Blindsight is something that the members of Biology 202-Neurobiology and Behavior at Bryn Mawr College have come across before. We briefly mentioned it in discussing the visual system, and said that the phenomenon of blindsight suggested-as we had seen in other contexts before- that our brain does things outside of the "I-function." Finding this very interesting but not feeling satisfied with what I had learned about blindsight and its relationship to consciousness in class, my last paper is dedicated to answering my lingering questions. First, I asked what blindsight is and why it happens. Then I wanted to know what the observation of the lack of awareness of a visual stimulus when there is damage to the striate cortex could tell us about consciousness.
As stated above, blindsight is seen clinically as a contrast between a lack of declarative knowledge about a stimulus and a high rate of correct answers to questions about the stimulus (1). People suffering from blindsight claim to see nothing, and are therefore unable to reach spontaneously for stimuli, cannot decide whether or not stimuli are present, and do not know what objects look like. In this sense, they are blind. However, they are able to give correct answers when asked to decide between given alternatives (1). Studies done with subjects that exhibit blindsight have shown that they are able to guess reliably only about certain features of stimuli having to do with motion, location and direction of stimuli. They are also able to discriminate simple forms, and can shape their hands in a way appropriate to grasping the object when asked to try. Some may show color discrimination as well (2). Subjects also show visual capacities, including reflexes (e.g. the pupil reacts to changes in light), implicit reactions and voluntary responses (3).
People suffering from blindsight are not "blind" because their eyes do not function. Rather they suffer from cortical blindness. People suffering from cortical blindness receive sensory information but do not process it correctly, usually due to damage in some part of the brain. The damage in blindsight patients has been shown to be in the striate cortex, which is part of the visual cortex. The striate cortex is often called the primary visual cortex (V1), and is thought to be the primary locus of visual processing (3). Destruction or disconnection of the striate cortex produces a scotoma, or a region of blindness, in the part of the visual field that maps to the damaged area of the cortex (3). Depending on the extent of the lesion, vision can be absent in anywhere between a very small section of stimulus field and the entire field (4). The person is unable to process the sensory input to the striate cortex, and does not recognize having seen the object.
If the person (or animal) with lesions to the striate cortex is not processing information about the stimulus, why does he seem to know something about the stimulus he didn't see? Studies with monkeys with striate cortex lesions and with brain damaged patients have shown that neural pathways outside of the striate cortex mediate voluntary responses (i.e. answers to questions) about stimuli presented in the scotoma (3). Namely, the superior colliculus, the lateral geniculate nucleus, and the pulvinar and extrastriate visual cortex seem to play important roles in the "seeing" aspect of blindsight. Also, studies of single neurons in extrastriate areas show that some neurons are activated by stimuli in the scotoma. A high proportion of these neurons are those that are responsible to pattern stimuli, and have been implicated in visual pattern recognition (3). Many of these neurons also respond to motion (3). When a lesion of the striate cortex occurs, many of these neurons are spared from degeneration or damage, and may be responsible for the ability to answer questions about stimuli in the scotoma (3).
Normal subjects also seem to exhibit a sort of blindsight. For example, when people blink, make eye scans, and eye vergences, people disrupt visual input. This disruption of visual input seems to inhibit visual processing and create a sort of "functional lesion" in the normal person (5). People with such "functional lesions" are also unable to answer questions about their vision field, but can answer questions like those that blindsight patients can answer.
What does this all imply? As Professor Grobstein says on his website, blindsight "provides evidence of an "unconscious" as a contributor to human behavior" (6). But what exactly does blindsight tell us about consciousness. Many ideas exist.
