Blindsight

From Psy3242

Revision as of 04:27, 25 April 2008

Definition

Blindsight is defined by the Oxford Concise Dictionary as "a condition in which the sufferer responds to visual stimuli without consciously perceiving them". Human patients with such a condition often have lost V1 functioning and claim they are blind. In primates and humans, the retina relays its major neural output to the thalamus and then to visual cortex (V1, or striate cortex). When the strait cortex is completely removed or blocked in the brain of a monkey, the animal can still discriminate between certain visual stimuli, though its overall capacity for normal functioning has changed. This is because the output from the eye also reaches various other brain regions in the midbrain and thalamus that often remain intact and undamaged when V1 is completely removed.

History

The history of blindsight stems from a focus on animal research and neuroanatomy, especially in the contrast of human and other primate visual cortex functions. One of the first hypotheses of residual vision in monkeys was made in 1886 by David Ferrier, who removed his subjects' entire visual cortexes and observed the 'blind' monkeys navigating around obstacles. More than a century and numerous studies later, the general consensus has been drawn that monkeys lacking a primary visual cortex can discriminate shapes, show sharp sensitivity to the detection of movement and contrast, and have measurable acuity. However, these abilities are discernibly reduced from normal functioning. In contrast to primates, evidence on the lack of V1 or damage to the area in human brains remained inconclusive until more recently.

The Case of DB

DB was a patient at the National Hospital in London who became the first major focus of research in human blindsight. His occipital lobe of his right hemisphere had been removed to alleviate headaches caused by a benign tumor in the area. While the surgery virtually cured him of his migraines, DB became blind in the left half of his visual field following the procedure. However, DB's 'blindness' was not normal in the sense that he had retained an ability to locate objects and detect some changes in his blind visual field. Dr. Larry Weiskrantz of Oxford University conducted much of the research on the case, and instead of relying on DB's personal descriptions of what he couldn't see, he applied 'monkey-type' tests, or methods of forced-choice more commonly used with animals to measure DB's visual capacity.

                                                                                    
                                                                                      Dr. Larry Weiskrantz