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Chapter 2

PSL440Y1 Chapter Notes - Chapter 2: Transcranial Magnetic Stimulation, Stimulus Modality, Achromatopsia


Department
Physiology
Course Code
PSL440Y1
Professor
A
Chapter
2

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1
What it means to perceive?
Sensation and perception provide the raw material for cognition
Our perception is not a simple registration of sensory stimuli but sophisticated cognitive
processes work on the sensory information almost immediately and produce the brain’s
interpretation, our existing knowledge guides these dynamic processes
The sensory stimuli are typically ambiguous and open to multiple interpretations
The two problems with sensation and perception:
1. Sensory input does not contain enough information to explain our perception
2. The world presents us with too much sensory input to include into our coherent
perceptions at any single moment
We do not just register the picture as a whole when we see an image, but we see only
relatively fine details in a small region point of fixation. Searching is one of the ways
to deal with excess input
We engage in selective attention which allows us to choose part of the current sensory
input for further processing at the cost of other aspects on input
In conclusion, whether too much or too little, cognitive processes are necessary to
interpret and understand the material we encounter
How it works: The case of visual perception
Vision, like hearing, is a distance sense that helps us sense objects without touching
them. It tells us what is out there and where is it.
Our senses also give us a nudge towards action e.g. where to grab the object from
Visual perception takes in information about the properties and locations of objects so
we can make sense of it and interact with our surroundings
Structure of the visual system:
The pattern of light intensity, edges and other features in the visual scene form an image
on the retina
Light hits the photoreceptors and nerve cells at the back of the eye [job: light gets
converted into electrochemical signals] transmitted to the brain via optic nerves (each
optic nerve is a bundle of long axons of ganglion cells in the retina optic nerve axons
contact with LGC (lateral geniculate nucleus) in thalamus; a structure lying on the
surface of the brain Primary visual cortex/striate cortex host of visual areas as well
as areas that are not exclusively visual in function.
Beyond the primary visual cortex, 2 pathways can be identified:
a. Dorsal pathway: reaches up into parietal lobes and is important in
processing information about where items are located and how we act on
them e.g. touch them

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b. Ventral pathway: reaches down into parietal lobe and processes
information that leads to recognition and identification of objects
Top-Down and Bottom-Up processing
Most visual areas that send output to another area also receive input from that area i.e.
they have reciprocal connections
Reciprocal connections lead to the idea that perception is the product of bottom-up
and top-down processes
Bottom-up processes are driven by sensory input from the physical world
Top-down processes seek and extract sensory information and are driven by our
knowledge, beliefs, expectations
All acts of perception involves both bottom-up and top-down processing e.g.
visual search depends on the surrounding objects
Perceptions are interpretations of what we see and, and the mental representations are
produced by the interaction of bottom-up and top-down processing
Learning to see
The interpretations of the world around us is determined by the interaction between
our biological brain and experience
Visual system of an infant typically fixates for about half a second and this visual
information starts to accumulate and force lasting mental representations of objects,
people and places
The early stages of life have critical period during which particular responses must
develop, otherwise that ability will fail to properly develop
The sensory input of a person competes for representation in the cortex
Building from the bottom up: From features to objects
Processing Features; the building block of perceptions
Visual features includes spots and edges, colors and shapes, movements and textures
they are the building blocks of perception
In bottom up processing, we look at an object and count the features so that we can figure
out the processing that takes place to comprehend the image
Spots and Edges
This feature is processed by the ganglion cells neurons in the retinas whose axon fibers
form the optic nerve
Light lands on the receptive field of the photoreceptors; clusters of photoreceptors are
linked to ganglion cells of retina that pass on the information to the optic nerve

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3
Each ganglion cell responds only to the light that falls on its photoreceptors i.e. each has
its own visual field
Receptive field: is the area of visual field in which the stimulus affects the activity of the
cell
Light in some portions of the receptive field excites the cell; makes it more active and
white in colour and at the same time it inhibits the surrounding the areas; makes it less
active and grey in colour. This is an effective way for a person to focus on the light spot
and ignore the surroundings.
In a bright field, the total amount of light response is determined by subtracting the
signals of inhibitory area from the signals of facilitatory area
In a total dark field, the amount of light response is always 0
However, conditions are best where the photoreceptors of the inhibitory are in total dark
so the illuminated center is clearly visible.
This leads us to conclude that the structure and arrangement of photoreceptors serves to
enhance the contrast at the edges
Mach bands is a visual illusion where colours of band lie adjacent to each other where it
looks like the bands are like a gradient of colours but in reality each band is a solid
colour. This illusion is due to the ganglion cells which are well designed to pick our
edges from the visual environment
Throwing away Information
The visual system is designed to collect information about features e.g. spots and edges
and does not spend unnecessary energy on nearly uniform areas where nothing is
happening. This is demonstrated by the Craik-O’Brien-Cornsweet illusion
Edge information is important for defining the shape of objects and providing cues for
where to direct attention
When we fixate, the image falls on the fovea which is the part of retina that is served by
many ganglion cells with tiny receptive fields, may even be 1 photoreceptor per ganglion
which helps in high resolution.
As we move away from the point of fixation, the receptive fields gets bigger and bigger
and we start to pick out important information
Neural processing of features
The visual information is passed on to the brain through optic nerves
Optic nerves meet before entering the brain and make up optic chiasm; a cross over
where each optic nerve sends the information to the opposite hemisphere of the brain
The whole visual field is contained in the primary visual cortex/striate cortex that
responds to variations in basic features e.g. orientation, motion and color.
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