Linking Brain Response and Behavior to Reveal Top-Down. (and Inside-Out) Influences on Processing in Face Perception. Heather A. Wild. Thomas A.

Transcription

1 Linking Brain Response and Behavior to Reveal Top-Down (and Inside-Out) Influences on Processing in Face Perception Heather A. Wild Thomas A. Busey Department of Psychology Indiana University Bloomington, IN Please send correspondence to: Heather Wild or Tom Busey

2 Contextual Influences on Face-Related ERPs 2 Abstract In two electrophysiological experiments we investigated contextual influences on face and word recognition. An event-related potential component previously identified with face processing (the N170) was shown to be modulated by task differences. Subjects viewed faces and words embedded in fixed visual noise, and produced a larger N170 to noise-alone trials when they expected a face. In a second experiment we found a larger N170 on noise-alone trials when observers thought they saw a face. The results demonstrate that the neurons responsible for the N170 are affected by a wider range of influences than previously thought. In addition, the size of the N170 is related to the behavioral response to an otherwise ambiguous stimulus, even in the absence of face-like features. The results point to the intriguing suggestion that the illusion of a face in an ambiguous display may result from greater activity in the temporal lobe face region.

3 Contextual Influences on Face-Related ERPs 3 Converging evidence from a variety of sources demonstrates that humans (and other primates) have a special facility for face recognition, perhaps supported by specialized areas in the inferotemporal cortex ( Kanwisher, N., McDermott, J. & Chun, M. M.,1997; Tanaka & Farah, 1993). Research on neural correlates of face processing suggests that areas of the inferotemporal (IT) cortex respond selectively to faces and other complex visual stimuli with which the observer has extensive experience and expertise (Gauthier, Tarr, Anderson, Skudlarski, & Gore, 1999). Single-cell recording in nonhuman primates also shows face selectivity in IT neurons (Perrett, Rolls, & Cann, 1979; Young & Yamane, 1992), which are at the end of a series of processing stages that extend along a pathway down the temporal lobe from earlier visual areas. In humans, there is evidence for slight right-hemisphere dominance (Watanabe, S. Kakigi, R., Koyama, S., & Kirino, E., 1999). In addition, prosopagnosic individuals show evidence of brain lesions in these areas and several show right-hemispheric specialization (Eimer & McCarthy, 1999; Farah, Rabinowitz, Quinn, & Liu, 2000). While much of the work has addressed the nature of the visual information that results in activity in area IT, relatively little research has focused on other sources of influence. In the present work we address the nature of contextual influences that are brought to bear by the perceiver during face and word recognition, and address the degree to which these contextual influences affect activity that occurs within milliseconds after stimulus onset. Electrophysiological (EEG) studies demonstrate that when a face stimulus is visually presented to observers during recording, a negative-going potential occurs around 170 ms after stimulus onset (Bentin, 1998; Bentin, Allison, Puce, Perez, &

4 Contextual Influences on Face-Related ERPs 4 McCarthy, 1996; Olivares & Iglesias, 2000). This downward deflection (which need not extend below zero) is known in the EEG/ERP literature as the N170. The N170 is largest at recording sites T5 and T6, roughly corresponding to the left and right temporal lobe respectively (Bentin et al., 1996). Intracranial EEG recordings in epileptic human observers reveal a face-specific negative-going potential at ms in IT and the fusiform gyrus (Allison, Puce, Spencer, & McCarthy, 1999; McCarthy, Puce, Belger, & Allison, 1999; Puce, Allison, & McCarthy, 1999). In addition, using a combination of MEG, EEG and source localization (BESA), Watanabe et al. (1999) have localized the N170 response to regions of the inferotemporal cortex, around the fusiform gyrus. Thus the locus of the N170 component appears to derive from the neurons in the inferotemporal cortex. The presentation of virtually any face or face-like stimulus results in an N170 component in the ERP. This is regardless of sex, age, emotional expression, or pose of the face (Allison et al., 1999; McCarthy et al., 1999; Puce et al., 1999). Inverted (upsidedown) faces also yield an N170, although the onset is slightly delayed and the negative deflection is slightly stronger (Bentin et al., 1996). Parts of faces (i.e., features alone) and cartoon and animal faces also yield an N170, although it is sometimes attenuated and the results are not entirely consistent (Allison et al., 1999; Bentin et al., 1996). Stimuli which elicit an attenuated or nonexistent N170 include other highly homogenous classes of symmetric complex visual stimuli such as flowers, cars, or butterflies; nonface bodyparts (e.g., hands; McCarthy et al., 1999); and pixelwise scrambled faces and various sinusoidal gratings (Allison et al., 1999). Components in the range of ms have been shown to be modulated by spatial attention (Hillyard & Anillo-Vento, 1998) but

