Neurons in the visual system:
Neurons at higher levels of the visual system respond to more complex stimuli.
light
Optic nerve neurons respond to spots of
oriented bars
Neurons in the visual cortex respond to
complex shapes and faces
Neurons in the temporal cortex respond to
Spatial Organization
how different locations in the environment and on the retina are represented by activity at specific locations in the visual cortex
Neurons in different places in the visual system respond best to specific stimuli is evidence that the visual system is organized in a number of ways.
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Spatial organization at the retinal image is essentially a picture of a the scene.
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Once the image has been transformed into electrical signals, the signals created by each object is then organized in the form of neural maps.
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Objects that create images near each other on the retina are represented by neural signals that are near each other in the cortex.
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Striate Cortex (area V1)
Points in the retinal image are represented spatially in this are
The neural map on the striate cortex is where we stimulate various places on the retina and note where neurons fire
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Retinotopic Map
map on the LGN or cortex in the visual system that indicates locations on the structure that correspond to locations on the retina.
In retinotopic maps, locations adjacent to each other on the retina are also represented by locations adjacent to each other on the structure.
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Cortical Magnification
small area of the fovea that is represented by a large area in the visual cortex.
Brain imaging
procedures that allow us to visualize areas of the brain that are activated by different types of stimuli,tasks or behaviors.
Magnetic Resonance Imaging (MRI)
Brain scanning technique that makes it possible to create images of structures of the brain.
Functional Magnetic Resonance Imaging (fMRI)
brain imaging technique that indicates brain activity in awake, behaving organisms.
fMRI enables researchers to determine
how various types of cognition activate different areas of the brain.
How does the fMRI response occur?
by detecting changes in the magnetic response of hemoglobin because blood flow increases in areas of the brain that are activated. hemoglobin has oxygen in the blood and contains an iron molecule that is magnetic.
The brain is divided into voxels, what are they?
small cube shaped areas of the brain that show colors to indicate whether brain activity increased or decreased in each.
Who used brain imaging to demonstrate cortical magnification in the human visual cortex?
Robert Dougherty and coworkers (2003)
During Dougherty's experiments, the stimulus light was presented in what two places and what did each illuminate?
1. Center(red area)-small area near fove
2. Far from center (blue area)- large area in peripheral retina
How do Dougherty's experiments represent cortical magnification?
Because stimulation of the SMALL foveal area activated a BIG area on the cortex (red) and stimulation of the LARGE peripheral area activated a SMALL area in the cortex.
Who carried out experiments that lowered electrodes perpendicular into a cat's cortex and recorded information?
Hubel and Wiesel (1965)
What did Hubel and Wiesel find?
Found that every neuron had its receptive field at about the same location on the retina. So the cortex is organized in columns. When they moved the electrode through the cortex obliquely, the neurons preferred orientations changed in an orderly fashion.
Location columns
column in the visual cortex that contains neurons with the same receptive field locations on the retina.
Orientation Columns
column in the visual cortex that contains neurons with the same orientation preference.
Adjacent orientation columns have cells with slightly different preferred orientations.
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Hypercolumn
location column with all of its orientation columns that receives information about all possible orientations the fall within a small area of the retina. Suited for processing information from a small area in the visual field.
How do orientation-sensitive neurons respond?
they simplify the task of determining how millions of neurons in the cortex respond by focusing on one small part of a scene. ex: tree trunk in a forest
Each location column contains a complete set of orientation columns.
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Tiling
the adjacent (overlapping)location columns working together to cover the entire visual field. So each circle has information that gets sent to one location column and each location column comes together to cover the entire visual field.
Steps of lesioning or Ablation experiments
First, animal is trained to indicate perceptual capacities. Second, part of the brain is removed or destroyed. Third,animal is retested to see which perceptual abilities still remain.
Purpose of lesioning/ablation experiments
to reveal which portions of the brain are responsible for specific behaviors.
Ungerleider and Mishkin experiments
1. Object discrimination problem
2. Landmark discrimination problem
1. Object discrimination problem (what) procedure
monkey is shown an object and presented with a 2 choice task and is rewarded if the target object is detected.
