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Disorders of Orienting
• Lesions to parietal cortex can produce some strange
behavioural consequences
– patients fail to notice events on the contralesional side
– Patients behave as if they are blind in the contralesional hemifield
but they are not blind
Disorders of Orienting
•
Called Hemispatial Neglect - patients appear unable to process
information in the contralesional hemifield
Disorders of Orienting
• Hypothesis: Parietal cortex somehow
involved in orienting attention into
contralesional space
Disorders of Orienting
• Posner and colleagues
– Use cue-target paradigm to investigate attentional abilities of
parietal lesion patients
Contralesional
Ipsilesional
Disorders of Orienting
invalid- contralesional target
Results: Severe difficulty with
invalidly cued contralesional
target
invalid - ispilesional target
valid - contralesional target
valid - ipsilesional target
Results: Valid cue in
contralesional field is effective
Disorders of Orienting
• Interpretation:
– Patients have difficulty disengaging attention from
good hemifield so that it can be shifted to
contralesional hemifield
Disorders of Orienting
• Interpretation:
– Patients have difficulty disengaging attention from
good hemifield so that it can be shifted to
contralesional hemifield
– Parietal cortex is somehow involved in
disengaging attention
Disorders of Orienting
• Disengage - Shift - Engage Model
– Parietal Cortex notices events and
disengages attention
Disorders of Orienting
• Disengage - Shift - Engage Model
– Parietal Cortex notices events and
disengages attention
– Superior Colliculus moves attention
Disorders of Orienting
• Disengage - Shift - Engage Model
– Parietal Cortex notices events and
disengages attention
– Superior Colliculus moves attention
– Pulvinar Nucleus reengages attention
Disorders of Orienting
• Disengage - Shift - Engage Model
– Parietal Cortex notices events and
disengages attention
– Superior Colliculus moves attention
– Pulvinar Nucleus reengages attention
– Entire process is under some top-down
control from Frontal Cortex
Disorders of Orienting
• Orienting mechanism can be interfered with in normal
brains
Disorders of Orienting
• Orienting mechanism can be interfered with in normal
brains
– changes that are not accompanied by transients are hard to
detect
Disorders of Orienting
• Orienting mechanism can be interfered with in normal
brains
– changes that are not accompanied by transients are hard to
detect
• e.g. building appearing slowly
• orienting mechanism scans the scene aimlessly
Disorders of Orienting
• Orienting mechanism can be interfered with in normal
brains
– changes that are not accompanied by transients are hard to
detect
• e.g. building appearing slowly
• orienting mechanism scans the scene aimlessly
– changes accompanied by full-field transients are hard to
detect
• e.g. change blindness
• orienting mechanism is blinded by the transient
Neural Correlates of Selection
• Since attention has a profound effect on perception,
one would expect it to have some measurable effect
on the brain
Neural Correlates of Selection
• Since attention has a profound effect on perception,
one would expect it to have some measurable effect
on the brain
• This has been confirmed with a variety of techniques:
EEG, fMRI/PET, Unit Recordings
Neural Correlates of Selection
• Electrical activity recorded at scalp (EEG) shows
differences between attended and unattended stimuli
in A1 within 90 ms
Hansen & Hillyard (1980)
Neural Correlates of Selection
• Single Unit Recordings measure signals
from individual neurons
Neural Correlates of Selection
• Single Unit Recordings measure signals
from individual neurons
• Remember that visual cortex neurons
have receptive fields that are tuned to
specific stimulus properties (e.g. color,
motion)
Neural Correlates of Selection
• Single Unit Recordings: Delayed Matchto-Sample task
MONKEY FIXATES CENTRE CROSS
Neural Correlates of Selection
• Single Unit Recordings: Delayed Matchto-Sample task
“CUE” APPEARS AT FIXATION
(not the same “cue” as in the cue-target paradigm)
Neural Correlates of Selection
• Single Unit Recordings: Delayed Matchto-Sample task
DELAY SEVERAL SECONDS
Neural Correlates of Selection
• Single Unit Recordings: Delayed Matchto-Sample task
MONKEY MAKES
EYE MOVEMENT
TO TARGET
Neural Correlates of Selection
• Single Unit Recordings: Delayed Matchto-Sample task
• Question: does attention modulate spike
rate of neurons that respond to visual
stimuli?
Neural Correlates of Selection
During presentation
of the “cue”, only
neurons tuned to it’s
properties are
excited
Neural Correlates of Selection
During the delay, those
neurons do not return to
baseline (is this
memory?)
Neural Correlates of Selection
During 1st 70 ms of
search array, any cell will
be excited if it’s preferred
stimulus is presented
Neural Correlates of Selection
Once attention is
focused, only cells tuned
to the attended object
remain active
Neural Correlates of Selection
• Results: Neurons in visual system respond vigorously to certain
stimuli but are then sharply suppressed if a different stimulus is
selected by attention
Neural Correlates of Selection
• Results: Neurons in visual system respond vigorously to certain
stimuli but are then sharply suppressed if a different stimulus is
selected by attention
• Interpretation: this selection might be a neural correlate of the
perceptual suppression of unattended information
Neural Correlates of Selection
• Is this a neural correlate of
consciousness?
Next Time
• Memory