Previous work suggests there are mechanisms at intermediate levels of visual processing specialized for the detection of radial and circular form. The evidence in favor of specialized global form mechanisms is derived from structure detection experiments that have told us very little about their bandwidth or number. To address these related questions, we examined the effects of configural backward masking on human observers' ability to detect global structure in arrays with different spiral forms. Each array consisted of 100 Gabors randomly positioned within a circular annular window. Observers judged which of two sequentially presented Gabor arrays contained global structure. One array contained Gabors with random orientations; the other contained Gabors with a variable proportion of orientations coherent with a randomly chosen spiral pitch. At its offset, each array was immediately followed by a backward masking Gabor array with a fixed spiral pitch angle. When mask and test had the same spiral pitch, we found an approximately three-fold elevation of structure detection thresholds that was not explained by local orientation masking. The magnitude and breadth of tuning around each masking angle was predicted by a simple model consisting of at least eight detectors broadly tuned for different spiral forms.

*Apple Macintosh G5*using custom software written in Python (Peirce, 2007). We presented the stimulus arrays on a

*Vision Master Pro 454*monitor at a resolution of 1,024 × 768 pixels, refresh rate of 100 Hz, and viewing distance of 70 cm. Each array (generated anew prior to each trial) consisted of 100 Gabors randomly positioned inside an annular window (outer diameter 10°, inner diameter 1°) at non-overlapping positions on a uniform background (luminance 95 cd/m

^{2}). Each Gabor (Michelson contrast 0.9, sinusoidal carrier frequency 6 cycles/°, circular Gaussian envelope SD 0.166°) was in sine phase and assigned an orientation consistent with a spiral pitch angle (Figure 1a). Gabors that fell outside the window were redrawn inside at a random location.

*y*is the proportion of correct trials,

*α*is an estimate of threshold at 75% correct, and

*β*is related to the slope of the function.

*) for each masking condition. This function has the form:*

_{i}*M*is the baseline, Amp is the amplitude, and

*σ*is the bandwidth of each masking function.

*i*th mechanism to a Gabor array with pitch angle

*θ*and coherence

*c*was defined by a Gaussian function of the form:

*θ*

_{I}is the mechanism's preferred pitch angle and

*σ*is the standard deviation of the tuning curve. Separate Monte Carlo simulations were run with (i) two mechanisms with preferences for radial (0 deg) and circular (90 deg) structure; (ii) four mechanisms (0, 45, 90, and 135 deg); and (iii) eight mechanisms (0, 22.5, 45, 67.5, 90, 112.5, 135, and 147.5 deg).

*w*is a weighting constant that sets the overall magnitude of the of masking effect (fixed at 0.35). In rare instances where the weighted response to the mask exceeded that to the stimulus, a normalized response of zero was assigned.

*N*) elicited in response to a stimulus was given by:

*N*

_{rest}and

*N*

_{peak}are resting (fixed at 5) and peak (fixed at 20) responses, respectively. The probability of

*n*spikes from a mechanism on a given stimulus presentation was defined by a Poisson probability density function with mean value

*N,*such that:

*I*detectors, the log likelihood of each potential stimulus was calculated (see Jazayeri & Movshon, 2006):

*SD*, range: 14–20.71°).

*σ*= 29, 27, and 12 deg for 2, 4, and 8 detectors, respectively). The results of these simulations are shown in Figure 4c. Clearly, predictions of the two cardinal detectors model do not provide an accurate description of the masking functions. In particular, the model erroneously predicts a broadening of masking functions at intermediate mask pitch angles (e.g., 45 deg). Overall, the cardinal model predictions provide a poorer fit of the empirical data set than a simple straight line through the mean (

*R*

^{2}= −0.39). The model consisting of four detectors provided a closer approximation of these data (

*R*

^{2}= 0.42), although some broadening of masking functions remains at angles falling between detector preferences (22.5 and 67.5 deg). The model consisting of eight detectors accounted for the largest proportion of variance in the pattern of tuning we observed across the spiral form dimension (

*R*

^{2}= 0.78).