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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
Results: Valid cue in contralesional field is effective
invalid- contralesional target
valid - contralesional target
invalid - ispilesional target
valid - ipsilesional target
Results: Severe difficulty with invalidly cued contralesional target
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 Match-to-Sample task
MONKEY FIXATES CENTRE CROSS
Neural Correlates of Selection
• Single Unit Recordings: Delayed Match-to-Sample task
“CUE” APPEARS AT FIXATION(not the same “cue” as in the cue-target paradigm)
Neural Correlates of Selection
• Single Unit Recordings: Delayed Match-to-Sample task
DELAY SEVERAL SECONDS
Neural Correlates of Selection
• Single Unit Recordings: Delayed Match-to-Sample task
MONKEY MAKES EYE MOVEMENT TO TARGET
Neural Correlates of Selection
• Single Unit Recordings: Delayed Match-to-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