PSY 310: Sensory and Perceptual Processes 1

Transcription

1 Processing streams PSY 310 Greg Francis 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 Lecture 10 A continuous surface infolded on itself. 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 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? 1

2 Craik-O Brien- Cornsweet effect Left rectangle looks brighter than right rectangle Brightness illusion A cross-section of luminance of the image shows what it is really like Why do we see it as we do? Consider the response of ganglion cells First to an image without an illusion Consider the response of ganglion cells First to an image without an illusion Consider the response of ganglion cells Second to an image with the illusion So why is our perceptual experience similar? True squares COC squares 2

3 Compare the ganglion cell responses to the two different images The neural patterns are very similar, which, in part, explains why the perceptual experience is very similar True squares COC squares True squares COC squares Perceiving patterns How does an arrangement of dots have an orientation? Take random dots, make a copy, shift in a particular direction Take random dots, make a copy, shift in a particular direction Take random dots, make a copy, shift in a particular direction 3

4 Look at responses from orientation sensitive cells Look at responses from orientation sensitive cells Compare to responses from real circles Test the explanation Make two types of 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 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 4

5 Prof. Greg Francis Describing a glove Brain structure 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. From visual cortex, information follows two paths through the brain 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! Dorsal stream From visual cortex to the parietal lobe Ventral stream From visual cortex to the temporal lobe Involved in working with objects Involved in recognizing or identifying objects Similar to Dr. P, but studied more carefully Now ask DF to order the barriers by height For example, ask her to walk over barriers of various heights Absolutely horrible Not a problem, toe clearance was fairly normal? 5

6 Similar behavior for another task Match the orientation of the card to be the same as the slot Very difficult to do for DF Similar behavior for another task Put the card into the slot Orientation matches just fine! DF s brain Damage to the ventral stream DF s brain Damage to the ventral stream 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 6