Review: a face-specific N170?--The controversy goes on.

Scott, Adam

Event Related Potential studies of face processing

Event Related Potential (ERP) studies on face recognition have isolated an early occipito-temporal negative component (N1) which is often associated with visual processing and encoding (see Bentin et al, 1996; Carmel & Bentin, 2002; Rousselet et al 2004; Rousselet et al 2005). This component falls within the low alpha range (7-8Hz), and has an early pick of 150-170ms after stimulus onset in visual tasks involving face processing. Contrary to recognition of non-facial objects, face detection produces a negative deflection in the N1 (see figure 1) and an early pick of 150-170 ms in the occipito-temporal electrodes after stimulus onset (Rousselet et al, 2004). Thus, when studied in connection with face recognition the N1 has often been termed 'N170' and it is found to be largely distributed at the posterior- inferior region of the temporal lobe and greater in the right than in left cerebral hemisphere (Bentin et al, 1996; Jeffreys, 1996; Kanwisher et al, 1997; Carmel & Bentin, 2002). It is, therefore, widely accepted that N170 responds preferentially to human faces as opposed to other objects (see Carmel & Bentin, 2002; Rousselet et al 2004; Itier & Taylor, 2004; Rousselet et al 2005 Bentin et al, 2007).

[FIGURE 1 OMITTED]

Figure 1 show ERPs elicited in one of the experiments conducted by Bentin et al (1996). Evoked potentials for faces and cars differed in latency range at locations T5 and T6. ERPs for both faces, scrambled faces and scrambled cars also differed in the 250-500ms latency range. Although not reaching statistical significance (p=0.3), the N170 for faces was more negative over the right (-4.07 [micro]V) than the left hemisphere (-3.02 [micro]V). Figure 1 also shows that the amplitude of the N170 at the left (T5) and right (T6) hemispheres is significantly larger for faces than for scrambled faces. These results show that face sensitive ERPs can be recorded with scalp electrodes, and also provide strong supportive evidence for a face-specific N170.

Controversies surrounding the notion of a face-specific N170

The N170 specificity to human faces has become paradigmatic for much debate, as some argued that this component is not exclusively sensitive to faces (Rousselet et al, 2004; Rousselet et al, 2005; Thierry et al, 2007a; Thierry et al, 2007b). A number of researchers asserted that this component is a product of complex and interrelated conceptual and perceptual factors. For instance, Thierry et al (2007a) proposed that, rather than faces, the N170 is sensitive to Interstimulus Perceptual Variance (ISPV). Their main thesis was that this component has not been controlled for variability across stimuli categories. Thierry et al asserted that in most experimental conditions faces are always presented in full front view and, therefore, they are less susceptible to perceptual variance. In both high and low ISPV conditions they claim to have found that the N170 did not occur within the usual 170ms after stimulus onset, but 70ms earlier. In this way, Thierry et al argued that differences found between experimental conditions do not reflect categorical inequalities, as these may arise as a consequence of ISPV.

However, Thierry et al results attracted critical attention from Bentin et al (2007) who demonstrated that the amplitude of the N170 does not vary linearly with the effect of ISPV. They also asserted that although the ISPV for cars and human faces are often identical, the N170 tend to be larger for faces than for cars. Furthermore, Thierry et al assertions have also been criticised on the grounds of not providing any theoretical explanation as to why the N170 was larger in amplitude low ISPV than for high ISPV (for a detailed discussion please see Bentin et al, 2007). Additionally, the N170 insensitivity to human faces found by Thierry et al can be explained through the way in which data was averaged across the different sites of participants scalp. Since six of the ten averaged data sites where located in the midline, some speculate that this factor might have contributed for a face 'insensitive' N170 (Bentin et al, 2007). Furthermore, in a reply to Thierry el al criticisms, Bentin (2007) argued that in fact ISPV was controlled in many studies and "yet the N170 remained conspicuous" (p. 801). Bentin also demonstrated that ISPV does not account for N170 sensitivity and, therefore he asserted that Thierry et al "failed to note the striking contradiction between their hypothesis and the existing literature" (p.801). Thierry et al speculations are also incompatible with the fact that a more negative N170 to faces is due to increases of power rather than phase-resetting of waveforms for faces as opposed to objects (see Bentin, 2007). Although one should not exclude the possibility that ERPs can be affected by stimulus variance, and factors such as ISPV it should be evident that control of ISPV does not abolish the 'face-specific' N170. In this way, Bentin asserted that the results reported by Thierry et al are "not valid and are not evidence against the well-established and reliable effect of larger N170s to faces than objects" (Bentin, 2007: 802). Therefore one must conclude that Thierry et al speculations were wrong and misleading.

