Fred,
For 2013, this will be the rule for head & neck restraint and lateral movement:
3.A.3 Driver’s Helmet Support:
It is important to control side motion of the hips, shoulders and head in the event of a crash.
Forward movement: All cars over 175 mph shall have an engineered, tested and approved SFI spec 38.1 type head and neck restraint system.
Lateral movement: The seat or roll cage structure shall provide restriction to lateral head and body movement of less than 2 in. per side inclusive of structure deflection for vehicles over 175 MPH. See Section 3.B.1.
Rear movement: See Section 3.C.