Electrophysiological constituents of the P100 and N170 ERP complex: De-constructing of face processing using independent component analysis and robust estimation.

The initial timing of face-specific effects in event-related potentials (ERPs) is a point of contention in face processing research. Although effects during the time of the N170 are robust in the literature, inconsistent effects during the time of the P100 challenge the interpretation of the N170 as being the initial face-specific ERP effect. The interpretation of the early P100 effects are often attributed to low-level differences between face stimuli and a host of other image categories. Research using sophisticated controls for low-level stimulus characteristics (Rousselet, Husk, Bennett, & Sekuler, 2008) report robust face effects starting at around 130 ms following stimulus onset. The present study examines the independent components (ICs) of the P100 and N170 complex in the context of a minimally controlled low-level stimulus set and a clear P100 effect for faces versus houses at the scalp. Results indicate that four ICs account for the ERPs to faces and houses in the first 200ms following stimulus onset. The IC that accounts for the majority of the scalp N170 (icNla) begins dissociating stimulus conditions at approximately 130 ms, closely replicating the scalp results of Rousselet et al. (2008). The scalp effects at the time of the P100 are accounted for by two constituent ICs (icP1a and icP1b). The IC that projects the greatest voltage at the scalp during the P100 (icP1a) shows a face-minus-house effect over the period of the P100 that is less robust than the N 170 effect of icN 1 a when measured as the average of single subject differential activation robustness. The second constituent process of the P100 (icP1b), although projecting a smaller voltage to the scalp than icP1a, shows a more robust effect for the face-minus-house contrast starting prior to 100 ms following stimulus onset. Further, the effect expressed by icP1 b takes the form of a larger negative projection to medial occipital sites for houses over faces partially canceling the larger projection of icP1a, thereby enhancing the face positivity at this time. These findings have three main implications for ERP research on face processing: First, the ICs that constitute the face-minus-house P100 effect are independent from the ICs that constitute the N170 effect. This suggests that the P100 effect and the N170 effect are anatomically independent. Second, the timing of the N170 effect can be recovered from scalp ERPs that have spatio-temporally overlapping effects possibly associated with low-level stimulus characteristics. This unmixing of the EEG signals may reduce the need for highly constrained stimulus sets, a characteristic that is not always desirable for a topic that is highly coupled to ecological validity. Third, by unmixing the constituent processes of the EEG signals new analysis strategies are made available. In particular the exploration of the relationship between cortical processes over the period of the P100 and N170 ERP complex (and beyond) may provide previously unaccessible answers to questions such as: Is the face effect a special relationship between low-level and high-level processes along the visual stream?

Photograph - B Squadron of the G.G.B.G. [Governor General’s Body Guard] at Niagara

Black and white photograph, mounted on board, 13 cm x 20 cm of the B Squadron of the G.G.B.G. [Governor General’s Body Guard] at Niagara.

Sketch of the ship "Jane C. Woodruff"

A black and white copy of a sketch of the ship "Jane C. Woodruff". This appears to have been in a scrapbook. There is a slight tear which affects the picture slightly. [The Jane C. Woodruff was a barquentine ship built in St. Catharines in 1866 by Lewis Shikeluna. The ship belonged to John Battle who was an associate of Samuel D. Woodruff. She was named in honour of Samuel and his wife, Jane Caroline. She originated as a square timber trade boat before being converted into a 3 masted schooner. She collided with the "Mary Battle" in a snow squall in Georgian Bay. The ship passed out of existence in 1902].

Niagara Guide looking into Cave of the Winds - Niagara Falls, U.S.A.

The description reads "(10) An old Niagara Guide - looking into the awe-inspiring Cave of the Winds - Niagara Falls, U.S.A.".

Maid of the Mist, Niagara Falls, U.S.A.

The image is described as "(5) 'Maid of the Mist' - Nymph of the mighty cataract, Niagara Falls, U.S.A.".

Sketch in the lock of the new canal above St. Catharines

Sketch in the lock of the new canal above St. Catharines. The sketch is unsigned, Aug. 18, 1899