Consciousness

• seCtIon tHRee: BoDY AnD WoRLD
  It was already known that a strong stimulus
  to somatosensory cortex could interfere
  with sensations coming from a touch on
  the skin. So if consciousness really takes
  half a second to build up, then it should be
  possible to touch someone on the skin and
  then block the sensation by stimulating the
  cortex up to half a second later. This was
  exactly what Libet found. He stimulated
  the skin first and then the cortex. When
  the cortical stimulus came between 200
  and 500 ms after the skin stimulus, the skin
  stimulus was not consciously felt. In other
  words, a touch on the skin that participants
  would otherwise have reported feeling was
  retroactively masked up to half a second
  later. This certainly seems to confirm the
  idea that neuronal adequacy for conscious
  perception is about half a second.
  But how can this be? We do not experience things as happening half a second
  behind, and half a second is long enough that surely we would notice the delay.
  Libet checked this intuition by asking participants to report the subjective timing
  of two sensations. One was an ordinary stimulus to the skin; the other was a cor-
  tically induced sensation (the two feel noticeably different). The interval between
  them was systematically varied and participants had to say which came first. They
  consistently reported that the skin stimulus came first, even when it came almost
  at the end of the train of pulses. This is what might be expected from previous
  findings, but is also very strange. If half a second of neuronal activity is required
  for conscious perception, why is the skin stimulus (which must also be followed
  by half a second of appropriate activity to produce a conscious sensation) felt
  first?

Libet’s controversial suggestion was that sensory experiences are subjectively

referred back in time once neuronal adequacy has been achieved. In other words,

what happens with any sensation is this. Information travels from, say, the skin,

up to the relevant sensory area of cortex. If, and only if, activity continues there for

the requisite half a second, the stimulus is consciously perceived. At that point it

is subjectively referred back to the actual time at which it happened. If neuronal

adequacy is not achieved (because the stimulus was not strong enough, because

other brain processes suppressed the activity, or because a devious experimenter

interfered directly in the cortex), nothing is consciously experienced.

How does subjective referral work? To what point in time is the experience

referred, and how? Libet surmised that the primary evoked potential might act

as a timing signal to which the sensation is referred back – or ‘antedated’. Because

evoked potentials occur so fast after peripheral stimulation, referring the sensa-

tion back to this point would mean no delay in conscious perception even though

half a second of activity is required for neuronal adequacy. To test this, Libet and

his colleagues (Libet et al., 1979) exploited two special features of what happens

when the medial lemniscus (part of the pathway from the cutaneous receptors to

C train (60 pps)

S-experience,

actually before C-experience

C-experience

S-pulse

S-experience expected

0100 200300 400500 600700 800900 msec

FIGURE 9.3 • Diagram of Libet’s experiment on
subjective time order. A continuous
stimulus train at 60 pulses per
second was applied to sensory
cortex (C), and a single pulse at
threshold to the skin of the arm
200 msec later (S). The conscious
experience of C (C-experience) was
reported to occur approximately
500 msec after stimulation
began, and was not reported at
all unless stimulation continued
for 500 msec. On this basis one
might expect S-experience to occur
200 msec after C-experience. In
fact it was reported to occur at
approximately the time of the skin
pulse, before the C-experience.
These findings led Libet to propose
the ‘subjective referral of sensory
experience backwards in time’
(after Libet et al., 1979, Fig. 1).