Local contrast and Pavlovian induction

Two experiments examined the effects of number and similarity of stimuli on local contrast. In the first experiment, local contrast effects differed in magnitude as a function of the similarity among stimuli; greater positive local contrast appeared when stimuli were less similar, though this effect sometimes reversed for very dissimilar stimuli. In the second experiment, both positive and negative local contrast appeared transiently during the course of training a discrimination including two quite dissimilar stimuli. When two new stimuli were added, both effects reappeared in several cases. The effects remained when the discrimination was rendered more difficult by substituting a new stimulus very similar to one of the original pair. These and other data suggest that local contrast depends on the same factors that produce Pavlovian induction; in the absence of an alternative account, Pavlov's interpretation may be useful in suggesting further research that will help identify the mechanisms involved in both classical and operant discrimination learning.     

Local contrast and maintained generalization.

Pigeons received variable-interval reinforcement for key pecking during presentations of horizontal and vertical line-orientation stimuli, while pecks during five intermediate orientations were extinguished. Lowest peck rates were observed during presentations of negative stimuli adjacent to the positive orientations while peck rate during 45 degrees (the intermediate negative orientation) was relatively high, i.e., there were negative contrast shoulders. When peck rates were manipulated in the positive orientations, peck rate in neithboring orientations changed in the opposite direction. Contrast shoulders faded after prolonged training. A second type of contrast, local contrast, was correlated with similarity of preceding stimulus and different average peck rates during different stages of the discrimination process. The data suggest that sequential local contrast accompanying the formation of a discrimination contributes to the form of generalization gradients. Blough's model of stimulus control predicts the changes in gradient form described here, but may not accurately depict the underlying process responsible for gradient form.     

Discrimination of spectrally blended natural images: optimisation of the human visual system for encoding natural images

We have developed a protocol for testing experimentally the hypothesis that the human visual system is optimised for making visual discriminations amongst natural scenes. Visual stimuli were made by gradual blending of the Fourier spectra of digitised photographs of natural scenes. The statistics of the stimuli were made unnatural to varying degrees by changing the overall slopes of the amplitude spectra of the stimuli. Thresholds were measured for discriminating small amounts of spectral blending at different spectral slopes. We found that thresholds were lowest when the spectral slope was natural; thresholds were increased when the slopes were either shallower or steeper than natural. A number of spurious cues were considered, such as differences in mean luminance or overall spectral power or contrast between test and reference stimuli. Control experiments were performed to remove such spurious cues, and the discrimination thresholds were still lowest for stimuli that were most natural. Thus, these experiments do provide experimental support for the idea that human vision and the human visual system are optimised for processing natural visual information [corrected].     

Local contrast in behavior allocation during multiple-schedule components

Allocation of responses between two keys was studied during two alternating multiple-schedule components. Responses were recorded in successive quarters of each component. Variable-interval reinforcer schedules on the two keys were constant throughout the experiment for one (constant) component and were varied over conditions on one key for the other, producing changes in reinforcer ratios for the varied component. Behavior allocation for the first quarter of the constant component was inversely related to varied-component reinforcer ratios, a form of local contrast, but this relationship was not observed later in the component. During the first quarter of the varied component, slopes of matching lines were high and decreased later in the component. It is argued that this form of local contrast cannot be explained in terms of reallocation of extraneous reinforcers between components, and that the matching law for concurrent operants does not capture some sources of control over behavior allocation. A simple extension of the matching law is offered that adequately describes behavior changes during both components. A version of this formulation can predict contrast effects in absolute response rates.     

Local contrast in multiple schedules: the effect of stimulus discriminability.

A three-ply multiple schedule assessed responding in a standard component as a function of the just-preceding schedule. The principal experimental condition was the difference among the wavelengths signaling the schedule components. Only the pigeons working in a narrow wavelength range showed persistent positive local contrast; that is, response rate during the standard component was higher when that component followed extinction than when it followed itself. Birds in both narrow- and medium-range groups showed persistent negative local contrast; that is, rate was lower following a relatively rich component. The dissipation of positive contrast appeared to be most clearly related to the establishment of differential responding. Negative contrast was inversely related to wavelength differences. Theories pertaining to contrast must account for the role of discrimination in both positive and negative types.     

