Midterm 1 Flashcards
Empiricism vs. Rationalism
Rationalism refers to the idea that we can only rely on our reason to know the world. By contrast, Empiricism proposes that we can only trust our senses.
Plato: rationalist
Locke: empiricist
Kant: both
Rationalism: We can’t trust our senses at all, what matters is the realm
of « ideal forms » that we can only know using our reason.
Empiricism: All knowledge originates from sensory
experience.
Plato and the allegory of the cave
The man coming out of the cave realizes that all the shadows he has seen his whole life were just reflections of true things, all illusions. The shadows are our ordinary everyday perception, the input from our senses that we should not trust according to Plato. It is through reason that you can get out of the cave and see the real world.
Locke: simple and complex ideas
information from our senses enters our minds as “simple ideas”— for instance the simple idea of “blue” or the simple idea of a “triangular shape”. These simple ideas can then be assembled together to form “complex ideas” – for instance the complex idea of a “blue triangle”
Hume
Inference of necessary cause and effect (B has to follow A)
relationship is invalid, but psychologically we believe there
is cause and effect.
Beliefs are caused by psychological « habits » (e.g. the sun
has risen every morning so far, we should expect it to rise
tomorrow again).
Kant
We may never really know the thing-in-itself (Noumenon: the world as it really is)
- All we can know is the impression that the noumenon
exerts on our senses (Phenoumenon).
- Our minds have to contribute innate knowledge in order
to make sense of our sensations (space, time, cause and
effect).
Kant: Psychology can be like history. History is based on facts so it is not a science and same for psychology.
What Kant was saying is that our senses don’t directly convey the four-dimensional nature of the world; it is our minds that have to reassemble the four dimensions from the rudimentary information received from our senses. In doing so, Kant managed to reconcile Rationalism with Empiricism. Our reason alone isn’t sufficient to know the world: without sensory information, our a priori structures are “empty”, they lack content. Conversely, sensory information by itself is meaningless, it is the a priori structures of our minds that give sensory information a meaningful form. We need both our reason and our senses to perceive the world.
Weber: just noticeable difference and fractions
discovered that for each sensory modality there was a fixed ratio that characterized the relationship between the just noticeable difference and the standard stimulus: Weber’s fractions.
Just noticeable difference (JND): What is the smallest
weight difference that someone can perceive ?
-> Another term for JND is the difference threshold
Weber fractions: the JND between the standard and
comparison weights is always close to 1/40 (or 0.025) of
the standard weight.
Fechner’s law
« Each JND is perceptually
equivalent !!! »
The 1 gram difference between 40 and 41 grams is exactly as
perceptually salient as the 10 gram difference between 410 and 400
grams.
Since the JND is the smallest detectable difference, it cannot be further
fragmented, i.e. perceptual « atom ».
Fechner’s law mathematically describes the relationship between physical intensity and subjective perception.
Fechner believed mind and matter were two sides of the same coin
Thresholds (absolute and difference)
Difference threshold: smallest difference you can perceive
- Absolute threshold: smallest physical intensity you can
perceive.
- The limit at which a stimulus is « detected» vs. « not
detected » is not sharp.
- The function has a logistic (« S ») shape!
- The threshold is defined as the sensory intensity at which a
stimulus is detected 50% of the time.
Thresholding procedures
Thresholding methods: what is the best way to quickly obtain
the most accurate threshold?
Methods of constant stimuli:
- Several intensities are systematically tested in a random order
- Most accurate, but takes a long time!
Methods of limits – ascending/descending
- Ascending/descending cycles; change direction when a « yes »
or a « no »
- A bit less accurate
- Some intensities still don’t contribute much information
Staircase method:
- Go back as soon as there is a change in response
- Faster than the method of limits, and almost as accurate.
Method of adjustment:
- Let the participant increase/decrease intensity in order to
identify the threshold
- Really fast
- Not very accurate
Concept of signal and noise
Signal and noise: Because what we subjectively perceive isn’t
100% determined by the « signal » present in the environment,
there is also « noise » in the signal.
