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Chapter 1: General Principles
• Although subjectively our perception seems direct, immediate, and effortless, it is very complex. The
cerebral cortex is devoted entirely to perception, and deficits wrought by brain damage demonstrate the
sophistication of brain processes (e.g. prosopagnosia, cannot recognize familiar faces, including one’s
• Our perceptual decisions are variable. In marginal conditions when stimuli are very close together or
indistinct, perceptual responses are probabilistic.
Sensation & Perception
• Sensation: Environmental information extracted and turned into neural, electrical signals
• Reception: Receiving environmental information using specialized receptors.
o Sensory systems acts as filters: high sensitivity to certain signals, such as male silk moths
having chemosensors for female pheromones; also high selectivity
o E.g. Vibrations in air become vibrations in fluid of inner ear, then mechanical stimulation of cilia
• Transduction: Energy signal is turned into electrical signals – action potentials or membrane potential
changes. This happens in the sensory periphery in order to transmit to the CNS.
o E.g. Mechanical vibrations of cilia in cochlea open ion channels to become electrical signals
o Transduction involves the amplification of energy: small amounts of target molecules have little
energy on their own and cannot trigger action potentials, but they activate receptors to amplify the
message to trigger electrical changes.
• Perception: Transforming electrical signals into representations, to support the experience of objects
and events in space and time. It is important for generating, controlling, and guiding adequate behaviour.
• Coding: E.g. Information about frequency, intensity, and timing of the sound are preserved.
• Transmission & Propagation: Electrical signals are relayed by neurotransmitters across synapses.
Signals are sent to the thalamus, to contralateral and ipsilateral nuclei, and then to the primary sensory
cortex, and to association cortices for additional processing. Even higher levels for bring sensory
information into consciousness.
o There are ascending connections (periphery to central), descending connections, and lateral
connections between neurons at the same level
• Doctrine of Specific Nerve Energy (Johannes Muller): Neural signals in the senses are all of similar
nature, being electrical signals -- there is one code. Rather, the nature of the sensation depends on
which pathways or sensory fibers in the nervous system are stimulated, by the receiving location and the
specific cortical receiving area.
The Big Debates
• Our brain never experiences the physical environment directly.
• Vitalism vs. Materialism: Can mental properties like reasoning and emotions be explained with the
machinery of the body? Descartes proposed that animals are purely mechanical, but he couldn’t say the
same for humans; proposed mind-body dualism with human higher capabilities due to the non-physical
• Nativism vs. Empiricism: Do we bring something to the world a priori before any experience, to make
sense of our experiences, or does everything we know come from experience?
o Hobbes: Human nature relies only on experiences. Thinking comes from memories of
experiences. Page 3-26, 23 pages Page 2 of9
o Locke: Everyone is born as a blank slate, tabula rasa. All thoughts are constructed from
experience with a collection of sensations. With Molyneux: If someone blind from birth was
suddenly cured, would they recognize objects by sight that he knew by other senses?
What is Reality?
• Constructivism: How much do we know of the world at all? Discrepancy between subjective
experience and the reality of what we are experiencing.
o Hume: We have no way of knowing if our perceived reality is valid, what we can test is only
reliability – the consistency of our perceptual measurements, whether they generate predictions
that are true when we interact with the world.
• Plato’s Allegory of the Cave: Prisoners only see a wall onto which shadows are projected, and never
see objects in the external world. They can reliably predict those shadows but have no valid experience
of it. The prison-house is the world of sight – we don’t know if our perceptions are just shadows of
something very different.
• “What is real? How do you define real? If you’re talking about what you can feel, what you can smell,
what you can taste and see, then real is simply electrical signals interpreted by your brain. This is the
world that you know.”
• Sensory systems develop depending on if that environmental energy is important for the organism, and
being able to process it will provide a survival advantage. Thus, human senses are limited to certain
kinds of energy, so our sense of reality is also limited.
o Bees see UV, rattlesnakes see infrared heat; dogs and cats hear higher frequencies, elephants
hear low frequencies; birds, reptile and amphibians can use magnetic fields to navigate.
Classification of the Senses
Sense Stimulus Sensory Structure Cortex
Vision Light: intensity, wavelength, Eye Visual cortex (occipital)
Audition Sound: air pressure, particle velocity Ear Auditory cortex (temporal)
Gustation Chemicals in liquids or solids Tongue, mouth Gustatory cortex
Olfaction Chemicals in the air Nose Olfactory cortex
Touch Pressure, vibration, temperature, pain Skin Somatosensory cortex
Proprioceptio Position, orientation, acceleration of Vestibular organs, Somatosensory cortex
n body and its parts in space receptors in muscles and
• CNS: Spinal cord, brainstem (medulla, pons, midbrain), diencephalon
(thalamus), and cerebral cortex
• The spinal cord has segmental organization, with 31 segments.
Sensory information inputs through the dorsal root ganglion, while
motor output leaves through the ventral root (dorsal in, ventral out).
