Midterm #1 Flashcards
Perception definition
The process or result of becoming aware of objects/relationships/events, by means of senses (recognizing, observing). Enable organisms to organize and interpret the stimuli into meaningful knowledge and act in a coordinated manner.
Sensation Definition
Experience produced by stimulation of a sensory receptor and the resultant activation of a specific brain centre, producing basic awareness.
Subjective experience
Particular to a specific person and is intrinsically inaccessible to the experience or observation of others.
Consciousness
An organism’s awareness of something either internal or external to itself.
Easy problem
Explanation of mental phenomena that are testable by standard methods of science.
Hard problem
What remains once the neurobiological mechanisms of a phenomenon have been explained. Eg: how do mechanisms give rise to subjective experience of colours?
What is it like to be a bat?
Reducing the complex experience of being a bat to mere physical or neuroscientific terms, may miss the true essence of being a bat.
The inverted spectrum thought experiment
There is no way to know that a person is experiencing the colour of something in a certain way.
Steps of The Perceptual Process
- Stimulus in the environment
- Light is reflected and focused
- Receptor processes
- Neural processing
- Perception.
- Recognition
- Action
PLUS the knowledge in the person’s brain.
Distal Stimulus
Environmental stimuli are all the objects in the environments that are available to the observer. Observer selectively attends to objects. Stimulus impinges on receptors resulting in internal representation. Step 1.
Proximal Stimulus
The representation of the distal stimulus on the receptors. Stimulus is “in proximity” to the receptors. Step 2.
Steps 1 and 2 of the Perceptual Process
Step 1: Information about the tree (the distal stimulus) is carried by light.
Step 2: The light is transformed when it is reflected from the tree, when it travels through the atmosphere, and when it is focused on by the eye’s optical system. The result is the proximal stimulus: the image of the tree on the retina, which is a representation of the tree.
Principle of transformation
When stimuli and responses created by stimuli are transformed, or changed, between the environmental stimuli and perception.
Receptor Processes: Step 3
Rod and cone receptors line the back of the eye, and they change light energy into electrical energy and influence what we perceive. Transduction occurs, which changes environmental energy to nerve impulses. End result is an electrical representation of the tree.
Rods
3 types: long, medium, short. Responsible for black and white vision. More sensitive than cones. Good in low light conditions. Not good for detail.
Cones
Colour vision. Good for detail. Not good in low light.
Neural Processing: Step 4
Changes that occur as signals are transmitted through the maze of neurons. Each sense sends signals to different areas of the brain. Signal that reaches the brain is transformed so that it represents the original stimulus (but not an exact copy).
Behavioural Responses (Steps 5-7)
Electrical signals are transformed into conscious experience.
Step 5: Perception occurs when electrical signals that represents object are transformed into experience of seeing object.
Step 6: Person recognizes it as the object (places object in category).
Step 7: Action.
Bottom-up processing
Processing based on incoming stimuli from the environment. Also called data-based processing.
Top-down processing
Processing based on the perceiver’s previous knowledge (cognitive factors). Also called knowledge-based processing.
Psychophysical approach to perception
Use of quantitative methods to measure relationships between stimuli (physics) and perception (psycho).
Physiological approach to perception
Measuring the relationship between stimuli and physiological processes and perception.
Observing perceptual processes at different stages
Relationship A: the stimulus-perception relationship
Relationship B: the stimulus-physiology relationship
Relationship C: the physiology-perception relationship
Example of the stimulus-perception relationship
Two coloured patches are judged to be different.
Example of the stimulus-physiology relationship
A light generates a neural response in the cat’s cortex.
Example of the physiology-perception relationship
A person’s brain activity is monitored as the person indicates what he is seeing.
Absolute threshold
The smallest amount of energy needed to detect a stimulus
Method of limits
Experimenter presents stimuli in either ascending order (intensity is increased) or descending order (intensity is decreased). Observer responds to whether she perceived the stimulus. Cross-over point is the threshold.
Phenomenological method
A researcher asks a person to describe what he or she is perceiving or to indicate when a particular perception occurs. Describes what we perceive.
Classical psychophysical methods
Limits, adjustment, and constant stimuli were the original methods used to measure the stimulus-perception relationship.