- attention fluctuates
- criterion for saying « yes » may also fluctuate
- spontaneous activity in the nervous system
Noise masks signals and sensations
Noise is conceptual, whatever is causing change in your perception that is not related to the signal and that we don’t control or measure
There are a number of factors other than the strength of the sensory signal that can influence our perception. When these factors are unknown, or unmeasured, we call them “noise”. Because of sensory “noise”, the absence or presence of a sensory signal can sometimes lead to the same subjective sensory experience.
Signal refers to the true sensory information coming from the external world. Noise refers to the various physiological or psychological processes influencing our perception of that external stimulus in an unpredictable manner.
The signal is the meaningful information that you’re actually trying to detect. The noise is the random, unwanted variation or fluctuation that interferes with the signal.
Signal detection theory
Signal detection theory: you will do more errors and wait less if you are rewarded for answering quicker, and the opposite if punished
Magnitude rating (relative and absolute)
In proportion, how much more/less intense are two stimuli of
different intensites perceived?
In relative magnitude rating, stimulus intensity is rated relative to a fixed comparison stimulus called a modulus. In absolute magnitude rating, stimulus intensity is rated relative to two fixed boundaries. In cross-modality matching and general labeled magnitude rating scales, stimulus intensity of one particular sensory modality is rated against another sensory modality.
absolute magnitude rating, and in which participants are asked to rate the intensity of physical stimuli using a scale that goes from the lowest intensity of stimulation that someone can perceive to the highest.
Stevens power law
[R = aSb] R: Response S: Stimulus magnitude b: controls the curvature of the function a: Corrects for the scaling of measurement units used for S
Cross-modality matching
In cross-modality matching and general labeled magnitude rating scales, stimulus intensity of one particular sensory modality is rated against another sensory modality.
one last development related to cross-modality matching was the creation of general labeled magnitude scales (gLMS) that are designed to estimate the intensity of all sorts of sensations with the same scale.
gLMS scale
general labeled magnitude scales (gLMS) that are designed to estimate the intensity of all sorts of sensations with the same scale.
Perception Step 1: Transduction
The physical stimulus interacts with a specific
receptor located on a peripheral sensory neuron and causes
the neuron to fire, i.e. the stimulus is transduced into a
electrical signal
Perception always starts with the transduction (step 1) of a physical stimulus into a nerve impulse.
Sensory neurons, which are almost always located in the periphery, have special receptors, called transducers, located at the surface of their cell membrane and that react to physical stimuli through various biochemical processes. Eventually, the biochemical processes initiated at the level of the transducer will cause the neuron to fire a series of action potentials. Action potentials are all-or-none nerve impulses that form the basis of all the activity in our nervous system (see video 2.1). Although there are many exceptions to this, stronger stimuli will tend to cause neurons to rapidly fire numerous action potentials, or to have a high firing rate (see figure 2.2). Therefore, at the end of the processes of transduction, the physical stimulus is said to have been encoded by the nervous system: it has been transformed into a train of action potentials which can be characterized by its firing rate.
Perception Step 2: Transmission
The signal is transmitted to the brain.
The second step of sensory perception consists in the transmission (step 2) of sensory information from the sense organs to the brain. Following transduction, nerve impulses will travel along the axon of the sensory neuron and enter the central nervous system (CNS). The central nervous system can be defined as the portion of the nervous system that is protected by the meninges – roughly the brain and the spinal cord. Almost all sensory neurons have their cell bodies located in the periphery with their axons projecting into the CNS. The only exception is the retina and optic nerve, which are considered part of the CNS. For most of our senses, the sensory neurons are located in the head, and they enter the CNS through their respective pair of cranial nerves. Cranial nerves can be either sensory, motor, or both (see fig. 2.3). By contrast, tactile information coming from the body reaches the CNS through peripheral nerves entering the spinal cord in between each of our vertebra (see figure 2.4). Most sensory neurons will then transmit their information by synapsing on a second neuron located in the thalamus. Neurons communicate with one another through the release and binding of neurotransmitters in the synaptic cleft, the small gap that exists between the axon terminals of the presynaptic neuron and the dendrites of the postsynaptic neuron. The term synapse technically refers to the functional structure composed of the axonal terminal, the synaptic cleft, and the dendrite, but it can also be used to refer more loosely to the process of one neuron communicating with another (see video 2.2). Neurotransmitters are molecules that are released by the presynaptic neuron and that have the potential to bind to their respective receptors located on the postsynaptic neuron, similar to a key to a lock. Neurotransmitters can have excitatory or inhibitory effects, that is they can either increase or decrease the chance that the postsynaptic neuron will fire an action potential. When the balance between excitation and inhibition reaches a certain threshold, the postsynaptic neuron will fire an all-or-none action potential that will transmit sensory information to the cerebral cortex.
