Evolution Flashcards
(63 cards)
1
Q
evolution
A
the change in genetic composition (and inherited characteristics) of a population over successive generations which may be mediated by mechanisms such as natural selection, inbreeding, hybridization or mutation
2
Q
natural selection
A
- changes in genetic composition that enhance reproduction are retained in successive generations i.e. traits that offer a competitive advantage are passed on
- Can act at the level of genes, cells, inds, groups of organisms & species
- Operates on more than one level
3
Q
the evolution of vision - when did we see light
A
- Emergence of homo sapiens is really recent in terms of evolution
- No ev of eyes in fossil remains of pre-Cambrian organisms
- Oldest eyes date back to Cambrian period (530-540m years ago)
- Abrupt appearance of a wide range of organisms - ‘The Cambrian Explosion’ - trilobites and arthropods abundant
- The Cambrian Explosion resulted in an entire fauna, including virtually all animal phyla we know today
- Visually guided predation may have been a trigger for ‘evolutionary big bang’
4
Q
cambrian fauna
A
- Trilobite - main skeleton and first eyes
- Morphology and structure very similar to the modern day fly
5
Q
trilobite compound eyes
A
- Trilolbites (extinct marine fauna) had early compound eyes
- Fossil ev suggests a large variety of visually guided animals evolved in a short space of time (~5m yrs)
- Parker’s ‘light-switch’ hypothesis: evolution of vision key driver for diversification - visually guided predation may have been a trigger for ‘evolutionary big bang’
- Body size increased during Cambrian period & skeletons & rigid protection seem to have evolved at the same time
o Increase mobility = predation as well as stopping predators eating it - Improved mobility & visually guided predation introduced selection pressure to develop protections (e.g. eyes, body armour)
6
Q
how did early visual systems evolve
A
- In order to ‘see’, need something to harvest light energy & an effective signalling system
1. All animal photoreceptors use opsin protein, bound to a light sensitive vitamin A derivative (chromophore), to detect light energy
2. Light sensitive opsins signal via a G-protein cascade and this may evolved very early in animal evolution - Material that captures and then signalling system
- Animal opsins may have originated as a modification of a chemoreceptor protein (generates a bio signal in response to a chemical) early in animal evolution
- At a molecular level - the 2 systems are almost identical
7
Q
what factors shaped the evolution of vision
A
- Selection acts on both the dev of the sensory detector (eye) and visually guided beh
- Genes allow to adapt morphology & physiology
- Both factors (causality and requirement) are linked with respect to evolution
o Causality: genes –> morphology & physiology –> beh guided by visual info –> fitness that selection acts upon
o Requirements: opposite to causality - Requirement to get some genetic mutation to show new behaviours
- To stop being preyed upon would lead to this idea of requirement
- In this view, beh is causal evolutionary link between fitness & morphology/physiology of sensory systems (Nilsson, 2009)
- –> to understand the evolution of the eye, need to consider the evolution of visually-guided beh
o Eyes are well matched to the visual tasks they serve
8
Q
visual behaviours constrained by
A
o Integration time - e.g. long integration time required for monitoring daily light cycle, but short integration is needed for avoiding motion blur
* At what time period does it catch photons to signal a change
o Detecting the angle of incoming light - e.g. unshielded receptors in transparent animals are non-directional and are useful for monitoring ambient radiance, but phototaxis responses require directionality
* Where in visual field is the light coming from
* e.g. phototaxis
o Detection accuracy - e.g. poor detection threshold (~30% change) may be sufficient for detecting the onset of dusk, but not for discriminating boundaries of an object in a visual scene (~3% change)
* Light differences between objects and background could be quite difficult (needs to be specific) or may be easier such as different lightness from sunlight to sunset
9
Q
class 1 - behaviours controlled by non-directional monitoring of ambient light
A
- Control of circadian rhythms
- Monitoring water depth
- Avoiding harmful levels of UV radiation
- Shadow detection to avoid predators
- Surface detection for burrowing
- Monitoring light levels for reproduction
- Class 1 tasks can be performed at very low light intensities (starlight) with unaided and morphologically unspecialised photoreceptors
o i.e. only opsin and signalling system
10
Q
class 2 - behaviours based on directional light sensitivity
A
- Phototaxis response