Method of adjustment
The observer or the experimenter adjusts the stimulus intensity continuously until the observer can just barely detect the stimulus. Procedure can be repeated several times and the threshold determined by taking the average.
Method of constant stimuli
The experimenter presents five to nine stimuli with different intensities in random order. The threshold is usually defined as the intensity that results in detection on 50% of the trials. Most accurate method, but time consuming.
The Difference Threshold
The smallest difference between two stimuli that a person can detect.
Determining the Difference Threshold
Participants are asked to determine whether they detect a difference between two stimuli. The DL is the difference between standard and comparison stimuli.
The Weber Fraction
The ratio of the difference threshold (DL) to the weight of the standard stimulus. It is constant. He found that when the difference was small, observers had difficulty detecting differences, but they easily detected larger differences. He found that the size of the DL depended on the size of the standard weight. As the magnitude of stimulus increases, so does size of the DL.
Magnitude Estimation
The experimenter first presents a “standard” stimulus, and assigns a value, they then present lights of different intensities, and observer is asked to assign a number to each light that is proportional to the brightness of the standard stimulus. Each light intensity has a brightness assigned to it by observer.
Response compression
Result of magnitude estimation that indicates that doubling the intensity does not necessarily double the perceived brightness. As intensity is increased, the magnitude increases, but not as rapidly as the intensity.
Response expansion
Result of magnitude estimation indicating that doubling the strength of a shock more than doubles the sensation of being shocked. As intensity is increased, perceptual magnitude increases more than intensity.
The Power Function
The relationship between the intensity of a stimulus and our perception of its magnitude follows the same general equation for each sense. These functions are called power functions, and are described by the equation P=KS^n.
Steven’s power law
Relationship of Perceived magnitude (P), equals a constant, K, times the stimulus intensity, S, raised to a power, n.
How physical events influence affect perception
We begin with: seeing in focus, seeing in dim light, seeing in fine details
Electromagnetic spectrum
A continuum of electromagnetic energy that is produced by electric charges and is radiated as waves. The energy in this spectrum ca be described by its wavelength.
Wavelength
The distance between the peaks of the electromagnetic waves. They range from extremely short-wavelength gamma rays, to long-wavelength radio waves.
Visible light
The energy within the electromagnetic spectrum that humans can perceive.
The eye
Where vision begins. Light reflected from objects in the environment enters the eye through the pupil and is focused by the cornea and lens to form sharp images of the object on the retina, which contains the receptors for vision.
Two kinds of visual receptors
Rods and cones.
Visual pigments
Rods and cones contain light-sensitive chemicals called visual pigments that react to light and trigger electrical signals. These signals then flow through the network of neurones that make up the retina.
Optic nerve
Signals emerge from the back of the eye in the optic nerve, which conducts signals towards the brain.
How light is focused by the eye
Focus in retina. Cornea accounts for 80% of the eye’s focusing power, but can’t adjust its focus. The lens accounts for the last 20%, and can adjust the eye’s focus for stimuli located at different distances.
Accommodation
A process that stops objects from being blurred. Accommodation increases the focusing power of the lens and brings the focus point for a near object back to point A on the retina, by bending the light rays passing through the lens.
Near point
The distance at which your lens can no longer adjust to bring close objects into focus.
Far point
The distance at which the spot of light become focused on the retina.
Myopia (nearsightedness)
Trouble seeing distant objects. Refractive myopia: Cornea and lens overland the light. Axial myopia: Eyeball is too long.
Hyperopia (farsightedness)
Trouble seeing near objects. Focus point beyond the retina. Eyeball is too short.
Presbyopia
Trouble seeing near objects due to aging. Lens becomes more rigid with age.
Isomerization
Before the light is absorbed, the retinal is next to the opsin. When a photon of light hits the retinal, it changes in shape, so it is sticking out from the opsin. Isomerization triggers the transformation of the light entering the eye into electricity in the receptors.
What triggers transduction
When the light-sensitive retinal absorbs one photon of light.
Hecht’s Psychophysical Experiment
Experiment enabled him to determine how many visual pigment molecules need to be isomerize for a person to see. He did this by using the method of constant stimuli to determine a person’s absolute threshold for seeing a brief flash of light. Conclusions: 1. A person can see a light if 7 rod receptors are activated simultaneously. 2. A rod receptor can be activated by the isomerization of 1 visual pigment molecule.
