Frog and Toad Eyes
by David D. Olmsted (Copyright - 2000, 2006. Free to use for personal and
Last Revised September 23, 2006
How the Frog Uses its Eye
How the frog
uses it eye is best described by Lettvin, Maturana, McCulloch, and Pitts (1959):
“A Frog hunts on land by vision. He escapes enemies mainly by seeing them. His eyes
do not move, as do ours, to follow prey, attend suspicious event, or search for
things of interest. If his body changes its position with respect to gravity or
the whole visual world is rotated about him, then he shows compensatory eye movements.
These movements enter his hunting and evading habits only, e.g. as he sits on a
rocking lily pad. Thus, his eyes are actively stabilized. He has no fovea, or region
of greatest acuity in vision, upon which he must center a part of the image....
The frog does not seem to see or, at any rate, is not concerned with the
detail of stationary parts of the world around him. He will starve to death surrounded
by food if it is not moving. His choice of food is determined only by size and movement.
He will leap to capture any object the size of an insect or worm, providing it moves
like one. He can be fooled easily not only by a piece of dangled meat but by any
moving small object. His sex life is conducted by sound and touch. His choice of
paths in escaping enemies does not seem to be governed by anything more devious
than leaping to where it is darker. Since he is equally at home in water and on
land, why should it matter where he lights after jumping or what particular direction
he takes? He does remember a moving thing provided it stays within his field of
vision and he is not distracted.”
Frog Eye Characteristics
The large round lens of the frog shown as a filled black oval in figure 1 gives
the animal a large field of view. The frog is naturally
nearsighted (myopic) to -6 diopters giving it a focus of approximately 6 inches. Frogs
and toads can change their focus by moving the lens out towards the cornea. In frogs
the focus range is a few diopters and in toads the focus range is 5 diopters giving
a best myopia of -1 diopters. During this accommodation the pupil also increases
in size. (Grusser and Grusser-Cornehl - 1976). The advantage of nearsightedness
is that it blurs the background clutter making foreground object characterization much
easier. Only 75%
of the light intensity entering the eye reaches the retina as shown in figure 2.
Top View of the Frog Eye (originally from Szent-Gyorgyi - 1914 but scanned from Grusser and Grusser-Cornehls - 1976)
Spectral Transmission Curve of the Eye of the Frog Rana esculenta (originally in Grusser-Cornehls and Saunders - 1975 but scanned from Grusser and Grusser-Cornehls - 1976)
The minimum separable
vertical stripe period in an optokinetic test is 6 to 7 minutes (Birukow - 1937
as reported by Grusser and Grusser-Cornehls - 1976). In this kind of test a drum
having vertical stripes in rotated around a frog who will normally track the stripes
for a certain distance before resetting its gaze back forward. Yet tectal neurons
can respond to moving dots of light as small as 0.1 to 0.2 minutes (Grusser and Grusser-Cornehls - 1976). This is in spite of the fact that the frog
Rana esculanta has an average of 35 visual receptors per degree
which is equivalent to approximately 1 every 2 minutes (Alexander-Schafer
- 1907 as reported by Grusser and Grusser-Cornehls - 1976) indicating that neural circuitry is able
to refine the native resolution of the retina.
A quiescent frog will close its eyelids
synchronously every 0.5 to 5 minutes. They will also be closed by:
whole body vibration
- sudden bright light
- large approaching dark objects
- prey swallowing
Frog eyes also exhibit a rhythmic motion possibly due to heartbeat or muscle drift.
This motion has period of 1.3 seconds and an amplitude of 28 minutes. Every 15 to
20 seconds the lungs empty causing a larger 1 degree movement of the eyes. (Grusser
and Grusser-Cornehls - 1976). One would expect the frog's brain to have neural circuitry
that allows it to ignore these types
of false motions.
Retinal Cell Anatomy
nerve of the frog consists of approximately 470,000 unmyelinated fibers and 15,000
myelinated fibers. In the toad it consists of 320,000 unmyelinated fibers and 10,000
The neurons of the retina which produces the output fibers are
the ganglion cells. The frog Rana pipiens has approximately 440,000 small ganglion
cells (7 to 10 microns in diameter) and 12,000 of the larger ganglion cells 14 to
20 microns in diameter. (Maturana - 1959). With the frog brain
consisting of only 16 million neurons one can see that the retina makes up a significant
portion of its brain's total neuron number.