o Movement towards or away from light - e.g. nematodes move their body in an “arc” to detect light
o A large field is sufficient (180deg)
o Spatial info is gathered by moving the field (scanning)
o This requires much faster responses from the photoreceptors
o Intensity diffs from scanning are small - need better ability to detect intensity change - Control of body position
- Predator detection
- Class 2 tasks require the addition of a screening pigment or photoreceptor shielding
o This makes them directionally selective
o Eye spots, or ocelli, are sufficient for class 2 tasks
11
Q
class 3 - visual tasks based on low spatial resolution
A
- Detecting ego or self-motion
- Anti-collision mechanisms
- Selecting a new habit
- Orientation towards large landmarks or celestial objects
- Properties
o Coarse spatial resolution is sufficient (5-25 deg)
o Stacking of photoreceptor membrane is required to increase photon catch
o Reduced angle of capture for each receptor
o Adding photoreceptors to pigment cup - ability to catch photons - Class 3 tasks require multiple photoreceptors that monitor light changes in diff directions
o Pigment cups or pits are sufficient for class 3 tasks
12
Q
class 4 - visual tasks based on high spatial resolution
A
- Detecting prey and pursuing them
- Evasion of predators
- Mate selection
- Orientation or navigation using fine-scale landmarks
- Recognition of objects & inds of same/diff species
- Visual communication
- Properties
o Require fine spatial resolution (1 arc min or 1/60 deg)
o Needs an optical system that can focus light
o Very small angle of light capture for each receptor - Class 4 tasks require the evolution of a lens (or other focussing system) and integration times need to be short to avoid motion blur
13
Q
evolution of visual behaviours
A
- Each higher level of task requires
o faster integration times
o Narrower angular selectivity
o Higher contrast sensitivity - This is aided by membrane stacking & focussing optics
14
Q
phylogeny of visual behaviours
A
- 4 phyla have developed high resolution vision (spiders, insects/crustaceans, cephalopods and vertebrates)
o Embryonic development of these groups is something quite different - This indicates that spatial vision has evolved independently in diff animal groups
15
Q
diversity of eye design
A
- Spatial vision can evolve in different ways
o More photoreceptors are added to the same pigment shield or cup
o The entire organ is multiplied - Diff modes of image formation used in both chamber & compound eyes
o Shadow eyes - light falls directly to retina
o Refraction eyes - bought to focus on an ind receptor
o Reflection eyes - light enters eye, bounces off reflective surface to be bought to a focus on an ind receptor
16
Q
how long does it take to evolve an eye?
A
- Assuming a continuous selection for improved spatial resolution, a patch of light-sensitive epithelium can be transformed into a fully focussed camera-type eye in less than 400,000 generations (or similar years in small invertebrate)
- Computational models looking at improved acuities
- Nilsson & Pelger (1994)
17
Q
primate evolution
A
- In each clade, a few representative present-day species and other groups are listed
- Primates are part of the Euarchontoglires radiation (4)
o 3 subdivisions (a,b,c)
o Primates in subdivision c - Time of the vertical axis is in millions of years ago (mya)
- 14 families and over 350 species of primates
- Primates 55-60mya
o 55-65 - Divergences
- Last common ancestor of monkeys & apes 25mya
- Las common ancestor of chimps & humans 6-8mya
- 350 species of primates - hugely successful species
18
Q
sims/diffs in primates evolution
A
- Primates evolved over 65mya
o Oldest fossil found in china in 2013 - Early primates small & most likely nocturnal
- Fed on insects, small vertebrates, fruits and buds & had good control of visuomotor hand movements
- Bush babies (galagos) & mouse lemurs may have changed body form least from early primates
- Many primates formed social groups to support diurnal living and aid protection
o Tarsier - reverted to nocturnal living
o Nocturnal vertebrates use a tapetum to increase light capture - lost in tarsier when it became diurnal
o Increases eye size to improve photon catch
o Tarsier eyes same as orangutan & equal to brain size - Line of apes diverged 6-8mya to form chimps & bonobos and hominins (including modern humans)
- Homo habilis emerged 2mya –> homo erectus (1.7mya) –> homo sapiens (0.25mya)
o Neanderthals died out 35,000ya
o Also another group - Denisovans
o Denisovans - share 3.5% of DNA w/humans and died out ~ 14.5Kya - All have forward facing eyes & well-formed digits
o Forward facing - overlap of visual fields, binocular vision, allows for depth perception - Huge variation of body size
19
Q
do primates have big brains?