The speed and accuracy of material recognition in natural images

We studied the time course of material categorization in natural images relative to superordinate and basic-level object categorization, using a backward-masking paradigm. We manipulated several low-level features of the images—including luminance, contrast, and color—to assess their potential contributions. The results showed that the speed of material categorization was roughly comparable to the speed of basic-level object categorization, but slower than that of superordinate object categorization. The performance seemed to be crucially mediated by low-level factors, with color leading to a solid increase in performance for material categorization. At longer presentation durations, material categorization was less accurate than both types of object categorization. Taken together, our results show that material categorization can be as fast as basic-level object categorization, but is less accurate.     

Human sensitivity to phase perturbations in natural images: a statistical framework

Fourier-phase information is important in determining the appearance of natural scenes, but the structure of natural-image phase spectra is highly complex and difficult to relate directly to human perceptual processes. This problem is addressed by extending previous investigations of human visual sensitivity to the randomisation and quantisation of Fourier phase in natural images. The salience of the image changes induced by these physical processes is shown to depend critically on the nature of the original phase spectrum of each image, and the processes of randomisation and quantisation are shown to be perceptually equivalent provided that they shift image phase components by the same average amount. These results are explained by assuming that the visual system is sensitive to those phase-domain image changes which also alter certain global higher-order image statistics. This assumption may be used to place constraints on the likely nature of cortical processing: mechanisms which correlate the outputs of a bank of relative-phase-sensitive units are found to be consistent with the patterns of sensitivity reported here.     

Processing of natural images is feedforward: A simple behavioral test

Natural images can be classified so rapidly that it has been suggested that their analysis is based on a first single pass of processing activity through the visuomotor system. We tested this theory in a visuomotor priming task in which speeded pointing responses were performed toward one of two target images containing a prespecified stimulus (e.g., animal vs. nonanimal, ellipse vs. rectangle). Target pictures were preceded by prime pictures of the same or an opposite category, linked to either the same or an opposite pointing response. We found that pointing trajectories were initially controlled by the primes alone, but independently of information in the actual targets. Our data indicate that prime and target signals remained strictly sequential throughout all processing stages, meeting unprecedentedly stringent behavioral criteria for feedforward processing (rapid-chase criteria). Our findings suggest that visuomotor priming effects capture the output of the very first pass of information through the visuomotor system, before output is affected by recurrent information.     

Depth from binocular half-occlusions in stereoscopic images of natural scenes

Over the past two decades psychophysical experiments have firmly established that binocular half-occlusions are useful sources of information for the human visual system. The existing literature has focused on simplified stimuli that have no additional cues to depth, apart from stereopsis. From this large body of work we can be confident that the visual system is able to exploit binocular half-occlusions to aid depth perception; however, we do not know if this signal has any influence on perception when observers view complex stereoscopic stimuli with multiple sources of depth information. This issue is addressed here with the use of stereoscopic images of natural scenes, some of which have been digitally altered to manipulate a major half-occlusion signal. Our results show that depth-ordering judgments for these relatively complex stimuli are significantly affected by the nature of the half-occlusion signal, but only when highly textured surfaces are viewed. Under such conditions, the replacement of a binocular half-occlusion with background texture slows reaction time relative to performance when the occluded region is consistent with the foreground object. This result is specific to conditions when the depth ordering is correct (ie not reversed) and depends upon the size of the half-occlusion. The influence of the half-occlusion information in the presence of potent depth cues such as perspective, texture gradient, shading, and interposition is convincing evidence that this information plays a significant role in human depth perception.     

