Ruth E. Propper

Bilateral eye movements enhance the retrieval of episodic memories.

Two experiments examining effects of eye movements on episodic memory retrieval are reported. Thirty seconds of horizontal saccadic eye movements (but not smooth pursuit or vertical eye movements) preceding testing resulted in selective enhancement of episodic memory retrieval for laboratory (Experiment 1) and everyday (Experiment 2) events. Eye movements had no effects on implicit memory. Eye movements were also associated with more conservative response biases relative to a no eye movement condition. Episodic memory improvement induced by bilateral eye movements is hypothesized to reflect enhanced interhemispheric interaction, which is associated with superior episodic memory (S. D. Christman & R. E. Propper, 2001). Implications for neuropsychological mechanisms underlying eye movement desensitization and reprocessing (F. Shapiro, 1989, 2001), a therapeutic technique for posttraumatic stress disorder, are discussed.

The role of sleep in false memory formation.

Memories are not stored as exact copies of our experiences. As a result, remembering is subject not only to memory failure, but to inaccuracies and distortions as well. Although such distortions are often retained or even enhanced over time, sleeps contribution to the development of false memories is unknown. Here, we report that a night of sleep increases both veridical and false recall in the Deese-Roediger-McDermott (DRM) paradigm, compared to an equivalent period of daytime wakefulness. But while veridical memory deteriorates across both wake and sleep, false memories are preferentially preserved by sleep, actually showing a non-significant improvement. The same selectivity of false over veridical memories was observed in a follow-up nap study. Unlike previous studies implicating deep, slow-wave sleep (SWS) in declarative memory consolidation, here veridical recall correlated with decreased SWS, a finding that was observed in both the overnight and nap studies. These findings lead to two counterintuitive conclusions - that under certain circumstances sleep can promote false memories over veridical ones, and SWS can be associated with impairment rather than facilitation of declarative memory consolidation. While these effects produce memories that are less accurate after sleep, these memories may, in the end, be more useful.

Increased interhemispheric interaction is associated with decreased false memories in a verbal converging semantic associates paradigm

Recent evidence indicates that task and subject variables that are associated with increased interaction between the left and right cerebral hemispheres result in enhanced performance on tests of episodic memory. The current study looked at the effects of increased interhemispheric interaction on false memories using a verbal converging semantic associates paradigm. In Experiment 1, strong right-handedness (which is associated with decreased interhemispheric interaction) was associated with higher rates of false memories. In Experiment 2, bilateral saccadic eye movements (which are associated with increases in interhemispheric interaction) were associated with fewer false memories. The results provide further support for an interhemispheric basis for episodic/explicit memory.

A mixed-handed advantage in episodic memory: a possible role of interhemispheric interaction.

Recent behavioral and brain imaging data indicate that performance on explicit tests of episodic memory is associated with interaction between the left and right cerebral hemispheres, in contrast with the unihemispheric basis for implicit tests of memory. In the present work, individual differences in strength of personal handedness were used as markers for differences in hemispheric communication, with mixed-handers inferred to have increased interhemispheric interaction relative to strong right-handers. In Experiment 1, memory for words was assessed via recall or word fragment completion. In Experiment 2, memory for real-world events was assessed via recall. Results supported the hypothesis, in that mixed-handers displayed better episodic memory in comparison with strong right-handers.

Degree of Handedness, but not Direction, is a Systematic Predictor of Cognitive Performance.

A growing body of evidence is reviewed showing that degree of handedness (consistent versus inconsistent) is a more powerful and appropriate way to classify handedness than the traditional one based on direction (right versus left). Experimental studies from the domains of episodic memory retrieval, belief updating/cognitive flexibility, risk perception, and more are described. These results suggest that inconsistent handedness is associated with increased interhemispheric interaction and increased access to processes localized to the right cerebral hemisphere.

Increased interhemispheric interaction is associated with earlier offset of childhood amnesia.