A
o Not really - large brains appear several times in mammalian radiation
o e.g. dolphins, elephants
o Afrotheria - elephant and manatee have large brains
* Human brain = large primate brain
20
Q
are primate brains relative to body size
A
- Brain size usually related to body size in animals
o Encephalization - ratio of brain weight to body weight - Encephalization quotient (EQ) = measure of relative brain size defined as the ratio between actual brain mass and predicted brain mass for an animal of a given size
o Residual factor once you take out body size - is it what you expect to see - Linear relationship in primates
o But humans (homo) appear to be exception
o Humans appear to sit above the line - sits above the line. Higher EQ than expected for a primate of their body size
o Proposed as basis for superior cog abilities - taken as an indirect measure for intelligence - May be a basic shared plan for mammalian brains as all follow a similar pattern
- Recent ev shows that mammalian brains of diff sizes aren’t similarly scaled-up or down versions of a shared basic plan (Herculanao-Houzel, 2009)
21
Q
do primate brains have more neurons
A
- Diffs in cellular scaling rules dictate that
o Primate brain size increases in proportion to number of neuronal cells (e.g. 11x larger brain = 10x more neurons and 12x more non-neurons)
o Rodent brains increase in size faster than they gain neurons - Primate brains have a larger number of neurons than rodent brains of similar size
o e.g. capybara vs capuchin monkey
o Capybara brain weight 76g, 1600M neurons
o Capuchin monkey brain weight 52g, 3690M neurons - Changes in neural density probably underlie differences in cognitive ability
22
Q
what does an early mammalian brain look like
A
- Small brain with little neocortex
- Dominated by structures that process olfactory info
- Hippocampus nearly large as all neocortex
o Critical for spatial mem - Small neocortex dominated by a few areas for processing sensory info (V1, A1, S1), that characterise the brains of all or nearly all mammals
- Has a primary motor area (M1) - not found in non-placental mammals
o e.g. marsupials and monotremes - Small cap of neocortex of tenrecs has ~15 functionally distinct areas of cortex, most of which have also been identified in members of other major clades of mammals
23
Q
what is special about primate brains
A
- Always have a lateral fissure and calcarine fissure
o Can be used to distinguish if a specific brain is a primate brain or not - Show a distinct pattern of layers in LGN
- 2 physiologically distinct pathways - magnocellular and parvocellular
- Visual areas of brain highly connected - up to 305 known pathways in macaque (van Essen et al., 1992)
- 32 different visual regions, making up about 50% of neocortex (van Essen et al., 1992)
o Highly connected to each other - M&P pathways
24
Q
are retinal projections the same as other species
A
- In many species: K & M classes dominate, P class small %
o In primates: 80% of ganglion cells are P class - In mammals, most RGCs project to superior colliculus (SC), but in primates nearly all RGCs project to the LGN, with virtually all P cells following this pattern
- The role of P cells has dramatically changed - primary target now LGN and not SC
- SC in primates connect to motor neurons in the brainstem that mediate eye & head movements, so that central vision is directed toward objects of interest
25
visual cortex
| primates
* Early primates characterised by large visual cortex divided into number of areas
o Large number of cortical areas dedicated to vision
* All primates have large primary visual area (V1)
* V1 in primates is 2-3 times larger than expected for mammals of similar body size
o V1 expanded up in primates (relative to other mammals)
* Primates devote 1/3-1/2 of V1 to the central 10 degrees of vision - more pronounced in diurnal primates (Rosa et al., 1997)
* All primates have a similar projection pattern from LGN layers to V1
* Primates & tree shrews have orderly arrangement of orientation-selective neurons in V1 (Bosking et al., 1997)
o As this arrangement is not found in rodents, systematic arrays of orientation selective "columns" of neurons likely evolved in the common ancestors of tree shrews & primates
* Primates are unusual in that LGN inputs related to the contralateral & ipsilateral eyes terminate in layer 4 forming ocular dominance columns
* V1 gets bigger with brain size, but only up to great apes
o Humans - V1 reduced in size relative to other primates
* Larger human brain has V1 of similar size to a chimpanzee - V1 size in humans actually decreased relative to other primates
* V1 of primates differs to other mammals by having more neurons for its size
o Increased density
* High densities of small neurons provide a framework for preserving the details of visual scenes
* V1 specialised for detailed vision in all primates, especially Old World monkeys, apes and humans
26
barton (1998)
| associations of brain size and visual specialisation
* Brain volume data available for 34 species
o V1, LGN & neocortex
* Brain data available for 14 species
o Number of neurons & volume of separate M and P layers of LGN
* Need to show that similar regimes of selection produce similar traits in separate taxa, i.e. the traits have evolved together in different lineages
* This discriminates between common inheritance and independent evolution
* To answer this we need to calculate
o 'relative size' of a brain structure: contrast the volume of brain structure of interest (e.g. V1) with rest of the brain [brain-(neocortex+LGN)]
o 'encephalisation'
* Are visual brain structures disproportionately expanded in species with large brains?
o Are there particular pathways to cause this?
