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Dear Donald, Thank you for your prompt response. One more query. Are we right in assuming that items are weighted in a parametric modulation by the number that they are assigned (e.g. easy = 1, hard = 2)? If this is true, provided your previous response, the contrast 0, 1 gives a linear increase from easy to hard (i.e. items weighted 1 to 2) while by comparison, a contrast of 0 -1 gives a linear decrease from easy to hard (i.e. items weighted 1 to 2)? A linear decrease does not mean a linear decrease from items rated 2 to 1? Correct? Best, Elsa Baena & Siobhan Hoscheidt, M.A. On Wed, Apr 13, 2011 at 4:38 PM, MCLAREN, Donald <[log in to unmask]>wrote: > Siobhan, > > The contrast for parametric modulators are: > 0 1 for linear increase > 0 -1 for linear decrease > > The first column is the canonical HRF (mean of overall task) and the second > column is the parametric modulator. > > If you only have 2 conditions, you could just code it as two different > event types too. > > Best Regards, Donald McLaren > ================= > D.G. McLaren, Ph.D. > Postdoctoral Research Fellow, GRECC, Bedford VA > Research Fellow, Department of Neurology, Massachusetts General Hospital > and Harvard Medical School > Office: (773) 406-2464 > ===================== > This e-mail contains CONFIDENTIAL INFORMATION which may contain PROTECTED > HEALTHCARE INFORMATION and may also be LEGALLY PRIVILEGED and which is > intended only for the use of the individual or entity named above. If the > reader of the e-mail is not the intended recipient or the employee or agent > responsible for delivering it to the intended recipient, you are hereby > notified that you are in possession of confidential and privileged > information. Any unauthorized use, disclosure, copying or the taking of any > action in reliance on the contents of this information is strictly > prohibited and may be unlawful. If you have received this e-mail > unintentionally, please immediately notify the sender via telephone at (773) > 406-2464 or email. > > > > On Wed, Apr 13, 2011 at 7:19 PM, Siobhan M. Hoscheidt < > [log in to unmask]> wrote: > >> Dear SPMers, >> We are currently working with a model using parametric modulation to >> examine a linear increase in brain activation given two levels of difficulty >> (easy vs. hard). We assume that giving the program a contrast of 1, 0 will >> yield a con image that will show significant areas of activation as a linear >> increase from easy to hard compared to the global brain mean. >> >> What does a negative contrast value mean in a parametric modulation? Would >> -1 0 yield a contrast in which a linear decrease in brain activation from >> easy to hard can be observed or will it yield a contrast in which a linear >> increase from hard to easy can be observed? Or, perhaps we are >> misinterpreting the meaning of -1 in a parametric modulation overall. >> >> Any help will be greatly appreciated. >> >> Best, >> Siobhan Hoscheidt, M.A. & Elsa Baena, M.A. >> > > -- Siobhan M. Hoscheidt, M.A. CNS Doctoral Program Psychology, Room 217J (520) 621-8792 Cognition & Neuroimaging Lab/ Anxiety Research Lab University of Arizona Tucson, AZ