Other studies opposing the 'face-specific' N170 paradigm have also claimed that the mechanism underlying face processing can be recruited for different object categories (see Gauthier, 2000; Russion et al, 2004; Rousselet et al, 2004; Busey & Vanderkolk, 2005). In this respect, Rousslelet et al (2004) investigated animal and human faces in natural scenes and found that the N170 was not only sensitive to both face categories (human and animal faces) but also to what has been termed the 'inversion effect'. Studies examining the 'inversion effect' found that this component is delayed when faces are inverted as opposed to upright position (Eimer, 2000; Itier & Tailor, 2002). This has led a number of researchers to claim that face recognition may reflect the operation of a module that holistically processes all objects of expertise and not just faces (see Itier & Taylor, 2002; Itier et al, 2003; Rossion et al, 2003). However, one can argue that the face-inversion effect is due to a disruption in the mechanisms responsible for configural processing (gluing together the different facial components) (for a detailed discussion please see Grand & Mondloch, 2002) rather than reflecting an 'non face-specific' N170. Furthermore, the 'inversion effect' occurs because normal processing of upright faces is holistic and is carried out by a face-specific module whereas decomposed facial parts are processed by a general recognition system (see Tanaka & Farah, 1993).

Another opposing view of the 'face specific' N170 paradigm was provided by Gauthier et al (2000) who argued that the neural substrates underlying the face-specific N170 can also be recruited for other object categories (Gauthier et al, 2000). This view was also shared by Russion et al (2002), who argued that the N170 response to faces reduces after expertise training with Greebles. Similarly, Gauthier and Tarr (1997) argued that the N170 is also sensitive to configuration changes in Greebles. This could lead some to speculate that face related effects associated with the N170 component can also be found for non-face objects when participants endure an extensive visual expertise training (see Russion et al, 2004; Busey & Vanderkolk, 2005). However, it has been argued that the aforementioned researchers' interpretation of data supporting the face-specific N170 paradigm ignored several important aspects (Bentin & Carmel (2002). In this respect Bentin and Carmel argued that the negative component N170 whilst strongly associated with face selectivity, "is most likely modulated also by non-selective visual activity which is always reflected by the N1" (p. 198). Therefore, one should not be surprised that to find visual stimuli elicited at this latency range.

Thus, due to the difference in the physical characteristics of the stimuli (i.e. Greebles, face-iversion, cars, etc), one should expect a considerable variation in amplitude of the N1 elicited by different stimulus categories (Bentin & Carmel, 2002). Furthermore, it has been found that the difference in amplitude of the N170 for faces and other stimulus categories (under similar conditions) exceeds the difference between other categories on their own (Tanaka et al, 1999). Thus, one can argue that data supporting a holistic processing paradigm, as well as the notion of an 'expert-specific' instead of 'face- specific' N170 is inconclusive and far from convincing. From this vantage point it appears that both face- and expert-specific N170 ERPs can be identified, but this factor does not abolish a conspicuous N170 face effect. However, it is still not clear if the face- and expert-specific N170s rely on the same neural substrates and whether they are a marker for similar cognitive functions (see Bentin & Carmel, 2002; Gazzaniga, 2004).

Another study also investigating visual expertise has also claimed to have demonstrated that there is a strong correlation between participants' level of car expertise and an amplitude decrease in the face-evoked N170 when pictures of cars and faces are presented simultaneously (Russion et al, 2007). Based on these results, Rossion et al (2007) proposed a "concurrent stimulation paradigm" which is concerned with the idea that there is a competition between faces and non-facial objects in a domain of expertise, and this tend to occur during the process of early visual categorization in the occipito-temporal cortex. Moreover, Rossion & Gauthier (2002) asserted that the N170 topographical distribution is similar for both faces and other object categories. However, some argue that due to the EEG's limited spatial resolution, topographical distribution, amplitude and latency, and the effects of expertise found on the previously mentioned N170 studies may not be reliable as these can simply reflect other potentials co-occurring at the same latency in the scalp (see Carmel & Bentin, 2002). Furthermore, it has also been proposed that the face-specific N170 is associated to structural encoding of face stimuli whereas later ERP components reflect other facial affects such as face processing (see Blau et al, 2007). This might explain why the studies conducted by Russion and Gauthier (2002) and Russion et al (2007) found in similar topographic distributions for faces and other objects.