Spotting animals in natural scenes: efficiency of humans and monkeys at very low contrasts

The ability of monkeys to categorize objects in visual stimuli such as natural scenes might rely on sets of low-level visual cues without any underlying conceptual abilities. Using a go/no-go rapid animal/non-animal categorization task with briefly flashed achromatic natural scenes, we show that both human and monkey performance is very robust to large variations of stimulus luminance and contrast. When mean luminance was increased or decreased by 25–50%, accuracy and speed impairments were small. The largest impairment was found at the highest luminance value with monkeys being mainly impaired in accuracy (drop of 6% correct vs. <1.5% in humans), whereas humans were mainly impaired in reaction time (20 ms increase in median reaction time vs. 4 ms in monkeys). Contrast reductions induced a large deterioration of image definition, but performance was again remarkably robust. Subjects scored well above chance level, even when the contrast was only 12% of the original photographs (≈81% correct in monkeys; ≈79% correct in humans). Accuracy decreased with contrast reduction but only reached chance level -in both species- for the most extreme condition, when only 3% of the original contrast remained. A progressive reaction time increase was observed that reached 72 ms in monkeys and 66 ms in humans. These results demonstrate the remarkable robustness of the primate visual system in processing objects in natural scenes with large random variations in luminance and contrast. They illustrate the similarity with which performance is impaired in monkeys and humans with such stimulus manipulations. They finally show that in an animal categorization task, the performance of both monkeys and humans is largely independent of cues relying on global luminance or the fine definition of stimuli.     

Rapid categorization of foveal and extrafoveal natural images: associated ERPs and effects of lateralization

Humans are fast and accurate at performing an animal categorization task with natural photographs briefly flashed centrally. Here, this central categorization task is compared to a three position task in which photographs could appear randomly either centrally, or at 3.6 degrees eccentricity (right or left) of the fixation point. A mild behavioral impairment was found with peripheral stimuli with no evidence in support of hemispheric superiority; but enlarging the window of spatial attention to three possible stimuli locations had no behavioral cost on the processing of central images. Performance in the central categorization task has been associated with a large difference between the potentials evoked to target and non-target correct trials, starting about 150 ms after stimulus onset on frontal sites. Present results show that this activity originates within extrastriate visual cortices and probably reflects perceptual stimuli differences processed within areas involved in object recognition. Latencies, slopes, and peak amplitudes of this differential activity were invariant to stimulus position and attentional load. Stimulus location uncertainty and lateralization did not affect speed of visual processing.     

Saliency on a natural scene background: Effects of color and luminance contrast add linearly

In natural vision, shifts in spatial attention are associated with shifts of gaze. Computational models of such overt attention typically use the concept of a saliency map: Normalized maps of center-surround differences are computed for individual stimulus features and added linearly to obtain the saliency map. Although the predictions of such models correlate with fixated locations better than chance, their mechanistic assumptions are less well investigated. Here, we tested one key assumption: Do the effects of different features add linearly or according to a max-type of interaction? We measured the eye position of observers viewing natural stimuli whose luminance contrast and/or color contrast (saturation) increased gradually toward one side. We found that these feature gradients biased fixations toward regions of high contrasts. When two contrast gradients (color and luminance) were superimposed, linear summation of their individual effects predicted their combined effect. This demonstrated that the interaction of color and luminance contrast with respect to human overt attention is—irrespective of the precise model—consistent with the assumption of linearity, but not with a max-type interaction of these features.     

Effects of stimulation by three-dimensional natural images on prefrontal cortex and autonomic nerve activity: a comparison with stimulation using two-dimensional images

Empirical evidence suggests that three-dimensional (3D) images of nature promote physiological relaxation in humans by providing more realistic effects compared with two-dimensional (2D) images. However, no studies have evaluated the physiological relaxation effects of nature-derived 3D images on prefrontal cortex and autonomic nerve activity. The present study aimed to clarify the physiological relaxation effects of visual stimulation by 3D flower images on prefrontal cortex and autonomic nerve activity. Nineteen male university students (22.2 ± 0.6 years) were presented with 3D and 2D images of the water lily for 90 s. Prefrontal cortex activity was measured using near-infrared spectroscopy, while autonomic nerve activity was measured using heart rate variability (HRV). Psychological effects were determined using a modified semantic differential method (SD). Compared with visual stimulation by 2D images, that by 3D images resulted in a significant decrease in oxyhemoglobin concentration in the right prefrontal cortex, lower sympathetic activity as calculated by the ratio of the low-frequency to high-frequency HRV component, and a significantly greater realistic feeling as evidenced by higher SD ratings. In conclusion, visual stimulation by realistic 3D floral images promotes physiological relaxation more effectively than the corresponding 2D image.     

