I don't know why you don't want to use hrf modeling here. But you can get the pure boxcar function by selecting the Fourier set and set the order to be 0, 
set length=TR/T, T is the number of bins (16 by default).

Ze
On Aug 29, 2011, at 11:01 AM, xiuqin jia wrote:

Dear SPM experts,

I have a 2by2 event-related (ER) design and the trial length is variable. What should I do if I want to conduct a boxcar analysis rather than canonical HRF with time derivatives?
Any comments are appreciated. Many thanks.

Best regards,

Xiuqin Jia

-- 
Xiuqin Jia, Ph.D Candidate
Department of Radiology,
Xuanwu Hospital,
Capital Medial University,
45 Changchun Street,
Xuanwu District,
Beijing, 100053 China 
Tel: (+086)-138-1012-5607