Insect Eyes and Vision Flashcards
2 major eye kinds
single lens (camera)- vertebrates, molluscs and spiders
compound (many individual lenses)- insects, crustacea, some analids
wide fields of vision; performance limited by small size of individual lenses
evolution
development of different types of eyes driven by evolution
cambrian explosion
arms race
photoreception using rhodopsins ancient property
different eye designs use same control genes
common challenges of seeing things
wide range of light intensities
detecting contrasts in time(movement) and space (objects)
Ramon
first drew peoples attention to this by comparing optic lobe of a honey bee with retina of a mammal
both show orderly progression of one layer to the next but also connections that go across for areas that are looking at different areas of space
good example of convergent evolution
visual behaviours include
stabilising course and body oreintation
chasing mates and prey
avoiding collisions and being caught
Flow field
coherent patter of movement from visual field when moving
compound eyes- locust
about 8500 individual 6 sided lenses per eye
pseudopupil
one lens is basic functional unit (ommatidium)- sample one point in space
8 photoreceptors per eye
Rhabdom light guide
contains photopigment which triggers phototransduction
pigment in regularly spaced finger-like microvilli
compound eye can fit more small lenses- more pigments
smaller lenses give worse images
animal can look in several directions at once
optic lobe= series of neuropile layers each containing several types of neuron
photoreceptor responses and coding
one photoreceptor experiences change in light intensity
receptor potential- electrical responses to light
depolarizes when light increases; hyperpolarizes when decreases
dark adapted photoreceptor
photon bumps- about 2mV
adaptation enables visual system distinguish objects by differences in the proportion of light they reflect- their contrast
Fast and slow flies
flesh fly in very bright light its photoreceptors respond quickly to changes in light
crane fly slow responses but sensitive to low levels of light
Lamina Neuron responses
most photoreceptors synapse with lamina neurons
in lamina neurons electrical signal is inverted, very phasic, helps pick out moving objects
neither photoreceptors nor lamina neurons make spikes
Neurons in lamina and beyond
photoreceptor- lamina- medulla- lobula
lobula- large neurons with fan shaped dendrites
combine info from many columns
respond to specific behaviourally relevant stimuli
Lobula Giant Motion Detector (LGMD) and DCMD- both excited in response to directly approaching stimuli
each LGMD spike triggers DCMD spike
in flying adult trigger drive connects with jump system
Collision warning
objects approaching on direct collision course
selects or filters images of objects directly approaching
feature detector
images distinguishing approaching from receding stimuli; not changes in brightness, edges must move, crossed lines, expanding rectangle, edges moving faster, race between image expansion and lateral inhibition
Borst et al (2009)
investigates the local motion preferences of 2 descending neruonsof the ocellar and vertical system; DNOVS1 and DNOVS2
• V2 output is necessary for DNOVS2 to differentiate between a roll rotation and a lift transition
• Integration from different receptive cells important
Fly vision
fly moving forward, eyes see a coherent pattern of movement as it goes past things
left and right flow field is very similar
gust of wind- makes fly twist around- images that left and right eye see moves, flow field is now different as the images moving over each eye are different
If fly rolls- different pattern of flow field
according to what happens on flow field over eyes fly will get different experience
Movement detecting neurons
Lobula Plate Tangential Cells (LPTCs) take signals from many columns
all selective to particular directions of flow field motion
suitable for mediating optomotor responses
Horizontal cells
respond to movements in the horizontal plane
H1
likes movement from back to front of fly on side of own eye
Vertical Cells
respond to movements in the vertical plane
sometimes respond to a combination of vertical movement
VS3
likes movement as if world was flowing from top to bottom of eye
VS5
pretty pure roll detecting neuron- likes when whole visual field is moved from bottom to top
VS8
likes fly pitching- image rotates around eye from one side bottom up and back down other side
How does sensitivity for movement direction arise?
single photoreceptor signals change in light
cannot distinguish flicker from movement, or movements from different directions
sequence of activation of different photoreceptors
Elementary Motion Detectors (EMDs)
signals from 2 photoreceptors are combined
neuronal and stimulus delay should match each other
if stimulus is going at the right speed the two signals will reach the next neuron at the same time signalling movement at particular speeds
only sensitive to one direction
Barlow and Levick (1950s)
though there was inhibition instead at the synapse from the neuron that is delayed
Drosophila
in each ommatidial column, 8 neurons carry excitation from medulla to LPTCs
these neurons likely to be output elements of EMDs
1 set responds to light-dark and the other to dark-light
each set includes 4 neurons
each has preferred direction of movement; up, down, forwards and backwards