The frog retina has three cell layers as shown in figure 3: the outer, middle, and
inner granular layers, and two fiber layers: the inner and outer plexiform layers.
The frog has a limited color discrimination as indicated by the red and green rods.
The red rods are the most numerous, closely followed by red sensitive cones. The
green rods are relatively scarce. Only the ganglion cells from the inner granule layer send axons to the brain. The bipolar cells (dendrites above and below the
cell body) in the middle granular layer are intermediate
processing elements. The bipolars connect
mostly vertically while another class of neurons called horizontals and amicrines (not shown) connect mostly horizontally. The ganglion cells themselves come in several varieties. Their
dendritic arbors can be thick or thin and be widespread horizontally or quite
restricted. Some of the thick arbors tend to separate into two vertical layers
Basic Retinal Cell Shapes (Lettvin, Maturana, Pitts, and McCulloch - 1961)
Response Classes of the Ganglion Cells
Class 1 and class 2 ganglion cells project to the
brain via unmyelinated fibers having conduction velocities of 20 to 50 centimeters
per second. They make up 97 percent of the optic nerve (Maturana - 1959). Typical
responses are shown in figure 4.
Class 1 neurons (edge detectors): These neurons
have oval receptive fields from 1.5 to 4 degrees in size. These neurons detect the
completeness and sharpness of an edge including that from a spot which can be either
brighter or darker than the background. If a boundary is brought into its receptive
field in total darkness and the light switched on a continuing response occurs after
an initial delay. The response is enhanced by edge movement. If an edge stops in
its visual field the neuron remains activated for several minutes. In this case
if the light is turned off the response ceases only to reappear less intensely when
the light is turned on again.
- Class 2 neurons (convexity detectors): These neurons
have oval receptive fields ranging in size from 2.5 to 5 degrees. Their purpose
seems to be the detection of the dark leading edge (head) of any worm or bug. If
the stimulus stops before the center of the receptive field the neuron exhibits
a slow decaying response (time constant between 0.3 to 3 seconds) . Unlike
the class 1 neuron a brief darkness or shadow will permanently stop the response
until the object moves again. These type of neurons have a strong inhibitory surround
region indicating that they are prevented from working in environments with substantial
- Class 3 neurons (change detectors): These neurons are also
known as on-off cells and they have receptive field sizes between 6 and 10 degrees.
The intensity of their response is governed by the amount of light intensity change.
This includes such parameters as as stimulus size but not shape.
- Class 4 neurons (dimming detectors): These neurons have oval receptive fields ranging
from 10 to 15 degrees. They respond to a dimming of the light.
- Class 5 neurons (tonic
darkness sense): These neurons are very few but they seem to indicate the general
light level of the frog’s surroundings. When the light level is reduces these neurons
slowly increase their firing rate.
The class 3 and class 4 ganglion cells project to the brain
on myelinated fibers which make up only 3% of the optic nerve. Class 3 fibers have
a conduction velocity of between 1 and 5 meters per second, class 4 have a conduction
velocity of 10 meters per second, and a small set of fibers having a conduction
velocity of 20 meters per second is thought to arise in the brain and project to
the retinal neurons.
For all classes of ganglion neurons the more rapid is the angular velocity of a
stimulus the greater is the frequency of the action potentials but with the consequence
of a shorter pulse length. Consequently, the total number of impulses probably remains
the same (figure 5).
Responses of the Four Retinal Ganglion Classes. Class 1 - edge detector, Class two - convexity detector, Class 3 - Change (on-off) detector, Class 4 - dimming detector. (from Grusser and Grusser-Cornehls - 1968a but scanned from Grusser and Grusser-Cornehls - 1976)
Signal Intensity vs. Stimulus Angular Velocity. Black spot moving on a light background. Spot 1.2 degrees for class 1 and 2. Spot 2.7 degrees for class 3. Spot 23 degrees for class 4. (from Grusser and Grusser-Cornehls - 1969 but scanned from Grusser and Grusser-Cornehls - 1976)
The left graph
in figure 6 shows how the latency of signal initiation decreases with increasing
light intensity. Top line (open circles) represent the “on” burst while the bottom
line (dark circles) represent the “off” burst. The right graph gives the frequency
in impulses per second of the first 6 impulses for both the “on” and “off” bursts.
