54. False Visual Mismatch Negativity

Recording of the visual mismatch negativity (vMMN) requires building of a visual temporal regularity using standard stimuli and afterwards interrupting of such regularity by a deviant stimulus. However, for the veridical vMMN it is necessary to control subject's attention by a task, because the attentional modulation might elicit a similar event related response (ERP) as the vMMN. In this contribution we point out an important and unexpected interaction between the attentional task and the regularity violation. We presented standard/deviant stimuli (a moving grating) in periphery of the visual field and targets/non-targets (numbers) stimuli in the central part of the visual field. Targets were presented randomly with preceding 2, 3 or 4 non-target stimuli (so the time between targets was randomly selected in the interval of 2460–4100 ms. Subjects (aged 21–61 years, 3 females) pressed a button when they spot the target. To make the recording session as short as possible the target was presented immediately (400 ms) after each deviant; this way deviants were unpredictable and also uninterrupted by a slow response to the target. ERPs were recorded from 68 unipolar electrodes, (four EOG channels) with the right earlobe as a reference. In results we observed a statistically significant vMMN-like response in the interval of 150–250 ms. This response, however, exhibited unexpected features, as its localisation was in the central area with prevalence to the left hemisphere (all subjects responded by the right hand) and it was absent in recordings where the subjects' response was not required. Further, we observed difference in a linear trend of the ERPs to standard/deviant stimuli. All these signs supported evidence that the vMMN-like potential was a false vMMN created by superposition of a preparatory potential for the target detection (the early readiness potential – Bereitschafts Potential) and the deviant response. The presented stimulation scheme with a randomised appearance of the deviant stimulus and the attentional task caused the false vMMN because the subjects' expectation of the target stimulus resulted in the readiness potential, which precedes the behavioural response even couple of seconds before subjects' reaction. Due to a suboptimal design, this preceding response was superimposed in a greater extent over the deviant response than over the standard one. The linear character of the readiness potential, however, allows its extraction from the ERP and identification of the veridical vMMN (Kremláček, 2013). Supported by P37/07 (PRVOUK) program of Charles University.