Interactions between working memory and visual perception: an ERP/EEG study

Abstract

How do working memory and perception interact with each other? Recent theories of working memory suggest that they are closely linked, and in fact share certain brain mechanisms. We used a sequential motion imitation task in combination with EEG and ERP techniques for a direct, online examination of memory load's influence on the processing of visual stimuli. Using a paradigm in which subjects tried to reproduce random motion sequences from memory, we found a systematic decrease in ERP amplitude with each additional motion segment that was viewed and memorized for later imitation. High-frequency (>20 Hz) oscillatory activity exhibited a similar position-dependent decrease. When trials were sorted according to the accuracy of subsequent imitation, the amplitude of the ERPs during stimulus presentation correlated with behavioral performance: the larger the amplitude, the more accurate the subsequent imitation. These findings imply that visual processing of sequential stimuli is not uniform. Rather, earlier information elicits stronger neural activity. We discuss possible explanations for this observation, among them competition for attention between memory and perception and encoding of serial order by means of differential activation strengths.

Figure 1

Stimuli and results. A: Example of a memory trial. Note that while viewing and reproducing the stimulus, the disc did not leave a trace, so subjects only saw its instantaneous position; the dashed lines are for illustration purposes. B: Behavioral results. Segment orientation error is plotted against segment serial position. Error bars are within-subject SEM (Loftus & Masson, 1994). C: ERPs at the midline electrode locations for the memory condition, time-locked to the onset of the disc’s appearance on the display. Vertical dashed lines indicate the onset of individual motion segments. The 4.75 seconds for which ERPs are displayed correspond to the period during which subjects viewed the stimulus models.

Figure 2

Modulation of ERP amplitude. A: Left and middle panels show ERPs time-locked to the onset of segments 2 to 5 in the memory and control conditions, respectively. Each color corresponds to a different serial position, and the horizontal black lines represent averaging epoch boundaries. Asterisks denote significance levels (ANOVA with factor segment) for the mean potential at each tested time epoch (p values above 0.05 are unmarked). The right panels show the energy of the ERPs from the left and middle panels; The solid and dashed lines represent the energy in the memory and control conditions, respectively. B: Mean energy of the ERPs across all 27 electrode locations, as a function of segment serial position. In all energy plots, Error bars are within-subject SEM for each condition, and the solid and dashed circles indicate the energy for the first segment, which is only given for reference; It was not included in any statistical analysis.

Figure 3

Modulation of high-frequency oscillations. A: Energy in four standard frequency bands in the memory and control conditions at midline electrode locations. Each group of bars corresponds to a different frequency band, and shows the energy for segments 3, 4 and 5 (left, middle and right bars, respectively) relative to the energy for segment 2. B: Mean energy across all electrodes in each band as a function of serial position, in the memory (solid line) and control (dashed line) conditions. The solid and dashed circles indicate the energy in the first segment. All significance markers are similar to Fig. 2. Error bars are within-subject SEM for each condition.

Figure 4

Correlation between ERP amplitude and behavioral performance. A: Imitation accuracy. Left panel shows orientation error as a function of segment serial position for each quarter of the trials in the memory condition. Right panel shows mean error across all five segments plotted against quartile. B: Mean ERP energy across all electrode locations. Left panel shows the energy of the ERPs time-locked to the onset of segments 2 to 5 in each group. Circles denote energy in segment 1. Right panel shows the energy of the averaged ERPs of all five segments as a function of imitation accuracy. Note that the energy values in the right panel are smaller, since the averaging of all segments reduces noise levels. All error bars are within-subjects SEM for each curve independently.

Figure A0

Interactions between Memory and Perception