5 Contextual Influences on Face-Related ERPs 5 these studies seem to indicate that modulation of these components by non-spatial features such color, motion or shape depends on spatial attention. Thus selecting a class of features (such as those in faces or words) without first selecting a spatial location may not be sufficient to modulate the N170. Researchers have examined whether there are top-down influences other than spatial attention on the N170 for faces and, until recently, have found none. There appears to be no modulation of the N170 by task demands, such as whether the face is a target stimulus or not (Cauquil, Edmonds, & Taylor, 2000), and no effect of familiarity of the face (Bentin & Deouell, 2000). This suggests that the N170 indexes purely perceptual or bottom-up processes. Additionally, fmri studies reveal that face-specific brain areas are active any time there is a face in a scene, regardless of whether the face is relevant to the task at hand (Gauthier et al., 1999). Thus, the neural response of cells in IT appears to arise from feed-forward perceptual processing of faces. In support of this, Bentin et al. (1996) suggested that the N170 indexes an automatically activated neural face detection mechanism. However, a recent study by Bentin and colleagues (Bentin, Sagiv, Mecklinger, Friederici, & von Cramon, 2002) has altered this view. Subjects were first shown pairs of dots, plus signs, or other simple shapes. These failed to elicit a strong N170. The pairs of dots were then place into a face context, where the dots became the eyes of a schematic face. When the observers were again shown just the pairs of shapes, they reported interpreting the shapes as eyes, and more importantly, the shapes and the complete faces produced equivalent N170s. These results suggest that the N170 can be modulated by contextual information.

6 Contextual Influences on Face-Related ERPs 6 Bentin et al. (2002) took advantage of the fact that pairs of dots can sometimes be interpreted as eyes. In the present work we address whether the N170 can be modulated in the absence of any face-like stimulus, even in the absence of any task demands or prior experience with the stimuli. If so, this would re-define the response properties of the neurons responsible for the N170. To accomplish this, we embedded faces and words in visual noise, and varied the task or other factors. The critical conditions derive from trials in which only noise is presented to the subject. Our noise is not resampled on each trial, and thus we hold the physical stimulus constant across the tasks for the noise-alone conditions. In Experiment 1 we ask whether the subject produces a larger N170 to the noise alone trials when they are looking for faces than when looking for words. In Experiment 2 we ask whether subjects produce a larger N170 to the noise-alone trials when they thought they saw a face, compared to a word. The results indicate that the N170 is not just the signature of a feed-forward face detector, but is affected by contextual information even in the absence of face-like features. Surprisingly, on noise-alone trials there appears to be a direct relation between the size of the N170 and whether the observer reports a face or a word was presented. Experiment 1 In Experiment 1 observers made face or word judgments to faces and words embedded in noise. The trials were blocked by task. The critical question is whether observers will make a larger N170 to the noise-alone trials when looking for faces than when looking for words. Methods Participants

