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Chapter 06

Chapter 06 - Perception


Department
Psychology
Course Code
PSYA01H3
Professor
Steve Joordens
Chapter
06

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Chapter 06 – Perception
Brain Mechanisms Of Visual Perception
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Perception – a rapid, automatic, unconscious process by which we recognize what is
represented by the information provided by our sense organs
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It’s a rapid, automatic, unconscious process; it isn’t a deliberate, effortful activity; we
simply perceive it
o
The distinction between sensation and perception is not easy; for some sensory
systems, such as pain and our vestibular sense, the distinction is arbitrary, since they
help us to react rather than to provide a representation of the world around us
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Visual perception is often described as a hierarchy of information processing
o
Circuits of neurons analyze particular aspects of visual information and send the results
of their analysis to another circuit, providing further analysis, evaluating more complex
features
The higher level interacts with memories; the viewer recognizes familiar objects
and learns the appearance of the new, unfamiliar ones
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The primary visual cortex was studied by David Hubel and Torsten Wiesel, who inserted
microelectrodes into various regions of the visual system of cats and monkeys to detect the
action potentials produced by individual neurons
o
They concluded that the surface of the retina is “mapped” on the surface of the primary
visual cortex, although this map is distorted, with the largest amount of area given to
the centre of the visual field, where our vision is precise
The map is like a mosaic; it is made of many modules
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Module – a block of cortical tissue that receives information from the
same group of receptor cells
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Each module contains approximately 150,000 neurons, and the primary
visual cortex contains approximately 2500 of these modules
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Receptive field – that portion of the visual field in which the presentation of a visual
stimuli will produce an alteration in the firing rate of a particular neuron
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Although the primary visual cortex is necessary for visual perception, the perception of objects
and of the totality of the visual scene takes place in the visual association cortex
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Circuits of neurons in the primary visual cortex analyze particular aspects of visual information
and send the results of their analysis to the other circuits, which perform further analysis
o
Neurons in the primary visual cortex send axons to the region of the visual association
cortex that surrounds the striate cortex, and then the visual association cortex divides
into two pathways
Ventral stream – the flow of information from the primary visual cortex to the
visual association area in the lower temporal lobe; used to form the perception
of an object’s shape, colour, and orientation (the “what” system)
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Perception of form
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Recognition of patterns and identification of objects takes place
in the inferior temporal cortex; and it’s there that the analyses
of form and colour are put together and perceptions of 3D
objects emerge
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o
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
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Fusiform face area (FFA) – a region of the ventral stream of the
visual system that contains face-recognizing circuits
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Extrastriate body area (EBA) – a region of the occipital cortex,
next to the primary visual cortex, that responds to forms
resembling the human body
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Parahippocampal place area (PPA) a region of the ventral
stream, below the hippocampus, that is activated by visual
scenes
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Perception of colour
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The ventral stream combines the information from red/green
and yellow/blue signals that originate in the retinal ganglion
cells
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Cerebral achromatopsia – the inability to discriminate among
different hues; caused by damage to the visual association
cortex
If brain damage occurs on only one side of the brain,
people will lose their colour vision in only half of their
visual field
If the damage is bilateral, all colour vision is lost and
they cannot even imagine colours or remember the
colours of objects they saw before the damage occurred
Dorsal stream – the flow of information from the primary visual cortex to the
visual association area in the parietal lobe; used to form the perception of an
object’s location in three-dimensional space (the “where” system)
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Perception of spatial location
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The parietal lobe receives visual, auditory, somatosensory, and
vestibular information and is involved in spatial and
somatosensory perception; damage to this lobe disrupts
performance on a variety of tasks that require:
Perceiving and remembering the location of objects
Controlling the movement of the eyes and the limbs
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Goodale and colleagues suggested that the primary function of
the dorsal stream of the visual cortex is to guide actions rather
than simply perceive spatial locations
They suggested the combination of “what” and “how”;
that is, how to perform the action
However this notion should not imply an absence of the
capacity to recognize spatial location
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Perception of movement
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Without the ability to perceive velocity, we could not predict
where they will be; we would be unable to catch or avoid them
o
Akinetopsia – an inability to see motion
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