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Connections Between Eye and Brain
· ? Coding of Visual Information in the Retina
· ? Analysis of Visual Information: Role of the Striate Cortex
Modular Organization of the Striate Cortex
· ? Analysis of Visual Information: Role of the Visual Association Cortex
Two Streams of Visual Analysis
Perception of Spatial Location
Dr. L., a young neuropsychologist, was presenting the case of Mrs. R. to a group of medical students doing a rotation in the neurology department at the medical center. The chief of the department had shown them Mrs. R.’s CT scans, and now Dr. L. was addressing the students. He told them that Mrs. R.’s stroke had not impaired her ability to talk or to move about, but it had affected her vision.
A nurse ushered Mrs. R. into the room and helped her find a seat at the end of the table.
“How are you, Mrs. R.?” asked Dr. L.
“I’m fine. I’ve been home for a month now, and I can do just about everything that I did before I had my stroke.”
“Good. How is your vision?”
“Well, I’m afraid that’s still a problem.”
“What seems to give you the most trouble?”
“I just don’t seem to be able to recognize things. When I’m working in my kitchen, I know what everything is as long as no one moves anything. A few times my husband tried to help me by putting things away, and I couldn’t see them any more.” She laughed. “Well, I could see them, but I just couldn’t say what they were.”
Dr. L. took some objects out of a paper bag and placed them on the table in front of her.
“Can you tell me what these are?” he asked. “No,” he said, “please don’t touch them.”
Mrs. R. stared intently at the objects. “No, I can’t rightly say what they are.”
Dr. L. pointed to one of them, a wristwatch. “Tell me what you see here,” he said.
Mrs. R. looked thoughtful, turning her head one way and then the other. “Well, I see something round, and it has two things attached to it, one on the top and one on the bottom.” She continued to stare at it. “There are some things inside the circle, I think, but I can’t make out what they are.”
“Pick it up.”
She did so, made a wry face, and said, “Oh. It’s a wristwatch.” At Dr. L.’s request, she picked up the rest of the objects, one by one, and identified each of them correctly.
“Do you have trouble recognizing people, too?” asked Dr. L.
“Oh, yes!” she sighed. “While I was still in the hospital, my husband and my son both came in to see me, and I couldn’t tell who was who until my husband said something—then I could tell which direction his voice was coming from. Now I’ve trained myself to recognize my husband. I can usually see his glasses and his bald head, but I have to work at it. And I’ve been fooled a few times.” She laughed. “One of our neighbors is bald and wears glasses, too, and one day when he and his wife were visiting us, I thought he was my husband, so I called him ‘honey.’ It was a little embarrassing at first, but everyone understood.”
“What does a face look like to you?” asked Dr. L.
“Well, I know that it’s a face, because I can usually see the eyes, and it’s on top of a body. I can see a body pretty well, by how it moves.” She paused a moment. “Oh, yes, I forgot, sometimes I can recognize a person by how he moves. You know, you can often recognize friends by the way they walk, even when they’re far away. I can still do that. That’s funny, isn’t it? I can’t see people’s faces very well, but I can recognize the way they walk.”
Dr. L. made some movements with his hands. “Can you tell what I’m pretending to do?” he asked.
“Yes, you’re mixing something—like some cake batter.”
He mimed the gestures of turning a key, writing, and dealing out playing cards, and Mrs. R. recognized them without any difficulty.
“Do you have any trouble reading?” he asked.
“Well, a little, but I don’t do too badly.”
Dr. L. handed her a magazine, and she began to read the article aloud—somewhat hesitantly but accurately. “Why is it,” she asked, “that I can see the words all right but have so much trouble with thingsand with people’s faces?”
As we saw in Chapter 3 , the brain performs two major functions: It controls the movements of the muscles, producing useful behaviors, and it regulates the body’s internal environment. To perform both these tasks, the brain must be informed about what is happening both in the external environment and within the body. Such information is received by the sensory systems. This chapter and the next are devoted to a discussion of the ways in which sensory organs detect changes in the environment and the ways in which the brain interprets neural signals from these organs.