Cardinal axes are not independent in color discrimination.

We measured chromatic discrimination under conditions where the target fields could be distinguished only by the ratio of excitation of the long- (L) and middle-wavelength (M) cones. The excitation level of the short-wavelength (S) cones was varied in the experiments, although for any given measurement the S-cone excitation was common to the two target fields and could not be directly used for discrimination. Adaptation was maintained by a steady neutral background metameric to Illuminant D65. Thresholds varied substantially and systematically with the S-cone level of the target probes, but in a complex way: when the ratio of L:M cone excitation was low, an increase in S-cone excitation reduced the thresholds, but when the L:M ratio was higher, an increase in S-cone excitation raised the thresholds. To account for the pattern of results, we postulate a neural channel that draws synergistic inputs from L and S cones and an opposed input from M cones. The proposed channel has a compressive response function and is most sensitive at the point set by the steady background.

What kind of network is the brain?

The different areas of the cerebral cortex are linked by a network of white matter, comprising the myelinated axons of pyramidal cells. Is this network a neural net, in the sense that representations of the world are embodied in the structure of the net, its pattern of nodes, and connections? Or is it a communications network, where the same physical substrate carries different information from moment to moment? This question is part of the larger question of whether the brain is better modeled by connectionism or by symbolic artificial intelligence (AI), but we review it in the specific context of the psychophysics of stimulus comparison and the format and protocol of information transmission over the long-range tracts of the brain.

Psychological Well-Being and Intra-personal Conflicts in Adolescence.

Background: Adolescence is a period characterized as transitional and as such, it is full of complications and conflicts. Research of Intra-Personal Conflicts in connection with Psycho-Emotional Well-being (PEW) comprising three kinds of indicators: personality, cognitive-evaluative and emotional represents new scientific approach. This approach provides the opportunity to define the role of PEW in Intra-Personal Conflicts: Motivation Value Conflict (MVC) and Self-Estimate Conflict (SEC). Objective: Our aim was to study the severity of MVC and SEC, the interrelationship of these types of conflicts, and their connection with various PEW components. Design: 237 high school students (ages 15-18; 99 boys, 138 girls) were surveyed. Tests of MVC, the Self-Estimate Scale (SE), and the Level of Aspiration Scale (LA) were applied to measure the conflicts. The Scale of Psychological Well-Being, the Scale of Life Satisfaction, and the Dominant Emotional States Test were employed to measure PEW. Results: The study revealed a high prevalence of Intra-Personal Conflicts in the sample. The adolescents all had high levels of Intra-Personal Conflicts; changes were found in all three blocks of PEW. In the group with a high level of MVC, the levels of Environmental Mastery and Self-Acceptance were significantly lower. Having high level of SEC went along with decreases in most indicators of the personal and cognitive-evaluative components of PEW: decreasing of Cheerfulness, Active Attitude to Life Situation and Life Satisfaction; there were changes in emotional blockage, including decreases in Stability and Emotional Tone, and increases in Despondency, Tension, and Anxiety. Conclusion: The study found the prevalence of Intra-Personal Conflicts in the adolescents. We showed that the personality and cognitive-evaluative components of PEW played the role of conflict moderators, while the emotional components were manifested as intra-personal conflict.

Parafoveal color discrimination: a chromaticity locus of enhanced discrimination.

Are boundaries between color categories associated with enhanced discrimination? In the present experiments, chromatic thresholds were obtained for discriminations along lines orthogonal to the yellow-blue axis of color space. The targets were parafoveal and thresholds were measured with a spatial two-alternative forced choice. In interleaved experimental runs, we also obtained empirical estimates of the subjective yellow-blue line by asking observers to categorize colors as reddish or greenish. Both types of measurement were made in the presence of a steady background that was metameric to equal-energy white. In a limited region from desaturated yellow to desaturated blue, an enhanced discrimination is found near the subjective transition between reddish and greenish hues. This line of optimal discrimination is not aligned with either of the cardinal axes of color space: In a MacLeod-Boynton chromaticity diagram, it runs obliquely with negative slope.

Separation in the visual field has divergent effects on discriminating the speed and the direction of motion.

Local motion in a visual scene allows the detection of prey or predator and predicts their future positions. Relative motion segregates objects and reveals their 3D relationships. 'Optic flow' - the motion of texture across the field - guides locomotion and balance. Given these several uses of visually perceived motion, it is unsurprising that many species have evolved hard-wired neural mechanisms to extract motion as a primitive feature of the visual world [1]. In the cortex (e.g. [2-4]), and even the retina [5], of primates, cells are found that respond selectively according to direction of motion. In visual areas V1 and MT, some directionally selective cells are also tuned for the second attribute of motion, speed [3]. It might be thought that the brain derives a single velocity signal from the activity in this population of neurons - since speed and direction must often be combined to predict an object's future position or to derive a 3D structure. However, we report here a striking difference in discrimination of the two attributes: Thresholds for direction, but not those for speed, increase with the spatial separation of the stimuli.

Foveal contour interactions and crowding effects at the resolution limit of the visual system.