2. Landmark discrimination problem (where) procedure
monkey is trained to pick the food well next to a cylinder.
What happened when one half of the monkeys' parietal lobe was removed and when the other monkeys' temporal lobe was removed?
Temporal lobe removal showed problems in the object discrimination task: what task/cannot pick the right shape
Parietal lobe removal showed problems in landmark discrimination task:where pathway./cannot pick food well close to a cylinder
What pathway:Ventral Pathway:Temporal Lobe
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Where pathway:Dorsal Pathway:Parietal Lobe
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Both the what and where pathways originate in the retina and go through two types of ganglion cells in the LGN. They are interconnected and receive feedback from higher brain areas.
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Milner and Goodale propose that the dorsal stream is for taking action, like picking up an object. (how)
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Dorsal stream shows function for both location and for action.
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Evidence about dorsal stream comes from:
1. Neuropsychology- study of the behavioral effects of brain damage in humans.
2.Double dissociation- two functions that involve different mechanisms and operate independently.
Double dissociation shows us that recognizing objects and locating objects operate independently of each other.
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Modularity
idea that specific areas of the cortex are specialized to respond to specific types of stimuli.
Areas that are specialized to specific types of stimuli are MODULES which is for processing information about these stimuli.
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Who measured the response of neurons in the inferotemporal (IT) cortex in monkeys?
Rolls and Tovee
Rolls and Tovee found neurons that
respond best to faces with little response to non-face stimuli.
Who presented 96 images of faces, bodies, fruits, gadgets, hands and scrambled patterns to two monkeys while recording from cortical neurons inside the face area.
Tsao and coworkers
What did Tsao and coworkers find?
97 percent of cells in the cortical area are face selective.
Who used fMRI to determine brain activity in response to picture of faces, hands, houses and household objects?
Kanwisher and coworkers
What did Kanwisher find?
found that the fusiform face area (FFA), located in the fusifrom gyrus on underside of the brain below the IT cortex is specialized to respond to faces.
Prospagnosia
difficulty recognizing faces of familiar people or even themselves due to damage to the temporal lobe.
Parahippocampal place area (PPA)
area in temporal lobe activated by indoor and outdoor scenes.
Extrastriate body area (EBA)
area of temporal lobe that is activated by pictures of bodies and parts of bodies.
Distributed Representation
neural activity that is distributed across the brain due to a stimulus.
Who performed two experiments that demonstrated Distributed Representation?
Huth and colleagues (2012)
Procedure of Huth experiments?
Subjects watched a 2 hours film while in a brain scanner and where told to determine which categories were present in each film scene. It confirmed that specific areas of the brain are responsible for perception of specific stimuli.
How did Huth do this?
He determined what kinds of stimuli each voxel responded too and the types of stimuli that causes each voxel to respond. Ex: one voxel responded well when streets, buildings, roads, interiors and vehicles were present.
Objects and actions similar to each other are located near each other in the brain.
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Faces are multidimensional- they cause many different reactions and these different reactions are associated with activity in many different places in the brain.
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Occipital cortex is for
initial processing
Fusiform face area
basic face processing
Amygdala
emotional reactions, face expressions and familiarity
Frontal lobe
evaluation of attractiveness
superior temporal sulcus (STS)
gaze direction, mouth movements and general face movements.
Where perception meets memory-hippocampus-
subcortical structure in the brain that is associated with forming and storing memories.
Anterograde amnesia
loss of the ability to create new memories after the event that caused the amnesia
Who showed that there are neurons in the hippocampus that respond to specific people's faces or buildings like Justin Bieber or the Eiffel Tower?
Quiroga and coworkers (2005,2008)
What did Quiroga do?
he recorded from eight patients with epilepsy who had electrodes implanted in their hippocampus or in the medial temporal lobe and a number of these neurons responded to only some of the stimuli due to past memory.
Where perception meets memory-sparse coding-
Small number of neurons are active for any given complex stimulus. The particular combination of neurons is what allows the brain to identify a specific stimulus.
Advantages to sparse coding:
only a small number of high level neurons need to be active which saves energy and we do not need to have a unique neuron assigned to every object we encounter.
