Blindsight

Seeing without Awareness

What is Blindsight

• ‘Blindsight’ (Weiskrantz): residual visual function after V1 damage in the lack of any visual awareness.

• Subject report that they don’t see anything, but still give correct responses.

• There is some activation of higher level visual areas. • Not going through V1. (In some cases residual V1

activity).

Blindsight: Example

X

X or O?

Subjects answer this correctly, but claim that they were merely guessing

Residual Functions

• Patients move their eyes towards the approximate position of a briefly presented stimulus they had not seen.

• point to the approximate location and say that they are merely guessing.

• Distinguish orientation (about 100 compared with 2-30)• Stationary from moving stimuli • Direction of motion • Wavelength (20-30 nm compared with a few nm)

Blindsight Shape Discrimination

Large shapes (7 degrees), not complex shapes or multiple shapes

Helen, the Zombie monkey

The bilaterally destriated monkey Helen roamed freely among the objects in the test arena.

D.F. orients without seeing

No Phenomenal Vision

• ‘When shown his results the patient expressed great surprise, and reiterated that he was only guessing’ (Weiskrantz 1974)

• The patients actually believed their performance was completely random’

• ‘How can I look at something that I haven’t seen?’ (Poppel 1973)

• the patient claimed he was only guessing, and could hardly believe that his performance was above chance

• The patients ‘consistently, repeatedly and firmly said that they did not experience anything’ (Stoerig 1985)

Learning to See (without seeing)

Monkeys also ‘reported’ that they were guessing

The Case of G.Y. The man who saw only motion

• G.Y. is a patient how had extensive visual damage, no residual activity in V1 at all.

• Was in this condition for 30 years • Could still see motion for strong signals,

but not other aspects of the stimulus • Had residual activity in area V5 (MT)• Could study relation between MT activity

and awareness without V1 and other perception.

G.Y. -- Physiology

• Slow motion weak activation of MT

• Fast motion stronger activation of MT

• No activation at all of V1

G.Y. -- Perception

• Stronger activation usually created awareness (not always).

• Discrimination without awareness clearly better than chance (classical blindsight).

• Some additional activation: V3, parietal, a region in right-middle-frontal gyrus

• Also medulla, in correlation with perceived trials.

G.Y. – Conclusions

• There can be aware perception without V1 – (V1 not necessary).

• Stronger activation (by fast motion) was correlated with awareness

• The ‘content’ of the motion awareness is in MT

(With some other motion-related areas, and the medulla which is non-specific.)

• More informative representation than awareness

Anatomy: Area MT

The Two Streams

Ventral and Dorsal Pathways

Alternate Route: via the SC

Affective blindsight: pairing unseeing stimulus with averse feeling

Anders et al Nature Neuroscience  7, 339 - 340 (2004)

Some faces were paired with an averse human scream. Following training, subject reported averse feeling when presented with the face in the blind region. They were not visually aware of the face.

Affective blindsight

BOLD responses were significantly increased in the left anterior parietal cortex [somatosensory association area, implicated in cortical affective processing and affective pairing with stimuli]

Affective Blindsight

• Negative emotional feelings significantly enhanced • All patients denied any visual sensation• fMRI: no visual activity in the region corresponding to the

calcarine sulcus

Blindsight – Summary and Questions

• There is residual visual function without awareness. • It can generate affective awareness without perceptual

awareness.• There is cortical activation going through an alternate

route. • It reaches both the dorsal (‘action’) stream and the

ventral (‘shape’) stream. • For DF there was dorsal activation and good visual-

based action without awareness. • Question: Does the ‘action’ stream operate in general

without awareness? – How can we tell? – Why – which cognitive processes do, which do not? – How come – which physical processes do, which do not?

Blindsight: Summary continued

• Other blindsight subjects depend on ‘shape’ activation (x vs O).

• Question: Why does this not reach awareness? • V1 is required? [no] Another crucial area? • The stimulation is weak and transient? • Not enough for perception but enough to invoke

affective awareness? • Meaningful informative activation is not enough