MENTAL REPRESENATIONS
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MENTAL REPRESENATIONS
Neur 3680 Midterm I review
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Mental Representations
• Mental representations can start with sensory input and progress to more abstract forms
– Local features such as colors, line orientation, brightness, motion are represented at low levels
A “labeled line”-Activity on this unit “means” that a line is present-Does the line actually have to be present?
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Mental Representations
• Mental representations can start with sensory input and progress to more abstract forms
– Local features such as colors, line orientation, brightness, motion are represented at low levels
A “labeled line”-Activity on this unit “means” that a line is present-Does the line actually have to be present?
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texture defined boundaries are representations arrived at by synthesizing the local texture features
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the representation is embellished and extended
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Mental Representations can be transformed
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SAME
MIRROR-REVERSED
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Mental representations
• Posner letter matching task– Same category or different?– Physically the same?
• Stroop task– interference
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First Principles
• What are some ways that information might be represented by neurons?
– Magnitude might be represented by firing rate (e.g. brightness)– Presence or absence of a feature or piece of information might be represented by
whether certain neurons are active or not – the “labeled line” (e.g. color, orientation, pitch)
– Conjunctions of features might be represented by coordinated activity between two such labeled lines
– Binding of component features might be represented by synchronization of units in a network
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Posner and his colleagues had participants view two letters and respond according to whether these letters were both vowels, both consonants, or one of each. Participants were fastest when viewing two physically identical letters, somewhat slower when viewing the same letter in two different fonts, and slowest in the case where two different consonants were presented. This finding shows that
a. we form multiple representations of stimuli.b. we form representations of stimuli based only on their
physical attributes.c. we form representations of stimuli based only on their
abstract category membership.d. more complex mental representations produce faster
reaction times.
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Posner and his colleagues had participants view two letters and respond according to whether these letters were both vowels, both consonants, or one of each. Participants were fastest when viewing two physically identical letters, somewhat slower when viewing the same letter in two different fonts, and slowest in the case where two different consonants were presented. This finding shows that
a. we form multiple representations of stimuli.b. we form representations of stimuli based only on their
physical attributes.c. we form representations of stimuli based only on their
abstract category membership.d. more complex mental representations produce faster
reaction times.
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VISION
Neur 3680 Midterm I review
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Visual Pathways• Image is focused on the retina
• Fovea is the centre of visual field– highest acuity
• Peripheral retina receives periphery of visual field– lower acuity– sensitive under low light
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Retina has distinct layers…
Photoreceptors:• Rods and cones respond to
different wavelengths
Amacrine and bipolar cells perform “early” processingconverging / diverging input from receptorslateral inhibition leads to centre/surround receptive fields - first step in shaping “tuning properties” of higher-level neurons
signals converge onto ganglion cells which send action potentials to the (LGN)
• two kinds of ganglion cells: • Magnocellular and Parvocellular
RETINA
LGN
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Lateral Geniculate Nucleus maintains segregation:
of M and P cells (mango and parvo)of left and right eyes
Primary visual cortex receives input from LGN
Primary cortex maintains distinct pathways – functional segregation
M and P pathways synapse in different layers
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The Role of “Extrastriate” Areas
1. System is hierarchical
2. System is analytic and parallel
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SUMMARY:
Dorsal stream area MT
M cells-interblobsRetina LGN V1 P cells- blobs
Ventral stream V4
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SUMMARY: movement and direction
Dorsal stream area MT WHERE/HOW
M cells-interblobsRetina LGN V1 P cells- blobs
Ventral stream V4 WHAT
color
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Subtract Voxel intensities during these scans… …from voxel
intensities during these scans
…etc.Time ->
Zeki et al.
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The receptive fields in the primary visual cortex are ________ the receptive fields in the visual association areas.
a. smaller thanb. larger thanc. equal in size tod. less selective than
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The receptive fields in the primary visual cortex are ________ the receptive fields in the visual association areas.
a. smaller thanb. larger thanc. equal in size tod. less selective than
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The term ________ is used to describe the situation in which group 1 is impaired on task X and unimpaired on task Y and group 2 is impaired on task Y and unimpaired on task X. In contrast, the term ________ is used when group 1 is impaired on task X and unimpaired on task Y and group 2 is unimpaired on both tasks X and Y.
a. single dissociation / double dissociationb. double dissociation / single dissociationc. double dissociation / triple dissociationd. triple dissociation / double dissociation
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The term ________ is used to describe the situation in which group 1 is impaired on task X and unimpaired on task Y and group 2 is impaired on task Y and unimpaired on task X. In contrast, the term ________ is used when group 1 is impaired on task X and unimpaired on task Y and group 2 is unimpaired on both tasks X and Y.
a. single dissociation / double dissociationb. double dissociation / single dissociationc. double dissociation / triple dissociationd. triple dissociation / double dissociation
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Subtract Voxel intensities during these scans… …from voxel
intensities during these scans
…etc.Time ->
Zeki et al.
Stroke = Achromatopsia
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Subtract Voxel intensities during these scans… …from voxel
intensities during these scans
…etc.Time ->
MOVING STATIONARY MOVING STATIONARY
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Subtract Voxel intensities during these scans… …from voxel
intensities during these scans
…etc.Time ->
MOVING STATIONARY MOVING STATIONARY
Stroke = akinetopsia
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Visual Neuron Responses• This conceptualization of the visual system was “static” - it did not take into account the
possibility that visual cells might change their response selectivity over time
– Logic went like this: if the cell is firing, its preferred line/edge must be present and…
– if the preferred line/edge is present, the cell must be firing
• We will encounter examples in which neither of these are true!
• Representing boundaries must be more complicated than simple edge detection!
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•Colour•Brightness
EDGES
•Texture•Motion cues•Depth cues
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The distinct modes of vision offered by feedforward and recurrent processing
Victor A.F. Lamme and Pieter R. Roelfsema
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Dorsal vs. Ventral stream- spatial and object vision
Pre-attentive vs. Attentive
Conscious vs. Unconscious
Dichotomies:
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The Feed-Forward Sweep• The feed-forward sweep is the initial response of each
visual area “in turn” as information is passed to it from a “lower” area
• Characteristics:– a single spike per synapse– no time for lateral connections – no time for feedback connections
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The Feed-Forward Sweep• The feed-forward sweep is the initial response of each
visual area “in turn” as information is passed to it from a “lower” area
• Characteristics:– a single spike per synapse– no time for lateral connections – no time for feedback connections
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Curve tracing– monkey indicates whether a particular
segment is on a particular curve– requires attention to scan the curve and
“select” all segments that belong together– that is: make a representation of the entire
curve– takes time
What is the binding problem?
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QUESTIONS: