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The Visual Perception System
There are five senses – touch, smell, vision, taste, hearing and kinaesthesia (body
movement and position)
The initial processes are mainly sensory, involving the detection, reception,
conversion and transmission of sensory information to the brain. The brain then
processes, analyses, organises and interprets the sensory info so that it becomes
meaningful.
The cognitive system of each of our senses involves a number of sensory and
cognitive processes that overlap and interact.
Visual sensation in 1700 was seen as a purely physiological process, now visual
sensation and visual perception is treated as one process.
The visual sensory input begins in the eye (the sense organ), where the light (a type of
energy) is detected by neurons that specialise in responding to light; it proceeds to the
brain, where it is integrated and interpreted in to a meaningful experience.
The visual perception system consists of the complete network of physical structures
involved in vision. These are all parts of the eyes, the specific neural pathways that
connect eyes to the brain and the visual processing areas in the cortex in the brain.
The perceptual experiences also involves many different psychological and cognitive
processes such as drawing on past experiences with similar stimuli; judging size,
shape and distance; and working out appropriate ways of interpreting and responding
to stimuli.
Characteristics of the visual perception system: processes reception, transduction,
transmission, selection, organization and interpretation of stimulus information.
Interpretation of stimulus information
All sensory systems must detect/receive transducer (convert) and transmit info to the
brain. Our Vestibular (receptors in the ears re balance, body position)
Various parts of the visual perception system are involved in processing the visual
info.
These can be examined from two perspectives – a sensory perspective (involving
physiological structures and processes) and a psychological perspective (involving
cognitive processes)
Sensory level – V P (visual perception) begins when the light that is reflected from an
object in our environment enters the eye and is focused on the light-sensitive area of
the retina. (Box 1 p.173)
This surface of the retina contains neurons called photoreceptors. Photoreceptors are
specialised neurons that detect and respond to light by converting it into neural
impulses that can be processed by the brain.
A minimum amount of energy is required to cause a response from the
photoreceptors. If the light is too low for instance the processing of vision with not
proceed.
Further processing occurs when the brain attempts to make sense of the sensory info
that has been detected and sent to it. This involves extremely complex cognitive
processes.
As many specific psychological experiences are unique to each individual, it is
possible for the same sensory info to be interpreted in a variety of ways.
STRUCTURE OF THE EYE
Cornea- a transparent convex-shaped covering (curved outwards) that protects the eye
and helps focus the light rays onto the retina.
Light passes through the Cornea then onto the aqueous humour a watery fluid that
helps to maintain the shape of the eyeball provides nutrient and oxygen to the eye and
carries waste products away.
Light continues through the pupil which appears as a black disc in the centre of the
eye. The pupil is an opening in the iris. The Iris is a ring of muscle that expands or
contracts to change the size of the pupil to regulate the amount of light entering the
eye. The iris is the coloured part of the eye.
In dim light the pupil dilates (expands) to allow more light into the eye, in bright light
it contracts to restrict the amount of light entering the eye.
After the pupil the light travels through the lens- a transparent, flexible, convex
structure. The lens focuses the light onto the retina. When focusing the light onto the
retina, the shape of the lens is adjusted as the distance of an object being viewed
changes. The ciliary muscles attached to each end of the lens contract and relax,
enabling the lens to automatically bulge (when the muscle contract) to focus near
objects onto the retina and to flatten (when the muscle relax) to focus distant objects
onto the retina.
After passing through the lens it continues through the vitreous humour. This is a
jelly-like substance that helps to maintain the shape of the eyeball and also has a role
in focussing. Finally, the light reaches the back of they eye where there are several
layers of neural tissue that form the retina. The layer of neurons at the very back, or
innermost part, of the retina consists of photoreceptors and once they are activated,
the response to light begins.
Response to light
Sensory receptors respond to specific forms of energy. For our sense of sight to occur,
light within a narrow band of electromagnetic energy known as the visible spectrum
is required in order to stimulate the retinal photoreceptors. These receptors then
convert this energy into neural impulses which are electrochemical in nature.
