Consciousness

SPLIT BRAINS, SPLIT CONSCIOUSNESS?

Epilepsy can be a debilitating disease, at its worst

causing almost continuous seizures that make a ful-

filling life impossible. For such serious cases, a drastic

operation was carried out many times in the 1960s,

before less invasive treatments were discovered.

To prevent seizures spreading from one side to the

other, the two halves of the brain were separated

in an operation known as a commissurotomy. In

some patients, only the corpus callosum, or part of

the corpus callosum, was cut; in others the smaller

anterior and hippocampal commissures were also

cut. Remarkably, these patients recovered well and

seemed to live a relatively normal life. Tests showed

that their personality was little changed and their IQ

and verbal and problem-solving abilities were hardly

affected (Sperry, 1968; Gazzaniga, 1992), but in the

early 1960s some clever experiments were designed

to test the two hemispheres independently. The

findings about the dramatic consequences

of this disconnection, and the work that

followed, earned a Nobel prize for pioneer-

ing psychobiologist Roger Sperry.

Information from the left visual field goes

to the right hemisphere and from the right

visual field to the left hemisphere (note,

this is not from the left and right eyes).

The left half of the body is controlled by

the right hemisphere (and vice versa), but

information from the right ear goes to the

right hemisphere (and vice versa). Know-

ing this, it is possible to feed information

to only one hemisphere, and obtain a

response from only one hemisphere. In

1961 neuroscientist Michael Gazzaniga

first tested the split-brain patient W. J. using

such a procedure. At that time, research on

cats and monkeys had shown that the two

hemispheres appeared to function almost

entirely separately when disconnected,

but no one expected this to be true of

humans  – after all, the patients appeared

to act and speak and think like ordinary unified people. But the research showed

that, as in the animals, each half-brain appeared to behave independently.

In a typical experiment, the patient fixated on the centre of a screen which was

divided into two. Words or pictures were then flashed to either visual field, thus

sending information to only one hemisphere. The patient responded verbally, or

(a)

‘Above’

(b)

Gas

(c)

Gas

Pressure

(d)

‘The greater

the pressure’

Gas

‘The faster

it dissolves

in water’

and pop-out in complex displays, so that synaesthetes

can detect concealed shapes, such as triangles or

squares, more easily than controls (Ramachandran and

Hubbard, 2001). this shows that synaesthetes are not

confabulating or relying on memory for associations.

Cytowic emphasises the connection with emotions and the

limbic system, while Ramachandran suggests that shape–

colour synaesthesia is caused by a mutation that creates

cross-activation between visual areas (especially V4 and

V8) and the number area, which lie close together in the

fusiform gyrus. other kinds of synaesthesia may depend

on crossover between other neighbouring areas of sen-

sory cortex. synaesthesia is more common in autism, sug-

gesting that both may result from neural over-connectivity

(Baron-Cohen et al., 2013).

FIGURE 6.6 • Luria (1968) read the following
sentence to S.: ‘If carbon dioxide is
present above a vessel, the greater
its pressure, the faster it dissolves
in water.’ S. was so distracted
by the mental images associated
with each word that he could not
understand this simple rule.