On the separability of two mechanisms involved in the detection of grating patterns in humans.

Abstract

SU1MMARY1. The delectability of contrast modulation (M) of sinusoidal gratings was explored at the rate of 8 Hz. The luminance profile of a contrast modulated sinusoidal grating is L = L(1 + C cos 27rFx). This stimulus may also be regarded as the sum of a steady grating pattern and counterphase flicker of the same spatial frequency.2. Contrast modulation sensitivity (1/M) was established in five observers at several levels of contrast and over a range of spatial frequencies, where M = AC/C of AC is the just detectable contrast change and C is the mean contrast of the grating. The slope of a modulation sensitivity function (C/AC v8. C) is 1 (i.e. AC = constant) near threshold contrast at each spatial frequency, but in the suprathreshold contrast range the slope flattens from close to 1 at 1.5 c/deg to almost 0 (AC/C = constant) at 12 c/deg.3. Adaptation to a high contrast steady grating of the same spatial frequency as the contrast modulated test gratings shifts each modulation sensitivity function to the right at low contrasts, but not at high. As a result the adapted curves cross their corresponding unadapted ones. At each spatial frequency the modulation sensitivity function is now fitted by a straight line of slope 1. While AC needs to be higher than half the detection threshold of the same grating at spatial frequencies above 3 c/deg, in the adapted condition the values are nearly equal at each frequency. Thus pattern adaptation unmasks the threshold of the counterphase component of the contrast modulated grating near threshold contrast as well as above it. The phase of the steady adapting grating, relative to the steady component of the test grating, does not make any difference.4. Apparently contrast modulation reveals differences beyond threshold sensitivity between spatial frequencies adjacent to the peak of the contract sensitivity curve. For each spatial frequency channel there must be different neural coupling between steady and modulated inputs. Electrophysiological studies using -contrast modulated gratings would be useful in the exploration of individual and ensemble properties of neurones of the visual cortex.

Local and global attention are mapped retinotopically in human occipital cortex

“…For one thing, the spatial frequency spectrum of the local and global features was largely overlapping-much more than one might expect from simply measuring the size of the two kinds of targets. Thus it is quantitatively difficult to suppose that the spatial frequency components required for local and global target detection stimulated different populations of neurons, even if partially segregated spatial frequency ''channels'' do exist in human visual cortex (41,42).…”

Cortical contrast gain control in human spatial vision.

“…1). Bodis-Wollner & Hendley (1979) found that near threshold contrast, sensitivity to this modulated contrast is independent of mean contrast over a range of spatial frequencies. At suprathreshold contrast levels, however, and at high spatial frequencies, contrast modulation detection depends on mean contrast.…”

“…2iT where C = contrast, L = mean luminance, F = spatial frequency, M = depth of modulation, x = distance horizontally, C(t) = instantaneous contrast, w/27r = temporal frequency in cycles per degree and C = mean contrast (Bodis-Wollner & Hendley, 1979). Figure 1 Fig.…”

“…This double-slit aperture ensured that the photocell sampled only input to about 2 mm of the screen of the CRT. The output voltage of the photodiode was measured and the contrast was calculated from these readings (Bodis-Wollner & Hendley, 1979), as detailed below. This procedure was followed for the Joyce oscilloscope and for the two Tektronix monitors.…”

“…by spatial contrast) (Shapley & Enroth-Cugell, 1984). Bodis-Wollner & Hendley (1979) used a contrast-modulated grating to explore the effect of mean contrast on the detection of contrast changes. This stimulus consists of a pattern whose deviation from the mean luminance is a product of a function of space and a function of time (see Fig.…”