The effects of the contrast, spatial frequency, and temporal frequency of a surrounding field upon the detection thresholds of gratings

Abstract

Three experiments, in which the temporal and spatial characteristics of square-wave gratings surrounding a central test field were varied, are reported. The detection thresholds of I-sec presentations of a 5-Hz counterphase flickering .5-cycle/deg (cpd) sinusoidal grating were measured under the different surround conditions. Threshold was found to increase with increasing surround contrast, and to be confined to surround spatial frequencies of2 cpd and below. Maximum threshold elevation occurred with surround drift frequencies at about 8 Hz, irrespective of spatial frequency.It was concluded that the surround effect is probably due to an inhibitory interaction between transient-type mechanisms in the central visual field and the pheriphery.It has recently been shown that sensitivity to a central low-spatial-frequency test grating is reduced by a moving grating or counterphase flickering blank field surrounding it. The reduction in sensitivity is confined to test spatial frequencies lower than about 2 cycle/deg (cpd) (Bowling, 1985;Green, 1983). The effect occurs both with counterphase flickering test gratings and with briefly pulsed presentations of the test stimuli. Bowling hypothesized that the surround effects would occur only in situations in which "transient" mechanisms were stimulated, since it was thought that temporal modulation of the surround altered the responsiveness of mechanisms with this type of time-course. 1 Flickering and pulsed presentations oflow-spatial-frequency gratings appear strongly to stimulate these mechanisms. The pronounced reduction in sensitivity to low-spatial-frequency flickering or pulsed gratings thus supported the hypothesis. However, Bowling (1985) also observed a low-spatial-frequency sensitivity reduction when the test presentation was a f-sec cosine temporal envelope, indicating that the same mechanism is perhaps responsive to low spatial frequencies under both flickering and slow onset conditions. The nature of the surround was also shown to have some influence on sensitivity reduction. A flickering blank field, and a moving low-frequency square-wave grating, both temporally modulated at 6.8 Hz, produced similar effects, but a greater reduction in sensitivity was observed when the orientations of the test and surround gratings were the same than when they differed by 90°. Consequently, it appears that although the sensitivity reduction is partly produced by the temporal modulation of the surround, This experimental work was made possible by the award of an Australian Research Grants Service grant to the author. The contribution of Daveena Brain in serving as subject is also gratefully acknowledged.