Psychological and stress hormone correlates in early life: a key to HPA-axis dysregulation and normalisation

Substantial recent research has focused on examining hormone indicators of psychosocial stress and on how relationships between stress and hormone changes might be linked to chronic illness. Particular attention has been paid to disease progression in cancer and HIV/AIDS. This focus has generated a plethora of research which has contributed both theoretically and clinically to the understanding of disease experience and the rate of disease progression. Measurement of salivary cortisol levels and diurnal variation has substantially advanced research methodology. Applying the unifying concept of allostasis and accumulated lifetime stress, this review attempts to assess the relevance of psychological and stress hormone correlates to disease resistance and health, through an examination of such correlates on the experience and outcomes of stress during childhood. Focus is on the role and importance of naturalistic social stress experiences such as school transition in healthy children, with emphasis on salivary cortisol as an endocrine marker of HPA-axis activation. It is argued that differing research perspectives offer valuable insight into the often assumed but largely unexplored links between early life experience and subsequent physical health outcomes in adulthood. Longitudinal studies incorporating measures of acute physical health outcome and of learning and memory are clearly needed.     

ERP responses to images of abstract artworks,photographs of natural scenes,and artificially created uncomfortable images

The idea of efficient coding in the visual brain allows for predictions for the processing of various types of images, including certain artworks, natural images and uncomfortable images. Efficient processing is thought to result in lower responses compared to less efficient processing. The efficiency of the processing is suggested to depend on the architecture of the visual system and the properties of the input image. In this study, neural responses were estimated using EEG across the categories of a set of five images of abstract artworks, a set of five photographs of natural images and a set of five computer-generated uncomfortable images. EEG responses to contrast-matched images were found to be lower for the set of five abstract artworks used in the study compared to the set of photographs of natural images, lending preliminary support for the idea that certain abstract artworks, for example the work of Pollock, may be processed efficiently.     

Behavioral contrast: Pavlovian effects and anticipatory contrast.

Two sources of behavioral contrast have been identified previously: Pavlovian stimulus-reinforcer relations and component sequence effects (anticipatory contrast). This study sought to isolate these sources of control procedurally in a four-ply multiple schedule composed of two fixed two-component sequences. Different cues were associated with the first component of each sequence, and contrast effects were studied in these target components. In Experiment 1, differential cuing of Component 2 between sequences and availability of reinforcement during target components were varied across three groups of pigeons; the stimulus-reinforcer relation between target-component cues and schedule of reinforcement in Component 2 was varied within subjects. Control by the Pavlovian relation was demonstrated under all conditions, and anticipatory contrast was not observed. In Experiment 2, target-component duration was systematically varied in the three groups of Experiment 1. Control by the Pavlovian relation was reliably obtained only when target-component behavior was unreinforced, and diminished with increases in component duration. Anticipatory contrast emerged in the two groups for which target-component reinforcement was available. These and other data indicate that Pavlovian effects in multiple schedules may be obscured when the requisite conditions for anticipatory contrast are present.     

Automatic detection of bunches of grapes in natural environment from color images

Despite the benefits of precision agriculture and precision viticulture production systems, its rate of adoption in the Portuguese Douro Demarcated Region remains low. We believe that one way to raise it is to address challenging real-world problems whose solution offers a clear benefit to the viticulturist. For example, one of the most demanding tasks in wine making is harvesting. Even for humans, the environment makes grape detection difficult, especially when the grapes and leaves have a similar color, which is generally the case for white grapes. In this paper, we propose a system for the detection and location, in the natural environment, of bunches of grapes in color images. This system is able to distinguish between white and red grapes, and at the same time, it calculates the location of the bunch stem. The system achieved 97% and 91% correct classifications for red and white grapes, respectively.