Acquisitive Visual Targeting Behavior in the Frog and Toad
by David D. Olmsted (Copyright - 2001, 2006. Free to use for personal and
Last Revised November 6, 2006
The Acquisitive Targeting Field
Frogs and toads must decide among two different classes of acquisitive behaviors:
approaching and orienting. The approach class of behavior can further be divided
into four subclasses: snap, turn - snap, jump-snap, and jump. Most of these are
shown in figure 1. A three inch frog will snap at straight ahead objects as far
out as 5 inches, and objects to the side as far out as 3 inches.
The aquisitive targeting field is relative to head position. Turning a frog’s head 30 degrees also turns
the aquisitive targeting field the same amount (Ingle - 1970).
For a given frog the acquisitive
behaviors are very consistent with the snap - jump
boundaries not varying by more than a half
inch (Ingle - 1970).
The Acquisitive Behavior Field of the Frog. (Ingle - 1970)
The various acquisitive actions can be elicited by electrical
stimulation of the tectum as shown in figure 2. Numbered locations show the region of the contralateral
visual field (the side opposite the side electrically stimulated) where the frog turned during electrical stimulation. Inner columns
are progressively more rostal (noseward). Other sites produced snapping and gulping.
This suggests that the tectum is
the trigger of these actions.
Electrical Stimulation of the Tectum in the Toad Bufo bufoProduces Acquistion Actions (Ewert - 1970).
An HRP study by Masino and Grobstein (1990) reported that the neural outputs from the
Tectum project to the Reticular Formation near the Superior Olive:
"Some of these collaterals (axon branches
from the Tectum) cross the midline to terminate in the vicinity of the Olive on
the opposite side of the brain. Previous reports have suggested that this
collateralization reflects a bilateral input to the Olive itself. Our material
shows the fibers to extend well beyond the olivary cell group and terminate as well
near cells of the Medial Reticular Nucleus. In fact, most of the observed
bouton-like swelling were closer to Medial Reticular
neurons than to olivary
Response Characteristics to Prey Stimuli
A test to determine the which visual stimulus produced the best turning response
in a toad is shown in figure 3 with each turn indicated by a vertical line. Disks
of varying sizes were rotated around the toad Bufo bufo at a rate of 30
degrees per second (5 rotations per minute). The contrast ratio was 0.95 given by:
(stimulus luminance - background luminance) / total luminance. The maximum orienting
response with a 6 degree spot was an average of 25 turns per minute (one every 2.4 seconds. The actual distance of the spots was not given. The turning responses have
a probabilistic component since they are not strictly periodic
the contrast ratio between the stimulus and the background the lower is the response
rate. The exact relationship depends on the shape of the stimulus and the motivation
of the toad. At a maximum contrast ratio of 0.95 a dark stimulus has a stronger
releasing efficiency than a white one (Ewert - 1970).
When a white optimal stimulus
(2 degrees by 16 degrees having a contrast ratio of 0.95) is used the maximum orientation
rates are elicited at stimulus angular velocities of between 30 and 60 degrees per
second. (Ewert - 1970).
When a strobe light is used on a stimulus circling at 30
degrees per second the inferred motion assumption used by the toad can be found
since the stimulus object appears to jump around the toad. At a flash rate of 5
per second the toad will still respond but the maximum releasing values are found
at flash rates between 10 and 20 per second (Ewert - 1970)
Test Mechanism Used on the Toad Bufo bufo for Determining Orienting Response Rates. (Ewert - 1970)
Response Characteristics to Partly Occluded Stimuli
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. A frog or toad is placed in a box having slits behind which is passed
light or dark cards. All cards were scanned at 4 to 6 degrees per second. Frogs
were usually placed 3 inches aways from the slits. Stimuli were presented after
the frog was facing at least 45 degrees away from a slit for 10 seconds.
In this occlusion situation frogs (Rana pipiens) had different best responses when orienting and snapping were compared.
