Re what you wrote about visual processing and stimulating the brain with
electrodes, a general rule seems to be that the further downstream of
processing (away from retina), the more complicated the circuits become.
They chose to implant the electrodes on primary visual cortex probably
because it is accessible and also topographical. I heard that he was able
to put a hat on a stand and also locate the door.
I also agree that the brain cannot be overloaded for sensory information.
It is a matter of choosing what to gain and to lose. Some blind/deaf
communities think they have their own culture and choose to remain blind;
but there certainly is a need for restoring vision for some patients.
"John H" <john at faraway> wrote:
> There are currently some very experimental trials in visual replacement. One
> case I vaguely remember is that of an individual fitted with a tiny video
> camera in the eyesocket. This was attached to a series of electrodes
> implanted into the rear of the skull for primary visual stimulation. The
> news report on this said the scientists claimed this will enable a person to
> see eventually. My problems with this are:
>> The primary visual cortex is but one of multiple processing modalities.
> Upstream of this are the 'higher' visual areas, the dorsal and ventral
> streams which deal with specific aspects of vision (place and identification
> I think), and then there's the frontal eye fields, and the prefrontal cortex
> also has a role in visual memory and attention(read some beautiful studies
> by Goldman Rakic on that!). Full vision is not recreating a picture in the
> mind. I don't know what it is, but it is not that. Simultaneous entrainment
> of all the relevant neural regions is my best guess but that says nothing
> and is a given ... .
>> Downstream (relative to retina) the optic nerves send projections to
> superior collicus (for saccade control among other things I think), the
> lateral geniculate nucleus, and with respect to the latter retains retinal
> geometry, as does happen in the primary visual cortex.
>> It is interesting that I have not seen any followups to the above
> experiment, my guess they are having all sorts of problems. With this
> external stimulation of the visual cortex how can visual attention be
> oriented? What about colour? Does the stimulation of the visual cortex only
> create the broadest possible geometric patterns(hence the early promise) and
> never allow for fine discrimination? Will the individuals ever be able to
> truly locate visual objects in geometric space?
>> Japanese scientists have overcome the above problems by inserting tiny light
> sensitive devices into the retina and these then fire up retinal ganglionic
> assemblies. Now that is a real possible solution but again we are a very
> long way from achieving the degrees of resolution that individual retinal
> cells achieve (down to 7 photons). My guess is that the greatest promise for
> this technology will be for those born blind at birth, but then one would
> have little time to have the device fitted because the visual cortex will
> 'shut down' and stay that way even after intensive stimulation (Hubel's
> 'monocular' cats).
>> I cannot reference this but remember reading of a 15 year old blind boy who
> could ride his push bike around the block via echolocation. Apparently it
> does happen with some blind people though rarely to that level of
> proficiency. My spin on this is:
>> The auditory cortex exists primarily in two layers, its processing is aimed
> at identifying sound source and frequency. However, in blind individuals
> there is a gradual recruitment of some visual processing areas for sound
> processing. This visual cortex is ideally designed for geometric and spatial
> properties of objects. The stimulation of the visual cortex via sound
> probably leads to downstream activation of the dorsal or ventral stream,
> thereby allowing for much greater discrimination in sound processing.
>> Consider synaethesia (spelling!), where sensory modalities overlap - eg.
> sounds generate colours. It is believed that babies experience this, and the
> studies on the Russian mnemonic(Luria's patient) indicate this as he appears
> to be one of the VERY FEW people who accurately recalls very early childhood
> experiences(at circa age 5 we forget most of our childhood). At this time
> the brain has not delinaeated the various sensory modalities, that takes
> time. In some individuals this process never quite finalises.
>> Consider 'gaze tinnitus' where if an individual stares in a certain
> direction then tinnitus ensues. This may relate to ocular nerve projections
> to auditory cortex (there is research indicating that tinnitus arises in the
> auditory cortex, not the cochlea itself). The brain is not as neat and tidy
> as all those diagrams suggest ... .
>> Consider the recent fascinating work of Ed Taub, wherein by forcing
> individuals to use their paralzyed limb some recovery of function could be
>> Clearly then sensory modalities are far more flexible then we are accustomed
> to think. However, I cannot see the value in directly acting on the brain to
> extend visual awareness for different frequencies. We have technology that
> can do that and do it much more powerfully and safely than mucking around
> with brains. Furthermore, such a technology would overload the brain, even
> if we could generate the expanded wavelength receptivity it would probably
> be at the cost of other modalities. There's no free lunch in the CNS and
> don't give me that 'but we only use 10% of our brain so why not ... . A
> myth, if you keep loading up the brain with too many tasks you'll probably
> end up with excitatory amino acid induced apoptosis and necrosis. Ask any
> poor chronic epileptic ... . Particularly re hippocampus (highest
> concentration of NMDA receptors) and the PFC.
>> In complete contradiction to my first subject I have not researched this.
> Off the top, speculative. Go for your life everyone!