The IPA is expected to appoint a replacement by 5 November. Nominees are being evaluated and consulted as to whether they are willing to take up the voluntary position. A vote by the IPA council will endorse the appointment.
Commenting on his decision, McCulloch said: "I've done it for three years and it's time someone with new ideas and energy was allowed to have a go."
McCulloch, who was made redundant from Zentropy in August, is still seeking employment. He said his redundancy came as a pragmatic move to cut costs due to a shortfall in revenue targets for the media part of the business.