Distribution of Rods and Cones
The small area of the fovea contains only cones - when we look directly at an object, its image falls on the fovea.
The peripheral retina (includes all of the retina outside of the fovea) contains both rods and cones. However, many more rods than cones in the peripheral retina.
Blind Spot
Area in the retina where there are no receptors. Where the optic nerve leaves the eye. Because of the absence of receptors, it is called the blind spot.
Why we are not aware of our blind spot
Located off to the side of our visual field, where objects are not in sharp focus. Most importantly: some mechanism in the brain “fills in” the place where the image disappears.
Dark adaptation
Causes the eye to increase its sensitivity in the dark.Two stages: an initial rapid stage, and a later, slower stage.
Dark adaptation curve
Function relating sensitivity to light to time in the dark. As adaptation proceeds, subjects become more sensitive to the light. Reveal the two stages of dark adaptation.
Measuring cone adaptation
Have to ensure that the image of the test light stimulates only cones. Achieve this by having observer look directly at the test light so its image will fall on the all-cone fovea, and by making the test light small enough so that its entire image falls within the fovea.
Measuring Rod Adaptation
Most use people who have rod monochromatic, who have no cones. Their all-rod retinas provide a way to study rod dark adaption without interference from cones. Once dark adaptation begins, the rods increase their sensitivity and reach their final dark-adapted level in about 25 mins.
Process of dark adaptation summary with both cones and rods
As soon as the light is extinguished, the sensitivity of both the cones and the rods begins increasing. The cones determine the early part of the dark adaptation curve, meanwhile the rods are increasing their their sensitivity. After about 3-5 mins, the cones are finished adaption, so their curve levels off. The rods sensitive continues to increase until about 7 minutes of dark adaption, when they catch up to the cones, and then become more sensitive than the cones. Once the rods become more sensitive, they begin controlling the person’s vision.
Rod-cone break
The place where the rods begin to determine the dark adaptation curve.
Visual Pigment Bleaching
When light hits the light-sensitive retinal part of the visual pigment molecule, it is isomerize and triggers the transduction produces. It then separates from the opsin part of the molecule. This separation cause the retina to become lighter in colour.
Visual Pigment Regeneration
In the light, as some molecules are absorbing light, isomerizing, and splitting apart, molecules that have been split apart are undergoing this process in which the retinal and opsin become rejoined.
William Ruston’s procedure
Measured the reservation of visual pigment in humans by measuring the darkening of the visual pigment that occurs after adaptation. Showed that cone pigment takes 6 minutes, whereas rod pigment takes 30 minutes.
Two important results demonstrated by Rushton
- Our sensitivity to light depends on the concentration of a chemical - the visual pigment.
- The speed at which our sensitivity is adjusted in the dark depends on a chemical reaction - the regeneration of the visual pigment.
Spectral sensitivity
An observer’s sensitivity to light at each wavelength across the visible spectrum.
Measuring spectral sensitivity
To determine special sensitivity, we use flashes of monochromatic light, which is light that contain only a single wavelength. We determine the threshold for seeing these monochromatic lights for wavelengths across the visible spectrum.
Results of threshold sensitivity curve
The threshold for seeing light is the lowest in the middle of the spectrum: less light is needed to see wavelengths in the middle than to see wavelengths at either the short or long wavelengths at the end of the spectrum.
Spectral sensitivity curve
Relative sensitivity versus wavelength. The cones are more sensitive the light, and the rods are more sensitive in the dark.
Measuring spectral sensitivity curve
Measure the cone spectral sensitivity curve by looking directly at test light, so that it stimulates only the cones in the fovea, and presenting test flashes of wavelengths across the spectrum. Measure the rod: measuring the sensitivity after the eye is dark adapted and presenting test flashes off to the side of the fixation point.
What does the difference in the sensitivity of the cones and the rods to different wavelengths mean?
As vision shifts from the cones to the rods during dark adaptation, we become relatively more sensitive to short-wavelength light. (near the blue and green end of spectrum)
Rod and Cone Absorption Spectra
Plot of the amount of light absorbed by a substance versus the wavelength of light. The difference between the rod and cone spectral sensitivity curves is caused by differences in the absorption spectra of the rod and cone visual pigments.
