Chapter 6.2 Flashcards
the retina converts light…
to neural signals, conducts them to the CNS, and participates in the processing of the signals.
The retina is composed of five layers of different types of neurons
receptors, horizontal cells, bipolar cells, amacrine cells, and retinal ganglion cells.
Amacrine and horizontal cells
specialized for lateral communication (communication across the major channels of sensory input).
Retinal neurons
communicate both chemically via synapses and electrically via gap junctions.
the retina is inside out
has to pass through four layers to reach the receptor layer, then once the receptors have been activated, the neural message is transmitted back out through the retinal layers to the retinal ganglion cells, whose axons project across the inside of the retina before gathering together in a bundle and exiting the eyeball. 2 problems:
inside out prob 1
incoming light is distorted by the retinal tissue through which it must pass before reaching the receptors
minimized by the fovea, which is an indentation, about .33 cm in diameter, at the center of the retina; it is the area of the retina that is specialized for high-acuity vision. The thinning of the retinal ganglion cell layer here reduces the distortion of incoming light
inside out prob 2
for the bundle of retinal ganglion cell axons to leave the cell axons to leave the eye, there must be a gap in the receptor layer; this is called the blind spot.
completion (blind spot)
visual system uses info provided by the receptors around the blind spot to fill in the gaps in your retinal images.
Surface interpolation
the process by which we perceive surfaces; the visual system extracts info about edges and from it infers the appearance of large surfaces
when you look at an object
your visual system extracts key info about the object, primarily info about its edges and their location, and conducts that info to the cortex, where a perception of the entire object is created from that partial info.
Duplexity theory of vision
the theory that cones and rods mediate different kinds of vision.
Photopic vision (cone-mediated vision)
predominates in good lighting and provides high acuity colored perceptions of the world.
Scotopic vision (rod-mediated vision)
lacks both detail and color that photopic vision provides.
differences between photopic and scotopic
in scotopic system, the output of several hundred rods converge on a single retinal ganglion cell, whereas in the photopic system, only a few cones converge on the retina ganglion cell.
dim light
simultaneously stimulating many rods can summate (add) to influence the firing of the retinal ganglion cell onto which the output of the stimulated rods converges whereas the effects of the same dim light applied to a sheet of cones cannot summate to the same degree and the retinal ganglion cells may not respond at all to the light.
less ambiguity in photopic system
of the location of a stimulus in the photopic system over the scotopic system where input from hundreds of rods makes it impossible to know which portion contributed to the change.
fovea
no rods; only cones. Density of rods reach a maximum at 20 degrees from the center of the fovea; more rods in the nasal hemiretina (half of each retina next to the nose) than in the temporal hemiretina (half next to the temples)
Spectral sensitivity curve
graph of relative brightness of lights of the same intensity presented at different wavelengths
Photopic spectral sensitivity curve
can be determined by having subjects judge the relative brightness of different wavelengths of light shone on the fovea. Maximally sensitive to wavelengths of about 560 nm so a light at 500 nm would have to be much more intense than one at 560 nm to be seen as equally bright.
Scotopic spectral sensitivity curve
can be determined by asking subjects to judge the relative brightness of different wavelengths of light shone on the periphery of the retina at an intensity to low to activate the few peripheral cones located there. Maximally sensitive to wavelengths of about 500 nm so a light of 560 nm would have to be much more intense than one at 500 nm to be seen as equally bright.
Purkinje effect
red flowers appear brighter in daylight over blue ones and blue ones appear brighter (brighter shade of grey) than red ones at night
why don’t we see a colored dot at the center of our visual field surrounded by grey?
our visual perception at any instant is a summation of recent visual info because of temporal integration.
fixational eye movements
involuntary; eyes continuously move even when we try to keep them still; three kinds: tremor, drifts, and saccades.
Important because when eye movements are blocked, visual objects begin to fade and disappear as most visual neurons respond only to changing images.
transduction
the conversion of one form of energy to another. Visual transduction is the conversion of light to neural signals by the visual receptors.
