Limits of Knowledge

The Limits of Our Knowledge about Our Brain

Blindsight

The term blindsight (Weiskrantz, 1986) is applied to patients with damage to the visual cortex of the occipital lobes – the primary visual cortex – which is where most of the nerve fibers from the retinae terminate. Such patients suffer from “cortical blindness”; they are blind because the part of the cortex that generates visual consciousness is no longer working. Blindness, then, means a lack of visual awareness. Thus, if you were to hold an object before these patients' eyes and ask them what they see, they would respond with the obvious: “I don't see anything; I'm blind.” But when they respond in this way, they are actually mistaken. They are erroneously equating “seeing” with “seeing consciously.” The distinction between vision and conscious vision is demonstrated when you ask these same patients to make a “forced choice” between various options (in other words, you encourage them to guess). The results of such experiments reveal that they guess correctly at a level well above chance, which demonstrates that these patients are seeing – are processing visual information – without realizing it (see Weiskrantz, 1986). They are seeing unconsciously. This occurs because some visual information is projected from the retina onto other parts of the cortex (intact in these patients) that do not generate visual consciousness but are, nevertheless, equipped to process the visual information they receive. In other words, these patients – as far as visual information is concerned – act like the “zombies”. Their brains compute visually, but they do not possess visual consciousness.

Implicit Memory

The same thing occurs with respect to other cognitive faculties. It is not all that rare for neurological patients to lose the ability to lay down new memories. This condition is called amnesia. These patients remember (recall consciously) nothing that happens to them after the onset of their brain disease or injury. If you were to read a list of words to such patients, after a few minutes they would not only forget the words, they would even forget the fact that you read them the list. However, as with cases of cortical blindness, such patients can be encouraged to “guess,” using the forced-choice paradigm. When they do so, they “randomly” select of generate words that were on the original list, at a very much higher rate than chance. So, just as we can see unconsciously, we can also remember unconsciously. The technical term for this unconscious type of remembering is implicit memory (conscious remembering is explicit memory).

What is the self in neurobiological terms? Photo by Elena.

Split-Brain Studies

In so-called split-brain patients, to treat otherwise intractable epilepsy the corpus callosum has been severed, thus separating the left (language-dominant) hemisphere of the brain from the right

By briefly flashing an image on a screen to such patients, it is possible to provide the right hemisphere with information that the left hemisphere cannot access. On this basis, it is possible to influence the patient's behavior without him or her being consciously aware of it. In one of Nobel Prize-winning neuroscientist Roger Sperry's famous cases, pornographic pictures were projected and giggled. When Sperry asked her why she why she was embarrassed, she was unable to account for it. This case (described in Galin, 1974, p.573) demonstrates that an entire cerebral hemisphere can process information “unconsciously.”

The case also reveals something else that is of crucial importance for understanding consciousness. The visual cortex was completely intact in Sperry's patient. This means that the pornographic pictures were perceived by the visual-consciousness generating part of her right hemisphere. Why, then, did she appear not to know what she saw? The answer to this question provides a good illustration of the “functional systems” concept. Although it is true that the primary visual cortex (in either hemisphere) is capable of generation simple visual consciousness, it does not do so in isolation. For someone to reflect consciously on visual experiences, he or she has to recode the visual experiences into words. This capacity is lost when the left (verbal) hemisphere is disconnected from the original visual experience. This shows that a distinction needs to be drawn between two levels or types of consciousness: Simple awareness and reflexive awareness. It also shows that the function of reflexive awareness is intimately connected with the left cerebral hemisphere and therefore with word (or, rather, “inner speech”).

The fact that an entire hemisphere (about half of the forebrain) can, in a sense, function unconsciously raises intriguing questions.

Neuropsychology aims to be entirely objective, and its great power, its advances, com from just this. Illustration by Elena.

The Brain and the Inner World, Introduction to Basic Concepts. Mark Solms, Oliver Turnbull.