- Explain blindsight and what it reveals about consciousness
If you have already studied about the brain (in the Biopsychology module) then the picture below of the four major lobes of the cerebral cortex should look familiar. Click on the part of the brain that is most heavily involved in vision.
What do you think would happen if your occipital lobes were damaged? Back in the 1970s, most scientists and physicians would have said, “you would become blind.” It turns out that the answer is more complicated than that.
When he was 8-years old, Graham Young from Oxford, England, was injured in a bicycle accident. Afterward, he reported that parts of his vision were gone. He told his doctors that he could no longer see anything to the right of his center of vision with either his left or right eye. The left side of his visual world in both eyes was normal. Although he says that he would sometimes walk into objects to his right because he couldn’t see them, when tested fifteen years later, an optician discovered that Mr. Young seemed to respond to visual movements in his “blind” area.
Graham Young was put into contact with Psychologists Larry Weiskrantz and Elizabeth Warrington, who had worked previously with a person (known as DB) who seemed to have a similar ability to see despite blindness. DB could report shapes, colors, movement, and the orientation of objects despite claiming that he could see nothing. He said that he was guessing, but he was usually right about colors and shapes and other characteristics of the objects.
Before we go on, please take a moment to theorize about what might be going on with Graham Young and DB.
People with blindsight have been tested for their ability to detect color differences, brightness changes, the ability to discriminate between various shapes, as well as tracking movement. Critically, people with blindsight have the conscious experience of blindness, often feeling like they are guessing despite their high level of accuracy.
Blindsight in Action
Here is a brief video of the man who experiences complete blindness because his visual cortex in both hemispheres has been damaged. The researchers (including Dr. Weiskrantz, mentioned above) set up an obstacle course for the man (whose face is blurred to protect his privacy). Watch how well he moves through the objects without help. The man behind him is just there as a safety precaution.
You can access the text alternative for “TN Blindsight” here (opens in new window).
How can blindsight happen?
Your conscious experience of the world around you, of the choices and decisions you make, and of the emotions and attitudes that motivate you are not the totality of your mental activity or of your brain’s processing of information. Many, perhaps most, psychologists believe that consciousness is only a small part of your total cognitive activity.
A person is considered to be blind if he or she has no conscious experience of the visual world. This conscious experience is based on the flow of information from the eyes through the thalamus in the middle of the brain to the primary visual cortex in the occipital lobe at the back of the brain. If the primary visual cortex is damaged or fails to receive input due to disruption of visual pathway, then the person will not “see” the objects and events that we normally associate with vision.
Blindsight occurs because the visual system has a primary pathway (retina to thalamus to primary visual cortex), but it also has secondary pathways (retina to thalamus to other brain areas). These “other brain areas” include parts of the frontal lobe that guide eye movements, parts of the midbrain that help guide visual attention, and parts of the occipital lobe that process features of the visual perception, including shape, movement, and color.
The existence of visual processing areas for isolated features of vision and the fact that these areas get some direct visual information (i.e, input that does not first go to the primary visual cortex) means that it is possible for a person to respond accurately to questions about color or motion or shape without consciously “seeing” the objects that have color or shape or are moving.
You can see Graham Young as he is tested in the lab in this video that shows him along with psychologist Larry Weizkrantz. The video clip (watch just the first 3 minutes), from a program hosted by neurologist V. S. Ramachandran, goes on to explain a theory as to why blindsight occurs.
It is important to remember that YOU have these same “unconscious” pathways in your visual system. That means your conscious experience of the visual world may not include all of the visual information you are processing. In other words, you may “know” more than you “see”.
Blindsight is not the only condition that involves unconscious or low-consciousness processing. Other neurological syndromes that have an unconscious element include amnesia, hemispatial neglect, dyslexia, aphasia, and various agnosias.
Creating Blindsight in the Laboratory
Wouldn’t it be great if we could produce blindsight in the laboratory, in order to better understand visual processing and conscious experience? Maybe with college student volunteers as our subjects? Crazy idea?
It turns out, researchers have already done it. Using precisely aimed magnetic pulses, researchers can temporarily disrupt specific areas of the primary visual cortex—the area responsible for conscious vision—without injury. This “blindness” lasts only a fraction of a second, after which vision returns to normal. Would you volunteer to be a participant?
Let’s look at how this works.
TMS: Transcranial Magnetic Stimulation
Transcranial magnetic stimulation (TMS) is a procedure used to stimulate neurons in the brain. A device referred to as a “wand” contains an electric coil that generates a magnetic field that in turn creates a small electric current in the brain. The electric current induces neurons (brain cells) to produce neural signals called action potentials. When action potentials are produced in normal brain processes, they allow neurons to communicate with one another. However, when action potentials are induced by an outside force—here by the TMS wand—they are meaningless and temporarily interfere with communication between neurons. If only a single pulse of electromagnetic energy is produced, then the disruption of the neurons in the targeted region lasts only a fraction of a second. Multiple pulses, called repetitive TMS (rTMS), can produce longer lasting effects. In fact, rTMS is now used by therapists as a treatment for depression and neuropathic pain.
The TMS pulse can be aimed very precisely at a small area of the brain. When the target is the primary visual cortex in the occipital lobe, the TMS pulse can be focused to interfere with neural communication in a tiny region of the of the visual field—so small and occurring for such a short time that you would not even notice. However brief the duration or tiny the affected area, the person receiving the TMS pulse is temporary blind in a small part of the visual field.
