Factors limiting large-scale localisation

Selective attention to specific location cues: the peak and center of a patch are equally accessible as location cues

Asymmetric patterns have several spatially distinct cues for spatial localization. These cues include the peak of the luminance distribution, the centroid of the contrast distribution, zero-crossings in the second derivative of the luminance profile, and the midpoint of the visible area. If these cues are represented as primitives in the visual system, the observer should be able to access them at will. To examine whether observers can selectively attend to particular cues, we measured perceived alignment for an asymmetric pattern with two distinct instructions: "align the peak", and "align the center". We found that observers could align the patterns in accord with the instructions with identical precision, suggesting that the peak and the center cues were equally accessible by the observer. We conclude that multiple localization cues are represented in and can be selectively accessed by the visual system.

Vernier acuity with non-simultaneous targets: the cortical magnification factor estimated by psychophysics

The eccentricity at which peripheral thresholds double their foveal value (E2) may relate to the visual system's anatomical organization. Using a variety of experimental approaches, previous estimates of E2 for vernier acuity have ranged from less than 0.1 deg to greater than 15.0 deg. This broad range of values seems to challenge the usefulness of E2 for determining visual topography. We explain that the varying contributions from at least two different regimes, spatial filter and local sign, may explain the broad range of E2 values found previously. We attempt to limit responses to the local sign regime, where it may be possible to determine the psychophysical analog to the gradient of the cortical spatial grain. In our experiments we measure how vernier task performance falls off with eccentricity. We hypothesize that if the vernier features are adequately separated in time, they will fall outside of the spatial filter's temporal integration span and the local sign regime would then predominate for precise positional processing. Using an interstimulus interval ranging from 20 to 200 msec between the two vernier features, we estimate that vernier thresholds in the local sign regime double at about 0.8 +/- 0.2 deg eccentricity, which is similar to anatomical estimates of the eccentricity at which the linear spacing of human cortical units doubles.

Localization of a peripheral patch: the role of blur and spatial frequency

Peripheral vision serves to direct our attention and fixation to objects of interest. This requires that the visual system be capable of accurately localizing peripherally presented targets having different spatial structures. The question we address is "to what extent does stimulus spatial structure influence the precision of peripheral localization?" To address this issue, we measured the precision of spatial localization (with reference to a foveal target) for a single Gaussian or Gabor patch briefly presented in the periphery. For both stimuli, we find that when the standard deviation of the stimulus envelope (SD) is less than 1/5 the stimulus eccentricity, localization thresholds are independent of SD and are approximately 1/50 of eccentricity. For larger values of SD, localization thresholds increase linearly with increasing SD, and are approximately 1/5 of SD. The results hold over a range of eccentricities (from 2.5 to 10 deg) and stimulus contrasts (from near detection threshold to 80%). In addition, for Gabor patches, the results are independent of frequency, phase and orientation of the carrier.