Dissociation of the N2pc and sustained posterior contralateral negativity in a choice response task

The N2pc, a greater negativity at posterior electrodes on the side contralateral to an attended visual stimulus, usually between 180 and 280 ms, is thought to reflect the moment-to-moment deployment of visual-spatial attention. In tasks that require the retention of information in visual short-term memory, there is also a sustained posterior contralateral negativity (SPCN) that often begins about 300-400 ms after stimulus onset and that persists for the duration of the retention interval. A positive-going deflection at around 300 ms often separates the N2pc and the SPCN. An SPCN is also observed in tasks that are not defined as memory tasks, but that presumably engage visual short-term memory as an intermediate processing buffer (e.g., in order to make a choice response to a briefly-presented visual stimulus). The SPCN in memory tasks has been shown to increase in amplitude as the memory load is increased. We used this property of the SPCN to verify that the SPCN observed during the performance of a choice task with a response following each stimulus display is related to the SPCN observed in tasks that are structured as memory tasks. Using identical physical stimuli, we manipulated the hypothesized memory load across trial blocks by instructions either to encode only one stimulus or two stimuli. We observed an increase of the amplitude of the SPCN as memory load increased, with no concomitant increase in the amplitude of the N2pc that immediately preceded it. The results provide a clear dissociation between the N2pc (spatial attention, not affected by memory load) and the SPCN (visual short-term memory, sharply sensitive to memory load).