Vision
Animals make use of some form of vision to provide info about the environment
Vision:
provides the earliest cues for the detection of a predator or prey
usually has a longer range than hearing
more precise and immediate than smell
almost always additionally involved in the pursuit of prey, guidance of a predatory strike, or the coordination of an escape maneuver
Many animals display a shadow withdrawal reflex in response to a sudden dimming of the light; image detail is irrelevant
In some animals the image is crucial because it enables the eye’s owner to determine where the object of interest is located
The Electromagnetic Spectrum
Not all animals possess color vision but, for those that do, it can be useful in detecting warning coloration
Many insects, birds, and fish can visualize shorter wavelengths of light in the UV range
the caribou, or artic reindeer, is one of the few mammals that can see in UV
Some animals can detect light in the longer wavelength infrared (IR) range
ex: fire-seeking beetles (to lay eggs in freshly burnt wood, vampire bats, and pit vipers, boas, and pythons
Many animals, including a number of arthropods and cephalopods, are also sensitive to polarization
ex: mantis shrimp (for navigation) and squid (for detecting prey)
Eyes: Acuity and Sensitivity
The quality of eye depends on:
Sensitivity - the amount of light energy that it can capture from a given source
Resolution (acuity) - the accuracy with which it can determine the spatial origin of that source
Compound eyes are composed of ommatidia that view a different part of the visual files so that each one represents a single pixel of the final image
Acuity of ommatidia is dependent on the diameter (bigger the facet size, better the diffraction power, better the image)
Many animals have a fovea where acuity is at its best
Animals for which most objects of interest occur in a 1-D horizontal line tend to have a broad visual streak fovea (infula)
ex: antelopes, gerbils, rabbits, cheetahs, and wolves
Animals that operate in a 2D visual environment, where the vertical and horizontal are equally important have an intermediate fovea
ex: primates, rodents, cats (except cheetahs)
Birds of prey (raptors) have a forward-looking shallow fovea in each eye to provide binocular vision, while a lateral looking deep fovea in each eye provides monocular vision
The fovea are connected by a visual streak
When hunting prey, the lateral deep fovea are used in prey tracking, until the forward shallow fovea takes over when in close range of prey
Feature Recognition and Releasing Behavior
Many animals possess a feature recognition system that extracts important elements of incoming visual signals to compute object identity, and then relays this info to the motor system to coordinate an appropriate behavioral response (attack, escape, or ignore)
Key visual stimuli - the combination of visual signals that release a behavior
Includes object movement, shape, form, speed, and direction
This process involves computation and filtering of the stimulus properties in brain circuits that function in an innate releasing mechanism
ex: using a cardboard cutout of a bird to measure turkey escape responses; turkeys escaped if the cutout resembled a hawk, but stayed still if the cutout resembled a goose (depending on the direction the cutout went [left or right])
Prey Capture in Toads
The common toad do not produce eye saccades, so their visual system is thought to be effectively blind to static scenes
It is interested in moving things, as its visual system is tuned to respond to simple moving objects
When the toad recognizes a moving object as prey, it initiates a sequence of behaviors in which one action triggers the next to form a stimulus-response chain
Orienting the head and body towards the prey
Approach the prey
Snap out its tongue and take the prey into its mouth
Gulp down the meal and wipe its mouth
In Ewert’s experiment, a horizontal rectangle moving along its long axis seemed to have a worm configuration, in contrast, if the same shape was flipped vertically and moved in the same direction it had an anti-worm configuration that caused the toad to ignore it or perceive it as a predator
Escape and avoidance behaviors include:
Crouching defensively
Standing up and inflating itself
Jumping out of the way
Most ganglion cells show a center-surround response property, where they are excited by an appropriate stimulus in the center of their receptive field, but inhibited by the same stimulus in the periphery of the field
Ganglion cells relay visual info to the brain and interpret said info, to a certain extent
Visual info from the retina passes through the optic nerve to the brain
Two main areas involved in feature recognition:
Optic Tectum
Of the nine classes of neurons here, two subtypes play an important role in signaling prey feature recognition
One fires to prey stimuli and the other decreases firing rate in response to threatening stimuli
Lesions to the second subtype results in animals attacking any moving stimulus perceived, predator or prey
Thalamic Pre-Tectal Area
One subtype of these thalamic neurons is important in recognizing threatening stimuli and initiating escape and avoidance behaviors
There is strong evidence for inhibitory connections from the thalamic neurons to the second subtype of tectum neurons
The neuronal network responsible for recognizing predators and prey appear to be innate, although plasticity in this network has been observed
Beetle larvae of the genus Epomis take advantage of the toads prey capture, by making movements similar to prey then striking the toad’s skin to suck the bodily fluids from the toad
