This study examines whether orienting attention to biologically based social cues engages neural mechanisms distinct from those engaged by orienting to nonbiologically based nonsocial cues. Participants viewed a perceptually ambiguous stimulus presented centrally while performing a target detection task. By having participants alternate between viewing this stimulus as an eye in profile or an arrowhead, we were able to directly compare the neural mechanisms of attentional orienting to social and nonsocial cues while holding the physical stimulus constant. The functional magnetic resonance imaging results indicated that attentional orienting to both eye gaze and arrow cues engaged extensive dorsal and ventral fronto-parietal networks. Eye gaze cues, however, more vigorously engaged two regions in the ventral frontal cortex associated with attentional reorienting to salient or meaningful stimuli, as well as lateral occipital regions. An event-related potential study demonstrated that this enhanced occipital response was attributable to a higher-amplitude sensory gain effect for targets appearing at locations cued by eye gaze than for those cued by an arrowhead. These results endorse the hypothesis that differences in attention to social and nonsocial cues are quantitative rather than qualitative, running counter to current models that assume enhanced processing for social stimuli reflects the involvement of a unique network of brain regions. An intriguing implication of the present study is the possibility that our ability to orient volitionally and reflexively to socially irrelevant stimuli, including arrowheads, may have arisen as a useful by-product of a system that developed first, and foremost, to promote social orienting to stimuli that are biologically relevant.

The present study used fMRI to investigate functional dissociations across frontal regions during incidental memory formation. Subjects were imaged while encoding materials with differential access to phonological codes (nonfamous faces and nameable famous faces) under task conditions that encouraged elaborate (deep) or superficial (shallow) encoding strategies. Results revealed a functional dissociation between dorsal posterior regions of the prefrontal cortex (BA 6/44) that were sensitive to material type (famous vs. nonfamous), irrespective of the encoding task, and ventral anterior regions of the prefrontal cortex (BA 45/47) that were uniquely sensitive to task demands (deep vs. shallow), regardless of material type. Further, subjects realized a memorial advantage to the extent that they recruited these dissociable frontal regions. These results demonstrate a posterior/anterior dichotomy in the frontal cortex that underlies separable code-based routes to human memory formation.

Previous neuroimaging studies have claimed a left hemisphere specialization for episodic “encoding” and a right hemisphere specialization for episodic “retrieval.” Yet studies of split-brain patients indicate relatively minor memory impairment after disconnection of the two hemispheres. This suggests that both hemispheres are capable of encoding and retrieval. In the present experiment, we examined the possible limits on encoding capacity of each hemisphere by manipulating the “depth” of processing during the encoding of unfamiliar faces and familiar words in the left and right hemispheres of two split-brain patients. Results showed that only the left hemisphere benefited from deeper (more elaborate) encoding of familiar words, and only the right hemisphere benefited from deeper encoding of unfamiliar faces. Our findings are consistent with the view that hemispheric asymmetries in episodic encoding are related to hemisphere-specific processing of particular stimuli. Convergent with recent neuroimaging studies, these results with split-brain patients also suggest that these hemispheric differences are not due to unique specializations in each half brain for encoding memories, but rather, are due to preferential recruitment of the synaptically closer prefrontal cortex to posterior regions processing material-specific information.