The three absorption spectra for the cones
Each of the three cone pigments are contained in its own receptor. They add together to result in a psychophysical spectral sensitivity curve that peaks at 560nm.
Absorption of the rod visual pigment
Closely matches the rod spectral sensitivity curve.
Five types of neurons that make up the retina’s layers
Signals generated in the receptors travel to the bipolar cells, and then to the ganglion cells. Receptors and bipolar cells do not have long axons, but the ganglia cells do. These axons transmit finals out of the retina in the optic nerve. There are also horizontal cells and amacrine cells, which connect neurons across the retina.
Neural Convergence
Occurs when one neuron receives signals from many other neurons. A great deal of neural convergence occurs in the retina.
Signal convergence in rods vs cones
Signals from the rods converge more than do the signals from the cones. 120 million rods in retina vs 6 million cones. 1) The rods result in better sensitivity than cones, and 2) the cones result in better detail vision than the rods/
Why do Rods result in greater sensitivity than cones?
The rods’ greater convergence. Many rods summate their responses by feeding into the same ganglion cell, but only one or a few cones send their responses to a single ganglion cell.
Why do we use cones to see detail?
The cones have better visual acuity because they have less convergence. Acuity is better in fovea than in the periphery.
Lateral inhibition
Inhibition that is transmitted across the retina.
What the Horseshoe Crab Teaches us About Inhibition
Used the Limulus (horseshoe crab) to demonstrate how lateral inhibition can affect the response of neurons in a circuit. The limulus eye is made up of hundreds of tiny structures called ommatidia, and each ommatidia has a small lens on the eye’s surface that is located directly over a single receptor. They found that illumination of the neighboring receptors inhibited the firing of receptor A, and this decrease is caused by lateral inhibition that is transmitted across the Limulus’ eye by the fibres of the lateral plexus.
The Hermann Grid: Seeing Spots at Intersections
Because the response associated with receptor A (at the intersection) is smaller than the response associated with receptor D (in the corridor between the black squares), the intersection should appear darker than the corridor. And this is what happens - we perceive grey images at the intersection. Lateral inhibition therefore explains the dark images at the intersection.
Mach Bands
Illusory light and dark bands near a light-dark border. Can be demonstrated using gray stripes, or by casting a shadow. You will see a dark Mach band near the border of the shadow and a light Mach band on the other side of the border. Because the intensities remain constant across the light stripe on the left and the dark stripe on the right, the small bands we perceive on either side of the border must be illusions. Can be explained by lateral inhibition.
How can mach bands be explained by lateral inhibition
Each of the six receptors in this circuit sends signals to bipolar cells, and each bipolar cell sends lateral inhibition to its neighbours on both sides. Receptors A,B, and C fall on the light side of the border and so receive intense illumination; receptors D,E, and F fall on the darker side and receive dim illumination.
Simultaneous Contrast
Simultaneous Contrast occurs when our perception of the brightness or colour of one area is affected by the presence of an adjacent or surrounding area.
Simultaneous Contrast explained by lateral inhibition
The receptors under the two small squares receive the same illumination. However, the receptors under the light area surrounding one square are intensely stimulated, so they send a large amount of inhibition to the receptors under the left square. The receptors under the dark area surrounding the other square are less intensely stimulated, so they send less inhibition to the receptors under the right square. Because the cells under the first square receive more inhibition than the cells under the right square, their response is decreased more, they fire less than the cells under the right square, and so it looks darker.
Reticular theory for the structure of the nervous system
The nervous system consisted of a larger network of fused nerve cells
Neuron theory on the structure of the nervous system
The nervous system consisted of distinct elements or cells.
Staining
A development that led to the acceptance of neuron theory. It is a chemical technique that caused nerve cells to become coloured so they stood out from surrounding tissue.
Doctrine of specific nerve energies
Johannes Mueller proposed this, which stated that our perceptions depend on “nerve energies” reaching the brain and that the specific quality we experience depends on which nerves are stimulated. He proposed that activity in the optic nerve results in seeing, activity in the auditory nerve results in hearing…etc.
