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Extracting Visual Evoked Potentials from EEG Data Recorded During fMRI-guided Transcranial Magnetic Stimulation

Transcranial Magnetic Stimulation (TMS) is an effective method for establishing a causal link between a cortical area and cognitive/neurophysiological ...

Tel-Aviv University

Despite the wide interest in the neural mechanisms of face processing and numerous event-related potential (ERP) and functional MRI (fMRI) studies of face-selective neural responses, no study, to date, has collected these two measures simultaneously. The main reason for the absence of such an investigation is that MRI data acquisition generates major artifacts, which completely conceals the EEG signal. Recently, artifact removal algorithms have been developed. Our goal was to examine the validity of the face-selective ERP component N170 and its functional effects such as category selectivity and hemispherical laterality, when recorded simultaneously with functional MRI. In our experiment, half of the scans were collected during fMRI acquisition and half without fMRI acquisition. The validity of the N170 was then measured for its amplitude, latency, face selectivity (the difference between the amplitude to faces and objects), laterality (the difference between the amplitude to faces over the right and the left hemispheres) and the laterality of the face selectivity effect, by correlating these measures across subjects between data collected without fMRI and with fMRI data acquisition, after applying artifact removal procedures. We found high validity coefficients for all N170 measures. Furthermore, ERP data collected outside the scanner on a different day were highly correlated with data collected during MR acquisition for the N170 amplitude, latency, and selectivity index but moderate for laterality indices. Our study demonstrates that face-selective ERP effects are preserved in simultaneous recording with fMRI. These findings will hopefully encourage researchers to combine the two complementary neuroimaging techniques in future research.

The validity of the face-selective ERP N170 component during simultaneous recording with functional MRI.

A face-selective neural signal is reliably found in humans with functional MRI and event-related potential (ERP) measures, which provide complementary information about the spatial and temporal properties of the neural response. However, because most neuroimaging studies so far have studied ERP and fMRI face-selective markers separately, the relationship between them is still unknown. Here we simultaneously recorded fMRI and ERP responses to faces and chairs to examine the correlations across subjects between the magnitudes of fMRI and ERP face-selectivity measures. Findings show that the face-selective responses in the temporal lobe (i.e., fusiform gyrus--FFA) and superior temporal sulcus (fSTS), but not the face-selective response in the occipital cortex (OFA), were highly correlated with the face-selective N170 component. In contrast, the OFA was correlated with earlier ERPs at about 110 ms after stimulus-onset. Importantly, these correlations reveal a temporal dissociation between the face-selective area in the occipital lobe and face-selective areas in the temporal lobe. Despite the very different time-scale of the fMRI and EEG signals, our data show that a correlation analysis across subjects may be informative with respect to the latency in which different brain regions process information.

Event-related potential and functional MRI measures of face-selectivity are highly correlated: a simultaneous ERP-fMRI investigation.

The amplitude of the ERP component N170 is larger in response to inverted than to upright faces. The current study tested two main hypotheses that have been suggested to explain this effect: according to the first hypothesis inverted faces lead to augmentation of activity of the same neural populations used for the processing of upright faces due to the increased processing difficulty that these stimuli impose; according to the second hypothesis the processing of inverted faces involves recruitment of additional neural mechanisms to those used for upright face processing. We employed an ERP competition paradigm in which the effect of a context stimulus on the ERP to a simultaneously presented target stimulus is measured. In Experiment 1, an upright target face was paired with an upright face, inverted face or a non-face context stimulus. ERPs time-locked to the presentation of the target showed reduced N170 amplitude when the context was an upright face more than in non-face context trials, replicating the ERP competition effect for faces. Interestingly, in contrast to the hypothesis that inverted faces recruit mechanisms used for upright faces to a greater extent, competition effects were similar in the context of inverted and upright faces. In Experiment 2, the target stimuli were inverted faces. This time, competition effect was larger in inverted than in upright face context. Taken together, these findings correspond with the hypothesis that inversion of the face leads to recruitment of additional mechanisms as early as at the N170 time-window.

Why is the N170 enhanced for inverted faces? An ERP competition experiment.

Neural selectivity to specific object categories has been demonstrated in extrastriate cortex with both functional MRI [1-3] and event-related potential (ERP) [4, 5]. Here we tested for a causal relationship between the activation of category-selective areas and ERP to their preferred categories. Electroencephalogram (EEG) was recorded while participants observed faces and headless bodies. Concurrently with EEG recording, we delivered two pulses of transcranial magnetic stimulation (TMS) over the right occipital face area (OFA) or extrastriate body area (EBA) at 60 and 100 ms after stimulus onset. Results showed a clear dissociation between the stimulated site and the stimulus category on ERP modulation: stimulation of the OFA significantly increased the N1 amplitude to faces but not to bodies, whereas stimulation of the EBA significantly increased the N1 amplitude to bodies but not to faces. These findings provide the first evidence for a specific and causal link between activity in category-selective networks and scalp-recorded ERP to their preferred categories. This result also demonstrates that the face and body N1 reflects several nonoverlapping neural sources, rather than changes in face-selective mechanisms alone. Lastly, because early stimulation (60-100 ms) affected selectivity of a later ERP component (150-200 ms), the results could imply a feed-forward connection between occipital and temporal category-selective areas.

Stimulation of category-selective brain areas modulates ERP to their preferred categories.

Faces and bodies are processed by distinct category-selective brain areas. Neuroimaging studies have so far presented isolated faces and headless bodies, and therefore, little is known on whether and where faces and headless bodies are grouped together to one object, as they appear in the real world. The current study examined whether a face presented above a body are represented as two separate images or as an integrated face-body representation in face and body-selective brain areas by employing a fMRI competition paradigm. This paradigm has been shown to reveal higher fMRI response to sequential than simultaneous presentation of multiple stimuli (i.e., competition effect), indicating competitive interactions among simultaneously presented multiple stimuli. We therefore hypothesized that if a face above a body is integrated to an image of a person whereas a body above a face are represented as two separate objects, competition effect will be larger for the latter than the former. Consistent with our hypothesis, our findings reveal competition effect when a body is presented above a face, but not when a face was presented above a body, suggesting that a body above a face is represented as two separate objects whereas a face above a body is represented as an integrated image of a person. Interestingly, this integration of a face and a body to an image of a person was found in the fusiform, but not the lateral-occipital face and body areas. We conclude that faces and bodies are processed separately at early stages and are integrated to a unified image of a person at midlevel stages of object processing.

Boaz Sadeh

School of Psychological Sciences

Tel-Aviv University

Extracting Visual Evoked Potentials from EEG Data Recorded During fMRI-guided Transcranial Magnetic Stimulation

Boaz Sadeh

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Tel-Aviv University

Extracting Visual Evoked Potentials from EEG Data Recorded During fMRI-guided Transcranial Magnetic Stimulation

School of Psychological Sciences