Hemiopic disturbance comes from lesion of either one of them, and total blindness, sensorial as well as psychic, from destruction of both. In any case, given the preserved anatomical similarities, why should such a major species difference obtain? Or does it? A move towards unity came when human subjects were tested in ways similar to animal tests, e. Even though their subjects said they could not see the stimuli in their field defects, there was a significant, albeit weak correlation between stimulus locus and target eye position, at least out to 20 degrees eccentricity.
DB's occipital cortex was removed surgically from his right hemisphere to ablate a benign tumour that had invaded it. The method of testing for visual capacity is usually deeply different in humans and in other animals. Humans are typically asked to give descriptions or to comment on visual appearances and differences, whereas animals are trained to make alternative choices for which they are usually rewarded, devoid of any commentary.
Even when a human subject is asked to make a discrimination between, say, two wavelengths, he is usually explicitly instructed verbally as to what colour attribute he should be responding, and more importantly there is an important implicit assumption that he will be aware of that attribute, or will tell us if he is not. But what would obtain if the human subject is tested in a manner that is closer to animal methodology, being asked simply to make a forced-choice "guess" or choice between the visual stimuli whether or not he cannot "see" them, e.
The result of using this methodology was that DB could succeed in a variety of discriminations by "guesswork" in his blind field, even though he said he did not "see" them. He could, for example, tell whether a grating was oriented in one or another direction, whether a stimulus was moving or stationary. His visual acuity could be measured by varying the spacing of a grating, with forced-choice guesses about whether there were "lines" or "no lines.
In fact, his ability matched reasonably well that of monkeys without primary visual cortex. The most remarkable feature undoubtedly was the high level of visual proficiency in making discriminations in the absence of acknowledged awareness by him.
He was astonished when shown his results, as were Weiskrantz and Warrington when conducting the tests that demonstrated his impressive performance without awareness. DB remained convinced that he was simply guessing and was at chance. These first results, by Weiskrantz and coworkers, were published in Brain in It was the first account of a hemianopic subject's performance over a wide range of visual tasks combined with his parallel "commentaries" about them.
In later research the pattern was formalized by the addition of "commentary response keys" as well as "discrimination response keys. The oxymoron "blindsight" was generated as a result of Weiskrantz having to respond urgently to produce a title for a seminar he was invited to give to Oxford neurologists, and under this pressure he spontaneously came up with "Blindsight and Hindsight" - "blindsight" to describe the striking behavioural results and "hindsight" to suggest an implication of the midbrain visual pathways for its mediation.
The term first appeared in print in a short article in Lancet in Sanders et al. The term stuck - so much so that it eventually appeared in standard dictionaries. DB continued to be the subject of sustained study over ten years by Weiskrantz in collaboration with Warrington, resulting in a book "Blindsight" in , with a second edition in , in which DB's capacity to detect, to discriminate orientation, movement, form, under a variety of conditions was studied and their possible implications considered..
It early became apparent, also, that rapid transient events, e. This was dubbed Blindsight Type 2 in contrast to the situation in which there was absolutely no reported experience, Type 1. DB later returned as a focus of interest and a number of follow-up studies appeared well into Weiskrantz et al. The basic phenomenon of "unconscious" visual capacity remains, although his sensitivity has improved markedly. Other subjects, e. More recently it has become clear that the phenomenon is not rare.
According to Dr Garrido, one possible explanation is that an independent pathway exists in the brain that bypasses the visual cortex and instead carries information from the eyes to the thalamus and from there straight to the amygdala, a brain region responsible for processing emotional information.
However, there has been very little evidence for such a shortcut and, as such, it has remained controversial. Now, in a new study published in eLife , Dr Garrido and her colleagues found that the proposed pathway does indeed exist in humans with normal vision that links the retina of the eye through two subcortical regions to the amygdala, hence bypassing the cortex. While the imaging demonstrated the existence of the structure, the researchers were still not certain it had a direct impact on behaviour.
Dr Garrido and her PhD student Jessica McFadyen then looked at functional magnetic resonating imaging fMRI data from a facial recognition task to see if there was a link. Stronger white matter connections in the subcortical pathway associated with better fearful face recognition would be evidence of this link.
But even then, he could not describe the content itself, meaning that his experience lacked almost everything we would normally associate with vision. Of all the questions these studies have posed, the most pressing has been why? What causes the conscious and unconscious to decouple so spectacularly? Tellingly, all the blindsight subjects had suffered damage to a region known as V1, at the back of the head, suggesting that it is this region that normally projects the stream of images into our awareness.
To test their ideas, scientists can use a form of non-invasive brain stimulation that disrupts different brain regions, in an attempt to induce a reversible form of blindsight in healthy participants.
You can see a video of the procedure below. The experiment began with Allen placing a magnet over the back of my skull, just above V1. Next, he began applying the magnetic field for short intervals at increasing strengths. At first, all I could feel was a slight tapping sensation the effect of the magnetic field on my skin but eventually I did notice a fleeting dark line crossing the centre of my vision, a bit like an old TV monitor just after you pressed the off switch.
It only lasted less than a second, however, and although it gave me a small shock, I soon became used to the sensation.
After Allen had found the right power, I sat in front of a computer screen, and he flashed up pictures of arrows for a split second: my job was to say whether they pointed left or right. The pictures were sometimes timed with the TMS signals causing the temporary blindness — and like Daniel in those original experiments, I often saw nothing and felt that I was guessing.
Nevertheless, once I had finished, Allen told me that I had answered many more correctly than would be expected by chance alone, suggesting the TMS had succeeded in giving me blindsight. Eventually, the researchers may even understand how the brain creates visual consciousness itself — and why V1 is so crucial. One idea is that consciousness relies on communication to and from many areas of the brain — and maybe V1 is working as a hub that helps orchestrate that broadcast.
People with blindsight cannot see what's in front of them, yet they can somehow "feel" the contents of a scene Credit: iStock. Picking apart the experience may also reveal further clues about the power of unconscious mind. To understand how, imagine that you are part of a strange puppet show. You have been blindfolded, and your limbs are tied to invisible strings. Every so often, they are tugged here or there by a hidden puppet master, leading you through a complicated dance.
That puppet show is essentially what happens when someone with blindsight navigates their way past obstacles — with the non-conscious mind acting as the puppet master. This, in turn, begins to cast doubt on some long-held assumptions about the very nature, and purpose, of consciousness. After all, it is by no means certain that other animals have a rich inner life like us, so it must have emerged for some reason. In this way, our heightened awareness helps highlight the most important parts of a scene, giving us the chance to respond.
By exploring their unconscious, blindsight patients have opened new paths for the study of the human mind Credit: iStock.
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