to s trike for the head in an attempt to compensate for the time delay needed to sum light energy. When given a choice between two stimuli frogs will tend to orient towards the nearest stimuli. The frogs were motivated to orient by first feeding them some mealworms.