Recording from Neurons
Recording from single neurons provides valuable info about what is happening in nervous system. Important to record from as many as possible because different neurons may respond differently to a stimulus or situation.
Modular organization
Specific functions are served by specific areas of the cortex.
Primary receiving areas
The first areas in the cerebral cortex to receive the signals initiated by each sense’s receptors. The primary receiving area for vision: occipital lobe, hearing: temporal lobe, skin senses: parietal lobe.
Frontal lobe role
Receives signals from all of the senses, and plays an important role in perceptions that involve the coordination of information received through two or more senses.
Receptors
Specialized to respond to environmental stimuli. Although receptors look different, they all have a part that reacts to environmental stimuli and triggers the generation of electrical signals, which eventually are transmitted to neurons with axons.
Nerve
Such as the optic nerve, which carries signals out the back of the eye, consists of the axons of many neurons. They contain many nerve fibres.
Resting potential
-70 millivolts. Stays the same as long as there are no signals in the neuron.
Action potential
Happens when the neurons receptor is stimulated so that a signal is transmitted down the axon. There is a rapid increase in positive charge until the inside of the neuron is +40 mV compared to the outside, followed by a rapid return the baseline of-70 mV. These changes are caused by the flow of sodium and potassium ions across the cell membrane.
Upward phase of the action potential
Change from -70mV to +40mV. Occurs when positively charged sodium ions rush into the axon.
Downward phase of the action potential
Change from +40mV back to -70mV. Occurs when positively charged potassium ions rush out of the axon.
Permeability
A property of the cells membrane that refers to the ease with which a molecule can pass through the membrane. The change in sodium and potassium flow that create the action potential are caused by changes in the fiber’s permeability to sodium and potassium.
Selective permeability
Occurs when a membrane is highly permeable to one specific type of molecule, but not to others.
Permeability during the action potential
Before the action potential occurs, the membrane’s permeability to sodium and potassium is low, so there is little flow of these molecules across the membrane. Stimulation of the receptor triggers a process that causes the membrane to become selectively permeable to sodium, so sodium flows into the axon. When the action potential reaches +40mV, the membrane suddenly becomes selectively permeable to potassium, so potassium flows out of the axon.
Propagated response
The action potential is a propagated response - once the response is triggered, it travels all the way down the axon without decreasing in size. This enables neurons to transmit signals over long distances.
Effect on action potential of changing the stimulus intensity
It does not affect the size of the action potentials, but it does affect the rate of firing.
Refractory period
The interval between the time one nerve impulse occurs and the next one can be generated in the axon. This is the cause of there being an upper limit to the number of nerve impulses per second that can be conducted by an axon.
Spontaneous activity
The action potentials that occur in the absence of stimuli from the environment. It establishes a baseline level of firing for the neuron.
What is the action potentials function
To communicate information.
Neurotransmitters
When action potentials reach the end of a neuron, they trigger the release of chemicals called neurotransmitters, that are stored in structures called synaptic vesicles in the sending neuron. The neurotransmitter molecules flow into the synapse to small areas on the receive neuron called receptor sites that are sensitive to specific neurotransmitters. When a neurotransmitter makes contact with a receptor site matching its shape, it activates the receptor site and triggers a voltage change in the receiving neuron.
Excitatory transmitters
Cause the inside of the neuron to become more positive, a process called depolarization. To generate an action potential, enough excitatory neurotransmitter must be released to increase depolarization to a certain level. Increases the chances that a neuron will generate action potentials and is associated with high rates of firing.
Inhibitory transmitters
Cause the inside of the neuron to become more negative, a process called hyper polarization. Hyperpolarization can prevent the neuron from reaching the level of depolarization needed to generate action potentials. Decreases the chances that a neuron will generate action potentials and is associated with lowering rates of nerve firing.
Why does inhibition exist?
The functions of neurons is not only to transmit information but also to process it, and both excitation and inhibition are necessary for this processing.
Convergence
The synapsing of more than one neuron into a single neuron.
Receptive Field of a neuron
The area on the receptors that influences the firing rate of the neuron.
Center-surround receptive field
The areas of the receptive field are arranged in a centre region that corresponds one way and a surround region that responds in the opposite way.
