In this example there are six authors: Kosslyn, Koenig, Barrett, Cave, Tang, and Gabrieli (1996)
For the first and subsequent citations write the first author’s name followed by et al.
This is how six or more authors would appear as part of a narrative:
Kosslyn et al. (1996)
And if you chose to use parentheses this how the citation would look:
(Kosslyn et al., 1996)
(American Psychological Association, 2010, p. 175)