Neural models of the sensory buffers
So-called “iconic” or (for the auditory sphere) “echoic” memory may
actually be a form of perception of the past, made possible by parallel
inputs with differential delays which feed independently into the
sensorium. This hypothesis goes well together with a set of related
psychological and phenomenological facts, as for example: Sperling’s
results about the visual sensory buffer, the facts that we seem to see
movement and hear temporal Gestalts, and the fact that we sometimes
seem to hear sounds only after they have stopped. In the most simple
form of this hypothesis, each one of a number of parallel input lines
with different delays feeds into a spatially separate sensory unit. The
set of such units then holds information about the immediate past in
what one might call a “chronotopic” sensory map. This contrasts with
the idea (common in sensory buffer theory) that the received sensory
information is kept (while possibly decaying) in the same unit for some
time after it occurred. The hypothesis also contradicts the
theory that all sensory information passes through the same unit but is
then successively passed through a unidirectional chain of separate
units, where the past experiences then become represented (the shift
register hypothesis). The main advantage of the new theory, beside the
natural explanations it offers for the above-mentioned kind of
phenomena, is that it postulates a parallel – and therefore robust –
rather than a serial mechanism for the registering of temporal
information. It can of course easily be modified to fit more complex
models of the sensory cerebral code(s) as well as of the chronotopic
representation as such.
It is also possible to formulate a corresponding hypothesis for those
motor commands which control brief movements. At closer inspection,
most so-called “ballistic” movements do not seem to be truly ballistic
(in the sense in which the movement of a cannonball is so) since the
brain must exert some kind of feedforward control over the later part
of their trajectory. It is suggested that this control is at least
sometimes realized by means of differentially delayed output from a
chronotopic representation of successive segments of the movement. Not
only could this be a biologically natural way of ensuring efficient
adaptability of the movement; the hypothesis also explains the not
uncommon experience of “seeing the whole movement laid out in advance”
when it is initiated.
.
References:
Karniel A & Mussa-Ivaldi FA, Sequence, time or state
representation: how does the motor control system adapt to variable
environments? Biological Cybernetics
89 (2003), 10-21.
Malmgren, H. Why
the past is sometimes perceived, and not only remembered.
Poster presentation at the Eighth International Conference on
Cognitive and Neural Systems, Boston, May 19-22, 2004 (pdf
format).
Sperling, G, The information available in brief visual
presentations. Psychological
Monographs 74 (1960), 11.
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Last update: January 11, 2005 by Helge Malmgren
