Shufflebrain

Nevertheless, there is a great deal of empirical evidence of a piece of the human brain that is involved in short-term memory. This structure is known as the hippocampus. Shaped like a zucchini, but about the size of a little finger, the hippocampus (Greek for sea-horse) is buried deep within the cerebrum's temporal lobe. A person with a damaged hippocampus exhibits defective short-term memory, whereas his or her long-term memory shows sign of being intact. One clinical sign of a lesion in the hippocampus is when a person can't repeat a name or a short sequence of randomly presented numbers but can, for instance, recite the Gettysburg Address. I will say more about the hippocampus in chapter 10. For now, I want to make this point: If we take the holist's classical position, we will have to dismiss important facts about the hippocampus.

Well, then, why can't we consider the hippocampus the seat of short-term memory? I've been asked sophisticated versions of this very question by several persons who work with the brain. There are correspondingly sophisticated reasons why we can't. But let me indicate some simple ones.

First of all, there are entire phyla of organisms whose brains lack hippocampi. Yet these same creatures often have splendid working, short-term memories. I can give another example from my own laboratory. Salamanders whose cerebrums, and therefore hippocampi, have been amputated learn as well as normal animals. Perhaps salamanders and various other forms of life are simply too lowly to count? Later in the book, I will summarize experiments whose results show that cats can learn, and thus exhibit working memory, without their hippocampi. The point, once again, is that structuralism is no more enlightening than holism, in regard to the role of the hippocampus.

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I mentioned earlier that we humans require the visual cortex in order to see. But on a summer forenoon, when I look out my office window, I sometimes observe a hawk, perhaps 600 feet up, gliding in circles above the meadowed and hardwood-forested Indiana University campus, searching the ground for a target less than a foot long. Why doesn't the hawk dive after that discarded Hershey bar wrapper or the tail of that big German shepherd? The hawk is up there in the clouds doing complicated data processing with its visual system. It's certainly seeing. Yet that hawk, unlike a human being, doesn't employ a visual cortex. It doesn't even have one. For the visual cortex in the occipital lobe is a mammalian characteristic.

Birds process their visual sensations in what is called the midbrain tectum. (Barn owls, in addition, handle depth perception in a mound of brain, on the front of the cerebrum, called the Wulst, the German word for pastry roll. Indeed, the Wulst[9] does look like something on a tray in a Viennese bakery.)

Mammals, humans included, also have tectums, which they use in pupillary light reflexes. A human who has suffered complete destruction of both occipital lobes, and loses the entire visual cortex as a consequence, becomes blind, although some evidence indicates that this person may be able to sense very strong light. Firm evidence shows that rats, rabbits, and even monkeys can sense diffuse light following complete destruction of their occipital lobes. Do the tectum and the visual cortex (and the Wulst, too, of course) comprise the seat of vertebrate vision? If a vertebrate lacks some, but not all, of these structures, it may lack certain special features of vision. If the creature lacks a tectum, a visual cortex, and a Wulst, will it have no vision at all?

The argument works, up to a point. Specific lesions in a frog's tectum produce specific deficits in its visual perception. But let me tell you a little anecdote from my own laboratory in the days before my experiments with shufflebrain.

I was doing experiments with larval salamanders. For control purposes, I had to have a group of neurologically blinded animals. That would be a cinch, I thought, since the tectum is the seat of vision in animals below mammals (the function of the Wulst hadn't been worked out yet). All I had to do, I thought, was go in and remove the tectum, which I did. Was I in for a surprise when the animals came out of anesthesia! Every single animal could see! I didn't even consider publishing the results, feeling certain that I must have goofed up somewhere. But a few years later, the animal behaviorist, G. E. Savage, reported basically the same thing, except in adult fish.