Due to disagreement over the status of this component, two broad research traditions can be identified: researchers that put forward the view that the N170 reflects the operation of a face-specific module, and proponents who argue that the N170 is a neural marker for an expertise-specific module theory. In this respect, a number clinical, PET and fMRI studies have provided converging evidence for a 'face-specific' module in the human brain, as well as the existence of a face-selective N170 (Kanwisher et al, 1997; McCarthy et al, 1997; Halgren et al, 1999; Sadeh et al, 2008). For instance, it has been previously established that faces are processed by a specific modular region of the ventral temporal cortex known as fusiform gyrus area (FFA) (Kanwisher et al, 1997; Kanwisher & Yove, 2006).

For instance, in a well known fMRI study conducted by Kanwisher et al (1997) it was found that regions of the occipito-temporal cortex were more active during face than object viewing. In this way, by comparing studies which used other imaging techniques it appears that the evidence for a face-specific become ever more solid and scientifically robust. Thus, one can be deduce that the N170 truly reflects the operation of a face-specific module. This establishes, therefore, that face-specific modules are a sine qua non for human face processing and that the neural marker for structural encoding and recognition of faces is undoubtedly the N170 (Bentin & Deouell, 2000; Tanaka & Curran, 2001).

One could even argue that there is a genetic predisposition for face expertise, since that infant studies showed an innate orienting of visual attention to faces (see Jonhson, 1997). This factor has led some to conclude that face processing differs from that of non face visual stimulus because due to a complex interaction of experiential and evolutionary forces (see Kanwisher & Moscovitch, 2000). This view is also shared by Farah et al, (2000) who found crucial evidence for the innateness of a face specific module. In their paper, they reported an incident of a boy who sustained a bilateral occipito-temporal lesion at 1 day of age. This boy became prosopagnostic at the age of 16 leading to impairment in face recognition whilst the recognition of other objects remained intact. Other clinical studies have found the precisely the opposite (an impairment in object but not in face recognition). For instance, McMullen et al, (2000) documented cases in which neurological patients had intact face recognition but found to have non-face object agnosia. Such impairment is often referred to as anti-prosopagnosia (see Moscovitch et al, 1997). This double dissociation between face and non-face object recognition provides strong supportive evidence for the existence of neural mechanisms that are exclusively responsible for face processing. This point is also corroborated by Yovel & Kanwisher (2006) who argued that face processing reflects the operation of a specialised cortical network that may not be shared by other cognitive operations. Such findings can only reinforce and establish the N170 as an electrophysiological manifestation and significant marker for a face-specific module in the human brain.

A critique of the above views was provided by Gauthier et al (1999) whose research challenged both theoretical and methodological foundations of neuropsychological evidence collected from clinical studies on prosopagnostic patients. After testing two prosopagnostic subjects, Gauthier et al has claimed that patients suffering from this kind of neurological condition sometimes show a deficit for both categories of stimuli (faces and non-face objects). They concluded by stating that contemporary literature on this area of research fails to adequately provide evidence regarding a disproportionate impairment for face recognition as compared to non-face objects, and vice versa. Obviously, the above criticisms did not take into account studies investigating this issue did not find significant changes in the amplitude of the N170 between faces and non-face categories (Bentin et al (1999). Thus, it appear that there is an apparent contradiction between Gauthier et al (1999) findings and the existing literature.

A further criticism of neuropsychological evidence for a face-specific module is provided by Tippet et al (2000). They argued that neurological patients can also loose other face-specific functions in cases such as prosoamnesia. As far as their paper was concerned, this particular neurological condition selectively impairs the learning of new faces whilst living intact the recognition of old ones. However the aforementioned criticisms failed to consider previous studies showing that ERPs in patients with prosopagnosia show a similar N170 for both face and non-face stimulus categories. For instance, a study carried out by Bentin et al (1999) it has been found that patients who suffer from this disorder (prosopagnosia) can demonstrate a non selective enhanced N170 for both stimulus categories (face and non-face). Perhaps even more compelling, is the evidence provided by Kress & Daum (2003) who demonstrated that congenital prosopagnosia goes along with alterations on the N170. Therefore, they asserted that the N170 is a useful and strong prognostic indicator of a face processing modular system in the brain. Further supportive evidence is also provided by Haan et al (2002) who investigated event related potentials associated with faces in human infants. They reliably demonstrated that there is an "infant N170" for the identification of faces. These findings rule out the possibility of an expert-specific modular system since that a face-specific region appears to be active in the human brain since birth.

