Chapter 6
Vision
Introduction
Sensory receptors – a specialized neuron
that detects a particular category of
physical events
Sensory transduction – the process by
which sensory stimuli are transduced into
slow, graded receptor potentials
The Stimulus
The perceived color of light is determined by 3
dimensions:
– Hue – the dominant wavelength
– Saturation - purity
– Brightness - intensity
Anatomy of the visual system
Eyes –
– Suspended in the orbits of
the skull
– Held in place by 6
extraocular muscles
– Retina – the neural tissue
and photoreceptive cells
located on the inner
surface of the posterior
position of the eye
– Fovea – the region of the
retina that mediates the
most acute vision; color
sensitive cones constitute
the only type of
photoreceptor here
Anatomy of the visual system
Eyes
– Photoreceptor – one of the receptor cells of the retina;
transduces photic energy into electrical potentials
Rod – sensitive to light of low intensity
Cones – maximally sensitive to one of 3 different wavelengths of
light and hence encodes color vision
– Optic disk – the location of the exit point from the retina of the
fibers of the ganglion cells that form the optic nerve; responsible
for the blind spot
– 3 types of movements:
Vergence movements – the cooperative movements that keep both
eyes fixed on the same target
Saccadic movements – rapid, jerky movements of the eyes used in
scanning a visual scene
Pursuit movement – the movement made to maintain an image of a
moving object on the fovea
Photoreceptors
Each photoreceptor consists of
an inner segment and an outer
segment, which contains
several hundred lamellae (thin
plate of membrane)
Visual info transduction
Photopigments are special
molecules embedded in the
lamellae (e.g. rhodopsin) and
consists of 2 parts:
– Opsin – protein
– Retinal – lipid
When a molecule of rhodopsin
is exposed to light it breaks
into its two parts, and this
causes a change in the
membrane potential of the
photoreceptor, which changes
the firing rate of glutamate
Visual info transduction
The membrane of photoreceptors is
different from others – the cation
channels (sodium and calcium) are
normally open
In the dark these ion channels are
open, and so the photoreceptors
continually release glutamate
When light strikes the photopigment,
the G protein transducin is activated,
which then activate the enzyme
phosphodiesterase which closes the
ion channels; this lowers the rate of
glutamate release
Light hyperpolarizes the
photoreceptor and then depolarizes
the bipolar cell
Connections between eye and
brain
The axons of the retinal ganglion cells project through the optic
nerves, cross over through the optic chiasm to the dorsal lateral
geniculate nucleus (LGN)
The LGN consists of 6 layers of neurons: the inner 2 layers are
called the magnocellular layers and the outer 4 are called the
parvocellular layers
The cells in the LGN project through the optic radiations to the
primary visual cortex (aka striate cortex)
Diagram of visual pathways
Coding of light and dark
The receptive field of a neuron in the visual system is the part of the
visual field that an individual neuron “sees”, i.e. the part in which
light must fall for the neuron to be stimulated
2 major types of retinal ganglion cells, ON center and OFF center
cells
– ON center cells are excited by light falling in the center of the field
(center), and inhibited by light falling in the surrounding field (surround)
– OFF center cells are excited by light in the surround, and inhibited by
light in the center
Coding of light and dark
The center-surround organization of the retinal ganglion cells
enhances our ability to detect the outlines of objects
Coding of color
The retinas of humans, Old World monkeys and apes contain 3
different types of cones which provide us with an elaborate form of
color vision
All visible colors (for humans at least) can be mixed from the 3 main
colors: red (long), green (medium), and blue (short); due to the
wavelengths absorbed by the 3 different cones (trichromatic theory)
Genetic defects can cause one or more of the 3 types of cones to
not function properly, resulting in either protanopia, where red and
green are confused because the red cones respond to green;
deuteranopia, where where red and green are confused also, but
because the green cones respond to red; or tritanopia, where they
lack blue cones
Anatomy of the striate cortex
Consists of 6 principle layers, arranged in bands parallel to the
surface
The striate cortex of one hemisphere contains info from the
contralateral visual field
Approx. 25% of the striate cortex is devoted to anlaysis of info from
the fovea
Neurons in the visual cortex selectively respond to specific features
of the visual world, not just to light
Orientation and Movement
Simple cell – an orientation-sensitive neuron whose receptive field is
organized in an opponent fashion
Complex cell – a neuron that responds to the presence of a line
segment with a particular orientation located within it s receptor field,
especially when the line moves perpendicularly to its orientation
Hypercomplex cell – a neuron that responds to the presence of a
line segment with a particular orientation that ends at a particluar
point within the cell’s receptive field