They oriented best towards 4 degree wide slits but snapped best at 15 degree wide slits.In the frog's environment distant
bugs will appear smaller while close bugs that are within range of its tongue will
appear larger. The response rates are shown below:
Testing for Orientation Towards Occluded Targets. (Ingle - 1968)
Yet angular size is only one factor in determining the probabilities of orientation
versus snapping behaviors. Frogs are able to adjust for distance in
order to maximally respond to their favorite sized prey of 3/16 inch thick worms
as shown in figure 5. The first and third columns have the same angular size (1.2 degrees) as do columns
two and four (3.6 degrees) despite being at distences of 3 and 9 inches respectively.
Best response rates are from the 3/16 inch thick “worms” irrespective of distance
(at 93%). The 1/16 inch thick “worm” produced a response of 41% while the 9/16 inch
thick “worm” produced a response of 34%. Not accounted for in this experiment is
the angular velocity.
Orientation Towards Stimuli Having the Same Angular Size but Differing Distences.
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. This trend
also holds for white “worms" which show snap rates of 88% for the disappearing ones
and 38% for the appearing ones.
Frogs and Toads Snap More at Disappearing “Worms”. (Ingle - 1968)
Despite orienting towards the head of white worms toads will tend to snap at the
middle of the worm.with the middle determined relative
to the endpoints of the worm as shown in figure 7. Yet the absolute light levels seem to determine where the
toad actuallys strikes the worm like object . See
Responses of the Toad in Low Light Levels
In Normal Light Levels Toads Tend to Strike Towards the Middle of a “Worm” as Determined by its Ends. (Ingle - 1968)
Decision Preferences - Distance Effects
When given a choice
between two stimuli frogs will tend to orient towards the nearest stimuli. In one
experiment (Ingle - 1973) two yellow stimuli 1/2 x 1/8 inches were wiggled at distances
2-1/2 inches or 3 inches away from the frog. The frogs were motivated to orient
by first establishing the appropriate environmental context
by feeding them some mealworms. This produced the following orientation rates
for the nearer stimulus when both targets were placed at equal angles on opposite sides.
Opposite side nearer: 45° from midline - 92%
- Opposite side nearer: 90° from midline - 84%
- Same side nearer: 45° from midline - 85%
- Same side nearer: 135° from midline- 85%
Frogs and toads compensate
for distance up to 15 cm as shown in figure 7. In this
experiment the frog Rana pipiens had to choose between the variable sized test prey and a constant 3 degree
sized prey. The optimum prey size for orientation
within this distance has a width of 0.8 cm. Notice that doubling the distance from 7.5 cm to 15 cm quartered the
optimum prey size from 6 degrees to nearly 1.5 degrees yet this prey object was
physically the same size. The 22.5 cm distance had the same optimum angular size
in degrees as the 15 cm distance indicating that size constancy broke down. Yet significantly the decision on
whether to approach a hole in a barrier is only based upon the visual angle (Ingle
and Cook - 1977).
The frog Rana pipiens Compensates for Prey Size up to a Distance of 15 cm.
Triangle Line - 22.5 cm distant prey, Square Line - 15 cm distant prey, Circle Line - 7.5 cm distant prey. Plain Line - Ewert’s data (1970) for toad Bufo bufo. (Ingle and Cook - 1977)
Decision Preferences - Horizontal Location Effects
Frogs tend to orient preferentially towards stimuli located directly in front. In one experiment (Ingle - 1973) two yellow stimuli 1/2 x 1/8 inches
were wiggled 3 inches away from the frog. The frogs were motivated to orient by
the appropriate environmental context by feeding them some mealworms. This produced the following orientation rates
for the more frontward (rostral) stimulus.
- Percent midline target selected compared to 45° target - 84%
- Percent 30° target selected compared to 90° target - 48%
Cutting the optic nerve from
one eye abolished this frontal preference and even reversed it. The midline vs.