7 Contextual Influences on Face-Related ERPs 7 Nine observers, eight of which were right-handed, participated in the study. These observers were research assistants from our lab and/or students at IU whose participation comprised part of their labwork or coursework. All observers were na ve as to the purpose of the study. Apparatus and EEG recording parameters The EEG was sampled at 1000 Hz and amplified by a factor of 20,000 (Grass amps model P511K) and band-pass filtered at Hz (notch at 60 Hz). Signals were recorded from sites F3, F4, Cz, T5, and T6, with a nose reference and forehead ground; all channels had below 5 kohm impedance. Recording was done inside a Faraday cage. Eyeblink trials were identified from channels F3 and F4 and removed from the analysis with the help of blink calibration trials. Images were shown on a 21 inch (53.34 cm) Macintosh grayscale monitor approximately 44 inches (112 cm) from participants. Data were collected by a PowerMac These details were identical for Experiment 2. Stimuli The stimuli for both experiments appear in Figure 1. Face stimuli consisted of frontal views of one male and one female face with neutral expressions, generated using the Poser TM application (Metacreations). Faces subtended a visual angle of 2.1 x 2.8 degrees. Two low imagery words were chosen for the second task ( Honesty and Trust ). Words subtended a visual angle of 1.1 x.37 degrees. All stimuli were embedded in white noise (4.33 x 4.33 degrees of visual angle) that was fixed (i.e. not resampled) on all trials. The faces and words were presented at three contrast levels (high, low, and zero). The zero contrast condition contained just noise and therefore had no correct answer. The identical stimuli were used in Experiment 2.

8 Contextual Influences on Face-Related ERPs 8 Procedure Participants completed the face task in the first half of the experiment and the word task in the second half. In the face task, observers made male/female judgments on 100 trials per contrast level that were presented in random order within the face block. The word task involved trust/honesty judgments, also with 100 trials per contrast level. Observers were told that there was a stimulus present on every trial, but that it might be difficult to see because of low contrast levels. Stimuli were presented for 1000 ms. EEG was recorded from 100 ms prior to stimulus onset to 1100 ms post-stimulus onset. The subject responded after the stimulus disappeared. Results and Discussion - Experiment 1 EEG signals were averaged across trials for each subject within condition, such that there were six ERP traces (high, medium, and zero contrast for faces and for words). As expected, the high-contrast face produced a much larger N170 than the high-contrast word, and had a latency of about 170 ms. The critical trials of the experiment are those where only noise was presented, because in these trials the physical stimulus is held constant and only the subjects expectations vary (i.e. looking for a face or looking for a word). The grand average ERPs collapsed across subjects for these conditions are presented in Figure 2 (sites T5 and T6). Our central question is whether we see a larger N170 in the noise-only trials when subjects expect a face than when they expect a word. At both T6 and T5 there is a greater N170 when subjects are looking for a face. To assess this difference statistically, we computed the average amplitude for each subject in the time window from ms for each condition. Across subjects, there was a significantly greater negative potential for the face condition for T5 (two-tailed t(8) =

9 Contextual Influences on Face-Related ERPs , p=.031) and T6 (two-tailed t(8) = 2.35, p =.047). We also analyzed the P100 and P300 components by averaging the amplitudes in the ms and ms windows and found no significant differences. We found a greater N170 on noise-alone trials where observers were expecting a face rather than a word, despite the fact that the physical stimulus was identical on these trials. These results demonstrate that top-down contextual factors can mediate the N170, and are consistent with the results of Bentin (2002). The current results extend previous results by demonstrating that the N170 can be modulated by contextual influences even in the absence of face-like features. Experiment 2 The contextual influences in Experiment 1 take the form of what might be thought of as perceptual set. During a face block observers are looking for facial features, and perhaps this involves attention to specific spatial frequency bands or features that fit the response properties of the N170 neurons. This attentional tuning could occur well before the start of a trial and be held constant throughout a block of trials. To remove this pre-trial contextual information, in Experiment 2 we switched to a mixed design. Subjects made a face/word discrimination on each trial, and again we included the noise-only trials. The critical question is whether the N170 is larger on those trials in which subject think they see a face in a noise-only display (relative to trials in which they think they see a word). If so, this would relate the N170 activity directly to the response and indicate how activity in this region might influence aspects of conscious behavior such as perception and overt responding. Methods