Center-surround antagonism effect
The fact that the centre and the surround of the receptive field respond in opposite ways causes this effect. Shows what happens as we increase the size of a spot of light presented to the neuron’s receptive field. A small spot of light presented to the excitatory centre causes a small increase in the rate of nerve firing, and increasing the light’s size so that it covers the entire centre of the receptive field increase the cell’s response.
When does centre-surround antagonism come into play
When the spot of light becomes large enough so that it begins to cover the inhibitory area. Stimulation of the inhibitory surround contracts the centre’s excitatory response, causing a decrease in the neuron’s firing rate. So, this neuron responds best to a spot of light that is the size of the excitatory centre of the receptive field.
Problem of sensory coding
How does the firing of neurons represent various characteristics of the environment? One answer: specificity coding.
Specificity coding
The representation of particular objects in the environment by the firing of neurons that are tuned to respond specifically to that object. There are neurons that are specifically tuned to each object in the environment.
Grandmother cell
A neuron that responds only to a specific stimulus.
Evidence from neurons that respond to specific stimuli
Recorded from eight patients with epilepsy who had electrodes implanted in their hippocampus or medial temporal lobe to help localize precisely where their seizures originated. A number of neurons responded to some of the stimuli that was showed to patients. But, some neurons responded to a number of different views of just one person or building. These neurons are not responding to visual features of the pictures, but to concepts. These neurons were in the hippocampus and MTL - areas associated with the storage of memories.
Are the neurons that respond to specific stimuli grandmother cells?
According to Quiroga, the answer is no. It is unlikely that these neurons respond to only a single object or concept. First, it would be extremely difficult to find the one neuron that responded to a particular person or concept. Second, if they had had time to present more pictures, they might have found other stimuli that caused their neurons to fire.
Distributed coding
The representation of a particular object by the pattern of firing of groups of neurons. Distributed coding allows the representation of a large number of stimuli by the firing of just a few neurons.
Sparse Coding
The idea that a particular object is represented by the firing of a relatively small number of neurons. This concept is somewhere between specificity coding and distributed coding.
Mind-body problem of perception
How do physical processes such as nerve impulses or sodium and potassium molecules flowing across membranes become transformed into the richness of perceptual experience?
Lateral geniculate nucleus (LGN)
Most signals from the retina travel out of the eye in the optic nerve to the LGN in the thalamus. From here, signals travel to the primary visual receiving area in the occipital lobe of the cortex.
Striate cortex
Other name for the visual receiving area, because of the white stripes that are created within this area by nerve fibres. From the striate cortex, signals are transmitted along two pathways, one to the temporal lobe, and the other to the parietal lobe.
Receptive fields of LGN Neurons
LGN neurons have the same centre-surround configuration as retinal ganglion cells. Neurons in the LGN, like neurons in the optic nerve, respond best to small spots of light on the retina.
Major function of LGN
Not to create new receptive field properties, but to regulate neural information as it flows from the retina to the visual cortex.
Information flow in the LGN
The LGN does not just receive signals from the retina and then transmit them to the cortex. The LGN receives info from many sources, including the cortex, and then sends its output to the cortex. One of the purposes of the LGN is to regulate neural info as it flows from retina to cortex.
Organization of LGN by Left and Right Eyes
The LGN is a bilateral structure, which means there is one LGN in left hemisphere and one in right hemisphere. Viewing one of the nuclei in cross section revels six layers. Each layer receives signals from only one eye. Layers 2,3,5 receive signals from ipsilateral eye (eye on same side of body), and layers 1,4,6 receive signals from contralateral eye (eye on opposite side of body from LGN).
Organization of LGN as a spatial map
The correspondence between points on the LGN and points on the retina creates a retinotopic map on the LGN.
Retinotopic map
A map in which each point on the LGN corresponds to a point on the retina. This correspondence means that neurons entering the LGN are arranged so that fibres carrying signals from the same area of the retina end up in the same area of the LGN, each location on the LGN corresponds to a location on the retina, and neighbouring locations on the LGN correspond to neighbouring locations on the retina.
Finding of Hubel and Wiesel
Found cells in the striate cortex with receptive fields that, like centre-surround receptive fields of neurons in the retina and LGN, have excitatory and inhibitory areas. But these areas are arranged side by side rather than in the centre-surround configuration - these cells are called simple cortical cells.
