question about ac-1 data set

Hello,
I have downloaded the ac-1 data set and opened it in Matlab, but am unsure what the various numbers refer to. I assume the "trace" matrix in "response" is where the principal data is, but I don't know what the numbers in "trace" mean or why they are all negative.
Any clarification would be greatly appreciated.
Thank you very much.
Hi Nicholas,
Thanks for your interest in our data. As you assumed, "response.trace" contains the raw data trace, which is the membrane potential of a rat primary auditory cortical neuron (in units of mV as in "response.unit") in response to sound (as in "stimulus" structure).
Please see "ReadMe.txt" for details of the data structure, see "Methods.txt" for general descriptions of our experimental procedures, and see "Contents.m" for a brief description of the data in each directory. For details, please refer to our papers:
Machens et al. (2004) J Neurosci 24:1089-1100.
Asari and Zador (2009) J Neurophysiol 102(5): 2638-56.
Hiroki