Spatial Frequency
Neurons in the primary visual cortex respond best to sine-wave
gratings, which are a series of straight parallel bands varying
continuously in brightness according to a sine-wave function, along
a line perpendicular to their lengths
A sine-wave grating is designated by its spatial frequency, or the
relative width of the bands, measured in cycles per degree of visual
angle
The most important visual information is that contained in low spatial
frequencies
Retinal disparity
Binocular vision (i.e. from 2 eyes) provides a vivid perception of
depth through the process of stereoscopic vision, or stereopsis
Most neurons in the primary visual cortex are binocular, they
respond to info of either eye
The cells respond most vigorously when each eye sees a stimulus
in a slightly different location, called retinal disparity
Color
In the striate cortex, info from color-sensitive ganglion cells is
transmitted through to special cells grouped into cytochrome
oxidase (CO) blobs
Modular organization of the striate
cortex
The striate cortex is divided into ~2500 modules, each containing
~150,000 neurons
The neurons in each module are devoted to the analysis of various
features contained in one very small portion of the visual field, and
combine together to form a complete whole
Info from the layers of the LGN project to the different layers of the
modules
The modules consist of 2 segments, each surrounding a CO blob
– The neurons in the CO blobs are sensitive to color and low spatial
frequencies
– The neurons outside of the blobs are sensitive to orientation,
movement, spatial freq, texture and binocular disparity
Role of the visual association
cortex
Two streams of visual analysis
– Visual info receive from the striate cortex is analyzed in the visual assc
cortex
– Neurons in the striate cortex project to the extrastriate cortex, which
surrounds the visual assc cortex
– The primate extrastriate cortex consists of several specialized regions
that respond to particular features of a visual stimulus
– Contains 2 streams of analysis:
Dorsal stream – a system of interconnected regions of visual cortex involved
in the perception of spatial location, beginning with the striate cortex and
ending with the posterior parietal cortex
Ventral stream – a system of interconnected regions of visual cortex
involved in the perception of form, beginning in the striate cortex and ending
in the inferior temporal cortex
Perception of color
Color constancy – the relatively constant appearance of the colors of
objects under varying lighting conditions; the visual system
compensates for the source of light when process visual information
about colors
Achromatopsia – inability to discriminate among different hues;
caused by damage to the visual assc cortex
Analysis of form
In primates the recognition of visual patterns and identification of particular objects
take place in the inferior temporal cortex, located at the end of the ventral stream on
the ventral part of the temporal lobe
–
–
Analyses of form and color are put together here and perceptions of 3D objects and
backgrounds are achieved
Consists of 2 major regions: TE and TEO
Damage to the human visual assc cortex can result in a visual agnosia, which results
in an inability to perceive or identify a visual stimulus
–
–
Apperceptive visual agnosia – failure to perceive objects, even though visual acuity is normal
(e.g. cannot name an object by looking at it, but can if allowed to touch it)
Prosopagnosia – failure to recognize particular people by the sight of their faces (i.e. can
recognize by voice, hair color, etc.)
Fusiform face area – region of the extrastriate cortex located at the base of the brain;
involved in perception of faces and other complex objects that require expertise to
recognize
Associative visual agnosia – inability to identify objects that are perceived visually,
even though the form of the perceived object can be drawn or matched with similar
objects; appears to involve difficulty in transferring visual info to verbal mechanisms
Perception of movement
Area V5 of the extrastriate cortex contains neurons that respond to
movement, and show directional sensitivity
An area adjacent to V5 receives info from V5 about movement and
respond to more complex movement features, such as radial,
circular or spiral motion
– Optic flow – the complex motion of points in the visual field caused by
relative movement b/t the observer and env’t; provides info about the
relative distance of objects from the observer and of the relative
direction of movement
Akinetopsia – inability to perceive movement, caused by damage to
area V5 of the visual assc cortex
Perception of spatial location
3 subareas of the extrastriate cortex send info through area V5 to
the parietal cortex, which is involved in spatial perception
Damage to this area disrupts performance on tasks that require
perceiving and remembering the location of objects
Balint’s syndrome – caused by bilateral damage to the parietooccipital region; includes optic ataxia (difficulty in reaching for
objects under visual guidance), ocular apraxia (difficulty in visual
scanning), and simultanagnosia (difficulty in perceiving more than
one object at a time)