Experiment 1 found that mixed-handedness, which is associated with increased interaction between the left and right cerebral hemispheres relative to strong right-handedness, was associated with an earlier offset of childhood amnesia. In Experiment 2, bilateral saccadic eye movements, which have been shown to enhance interhemispheric interaction, were also associated with an earlier offset of childhood amnesia. These results build upon a growing body of research indicating an interhemispheric basis for the retrieval of episodic memories. Moreover, the results of Experiment 2 suggest that interhemispheric interaction has its effect on the retrieval, not encoding, of episodic memories.

A combined fMRI and DTI examination of functional language lateralization and arcuate fasciculus structure: Effects of degree versus direction of hand preference

The present study examined the relationship between hand preference degree and direction, functional language lateralization in Brocas and Wernickes areas, and structural measures of the arcuate fasciculus. Results revealed an effect of degree of hand preference on arcuate fasciculus structure, such that consistently-handed individuals, regardless of the direction of hand preference, demonstrated the most asymmetric arcuate fasciculus, with larger left versus right arcuate, as measured by DTI. Functional language lateralization in Wernickes area, measured via fMRI, was related to arcuate fasciculus volume in consistent-left-handers only, and only in people who were not right hemisphere lateralized for language; given the small sample size for this finding, future investigation is warranted. Results suggest handedness degree may be an important variable to investigate in the context of neuroanatomical asymmetries.

Mixed-handed persons are more easily persuaded and are more gullible: Interhemispheric interaction and belief updating

Research has shown that persons with mixed hand preference (i.e., who report using their non-dominant hand for at least some manual activities) display an increased tendency to update beliefs in response to information inconsistent with those beliefs. This has been interpreted as reflecting the fact that the left hemisphere maintains our current beliefs while the right hemisphere evaluates and updates those beliefs when appropriate. Belief evaluation is thus dependent on interhemispheric interaction, and mixed-handedness is associated with increased interhemispheric interaction. In Experiment 1 mixed-handers exhibited higher levels of persuasion in a standard attitude-change paradigm, while in Experiment 2 mixed-handers exhibited higher levels of gullibility as measured by the Barnum Effect.

Mixed- versus strong right-handedness is associated with biases towards "remember" versus "know" judgements in recognition memory: Role of interhemispheric interaction

Growing evidence supports the hypothesis that episodic versus semantic memories rely primarily on interhemispheric versus intrahemispheric processing, respectively. For example, a recent study found that individuals with presumed greater interhemispheric interaction were superior in episodic recall but inferior at semantic word fragment completion; however, tests of recognition memory yielded no group differences. Interestingly, recognition memory can be based on either explicitly remembering a stimulus or implicitly knowing that a stimulus had been presented. The current experiments administered recognition memory tests to strongly versus mixed handed participants who judged for each recognised item whether their response was based on remembering (episodic memory) or knowing (semantic memory) (Tulving, 1983). Results indicate that strong versus mixed handers are biased towards basing recognition responses on judgements of knowing versus remembering, respectively. As strong versus mixed handedness is associated with greater versus lesser interhemispheric processing, the results support the original hypothesis.

Effect of Bilateral Eye Movements on Frontal Interhemispheric Gamma Eeg Coherence: Implications for Emdr Therapy

The use of bilateral eye movements (EMs) is an important component of Eye Movement Desensitization and Reprocessing (EMDR) therapy for posttraumatic stress disorder. The neural mechanisms underlying EMDR remain unclear. However, prior behavioral work looking at the effects of bilateral EMs on the retrieval of episodic memories suggests that the EMs enhance interhemispheric interaction. The present study examined the effects of the EMs used in EMDR on interhemispheric electroencephalogram coherence. Relative to noneye-movement controls, engaging in bilateral EMs led to decreased interhemispheric gamma electroencephalogram coherence. Implications for future work on EMDR and episodic memory are discussed.