One must also notice that previous studies investigated electrophysiological activity in the brain and familiarity of faces. For instance, Bentin et al 1996 found that the N170 was not sensitive to familiarity of the face. This means that an identical N170 is elicited for both unfamiliar and familiar faces, regardless of whether there is an intention to recognise faces or not (see Kanwisher & Moscovitch, 2000). These findings added further weight to the argument that the N170 is involved in the process of face recognition. Thus, by weighing both supportive and evidence against a 'face-specific' N170 one must conclude that there is reliable face specificity in this component corroborating the existence of a face-specific module, as well as specialized cortical network for faces that may not be shared by other cognitive functions. To put simply, the N170 electrophysiological 'specificity' for faces may involve extra neural generators compared to objects (Itier & Taylor, 2004). This explains why sometimes researchers find an N170 also sensitive to non-face objects, but yet, as demonstrated in this paper, this component is more conspicuous in the presence of face stimuli.

Conclusion

This paper discussed some of recent research findings surrounding the N170 and its specificity to human faces. Recent evidence on face recognition and encoding of face stimuli postulates that specialized brain systems meditate face processing. Specifically, evidence from behavioural (Yin, 1969; Bruce et al, 1992; Tanaka & Farah, 1993), neurophysiological (see Bentin et al, 1996; Carmel & Bentin, 2002; Rousselet et al 2004; Rousselet et al 2005) and neuroimaging (Kanwisher et al, 1997; McCarthy et al, 1997; Halgren et al, 1999; Sadeh et al, 2008) studies suggests that the recognition of faces relies upon specific processes implemented by specialized brain mechanisms. In this way, it has been demonstrated that this component is relatively specific to face processing compared to other objects. As previously mentioned, critics may argue that instead of face-specific, the N170 is sensitive to Interstimulus Perceptual Variance (see Thierry et al, 2007a), or that it simply reflects a global marker for perceptual expertise (see Gauthier, 2000; Russion et al, 2004; Rousselet et al, 2004; Busey & Vanderkolk, 2005, Rossion et al, 2007). Although some of these arguments are well taken, their underlying criticism is not persuasive, as their interpretation of the current data ignores very important aspects. For example, due to the difference in the physical characteristics of the stimuli, one should expect a considerable variation in amplitude of the N1 elicited by different stimulus categories simply because the N1 is a neural marker for processing visual stimuli in general (Bentin & Carmel, 2002). Nevertheless, this factor does not abolish the conspicuousness of the N170 face effect.

It is also worth highlighting here that the existence of a face specific module has been strongly corroborated by double dissociations found in agnostic patients that could not recognise objects but retained their ability to recognise faces (Moscovitch et al, 1997), and patients with prosopagnosia who could not identify familiar faces whilst still able to recognise objects (Bentin et al, 1999). Therefore, in the light of the wealth of empirical evidence presented in this paper, it must be concluded that the N170 is not only an ERP marker for face selectivity, but also a strong prognostic indicator of a 'face-specific' module in the human brain. In other words, it constitutes an electrophysiological manifestation of the neural mechanisms underlying the structural encoding of faces. However, given the diversity and complexity of the extrastriate cortical regions, one should not exclude the possibility that the N170 can be modulated by different neural sources involved in distinct perceptual processes. Finally, it is hoped that this paper will act as a catalyst for more and different studies in this area. Future research should seek to investigate if face- and expert-specific N170s rely on the same neural substrates.

References:

Bentin, S. & Deouell, L. Y. (2000) Structural Encoding and Identification in Face Processing: Evidence for Separate Mechanisms. In Kanwisher, N & Moscovitch, M.(eds) (2000) The Cognitive Neuroscience of Face Perception. UK: Psychology Press

Bentin, S.; Allison, T.; Puce, A.; Perez, E. & McCarthy, G. (1996) Electrophysiological studies of face perception in humans. Journal of Cognitive Neuroscience. Vol. 8, pp. 551-565

Bentin, S. Deouell, L. Y. & Soroker, N. (1999; Selective visual streaming in face recognition: evidence from developmental prosopagnosia. Neuroreport. Vol.10, (4), pp. 823-827

