Sensation/Perception and Object Recognition Flashcards
Why is vision different from our other senses?
A lot of real estate in the brain related to visual processing
vision allows us to process and evaluate our environments
Cornea
the eye’s outermost layer. It is the tran
Iris
a thin circular structure in the eye, responsible fro controlling the diameter and size of the pupil and thus the amount of light reaching the retina (gives the eye its color)
Lens
is a transparent, biconvex structure in the eye that, along with the cornea, helps to refract light to be focused on the retina
Fovea
Specialized for high-resolution visual information
Retina
10 densely packed layers of neurons
the deepest layer is made up of millions of photoreceptors
Optic Nerve
transmits impulses to the brain from the retina at the back of the eye
photoreceptors
rods and cones
Rods
contains the photopigment rhodopsin which is destabilized by low levels of light (night vision/black and blues)
Cones
contain photoreceptor photopsin which requires more intense levels of light (daytime/colors)
Ganglion Cells
the output layer of the retina, their axons form a bundle known as the optic nerve and that carries information to the brain
Connection to the CNS
the rods and cones are connected to bipolar neurons that then synapese with ganglion cells –> axons of ganglion cells form the optic nerve –> optic chiasm –> 90% of the axons go to the retinogeniculate pathway (Lateral geniculate nucleus of thalamus) and 10% fots to the pulvinar nucleus of the thalamus and superior colliculus of the midbrain
Geniculocortical Pathway
Terminates in the primary visual cortex (V1/striate cortex) of the occipital lobe
Retinogeniculate pathway
lateral geniculate nucleus of the thalamus
contains more than 90% of the axons from the optic nerve
Pulvinar Nucleus & Superior Colliculus
play a role in visual attention
other 10% of the axons
Retinotopic Maps
Neurons in the visual system represent space in an orderly manner
Visual neurons only respond when a stimulus is presented in a specific region of space (that neuron’s “receptive field”)
Hubel and Wiesel
Individual neurons respond to very specific stimuli
Where does visual information go once it’s processed by V1?
to many different extrastriate visual areas (30+)
Sensation
Early processing that goes on
Most clearly linked to V1
Perception
begins with a stimulus from the environment, which stimulates one of the sense organs –> the input is transduced into neural activity and sent to the brain for processing
Most closely linked to extrastriate cortex
Enigma Pattern
illusory motion
when human view this pattern while undergoing an fMRI scan, there is activation in the MT (responsible for motion)
Patient G.S.
patient that proves that sensation is different from perception and recognition
his sensation was completely intact but have perception and object recognition failures
Shown a photo of a combination lock, started making a rotary motion with his hands but couldn’t make the leap from the sensation to what the object really was (guessed it was a telephone)
What vs Where
Output from V1 is contained in two major fiber bundles called fasciuli (purple bundles)
Neuropsychological support for What vs Where
animal evidence:double dissociation
Bilateral lesion of the temporal lobe leads to a behavioral deficit in a task that requires the discrimination of objects
vs
bilater lesion of the parietal lobe leads to a behavior deficit in a task that requires the discrimination of locations (landmarks)
Patient D.F
had lesion in ventral stream
in the perception condition: she failed to correctly orient her hand to match the orientation of the card to that of the slot
In the action condition: she correctly inserted the card when asked to do so (failing at the recognition of the object in front of her)
Perhaps her dorsal stream wasn’t damaged and also helps guide interactions with objects, maybe better labeled “how”
Neuronal-Based Evidence for What vs Where
Dorsal Stream: The receptive fields of neurons encompass not only the fovea but also non-foveal regions
vs
Ventral Stream: Receptive fields of the neurons always encompass the fovea
Neuroimaging (PET) Evidence for What vs Where
Subject views image A
Object Task/ventral: Subject is asked if the three objects are the same as the screen they viewed
Spatial Task/dorsal: Subject is asked if the threeobjects are in the same positions as the screen they viewed
View-Dependent Recognition
We have a stored representation of object in memory
When we see an object, we “match” it to our stored representation
View Invariant Recognition
Perceptual system extracts information about the components of an object and the relationship between those components
critical properties of an object stay independent of viewpoint
Shape Encoding
simple features can be combined into edges corners and intersections, which can then be grouped into parts and then grouped into objects
What part of the brain is critical for shape and object recognition
Lateral Occipital Cortex
Grandmother Cells/ Hierarchical Representationsof Objects
The final percept of an object is coded by a single cell
aka “grandmother”
Hallie Berry Study (individual electrodes implanted in cortex, neurons would fire specifically to images of hallie berry)
Ensemble Coding of Objects
Cells coding for different features of an object or face work together to permit perception and recognition
Visual Agnosia
Object recognition problems eventough visual information continues to be registered at the cortical level
Sensation is still intact (dont know what keys are when visually looking at it but when holding them in her hand she knows what it is)
A perceptual problem, not a memory problem.
Unusual Views test
Test for apperceptive agnosia
patients would have trouble recognizing a chair from above
Test for Appericeptive Agnosia
some patients would have trouble drawing a chair if missing contours
Patients can draw a photo but not in typical order (cannot fully integrate it and copy it accurately)
Associative Agnosia
Inability to understand or assign meaning to objects
Subject can copy an image accurately but cannot tell you what it is
The border of occipital and temporal lobes and bilaterally
Matching by function Task
Prosopagnosia
Impairment in face recognition
Patients are still able to accurately recognize objects (though below normal)
Can be congential or result from brain damage
Generally involves the occipital and temporal lesions
Fusiform gyrus (in ventral stream) –> fusiform face area(FFA)
Is there a specialized system for face processing?
prosopagnosia suggest so but this hypothesis must be approached cautiously because it is hard to find control objects that match faces in complexity
Patient C.K.
Had severe object recognition deficits but could perceive faces with relatively little trouble
we prefer to integrate all the parts of the face