10 Contextual Influences on Face-Related ERPs 10 Participants Ten right-handed observers participated in the study. These observers were research assistants from our lab and/or students at IU whose participation comprised part of their labwork or coursework. All observers were na ve as to the purpose of the study. Procedure The procedure was similar to that of Experiment 1, except that face and word trials were intermixed and the task was to indicate whether a face or a word was embedded in the noise. Observers responded via a joystick using a single finger, and were asked to make speeded responses. This change was made to eliminate additional guessing strategies not tied to the initial perceptual processing of the stimulus. The same stimuli were presented as in Experiment 1 (high, low, or zero contrast faces and words embedded in noise), and observers were told that there was a stimulus present on every trial, despite the fact that one-third of the trials were noise-alone. Observers were also told that faces and words appear equally often. There were a total of 720 trials. Results and Discussion - Experiment 2 EEG signals were averaged across trials for each subject based on the stimulus category for high or medium contrast words and faces, and the noise-only trials were binned according to the subjects response (either face or word ). Again we found a large N170 for the high contrast face, while the high contrast word showed a much later onset and a reduced amplitude. There was a slight bias to say face such that observers made this response on 62% of the noise-alone trials. As shown in Figure 3, on these noise-alone trials, face responses are associated with a greater N170 than word responses in the right temporal channel (T6) (two-tailed, t(9) = 2.74, p =.023), but not for the left

11 Contextual Influences on Face-Related ERPs 11 temporal channel (T5) (t(9) = 1.54, p =.157, ns). We also analyzed the P100 and P300 by averaging the amplitudes in the ms and ms windows and found no significant differences for either channel. Thus the differences in the ERPs between face and word responses are confined to the right temporal lobe at about 170 ms after stimulus onset. The results of Experiment 2 demonstrate that the N170 is related to the response given by subjects to an ambiguous stimulus (the noise-alone image). That the differences were confined to the right temporal lobe at about 170 ms after stimulus onset puts strong constraints on the nature of the processing that causes the differences in the EEG. Below we discuss possible explanations for these differences at the N170 for the two responses. General Discussion In Experiment 1, we demonstrated that modulation of the EEG signal at 170 ms can result from contextual influences in the absence of face-like features. Our noise-alone display bears no resemblance to the perceptual stimuli that are typically reported in the literature as generating an N170. First, there are no clearly identifiable face-like features. Second, the display has a flat frequency spectrum, whereas faces have approximately a 1/f frequency spectrum. Third, pixelwise scrambled faces (which produces displays similar to our white noise) do not yield an appreciable N170 (McCarthy et al., 1999). Thus, we feel it is reasonable to conclude that we have excluded possible face-like features from our noise-alone stimuli. The greater N170 on noise-alone trials in the face block appears to be driven by expectations or differences in the nature of the perceptual information acquired by the subject, rather than by bottom-up perceptual differences since the physical stimulus was constant in the two blocks.

12 Contextual Influences on Face-Related ERPs 12 In Experiment 2 we found that observers show a greater N170 when they think they see a face in the noise rather than a word, an effect which is localized to the right temporal lobe. These results establish a link between the magnitude of the neural response at 170 ms and the behavioral response (i.e., whether observers say they saw a face). This is neither the result of bottom-up nor top-down influences, but rather demonstrates what might be thought of as inside-out processing. Under this account, the internal response of N170 neurons varies from trial to trial due to internal noise or other stochastic processes. When faced with a noise-only trial that occurs in conjunction with a greater activity level in the N170 neurons, the observer may experience the illusion of the presence of a face and respond accordingly. In support of this hypothesis, several studies have found that observers report face and face-like percepts following intracranial stimulation of face-specific areas of the cortex (Puce et al., 1999; Vignal, Chauvel, & Halgren, 2000). In our experiments this illusion may be quite faint, but sufficient to bias the subject s response on noise-alone trials. Before accepting this candidate hypothesis, we must first rule out several other plausible explanations for this effect. First, the activity at 170 ms occurs too early to simply be a signature of the observer s response after it had been executed, since the median reaction times were around 600 ms for both conditions. Thus while it is possible (even likely) that the N170 neurons influence the decision, it is unlikely that the subject s decision influences the electrophysiological response at 170 ms. The inside-out account assumes that the decision process starts once the trial begins. One candidate mechanism that would allow the process to start earlier would be priming from the previous trial. We explored this possibility by examining whether the

