Chapter 9 - Vision Flashcards
reception
reception of information
transduction
the conversion of information to neural activity. can be done in different ways such as electromagnetic (eye), acoustic (ear), mechanical (skin), thermal (skin) and chemical (nose, tongue)
distinction between information during coding
done in several ways such as frequency, modulation, and rhythm of activity or action potentials. another way is via the place of arrival or spatial coding
what determines how we perceive the world around us?
our brain, and not our senses.
visual field
a piece of visual space that is seen through the eyes, so it is about a space seen by a person or what a person sees
receptive field
a piece of visual space that activates a particular cell, a specific part of the world to which it responds. this is a sensory region that stimulates a receptor cell or neuron. a receptor field of a cell is the visual field of that cell.
sensory neurons
sensory receptor neurons specialize in the transduction of sensory energy, such as light, into neural activity. each sensory receptor specializes in filtering from a different energy:
- vision
- auditory system
- touch
- taste and smell
sensory receptor neurons for vision
the photoreceptors in the retina convert light into chemical energy, which in turn is converted into action potentials
sensory receptor neurons for the auditory system
pressure waves are first converted into mechanical energy, activating the auditory receptors, which produce action potentials in the auditory receptor neurons.
sensory receptor neurons for touch
mechanical energy activates receptors that are sensitive to touch, pressure, or pain, these somatosensory neurons in turn generate the action potentials in somatosensory receptor neurons
sensory receptor neurons for taste and smell
various chemical molecules in the air or in food fit themselves into receptors of different shapes to activate action potentials in the respective receptor neurons
sensation
the registration of physical stimuli from the environment by the sensory organs
perception
the subjective interpretation of sensations by the brain. perception is an idiosyncratic representation of reality, it depends on the individual and the culture
functional anatomy of the visual system
our primary sensory experience is visual. light consists of electromagnetic waves, which are visible to us between the wavelength 400 to 700nm. we can perceive light directly (staring at a lightbulb) or indirectly (reflection). the eye works like a camera but is reversed.
structure of the retina
light travels from outside through the pupil in the eye, where it falls on a light-sensitive surface called the retina in the back of the eye. from this stimulation of photoreceptor cells, a construction of the visual world is created. the image of objects in the retina is upside-down and reversed, so the brain corrects it
blind spot
a small area of the retina where blood fibers enter the eye and the axons forming the optic nerve leave the eye to bring information to the brain. the blind spot is located on the nasal side of the retina, medial to the fovea. there are no photoreceptors here.
our right eye sees the blind spot of our left eye and vice versa, so we don’t notice any gaps in our vision
fovea
center of the retina, where photoreceptors are more densely packed (no rods), so our vision is better in this area. peripheral visual field is around the fovea, predominantly consisting of rods.
photoreceptors
photoreceptor cells of the retina convert light energy first into chemical energy and then into neural activity. light reaching a photoreceptor causes a series of chemical reactions that lead to a change of the membrane potential. this potential leads to a change in the release of neurotransmitters in nearby neurons
rods
photoreceptors specilized for functioning at low light intensities and therefore important for night vision. they do not see color. rods are not present in the fovea, so they are important for peripheral vision
cones
photoreceptors specialized in color and visual acuity (fine details). overall, there are more rods than cones. the fovea only contains cones. the density of these cones drops outside the fovea. that is why vision is not so sharp at the edges of the field of vision
photosensitive retinal ganglion cells
have a synchronised circadian rhythm and regulate pupil size. they regulate the release of melatonin by the pineal gland
types of cones
there are three kinds of cones, sensitive to three kinds of light: red, green, and blue. one cone does not only react to one color but reacts extra strongly to a certain color
pigments of vision
all rods have the same pigments and each cone has one of the three pigments so this leads to a total of four pigments. these four pigments form the basis of vision
myopia
people with myopia can see things well if they are up close but can not see well from far away
the focal point of light falls short of the retina, eye is too long
hyperopia
people can not see up close, but they can see well from a distance
the focal point of light falls beyond the retina, eye is too short.