* as brains get bigger, both V1 and LGN get bigger
* p-cells in LGN +vely corr encephalisation
* not true for M cells
* the identity between the ecological correlates of P LGN size in primates
* Correlations between social group size & neocortex size have been interpreted as ev for evolutionary selection on social cog
* P processing of fine details of dynamic social stimuli, including facial expressions, gaze direction, posture, and subject/object interaction, likely to occur in extra-striate areas such as inferotemporal cortex (IT)
* The expansion of neocortex appears to be associated with changes in a functionally specific visual pathway which is linked to P neurons in LGN
27
why so many areas - the case for specialisation
* Primate extra-striate cortex contains large number of smaller specialised visual areas - good example in MT (called V5 in humans)
* MT easily identified in primates by its projection pattern from V1 - dominated by relay of the M-cell pathway through V1
* Found in all studied primates, including humans, but not in tree shrews, rodents, or rabbits
* Visual area that first evolved in early primates
* Lesions to MT lead to severe deficits in perceiving visual motion
* Microstimulation of MT produces visual motion perception
28
zihl et al. (1983)
o A patient with bilateral lesions to MT can no longer perceive motion - called akinetopsia
o Static objects appear at various locations along their trajectories
29
beckers & Homberg (1992)
o Selective disruption of V5 (homolog of MT) interferes with the ability to perceive motion
o Introduction of TMS - performance drops to chance
30
functional specialisation in the primatebrain
* Primate brain contains 2 pathways beyond V1:
* Ventral stream - 'what' - object recognition
o Main input = P system
o Temporal lobe - specialised areas for different sorts of objects e.g. faces
* Dorsal stream - 'where' - spatial perception and motor planning
o Main input = M system
o Parietal lobe - many specialised areas for using vision to guide actions in space
31
the what pathway and neurons in IT cortex
* Cells in IT cortex of macaque brain respond to faces from same species and to related stim (Gross et al., 1972)
* Same species - robust neural response
o Likes that particular stimulus (preference)
* Also faces of other species
o Human face - similar features
* As you get less away from a living face - responses get poorer (activity levels)
* The human brain has a region of extra-striate cortex that is specifically and reliably activated by faces (Spiridon et al., 2006)
* Fusiform face area = FFA
* Doesn’t matter if forward of side profile - even if not humans - activations in FFA
* Specificity in human brains for face like stimuli
* Areas that become specialised down the visual pathway
32
colour vision - how does it work
* Under scotopic (dark) conditions we don't perceive colour
* At scotopic light levels only rod photoreceptors function - single photopigment (rhodopsin)
* Lights of different wavelengths can elicit the same photoreceptor response
* Different wavelength-intensity combinations can produce identical responses (univariance)
* Single photoreceptor type (e.g. rods) is not adequate for signalling differences in wavelength
* >1 receptor type
* Diff wavelengths of light now produce diff patterns of activity across receptor population
* --> need more than a single receptor type to mediate colour discrimination
* Combine outputs of 3 receptors, 3 pigment (trichomatic), discrimination of 10m colours
* Most objects we look at are composed of a range of wavelengths
* If 2 sets of lights produce same combined response = metamers
* Will appear identical - mixture of red and green indistinguishable from yellow light
* Physical light not altered: combination results in same neural response
* More photopigments you add, fewer metamers experienced
* Perceived colour of objects result of wavelengths in the illuminant & reflectance properties of objects surface
* Changes in either can --> change in colour appearance
* Spectral content of light reaching eye determined by the surface reflectance multiplied by illuminant
* Outputs of diff cone types are selectively computed at level of RGCs to create opponent pathways
* Colour perception is based on output of these mechanisms
* Achromatic system [L+M] signals the intensity of the light
33
herring's idea of opponent colours
* colours can be represented by the 2 pairs of opposing colours
o Red/green
o Blue/yellow
34
detection
selective absorption of electromagnetic radiation (photons) using cone photoreceptors containing unique photopigment
35
discrimination
relative activity across diff phone types necessary to discriminate between lights of diff wavelength
36
colour appearance
recombination of colour signals from retinal photoreceptors to create colour-opponent mechanisms
37
colour vision useful in
* foraging for food
mate selection
38
variations in colour perception
* marine mammals: single M/L cone, monochromatic
* terrestrial mammals: S & L cone, dichromatic
* old world primates: S, M & L, trichromatic
39
bees
* Bees have photopigments that extend into ultraviolet range
* Appearance of flowers (b) - in this case black-eyed Susans - relative to the image we perceive (a) can be dramatically altered
* May offer a visual advantage to bee (larger central target)
40
evolution of colour vision in primates
* Colour visions shows diversity within the primate lineage
* ~100m years ago: Africa & South America split due to movement of tectonic plates
