The mode of short-term memory encoding as indicated by event-related potentials in a memory scanning task with distractions

Objective: Auditory event-related potentials (ERPs) were recorded during the performance of a memory scanning task, with and without distracters between the memorized items and the probe. The effect of distracters with different phonological/semantic characteristics was tested, to indicate the encoding mode in short-term memory. Methods: Three types of sets (‘memorized sets’) were presented to the subject before the probe: 4 memorized digits, two memorized digits with two distracter digits and two memorized digits with two noise distracters. Potentials in response to the set items were averaged separately according to stimulus type and position in the set. Potentials in response to the probe were averaged according to the preceding stimulus sequence: 4 memorized digits, two distracter digits or two noise distracters. Results: The early components (N1, P2) differed between distracter items and memorized items, indicating lower attention allocation to distracter items. In contrast, the late components (N2, P3) indicated similar processing of distracters and memorized items. Behavioral measures indicated shorter scanning times of sets with distracters. The early ERP components in response to the probe (P2, N2) indicated differences among probes according to the preceding combinations of memorized items and distracters. The late component (P3) indicated different speeds (latencies) of scanning and comparison for series with compared to without distracters, but similar processing resource allocation (amplitudes). Processing was prolonged when the distracter items were phonological. Conclusions: This study shows that distracters affect both the memorization process and the scanning and comparison in short-term memory. The stronger distraction by stimuli that are phonologically similar to the memorized items supports phonological processing in short-term memory.