Face Recognition Turned Upside-Down
Authors: Geoffrey K. Aguirre
Journal: Neuron (1999)

Imagine that you have arrived at a cocktail party to find all the other guests hanging upside-down from the rafters (perhaps you should have read the invitation more carefully). How difficult would it be to recognize your friends? Would inverted hors d’oeuvres pose a challenge? If you selected an upside-down cheese blintz without pause but took a moment longer to recognize a coworker, you would be in good company. Psychophysical studies have shown that inversion of visually presented stimuli disproportionately impairs face recognition compared to recognition of other objects. This finding has been interpreted as evidence that face perception is subserved by computational processes that differ, at some point, from the larger realm of object recognition (7Valentine T. Br. J. Psychol. 1988; 79: 471-491Crossref PubMed Scopus (663) Google Scholar). In this issue4Haxby J.V. Ungerleider L.G. Clark V.P. Schouten J.L. Hoffman E.A. Martin A. Neuron. 1999; 22 (this issue,): 189-199Abstract Full Text Full Text PDF PubMed Scopus (494) Google Scholar report the results of a functional neuroimaging experiment designed to elucidate the neurocomputational correlates of the inversion effect. Behavioral studies of stimulus inversion dovetail nicely with neuropsychological studies of object recognition. Following brain lesions in the posterior, ventral portion of the neocortex, patients occasionally demonstrate isolated impairments in their ability to recognize certain types of objects. Well documented are cases of prosopagnosia, in which the patient cannot recognize faces but can recognize other objects, and object agnosia, in which the patient can recognize faces but not general objects. These perceptual deficits seem to result from damage to “specialized” cortical areas that are necessary for the perception of the given stimulus class (i.e., faces versus general objects) (2Farah M.J. Visual Agnosia. MIT Press, Cambridge, MA1990Google Scholar). Interestingly, prosopagnosic patients handle inverted face stimuli with the same (or even better) facility as do normal controls (3Farah M.J. Wilson K.D. Drain H.M. Tanaka J.R. Vision Res. 1995; 35: 2089-2093Crossref PubMed Scopus (296) Google Scholar), in contrast to an agnosic patient who was found to be severely impaired at the recognition and manipulation of inverted faces (6Moscovitch M. Winocur G. Behrmann M. J. Cogn. Neurosci. 1997; 9: 555-604Crossref PubMed Scopus (551) Google Scholar). This pattern suggests that, when presented in an upright orientation, faces are preferentially processed by a “face-specific” cortical system. When, however, the face is inverted, the stimulus is handled by more general “object” recognition systems. Haxby and colleagues used functional magnetic resonance imaging (fMRI) to define in each of several subjects cortical areas that respond more to faces or to objects (in this case, houses). These regions were assumed to correspond to the “face” and “object” cortical sites that, when damaged, result in prosopagnosia and general object agnosia. The critical question then posed was: will the presentation of inverted faces, as compared to upright faces, shift the site of active neural processing from the “face” region to the “object” region? An affirmative answer would, first, confirm that inverted faces call upon the neural hardware normally used for the perception of general objects and, second, further demonstrate the highly specialized nature of the face-responsive regions. Interestingly, a mixed result was obtained. While face inversion did increase activity within the object areas, consistent with the hypothesis, it did not greatly change activity within the face areas (especially as compared to an upright/inverted object control). Haxby and colleagues consider several possible scenarios by which responses within the face area might be driven either directly or indirectly by inverted face responses in other areas. While these notions await further testing, they highlight an interesting and important aspect of the results: even though an intact “face” region is not necessary for normal perception of inverted faces (as demonstrated by studies of prosopagnosic patients), the region responds vigorously to these stimuli nonetheless. The current study joins two other neuroimaging investigations of stimulus inversion. Kanwisher and her colleagues (1998) also found that inversion of grayscale face pictures had only a slight effect upon the neural response of the face region. The study of 1Aguirre G.K. Singh R. D’Esposito M. Neuroreport. 1999; 10: 189-194Crossref PubMed Scopus (125) Google Scholar used an event-related fMRI design to examine the responses of face and object regions to upright and inverted face and object stimuli. In broad agreement with the current study, face inversion did not alter the response within face-responsive areas but did increase the evoked response within object areas. In contrast to the current study, however, inversion of the object stimuli also resulted in an increase in response in object regions. This discrepancy provides a speculative opportunity. In the study of Aguirre and colleagues, a target detection behavioral paradigm was used in which equal reaction time (response) penalties result from inversion of faces or objects. This is in contrast to the current study, in which a recognition design was used that produced a disproportionate penalty for face inversion. Therefore, the possibility exists that there is a relationship between the presence of a reaction time penalty with stimulus inversion and the observation of a greater signal response from cortical “object” regions. A facile explanation of such a relationship is that the effect of stimulus inversion is to lengthen the duration of neural activity within these object-responsive regions, as opposed to changing its intensity. The ability to distinguish between duration and intensity changes in neural activity is important, as it provides another measure against which we might evaluate the neural implementation of cognitive processes. Future studies that examine the parametric relationships between reaction time and neural response might add to the computational scaffolding of object recognition that Haxby and colleagues have erected here.