In looking over Mele's response to
Libet's work in Conscious
Will and Responsibility (23-33),
he wonders what happens in that
300 ms during the onset of the readiness potential (RP I) before the
activation of the motor response to move (RP II). Is RP I a reliable
predictor of the movement? As of his writing it was not known (27) but he guessed not.
And guess what? That is exactly what Schurger
et al. explored. Recall from their paper they differentiated the
onset of the RP (RP I) with the neural decision to move (RP II). This gap is the very basis for the whole specious argument against conscious intent to move. But not
only does RP II coincide with the awareness of intention to
move, RP I's causal role is incidental
to the movement. How can this not be any clearer unless you have
serious confirmation bias? Unless of course Schurger et al are the ones with confirmation bias and designed their experiment to prove it? Uh huh... They said:
“It
is widely assumed that the neural decision to move coincides with the
onset of the RP (which, given its slow nonlinear character, is
difficult to pinpoint) (11). Our model challenges that assumption by
suggesting that the 'neural decision to move now' might come very
late in the time course of the RP.
“Thus,
according to our model, uncued movements in a task like Libet’s
tend to be preceded by a gradual increase in neural activity
[measured at the scalp (8, 9) or the single-neuron level (16)] whose
causal role is incidental—not directed (consciously orotherwise) at
producing a movement.
“Given
that such spontaneous fluctuations are always present (55), even when
we are not even thinking about moving, is it reasonable to conclude
that the brain “decided” to move 2 s before the threshold
crossing? We suggest reserving the term “decision” to the
commitment to move achieved once neural activity (spontaneous or goal
directed) crosses a specific threshold. Libet et al.’s (9) findings
were surprising because they suggested that the neural decision to
move happens well before we are aware of the urge to move, by 1/2 s
or more. According to our model, this conclusion is unfounded. The
reason we do not experience the urge to move as having happened
earlier than about 200 ms before movement onset is simply because, at
that time, the neural decision to move (crossing the decision
threshold) has not yet been made. A very similar fluctuation in
neuronal firing could equally well, at some other time, have not
preceded a movement.
“Finally,
although our model is silent with respect to the urge to move and its
temporal relation to motor decisions, it helps dissolve another
puzzling question that seemed to arise from Libet’s paradigm. Libet
himself found that subjects were able to estimate the time of a
tactile sensory decision in relation to a quickly rotating clock dial
with only about 50 ms of error on average (9). Why then should there
be such a long and variable gap between the time of a motor decision
and the subjective estimate of the time of the motor decision,
whereas no such gap exists for sensory decisions? In fact, this
question arises only when we assume that the motor decision coincides
in time with the onset of the RP. We have argued that this need not
be the case and that the neural decision to move may come much closer
in time to the movement itself (e.g., −150 ms). We propose that the
neural decision to move coincides in time with average subjective
estimates of the time of awareness of
intention to move (9, 11) and that the brain produces a reasonably
accurate estimate of the time of its movement-causing decision
events.”
No comments:
Post a Comment
Note: Only a member of this blog may post a comment.