Nativism: The idea that the mind produces ideas that are not derived from external sources, and that we
have abilities that are innate and not learned
Empiricism: The idea that all knowledge comes through the senses
perception is the source of all knowledge about the world.
Each of our senses follows the same order of events:
1.Capturing physical energy
3.Transduction into neural signal: Sensation
4.Transmission of neural signal to the brain
5.Cortical Processing: Perception
Synapse: The junction between neurons that permits information transfer
Neurotransmitter: A chemical substance used in neuronal communication at synapses
Dualism: The idea that mind has an existence separate from the material world of the body
Materialism: The idea that the only thing that exists is matter, and that all things, including mind and
consciousness, are the results of interactions between bits of matter
What most scientist believe to be true; what we will focus on in this class.
Panpsychism: (Fechner) The idea that the mind exists as a property of all matter—that is, that all
matter has consciousness
Absolute threshold: Minimum amount of stimulation necessary for a person to detect a stimulus
50% of the time.
Twopoint threshold: the minimum distance at which two stimuli (e.g., two simultaneous touches) can
More nerve endings on fingertips then arms so the twopoint threshold is smaller distance on
fingertips JND (just noticeable difference): The smallest detectable difference between two stimuli, or the
minimum change in a stimulus that can be correctly judged as different from a reference stimulus. also
known as difference threshold.
Fechner’s law: Subjective sensation increases proportionally to the logarithm of the stimulus intensity
1.Method of constant stimuli: Many stimuli, ranging from rarely to almost always perceivable, are
presented one at a time. (Threshold: Value at which stimulus is perceived 50% of the time.) need more
trials/ patients then the other methods
2.Method of limits: The magnitude of a single stimulus or the difference between two stimuli is varied
incrementally until the participant responds differently.
3.Method of adjustment: Similar to the method of limits, but the participant controls the stimulus
4.Magnitude estimation: The participant assigns values according to perceived magnitudes of the
5.Crossmodality matching: The participant matches the intensity of a sensation in one sensory
modality with the intensity of a sensation in another.
Signal detection theory: A psychophysical theory that quantifies the response of an observer to the
presentation of a signal in the presence of noise.
Hit, Miss, Correct Rejection, False Alarm
Sensitivity: A value that defines the ease with which an observer can tell the difference between the
presence and absence of a stimulus or the difference between stimulus 1 and stimulus 2.
d’ (dprime): A measure of signal separability (sensitivity of the measurement). It tells us by how many
standard deviations the two signals are separated. It is defined as:
Criterion: An internal threshold that is set by the observer. If the internal response is above criterion, the
observer gives one response (e.g., “yes, I hear that”). Below criterion, the observer gives another response
(e.g., “no, I hear nothing”).
Receiver operating characteristic (ROC): In studies of signal detection, the graphical plot of the
hit rate as a function of thefalse alarm rate.
Electroencephalography (EEG): measures electrical activity from populations of many neurons
using many electrodes on the scalp.
Eventrelated potential (ERP): A measure of electrical activity from a subpopulation of neurons in
response to particular stimuli that requires averaging many EEG recordings.
Magnetoencephalography (MEG): similar to EEG, but measures changes in magnetic activity
across populations of many neurons in the brain. Much more expensive than EEG. Functional magnetic resonance imaging (fMRI): A variant of MRI that makes it possible to
measure localized patterns of activity in the brain. Activated neurons provoke increased blood flow, which
can be quantified by measuring changes of oxygenated and deoxygenated blood to strong magnetic fields.
Positron emission tomography (PET): allows us to define locations in the brain where neurons
are especially active, by measuring the metabolism of brain cells using safe radioactive isotopes.
Light: A wave; a stream of photons, tiny particles that each consist of one quantum of energy
Light can be:
Absorbed: Energy (e.g., light) that is taken up, and is not transmitted at all
Reflected: Energy that is redirected when it strikes a surface, usually back to its point of origin
Transmitted: Energy that is passed on through a surface (when it is neither reflected nor absorbed by
Refracted: Energy that is altered as it passes into another medium, (e.g., light entering water from the
Refraction is necessary to focus light rays and this is done by the lens
The lens can change its shape, and thus alter the refractive power: Accommodation
Astigmatism: A visual defect caused by the unequal curving of one or more of the refractive surfaces of
the eye, usually the cornea
Cataracts: loss of transparency in lens (solved with silicone implants)
Presbyopia: "old sight". Inability to accommodate nearby objects.
Retinitis pigmentosa: A family of hereditary diseases that involve the progressive death of
photoreceptors and degeneration of the pigment epithelium
But the resemblance stops at the RETINA, where transduction occurs.
Light is transduced by rod and cone photoreceptors
Photoreceptors: Cells in the retina that initially transduce light energy into neural energy
Slice of the retina, about half the thickness of a credit card.
photoreceptors in LAST LAYER, mostly because of the pigment epithelium (provide vital nutrients to the
Photoreceptors: Cells in the retina that initially transduce light energy into neural energy.