• The brainstem also has segmental organization, with 12 segments
and 12 cranial nerves. This includes information from the olfactory,
optic, auditory, and glossopharyngeal (taste) nerves for the senses. Page 3-26, 23 pages Page 3 of 9
• Receptors relay this signal to specialized areas of the cerebral cortex. Activity in the cortex leads to
conscious perceptual experience.
• The cerebral cortex is divided into 4 major lobes: frontal, parietal (somatosensory), temporal, and
• Relative area of cortex devoted to different senses is indicative of relative importance in survival of the
species, e.g. vision is largest for primates, while auditory is for bats and somatosensory for moles.
Methods Used to Study Perception
• We must ask: Where? How (the machinery)? How (the principles)? Why (purpose, evolution)?
Where: Lesion Experiments
• Lesions are abnormalities in structure or function in the body. Lesion experiments where areas of an
animal’s brain is removed or destroyed, and the consequences are observed.
• Localization of Function: View that neurons underlying a specific sensory or cognitive function are
located in a circumscribed brain area.
• Damage to the angular gyrus caused monkeys to not be able to locate food. Was not due to damage to
vision, but to disruption of visually guided action – monkey could see the food but could not perform
actions to obtain it.
Where: Clinical Studies (Neuropsychology)
• Clinical investigations into consequences of cases of damage or disease to human brains.
• Inouye: Studied Japanese soldiers wounded in combat to assess degree of blindness due to bullet
wounds. Impairments are related to the path of the bullet and the areas damaged, and inferences about
the occipital cortex are made.
Neuronal Basis: Single-Unit Recordings
• Hubel & Weisel: Visual cortex is mapped topographically – neurons are arranged spatially such that
nearby cells respond to nearby locations in the visual field. Visual patterns trigger corresponding fields of
electrical activity in their cortical neurons.
• Single-Unit Recording: A stimulus is presented to the animal (visual) while a fine microelectrode
records electrical activity from a single cell in the sensory system.
• Despite anatomically uniformity, functional properties and stimulus preferences vary widely – in terms of
intensity, size, orientation, movement direction, colour, etc.
• Feature Detectors: View that individual neurons in the brain act as detectors for individual stimulus
• The neuronal basis of information processing can also be studied by neuropharmacology’s
manipulation of synaptic transmission
Where: Brain Imaging
• Computerized Tomography (CT): X-rays are passed through the body at different angles, and the
resulting pattern of X-ray transmission is collected and processed to create detailed images of body
• Magnetic Resonance Imaging (MRI): Short bursts of powerful radio waves are passed through the
body, and signals emitted by body molecules are processed to create detailed images of body structure.
• Functional MRI (fMRI): MRI is used to detect magnetic changes in hemoglobin induced by variation in
blood oxygen concentration (BOLD – blood oxygen level-dependent response). fMRI is a measure of
brain activity, and can identify brain areas associated with certain tasks.
When: Electroencephalogram (EEG), Magnetoencephalogram (MEG) Page 3-26, 23 pages Page 4 of 9
• EEG and MEG have better temporal resolution than fMRI, which has good spatial resolution. This
allows us to study when an event occurs, and in which order.
• Computation: The manipulation of quantities or symbols according to a set of rules.
• One can attempt to solve problems theoretically – how could facial recognition work in principle?
• Artificial Intelligence (AI): Computer science which aims to produce a device capable of behaviour
normally associated with human cognition, such as language understanding, reasoning, and perception.
• The brain is a computational device, with inputs and outputs able to be expressed mathematically.
• Connectism: Computational modeling based on simulated networks of simple processing units, like
General Principles of Sensation & Perception
Neural Impulses & Transduction
• The nervous system transmits information by neural impulses, brief, discrete electrical signals of
action potentials that travel rapidly along a cell’s axon, passing from one cell to another.
• Each sense requires specialized cells that receive one particular form of energy. The eye contains
photoreceptors that have photopigments that breakdown when struck by light, producing electricity. The
mechanoreceptors of the inner ear have cilia, that when moved by sound vibrations are deflected and
trigger electrical changes.
• Between transduction and arrival at the cortex, signals pass through many synapses at successively
higher levels of neural processing.
• In all senses except olfaction, one of the synapses is located in the thalamus, located in the
diencephalon at the top of the brainstem below the cerebral cortices. The thalamus is an information
• After afferent neurons from the sense organ arrive at the primary sensory cortex, they are passed
onto cortical association areas for higher processing.
• Signal flow is unidirectional below the thalamus, and bidirectional above. Each stage contains a large
population of interconnected neurons which through processing changes the signal that is passed onto
the next stage.
• Each sensory system responds only to a particular range of stimuli; this range is the sensory space.
E.g. Human auditory system can respond to sound pressure of frequencies between 20 Hz and 16,000
• Single-unit recording examines the particular range of stimuli of an individual neuron to pinpoint where
in the system’s