The purpose of the study was to find out what spatial frequency information human observers use in the recognition of face images. Signal-to-noise ratio thresholds for the recognition of facial images were measured as a function of the centre spatial frequency of narrow-band additive spatial noise. The relative sensitivity of recognition to different spatial frequencies was derived from these results. The maximum sensitivity was found at 8-13 c/face width and the bandwidth was just under two octaves. Qualitatively similar results were obtained with stimuli in which Fourier phase was randomised within a narrow band of different centre spatial frequencies. This resulted in a considerable increase of energy threshold around 8 c/face width and less elsewhere. Further, contrast energy thresholds were measured as a function of the centre spatial frequency of band-pass filtered face images. As a function of object spatial frequency (c/face width), energy threshold first decreased and then increased. The lowest energy thresholds found around 10 c/face width were lower than the energy threshold for unfiltered images. This is what one would expect if face recognition is narrow-band, since band-pass filtered images of optimal centre spatial frequency do not contain unused contrast energy at low and high spatial frequencies. In conclusion, the results suggest that the recognition of facial images is tuned to a relatively narrow band (< 2 octaves) of mid object spatial frequencies.

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A new test chart was developed for the measurement of visual acuity of pre-school children. The symbols of the test are circle, square, apple and house. These were so designed that each symbol measures visual acuity similarly. This feature of the test was verified experimentally. The visual acuity values measured by the individual symbols correlated highly with the visual acuity values measured with the whole test (0.82-0.86). The correlation between the visual acuity values measured repeatedly, the reliability of the new test, was found to be 0.94 for adult subjects. The new visual acuity test thus fulfils the statistical criteria of a good visual acuity test. Because both children and nurses seem to like the new test, it may be useful in the assessment of visual acuity in pre-school children.

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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.