Bentin, S.; Taylor, M. J.; Rousselet, G. A.; Itier, R. J.; Caldara, R; Schyns, P. G.; Jacques, C. & Rossion, B. (2007) Controlling interstimulus perceptual variance does not abolish N170 face sensitivity. Nature Neuroscience. Vol. 10, (7), pp. 801-802

Blau, V. C.; Maurer, U.; Tottenham, N. & McCandliss, B. D. (2007) The face-specific N170 component is modulated by emotional facial expression. Behavioural & Brain Functions. Vol.3, (7), pp. 1-13

Bruce, V. & Young, A. (1996) Understanding face recognition. British Journal of Psychology. Vol. 77, pp. 305-327

Busey, T. A. & Vanderkolk, J. R. (2005) Behavioural and electrophysiological evidence for configural processing in fingerprint experts. Vision Research. Vol. 45, pp.431-448

Busey, T. A., & Vanderkolk, J. R. (2005). Behavioral and electrophysiological evidence for configural processing in fingerprint experts. Journal of Vision. Vol. 5, (8), pp. 635, 635

Carmel, D. & Bentin, S. (2002) Domain specificity versus expertise. Factors influencing distinct processing of faces. Cognition. Vol. 86, (1), pp.11-14

Eimer, M. (2000) Effects of face inversion on the Structural Encoding and Recognition of Faces: Evidence from Event-Related rain Potentials. Cognitive Brain Research. Vol. 10, (1-2), pp. 145-158

Eimer, M., & McCarthy, G. (1999). Prosopagnosia and structural encoding of faces: Evidence from event-related potentials. NeuroReport. Vol. 10, pp. 25-259

Farah M.J., Rabinowitz C., Quinn G.E., Liu G.T. (2000) Cognitive Neuropsychology. Vol.17, (1-3), pp. 117-123

Gauthier, I. (2000) What constrains the organization of the ventral temporal cortex? Trends in Cognitive Sciences. Vol. 4, pp. 1-2

Gauthier, I., & Tarr, M.J. (1997). Orientation priming of novel shapes in the context of viewpoint-dependent recognition. Perception. 26(1), pp.51-73

Gauthier, I., Tarr, M. J., Anderson, A. W., Skudlarski, P., & Gore, J. C. (1999). Activation of the middle fusiform "face area" increases with expertise in recognizing novel objects. Nature Neuroscience. Vol. 2, (6), pp.568-573

Gauthier, I.; Behrmann, M. & Tarr, M. J. (1999) Can Face Recognition Really be Dissociated from Object Recognition? Journal of Cognitive Neuroscience. Vol. 11, pp. 349-370

Gazzaniga, M. S. (2004) The Cognitive Neurosciences III. US: MIT Press

Grand, R., & Mondloch, C.J. (2002). The many faces of configural processing. Trends in Cognitive Sciences. Vol. 6, pp. 255-260.

Haan, M., Pascalis, O & Johnson, M.J. (2002) Specialization of neural mechanisms underlying face recognition in human infants. Journal of Cognitive Neuroscience. Vol. 14, pp. 99-209.

Haan, M.; Pascalis, O. & Johnson, M. H. (2002) Specialization of Neural Mechanisms Underlying Face Recognition in Humans Infants. Journal of Cognitive Neuroscience. Vol. 14, (2), pp. 199-209

Halgren, E.; Dale, A.M.; Sereno, M.I.; Tootell, R.B.; Marinkovic & K.; Rosen, B.R. (1999) Location of human face-selective cortex with respect to retinotopic areas. Human Brain Mapping. Vol. 7, pp. 29-37

Itier, R. J.; Latinus, M. & Taylor, M. J. (2002) Inversion and contrast polarity reversal affect both encoding and recognition processes of unfamiliar faces: A repetition study using ERPs. Neuroimage. Vol. 15, (2), pp. 353-372

Itier, R. J.; Latinus, M. & Taylor, M. J. (2006) Face, eye and object early processing: What is the Face Specificity? Neuroimage. Vol. 29, pp. 667-676

Jeffreys, D. A.(1996) Evoked studies of face and object processing. Visual Cognition. Vol. 3, pp.1-38.

