Visual Physiology Flashcards
Compare rods and cones as to: distribution, numbers, pigment, ability to discriminate color, acuity, wiring to ganglion cells, temporal resolution and light sensitivity.
rods are more numerous overall in the eye, concentrated outside of fovea, have a many rod to one ganglion cell wiring (termed convergent wiring), higher light sensitivity, lower acuity, only one photpigment (rhodopsin), poor temporal resolution
cones have three color pigments, concentrated at fovea, high acuity and temporal resolution, less overall number, one-to-to wiring withe ganglion cells, low sensitivity to light
Compare and contrast scotopic, mesopic and photopic vision with regard to receptors involved, acuity, color perception.
scotopic is low light with rods only, low acuity, no color perception
mesopic vision is somehwat increased light intensity with both rods and cones functioning, some color vision and poor but improved acuity
photopic vision is full light, rod system is saturated and not working, vision is all cones with high acuity and color vision
State whether a photoreceptor’s membrane potential hyper- or depolarizes when illuminated and the effect of this change in polarization on the amount of transmitter released.
light produces hyperpolarization of photoreceptors which reduces the amount of transmitter released
cells normally depolarize via cGMP-gated sodium channels, activated photopigment closes these channels and leads to hyperpolarization
Compare and contrast the direct and the modulated pathways of information flow in the retina.
direct pathway involves rods and cones synapsing directly with bipolar cells, whether the transmitter causes IPSP or EPSP is determined by wheter the photoreceptor is part of an on- or off-center pathway (respectively); biploar cells synapses directly with ganglion cells, EPSP, has a slow basal firing rate; optic nerve fibers synapse in lateral geniculate body, superior colliculus, and pretectal olivary nucleus of midbrain
Modulated pathway: horizontal cells go between photoreceptors and bipolar cells
Describe the firing rate of a retinal ganglion cell with an on-center, off-surround receptive field when uniformly illuminated, when the on-center only is illuminated and when the off-surround only is illuminated.
short answer: uniform illumination causes no change in firing rate, on center only olluminated causes highest firing rate, off center only illuminated causes decreased firing rate (the whole point is it increases contrast)
on-center off-surround field is how the on-center cone ultimately excites the bipolar cell (light depolarizes center cone causes less inhibitory NT release) whereas light on off center cones inhibits bipolar cell (light causes off center depolarization, less excitatory NT on the horizontal cell, less inhibitory NT on center cone, center cone releases more inhibitory NT on bipolar cell, less ganglion cell firing)
key:
ganglion cell fires when given excitatory NT from bipolar cell
bipolar cell recieves inhibitory NT from on center cone but always fires at a basal rate
on center cone always secretes inhibitory NT on bipolar cell
light hyperpolarizes all cones
off center cones releases excitatory NT to horizontal cells
horizontal cells release inhibitory NT on center cone
Describe what happens to a on-center off-surround ganglion cell’s firing rate as a light-dark boundary passes across its receptive field.
when it’s uniformly illuminated there’s no change in firing rate, as incident light crosses the off center border firing rate decreases, when light hits all of on center but just some of off center the firing rate is increased, and when entire field is illuminated there’s no change in firing rate
Explain why an individual cone can’t distinguish color.
because one cone can only send a signal of firing rate in response to only one absorption curve
ie, the brain can’t tell if the cone is firing at a certain rate because it’s absorbing light of intensity x at wavelength x or at intensity y of wavelength y
thus, 3 cones allows the brain to compare different intensity responses to the same wavelength and derive from the different intensities which color the light is
red and green cones have the most overlapping absoprtion spectra, and they populate the central fovea. This avoids chromatic abberation, which is the process of an image blurring as different wavelengths of light refract at different angles (like how a prism separated the rainbow). (ie, if blue was at the central fovea the image would be blurry)
Explain colorblindness.
the lack of a color cone makes someone unable to distinguish that color from the color of the overlapping cone they do have
red-green colorblindness is most common