* New evolutionary path for new world (South America) and old world primates
* To go from a dichromatic to trichromatic system we need
o To acquire a new photopigment with unique spectral sensitivity
o Express photopigment in class of cones
o Develop neural mechanism that compares the output to other types of cones - thought to be missing in those that use the oil droplets (earlier in notes)
* In vertebrates there are 5 distinct families of visual photopigments
41
2 subsystems which are parallel & independent at early stages of visual pathway
1. The primordial or ancient subsystem
o Shared with most mammals
o Depends on a comparison of the rates of photon catch in S and M-wave cones
o This system exists almost exclusively for colour vision
o Humans and OW Primates [S-(L+M)]
2. Depends on a comparison of the rates of photon catch in L and M-wave receptors
o At early stages of visual pathway, this chromatic info is carried by a channel that is also sensitive to spatial contrast
o Humans and OW Primates (L-M)
42
do new world primates have trichromatic vision
* To avoid over expression of genes - female mammals selectively inactivate 1 of the 2 genes on X-chromosome
* Done early in embryonic development
* Female mammals are genetic mosaics
* New world primates - only females have trichromatic vision (2/3 of population) but not in males
* Have a single gene, but it is polymorphic
o 3 alleles can be expressed (535, 550 & 563nm)
o 3 variants of this gene
* New world monkeys have taken a different route to trichromacy:
o heterozygous females gain trichromacy as a result of X-chromosome inactivation
o ensures that diff photopigments are expressed in 2 subsets of retinal photoreceptor
43
evolutionary sequence for colour vision
* Dichromatic system: blue/yellow pathways
* Polymorphisms: idea of 3 allelic forms
* The fact that howler monkeys were able to all become trichromatic by 1990s shows evidence that polymorphism of X-linked photopigment gene happened prior to the duplication of X-linked photopigment gene
44
can dichromatic animals develop the necessary neural circuits to extract new information from a trichromatic system?
* Yes
* Mancuso et al. (2009)
* Squirrel monkeys - red green colour-blindness (dichromatic), run into the problem of univariance
* Addition of a third opsin, via gene therapy, resulted in the rapid onset of trichromatic vision
45
physiology of eye movements
* Humans have 6 extra ocular muscles
o Eye position relative to orbit managed by these
* Operate in antagonistic pairs
o Abduction and adduction
o Cranial nerves
o Can lead to abnormal eye movements
* Brain circuits & innervation of extra-ocular muscles mediating eye movements
* LIP involved in guiding visual attention
46
types of eye movements
* fixational
* saccadic
* smooth pursuit
* vergence
47
fixational eye movements
* Our eyes are never stationary
* 3 types of fixational eye movement
o Ocular drift
o Tremor
o Microsaccades
* Functional role
o Control of fixation position
o Reduction of perceptual fading (Troxler effect)
o Generation of synchronised transient visual activity
o Enhancing visual acuity and scanning across visual space
48
spine drift illusion
Although this is a static image, it is constructed in a way that any eye movement causes illusory motion between the centre and the surround
49
saccadic eye movements
* Ballistic eye movement (up to 1000deg/s) that moves fixation from one point in space to another
o Eye movements are purposeful
* Make 3-4 saccades every sec
* Saccades move our fixation around interesting parts of an image, but we suppress (reduce visual sensitivity) during the saccade to eliminate motion blur
50
smooth pursuit eye movements
* Eyes follow an object moving through space to keep the object imaged on the fovea e.g. there is no net retinal motion
* Found in humans and other primates
51
vergence eye movements
* The angle of the eyes are moved in opposite directions to put a near object of interest on the fovea of each eye
* Have to move in opposite directions
* As bring object closer, got to converge eyes
52
problem: the visual world is dynamic
* Body moves --> head moves relative to body --> eyes move relative to head
* Major challenge for oculumotor system is to keep gaze stable
53
vestibular ocular reflex
* Prevents involuntary rotation of eyes relative to surroundings
* Initiated by rotation detectors in semi-circular canals (inner ear) - have cells which fire in proportion to head velocity
o Head rotations detected by SSC
* Results in movement of eyes in equal & opposite direction to head
o Counter rotations of the eyes
* VOR is fast, operates up to 10Hz
* Well calibrated
* Stabilisation mechanism which doesn’t need visual input
54
opto-kinetic response
* Uses signals from motion detectors to cancel out residual movement between image & retina
* If object moves rightwards across retina, a rightwards eye movement will limit amount of relative motion
* We 'assume' the world is stationary & we fix gaze relative to it
* Causes induced nystagmus (sawtooth movements of the eyes)
* Involves whole field stimuli & must be suppressed when pursuit eye movements track small foveal target
* Slow - effective up to 1Hz
55
primates use a saccade and fixate strategy
* See both fast saccadic movements of eye & slow movements opposite to head (VOR)
* Gaze fixations are steady - traces displaced relative to one another for clarity
56
do other animals use a saccade and fixate strategy?
