To find cortical correlates of face recognition, we manipulated the recognizability of face images in a parametric manner by masking them with narrow-band spatial noise. Face recognition performance was best at the lowest and highest noise spatial frequencies (NSFs, 2 and 45 c/image, respectively), and degraded gradually towards central NSFs (11-16 c/image). The strength of the 130-180 ms neuromagnetic response (M170) in the temporo-occipital cortex paralleled the recognition performance, whereas the mid-occipital response at 70-120 ms acted in the opposite manner, being strongest for the central NSFs. To noise stimuli without faces, M170 was small and rather insensitive to NSF, whereas the mid-occipital responses resembled closely the responses to the combined face and noise stimuli. These results suggest that the 100 ms mid-occipital response is sensitive to the central spatial frequencies that are critical for face recognition, whereas the M170 response is sensitive to the visibility of a face and closely related to face recognition.

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The purpose of the study was to estimate the temporal processing capacity of human object identification under different stimulus conditions. Objects, either facial images or characters, were shown in a rapid sequence on a computer display using a rapid serial visual presentation (RSVP) method. One of the images was a target and the other images were distracters. The task of the observer was to identify the target. A staircase algorithm was used to determine the threshold frequency of image presentation in the RSVP sequence. The threshold frequency was determined as a function of image contrast, size, and mean luminance. The results showed that the threshold frequency, around 10 Hz for faces (100 ms per face) and about 25 Hz for characters (40 ms per character), was independent of contrast and size at medium and high contrast values, medium and large sizes, and high luminances, but decreased at very low contrasts or small sizes and medium or low levels of luminance. Computer simulations with a model, in which temporal integration limited perceptual speed, suggest that the experimentally found difference in processing time for faces and characters is not due to the physical differences of these stimulus types, but it seems that face-specific sites in the brain process facial information slower than object-specific areas process character information. Contrast, size, and luminance affect the signal-to-noise ratio and the temporal characteristics of low-level neural signal representation. Thus, the results suggest that at low contrasts, low luminances and small sizes, the processing speed of object identification is limited by low-level factors, while at high contrasts and luminances, and at large sizes, processing speed is limited by high-order processing stages. Processing speed seems to depend on stimulus type so that for faces processing is slower than for characters.

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