13 Contextual Influences on Face-Related ERPs 13 presentation of a face on the previous trial resulted in a larger N170 on the current noisealone trials. As shown in the left panel of Figure 4, the size of the N170 to noise-alone trials has only a slight dependence on whether a face or a word was presented on the previous trial. Noise-alone trials preceded by a face produced a reduced N170, which contradicts the priming hypothesis. This difference is significant (t(9) = 2.43, p = 0.038) mainly due to differences that occur late in the window. However, as shown in the right panel of Figure 4, this effect is due entirely to differences on trials in which observers responded word to the noise-alone stimulus. For face responses to noise-alone trials, there were no differences in the EEG depending on whether the prior trial had been a face or a word (t(9) =.04, p=0.96). However, there was a slightly smaller N170 for wordresponse noise-alone trials that were preceded by a face stimulus (t(9) = 3.24, p =.01), which again contradicts the priming hypothesis. One possible explanation is negative priming from the stimulus on the prior trial, but it is not clear why the effects of priortrial priming should be restricted to noise-alone trials in which the observer responded word. In addition, this effect is only a small part of the large difference seen in Figure 3 (right panel). This can be observed in the right panel of figure 4 by collapsing the dark lines together and the light lines together to recover the original effect in Figure 3. This shows that the overall differences between the two responses are much larger than the differences conditioned on a word response. Thus we rule out the prior-trial priming explanation as a major influence on our results in Experiment Of course, we could also look at whether the response on the previous trial is correlated with the magnitude of the N170 on the present trial. However, as observers had extremely high accuracy levels, the responses and stimuli on previous trials were strongly correlated and thus the analysis would yield the same conclusions.

14 Contextual Influences on Face-Related ERPs 14 A second possible explanation for the results of Experiment 2 is that observers attend to face-like features or face-specific spatial frequency bands of the noise on some trials, which provides a stronger input to the N170 neurons, leading to a larger N170 and an overt face response. This explanation is of interest because it still ties the N170 activity to the behavioral response, although it doesn t explain why subjects should decide to look for a face on a particular trial. However, Gold, Bennett, & Sekuler (1998) have shown that faces and words are processed by humans using the similar spatial frequency filters despite the vastly different spatial frequency content. Thus attention to a particular spatial frequency band by the observer in order to detect one type of stimulus may not provide greater input to the N170 neurons. In addition, Puce et al. (1999) showed that bandpass filtered faces, which include just high or just low spatial frequencies, give equivalent N170 responses that are as strong as the N170 to the unfiltered face, and so changing the spatial frequency content in the input to the N170 via selective attention to different spatial frequency bands may not be sufficient to modulate the N170. Having at least tentatively eliminated explanations based on prior-trial priming and attention to different features in the noise, we are left with the intriguing possibility that the behavioral response is directly related to the activity in the N170 neurons. That is, when presented with an ambiguous stimulus, observers may have a tendency to think they saw a face when the activity of the N170 neurons is higher. The data from both experiments lead to the view that the processing of faces, as indexed by the N170, is much more flexible and amenable to contextual influences than has previously been reported in the literature. The N170 does not seem to simply reflect a feedfoward face detector, but instead responds to the demands of the task and perhaps

