No abstract

Humans are remarkably adept at recognizing objects across a wide range of views. A notable exception to this general rule is that turning a face upside down makes it particularly difficult to recognize. This striking effect has prompted speculation that inversion qualitatively changes the way faces are processed. Researchers commonly assume that configural cues strongly influence the recognition of upright, but not inverted, faces. Indeed, the assumption is so well accepted that the inversion effect itself has been taken as a hallmark of qualitative processing differences. Here, we took a novel approach to understand the inversion effect. We used response classification to obtain a direct view of the perceptual strategies underlying face discrimination and to determine whether orientation effects can be explained by differential contributions of nonlinear processes. Inversion significantly impaired performance in our face discrimination task. However, surprisingly, observers utilized similar, local regions of faces for discrimination in both upright and inverted face conditions, and the relative contributions of nonlinear mechanisms to performance were similar across orientations. Our results suggest that upright and inverted face processing differ quantitatively, not qualitatively; information is extracted more efficiently from upright faces, perhaps as a by-product of orientation-dependent expertise.

Perceptual discrimination improves with practice. This 'perceptual learning' is often specific to the stimuli presented during training, indicating that practice may alter the response characteristics of cortical sensory neurons. Although much is known about how learning modifies cortical circuits, it remains unclear how these changes relate to behaviour. Different theories assume that practice improves discrimination by enhancing the signal, diminishing internal noise or both. Here, to distinguish among these alternatives, we fashioned sets of faces and textures whose signal strength could be varied, and we trained observers to identify these patterns embedded in noise. Performance increased by up to 400% across several sessions over several days. Comparisons of human performance to that of an ideal discriminator showed that learning increased the efficiency with which observers encoded task-relevant information. Observer response consistency, measured by a double-pass technique in which identical stimuli are shown twice in each experimental session, did not change during training, showing that learning had no effect on internal noise. These results indicate that perceptual learning may enhance signal strength, and provide important constraints for theories of learning.

Discriminating the direction of motion of a low-contrast pattern becomes easier with increasing stimulus area. However, increasing the size of a high-contrast pattern makes it more difficult for observers to discriminate motion. This surprising result, termed spatial suppression, is thought to be mediated by a form of center-surround suppression found throughout the visual pathway. Here, we examine the counterintuitive hypothesis that aging alters such center-surround interactions in ways that improve performance in some tasks. We found that older observers required briefer stimulus durations than did younger observers to extract information about stimulus direction in conditions using large, high-contrast patterns. We suggest that this age-related improvement in motion discrimination may be linked to reduced GABAergic functioning in the senescent brain, which reduces center-surround suppression in motion-selective neurons.

The sensory information specifying objects is often optically incomplete: Objects occlude parts of themselves and other objects. However, people rarely experience difficulty in perceiving complete 3-dimensional forms. This report describes a paradigm for the objective study of completion effects and their microgenesis. A well-known priming method was extended to determine whether a partly occluded object produces priming effects more like one of its possible interpretations than like another. For prime durations > 200 ms, a partly occluded object produces priming effects equivalent to its complete interpretation. The results provide objective evidence for the perceptual completion of partly occluded objects and imply that the complete interpretation develops over time, possibly proceeding through a preliminary mosaic interpretation. The theoretical implications for visual perception and cognition are discussed.We can easily imagine the following scene being enacted in court:Counsel: "Where was the book?"

To better understand how the visual system makes use of information across spatial scales when identifying different kinds of complex patterns, we measured human and ideal contrast identification thresholds to estimate identification efficiency for 1- and 2-octave wide band-pass filtered letters and faces embedded in 2-D dynamic Gaussian noise. Varying stimulus center frequency from 1 to 70 c/object had different effects on letter and face identification efficiency. In the 2-octave conditions, identification efficiencies decreased by 0.25-0.5 log units for letters and 0.5-1.2 log units for faces as center frequency increased from 6.2 to 49.5 c/object, but only letters were identifiable at center frequencies below 6.2 c/object. In the 1-octave conditions, letter identification efficiencies increased by about 0.5 log units as center frequency increased from 1.1 to 2.2 c/object, and were nearly constant from 2.2 to 35 c/object. Letters were unidentifiable by human observers at 70 c/object. Surprisingly, face identification was impossible for human observers at all center frequencies except 8.8 c/object for one observer, and 8.8 and 17.5 c/object for a second observer. Ideal observer thresholds were obtained for both letters and faces in all conditions, so information was always available to perform the task. Thus, the failure to identify faces reflects constraints on visual processing rather than a lack of stimulus information. Selective spatial sampling may account for some of the differences between letter and face identification efficiencies.

Random dot cinematograms were used to probe motion perception in human observers ranging from 23 to 81 years of age. Stimuli were either broadband directional Noise, which produces no experience of global motion flow, or a narrower band directional Signal, which tended to produce experiences of coherent, global direction flow. On each trial, subjects rated their certainty that a Signal had been presented, and used a computer mouse to indicate the direction of perceived global flow. At all ages, sensitivity to motion and accuracy of perceived direction improved significantly as stimulus duration increased from 75 to 470 ms. However, older subjects (>70 years of age) were significantly less sensitive to motion, and were significantly less accurate at identifying the direction of movement. A control experiment, which found that older subjects accurately perceived and remembered the orientation of a line, ruled out the possibility that the observed deficits in motion perception were due to an inability on the part of older subjects to manipulate the computer mouse. Those control results also showed that both younger and older observers maintained robust visual representations over durations ranging from .24 to 6.0s. The motion detection and identification results obtained from subjects less than 70 years of age were well fit by a simple multichannel model of motion, although different levels of additive internal noise were needed to fit detection data and direction-identification data, suggesting that motion direction and identification are constrained by different mechanisms. To fit the data from the oldest subjects, however, the values of model parameters had to be significantly altered, either by increasing the level of additive internal noise substantially, or by a smaller increase in noise coupled with an increase in the bandwidth of the model's directionally selective channels. These results are qualitatively consistent with recent neurophysiological studies showing weaker directional selectivity and higher spontaneous noise in visual neurons of senescent monkeys and cats.

The visual system is constantly faced with the problem of identifying partially occluded objects from incomplete images cast on the retinae. Phenomenologically, the visual system seems to fill in missing information by interpolating illusory and occluded contours at points of occlusion, so that we perceive complete objects. Previous behavioural [1] [2] [3] [4] [5] [6] [7] and physiological [8] [9] [10] [11] [12] studies suggest that the visual system treats illusory and occluded contours like luminance-defined contours in many respects. None of these studies has, however, directly shown that illusory and occluded contours are actually used to perform perceptual tasks. Here, we use a response-classification technique [13] [14] [15] [16] [17] [18] [19] [20] to answer this question directly. This technique provides pictorial representations - 'classification images' - that show which parts of a stimulus observers use to make perceptual decisions, effectively deriving behavioural receptive fields. Here we show that illusory and occluded contours appear in observers' classification images, providing the first direct evidence that observers use perceptually interpolated contours to recognize objects. These results offer a compelling demonstration of how visual processing acts on completed representations, and illustrate a powerful new technique for constraining models of visual completion.