* Stationary gaze & fast saccades move the goldfish eyes about 10-20 degrees
* Humans & goldfish (vertebrates) have common ancestor
* Rock crabs evolved independently of vertebrates and have diff eye design (compound)
* Rock crabs use saccades and counter rotations to produce periods of stable gaze
* Goldfish and crabs, despite evolutionary differences, use saccade and fixate behaviour
* Insects don’t have eye movements - they move their head
* Stalked eye fly rotates through 90 deg (a)
* Bodily movement is continuous
o Head and eyes rotate in saccade-like movement (e.g. 80-120 deg)
o Fly keeps gaze steady while body rotates
o Even in flight, blowflies make saccadic head movements (b)
57
flight behaviour of cerceris leaving nest
* Wasp makes sweeps in an arc pattern that increase in radius as moves away from nest - flies sideways
* Head direction changes in saccadic fashion w/ intervening periods where head & body orientation remain constant
* During periods of stable fixation, wasp records position & distances of landmarks from the nest to facilitate return (Zeil et al., 2007)
58
head bobbing in birds - translational saccades
* Do any animals limit blur from translational motion rather than gaze rotation?
o This would limit movement of visual field, w/o impeding locomotion
* Many birds achieve this using head-bobbing technique - head thrust then held for body catch-up
* If background motion is removed (treadmill) pigeons do not head-bob
o Info from retinal slip drives compensatory head movement (Frost, 1978)
* Birds with frontal eyes don't head-bob
* Smaller birds show similar beh by hopping (jump and hold phases)
* Head-bobbing gives periods of clear vision between those where vision is compromised by motion
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the consequences of image motion
* Resolution is degraded when motion blurs an image
* Easier to detect foreground motion when background is stationary
* Heading & distance info are easier to recover from pattern of retinal stimulation when translation is removed
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motion detection
* most eyes or heads dont move enough to get close to blur rule limit
* many animals have to detect moving objects against a static background
* relative motion between 2 surfaces is readily detected when rest of visual scene is static
* addition of common motion makes motion detection much more difficult (Nakayama, 1981)
* keeping eyes still ensures optimum detection of moving objects
## Footnote
compensatory eye movements may enhance sensitivity to motion parallax
61
estimating distance using flow fields
* (a) retinal velocity flow field from animal moving in a straight line (translation)
* (b) flow field from horizontal rotation
* (c) resulting flow field from combining (a) + (b)
* Centre of expansion (heading direction) is easy to determine in (a) but not (c)
* Good ev that honey bees use this info (Lehrer et al., 1988)
62
generating motion parallax movements
* Lateral translation (without rotation) allows animals to estimate the distance of objects
* Locusts & mantids (peering)
* Bees (roll movements of thorax)
* Gerbils (head bobbing)
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exceptions to the rule: scanning eyes
* Some species actively rotate their eyes in order to acquire visual information
a. Mantis shrimp - ambush predators that smash or spear prey
b. Each eye has a mid-band of enlarged facets that contains colour vision system
c. Mid-band has small field of view, so eyes need to continuously move - small (10 deg)
d. Each eye makes independent movements
* Jumping spider - has pairs of secondary eyes and large forward pointing principal eyes
* Secondary eyes used only for motion detection
* Principal eyes move (horizontal, vertical and torsional) & have a field of view that extends 20 deg vertically and 1 deg horizontally
* Scanning creates a slow (3-10 deg/s) side-to-side oscillation and torsional rotation
* Movements designed to detect the orientation of linear structures - legs
