Responding to an increasingly urgent need for collaboration between mathematicians and mathematics educators, the study reported in this paper engages mathematicians as educational co-researchers in a series of themed Focus Group interviews where a pre-distributed sample of mathematical problems, typical written student responses, observation protocols, interview transcripts and outlines of relevant bibliography is used as a trigger for reflection upon and exploration of pedagogical issues. In the extract exemplified here, a part of the sample that includes a question involving the concept of det(M), the determinant of an nxn matrix, triggers a conversation that reveals various images of the concept held by members of the group as well as their beliefs about personal and shared concept images. (Contains 1 figure.) [For complete proceedings, see ED500858.]