Sight needs light
ROY G. Biv = red, orange, yellow green, blue, indigo and violet
The human eye responds only to a limited range of wavelengths in the
electromagnetic spectrum, while other species like insects detect wavelengths shorter
and fish longer than us.
There can be variations within a species in the limitations of the photoreceptors
responsiveness to light; and these are referred to limitations.
Red on top (highest/longest) wavelength to violet (lowest/shortest)
The way in which light is detected and focussed on the retina
The Retina at the back of the eye responds to the light and it is made up of layers of
neural tissues and the very back which is the photoreceptors. The photoreceptors
receive and absorb light. The light energy detected by the photoreceptors is changed
from electromagnetic energy in neural impulses so that it can be sent to the brain for
further processing.
The two types of photoreceptors are rods and cones, so named because of their
respective shapes. Estimated to be 6.5 million cones and 125 million rods in each
retina.
Cones are specialised photoreceptors that are important for daylight vision, visual
acuity (fine detail) and colour vision. They do not operate in dim light. The central
Fovea contains only cones and finely focuses vision onto the retina. The sharpest
images are focused on this small area, the outside has some rods.
Rods are important for night vision because they are more sensitive than cones to dim
light. Also important for peripheral (non-central) vision because rods are distributed
in large numbers to the outer reaches of the retina (cones centrally located) to see
better at night it is best to look slightly above or below or to one side of the object.
This will focus the image on the periphery of the retina where the rods are.
The retina has a ‘blind spot’ where there are no cones or rods and this is where the
optic nerve leave the retina.
THRESHOLDS: ABSOLUTE AND DIFFERENTIAL
Threshold refers to our ability to detect a stimulus or changes in a stimulus.
Absolute: for vision refers to the minimum amount of light energy that is necessary in
order for a visual stimulus to be perceived. The weakest stimulus can be perceived has
the light energy equivalent to a candle at about 50 kms viewed under ideal conditions
( a clear, pitch-black night) This is only a hypothetical description of the min light
energy required by the human visual perception system at absolute threshold. (There
is no specific level at which our eyes ‘switch’ from seeing no light to seeing light,
rather the light stimulus gradually increases in intensity)
In statistical terms it is the lowest or weakest level of a particular stimulus that ca be
detected on average, 50% of the time.
Differential (also called noticeable difference threshold or JND) for vision is the
smallest perceptible difference (or perceptible change) that can be detected between
two visible stimuli by the eye.
I.e. during an experiment a researcher might investigate when a difference in the
brightness of a light undergoing slight increases or decreases in its intensity can just
be perceived. (50% of the time)
Differential thresholds for vision can also be applied to specific visual capabilities:
Visual acuity; the sharpest of our vision and our ability to detect fine detail, and as we
get older this usually decreases for objects that are close, like small print in books or
newspapers.
Test was done by Lindenberger and Baltes in Berlin (1994) of the older population
between 70 -103 yrs. They used a Snellen chart (used by optometrists to test
eyesight). Their results concluded that the differential threshold for the ‘very old’
group (85-103) was greater than for the ‘old group’
The nature of processes in visual perception
Process reception: to the process by which the structures of the eye capture an image
of a visual stimulus and focus it on the photoreceptors in the retina.
The image focused onto the retina is an inverted (upside-down) and reversed (back-tofront) image of the object being viewed. Because light rays only travel in straight
lines, rays from the top of an object being viewed are bent to fall at the bottom of the
retina, and light rays from the bottom of an object being viewed hit the top of the
retina. Likewise, light rays from the left side of an object are bent by the lens to the
right side of the retina and vice-versa for the light rays from the right side. This
left/right reversal from the actual object to its image on the retina is the key to
understanding why an object in our left visual field is processed in the right cerebral
hemisphere and vice versa.
Transduction: the process by which the sensory receptors change one form of
physical energy (e.g. light/electromagnetic energy) into another form (e.g. neural
impulses/electrochemical energy) that can be used by the nervous system.
Transduction results in perception. Raw sensory data is transformed into a form that
the brain can process.