types of retinal neurons (2 layers)
first layer contains three types of cells; bipolar, horizontal and amacrine cells. horizontal cells connect photoreceptors to bipolar cells, while amacrine cells connect bipolar cells to cells in the second neural layer; the retinal ganglion cells (RGCs)
two types of retinal ganglion cells
- large magnocellular (M)
- small parvocellular (P)
magnocellular cells
found all throughout the retina and receive their input mainly from rods and are thus sensitive to light and movement but not color. send information to layers 1 and 2
parvocellular cells
are found mostly in the fovea and receive their input mainly from cones, and are therefore sensitive to colors and fine details. send information to layers 3, 4, 5, and 6
visual pathways
from both eyes, the optic nerve travels to the brain. the optic nerve partly (about half of the fibers) crosses the optic chiasm. after the junction, is called the optic tract because it has entered the brain. the medial pathway of each retina crosses to the other side with the lateral pathways stay on the same side
two main pathways to the visual brain
they lead from the eyes to the visual cortex in the occipital lobe, namely the geniculostriate pathway for processing the image of the object and the tectopulvinar pathway for directing rapid eye movements. another smaller path (retinohypothalamic tract) leads to the hypothalamus
geniculostriate system
all P-ganglion axons and some M-ganglion axons form a pathway called the geniculostriate system. this pathway leads from the retina to the lateral geniculate nucleus of the thalamus and then to layer IV of the primary visual cortex (V1/striate cortex) in the occipital lobe.
lateral geniculate nucleus
each LGN consists of six layers. layers 2, 3, and 5 receive input from the ipsilateral eye (same side), and layers 1, 4, and 6 receive input from the contralateral eye (opposite side):
- information that crosses is in the V1 in 1, 4, 6
- information that does NOT cross is in the V1 in 2, 3, 5
spatial coding
different aspects of vision remain separable in the brain. e.g. segregation of visual input remains in the ovular dominance columns in layer IV of the striate cortex.
tectopulvinar system (where system)
the remaining M cells form the second path. these cells send their axons to the superior colliculi in the tectum, which connects to the pulvinar region of the thalamus, which sends connections directly to the parietal and temporal lobes
retinohypothalamic tract
a small proportion of RGCs are unique because they behave like photoreceptors. involved in the circadian rhythm and in the pupillary reflex that expands or contracts the pupil in response to the amount of light present in the environment
dorsal and ventral visual flows
two distinct visual pathways originate from the striate cortex in the occipital lobe.
ventral stream leads to the temporal lobe. linked to stimulus identification (what function) dorsal stream leads to the parietal lobe. linked to the controlling movements to or from the stimulus (how function)
occipital lobe
consists of 6 visual regions: V1, V2, V3, V3A, V4, and V5. V1 or primary visual cortex, is the striate cortex in the occipital lobe that receives input from the lateral geniculate nucleus. regions V2 through V5 from the extrastriate cortex.
3 types of information
visual cortex receives 3 types of information, which are spatially coded:
- color
- form
- movement
blob
a region in V1 that contains color sensitive neurons. in V1 there is a blob pattern
interblob
sensitive to form and movement
V2 has a striped pattern (types)
- thick stripes: input of movement-sensitive neurons
- thin stripes: input of color-sensitive neurons
- interstripes: input of form-sensitive neurons
visual field
the part of the visual field that is visible without moving your head is your visual field
corpus callosum
we perceive the world as a whole instead of two halves, corpus callosum connects the two sides of the visual field
types of ganglion cells
donut shaped receptive field
- on-center cells
- off center-cells
luminance contrast
the RGCs tell the brain about the amount of light hitting a certain spot on the retina compared with the average amount of light falling on the surrounding retinal region
simple cells
simple V1 cells have an on-off arrangement and respond to line segments with a certain orientation
hypercomplex cell
reacts maximally to movement but also has a strong inhibitory area at the end of the receptive field. hypercomplex cells are like complex cells but they fire much less if there is no light at the end of the receptive field
complex cells
maximally excited by light moving in a certain direction through the visual field. complex V1 cells react to moving line segments with a certain orientation
stimulus equvilence
recognizing an object as remaining the same despite being viewed from different orientations
trichromatic theory of Helmholtz
seeing color is determined by the relative reaction of the corresponding cone types. when all 3 types - red, green, blue - are equally active, we see white.
orientation columns
neurons respond to the same line orientation
ocular dominance columns
neurons receive input from the left and right eye
prosopagnosia
people cannot recognise faces due to a lesion in the fusiform face area
scotoma
small blind spots in the visual field, often because of V1 lesions
blindsight
complete loss of striate cortex in the geniculustriate system (what) with an intact tectopulvinar system (where)
quadrantanopia
blindness in a quarter of the visual field
homonymous hemianopia
blindness in one visual half-field. this is caused by lesion in the optic tract
