Processing streams PSY 310 Greg Francis. Lecture 10. Neurophysiology

Transcription

1 Processing streams PSY 310 Greg Francis Lecture 10 A continuous surface infolded on itself. Neurophysiology We are working under the following hypothesis What we see is determined by the pattern of neural activity in the brain This leads to several interesting questions about the connection between neurophysiology and perceptual experience 1

2 Neural patterns Spatial scenes lead to particular patterns of neural activity Neural patterns Different stimuli give rise to different neural patterns and thus different percepts 2

3 Neural patterns Different stimuli give rise to different neural patterns and thus different percepts Beginning, not end It becomes clear that the orientated edges representation is the beginning rather than the end of the neurophysiological representation of information For example, all of the previous pictures can be described as photos of tables But what is common about the neural representation? When can we say that we understand how a percept is represented in the brain? 3

4 Brightness illusion Craik-O Brien- Cornsweet effect Left rectangle looks brighter than right rectangle COC illusion A cross-section of luminance of the image shows what it is really like Why do we see it as we do? 4

5 COC illusion Consider the response of ganglion cells First to an image without an illusion COC illusion Consider the response of ganglion cells First to an image without an illusion 5

6 COC illusion Consider the response of ganglion cells Second to an image with the illusion COC illusion So why is our perceptual experience similar? True squares COC squares 6

7 COC illusion Compare the ganglion cell responses to the two different images True squares COC squares COC illusion The neural patterns are very similar, which, in part, explains why the perceptual experience is very similar True squares COC squares 7

8 Perceiving patterns How does an arrangement of dots have an orientation? Glass patterns Take random dots, make a copy, shift in a particular direction 8

9 Glass patterns Take random dots, make a copy, shift in a particular direction Glass patterns Take random dots, make a copy, shift in a particular direction 9

10 Glass patterns Look at responses from orientation sensitive cells Glass patterns Look at responses from orientation sensitive cells Compare to responses from real circles 10

11 Test the explanation Make two types of Glass patterns Orientationally tuned cells respond less selectively to the pattern on the right Higher brain areas We could spend a lot of time relating perception to properties of ganglion cells or orientation-sensitive cells But there are some perceptual experiences that can only be related to cells in other areas of the brain Processing streams identify organization of brain 11

12 Agnosia An inability to identify objects in the world Dr. P. (The Man Who Mistook His Wife for a Hat) Excellent musician and painter Became unable to recognize people s faces Facial agnosia Speaker voices quickly lead to identification So did body movements Also saw faces where there were none Patting tops of fire hydrants as if they were small children Agnosia During examination could not put shoe back on foot Was unsure which was the shoe and which was the foot 12

13 Describing a glove What is this? I asked, holding up a glove. May I examine it? he asked, and, taking it from me, he proceeded to examine it as he had examined the geometrical shapes. A continuous surface, he announced at last, infolded on itself. It appears to have he hesitated five outpouchings, if this is the word. Yes, I said cautiously. You have given me a description. Now tell me what it is. A container of some sort? Yes. I said, and what would it contain? It would contain its contents! said Dr P. with a laugh. There are many possibilities. It could be a change-purse, for example, for coins of five sizes. It could I interrupted the barmy flow. Does it not look familiar? Do you think it might contain, might fit, a part of your body? No light of recognition dawned on his face. Later, by accident, he got it on, and exclaimed, My God, it s a glove! Brain structure From visual cortex, information follows two paths through the brain 13

14 Dorsal stream From visual cortex to the parietal lobe Involved in working with objects Ventral stream From visual cortex to the temporal lobe Involved in recognizing or identifying objects 14

15 Patient DF Similar to Dr. P, but studied more carefully For example, ask her to walk over barriers of various heights Not a problem, toe clearance was fairly normal Patient DF Now ask DF to order the barriers by height Absolutely horrible? 15

16 Patient DF Similar behavior for another task Match the orientation of the card to be the same as the slot Very difficult to do for DF Patient DF Similar behavior for another task Put the card into the slot Orientation matches just fine! 16

17 DF s brain Damage to the ventral stream DF s brain Damage to the ventral stream 17

18 Conclusions Relating perception to neurophysiological responses Ganglion cells Orientation sensitive cells Other areas of the brain Processing streams Dorsal Ventral Agnosia Next time Neural codes in the different streams Motion Object identification 18