Mind-body Problem
How do physical processes such as nerve impulses or sodium and potassium molecules flowing across membranes (the body part of the problem) become transformed into the richness of perceptual experience (the mind part of the problem)?
Ex: How does the flow
Experience in perceiving the environment plays a role in perceptual development.
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Experience-dependent plasticity
Blakemore and Cooper's (1970) experiments.
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The oblique effect
people perceive horizontal and vertical orientations more easily that other orientations.
What lines occur more frequently?
Horizontal and vertical lines because there are more cortical neurons that respond to these orientations.
The oblique effect is a link between stimuli that typically occur in the environment, neurons that prefer these stimuli, and our ability to perceive these stimuli.
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Expertise hypothesis
the idea that human proficiency in perceiving certain things can be explained by changes in the brain caused by long exposure,practice or training.
Who demonstrated an expertise effect by using fMRI to determine the level of activity in the fusiform face area in response to faces and objects called Greebles?
Gauthier and coworkers (1999)
What did this experiment show in regards to the expertise hypothesis?
that the FFA responds not just to faces but also complex objects which can be established by experience with those objects.
It is still very difficult to design a perceiving machine. But we have robotic vehicles, artificial visual systems like cameras, face recognition.
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The perceptual system start with the
image on the retina which is to determine the object "out there" that created the image.
Inverse projection problem
an image on the retina can be caused by an infinite number of objects.
Occlusions are common in the environment and we are able to recognized blocked or blurred objects while computers cant.
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Viewpoint invariance
the ability to recognize an object regardless of the viewpoint, which is difficult for computers to do.
Perceptual organization
the process by which small elements become perpetually grouped into larger objects.
Perceptual organization involves 2 components-
grouping and segregation
Grouping
the process by which visual events are put together into units or subjects.
Segregation
the process of separating one area or objection from another.
Gestalt psychologists
an approach the proposes principles of perceptual organization and figure ground segregation and states that the whole is different than the sum of its part. this developed as a reaction to structuralism.
Structuralism
States that perceptions are created by combining elements called sensations.
Structuralism distinguished between
Sensations�elementary processes that occur in response to stimulation of the senses�and Perceptions, more complex conscious experiences such as our awareness of objects.
Who was structuralism establised by?
Wundt (late 1800s)
The Gestalt psychologists rejected the idea that perceptions were formed by "adding up" sensations.
The whole differs from the sum of its parts.
Perception is not built up from sensations, but is a result of perceptual organization.
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Apparent movement
an illusion of movement that occurs when two objects separate in space are presented rapidly, one after another between a brief time interval. Flip book.
The whole is different than the sum of its parts, because the perceptual system creates the perception of movement where there actually is none.
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Illusory Contours
contour that is perceived even though it is not present in the physical stimulus.
Gestal Principles produced the principles of perceptual organization
principles that describe how elements in a scene become grouped together.
Principle of good continuation
points that, when connected, result in straight or smoothly curving lines are seen as belonging together and that lines tend to be seen that way to follow a smooth path.
Principle of good figure
every stimulus pattern is seen in such a way that the structure is as simple as possible. contributes to perceiving the five circles
Principle of Similarity
similar things appear grouped together.
Proximity
things that are near each other are grouped together.
Common fate
things moving in the same direction are grouped together
Common Region
elements in the same region are grouped together
Uniform connectedness
connected region of visual properties are perceived as single unit.
The Gestalt psychologists wanted to determine characteristics of the environment responsible for perceptual segregation.
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Perceptual segregation
perceptual organization in which one object is seen as separate from the other objects.
Figure-ground segregation
the perceptual separation of an object and its background. We see an object because the FIGURE stands out from its background, which is called the GROUND.
How do we determine the difference between figure and ground?
reversible figure-ground image...
The figure is more "thinglike" and more memorable than the ground.
The figure is seen in front of the ground. the ground is more uniform and extends behind figure and the contour separating figure from the ground belongs
When the vase is perceived as figure, it is seen in front of a homogeneous dark background.
When the faces are seen as figure, they are seen in front of a homogeneous light background.
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Peterson and Salvagio (2008) presented displays and asked observers to indicate whether the red square was "on" or "off" a perceived figure.
