Greg Detre
22/10/01
Dr Iles, week II
reaching � in order to lift up, e.g. primates
movement made under visual control
learned to some extent, probably takes a few months to co-ordinate, partly trial + error
open loop or pre-programmed � visual input is used to specify the motor programme, but corrections are being continually made if the objects move
almost irrepressible corrections (see reading list Pisella, Grea et al 2000, Nature neurosci)
direction of object � needs to be body-centred
good evidence for those kind of coordinate transformations in the PPC
object distance � stereopsis (best at 1m), needs independent information about where your eyes converge � cells in the parietal cortex (???)
starting position of limb provided by kineshetic � muscle spindle, cutaneous, joints etc. � all project to the PC
start and end position defines a trajectory � close to straight line trajectories , bell-shaped velocities
also: grip force, hand shape, finger aperture, posture, lift force etc.
some evidence tha they might be organised in the PC
Goodale: shown that lesions in the PC affect these, whereas lesions in the ventral stream do not affect them
effects of illusions, e.g. size illusions fool perceptual but not reaching system
some that effect estimates of weight (which is probably ventral anyway)
most of the necessary components/information fits/feeds into the dorsal system
where else is involved in reaching?
motor + pre-motor
lesion evidence, functional imaging, PET (premotor related to self-paced movements)
main approach = chronic recording in conscious monkeys
(first done by Evarts)
Georgeopolous � studied reaching on conscious operant trained monkeys � suggested that the firing varied according to direction of movement � overall movement out of the vector sum of individually-tuned populations of neurons
Kadaska and Crammond tried movement from A-B with different patterns/postures � argued that he from iring of some of the neurons changed, so they weren�t coding just for directions � indeed, some were affected by load-bearing
Kakei (recent) monkey different grips � different muscles to more up-/down � in the premotor, mainly specifies overall direction of movement oblivions of which muscles do what int eh motor, a significant number were related to muscle activation (2001, Nature Neurosci, vol 4, pg 1020) (1999, Sci, vol 285, pg 3136), though there were a small number coding for direction in space
so where are the details of the motor output mainly specified?
possibly sub-cortical, but because they�re highly-learned, could go on with the cerebellum
massive projections from PPC � pons � cerebellum, and back to motor cortex via thalamus + caudate, or even the basal ganglia (there are projections from PPC � BG � premotor (e.g. bradykinesia akinesia in Parkinson�s))
or even in the spinal cord? Lundberg et al � hindlimb of the cat, but some on the forelimb of the cat, even though we don�t normally think of cats reaching out
transparent tube with food at the end, and cats had to put their paw through tube + manipulate � showed that there is manipulation masked by the fur
looked to se how primary motor is wired to spinal � no direct connecitons, though there are via 1 or 2 inter-neurons, plus an indirect paht that has a different function
CST travels down different part of spinal cord so could lesion
propriospinal lesion affected reaching, direct affected manipulation
giant fibres in lobster � almost complete tail-flip escape respnose from command neurons in spine
cats are carnivores + we�re primates � very distant common ancestor with limited vision + reaching
opposite argument in �On the origin of skilled forelimb movements�
says that reaching + manipulation movements are actually quite widespread, so the machinery underlyingn might be distributed + conserved in the spinal cord (???)
Dessolini � human neurophsyiology experiments � many phenomena attributed to propriospinal in cat, so �/span> hope for some simple reaching located in the spinal cord
Lemon worked on monkeys � should be easy enough to reveal propriospinal tract
found no evidence for it in rhesus anaesthetised monkeys
Lemon on conscious monkeys, found no evidence for propriospinal mediated actions
some responses though in squirrel monkesy, so Whisaw + Iwaniak might be right in attributing same movements further back � but, squirrel monkeys have poor control of movement
rubrospinal (from the mid-brain red nucleus)
rubrospinal said to be absent, so might be a need for propriospinal in man
frogs � highly specialised amphibia
frogs are adept at reoving things from body surface, even spinal frogs
stimulating part of spinal cord �/span> hindleg movement to a particular position (Bitzy)
but can't be confident that this is activating an unnatural part of the spinal cord
rats flexor reflex � similar to frogs� reflex
might be stimluating cutaneous afferents by accident
Anderson � intention in the PC?
intention seems less related to irrepressible corrections
self-paced � more medial premotor areas, like supplementary areas � probably responsible for readiness potentials
BG may also be implicated
intention probably comes in many different guises, including perhaps PC
parahippocampal gyrus � involved in memory
cingulate � memory + intention (just underneath supplementary motor cortex)
place fields in the rat hippocampus � probably more for finding its way round the environment, spatial learning etc.
(Paus(???), 2001, Nature Neurosci � anterior cingulate cortex � motor + cognitive interface)
accept the 2 stream model of vision as useful simplifying model but be careful
Livingstone + Hubel � inputs are not purely p- and m- though � that�s mixedu p
how distinct are they anatomically � Young (Nature, 1992, pg 155)
parallel processing � look at the IT side, and face recognition
open loop vs pre-programmed
alien hand syndrome
PC shows that � of the visual system at least, needs embodiment
NN constraint satisfaction = good for ill-posed problems
where is premotor/somatosens???
spinal cord is conservative
readiness potentials � signal couple of seconds before execution
Sakata � important for distance of object