The Interaction of Avoidance and Acquisition Behaviors in the Frog and Toad
by David D. Olmsted (Copyright - 2001, 2006. Free to use for personal and
Last Revised October 25, 2006
Stimulus Characteristics Determine
the Choice of Avoidance or Orientation
Frogs and toads orient towards small visual
stimuli while avoiding larger visual stimuli. This is shown in figure 1. 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 the equation: (stimulus luminance - background
luminance) / total luminance. Maximum orienting response with a 6 degree spot occurred
at an average rate of 25 turns per minute (one every 2.4 seconds) while the maximum
avoidance response occurred at an average rate of 20 responses per minute.
Optimal Avoidance Size is a Spot 45 Degrees in Diameter while Optimum Orienting Size is a Spot 6 Degrees in Diameter. (Ewert - 1970)
Yet these response rates can be altered by changing the motivation level of the
toad. This can be done by presenting the toad with the odor of mealworm excrements.
When stimulated by this odor toads will orient and snap at objects as large as 20
degrees in size, objects they would normally ignore (Ewert - 1970).
The larger the
size of the spot up to 6 degrees the less motivation was required to trigger a response.
Damage to the
forebrain of toads which processes the olfactory information and modulates the motivational
state of the animal depresses orienting activity to potential prey. The effect is
proportional to the amount of brain removed along the rostal-caudal axis. Removal
of only one side of the forebrain produces a complete depression to responses viewed
through the contralateral (opposite) eye although some depression is seen via the
ipsilateral (same side) eye (Ewert - 1970).
Yet even if the whole forebrain is removed
thus eliminating any acquisitive behavior the avoidance behavior remains unaffected.
(Ewert - 1970).
Both avoidance and acquisitive behaviors can be triggered by electrically
stimulating the tectum as shown in figure 2. The avoidance movement sites tend to
be elicited towards the noseward (rostal) side of the tectum which represents the
forward visual field. This simply means that the triggering threshold is less in
that area for avoidance than it is for acquisition.
Different Tectal Areas Show Differing Sensitivities for Producing Either Acquisition or Avoidance Responses. (Ewert - 1970)
The Different Behavioral Sensitivities
of the Tectum are Modulated by the Pretectum
Toads normally orient towards
worm like stimuli in which the horizontal dimension is longer than the vertical
dimension. Yet this preference is abolished when connections to the pretectal area
are severed as shown in figure 3. The toad losses all inhibition and orients at
a high rate towards all stimuli as if each stimulus was an optimum behavior releaser.
Destruction of the Pretectal Area Inputs to the Tectum in the Toad Bufo bufo Destroys Prey Selectivity. (Ewert - 1970)
This release of tonic inhibition by pretectum damage also affects the context inhibitory
of “worm” recognition. Reducing false targeting due to background clutter
saves energy. Yet pretectum damage prevents such inhibition causing
the toad to react to it as if it again was an optimum releaser of acquisitive behavior. The
left image in figure 4 shows normal response to a “worm” stimuli and a "clutter
worm" stimuli as a function of their closeness given as the ratio of the response
rate of both “worm” stimuli (Rab) divided by
the response rate to one worm stimuli (Ra).
So the lower the ratio the better is the clutter inhibition. In the case of the
left figure when the "worms" are close together they are viewed as one fat worm.
As they are spread apart the clutter inhibition kicks in. As they are spread even
further apart the toad views the "worms" as separate entities.. In contrast pretectal
damage as shown in the right figure shows no clutter inhibition.
Pretectal Destruction also Destroys Context Selectivity in the Toad Bufo bufo. (Ewert - 1970)
Pretectum damage will also allow acquisition behaviors to re-emerge after complete
forebrain removal which normally prevents the expression of such behavior. This
double lesion produces an even greater release from inhibition than the pretectum
lesion alone (Ewert - 1970).
The pretectum area is also known as the caudal thalamic
area because it is located between the tectum and the central thalamus. Cuts affecting
pretectal projections to the tectum cause the same disinhibition as does tectal
damage as shown in figure
5. The blacked out areas indicate tissue destruction. Blacked out visual fields
(cicles) show disinhibited responses
The Location of Cuts Which Release Inhibition of the Tectal Responses in the Toad Bufo bufo. (Ewert - 1970)
Ewert, J.-P (1970). Neural Mechanisms of Prey-catching
and Avoidance Behavior in the Toad (Bufo bufo). Brain, Behavior, and Evolution 3:36-56.