We describe several experiments on contour interactions and crowding effects at the resolution limit of the visual system. As test stimuli we used characters that are often employed in optometric practice for testing visual acuity: Landolt C's, Snellen E's, and rectangular gratings. We tested several hypotheses that have been put forward to explain contour interaction and crowding effects. In Experiment 1 and Experiment 2, Landolt C's were the test stimuli, and bars, or Landolt C's, or gratings served as distractors. In Experiment 1, we showed that neither scale invariance nor spatial frequency selectivity is a characteristic of foveal crowding effects. These results allowed us to conclude that mechanisms other than lateral masking contribute to observers' performance in 'crowded' tasks. R. F. Hess, S. C. Dakin, and N. Kappor (2000) suggested that the spatial frequency band most appropriate for target recognition is shifted by the surrounding bars to higher spatial frequencies that cannot be resolved by observers. Our Experiment 2 rejects this hypothesis as the experimental data do not follow theoretical predictions. In Experiment 3, we employed Snellen E's, both as test stimuli and as distractors. The masking functions were similar to those measured in Experiment 1 when the test Landolt C was surrounded by Landolt C's. In Experiment 4, we extended the range of test stimuli to rectangular gratings; same-frequency or high-frequency gratings were distractors. In this case, if the distracting gratings had random orientation from trial to trial, the critical spacing was twice larger than in the first three experiments. If the orientation of the distractors was fixed during the whole experiment, the critical spacing was similar to that measured in the first three experiments. We suggest that the visual system can use different mechanisms for the discrimination of different test stimuli in the presence of particular surround. Different receptive fields with different spatial characteristics can be employed. To explain why crowding effects at the resolution limit of the visual system are not scale invariant, we suggest that a range of stimuli, slightly varying in size, may all be processed by the same neural channel--the channel with the smallest receptive fields of the visual system.

The comparison of spatially separated colours.

We have measured chromatic discrimination as a function of the spatial separation of the stimuli within the visual field. Pairs of stimuli were presented on an imaginary circle of 5 degrees radius and the distance between their centres was varied up to 10 degrees. Stimulus duration was 100 ms. Constructing an analogue of the MacLeod-Boynton diagram for an extra-foveal observer, we made separate series of measurements for the L/(L+M) and S/(L+M) axes of colour space. For both these axes, discrimination was optimal when there was a small spatial interval between the boundaries of the stimuli; thereafter thresholds rose moderately with increasing separation. Nevertheless, even at a separation of 10 degrees , subjects exhibited impressive discrimination, achieving thresholds in the range 0.4-2% on the L/(L+M) axis and in the range 3-6% on the S/(L+M) axis. Even when the two stimuli fell in different hemifields and transmission of information across the corpus callosum was required, accuracy did not differ significantly from that obtained when both stimuli fell within one hemifield. The human ability to compare remote stimuli requires an explanation. We argue that the discrimination is unlikely to depend on hard-wired neural comparators and may depend on neural representations that can be transmitted on a cerebral bus independently of the particular neurons carrying the code. Contrary to earlier reports, chromatic discrimination was not systematically better in the left visual field than in the right. And only one subject showed a significant advantage of the lower hemifield over the upper hemifield.

Complexity of images: experimental and computational estimates compared.

We tested whether visual complexity can be modeled through the use of parameters relevant to known mechanisms of visual processing. In psychophysical experiments observers ranked the complexity of two groups of stimuli: 15 unfamiliar Chinese hieroglyphs and 24 outline images of well-known common objects. To predict image complexity, we considered: (i) spatial characteristics of the images, (ii) spatial-frequency characteristics, (iii) a combination of spatial and Fourier properties, and (iv) the size of the image encoded as a JPEG file. For hieroglyphs the highest correlation was obtained when complexity was calculated as the product of the squared spatial-frequency median and the image area. This measure accounts for the larger number of lines, strokes, and local periodic patterns in the hieroglyphs. For outline objects the best predictor of the experimental data was complexity estimated as the number of turns in the image, as Attneave (1957 Journal of Experimental Psychology 53 221-227) obtained for his abstract outlined images. Other predictors of complexity gave significant but lower correlations with the experimental ranking. We conclude that our modeling measures can be used to estimate the complexity of visual images but for different classes of images different measures of complexity may be required.

The gap effect is exaggerated in parafovea.

In central vision, the discrimination of colors lying on a tritan line is improved if a small gap is introduced between the two stimulus fields. Boynton et al. (1977) called this a "positive gap effect." They found that the effect was weak or absent for discriminations based on the ratio of the signals of long-wave and middle-wave cones; and even for tritan stimuli, the gap effect was weakened when forced choice or brief durations were used. We here describe measurements of the gap effect in the parafovea. The stimuli were 1 deg of visual angle in width and were centered on an imaginary circle of radius 5 deg. They were brief (100 ms), and thresholds were measured with a spatial two-alternative forced choice. Under these conditions we find a clear gap effect, which is of similar magnitude for both the cardinal chromatic axes. It may be a chromatic analog of the crowding effect observed for parafoveal perception of form.

Comparison at a distance.

The visual system is known to contain hard-wired mechanisms that compare the values of a given stimulus attribute at adjacent positions in the visual field; but how are comparisons performed when the stimuli are not adjacent? We ask empirically how well a human observer can compare two stimuli that are separated in the visual field. For the stimulus attributes of spatial frequency, contrast, and orientation, we have measured discrimination thresholds as a function of the spatial separation of the discriminanda. The three attributes were studied in separate experiments, but in all cases the target stimuli were briefly presented Gabor patches. The Gabor patches lay on an imaginary circle, which was centred on the fixation point and had a radius of 5 deg of visual angle. Our psychophysical procedures were designed to ensure that the subject actively compared the two stimuli on each presentation, rather than referring just one stimulus to a stored template or criterion. For the cases of spatial frequency and contrast, there was no systematic effect of spatial separation up to 10 deg. We conclude that the subject's judgment does not depend on discontinuity detectors in the early visual system but on more central codes that represent the two stimuli individually. In the case of orientation discrimination, two naive subjects performed as in the cases of spatial frequency and contrast; but two highly trained subjects showed a systematic increase of threshold with spatial separation, suggesting that they were exploiting a distal mechanism designed to detect the parallelism or non-parallelism of contours.