Jemel, B (2003) Stepwise emergence of the face-sensitive N170 event-related potential component Neuroreport. Vol.14, (16), pp.2035-2039

Johnson, M.H. (1997) Developmental Cognitive Neuroscience: An Introduction. Oxford: Blackwell

Kanwisher, N.; McDermott, J. & Chun, M. M. (1997) The Fusiform Face Area: A Module in Human Extrastriate Cortex Specialized for Face Perception. The Journal of Neuroscience. Vol. 17, (11), pp. 4302-4311

Kanwisher, N. & Moscovitch, M. (2000) The Cognitive Neuroscience of Face Processing: An Introduction. Cognitive Neuropsychology. Vol. 17,(1), pp.1-11

Kanwisher N, Yovel G. (2006). The fusiform face area: a cortical region specialized for the perception of faces. Philos Trans R Soc LondB Biol Sci. Vol. 361, pp.2109-28.

Kanwisher N. (2006). What's in a face? Science. Vol. 311, pp.617-18.

Kanwisher, N & Moscovitch,M.(eds) (2000)The Cognitive Neuroscience of Face Perception. UK: Psychology Press

Kress T., Daum I. (2003) Event-related potentials reflect impaired face recognition in patients with congenital prosopagnosia. Neuroscience Letters. Vol. 352 (2), pp. 133-136

McCarthy, G.; Puce, A.; Gore, J.C. & Allison, T. (1997) Face-specific processing in the human fusiform gyrus. Journal of Cognitive Neuroscience. Vol.9, pp.605-610

McMullen, P. A; Shore, D. I. & Henderson, R. B. (2000) "Testing a two-component model of face identification: Effects of inversion, contrast reversal, and direction of lighting" Perception. Vol. 29, pp.609-619

Moscovitch M, Winocur G, Behrmann M (1997) What is special about face recognition? Nineteen experiments on a person with visual object agnosia and dyslexia but normal face recognition. Journal of Cognitive Neuroscience. Vol. 9, pp.555-604

Russion, B. & Gauthier, I. (2002). How does the brain process upright and inverted faces? Behavioral and Cognitive Neuroscience Reviews. Vol.1, pp. 63-75.

Russion, B.; Kung, C. C. & Tarr, M. J. (2004) Visual expertise with nonface objects leads to competition with the early perceptual processing of faces in the human occipitotemporal cortex. PNAS. Vol. 101, (40), pp. 14521-14526

Russion, B.; Collins, D.; Goffaux, V. & Curran, T. (2007) Long-term Expertise with Artificial Objects Increases Visual Competition with Early Face Categorization Processes. Journal of Cognitive Neuroscience. Vol. 19, (3), pp. 543-555.

Rousselet, G. A.; Mace, M. J. & Fabre-Thorpe, M. (2004) Animal and human faces in natural scenes: How specific to human faces is the N170 component? Journal of Vision. Vol. 4, pp. 13-21

Rousselet, G. A.; Husk, J. S.; Bennett, P. J. & Sekuler, A. B. (2005) Spatial scaling factors explain eccentricity effects on face ERPs. Journal of Vision. Vol. 5, pp. 755-763

Sadeh, B.; Zhdanov, A.; Podlipsky, I.; Hendler, T. & Yovel, G. (2008) The validity of the face-selective ERP N170 component during simultaneous recording with functional MRI. Neuroimage. Vol. 42, pp. 778-786

Sagiv, N. & Bentin, S. (2001) Structural encoding of human and schematic faces: Holistic and part-based processes. Journal of Cognitive Neuroscience. Vol. 13 (7), pp. 937-951

Tanaka, J. W. & Farah, M. J. (1993) Parts and wholes in face recognition. Quarterly Journal of Experimental Psychology. Vol. 46, pp. 225-245

Tanaka, J. W., & Curran, T. (2001). A neural basis for expert object recognition. Psychological Science. Vol.12, pp. 43-47

Thierry, G. ; Martin, C. D. ; Downing & Pegna, A. J. (2007a) Controlling for Interstimulus Perceptual Variance abolishes N170 face selectivity. Nature Neuroscience. Vol. 10, pp. 505511

Thierry, G.; Martin, C. D. ; Downing & Pegna, A. J. (2007b) Is the N170 sensitive to human face or to several intertwined perceptual and conceptual factors? Nature Neuroscience. Vol. 10, pp. 802-803.

Tippett L.J., Miller L.A. & Farah M.J. (2000). Prosopamnesia: A selective impairment in face learning. Cognitive Neuropsychology. Vol.17, (1/2/3), pp.241-255.

Yin, R. K. (1969) Looking at upside-down faces. Journal of Experimental Psychology. Vol.81, pp.141-145

Adam Scott

E-mail address: adam.scott@oxford-mosaic.co.uk