The total number of impulses in the signal is also proportional to the light intensity
Responses to Light Intensity of the Class 3 (Change Detector) Neuron. (Hartline - 1938)
The more central is the stimulus in a receptive
field the more intense is the neural signal. This is shown in Figure 7 which gives the average impulse rate when a black spot is moved at a
constant velocity in a horizontal direction. Yet the size the receptive
field does not seem to correlate to the breadth of the respective ganglion cell
dendritic arbors. This is shown in figure 8 which shows the top view of some
class 4 ganglion cells called A, B, and C. Their receptive fields are shown in the
illustration in the upper left. Notice how small is the receptive field for C and
how the receptive field for B has some inhibitory flanks as exposed by the small
spots of lights used as probes.
Class 2 Neuron has Greatest Response from the Center of its Receptive Field. Each division represents 0.5 degrees. (from Grusser and Grusser-Cornehls - 1968a but scanned from Grusser and Grusser-Cornehls - 1976)
Size of the Ganglion Cell’s Dendritic Arbor does not Correlate with Receptive Field Size for the Class 4 Cells (Dimming Detectors). (Stirling and Merrill - 1987)
The rare class 5 neurons do not seem to be
the only tonic darkness detectors for such neurons have also been found in the frontal
organ of the frog. The frontal organ is analogous to the reptilian pineal eye and
is located on the dorsal surface of the head where light diffuses through the skin.
The response of the frontal organ nerve is shown in figure 9. The 100% firing rate represents the mean of 76.5
spikes per second obtained after 15 minutes of dark adaptation. As light levels
increase the firing rate declines. The data was taken from the frog Rana pipiens
collected during August and September.
Response of the Frontal Organ Nerve to Steady White Light. (Hamasaki and Esserman - 1976)
class 3 and 4 neurons are inhibited by eye blinking as shown in figure 10.
This is probably accomplished by the fibers projecting to the retina.
In figure 10: A = normal class 4 response, B = normal class 3 response, C1 = class
4 response to eye blink, C2 = class 3 response to eye blink, D = class 4 response
to a slow increase in light intensity. Using a photocell the eyelid was found to
transmit 50% to 60% of the incident light.
Retinal Class 3 (Change Detectors) and Class 4 (Dimming Detectors) are Inhibited by Blinking in the toad Bufo bufo. (Ewert and Borchers - 1974)
Responses of the Toad in Low Light Levels
Compared to frogs, toads have a more complex and versatile
visual system due to their more general eating behavior. Toads not only eat
moving insects like frogs but will also hunt for white grubs crawling on the
ground. Insects are normally dark when highlighted against the sky while grubs are
normally light when highlighted against the dark ground. Consequently, the convexity
detector (class 2) neurons of toads respond to small light objects in addition to
small dark objects. As shown
in figure 11 toads can respond to light objects even when in a dark environment (-2 is just less
than full moonlight). This is equivelent to responding at
a low 0.01 Rh per second where Rh is a measure of the number of
photo-isomerizations per rod (molecular transformation in response to a photon of light). In figure 11
the proportion of test sessions in which snaps occurred is given
by the vertical ordinate while the light illumination level is given by the horizontal
abscissa in log units.
Snapping in the Toad Bufo bufo Requires That Light Levels Exceed a Threshold. (Aho, et al - 1993)
The class 2 retinal neurons sum light energy over time. The rods and cones wait until they have collected enough light energy
before producing a signal. The greater is the light energy the shorter is this latency
as shown in figure 12. In figure
12 the response latencies are given in seconds vs light stimulus intensity. The
light intensity of 100 is slightly less than the light level during full moonlight
which according to figure 11 is the snapping threshold.
