On Sun, 22 Oct 2006 14:20:28 -0500, "Fijoy George"
<tofijoy from yahoo.co.in> wrote:
>>"Matthew Kirkcaldie" <m.kirkcaldie from removethis.unsw.edu.au> wrote in message
>news:m.kirkcaldie-B07890.10410122102006 from news.sydney.pipenetworks.com...>> In article <ehdkh3$are$1 from mailhub227.itcs.purdue.edu>,
>> "Fijoy George" <tofijoy from yahoo.co.in> wrote:
>>>>> Hi all,
>>>>>> Would someone be able to help me with the following question?
>>>>>> When we look at the neuronal activation areas in Functional Magnetic
>>> Resonance Images obtained from various experiments, we see that in each
>>> area, the activity is most intense at the center and slowly declines
>>> towards
>>> the boundary. Now, suppose we were to actually measure the neuronal
>>> currents
>>> in the activation areas. Will the current amplitudes follow the same
>>> pattern? That is, will the current amplitudes be highest near the center
>>> of
>>> the area, and smoothly decline toward the boundary?
>>>>>> Thank you very much
>>> Fijoy
>>>> Fairly unlikely, in my estimate. The nearest correlate to fMRI signal I
>> know of is firing rate in neurons, but the decrease at the edges of the
>> fMRI signal is likely to be a combination of measurement artifact
>> (limited resolution of the technique) and the anatomy of the blood
>> vessels - in particular, anastomoses between vessels, spreading
>> increased blood flow around the activated area. I'm not exactly sure
>> what you mean by "neuronal currents" - firing rates? Evoked
>> potentials? More info would allow a better answer.
>Thank you for your reply, and sorry about using loose terminology in my
>question. I hope I am making myself clear in the following.
>>We know that the primary sources of electrical currents in the brain are the
>pyramidal cells in the cortex. We also see that the pyramidal cells are
>normally oriented to the cortical surface. Suppose that a particular portion
>of the cortex (a cortical patch) is activated during a task. Suppose we
>measure the *current density* normally oriented to the cortical surface at
>various locations in the patch. Now, will the amplitudes of the current
>density be maximum at the center of the patch and slowly decline as we move
>along the cortical surface away from the center?
>
Note: I "corrected" your top posting.
You still don't define 'electrical current'. How is this measured?
Do you mean extracellular currents that are produced by action
potentials? by synaptic activity? In fact, what do you mean by
"neuronal activity" in general? Is it action potentials? Note: an
enormous number of local cell-cell interaction within the cortex can
function purely on local graded currents. Do you mean synaptic
activity? Do you mean something related to general cell metabolism
and oxygen consumption, what is measured by fMRI?
As Matthew indicated, of fMRI really averages activity over a volume
of perhaps several hundred micrometer diameter (or more), then
obviously it will peak in a region of high "activity" but drop off
gradually towards the edges. That is an artifact of volume
averaging.
