The existence of individual differences in the retrieval of episodic memories as a function of degree of handedness is well documented. Mixed (or inconsistent) handedness, relative to strong (or consistent) handedness, is associated with superior recall of lab-based and real world-based memories (Parker and Dagnall, 2010 and Propper et al., 2005), superior source memory (Christman et al., 2004, Lyle and Jacobs, 2010, Lyle et al., 2008 and Lyle et al., 2008), an increased tendency for recognition to be accompanied by recollection, as indexed by “Remember” judgments, versus familiarity, as indexed by “Know” judgments (Propper & Christman, 2004), and an earlier offset of childhood amnesia (Christman, Propper, & Brown, 2006). There is also tentative evidence linking mixed-handedness to a higher capacity episodic buffer in working memory (Kempe, Brooks, & Christman, 2009), a decreased tendency to experience dissociative amnesia (Christman & Ammann, 1995), and better self-reported everyday memory (Christman & Propper, 2008).
These findings have been interpreted in terms of two related phenomena. First, strong right-handedness is associated with smaller corpus callosum size (e.g., Clarke and Zaidel, 1994, Cowell et al., 1993, Habib et al., 1991 and Witelson and Goldsmith, 1991), suggesting decreased interhemispheric interaction in strongly right-handed individuals. Moreover, behavioral evidence indicates that strong right-handedness is associated with decreased functional interaction between processes known to be lateralized to the opposite hemispheres, including left hemisphere-based belief maintenance and right hemisphere-based belief updating mechanisms (e.g., Christman et al., 2007, Christman et al., 2008, Niebauer et al., 2002 and Niebauer et al., 2004), left hemisphere-based approach and right hemisphere-based withdrawal mechanisms (Christman, Jasper, Sontam, & Cooil, 2007), and left hemisphere-based word reading and right hemisphere-based color processing mechanisms (Christman, 2001).
The second relevant phenomenon involves a between-hemisphere division of labor in episodic memory, with fMRI studies indicating that the left hemisphere is more involved in encoding than retrieving episodic memories, and vice versa for the right hemisphere (e.g., Cabeza and Nyberg, 2000, Habib et al., 2003 and Tulving et al., 1994). Similarly, a recent study using transcranial magnetic stimulation found that disrupting left versus right prefrontal activity was more detrimental to the encoding versus retrieval of episodic memories, respectively (Gagnon, Blanchet, Grondin, & Schneider, 2010). Thus, to the extent that strong right-handedness is associated with decreased interhemispheric interaction and episodic memory is associated with interaction between left hemisphere-based encoding and right hemisphere-based retrieval, the findings of inferior episodic memory in strong right-handers have been hypothesized to reflect decreased interhemispheric interaction.
The majority of the literature on handedness differences in episodic memory has focused on conditions involving intentional learning (i.e., participants knew their memory for material would be subsequently tested). The only exceptions are studies demonstrating handedness differences in childhood amnesia (Christman et al., 2006), real world-based memories (Propper et al., 2005), and incidental memory for slideshows (Lyle & Jacobs, 2010). However, these were not experimentally controlled examples of incidental learning (e.g., they did not manipulate the nature of the orienting task during encoding). The present paper presents an initial investigation into potential handedness differences in episodic memory under various conditions of incidental as well as intentional learning, in order to test the generality of the mixed-handed advantage in episodic memory.
In addition, the current experiments can address the question of the extent to which the mixed-handed advantage arises at the encoding versus retrieval stages. While past research has been interpreted in terms of mixed-handers’ greater access to right hemisphere-based retrieval mechanisms, it is possible that the observed handedness differences arise at least in part from superior encoding in mixed-handers. Under conditions of intentional learning, perhaps mixed-handers spontaneously engage in relatively deeper levels of processing, leading to their observed memory advantage, while under conditions of incidental learning, perhaps mixed-handers will not spontaneously engage in deeper levels of processing than strong right-handers, and the mixed-handed advantage would be attenuated or eliminated. Conversely, if the memory advantage for mixed-handers is based on superior retrieval processes, it should extend to conditions of incidental learning.
Experiment 1 is based on the classic levels of processing effect demonstrated by Craik and Tulving (1975), in which intentional learning was compared to incidental learning under conditions of shallow versus deep processing. The second experiment is based on the procedure developed by Buckner, Wheeler, and Sheridan (2001), in which participants are first given a standard recognition memory test, and are then tested for their incidental memory for distracter items from the initial recognition test.
