Observers searched for a horizontal line segment through displays containing varying numbers of elements differing from the target and from each other in terms of orientation. These elements were always positioned on imaginary concentric circles centered in the middle of the display. They were allocated to these positions either randomly or in such a way that their orientation was equal to that of the tangent to the circle at that position. The search for the target line appeared to proceed spatially in parallel with the latter class of displays, and serially with the former. These findings are explained and discussed within the context of the attentive-preattentive dichotomy that characterizes spatial vision.

Conditions under which binocular unmasking (BU), as an analogue of binaural unmasking, occurs have been explored. Observers were to detect through a stereoscope a Gabor signal in patches of two-dimensional broadband gaussian noise surrounded by a frame of uniform noise. The right-eye gaussian field was displaced relative to the left eye so that it appeared either in front of or behind the frame. Performance when signal disparity was equal to that of the noise--a condition functionally equivalent to monocular processing--was compared to that obtained when signal disparity was zero--a case in which BU should occur. Enhanced signal detectability of up to 12 dB and of nearly constant magnitude was observed in the latter condition when uncrossed disparities of up to 67.60 min visual angle and display durations of 1 s were employed. Signal detectability declined appreciably with increasing disparity (both crossed and uncrossed) when display duration was reduced to 90 ms, thus preventing the occurrence of compensatory vergence eye movements. It is suggested that BU effects may result from a process of linear summation of monocular inputs.

This study investigated the effects of age on binocular unmasking. This term denotes the fact that a visual signal embedded in noise is detected appreciably better when the stimulus complex contains interocular cues (dichoptic condition) than when such cues are absent (binoptic condition). Detection thresholds for two Gabor signals differing in spatial frequency were determined in young and old adults with no identifiable ocular pathologies. The signals were embedded, in both conditions, in two-dimensional Gaussian noise. Binocular Masking Level Differences, defined as the difference between the binoptic thresholds and the dichoptic thresholds, did not change with age; however, the older adults showed higher binoptic thresholds with both signals and higher dichoptic thresholds with only the lower-spatial-frequency signal. For both groups, binoptic and dichoptic thresholds increased with spatial frequency. The implications of these results are discussed.

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In this paper we consider the processes by which observers can detect and recognize signals embedded in natural scenes and images. Although our results do not strongly support detection processes based strictly upon the cross-correlation (template-matching) of the signal and image luminance profiles, they do support a version of cross-correlation based upon the comparison of their "structural similarities." This latter correlational measure may be separated from the energy component in the cross-correlation function. Of particular importance to the structural similarity between the signal and image is their characteristic edge components, and we also show that recognition performance was consistent with the cross-correlations between the signal and image edge-only versions. This finding, again, exemplifies the importance, for the analysis of pattern detection and recognition, of isolating structural components from energy detection processes per se. Although the task of detecting a signal embedded in an image is a common one for the human visual system, the spatial determinants of such a task are rarely studied, particularly with naturally occurring images. Exceptions to this are the investigations of Julesz (l98l, 1985; Julesz & Bergan, 1983), who studied the conditions for detecting specified micropatterns in textures, and Burgess and Ghandeharian (1984a, 1984b), who determined the efficiency of observers in detecting signals embedded in noise. According to Julesz, such "attentional" processes are dependent on the ability of the human visual system to differentiate among specific spatial codes (e.g., orientation, aspect ratios, etc.), whereas, for Burgess and Ghandeharian, detection of signals in noise is adequately determined by simple cross-correlation, or matched filtering, between the signal and signal-plus-noise image. Although it seems obvious that simple energy-detection models, based on the direct perceptual cross-correlation of the input luminance profiles, are inadequate in attempting to explain all types of pattern recognition behavior, some form of correlation seems necessary for any reasonably broad model for this task. It is therefore of central interest to this study to consider the type of spatial information that is cross-correlated in such tasks when more natural images are used. According to Burgess and Ghandeharian's (l984a, 1984b) formulation, the ideal observer, using a maximum