Simple cortical cells
Cells that are arranged side by side. We can tell from the layout of the excitatory and inhibitory areas of the simple cell that a cell with this receptive field would respond best to vertical bars.
Orientation tuning curve
The relationship between orientation and firing is indicated by a neuron’s orientation tuning curve, which is determined by measuring the responses of a simple cortical cell to bars with different orientations. There are neurons that respond to all of the orientations that exist in the environment.
Complex cells
Like simple cells, respond best to bars of a particular orientation. However, unlike simple cells, most complex cells respond only when a correctly oriented bar of light moves across the entire receptive field, and many respond best to a particular direction of movement.
End-stopped cells
Cells that fire to moving lines of a specific length or to moving corners or angles. The neuron responds when the corner moves up-ward. The neuron’s response increases as the corner-shaped stimulus gets longer, but then stops responding when the corner becomes too long.
What do Hubel and Wiesel’s finding that some neurons respond only to oriented lines indicate? And why does this make sense?
Indicates that neurons in the cortex do not simply respond to “light”; they respond to some patterns of light and not to others. Makes sense because: the purpose of visual system is to enable us to perceive objects in the environment, and many objects can be represented by lines of various orientations.
Retinal ganglion cells respond best to
Spots of light
Cortical end-stopped cells respond best to
bars of a certain length that are moving in a particular direction
Selective adaptation
If the neurons fire for long enough, they become fatigued, or adapt. This adaptation causes two phycological effects: 1) the neuron’s firing rate decreases, and 2) the neuron fires less when that stimulus is immediately presented again. According to this idea: presenting a vertical line causes neurons that respond to vertical lines to respond, but as these presentations continue, these neurons eventually begin to fire less to vertical lines.
Why is adaptation selective
only the neurons that respond to verticals or near-verticals adapt, and other neurons do not.
Basic assumption behind a psychophysical selective adaptation experiment
If these adapted neurons have anything to do with perception, then adaptation of neurons that respond to verticals should result in the perceptual effect of becoming selectively less sensitive to verticals, but not to other orientations.
Grating stimuli/contrast threshold
Alternating bars. High-contrast gratings are on the left, and lower-contrast on the right. A grating’s contrast threshold is the difference in intensity at which the bars can just barely be seen.
Selective rearing
If an animal is reared in an environment that contains only certain types of stimuli, then neurons that respond to these stimuli will become more prevalent.
Neural plasticity/experience-dependent plasticity phenomenon
The response properties of neurons can be shaped by perceptual experience. According to this idea, rearing an animal in an environment that contains only vertical lines should result in the animal’s visual system have neurons that respond predominantly to verticals.
Cortical magnification factor
The area representing the cone-rich fovea is much larger than one would expect from the fovea’s small size. The apportioning of the small fovea with a large area on the cortex is called the cortical magnification factor.
Brain imaging
Refers to a number of techniques that result in images that show which areas of the brain are active.
Positron emission tomography (PET)
A person is injected with a low does of radioactive tracer that is not harmful. The tracer enters the blood stream and indicates the volume of blood flow. Changes in the activity of the brain are accompanied by changes in blood flow, and monitory the radioactivity of the injected tracer provides a measure of this blood flow.
fMRI
Based on the measurement of blood flow, like PET. Determines the relative activity of various areas of the brain by detecting changes in the magnetic response of the hemoglobin that occurs when a person perceives a stimulus or engages in a specific behaviour.
Connection between cortical area and acuity experiment (Robert Duncan and Geoffrey Boynton)
Good acuity is associated not only with sharp focusing of images on the retina, and the small amount of convergence of the cones, but also with the relatively large amount of brain area devoted to the all-cone fovea.
Location columns
Hubel and Wiesel concluded that the cortex is organized into location columns that are perpendicular to the surface of the cortex so that all of the neurons within a location column have their receptive fields at the same location on the retina.
Orientation columns
The cortex is organized into orientation columns, with each column containing cells that respond best to a particular orientation. Also, adjacent columns have cells with slightly different preferred orientations.
Ocular Dominance Columns
Neurons in the cortex are also organized with respect to the eye to which they respond best. Means that each neuron encountered along a perpendicular electrode track responds best to the same eye.
