Vision: Outline

• Eye – Color vision– Receptive Field – Edge Detection

• Visual Path– thalamus (LGN) – primary visual cortex

• Orientation sensitive; Spatial frequency

– Ventral Pathways • Area MT (motion), Object Recognition, Area V4 (color)

• synesthesia

– Dorsal Pathway • Spatial Attention

• Hemispatial Neglect

Ventral

Dorsal

Visual Cortex

Dorsal

Dorsal Pathway

Retina

LGN

V1

V4 ParietalCortex

InferotemporalCortex

(Relay Station)

(Detects Edges)

(Color,Form)

(Shape,Object Recognition)

(Location,How to reach oract upon)

- Line-bisection task

- Extinction

hemispatial neglect syndrome

- Drawing

- Line cancellation

time

Memory/Attention Task(fMRI / ERP)

Regions of Interest

RightVisualField

LeftVisualField

Stim 1 Stim 2

Single-Unit Recording

“spike” = single neuron’s action potential

(Macaque monkey)

SignalAnalysis

Receptive Field

Attention Effects inSingle Neuron Responses

100 msec

Frequencyofspikes

Attended bar

Unattended bar

(Robert Desimone, NIH)

• move your eyes (& your attention)• you ‘see’ the change -- you are

‘perceptually aware’-- at the attended location

• You are unaware at the unattended locations.

Try to find the difference between these two pictures

Attention contributes to object perceptual awareness

‘look for blue horizontal line”

• Inattentional blindness (here)

http://viscog.beckman.uiuc.edu/grafs/demos/15.html

Count how many passes the ‘white’ team makes

• Continuity errors (Terminator 2)

Atypical Alzheimer’s Disease • Also known as Posterior Cortical Atrophy (PCA)• Similar pathology to AD, but:

– Localized in Parietal cortex

• Some clinical similarities to:– Right parietal stroke

(hemispatial neglect)– Bi-parietal stroke (Balint’s syndrome)

Hemispatial neglect

Atypical AD Patient • 68 y-old male • Local bias

c c c

c

cc

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c

• Impaired drawing

• bilateral posterior parieto-temporal atrophy, most pronounced in right hemisphere

• Left visual field extinction

• ‘Perception’ Tasks: • Can the patient ‘discriminate’ two simultaneous shapes? (circle,

square)• Can the patient ‘see’ two simultaneous shapes?

Buffer

Shape Discrimination Task: same / different

Display: Sequential

(duration tailored for each subject)

T 1

Ts 1 & 2

Mask

Interval

T 2

Simultaneous

Buffer

Mask

Buffer

Mask

88% correct 50% correct

Count Task: one / two

Accuracy

R.D. (right TPJ) 50 % (400 ms)

Follow-up study - Single Figure

Accuracy 61% 46%

Conclusion:- R.D. cannot ‘see’ two objects

Control study: Single complex figure90% accuracy

Interactions between Dorsal and Ventral Pathways

Objects exist in space