PubMed · 1408155
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A Bradley, L Thibos, Y Z Wang, K Haggerty, A Poorman. 1992. Imaging FWC.. https://doi.org/10.1111/j.1475-1313.1992.tb00272.x
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The detection of small radially symmetric targets was studied using a subthreshold summation paradigm. Small disc and disc-like patterns with diameters up to 0.60 were used for superposition on Bessel functions of zero order, subthreshold contrast and various spatial frequencies. Contrast interrelation functions prove linear over the whole range of contrasts used for the Bessel functions while their slopes show systematic variation with spatial frequency. An extrapolation of sensitivity from the slopes reveals that sensitivity can be predicted by a simple model assuming detection to be mediated by a transfer function made up as a cascade of an even bandpass function and the disc pattern spectrum, as has been found previously using one dimensional luminance distributions. Problems concerning the formation of pattern-specific radial symmetric filters are discussed.
Interactions between motion sensors tuned to the same and to opposite directions were probed by means of measuring summation indexes for sensitivities (d') to contrast increments and/or decrements applied to drifting gratings presented in binocular and in dichoptic vision. The data confirm a phenomenon described by Stromeyer, Kronauer, Madsen & Klein (1984, J. Opt. Soc. Am. A 1, 876-884), whereby opposite polarity contrast changes applied to binocular gratings drifting in opposite directions yield sensitivities significantly higher than same sign changes for which performance complies with probability summation (PS). The effect disappears in dichoptic vision where opposite sign contrast changes yield a performance close to, or below PS, whether they are applied to same or to opposite direction stimuli. Same sign changes in dichoptic drifting stimuli yield a performance higher than PS independently of their relative directions and close to the performances obtained when these same sign changes are applied to dichoptic, static +/- 45 degrees gratings. Opposite sign changes applied to such static gratings yield PS. The data support the view according to which: (i) motion direction is extracted at the monocular site; (ii) motion sensors exhibit mutual inhibition within each eye when tuned to opposite directions; and (iii) binocular summation when tuned to the same direction. The data also suggest that (iv) independently of their directional tuning, all motion sensors converge on a binocular, motion non-specific ('flicker') unit; and that (v) signals carried by ON and OFF pathways are slightly inhibitory to each other.
Orientation detection and discrimination thresholds were measured for Gabor 'envelopes' formed from contrast-modulation of luminance 'carriers'. Consistent with previous research differences between carrier and envelope orientation had no effect on sensitivity to envelopes. Using plaid carriers in which the proportion of contrast modulation 'carried' by each plaid component was systematically manipulated, it was shown that this tolerance to carrier-envelope orientation difference reflects linear summation across orientation indicative of a single second-stage channel coding for contrast-defined structure. That contrast envelopes did not exhibit linear summation across spatial-frequency, nor across combinations of orientation and spatial-frequency differences, suggests that these second-order channels operate only within certain spatial scales. Using arrays of Gabor micropatterns as carriers in which the orientation distribution of the carriers was manipulated independently of the difference between envelope orientation and mean carrier orientation, it was further demonstrated that the locus of orientation integration must occur prior to envelope detection. In the context of two-stage models that incorporate a non-linearity between the stages, the pattern of results obtained is consistent with the operation of an orientation pooling process between first-stage and second-stage channels, analogous to having all filters of the first-stage feed into all filters of the second-stage within the same spatial-frequency band.