Class 2 Ganglion Cell Response Latencies Decrease with Increasing Light Levels in the Toad Bufo bufo. (Aho, et al - 1993)
When a toad is very hungry so that it is motivated to snap in low light levels the
tongue strike location onto a moving worm like target
moves progressively backwards as shown in figure 13. The light rectangle is moving towards the right . Light reduction
in the figure progresses from the upper left (200), lower left(2), upper right (0.1),
to lower right (0.2). Despite the increase latency from 1.5 seconds to 3 seconds
from light levels 0.1 to 0.02 the targeting does not get significantly worse suggesting
some sort of compensation is made
for the darkness.
The Response Latencies Cause Progressively Less Accurate Tongue Targeting in the toad Bufo bufo. (Aho, et al - 1993)
Variations in Toad Retinal Responses
A variation in the dark object
or light object toad acquisition response is found if a less then ideal prey stimulus
is used such as a square. Ewert and Siefert say this (1974):
“To be sure, square
patterns were not so effective for prey-capturing behavior as horizontal stripes
(“worm” like patterns). Nevertheless, the squares proved to be especially advantageous
in these experiments: In the case of an optimal target shape, the toad seldom betrays
through behavioral changes a recognition of limited variations in stimulus pattern
contrast. Horizontal stripes, whose shape fit the category of “worm” prey, attracted
equally in both black and white. On the other hand, when the form of a stimulus
less accurately represents prey from nature (square objects), the importance to
prey evaluation of the other stimulus parameters - such as the direction of the
background contrast - increases.”
The effect of contrast is shown in figure 14 where the impulses per second are compared
at different contrasts with black on white and white on black during the summer.
The square size is 4 degrees and it was moved at a constant angular velocity. The
greater is the contrast the more intense is the neural signal especially for white
Reduced Contrast Reduces the Intensity of the Neural Signal for Class 2 (Convexity Detector) Neurons in the Toad Bufo bufo. (Ewert and Siefert - 1974)
In the summer the neural response to to the squares changes with the location of
the square in the eye's visual field as shown in figure 15. The sensitivity to white
on black squares increases towards the lower visual field where the ground is usually
in view. In contrast the sensitivity to black on white increases towards the upper
visual field where the sky is usually in view. Summer responses are shown on the
left of the figure while winter responses shown on the right. The top row
gives the responses for the upper visual field, the middle row gives the responses
for the middle visual field, and the bottom row gives the responses for the lower
visual field. The square was moved at a rate of 7.6 degrees per second. During
the winter when the toads normally hibernate the responses do not show any significant
variation indicating that this selective sensitivity is probably modulated by hormonal
Figure 16 shows that this retinal effect is mirrored in the toad’s prey orientating behavior as well by showing the turning responses per minute vs. square size. The
summer response shown on the left and the winter response shown on the right. White
open circles represent white on black stimuli while back filled circles represent
back on while stimuli. The open circle line is below the closed circle line in the
upper right graph. Squares moved at a rate of 20 degrees per second. About half
of the 20 frogs tested showed greater overall responsiveness and these are shown
in the top row. In contrast the other half showed little responsiveness to the square
and these are shown in the bottom row (perhaps due to their motivational or mental
Responses of Toad Class 2 (Convexity Detector) Neurons in Summer and Winter for Different Regions of the Visual Field in the Toad Bufo bufo. (Ewert and Siefert - 1974)
Behavioral Response of the Toad Reflects the Retinal Signal in the Toad Bufo bufo. (Ewert and Siefert - 1974)
Aho, A. -C, Donner, K., Helenius, S., Oleson-Larson, L., and Reuter, T. (1993).
Visual Performance of the Toad (Bufo bufo) at Low Light Levels: Retinal Ganglion
Cell Responses and Prey-Catching Accuracy. Journal of Comparative Physiology A 172:671-682)
Ewert, J.-P. and Borchers, H.-W. (1974). Inhibition of Toad (Bufo bufo)
On-Off and Off Ganglion Cells Via Active Eye Closing. Vision Research 14:1275-1276
Ewert, J.-P, and Siefert, G. (1974) Seasonal Change of Contrast Detection in the
Toad’s (Bufo bufo) Visual System. Journal of Comparative Physiology 94:177-186
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Vertebrate Eye to the Illumination of the Retina. American Journal of Physiology
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in Embodiments of Mind (1965) edited by Warren S. McCulloch, MIT Press
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