PSYC10003 Lecture Notes - Lecture 7: Vitreous Body, Color Blindness, Retina
12 Jun 2018
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Lecture 7, Wednesday 16 March 2016
PSYC10003 - MIND, BRAIN & BEHAVIOUR 1
LECTURE 7
THE HUMAN VISUAL SYSTEM
THE ELECTROMAGNETIC SPECTRUM
•Our eyes detect the presence
and pattern of light reflected
off objects in the world.
•We are sensitive to a very
narrow range of wavelengths
in the electromagnetic
spectrum, known as the
visible spectrum (i.e., white
light, or daylight).
‣The visible spectrum
extends from 380
nanometres (billionths of
a metre) to 760 nm.
•By contrast, honeybees can
detect light within the
ultraviolet range.
•The range of wavelengths we
can see is not qualitatively
different from the rest of the electromagnetic spectrum; it’s just the part of the continuum of
electromagnetic radiation that we are sensitive to.
•The colour of light is determined by three dimensions:
‣Hue; the wavelength of electromagnetic radiation.
‣Brightness; the intensity of electromagnetic radiation.
‣Saturation; the purity of electromagnetic
radiation.
THE HUMAN EYE
•The eye contains the peripheral apparatus
necessary for transferring light into a neural
signal.
•Light enters the eye through the transparent
outer layer known as the cornea.
•Immediately behind the cornea is the lens,
which is made up of a number of transparent
layers, much like an onion.
‣The shape of the lens can be altered to help
focus the image onto the back of the eye, which is lined by a light sensitive structure called the
retina.
•The eyeball itself is filled with a clear gelatinous fluid called the vitreous humour.
•Note that the light sensitive cells in the retina (the rods and cones) send their axons out of the eye
from a common point, known as the optic disk.
‣Because there are no photoreceptors at the optic disk, it causes a blind spot (i.e., the region of
space from which an object is not visible).
‣The axons that are bundled together at the optic disk are known collectively as the optic
nerve.
•Our primary concern in this lecture is with the structure and function of the light sensitive retina,
and with the manner in which neural signals from the retina are elaborated by the rest of the
brain to allow perception.
Lecture 7, Wednesday 16 March 2016
PSYC10003 - MIND, BRAIN & BEHAVIOUR 1
CELLS OF THE RETINA
•A closer view of a cross-
section through the light
sensitive retina reveals a series
of layers, each containing
specialised neurons, their
axons and dendrites, and the
photoreceptors (the retina is
in fact part of the brain).
•The light sensitive
(photosensitive) cells are
located at the back of the
retina, so light must pass
through each of the other
layers to get to them.
‣There are two types of
these photoreceptors: rods
and cones.
‣The rods and cones contain photopigments.
‣These pigments break down when exposed to light, and this breakdown process
triggers a series of stages that leads to the neural impulses that are eventually conveyed
to the brain by the optic nerve.
•The human retina has about 120 million rods and about 6 million cones.
‣Even though there are fewer cones, these are the most important for seeing fine detail, and
they are most active in the daylight.
•Cones are concentrated in a region of the retina called the fovea, which is responsible for the
central few degrees of our visual field.
‣Different types of cones are also sensitive to different wavelengths of light, and so they are
responsible for our ability to see colour.
•Rods do not discriminate between different wavelengths, and they cannot discriminate fine visual
detail.
‣But rods are much more sensitive to light than cones, and so rods are used in dimly
illuminated environments (hence our failure to perceive colour or fine detail in semi-
darkness).
•The retina can be divided into three distinct layers:
‣Photoreceptor layer
‣Bipolar cell layer
‣Ganglion cell layer
•The rods and cones form synapses with bipolar cells, which in turn form synapses with ganglion
cells.
‣Ganglion cells send their axons through the optic nerve (the second cranial nerve), which
conveys visual information to the brain.
•Two other cell types in the middle layer of the retina, horizontal cells and amacrine cells, serve the
function of combining messages from several photoreceptors.
•Photoreceptors and bipolar cells do not produce action potentials.
‣Rather, they release neurotransmitters that increase or decrease the firing rate of action
potentials generated by the ganglion cells.
Lecture 7, Wednesday 16 March 2016
PSYC10003 - MIND, BRAIN & BEHAVIOUR 1
TYPES OF CONES
•As already outlined, there are
three types of cones, each
containing a photopigment that is
sensitive to a different range of
wavelengths within the visible
spectrum.
•Short-wavelength (S) cones;
peak sensitivity at 440 nm
(blue light)
•Medium-wavelength (M)
cones; peak sensitivity at 530
nm (green light)
•Long - wavelength (L) cones;
peak sensitivity at 560 nm (red
light)
ISHIHARA COLOUR PLATES
•Different types of colour blindness, a genetic condition, arise from anomalies in the pigments of
one or more cone types in the retina.
•The two most common forms of colour blindness are more common in males than females
because the responsible gene is located on the X chromosome.
‣Males have just one X chromosome and so the defective gene is expressed.
‣Females have a pair of X chromosomes, one of which is likely to have a normal gene that can
mask the expression of the defective one.
•In fact most people with colour blindness are not literally ‘blind’ to colour.
‣They still see the world in colour, but they are deficient in discriminating between certain
hues.
•The most common kind of colour deficiency is one in which a person is poor at discriminating
red and green (red-green deficient; this affects around 10% of males and about 1% of females).
•People who are colour deficient have anomalies in the photo-pigments of one or more of the
three cone-types (S, M or L).
•The Ishihara Colour Plates are used to test anomalies of colour perception.
‣The disk on the left of the slide contains a digit that can be seen by both normals and those
with colour deficiencies.
‣The central and right disks contain digits that can be seen by normals; individuals with red-
green deficiency may see an
incorrect digit, others with
anomalous colour vision may not
see any digits at all.
•Colour deficiency is not uncommon,
especially in males, and is no cause for
alarm.
•Other than an unusual taste in
coordinating the colours of their
clothes, there are few (if any) everyday
problems for most people with
anomalous colour vision.
