Cognitive Neuroscience NeuroReport 0959-4965 # Lippincott Williams & Wilkins An event-related brain potential correlate of visual short-term memory Peter Klaver, CA Durk Talsma, Albertus A. Wijers, Hans-Jochen Heinze and Gijsbertus Mulder Clinic for Neurology II, Otto-von-Guericke University, Leipzigerstrasse 44, 39120 Magdeburg, Germany CA Corresponding Author EVENT-RELATED potentials (ERPs) were recorded as 12 subjects performed a delayed matching to sample task. We presented two bilateral abstract shapes and cued spatially which had to be memorized for a subsequent matching task: left, right or both. During memoriza- tion a posterior slow negative ERP wave developed over the hemisphere contralateral to the memorized shape. This effect was similar in high and low memory load trials while the memory ®gures were visible (for 1000 ms). As the ®gures disappeared (for 1500 ms), the effect persisted only in the low memory load conditions. We suggest that the contralateral negativity re¯ects a visual short-term memory process and that capacity limitation in the high memory load condition causes this process to collapse. NeuroReport 10:2001±2005 # 1999 Lippincott Williams & Wilkins. Key words: Event-related potential; Memory load; Spatial cueing; Visual memory Introduction One of the theories on the organization of visual memory assumes that memory traces develop mainly in the cells with the receptive ®eld corre- sponding to the location where the information is originally presented [1±4]. This means that the memory trace develops predominantly at the con- tralateral side of memorization. This hypothesis was recently supported in a recognition task with human subjects [5]. In that study subjects memorized abstract objects that were presented either on the left side or on the right side. After memorization a series of centrally presented probes had to be recognized. Event-related brain potentials (ERPs) showed that recognizing such probes correlated with asymmetrical ERP correlates: more brain activity was seen at electrodes contralateral to the side where the ®gure was memorized. As an alternative to this hypothesis, visual information could be represented dominantly in one hemisphere, as would be pre- dicted by Smith and Jonides [6]. These authors showed, in short-term memory functional brain imaging studies, that spatial information is predomi- nantly represented in the right hemisphere whereas object information is mainly represented in the left hemisphere. In the present task we investigated if similar asymmetrical ERP correlates can be found during memorization of abstract ®gures. We used a delayed matching-to-sample task in which we presented two objects simultaneously at the left and right side of ®xation. Spatial cues indicated which object(s) had to be memorized, the left, right or both. The location of the memory stimuli was not relevant for the subsequent matching task, because during recog- nition one object was presented at the same or at the opposite hemi®eld of memorization. ERPs were recorded from the start of the memory set presenta- tion to the start of the probe stimulus. The rationale was that if visual memory is developing contralateral to the side of the to be memorized object, then one would expect memory related ERPs that are max- imal at the contralateral side of memorization. Memory load was varied to investigate if the same processes are engaged when two objects have to be memorized as when one object has to be memor- ized. Materials and Methods Fourteen paid volunteers (age range 20±27, ®ve female) with normal or corrected-to-normal vision participated in the experiment. The experiment con- sisted of 24 blocks of each 48 trials. A trial started with two vertical lines (48) that were presented (1250 ms) on either side (68) of a constantly present ®xation point. Either both were red, or one was red and one green. Two memory set stimuli were presented (1000 ms) on either side of the cues (128), followed by a delay (1500 ms), a probe on one of the sides (500 ms), and a pause (500 ms). The stimuli NeuroReport 10, 2001±2005 (1999) Vol 10 No 10 13 July 1999 2001