15 Contextual Influences on Face-Related ERPs 15 internal influences. In Experiment 1, observers performing face or word tasks may tune the behavior of the N170 neurons through top-down connections. In Experiment 2, the differences may come from internal sources at the N170 neurons, but still influence behavior. Thus we propose that, given no other information, internal activity may bias an overt response, through processes we term inside-out influences. The fact that we can link the activity in the N170 neurons to the behavioral response suggests that the output of these neurons eventually becomes available to conscious awareness. The source of this internal activity may be viewed as a form of internal noise that varies from trial to trial, influencing the processing of perceptual input. One way to investigate the nature of internal noise sources is to add external noise to the stimulus, which grown recently as a method for measuring internal noise and perceptual templates (Ahumada, 1987; Dosher & Lu, 1999; Gold, Bennett, & Sekuler, 1999a, 1999b; Gold, Murray, Bennett, & Sekuler, 2000; Skoczenski & Norcia, 1998; see Pelli & Farell, 1999, for a review of methods). The current study used external noise simply to create an ambiguous stimulus. However, the results point to a type of internal noise in the face processing neurons that ultimately influences behavior, rather than a noise source that simply limits performance. It seems likely that EEG recording may help define the nature of the internal noise, and when used in conjunction with modeling, may help tease apart current debates on the nature of internal noise (i.e., additive or multiplicative noise; e.g. Dosher & Lu, 1999). By relying on the time course of the signal, the nature of the internal noise at different stages of processing might be revealed. We are currently investigating how EEG data can be used with resampled noise to compute classification images (Gold et al., 2000). This is the EEG analog of the reverse

16 Contextual Influences on Face-Related ERPs 16 correlation technique in single-cell recording that has been used to map out the receptive fields of single neurons (e.g. DeAngelis, Ohzawa & Freeman, 1995). The hope is that the EEG signal will reveal, at a gross level, the response properties of the N170 neurons. The strength of the current approach is that it holds the physical stimulus constant in order to remove explanations based on different attributes of the stimuli. This allows us to tie the response of the neurons in area IT to the behavioral response and reveal evidence of non-perceptual influences. Similar techniques could be applied to other domains such as spatial attention and object recognition to disambiguate bottom-up, topdown and inside-out processes.

17 Contextual Influences on Face-Related ERPs 17 References Ahumada, A. J. (1987). Putting the visual system noise back in the picture. Journal of the Optical Society of America A, 4, Allison, T., Puce, A., Spencer, D. D., & McCarthy, G. (1999). Electrophysiological studies of human face perception. I: Potentials generated in occipitotemporal cortex by face and non-face stimuli. Cerebral Cortex, 9(5), Bentin, S. (1998). Separate modules for face perception and face recognition: Electrophysiological evidence. Journal of Psychophysiology, 12(1), Bentin, S., Allison, T., Puce, A., Perez, E., & McCarthy, G. (1996). Electrophysiological studies of face perception in humans. Journal of Cognitive Neuroscience, 8(6), Bentin, S., & Deouell, L. Y. (2000). Structural encoding and identification in face processing: ERP evidence for separate mechanisms. Cognitive Neuropsychology, 17, Bentin, S., Sagiv, N., Mecklinger, A., Friederici, A., & von Cramon, Y. D. (2002). Priming visual face-processing mechanisms: Electrophysiological evidence. Psychological Science, 13(2), Cauquil, A. S., Edmonds, G. E., & Taylor, M. J. (2000). Is the face-sensitive N170 the only ERP not affected by selective attention? NeuroReport, 11, DeAngelis, G.C., Ohzawa, I. & Freeman, R. D. (1995). Receptive field dynamics in the central visual pathways. Trends in Neuroscience, 18, Dosher, B. A., & Lu, Z.-L. (1999). Mechanisms of perceptual learning. Vision Research, 39,

