20
2 The Visual World
the table were now a body part. The idea is that you can project your sensations
onto external objects.
The Binding Problem
Nothing, it seems, is more natural, easier and more spontaneous than seeing.
It strikes us as self-evident that the instant the optical array is formed on our
retina, the image is transferred to our understanding and we see. The feeling
of this is so compelling that until relatively recent times there seemed no reason
to question it. However, modern research into the neurophysiology of eyesight
discloses a reality which is starkly different from what has just been described.
In particular, many layers of mental processing are interposed between the eyes
and the visual world that we experience, so what we see is indirect at best.
Based largely on research with monkeys, the Nobel laureate David Hubel
and his collaborator Margaret Livingstone in 1988 published a surprising and
far-reaching discovery: They found that the nerve fibers from the eyes, after
entering the brain, divide into many separate and independent streams, or
pathways, that analyze different aspects of the same retinal image. It turns out,
then, that the retinal image is split apart at its very inception into disembodied
aspects each of which is analyzed in a different and specialized part of the
brain: One pathway is devoted entirely to motion, another analyzes shape, and
so on. 7
The fact that different brain areas carry out specialized tasks has been known
at least since 1860, when the French physician Paul Broca reported on two
patients with injuries to an area of the left brain, who had developed incapacitating deficiencies in language comprehension. It rapidly became clear that
the brain area in question, known today as Broca’s area, is the locus of speech
production. A nearby area called Wernicke’s area is the area for speech comprehension. In succeeding years, right up to the present time, clinical studies
have revealed that the brain is divided into a large number of modules, or
functionally specialized areas, each of which carries out a very specific task.
Many of these facts emerge from clinical studies of patients who have suffered
strokes or head injuries. From these studies it is possible to associate particular
brain areas with the functions they serve—because the functions they serve are
precisely those that fail when the area is damaged. Over the years, and from
case studies of a large number of patients, it has been possible to piece together
cortical maps linking specific regions of the cortex to specific faculties.
An impairment in vision due to brain injury is known as a visual agnosia.
Patients whose lesion is in the shape-processing area of the brain are impaired
2 The Visual World
the table were now a body part. The idea is that you can project your sensations
onto external objects.
The Binding Problem
Nothing, it seems, is more natural, easier and more spontaneous than seeing.
It strikes us as self-evident that the instant the optical array is formed on our
retina, the image is transferred to our understanding and we see. The feeling
of this is so compelling that until relatively recent times there seemed no reason
to question it. However, modern research into the neurophysiology of eyesight
discloses a reality which is starkly different from what has just been described.
In particular, many layers of mental processing are interposed between the eyes
and the visual world that we experience, so what we see is indirect at best.
Based largely on research with monkeys, the Nobel laureate David Hubel
and his collaborator Margaret Livingstone in 1988 published a surprising and
far-reaching discovery: They found that the nerve fibers from the eyes, after
entering the brain, divide into many separate and independent streams, or
pathways, that analyze different aspects of the same retinal image. It turns out,
then, that the retinal image is split apart at its very inception into disembodied
aspects each of which is analyzed in a different and specialized part of the
brain: One pathway is devoted entirely to motion, another analyzes shape, and
so on. 7
The fact that different brain areas carry out specialized tasks has been known
at least since 1860, when the French physician Paul Broca reported on two
patients with injuries to an area of the left brain, who had developed incapacitating deficiencies in language comprehension. It rapidly became clear that
the brain area in question, known today as Broca’s area, is the locus of speech
production. A nearby area called Wernicke’s area is the area for speech comprehension. In succeeding years, right up to the present time, clinical studies
have revealed that the brain is divided into a large number of modules, or
functionally specialized areas, each of which carries out a very specific task.
Many of these facts emerge from clinical studies of patients who have suffered
strokes or head injuries. From these studies it is possible to associate particular
brain areas with the functions they serve—because the functions they serve are
precisely those that fail when the area is damaged. Over the years, and from
case studies of a large number of patients, it has been possible to piece together
cortical maps linking specific regions of the cortex to specific faculties.
An impairment in vision due to brain injury is known as a visual agnosia.
Patients whose lesion is in the shape-processing area of the brain are impaired