Rods: Photoreceptors that are specialized for night vision
Cones: Photoreceptors that are specialized for daylight vision, fine visual acuity and color
90 million rods 4 million cones.
Various regions of the retina interact via lateral inhibition: Horizontal cells and amacrine cells
These form the horizontal pathway of information processing in the retina. The vertical pathway describes the information processing from the photoreceptors to the ganglion
Photoreceptors > bipolar cells > ganglion cells
The Receptive Field
Oncenter ganglion cell: response is high when light is on center
Offcenter ganglion cell: response is high when light spot is in the surround
Two mechanisms for dark and light adaptation:
Photoreceptors and their replacement
Neural circuitry of the retina accounts for why we are not bothered by variations in overall light levels
rods: sensitive but slow
cones: fast, wide range, but only after a minimum of light (10 photons per second).
wiring: ganglion cells can still report activity as they sumup what's going on at the photoreceptor level (plus
onoff surround differences).
The visual system regulates the amount of light entering the eye (pupil), and ignores whatever variation in
overall light level is left over (ganglion cells).
RODS can respond to one photon. But that uses up their photopigment and we have to wait until it is
replenished. In low light conditions, that works great, we have time to replenish each photopigment between
When light increases, the photopigments cannot regenerate fast enough, so the rods are bleached. We
become insensitive to changes in overall illumination.
But, the cone system picks up: 10 photons per second (minimum light to see color) to 1 million photons per
the path of image processing from the eyeball to the brain?
1.Eye (vertical path)
Retinal ganglion cells 2.Lateral geniculate nucleus
Acuity: The smallest spatial detail that can be resolved
Measuring visual acuity:
Eye doctors use distance (e.g., 20/20)
Vision scientists use the smallest visual angle of a cycle of a grating
Spatial Frequency: The number of cycles of a grating per unit of visual angle (usually specified in
Cycles per degree: The number of dark and bright bars per degree of visual angle
Contrast: The difference in illumination between a figure and its background.
Contrast Threshold = minimum difference in contrast for you to detect a pattern.
Contrast Sensitivity = inverse of Contrast Threshold
Contrast sensitivity function (CSF): a plot of the threshold contrast to detect the grating (as
opposed to seeing a uniform gray) as a function of spatial frequency.
Contrast sensitivity is the inverse of the contrast Threshold.
For 1 cycle / degree, the light must be 0.01 lighter than the background dark region (surface reflecting 1000
photons, the background reflecting 990).
A contrast of 100% (All light vs No light) corresponds to a contrast sensitivity of 1. We can see about 60
cycles / degree at that sensitivity. 60 cycles per degree is 0.017 (as mentioned before).
Patterns of stripes with fuzzy boundaries are quite common
Trees in a forest, books on a bookshelf, pencils in a cup
The edge of any object produces a single stripe, often blurred by a shadow, in the retinal image
The visual system breaks down images into a vast number of components; each is a sine wave grating with
a particular spatial frequency
This is called “Fourier analysis,” which is also how our perceptual systems deal with sound waves
Phase: The phase of a grating refers to its position within a receptive field Not only is the spatial frequency important, but so is the phase
Two lateral geniculate nuclei (LGNs).
Axons of retinal ganglion cells synapse there.
Relays between retina and cortex.
6 layered structure. Geniculate means bent.
Bottom two layers have larger cells: MAGNOCELLULAR layers.
Top four are PARVOCELLULAR (small in latin).
Two types of layers in LGN: Magnocellular vs. Parvocellular
Fast, large moving objects
Details in static objects
The world is divided at the LGN:
Left side ospace goes right.
Right side ofspace goes left.
But note: Botheyes project to both LGNs!
Each layer: input from ONE eye.
Each layer = organized map of half of the visual field Ipsilateral: Referring to the same side of the body (or brain)
Contralateral: Referring to the opposite side of the body (or brain)
LGN is not only a “relay” between eyes and visual cortex, it also receives information from a number of
other brain areas, functioning as a gate to the cortex.
Also known as primary visual cortex or V1
Major transformation of visual information takes place in striate cortex
It has about 200 million cells! MORE THAN 100 times the number in the LGN.
Two important features of striate cortex:
Dramatic scaling of information from different parts of visual field
1 degree of visual angle at fovea is processed by 15 times more neurons than 1 degree of visual angle
just 10 degrees away from fovea.
This makes sense, because acuity is lower at the periphery. Visual acuity declines in an orderly fashion with
One consequence of cortical magnification is that images in the periphery have much lower resolution than
images at fixation.
This can lead to isual crowding : the deleterious effect of clutter on peripheral object detection.
Stimuli that can be seen in isolation in peripheral vision become hard to discern when other stimuli are
This is a major bottleneck for visual processing.
When we can’t see an object due to crowding, we have to move our eyes to look directly
at it with our high acuity foveal receptive fields.
Selective Responsiveness: Orientation tuning: tendency of neurons in striate cortex to respond