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1. We confirm Keesey's (1972) observation that, when a flickering line is viewed, there are distinct thresholds for detecting flicker (or movement) and for detecting a well localized line (pattern detection). Our measurements of the temporal sensitivity of these two mechanisms are similar to Keesey's. 2. The flicker and pattern detection mechanism have been analysed using subthreshold summation, i.e. by observing the effect of subthreshold flickering stimuli (lines and gratings) on the contrast threshold for a flickering test line. 3. The pattern detector shows linear spatial summation of contrast while the flicker detector is non-linear in this respect. 4. The receptive field of the (most sensitive) flicker detector is about two to four times broader than that of the pattern detector. 5. The flicker detector has relatively weak surround inhibition and so, unlike the pattern detector, it is sensitive to a uniform flickering field. 6. The spatial arrangement of the pattern detector is the same at all temporal frequencies (including steady presentation); for flicker detection, the width of the receptive field increases with temporal frequency and the strength of lateral inhibition decreases at high frequencies. 7. Flicker detectors of various widths were demonstrated by using different test stimuli (for 12 Hz modulation); surround ingibition was relatively weak for the broadest detector. 8. There is a delay of surround inhibition of about 3 ms for both flicker and pattern detection. 9. By using a broad test stimulus modulated at a high frequency, a detector can be found with no significant surround inhibition. At threshold, this stimulus produces a sensation of flicker without the appearance of lateral motion observed for finer test lines at lower frequencies. 10. The characteristics of pattern and flicker (movement) detection are compared to electrophysiological studies on X (sustained) and Y (transient) neurones respectively, and correlations are described for studies of temporal frequency response, non-linearity, width of receptive field, strength of the inhibitory surround and motion sensitivity.