Face recognition and cortical responses: Effect of stimulus duration Topi Tanskanen, a, ⁎ Risto Näsänen, b Helena Ojanpää, b,c and Riitta Hari a,d a Brain Research Unit, Low Temperature Laboratory, Helsinki University of Technology, PO Box 2200, FI-02015 HUT, Espoo, Finland b Institute of Occupational Health, Topeliuksenkatu 41a A, FI-00250 Helsinki, Finland c Department of Psychology, University of Helsinki, PO Box 9, FI-00014, Helsinki, Finland d Department of Clinical Neurophysiology, Helsinki University Central Hospital, FI-00290 Helsinki, Finland Received 7 July 2006; revised 23 January 2007; accepted 25 January 2007 Available online 6 February 2007 To clarify the relationship between face perception and cortical activation, we manipulated the face recognition performance of 9 subjects by varying the duration (DUR) of stimuli while cortical neuromagnetic responses were recorded. A face image replaced a continuous pixel-noise mask for 17–200 ms, and the subject reported which of the pre-learned faces had been presented. Two cortical responses were clearly stronger to intact than phase-scrambled faces: the temporo-occipital response peaking at 140–200 ms (M170) and a more widely distributed response peaking at 200–500 ms (M300). For the shortest DURs (17–33 ms), face recognition was at chance level and the cortical responses negligible. For DURs of 50–83 ms, the proportion of recognized faces as well as the strength of cortical responses increased steeply. Recognition performance saturated at DURs of around 100 ms, whereas cortical responses continued to increase until the longest DUR of 200 ms. Amplitudes of both M170 and M300 were thus tightly correlated with recognition performance (r = 0.98), but comparison of the increment rates as a function of DUR showed the recognition performance to have an even closer similarity to M170 than to M300. In single-trial analysis the variability of response strengths increased in a direct proportion to response amplitude, demonstrating the averaged responses to be composed of graded rather than of all-or-nothing-type single responses. © 2007 Elsevier Inc. All rights reserved. Keywords: Cortex; Human; Masking; MEG; Visual Introduction A dynamic network of cortical areas is activated when we see faces (for a review, see Haxby et al., 2004). Some of these activations, pinpointed by means of functional magnetic resonance imaging (fMRI), positron emission tomography (PET), as well as electroencephalography (EEG) and magnetoencephalography (MEG), appear to be face-specific. The best-characterized electro- physiological face-selective response, called M170 in MEG and N170 in EEG recordings, peaks temporo-occipitally around 170 ms after stimulus onset (Lu et al., 1991; Allison et al., 1994, 1999; Bentin et al., 1996; George et al., 1996; Sams et al., 1997; Halgren et al., 2000; Liu et al., 2000). M170 is clearly stronger to faces than to other stimuli tested so far, and this apparent relationship to face processing is reinforced by observations of stronger M170 to recognized than unrecognized faces (Liu et al., 2002) and by the covariation of the strength of M170 and the visibility of faces (Tarkiainen et al., 2002; Jemel et al., 2003; Horovitz et al., 2004). Specifically, the strength of M170 correlates strongly with recognition of facial images that are manipulated by masking them with noise of different spatial frequencies (Tanskanen et al., 2005). Faces elicit cortical responses also beyond 200 ms. These long- latency responses are affected by the familiarity of the faces (Puce et al., 1999; Bentin and Deouell, 2000; Eimer, 2000; Henson et al., 2003; Paller et al., 2003; Herzmann et al., 2004), and they are more sensitive than M170/N170 to attention (Lueschow et al., 2004; Furey et al., 2006). To our knowledge, the relationship between these long-latency face-sensitive responses and face recognition has remained uncharacterized so far. It is still unknown whether neural responses contributing to M170 are categorical or graded by nature. The gradually varying averaged cortical responses could either be averages of varying proportions of all-or-nothing-type single responses (either full response or no response), or averages of single responses that are graded as such. The improved signal-to-noise ratio of modern neuromagnetometers, together with recent noise-attenuation tech- niques (Taulu et al., 2004) permits us to approach this issue by analyzing the variability of the amplitudes of single responses. The present study had three purposes. First, we sought for further support for our earlier findings on the correlation of M170 strength and face recognition performance (Tanskanen et al., 2005), now using temporal instead of spatial masking. Second, we attempted to find out whether the more task- and familiarity- dependent responses peaking after 200 ms would also correlate with recognition performance. Third, by single-response analysis, we characterized the trial-by-trial variability of the cortical face responses to better understand the build-up of sub-maximal www.elsevier.com/locate/ynimg NeuroImage 35 (2007) 1636 – 1644 ⁎ Corresponding author. Fax: +358 9 451 2969. E-mail address: topi.tanskanen@tkk.fi (T. Tanskanen). Available online on ScienceDirect (www.sciencedirect.com). 1053-8119/$ - see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.neuroimage.2007.01.023