Perception Step 3: Perception
The signal reaches the cortex and produces a counscious
perceptual experience.
Perception Step 4: Modulation
Cognitive factors, like expectations, attention, etc. will
influence how sensations are perceived.
Modulation in the central nervous system is mainly implemented by means of reciprocal feedback connections. For instance, the connections between the thalamus and cortex are far from being unidirectional. It is estimated that the number of connections from the cortex to the thalamus can be 10 times as much as the number of connections from the thalamus to the cortex. The cortex therefore seems to be in a good position to control how much input it receives from the thalamus. This way, psychological factors such as expectations, motivation and attentional focus can influence perception by determining which type of sensory information to prioritize.
Accomodation
The process in which the lens
changes its shape, thus altering its refractive
power.
Nearby objects require more refraction
because light rays from near objects diverge
more.
Diopter: The focusing force of a lens can be
measured in diopters, i.e. the reciprocal of the focal
length.
Focal length: Distance between the lens and the
point at which light rays converge (the focus).
maximal accomodation decreases with age
Different types of visual deficits
Problems of refraction
Emmetropia: The happy condition of no refractive error.
Myopia: When light is focused in front of the retina and distant objects
cannot be seen sharply; nearsightedness.
Hyperopia: When light is focused behind the retina and near objects
cannot be seen sharply; farsightedness.
-> Presbyopia is a form of hyperopia associated with old age:
eventually the lens will loose its elasticity.
Astigmatism: Unequal curving of one or more of
the refractive surfaces of the eye, usually the
cornea.
Rods and cones
Rods: Photoreceptors specialized for night vision. - Respond well in low luminance conditions - Do not process color Cones: Photoreceptors specialized daytime vision, fine visual acuity, and color. - Respond best in high luminance conditions Cons pigments regenerate much faster then rods Much more rods (~90 millions) than cones (~4-5 millions) in your retina - More cones in your fovea, almost no cones outside of the fovea. - This means that you have very poor color vision in your periphery. It may seem as if your entire field of view has full-resolution color, but it does not
Visual angle
The visual angle is the place on the retina taken by an object. One unit of arc corresponds to one degree. A degree (or an arc) can be divided in minutes. There are 60 minutes in a degree (or in a unit of arc).
The visual angle of an object is a
function of both its actual size and distance from
the observer and it corresponds to the size of the
object on the retina.
Phototransduction
the conversion of light into a change in the electrical potential across the cell membrane. This process involves the sequential activation of a series of signaling proteins, leading to the eventual opening or closing of ion channels in the photoreceptor cell membrane.
The process by which photoreceptors transform light into electrical signals is known as phototransduction: Each photopigment consists of an opsin and a chromophore. The structure of the opsin determines the wavelength of light that the photoreceptor responds to, and it will differ between rods and cones, as well as between different types of cones. Mammals have an 11-cis retinal as their chromophore. When a photopigment absorbs a photon, it undergoes photoisomerization, which is the trigger for phototransduction. In this state, the photoreceptor is “bleached”, i.e. it cannot absorb any more light. It will therefore undergo the process of photopigment regeneration.
What is important to remember here is that the opsin part of the photopigment captures light, which produces a chain of chemical reactions that temporarily “bleaches” the chromophore part of the photopigment. When the number of “bleached” photopigments reaches a certain threshold, the photoreceptor will change the rate at which it is releasing neurotransmitters to indicate that it has received light: this is how light becomes transduced into neuronal activity.
Dark and light adaptation
Dark adaptation: When entering a dark room
(cinema), your visual system slowly adjusts
itself.
Four mechanisms for dark adaptation:
1. Pupil changes its size; fast, but effects are
relatively limited.
2. Rods and cones will gradually become more
sensitive to light.
