Reporter: 周蔚倫 2005/9/19

1. Tong, F. (2003).Primary visual cortex and visual awareness.Nature Reviews Neuroscience, 4, 219-229. Reporter: 周蔚倫2005/9/19

2. Introduction • visual perception consists of two key components: • information analysis • subjective awareness • information analysisalone could support visually guided behaviourbut many of the analyses are closely tied to the visual features and objects that reach our awareness • This review focuses on the relationship between activity in V1 and visual awareness, and discusses theories and evidence pertaining to whether V1 might have a direct and necessary role in conscious vision.

3. Retina  LGN  V1 (lateral geniculate nucleus) • Left visual field  right visual cortex(vice versa) retina LGN V1

4. PRIMARY VISUAL CORTEX • The first cortical area to receive inputs from the eye via the geniculostriate pathway (primary visual cortex =V1, area 17, striate cortex) EXTRASTRIATE CORTEX • A belt of visually responsive areas of cortex surrounding the primary visual cortex.

5. Overview of the primate visual system • Connectivity • hierarchical models: only extrastriate areas such as MT, V4 and IT cortex can contribute to visual awareness • interactive models: recurrent loops between V1 and extrastriate areas are essential for maintaining a visual representation in awareness (frontal eye fields) (lateral intraparietal) inferotemporal cortex

6. Response properties Retina  LGN  V1 • receptive fields & prefer stimuli retina & LGN: small centre–surround concentric receptive fields; prefer disk lightV1: ellipse; selectivity for orientation, motion direction RFs of V1 RFs of retina & LGN

7. V1 V2 V4, TEO TE The size of receptive field • Effects of lesions • Lesion studies  V1 is necessary for normal visual function and awareness

8. Neural localizationist theories of visual awareness (1) • specific neural regions or circuits are important for awareness • visual awareness: consciousness for items in immediate sight • attention is necessary but not sufficient for visual awareness • even during sustained attention, awareness can fluctuate (ex., binocular rivalry ***) or fail to isolate the target stimulus (ex., perceptual crowding demo)

9. Neural localizationist theories of visual awareness (2) • Hierarchical models • Only higher-level extrastriate areas are directly involved in visual awareness • Extrastriate areas, such as V4, MT and inferotemporal cortex, directly represent conscious information about colour, motion and object identity, respectively • Interactive models • V1 participates directly in visual awareness by forming dynamic recurrent circuits with extrastriate areas • an essential role for recurrent V1–extrastriate activity

10. Neural localizationist theories of visual awareness (3) • Alternative models • a distributed model: any given brain region, such as V1, can participate in awareness if its information is widely broadcast across many brain areas • oscillation models: emphasize temporal structure rather than localization of function, and do not address the role of V1 in awareness

11. Lesion studies (1) • Blindsight • V1 lesion *** loss of awareness (reporting no awareness of the stimulus), but residual visual function • a dissociation between awareness and information processing • whether visual awareness is completely absent in blindsight? severely degraded but not always completely absent • How? subcortical pathways project to extrastriate areas  visual information can still reach extrastriate areas after V1 has been lesioned ***

12. Lesion studies (2) • Extrastriate lesions • V1 lesion  devastating effects • other cortical visual areas lesion  restricted deficits ex., V2 perceptual grouping; V4  color; MT  motion; IT  object recognition • posterior parietal lobe & superior temporal gyrus global deficits in visual attention and awareness(ex., visual neglect) • independent lesions of either V1 or parietal-temporal regions can greatly impair conscious vision  no single visual area is sufficient for visual awareness

13. Neural correlates of visual awareness (1) • Binocular rivalry • population activity in human V1 is tightly linked to conscious perception *** • Visual detection • V1 activity is linked to the conscious detection of a visual target ***

14. RF Neural correlates of visual awareness (2) • Bistable perception • the correlation is greater in extrastriate areas than in V1 • ambiguous rotating cylinder: only 20% of V1 neurons; 60% of MT neurons were modulated by perception • Rubin’s face-vase (fMRI study): ventral extrastriate, parietal & frontal regions↑; V1 ↓ • V1 response properties and perception • orientation learning • contour integration • subjective contours • adaptation  V1 ↓

15. Dissociations between V1 activity and awareness • V1 activity is not sufficient for awareness • the responses can follow flickering colour gratings at temporal rates that are too fast to be perceived • orientation-specific adaptation can occur for high-spatial frequency gratings that are too fine to be perceived • Internally generated visual experiences • Visual hallucinations (fMRI; schizophrenic patients) ventral extrastriate areas↑, but not V1 • PET study; REM sleep stage: extrastriate activity↑, but not V1 • Other similar examples: migraine (V3A) ; color synaesthesia (V4)

16. Creating and disrupting visual experiences • patients with severed connections between LGN and V1: cortical stimulation of V1  conscious experience • stimulation of occipital areas evokes vivid impressions of basic visual sensations (points of light, motion, colour) • perception can be disrupted when TMS (transcranial magnetic stimulation) of the occipital lobe (80–120 ms after a stimulus) • feedback projections from MT to V1 might be necessary for conscious perception ***

17. Concluding remarks • V1 is necessary for normal visual awareness (but not sufficient) • V1 activity is tightly correlated with awareness under various conditions • feedbackactivity from higher areas might be crucial for conscious vision • many questions remain to be addressed: • the contributions of individual visual areas to awareness? • the relative contributions of feedforward and feedback pathways? • binding problem

18. Evidences from binocular rivalry • Fig. a & b: fMRI modulations: rivalry = physical alternation • Fig. c: equally robust fMRI modulations in V1 to V4 (face- & house-selective areas: rivalry = control) =

19. neural modulations are linked to the monkey’s conscious perception “not seen”figure-present trials (guess) Average of figure-present trials “seen”figure-present trials

20. feedback connections from MT to V1 might be necessary forawareness of motion • relationship between timing of V1 disruption and visual awareness clearly moving phosphene 1st TMS no phosphene 2nd TMS

21. Binocular rivalry • different monocular patterns are simultaneously presented to the two eyes perception alternates between one iand the other

22. Q L N G K R F H A E P N O P G X N A E B S D F A G L R F S A Crowding effect

23. V1 lesion  associated scotomas

24. functional magnetic resonance imaging (fMRI) responses • patients with unilateral V1 damage • sustained extrastriate cortical activation without visual awareness (lateral occipital)

25. left cortex right cortex perceived left hemifield objects (green line) unperceived right hemifield objects (red line).

26. Interocular rivalry revealed in thehuman cortical blind-spot representation(Frank Tong & Stephen A. Engel)