Laboratory Research on Unconscious Visual Processing
Dr. Tony Ro is a professor of psychology at the City University of New York. He started studying the connection between consciousness and brain processing more than 20 years ago, and he was one of the earliest researchers to apply TMS technology to the study of visual perception.
In one study, Dr. Ro and graduate students Jennifer Boyer and Stephenie Harrison used TMS technology to see if normal people could process features of visual stimuli without conscious awareness of those stimuli. In other words, they wanted to know if they could they create temporary blindsight in normal subjects in a laboratory.
Remember that blindsight involves unconscious awareness of “features” of objects and events, such as the shape of an object or the direction of its movement. This study focused on two visual features: orientation and color. You and I see orientation (horizontal or vertical) or color (red or green) as part of the experience of some object. A line is horizontal. A box is red. For a person with blindsight, “horizontal” is experienced without any shape associated with it. “Red” is experienced without awareness of the thing that is red. This is the blindsight condition that Dr. Ro and his colleagues wanted to reproduce in the laboratory with the help of volunteer subjects.
Let’s walk through the experiment to understand how it was designed and conducted.
Experiment 1: Unconscious Detection of Orientation
SETUP: The TMS wand was precisely adjusted so the TMS pulse was aimed at the back of the brain (primary visual cortex in the occipital lobes) affecting a very small area of the visual field. For example, imagine the gray box below as a computer screen. The plus sign in the middle is a fixation point. You (the participant in the study) fixate your eyes on this plus sign and hold them there during each trial. The TMS pulse is adjusted to your individual brain so that the area shown as a blue circle (used here only for explanation purposes) is momentarily “blind” when the pulse is active. This is a painstaking process that involves fine calibration of the wand based on feedback from the participant about what he or she can see when different targets are shown on the screen.
TESTING: In one of Dr. Ro’s experiments, participants had to guess the orientation of a line, sometimes when they were temporarily blinded (in a tiny area of the visual cortex) by a TMS pulse. The study consisted of a series of trials. On each trial, either a horizontal or a vertical line was flashed for a fraction of a second on the computer screen in front of the participant. On some of these trials, a TMS pulse disrupted the neurons in the visual cortex. On other trials, there was no TMS pulse. The no-pulse trials served as a kind of control condition.
Click on the slideshow below to see the steps in the vertical line condition. You can use the arrows at the bottom to navigate through the slides.
RESULTS: By chance, if you have to choose between two equally likely options (horizontal or vertical), you would be correct about 50% of the time. On the trials when the subjects reported that they did not “see” anything at all, they correctly guessed the orientation of the line 75% of the time, performance that is significantly better than chance. There was also a strong positive correlation (r = +0.93) between accuracy and confidence: the more confident the subject in his or her guess, the more likely it was that the guess was correct. Keep in mind that, in all of these cases, the subjects started by saying that they saw nothing. That was about 60% of the trials. On the other 40% of trials, the subjects reported seeing something, even if it was a slight blur, and these trials did not count. Not surprisingly, accuracy was near perfect when subjects were conscious of seeing the bar and its orientation.
Variations of the Experiment
A second study using the color of a circle rather than the orientation of a bar was reported in the same paper. Otherwise, the procedures were the same as in the first experiment and the results consistent with the results for the bar orientation experiment.
Testing Blindsight with TMS
Here is a video about a similar experiment conducted by Dr. Ro and his colleagues. The experiment in the video involves detecting yet another feature of objects: their shape. The basic procedures and results are similar to the ones you have just read.
Conclusions from the Research
The experimenters succeeded in producing the experience of blindness using the TMS apparatus, and they also succeeded in producing evidence for unconscious processing of features of the visual experience in normal (college student) volunteers. These results, when put together with the experiences of people with neurological damage, strengthen the case for the theory that some of our visual perception of the world takes place outside of our awareness. The college students have shown that this unconscious processing is not the result of brain damage, but rather is part of our normal perception of the world.
Some Final Words
This module is about consciousness. It is common to assume that everything we know about the world around us and about our own thoughts and internal experiences must go through the doorway of our conscious mind. Evidence from blindsight is just one of several lines of research that shows that we process more information than we are aware of. Learning just how much this unconscious information can influence our thoughts and actions, our preferences and beliefs, is an important challenge for the rising generation of scientists.
- Source: http://marketingland.com/wp-content/ml-loads/2014/09/iceberg-ss-1920.jpg ↵
- A recent literature review of evidence for the existence of the pathways to the cerebral cortex: Rabbo, F. A., Koch, G., Lefevre, C., & Seizeur, R. (2015). Direct geniculo-extrastriate pathways: A review of the literature. Surgical and Radiologic Anatomy, 37(8), 891-899. ↵
- See Consciousness Lost and Found: A Neuropsychological Exploration by Larry Weiskrantz (1997, Oxford University Press). Dr. Weiskrantz is one of the scientists who first described blindsight and studied people with the condition. ↵
- The physics of electromagnetism is fascinating, but we will spare you the details here. You may have studied it in some other class, and there are many readable online sources (e.g., Wikipedia). TMS is a great example of the convergence of technology and psychology that is the basis of modern neuroscience. ↵