3. The duplex retina: rods will take over cones
4. Neural circuits enhance contrast, making vision
possible regardless of global luminance levels.
The shift from photopic to scotopic vision is indeed the one that takes most time. It can take up to 30 minutes for the human eye to complete the process of dark adaptation
Receptive fields
The region on the retina in which stimuli influence a neuron’s
firing rate.
- Receptive fields have a center-surround organization (ON/OFF or OFF/ON)
-> This allows to increase contrast
-> This organisation depends on horizontal cells
1) Light hyperpolarizes (renders polarity more negative) the center cone
2) On-center bipolar cells reverse the sign of the cone (Off-center bipolar
would keep the same sign).
3) Dark depolarizes (renders polarity more positive) surround cones
4) This activates horizontal cells which in turn inhibits all the cones. Because
the centre cone is placed in the middle of two horizontal cells, it receives
+ more inhibition than surround cones.
5) This amplifies the bipolar ON cell activity, and subsequently retinal
ganglion cell activity.
ON/OFF cells
There are two types of bipolar cells and twotypes of ganglion cells: ON-center and OFFcenter.
-> ON-center bipolar cells reverse the sign of the
photoreceptor (as seen previously).
-> OFF-center bipolar cells don’t reverse the sign
of the photoreceptor.
This allows black-on-white to « stand out » as
much as white-on-black.
ON-center ganglion cells—excited by light that falls
on their center and inhibited by light that falls in
their surround.
• OFF-center ganglion—inhibited when light falls in
their center and excited when light falls in their
surround.
Lateral inhibition
The center-surround configuration that amplifies contrast comes from a phenomenon that implicates horizontal cells. This mechanism is known as lateral inhibition. As the name suggests, horizontal cells are connected to many photoreceptors. When there is no light, photoreceptors release glutamate, and this causes horizontal cells to be excited. When horizontal cells are excited, they send high levels of inhibitory feedback to all of the photoreceptors that they are connected with, causing them to release less glutamate. When everything is dark, the balance between excitatory activity from photoreceptors and inhibitory activity from horizontal cells produces an even level of glutamate release across all photoreceptors. However when light is shone on a photoreceptor surrounded by darkness, the light will cause the photoreceptor to release less glutamate, as seen previously. However, this reduction in glutamate release will be amplified by the surrounding darkness through horizontal cells because 1) darkness causes horizontal cells to reduce glutamate release in the photoreceptors that they are connected with, and 2) because geometrically-speaking the center photoreceptor receives more inhibitory feedback from surround photoreceptors through horizontal cells than the other way around. This will cause even less glutamate release in the photoreceptor that is receiving light, as if it was receiving more light than it actually does. This is how the center-surround organization of ganglion cells is generated. By contrast, if there is light all over, the center photoreceptor won’t benefit from the extra inhibition coming from horizontal cells connected with surround photoreceptors, and it will therefore release more glutamate than if the surround was in darkness. Lateral inhibition can be said to be a form of normalization because it subtracts the average level of activity from the larger surround zone from activity at the center. The “subtraction” is implemented by the inhibitory feedback enacted by horizontal cells, and the “averaging” results from the geometrical organization of the center-surround receptive field: there are many receptors surrounding the central one.
Edge enhancement.
The process by which “edges”, that is the transition between two surfaces, is enhanced. Edge enhancement is due to center-surround organization, which is due to lateral inhibition. edge enhancement explains certain phenomena like Mach bands and Hermann grid illusion.
The purpose of the center-surround
organisation is to help us perceive the edges
of objects
Center Surround Receptive fields
Midget vs diffuse bipolar cells
• Diffuse bipolar cell: Receives input from multiple
photoreceptors.
• Midget bipolar cell: Receives input from a single
cone.
Parvocellular vs Magnocellular pathway
P ganglion cells: Connect to the parvocellular
pathway.
• Receive input from midget bipolar cells
• Parvocellular pathway is involved in fine visual
acuity, color, and shape processing; poor
temporal resolution but good spatial resolution.
– M ganglion cells: Connect to the magnocellular
pathway.
• Receive input from diffuse bipolar cells
• Magnocellular (“large cell”) pathway is involved in
motion processing; excellent temporal resolution
but poor spatial resolution