Electrochemical energy is a form of energy involving tiny electrical charges that
move from one neuron to the next with the aid of a chemical substance
(neurotransmitter) All sensory info must be transduced at the receptor sites before it
can be transmitted to the brain as it only receives electrochemical energy. If
transduction does not occur the incoming info will travel no further than the sensory
receptors.
The process of transduction is similar to that which occurs when a TV receiver picks
up signals and converts them into electrical impulses that are organised and displayed
on a TV screen.
Conversion of the information into a form that can be processed
Transmission: the process of transferring or moving info from one location to
another. (I.e. from receptor cells to the brain)
It is the process of sending and receiving visual info in the form of electrochemical
energy from neuron to neuron along the neural pathways to the visual cortex in the
brain.
The photoreceptors relay their messages through at least two other types of neurons in
the retina – the bipolar cells that are connected to the photoreceptors, and the ganglion
cells.
Bipolar cells are like ‘spark plugs’ once ignited by photoreceptors they trigger the
firing of the ganglion cells. The axons the fibres that take neural impulses away from
the cells of the ganglion cells, collect from all over the retina and converge to make a
bundle of about one million fibres called the optic nerve.
The optic nerve transmits neural impulses from the retina to the primary visual cortex
in the occipital lobe of the brain. The point at which the optic nerve leaves the retina
is the ‘blind spot’ or the optic disc.
The fibres of the optic nerve that originate from the left side of each retina transmits
visual info to the left visual cortex. Visa versa with right side. The transmission of
visual info along the optic nerve from each retina involves a partial cross over of
neural pathways.
Selection: a process which involves coding information to specific features of a
stimulus such as size, colour and direction of movement. Occurs during the
transmission of info and in the brain.
- involves discrimination or differentiating between the various features that make up
a visual stimulus. The most basic selection occurs during transduction as the
photoreceptors are very selective to the range of electromagnetic energy to which they
respond. There are three different types of cones and each type is selective in terms of
its responsiveness to specific wavelengths of light. The selection process involved at
the photoreceptor level is so sophisticated that the differential threshold for colours
allows us to discriminate between about seven million different colour variations.
Selection also occurs in terms of the difference between cones and rods in the overall
sensitivity to wavelengths of light. Cones have a maximum sensitivity to light with a
wavelength of about 560 nanometres (yellow-green) rods a maximum of 500
nanometres (blue). Although rods don’t perceive colour they are more strongly
activated by light with a wavelength that produces blue than by any other wavelength.
Rods to not respond to long wavelengths at all (red)
Selection also occurs in the visual cortex, which has highly specialised cells called
feature detectors. Feature detectors are specialised neurons that detect specific
features or parts of a visual stimulus. Each type of feature detector specialises in
detecting only certain features of a stimulus. Some type of F D might respond most
often to a horizontal line in the upper part of the visual field, others to a vertical line
in the lower left part of the visual field etc.
A receptive field is an area of visual info received by a cluster (or group) of
photoreceptors and this info is processed individually in the visual cortex.
Organization: the process of reforming appropriate or meaningful way incoming
sensory information. Organisation occurs in the brain and may occur in a variety of
ways.
-is the reassembling of elements or features of visual info in an appropriate or
meaningful way. A perceptual principle involves organising elements of a visual
stimulus with similar features into groups or categories because they ‘go together’ i.e.
the horizontal, vertical and diagonal lines which make up a chair. The way in which
sensory info is organised is also influenced by psychological factors that vary from
one person to the next.
Interpretation – the process of assigning meaning to sensory information detected by
photoreceptors and transmitted to the brain. The meaning is influenced by
psychological factors that are unique to each individual. Incomplete stimuli might
result in educated guesses using existing knowledge, involving cognitive processes
(memory and problem solving) and the use of perceptual principles (such as knowing
the object wont change its physical size when it is moving away)
Two factors that influence the organisation and interpretation of sensory info are
specific features of the visual stimulus and expectations we have about the stimulus
being perceived. When features are difficult to identify or there is conflict between
our expectation of the stimulus and its features, interpretation of the info may be
difficult. Our interpretations are influenced by many psychological factors, including
past experiences and the context in which the object/event is being perceived. See
figure 5.16
The Distal stimulus – is the environmental stimulus outside the body that is detected.