45° decision produced the following results for the given number of days after
4 days - 16%
- 4 weeks - 29%
- 4 months - 34%
In the same experimental setup to that described
above David Ingle (1973) measured the following average orientation latencies:
Stimuli 30° from midline:
- One stimulus - 2.5 seconds
- Two stimuli - 6.4 seconds
- One stimulus - 2.1 seconds
- Two Stimuli - 2.1 seconds
Stimuli on same
- One stimulus (front or back) - 2.3 seconds
- Two stimuli - 2.4 seconds
From these experiments indecision delay seems to only be produced when the stimuli
are close to the midline, that is near optimal. This might be somewhat analogous
to the indecision delay found even in
humans who tend to “freeze” when confronted with some novel and emotionally intense
Frogs often tilt their head upwards as much as 90°
during orientation to get a better view. Yet they still make accurate
turning actions despite objects having differing locations in their visual field
(on the retina). Since turning is a bodily referenced action some sort of remapping
from retinal coordinates to body coordinates seems to be taking place. This may
be one of the functions of the
nucleus isthmus which has a topographic representation
of all the space surrounding the frog.
Correct orientation seems to require input
from both eyes via the tectums on each side of the brain. If the frog is forced
to use only one eye it will overshoot the target as shown is figure 9. It appears
as if the tectum's average the visual locations as seen by each eye to arrive at
the correct stopping location. Frogs turning with only one tectum overshoot their
target by about 50% of the required angle yet they show no error for a direct snap
(Ingle - 1970).
Toads Overshoot the Target When Forced to Use One Eye. (Ingle - 1970)
Habituation of Responses
Repeated presentation of a stimulus to
the same part of the visual field will cause the frog to cease snapping but it will
still sometimes orient towards the object (Ingle - 1970). Yet these same frogs will
still snap at objects located only 20 degrees away.
Habituated areas, like the acquisitive targeting field shown in figure 1 are relative to the head. A frog will snap at a habituated stimulus placed
in the same position relative to the ground if its head is moved (Ingle - 1970).
An example of the rate of visual habituation is shown at the top of figure 10 while
the recovery from that habituation is shown at the bottom. The stimulus is
the optimum 2 degree by 16 degree “worm”. The top illustration shows how the number
of orientations per each minute declines over time if presented to the same part
of the visual field. Notice the log scale on the left. The recovery pattern shown
at the bottom
is similar to that a damped oscillatory system. The recovery ratio
(E) is the (number of responses of a second habituation stimulus series separated
from the first by the recovery time - t) / (the number of responses of the first
habituation stimulus series).
Orientation Habituation in the Toad Bufo bufo. (Ewert - 1970)
Approach Behavior is Divided into Segments
If prey is farther away than the snap zone the frog or toad will have
to approach it. The approach is usually divided into several ballistic segments.
This means that once the direction and length of the walk has been determined it
is not adjusted even if the prey changes velocity, stops, or disappears (figures 11).
The Toad Bufo marinus Does Not Compensate for Changes in Prey Motion.
Toad position (arrows) and prey postion shown every 0.2 seconds. (Lock and Collett - 1979)
Figure 12 shows the distance of the initial walking segment vs. target distence
when the target is moving (a) and when the target vanishes or stops (b ). Bottom
graph (c) shows that walking distance is unaffected by the length of time between
the initiation of walking and target stop (or vanish).
The angle of the initial turn and the length of the walking segment
are determined when the decision to approach the target is made. The length of the
walk segment is proportional to the distance of the target showing a fine visual
depth perception on the part of the toad.
The Length of the Walking Segment in the Toad Bufo marinus Varies with Target Distance. (Lock and Collett - 1979)
Despite the ballistic nature of the walk
segment the toad still closes its eyes during the fastest part of the walking cycle
as shown in figure 13. Perhaps this is to inhibit visual balance reflexes such as
The Toad Bufo marinus Shuts Its Eyes At the Periods of Greatest Motion During the Walking Cycle (Lock and Collett - 1979)
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