Visual Perception and Imagery
Retina
The retina is a thin layer of tissue that lines the back of the eye on the inside.
It is located near the optic nerve. The purpose of the retina is to receive light
that the lens has focused, convert the light into neural signals, and send these
signals on to the brain for visual recognition
Photoreceptors:
The retina processes light through a layer of photoreceptor cells. These are
essentially light-sensitive cells, responsible for detecting qualities such as
colour and light-intensity. The retina processes the information gathered by
the photoreceptor cells and sends this information to the brain via the optic
nerve. Basically, the retina processes a picture from the focused light, and the
brain is left to decide what the picture is
Photoreceptors are the cells in the retina that respond to light. Their
distinguishing feature is the presence of large amounts of tightly packed
membrane that contains the photopigment rhodopsin or a related molecule.
The tight packing is needed to achieve a high photopigment density, which
allows a large proportion of the light photons that reach the photoreceptor to
be absorbed. Photon absorption contributes to the photoreceptor’s output
signal
There are two types of
photoreceptors in the human
retina, rods and cones:
Rods are responsible
for vision at low light
levels (scotopic vision).
They do not mediate
colour vision and have a
low spatial acuity. One
type
Cones are active at
higher light levels
(photopic vision), are
capable of colour vision
and are responsible for
high spatial acuity. The central fovea is populated exclusively by cones. There
are 3 types of cones which we will refer to as the short-wavelength sensitive
cones, the middle-wavelength sensitive cones and the long-wavelength
sensitive cones or S-cone, M-cones, and L-cones for short
The light levels where both are operational are called mesopic
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