A Mind's Eye in Still Frames
In 1988, a 34-year-old woman known in scientific literature as Patient D.F. suffered severe brain damage from accidental carbon monoxide poisoning due to a faulty water heater. The resulting hypoxia—a lack of oxygen to the brain—caused focused, bilateral lesions to her lateral occipital cortex (LOC). This specific area is part of the brain's ventral visual stream. The damage left D.F. with a rare and serious condition: visual form agnosia and akinetopsia, or "motion blindness".
For D.F., the world appears not as a continuous flow of movement, but as a series of static snapshots. A car moving down the street might appear in one spot, and then suddenly further down the road, without any perception of the travel in between. Pouring a cup of coffee is a challenge because the rising liquid appears frozen, making it impossible to know when to stop. Despite this, her visual acuity for stationary objects, her color vision, and her general intelligence remained normal. She can identify friends by their voices, but not by seeing their faces. She can draw familiar objects from memory, but she cannot copy a simple line drawing of the same object.
Two Pathways for Sight
The most important aspect of D.F.'s case was discovered by neuropsychologists Melvyn A. Goodale and A. David Milner at the University of Western Ontario. They devised a simple experiment. They presented D.F. with a disc that had a slot cut into it and asked her to describe the slot's orientation. She was unable to do so, performing at levels of pure chance. When they handed her a card and asked her to "post" it through the slot, she oriented her hand and wrist perfectly to match the angle, executing the motion flawlessly.
The difference between what she could consciously perceive and what her body could automatically do showed the "Two Visual Streams Hypothesis". First proposed in a less-developed form by Ungerleider and Mishkin in 1982, Goodale and Milner's 1992 model, based on D.F.'s case, refined the theory. It posits that our brains have two distinct visual processing systems.
The Ventral Stream (the "what" pathway) runs from the primary visual cortex to the temporal lobe. This is the system responsible for conscious object recognition and identification. It is the pathway that was damaged in D.F. The Dorsal Stream (the "how" or "where" pathway) runs from the visual cortex to the parietal lobe. This system handles the unconscious, moment-to-moment visual information needed to guide our physical actions, like reaching for and grasping an object. In D.F., this stream remained largely intact. Her eyes saw the slot, and even though her conscious mind couldn't "see" its orientation, the information was processed by her dorsal stream, which guided her hand to the correct angle.