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Chapter 7 Notes
The primary function of the sense organs is to provide information to guide behaviour.
Perception – A rapid, automatic, unconscious process by which we recognize what is represented by the
information provided by our sense organs.
We do not first see the object and then perceive it; we simply perceive the object. The ambiguous objects
that require some research are more similar to problem solving than perception.
Visual perception takes place in the brain. The optic nerves send visual information to the thalamus,
which relays information to the primary visual cortex, located in the occipital lobe at the back of the
brain. In turn, neurons in the primary visual cortex send visual information to 2 successive levels of the
visual association cortex. The first level, located in the occipital lobe, surrounds the primary visual cortex.
The second level is divided into 2 parts, one in the middle of the parietal lobe and one in the lower part of
the temporal lobe.
Visual perception by the brain is usually described as a hierarchy of information processing. The higher
levels of the perceptual process interact with memories: the viewer recognizes familiar objects and learns
the appearance of new ones.
David Hubel and Torsten Wiesel inserted microelectrodes (small wires with microscopically sharp points)
into the various visual systems of cats and monkeys to observe the action potentials produced by
individual neurons. They concluded from their research that the geography of the visual field is retained
in the primary visual cortex.
The map is like a mosaic i.e. a picture made of individual tiles or pieces of glass. Each tile or module
consists of a block of tissue, approximately .5 x .7 mm in size and containing approximately 150 000
neurons. The primary visual cortex contains approximately 2500 of these modules. Hubel and Wiesel
found that neural circuits within each module analyzed various characteristics of their own particular part
of the visual field.
Receptive field – That portion of the visual field in which the presentation of visual stimuli will produce
an alteration in the firing rate of a particular neuron.
Some detected lines passing through the region, some detected thickness while some detected colours.
The neuron responds when a line oriented at 50 degrees to the vertical is placed in this location and has
very little response when a line having 70 or 30 degree orientation is passed through the receptive field.
The combination of the visual information in different modules occurs in the visual association cortex.
The first level of the visual association cortex contains several subdivisions, each of which contains a map
of the visual scene. Each subdivision receives information from different types of neural circuits within
the modules of the primary visual cortex. One subdivision receives information about the orientation and
widths of lines and edges and is involved in perception of shapes. Another subdivision receives
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information about movement and keeps track of relative movements of objects and another subdivision
receives information concerning colour. (Hadjikhani)
The second level puts together the bits and pieces provided by the subdivisions of the first level.
Information such as shape, movement and colour are combined in the visual association cortex in the
lower part of the temporal lobe. 3-D form perception takes place here. The visual association cortex in the
parietal lobe is responsible for perception of the location of objects. It integrates information from the
first level along with the information from the motor system and the body senses about movements of
eyes, head and body.
When the primary visual cortex is damaged, a person becomes blind in some portion of the visual field.
The location depends on what part of the brain has been damaged. But even if a person loses most of their
sight, they are still able to perceive objects and their backgrounds. (Telescope example) It also works vice
Achromatopsia – The inability to discriminate among different hues; caused by damage to the visual
association cortex. (Black and white movie for the rest of their lives)
Damage to the first level can lead to the difficulty of perceiving movements of objects. (The lady who
couldn’t cross the street example by Zihl, von Cramon, Mai and Schmid in 1991.
Balint’s syndrome – A syndrome caused by bilateral damage to the parietooccipital region of the brain;
includes difficulty in perceiving the location of objects and reaching for them under visual guidance.
They can see every piece of the puzzle (the scene) but the whole puzzle is scrambled instead of a finished
Visual agnosia – The inability of a person who is not blind to recognize the identity of an object visually;
caused by damage to the visual association cortex.
Prosopagnosia – A form of visual agnosia characterized by difficulty in the recognition of people’s faces;
caused by damage to the visual association cortex.
Most people who have prospagnosia have difficulty recognizing other complex visual stimuli as well.
They can easily recognize categories of objects (automobiles, animals or houses) but have difficulty
distinguishing between particular individual stimuli (their car, their dog or their house).
The visual system is able to perceive shapes, determine distances and detect movements; it tells us what
something is, where it is located and what it is doing.
Objects are things having particular shapes and particular locations in space. (People can be considered
objects). Backgrounds are essentially formless and serve mostly to help us judge the location of objects
we see in front of them.
Figure – A visual stimulus that is perceives as a self-contained object.
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