A Loss of imagery Case Study: No Modulation of Rotation-related Negativity in Mirror Letter Rotation

Patient MX reported a sudden loss of visual imagery, yet performed normally on a wide range of mental imagery tasks except for mental rotation. He was accurate, but showed a non-linear pattern of reaction times with increasing rotation angle. This suggested that he may have used an atypical strategy to perform the task (Zeman et al., 2010). To further investigate this account, MX and ten age and IQ matched controls were assessed in an event-related brain potentials (ERP) experiment while performing a normal-mirror image discrimination of letters in different rotation angles. MX performed as the controls in the normal condition, but showed reduced difference across angles for mirrored letters. A difference between MX and controls was revealed by the corresponding ERP analysis of angular disparity effect, a parietal negativity which becomes more pronounced with increasing angular disparity (Heil, 2002). The controls showed the expected response of the rotation-related negativity (RRN) in both normal and mirror conditions. Little modulation of the RRN by angular disparity was observed in MX in mirrored condition, suggesting that he used some other cognitive source/process in addressing this task rather than mental rotation.