Figures are more likely to be perceived on the convex side of borders.
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Segregation is determined not by just what is happening at a single border but by what is happening in the wider scene.
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The meaning of a stimulus can affect both figure-ground formation and our perception of objects in a scene.
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Scene
view of a real-world environment that contains background elements and objects organized in meaningful ways
Objects
compact and acted upon
Scenes are
extended in space and acted within and are large and complex.
Gist of a scene
general description of a scene.
Who showed observers a target picture and asked them to indicate whether they saw that picture as they viewed a sequence of 16 rapidly presented pictures?
Potter (1976)
People could do this with almost 100% accuracy even when the pictures were flashed for only 250 ms.
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Who presented pictures of scenes ranging from 27 ms to 500 ms and asked observers to write a description of what they saw by using masking to be sure the observers saw the pictures for the exact duration?
Fei-Fei
Visual masking stimulus
a visual pattern that when presented immediately after a visual stimulus, decreases a persons ability to perceive that stimulus. This stops the persistence of vision and therefore limits the effective duration of the stimulus.
Fei-Fei and coworkers (2007) Results
The overall gist of the scene is perceived first, followed by perception of details and smaller objects within the scene.
Regularities in the environment
Characteristics of the environment that occur regularly and in many different situations.
Our past experiences in perceiving properties of the environment play a role in determining our perceptions.
Oblique effect: People perceive horizontals and vertical
Light-from-above heuristic
light in natural environment comes from above us.
Light-from-above assumption
the assumption that light usually comes from above which influences out perception of form.
Our perception of illuminated shapes is influenced by how they are shaded, combined with the brain's assumption that light is coming from above
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Semantic regularities
semantics refers to the meaning of a scene. characteristics associated with the functions associated with different types of scenes which are learned from experience.
Scene schema can influence perception and is the
knowledge of what a given scene typically contains. Ex: office having a desk and computer and bookshelves.
Palmer experiments tested semantic regularities.
The flashed a context scene (kitchen) and 2 target objects (bread,mailbox and drum). Observers could only identify the loaf of bread because it fit in with the scene.
Results of palmer experiments
Targets congruent with the context were identified 80% of the time.
Targets that were incongruent were only identified 40% of the time.
We direct our attention toward specific objects or locations within a scene and ignore other objects or locations.
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Attention
The process of focusing on some objects while ignoring others and it can enhance the processing of the attended object.
Our perceptual system has a limited capacity for processing information.
So we need to pay attention to something by ignoring others.
Visual scanning-looking from place to place
moving your eyes to focus attention on different locations on objects or in scenes to select certain things in our visual environment for enhanced processing.
Fixation
the brief pause of the eye that occurs between eye movements as a person scans a scene.
Saccadic eye movement
rapid eye movement between fixations that occurs when scanning a scene.
Scanning a scene we ignore some objects.
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Overt attention
looking directly at the attended object. fixation and attention are couple in general, not always.
Covert attention
refers to attention without looking.
Binding
the process by which features are combined to create perception of coherent objects.
Binding Problem
features of objects are processed separately in different areas of the brain.
Feature integration theory (FIT)
explains how an object is broken down into features and how these features are recombined to result in a perception of the object.
FIT process
object --> preattentive stage(features distinguished) --> focused attention stage (features combined) --->perception
Preattentive stage
automatic,unconscious and effortless stage of processing during which a stimulus is decomposed into individual ones. This happens before we focus attention on an object. The features of objects are analyzed independently in separate areas of the brain and
Focused attention stage
features are bound into a coherent perception, the independent features derived in the preattentive stage are combined.
Illusory conjunctions
features that should be associated with an object become incorrectly associated with another due to the task reducing the ability to focus attention to those items.
Balints Syndrome
patients with parietal lobe damage show lack of focused attention. unable to do real selective attention and can lead to illusory conjunctions.
Visual search
procedure in which a persons task is to find a particular element in a display that contains a number of elements. feature search and conjunction search.
Feature search
a visual search task in which a person can find a target by searching for only one feature.
Conjuntion search
a visual search task in which it is necessary to search for a combination or conjunction of 2 or more features on the same stimulus to find the target.