Ocular dominance
Neurons having a preferential response to one eye than to another. The cortex consists of a series of columns that alternate in ocular dominance in a left-right-left-right pattern.
Hypercolumns
All three types of columns could be combined into one larger unit called a hypercolumn. Each hypoercolumn contains a single location columns, left and right ocular dominance columns, and a complete set of orientation columns that cover all possible stimulus orientations from 0-180 degrees.
Hubel and Wiesel’s “processing model”
They thought of a hyper column as a “processing model” that processes information about any stimulus that falls within the location in the retina served by the hyper column. They based this on: each hyper column contains a full set of orientation columns, so that when a stimulus of any orientation is presented to the area of retina served by the hyper column, neurons within the hyper column that respond to that orientation will be activated.
How is an object represented in the striate cortex?
The pattern of activation of an object on the striate cortex is distorted compared to the actual object. The magnification factor allots more space on the cortex to the parts of the image that fall on the observer’s fovea. A large stimulus, which stretches across the retina, will stimulate a number of different orientation columns, each in a location in the cortex that is separated form the other columns.
Brain Ablation experiment
Once the animals perception has been measured, a particular area of the brain is ablated (removed or destroyed), either by surgery or by injecting a chemical at the area to be removed. The monkey is then retrained to determine which perceptual capacities remain and which have been affected by the ablation.
Brain ablation on monkeys experiment results (what/where pathways)
Part of temporal lobe was removed in some monkeys: testing showed that the object discrimination problem was difficult for them. Indicates that the pathway that reaches the temporal lobe is responsible for determine and object’s identify. Therefore, called the path leading from striate cortex to temporal lobe the what pathway.
Other monkeys had their parietal lobes removed, had difficulty solving landmark discrimination problems. This indicates that the pathway that leads to parietal lobe is responsible for determine objects location - called where pathway.
Ventral pathway / dorsal pathway
What pathway is also called the ventral pathway, and where pathway is also called the dorsal pathway.
Neuropsychology
The study of the behavioral effects of brain damage in humans.
The behaviour of patient D.F
The method of determining dissociations used on DF, who suffered damage to her ventral pathway. She had trouble matching the orientation of a card held in her hand to different orientations of a slot. But, she was able to place the card through the slot. Thus, she performed poorly in the static orientation-matching task, but did well as soon as action was involved. This behaviour was interpreted as there being one mechanism for judging orientation and another for coordinating vision and action.
How pathway suggestion
Dorsal pathway (where) should actually be called the how/action pathway, because it determines how a person carries out an action.
Rod and frame illusion
Two small lines inside the tilted squares appear slightly tilted in opposite directions, even though they are parallel vertical lines. Observers were presented with a matching task and a grasping task. Results showed that observers had to adjust the matching stimulus in order to make it match their perception of the rod, but the tilted square did not affect the grasping task.
Rationale behind rod and frame illusion
Because these two tasks involve different processing streams (matching task = ventral/what, grasping = dorsal/how), they may be affected differently by the presence of the surrounding frames. So, conditions that created a perceptual visual illusion (matching) had no effect on the person’s ability to take action (grasping). Supports the idea that perception and action are served by different mechanisms.
Module
A structure that is specialized to process information about a particular type of stimulus is called a module.
Fusiform face area (FFA)
Area located in the fusiform gyrus on the underside of therein directly below IT cortex. FFA is specialized to respond to faces.
Prosopagnosia
Caused by damage to the temporal lobe. Difficulty recognizing the faces of familiar people.
How can the nervous system achieve adaptation to the specific environment?
Through a process that causes neurons to develop so that they respond best to the types of stimulation to which the person has been exposed. This is the process of experience-dependent plasticity. Brain imaging experiments have also demonstrated a shift in responding of neurons in the FFA due to training.
Greebles experiment
Observers were shown both human faces and Greebles. Results indicated that the FFA neurons responded poorly to the Greebles but well to the faces. The participants were then trained in “Greeble recognition”. After the training, participants had become “Greeble experts”. After the training, the FFA neurons responded about as well to the Greebles as to faces.
How do neurons become specialized?
Specialized tuning is at least partially the result of experience-dependent plasticity. This makes it possible for neurons to adapt their tuning to objects that are seen often and that are behaviourally important.
