Most studies using a recognition memory paradigm examine the neural processes that support the ability to consciously recognize past events. However, there can also be nonconscious influences from the prior study episode that reflect repetition suppression effects—a reduction in the magnitude of activity for repeated presentations of stimuli—that are revealed by comparing neural activity associated with forgotten items to correctly rejected novel items. The present fMRI study examined the effect of emotional valence (positive vs. negative) on repetition suppression effects. Using a standard recognition memory task, 24 participants viewed line drawings of previously studied negative, positive, and neutral photos intermixed with novel line drawings. For each item, participants made an old–new recognition judgment and a sure–unsure confidence rating. Collapsed across valence, repetition suppression effects were found in ventral occipital-temporal cortex and frontal regions. Activity levels in the majority of these regions were not modulated by valence. However, repetition enhancement of the amygdala and ventral occipital-temporal cortex functional connectivity reflected nonconscious memory for negative items. In this study, valence had little effect on activation patterns but had a larger effect on functional connectivity patterns that were markers of nonconscious memory. Beyond memory and emotion, these findings are relevant to other cognitive and social neuroscientists that utilize fMRI repetition effects to investigate perception, attention, social cognition, and other forms of learning and memory.

The existence of a functional-anatomic dissociation for retrieving item versus contextual information within subregions of the medial temporal lobe (MTL) is currently under debate. We used a spatial source memory paradigm during event-related functional magnetic resonance imaging to investigate this issue. At study, abstract shapes were presented to the left or right of fixation. During test, old and new shapes were presented at fixation. Participants responded whether each shape had been previously presented on the “left,” the “right,” or was “new.” Activity associated with contextual memory (i.e., source memory) was isolated by contrasting accurate versus inaccurate memory for spatial location. Item-memory-related activity was isolated by contrasting accurate item recognition without contextual memory with forgotten items. Source memory was associated with activity in the hippocampus and parahippocampal cortex. Although item memory was not associated with unique MTL activity at our original threshold, a region-of-interest (ROI) analysis revealed item-memory-related activity in the perirhinal cortex. Furthermore, a functional-anatomic dissociation within the parietal cortex for retrieving item and contextual information was not found in any of three ROIs. These results support the hypothesis that specific subregions in the MTL are associated with item memory and memory for context.

Kosslyn (1987) proposed that the left hemisphere is better than the right hemisphere at categorical visuospatial processing while the right hemisphere is better than the left hemisphere at coordinate visuospatial processing. In 134 patients, one hemisphere (and then usually the other) was temporarily deactivated by intracarotid injection of sodium amobarbital. After a hemisphere was deactivated, a cognitive test battery was conducted, which included categorical and coordinate visuospatial tasks. Using this technique, the processing capabilities of the intact hemisphere could be determined, thus directly testing Kosslyn's hypothesis regarding hemispheric specialization. Specifically, if the left hemisphere does preferentially process categorical visuospatial relationships, then its deactivation should result in more errors during categorical tasks than right hemisphere deactivation and vise versa for the right hemisphere regarding coordinate tasks. The pattern of results obtained in both categorical and coordinate tasks was consistent with Kosslyn's hypothesis when task difficulty was sufficiently high. However, when task difficulty was low, a left hemispheric processing advantage was found for both types of tasks indicating that: (1) the left hemisphere may be better at “easy” tasks regardless of the type of task and (2) the proposed hemispheric processing asymmetry may only become apparent during sufficiently demanding task conditions. These results may explain why some investigators have failed to find a significant hemispheric processing asymmetry in visuospatial categorical and coordinate tasks.