Brain and Cognition 42, 37–40 (2000)

doi:10.1006/brcg.1999.1155, available online at http://www.idealibrary.com on

Searching for the Neural Correlates of Consciousness:Clues from Face Recognition Research

Steven Z. Rapcsak

Neurology Section, VA Medical Center, Tucson, Arizona; and Department of Neurology,and Department of Psychology, University of Arizona, Tucson

and

Alfred W. Kaszniak

Department of Neurology and Department of Psychology, University of Arizona, Tucson

Research has afforded irrefutable proof that mental activity is bound up with thefunction of the brain . . . but every attempt to deduce from these facts a localizationof mental processes, every endeavor to think of ideas as stored up in nerve cellsand of excitations as passing along nerve-fibers has completely miscarried. The samefate would await any doctrine which attempted to recognize, let us say, the anatomi-cal position of the system Cs—conscious mental activity—in the cortex and to local-ize the unconscious processes in the subcortical parts of the brain. Here there is ahiatus which at present cannot be filled, nor is it one of the tasks of psychology tofill it.

Sigmund Freud

Few students of cognitive neuroscience today would share Freud’s pessi-mism concerning our prospects for identifying specific neural correlates ofconsciousness. On the contrary, most would probably agree that recent ad-vances in neurophysiology, functional imaging, and experimental psychol-ogy have brought us closer than ever to attaining this goal. These excitingdevelopments have effectively and irrevocably transformed the study of con-sciousness into a legitimate scientific enterprise, and we are confident thatthis line of research will produce many important discoveries during the firstcentury of the next millenium.

In this paper we focus on one particularly active area of investigation:

This work was supported by the Cummings Endowment Fund to the Department of Neurol-ogy at the University of Arizona.

Address correspondence and reprint requests to Steven Z. Rapcsak, M.D., Neurology Sec-tion (1-127), VA Medical Center, Tucson, AZ 85723. E-mail: szr@u.arizona.edu.

370278-2626/00 $35.00

Copyright 2000 by Academic PressAll rights of reproduction in any form reserved.

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human face recognition. Our goal is to highlight empirical findings we be-lieve have general implications for our understanding of the neural substratesof conscious vs unconscious information processing and to suggest possibledirections for future research. We limit our discussion to two closely relatedtopics: perceptual awareness of faces and awareness of face memories.

Perceptual Awareness

Neurophysiological and functional imaging studies have provided evi-dence that face perception is mediated by a specialized cortical module lo-cated in the region of the fusiform gyrus. Under conditions of binocularrivalry, awareness of a face percept is associated with increased activitywithin the fusiform face area (FFA) (Tong et al., 1998). By contrast, whena competing nonface stimulus becomes perceptually salient, FFA activationis suppressed even though visual input remains unchanged. FFA activity,therefore, is correlated with the consciously perceived rather than with theretinal stimulus. Perceptual shifts during binocular rivalry may be mediatedby top-down modulation of the FFA by fronto-parietal systems (Lumer etal., 1998), consistent with the hypothesis that the neural substrates of con-scious visual awareness extend well beyond primary visual cortex and mayinvolve synchronized activity between higher order visual areas and prefron-tal cortex (Crick & Koch, 1995). The specific components of this neuralnetwork and their functional interactions could be elucidated further by com-bining neurophysiological and imaging studies in normal subjects with psy-chophysical experiments of multistable perception in patients with focal le-sions.

Other imaging studies have used backward masking of faces to explorethe neural correlates of conscious vs unconscious visual processing (Whalenet al., 1998; Morris et al., 1998). Preliminary evidence indicates that patternsof brain activation may differ depending on the subject’s phenomenal aware-ness of the relevant stimuli. These observations, and complementary findingsin blindsight (Sahraie et al., 1997), suggest that the difference between awareand unaware modes of processing is not merely quantitative, and that thesetwo types of mental activity may have distinct neural substrates. Additionalimaging studies of subliminal perception are needed to define the relationshipbetween brain activation, subjective report, and objective measures of dis-crimination capacity.

Awareness of Face Memories

Inferotemporal (IT) visual cortex also plays a critical role in face recogni-tion memory. Although the responses of IT neurons are modified by priorexposure to a face, this differential activity is not necessarily accompaniedby conscious recollection (Seeck et al., 1997). Functional imaging studiessuggest that conscious recognition of previously encountered faces also re-

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quires the participation of medial temporal lobe structures and prefrontalcortex (Haxby et al., 1996).

Although the activation of IT face records may not always result in con-scious remembering, it may be sufficient to mediate performance on implicitmemory tasks such as priming, which can be preserved in amnesia. Uncon-scious access to face memory representations is also believed to underliecovert recognition in prosopagnosia. Converging evidence from normal sub-jects and neurological patients suggests that some covert face memory effectsare mediated by neural systems separate from those involved in overt recog-nition. For instance, normal individuals generate covert skin conductanceresponses to familiar faces even when overt identification is prevented bysubliminal stimulus presentation, demonstrating that autonomic discrimina-tion can occur without conscious awareness (Ellis et al., 1993). Dissociationsbetween overt and covert measures of memory have also been documentedin patients with face recognition impairment. Specifically, covert autonomicdiscrimination may remain accurate regardless of whether overt recognitionis characterized by memory loss (i.e., prosopagnosia) (Bauer, 1984) or mem-ory distortion (i.e., false recognition) (Rapcsak et al., 1998). The hypothesisthat overt and covert recognition are mediated by different neural systemsreceives additional support from a double dissociation between verbal andautonomic measures of face memory in patients with frontal vs occipito-temporal damage (Tranel et al., 1995).

In summary, face recognition research has provided some intriguing cluesabout the neural mechanisms of conscious and unconscious visual pro-cessing. The challenge that lies ahead now is to develop these promisingleads into a coherent research program, the ultimate goal of which is to un-derstand the neurobiological basis of human consciousness.

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