Hi,
yes, unless FEAT complains about rank deficiency I would ignore the
eigenvalues and just use the "effect required" efficiency calculations.
It is fine to reduce the number of timepoints in your data using
fslsplit (I'm not sure why you'd want to use fslmerge) to simulate a
shorted experiment and see what effect that makes on the efficiency
calculations.
Note though, as described in the paper, the efficiency calculations
don't know anything about the nonlinearities that occur as the events
get closer and closer together, so be careful there.
Cheers.
On 2 Jun 2008, at 15:44, Jose Paulo Santos wrote:
> Hello,
>
> Sorry for the confusion. I use the term “efficacy” in the sense that
> I use
> these values to determine if a paradigm is efficacious or not, i.e.,
> if
> they look OK and then I can pursuit with the paradigm, or not, and
> changes
> to the paradigm are required.
>
> It’s acceptable to use fslsplit and fslmerge to reduce the number of
> volumes in the data file to compare the resulting values of Effect
> Required and “Efficacy”? Or these comparisons aren’t possible?
>
> Kind regards,
> Jose Paulo Santos
>
>
> On Mon, 2 Jun 2008 07:27:16 +0100, Steve Smith <[log in to unmask]>
> wrote:
>
>> Hi - I'm still a little confused as we never call this "efficacy" -
>> this is a plot of the eigenvalues from a singular value decomposition
>> of the design matrix - it's not as directly useful or interpretable
>> as
>> the "Effect required" efficiency calculations - if they look ok then
>> that's good enough.
>>
>> Cheers.
>>
>>
>> On 1 Jun 2008, at 23:29, Jose Paulo Santos wrote:
>>
>>> Hello,
>>>
>>> Yes, I got these values from FEAT (please see attachment; this
>>> example is
>>> for 250 volumes). The Efficacy is the right most value in the
>>> diagonal (I
>>> didn’t consider the others) and the Mean Effect Required is the mean
>>> of
>>> Ci’s.
>>>
>>> Kind regards,
>>> Jose Paulo Santos
>>>
>>>
>>> On Fri, 30 May 2008 08:19:03 +0100, Steve Smith <[log in to unmask]
>>> >
>>> wrote:
>>>
>>>> Hi - I'm not sure where these Efficacy numbers come from - not from
>>>> the FEAT efficiency calculations I think? I'm not sure what these
>>>> are.......
>>>> Cheers.
>>>>
>>>>
>>>>
>>>> On 28 May 2008, at 19:10, Jose Paulo Santos wrote:
>>>>
>>>>> Hello,
>>>>>
>>>>> I'm trying to establish some kind of method to achieve to the
>>>>> optimal
>>>>> length in event-related paradigms.
>>>>>
>>>>> I designed a paradigm were I collected 500 volumes. The
>>>>> participant
>>>>> reported at the end that he was exhausted. So I know that 500
>>>>> volumes were
>>>>> too much in the human perspective. By the end of the session, the
>>>>> probability of the participant still being concentrated in the
>>>>> task
>>>>> was
>>>>> quite low.
>>>>>
>>>>> With fslsplit and fslmerge commands, I reconstructed several
>>>>> files,
>>>>> until
>>>>> 250 volumes, and I analyzed each of them with Feat. The numerical
>>>>> data was:
>>>>>
>>>>> Volumes Efficacy Mean Effect Required
>>>>> 250 0.253 0.938
>>>>> 312 0.241 0.847
>>>>> 375 0.251 0.752
>>>>> 438 0.245 0.740
>>>>> 500 0.243 0.722
>>>>>
>>>>> The rule of thumb for the Efficacy determines that it must be more
>>>>> than
>>>>> 0.200, and it was for all volumes. The rule of thumb for the
>>>>> Effect
>>>>> Required determines that it must be less than 0.800. This
>>>>> requirement is
>>>>> true only for 375 volumes or more. When this data is plotted, it
>>>>> seems
>>>>> that there aren’t significant increases in the Effect Required for
>>>>> more
>>>>> than 375 volumes. This is in line when the maps are analyzed: the
>>>>> 375
>>>>> volumes map has activations more intense (just a bit) and more
>>>>> extended
>>>>> (just a bit) than the 438 and 500 maps. So, 375 volumes would be
>>>>> the
>>>>> optimal length (and duration) of the session. If there is some
>>>>> clearance,
>>>>> it would be better to increase the inter stimulus interval (null
>>>>> event)
>>>>> than to show more stimuli.
>>>>>
>>>>> Is this acceptable? Why the Efficacy didn’t change so much?
>>>>> Increasing
>>>>> ISIs would increase the Efficacy?
>>>>>
>>>>> Thanks in advance,
>>>>> Jose Paulo Santos
>>>>>
>>>>
>>>>
>>>> -----------------------------------------------------------------------
> ---
>>> -
>>>> Stephen M. Smith, Professor of Biomedical Engineering
>>>> Associate Director, Oxford University FMRIB Centre
>>>>
>>>> FMRIB, JR Hospital, Headington, Oxford OX3 9DU, UK
>>>> +44 (0) 1865 222726 (fax 222717)
>>>> [log in to unmask] http://www.fmrib.ox.ac.uk/~steve
>>>> -----------------------------------------------------------------------
> ---
>>> -
>>>> =
>>>> =
>>>> =
>>>> =
>>>> =
>>>> ===================================================================
>>> <design_cov.png>
>>
>>
>> --------------------------------------------------------------------------
> -
>> Stephen M. Smith, Professor of Biomedical Engineering
>> Associate Director, Oxford University FMRIB Centre
>>
>> FMRIB, JR Hospital, Headington, Oxford OX3 9DU, UK
>> +44 (0) 1865 222726 (fax 222717)
>> [log in to unmask] http://www.fmrib.ox.ac.uk/~steve
>> --------------------------------------------------------------------------
> -
>> =
>> =
>> =
>> =====================================================================
>
>
---------------------------------------------------------------------------
Stephen M. Smith, Professor of Biomedical Engineering
Associate Director, Oxford University FMRIB Centre
FMRIB, JR Hospital, Headington, Oxford OX3 9DU, UK
+44 (0) 1865 222726 (fax 222717)
[log in to unmask] http://www.fmrib.ox.ac.uk/~steve
---------------------------------------------------------------------------
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