Some people suggest that the ability of subjects with striate cortex lesions to answer the question without being aware that they saw the object implies that conscious awareness of visual stimuli depends on an intact striate cortex, which is damaged in these patients (3). Unconscious vision is mediated by parts of the cortical and subcortical areas involved in the visual system that are undamaged (4), (7). These pathways receive information is damaged from a direct pathways from the retina even when the striate cortex, so they are still able to activate a response. However, these subcortical structures and pathways are unable to support consciousness alone (3). Because consciousness is the missing element in seeing that is lacking when the striate cortex is removed, people have theorized that visual consciousness is somehow IN the striate cortex.
Although the above evidence does fit with the idea that visual consciousness is in the striate cortex, I thought of some other plausible explanations for the lack of awareness of seeing that occurs when the striate cortex is lesioned. Consciousness theories, starting with Descartes (8), have for much of the past consisted of the idea that one center of consciousness exists. The fact that lesions, whether actual or functional, seem to produce a loss of visual consciousness and not any other "consciousnesses" initially suggests that consciousness cannot be all in one place. Upon more thought, however, another possibility arises in my thoughts. Perhaps the striate cortex is the relay center to the larger "center" of consciousness in the brain. Different kinds of information about experiences could be processed by different "boxes" and then sent to the "primary center." If this were true, dysfunctions, such as lesions, in the striate cortex would keep the information about the experience from continuing on to the "center." Since the information never got to the "center" to become a part of consciousness, this part of consciousness would be missing. The observation that striate cortex damage causes a lack of visual consciousness may not mean that visual consciousness is in the striate cortex itself, but that the striate cortex is part of the pathway through which information reaches a center of consciousness.
A third possibility struck me while reading Dennett's philosophical model of consciousness, the Multiple Drafts theory (9) (10). In his theory, consciousness is not localized, but exists as a narrative stream that is brought into consciousness from prompting. Many drafts of experience are occurring in parallel in the brain, but our "experience" results from the one narrative that is brought to the forefront through prompting. He explains blindsight as a representation of absence of information about visual stimuli, so that no information about the visual stimuli enters in to the drafts while they are being written in the brain (4). Perhaps, as in the previous example, the striate cortex somehow mediates the passing of visual information. Here, the striate cortex would be responsible for passing the information into the system that "writes" the drafts. Since the information never makes it into the draft pool, it cannot become part of the stream of consciousness through prompting. Thus, an observation that damage to the striate cortex represents a loss of consciousness could support the idea of a consciousness spread throughout the brain as well.
So what has making the observation of striate cortex damage causing blindsight taught us? It certainly shows us that consciousness is only a part of what goes on in the brain, and that consciousness is not needed for behavior. The observation does not, however, tell us where consciousness is located. The observation could support theories of consciousness being in one structure, of various types of consciousness being in different structures with vision's consciousness in the striate cortex, or of consciousness that is not localized at all. We have learned that we cannot answer the question of where consciousness is with only this observation. Although the observation has taught us an important lesson about consciousness, there is a great deal about consciousness that we cannot learn from this observation alone.
(2) On a Confusion about a Function of Consciousness, from MIT Dept. of Philosophy
(3) Blindsight and Visual Awareness , from Consciousness and Cognition v. 7, p. 292-311 (To access from Bryn Mawr, go to site, then select back issues and click on GO. Then click 1998, then on Vol.6 No. 1. Scroll down to article and click on Article)
(4) Blindsight in Hindsight , from Consciousness and Cognition v.6, p. 67-74 (To access from Bryn Mawr, go to site, then select back issues and click on GO. Then click 1997, then on Vol.7, No. 3. Scroll down to article and click on Article)
(5) Another Variety of Vision , from the Psychology Dept. University College London
(6) Seeing what you don't see?, from the Serendip website (includes a demo of blindsight-try it out!)
(7) Invited Review. Blindsight in Man and Monkey , abstract from Brain, Vol. 120, Issue 3
(8) The Philosophy Behind Ordinary Consciousness , from The Brain Project
(9) Time and the Observer: The Where and When of the Conscious Brain , Dennett's article draft
(10) Multiple Drafts..., Dennett's response to criticism