Multiple visual memory phenomena in a memory search task MONICA FABIANI, a JONATHAN HO, b ALEX STINARD, c AND GABRIELE GRATTON a a Beckman Institute, University of Illinois at Urbana-Champaign , Urbana, Illinois, USA b Psychology Department, Columbia University, New York, New York, USA c Department of Psychology, University of Missouri–Columbia, Columbia, Missouri, USA Abstract This paper reports evidence of the existence of multiple and distinct visual memory processes in a memory search task in which a divided field stimulus presentation was used at study (Experiments 1–3) and either a foveal (Experiments 1 and 2) or a lateralized (Experiment 3) stimulus presentation was used at test. These memory processes can be distinguished on the basis of (1) whether or not they are hemispherically organized; and (2) the locus of their underlying brain activity, as evidenced by the scalp distribution of the event-related brain potentials and by the localization of the event- related optical signal that accompany them. These memory effects are discussed in the context of visual form memory. Descriptors: Hemispheric organization, Encoding-related lateralization, Sensory signature, Visual working memory, Memory search task, Event-related brain potentials (ERPs), Event-related optical signal (EROS) A substantial body of literature reports the effects of previous stimulus exposure in early visual processing, from passive phenomena such as habituation (e.g., Colon, Boumen-Van den Eerden, & Cuyten, 1983; Megela & Teyler, 1979) to memory effects such as visual form and repetition priming (e.g., Doyle, Rugg, & Wells, 1996; Paller & Gross, 1998; Paller, Kutas, & McIsaac, 1998; Rugg, 1985; Walsh, Le Mare, Blaimire, & Cowey, 2000). In addition, a recent series of experiments, using a divided-field presentation at study and a foveal presentation at test, has shown that some visual memory processes are hemispherically organized, in that the behavior and/or the brain activity at test maintains a ‘‘signature’’ of the lateralized encoding condition (Fabiani, Stadler, & Wessels, 2000; Gratton, Corballis, & Jain, 1997; Gratton, Fabiani, Goodman-Wood, & DeSoto, 1998; Talsma, Wijers, Klaver, & Mulder, 2001). In this paper we present three experiments using the polarity and scalp distribution of event-related brain potentials (ERPs; e.g., Fabiani, Gratton, & Coles, 2000), the localization and time course of the event-related optical signal (EROS; Gratton, Corballis, Cho, Fabiani, & Hood, 1995; Gratton & Fabiani, 1998, 2001), and the presence of hemispheric organization to discriminate among these early visual memory effects. The results suggest that at least two distinct types of visual memory effects are present, one of which is hemispherically organized whereas the other is not. Several investigators have proposed that the successful retrieval of a memory trace involves an overlap between encoding and retrieval processes, and possibly the reactivation of the same brain areas that were used at encoding (e.g., Kosslyn, 1980; Roediger, Weldon, & Challis, 1989). Recently, Gratton and colleagues have suggested that, because the visual processing system is contralaterally organized, the memory traces left by laterally presented visual stimuli should maintain a ‘‘sensory signature’’ of the hemifield of initial stimulus presentation, in the form of a performance advantage and/or of the existence of differential brain activity (Gratton et al., 1997; see also Fabiani, Stadler, et al., 2000; Gratton, 1998). This hypothesis has been tested in recognition paradigms using verbal (Fabiani, Stadler, et al., 2000) and nonverbal (Gratton et al., 1997) visual stimuli. The results indicate that, for both verbal and nonverbal stimuli, there is evidence of encoding-related lateralizations in the ERPs elicited by test stimuli presented foveally. Note that this lateralized activity observed at test switches sides according to the hemifield stimulated at encoding, and that the test stimuli vary only with respect to encoding side and are identical in every other way (including the fact that they require the same manual response). Thus, these data suggest that memory traces may retain some of the information contained in the sensory world, and that, if this information implies a differential early involvement of the two cerebral hemispheres, the retrieval activity will be hemispherically organized (for related work, see also Senkfor, Van Petten, & Kutas, 2002). The ERP experiments mentioned above have investigated the hemispheric organization of visual memory in recognition paradigms in which study and test are separated by several This research was supported by NIMH Grant MH 57125 to Gabriele Gratton, and by McDonnell-Pew Grant 97-32 to Monica Fabiani. We thank Marsha Goodman-Wood, Ted Moallem, and M. Catherine DeSoto for help with some of the data collection, and the Max Planck Institute for Cognitive Neuroscience and the University of Leipzig, Germany, for hosting us during the preparation of this article. Address reprint requests to: Monica Fabiani, University of Illinois, Beckman Institute, 405 North Mathews Avenue, Urbana, IL, 61801, USA. E-mail: mfabiani@uiuc.edu. Psychophysiology, 40 (2003), 472–485. Blackwell Publishing Inc. Printed in the USA. Copyright r 2003 Society for Psychophysiological Research 472