Beyond the Visible Spectrum
Ambush predators, such as snakes, used infrared vision to produce a thermal image of their environment
This helps them target warm-blooded prey with remarkable accuracy
Snakes use their pit organs to detect IR radiation
A thermal “eardrum” is stretched across the inner chamber of each pit organ as a thin, suspended membrane
It has a inner and outer epithelial layer, with a region lying between them densely innervated by terminal nerve masses, thermosensitive trigeminal endings
These are the IR detectors that carry thermal info from the pit membrane to the optic tectum
The pit organ functions like a pinhole camera, the entry hole into the cavity focuses EM waves onto the sensitive and highly innervated membrane
Forming a 2D map of incoming IR info analogous to that of visible light on the retina
Thermosensitive axon endings distributed throughout the membrane of each pit organ provide info to enable signal-processing circuitry in the medulla of the hindbrain to de-blur the image
The pit organ membrane is sensitive to thermal changes in the environment as small as 0.001-0.003 degrees Celsius
The TNM endings are extremely close to the surface of the outward-facing side of the membrane, making them more sensitive to temperature changes
The suspension of the organ membrane creates an important pocket of air between the pit membrane and the snake’s head
prevents the unwanted loss of IR energy through tissue absorption
The presence of an extensive capillary network surrounding the TNMs acts an effective heat exchanger (essential for pit organ function)
Retaining heat would create an “after-image” in the snake’s brain
Ensure that the pit organ remains sensitive to real-time changes in the thermal environment
The capillary network upregulates local blood flow in response to real-time increases in IR radiation
The pit organ is covered in nanopits and micropits, whose structure may function as a spectral filter of unwanted wavelengths of EM radiation, protecting the organ from non-infrared thermoreceptor heating
The embedded terminal endings of the trigeminal nerve fibers act as the sensory receptors for IR vision
The transient receptor potential Ankyrin 1 is highly enriched within the trigeminal ganglia of thermosensitive snake species
This receptor is a highly effective sensor of IR energy and is the most temperature-sensitive vertebrate ion channel ever discovered
IR visual info pathway:
TNMs at the pit organ project ipsilaterally via two ophthalmic branches and one maxillary branch of the trigeminal nerve
→ later descending tract of the trigeminal nerve in the medulla → reticulis caloris
signals from the pit organ are refined to improve image quality
→ deep layers in the contralateral optic tectum
IF info is mapped with the same orientation as the visual info, but at higher magnification
This system is evidence of cross-modal integration between visual and IR sensory info
Six classes of bimodal tectal neurons:
AND
OR
visual-enhanced infrared
infrared-enhanced visual
visual-depressed infrared
infrared-depressed visual
Infrared snakes that are blinded in the normal visual spectrum are still able to strike a target with only slightly reduced accuracy
Infrared alone provides sufficient information for accurate coordination of predatory strikes
When pit organs are blocked, normal vision alone is also sufficient for predatory strikes in daylight
When both eyes and the pit organs are occluded, the snakes are unable to initiate a predatory strike at all
Pit organs are also used to identify cool spots in the environment to which the snake can retreat in the heat of the day for thermoregulation
California ground squirrels and rattlesnakes have an ancient predator-prey relationship that has led to fascinating specializations in both animals
The squirrel sends predator deterrent signals to communicate with the snake
The tail wagging and blood to the tail play on both of the snakes sensory modalities when hunting
Aerial Predators: Dragonfly Vision
Each eye of the dragonfly are not uniform, but instead contain a specialized region called the dorsal acute zone
Contains larger ommatidial facets and closely aligned optical axes
During hunting, the dragonfly rotates its head to keep the target prey within this zone
Aerial pursuit techniques:
Constant bearing, decreasing range - the predator steering itself so as to maintain its target at a fixed angle relative to its own direction of travel
Tracking (classical pursuit) - the predator aims directly at the target
Interception - the predator aims for a point in front of the target
Reactive strategy - deviations in prey visual angle are detected by specific neurons in the visual system and responded to with compensatory movements
Predictive strategy - movements are pre-planned, based on internal models of self and prey motion
Dragonflies possess four wings, which can be controlled independently
Predictive foveation - they perform compensatory head movements independent of body movements, so as to cancel out the image drift and keep the prey within the dorsal acute zone of the retina
Insects have feature detectors in their visual pathway
The lobula complex contain small target motion detector neurons that sensitive to the movements of small objects in a broad range of directions
STMDs drive eight pairs of descending interneurons called target-selective descending neurons, which transmit target motion info to the thoracic motor centers and result in adjustments to wing movement
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