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19 Contextual Influences on Face-Related ERPs 19 McCarthy, G., Puce, A., Belger, A., & Allison, T. (1999). Electrophysiological studies of human face perception. II: Response properties of face-specific potentials generated in occipitotemporal cortex. Cerebral Cortex, 9(5), Olivares, E. I., & Iglesias, J. (2000). Neural bases of perception and recognition of faces. Revista De Neurologia, 30(10), Pelli, D. G., & Farell, B. (1999). Why use noise? Journal of the Optical Society of America A, 16, Perrett, D. I., Rolls, E. T., & Cann, W. (1979). Temporal lobe cells of the monkey with visual responses selective for faces. Neuroscience Letters, Suppl. 2, 340. Puce, A., Allison, T., & McCarthy, G. (1999). Electrophysiological studies of human face perception. III: Effects of top-down processing on face-specific potentials. Cerebral Cortex, 9(5), Skoczenski, A. M., & Norcia, A. M. (1998). Neural noise limitations on infant visual sensitivity. Nature, 391, Tanaka, J. W., & Farah, M. J. (1993). Parts and wholes in face recognition. Quarterly Journal of Experimental Psychology: Human Experimental Psychology, 46A, Vignal, J. P., Chauvel, P., & Halgren, P. (2000). Localised face processing by the human prefrontal cortex: Stimulation-evoked hallucinations of faces. Cognitive Neuropsychology, 17, Watanabe, S. Kakigi, R., Koyama, S., & Kirino, E. (1999). Human face perception traced by magneto- and electro-encephalography. Cognitive Brain Research, 8,

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21 Contextual Influences on Face-Related ERPs 21 High Contrast Faces Low Contrast Faces Noise Alone Low Contrast Words High Contrast Words Figure 1. Stimuli for experiments 1 and 2. From left to right: high- and low-contrast female and male faces; noise alone; and low- and high-contrast words ( Honesty and Trust ). Note that the noise is identical for all stimuli.

22 Contextual Influences on Face-Related ERPs 22 6 Left Temporal Lobe (T5) 6 Right Temporal Lobe (T6) 4 4 Amplitude (µv) * Amplitude (µv) * -4 Expected a Word Expected a Face -4 Expected a Word Expected a Face time (ms) time (ms) Figure 2. Experiment 1. Event-related potentials (ERPs) elicited at temporal lobe sites T5 (left panel, left hemisphere) and T6 (right panel, right hemisphere). Solid lines indicate noise-alone trials where observers are expecting a face; dashed lines indicate noise-alone trials where observers are expecting a word. The two asterisks indicate significant effects at the N170 component.

23 Contextual Influences on Face-Related ERPs 23 Left Temporal Lobe (T5) Right Temporal Lobe (T6) Amplitude (µv) 0-2 n.s. Amplitude (µv) 0-2 * -4 Responded 'Word' To Noise-Alone Trial Responded 'Face' To Noise-Alone Trial -4 Responded 'Word' To Noise-Alone Trial Responded 'Face' To Noise-Alone Trial time (ms) time (ms) Figure 3. Experiment 2. ERPs elicited at temporal lobe sites T5 (left panel, left hemisphere) and T6 (right panel, right hemisphere). Solid lines indicate noise-alone trials where observers thought they saw a face; dashed lines indicate noise-alone trials where observers thought they saw a word. The asterisk in the right panel indicate significant differences between the two responses at the N170 component.

24 Contextual Influences on Face-Related ERPs 24 Noise-Alone Trials Conditioned on Prior Trial Noise-Alone Trials Conditioned on Prior Trial and Response Amplitude (µv) 0-2 * Amplitude (µv) 0-2 Responded 'Face' Responded 'Word' Face on Prior Trial -4 Word on Prior Trial Face on Prior Trial -4 Word on Prior Trial time (ms) time (ms) Figure 4. Data from Experiment 2, channel T6 (right hemisphere). ERP traces from noise-alone trials conditioned on the stimulus presented on the prior trial (left panel) and on prior trial and response (right panel). Note that the time scale is different than in prior figures to emphasize effects at the N170. The asterisk in the left panel indicates a significant difference between the two conditions at the N170, due mainly to the differences late in the averaging window.