Hi everyone,
I'm a newbie to R and to linear mixed effects modeling, so please have mercy
:-)
Just wanted to check, whether what I'm doing is alright.
I've collected data concerning tonotopic organization of the auditory cortex
in humans, and I have approximately 1400-1800 data/time points per person
(13 in total). Observations were made how the focus of neuronal activity
changed spatially while processing of a frequency-modulated tone.
Regression analysis was performed for each individual using orthogonal
polynomials ...
regtemp <- lm(tempmat[,j] ~ poly(tempmat[,1],degree=i))
... up to degree = 5.
Based on the median (i.e. over all individuals) adjusted R-square, it could
be seen that a linear approach yielded about .5 adj-R2, adding a quadratic
term increased R2 to about .7 (adding further terms didn't increase adj-R2
in a significant manner).
SO: the next step is to apply a linear mixed effects model of the kind:
y ~ x + x^2
My data.frame looks something like this:
Latency medlat Subject
1 124.1 NA 1
2 125.6 NA 1
....
306 573.9 -3.83 1
307 575.3 -3.83 1
....
3000 1859.7 -6.04 2
3001 1861.2 -6.04 2
3002 1862.6 -6.03 2
..... etc. until subject 13
I.e. medlat is the dependent variable, latency my independent variable,
subject is the grouping variable (however I didn't group specifically before
calling lme --> was this wrong?).
There are 23686 observations in total, and depending on the subject some NA.
Following function call was used:
dummy.lme <- lme(medlat ~ poly(Latency, degree = 2), data = dummy,
+ random = ~ Latency | Subject, na.action = na.omit)
Questions:
- Is this approach o.k., or have you lost all your hair already?
- Can one of the lme-experts see if there's something overtly wrong with my
lme-call (especially the fixed and random term)
- I wanted to see how the fits look like for every individual. Using
plot(dummy.lme), I thought this should yield a trellis plot, with each
individual in a separate plot (and fitted line). However it didn't (it was
just a big mess). Any hints?
Thanks for your patience and all the best,
Nathan
---------------------------------------
Nathan Weisz
Institute for Clinical Psychology and
Behavioral Neuroscience
University of Konstanz
P.O. Box D25
D - 78467 Konstanz
GERMANY
Tel: +49 (0)7531 88- 4612
Fax: +49 (0)7531 88- 2891
E-mail: Nathan.Weisz at uni-konstanz.de
http://www.clinical-psychology.uni-konstanz.de
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