disorders of orienting lesions to parietal cortex can produce some strange behavioural consequences...

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