Six shortlisted commissions for the Fourth Plinth in Trafalgar Square, London, have been revealed as part of a free exhibition now being held in the foyer of St Martin-in-the-Fields.

The exhibition, which is scheduled to run until 31 October, features models of each of the six proposals for the empty plinth, which will be occupied by Yinka Shonibare's Nelson's Ship in a Bottle until the end of 2011. Among the shortlisted commissions is Elmgreen and Dragset's Powerless Structures, Fig. 101 - a brass sculpture of a boy on a rocking horse, while Brian Griffiths' Battenburg is a large-scale model of the eponymous cake.

Allora and Calzadilla's Untitled (ATM/Organ) will see a working cash machine embedded within the plinth, triggering a pipe organ when accessed. Katherina Fritsch's Hahn/Cock features an oversized blue cockerel to symbolise regeneration. Other potential commissions include Hew Locke's Sikandar, a replica of the Portland Square statue of Sir George White adorned with charms and medals, while Mariele Neudecker's It's Never Too Late and You Can't Go Back is a mountain-scape presenting two images of the British Isles.

Image: Model of Katharina Fritsch's Hahn/Cock - James O Jenkins