Neural recoding in human pattern vision: model and mechanisms.
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We describe a model of neural recoding in spatial vision that specifies how the outputs of selected units akin to VI cells are normalized and combined to signal information about particular stimulus attributes. The recoding portion of the model is linked to psychophysical behavior via a two-stage signal-detection decision module that specifies how the outputs of the combining mechanisms are used in making fine spatial discriminations. We describe how masking and cue summation experiments isolate each of the processing stages, how earlier results from such studies guided development of the model, and we demonstrate how these procedures permit empirical estimates of model parameters as well as tests of alternative formulations. An important part of our work describes the characteristics of two complementary types of higher-level mechanisms isolated from previously published discrimination data. One sums normalized primary-level responses over disparate frequencies to signal precise information about the orientation of a stimulus; the other sums over all orientations to signal the spatial grain of texture-like patterns. We demonstrate how the model accounts for a large body of previously published discrimination data, and present the results of a new quantitative test of model predictions. (+info)
Noisy templates explain area summation.
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The noisy template model is a variant of an ideal detector for a signal known except for contrast. The ideal detector cross-correlates the stimulus with a normalised template which is matched to the known signal pattern. The noisy template model simply adds noise to the matched template every time it is cross-correlated with the signal. This paper outlines the predictions of the noisy template model for area summation. The noisy template model explains Piper's Law, as does the ideal-observer, but it also explains critical area phenomena and the lack of area summation for contrast discrimination. (+info)
Adolescents' pregnancy intentions: relations to life situations and caretaking behaviors prenatally and 2 years postpartum.
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PURPOSE: This study explores if and how adolescents' pregnancy intentions relate to life situations and health-related behaviors prenatally and up to 2 years postpartum. METHODS: Adolescent girls who reported that they had "wanted a baby" (n = 75) as their reason for pregnancy were compared with those who reported that the pregnancy "just happened" (n = 79) at four separate time periods: prenatally, at 6 and 24 months postpartum, and at 18 months postpartum for teens who became pregnant again subsequent to the study pregnancy. RESULTS: Those who stated that they wanted a baby were more likely to be Hispanic, married, and out of school before becoming pregnant. They were less likely to receive welfare as their primary means of support and to have run away from home in the past than teens who stated that their pregnancy just happened. Self-reported reason for pregnancy was unrelated to repeat pregnancy by 18 months postpartum, but those who had wanted the study baby were less likely to undergo elective termination of a subsequent pregnancy and less likely to become pregnant by a different partner. The groups diverged at 24 months postpartum when those who wanted a baby were more likely to be married to the father of the baby, be financially supported by him, receive child care assistance from him, and have attempted or succeeded at breastfeeding the study child. CONCLUSION: Self-reported reason for pregnancy reveals many important characteristics of pregnant adolescents both at the time of presentation and up to 2 years postpartum. Young women in this study who reported intentional pregnancy seem to fare better with regard to their financial status and their relationship with the father of the baby. (+info)
Evidence for the stochastic independence of the blue-yellow, red-green and luminance detection mechanisms revealed by subthreshold summation.
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We investigated the manner in which the outputs of the three postreceptoral mechanisms (red-green, blue-yellow and luminance) combine to determine contrast threshold. We used a subthreshold summation paradigm to test whether the combination of the postreceptoral mechanism outputs could be described by a probability summation model which assumes stochastic independence of the mechanisms, and determined the best fitting summation exponent. Stimuli were Gaussian enveloped 1 c/d sinusoidal gratings represented in a 3D cardinal space transformed from cone contrast axes, and normalized to detection threshold. The use of this space avoids the presence of elongated threshold contours, allowing a reliable model fit to include the less sensitive blue-yellow and luminance mechanisms. Our results were well fitted by the probability summation model and hence support the underlying stochastic independence of the three postreceptoral mechanisms. (+info)
Alignment of separated patches: multiple location tags.
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Gaussian and Gabor patches can be accurately localized; however, it is not yet clear which cues (or location tags) the visual system utilizes for localization. To determine the cues used in spatial alignment, we measured and modelled the perceived shifts for asymmetric Gaussian and Gabor patches over a wide range of separations, patch sizes and orientations. For Gaussian patches we observed perceived shifts that were generally consistent with that of the centroid of the envelope. For Gabor patches we found that the perceived shift depends on the carrier orientation (whether co-axial or orthoaxial with the patch arrangement), separation (in units of carrier wavelength) and patch size (number of cycles per standard deviation). Gabor patches with the carrier orthoaxial (horizontal) to the three vertically arranged patches, were similar to Gaussian patches. However, Gabor patches with the carrier coaxial (vertical) to the three vertically arranged patches resulted in perceived shifts that were consistent with a number of alternate localization primitives. The selection of primitives was dependent on mainly the separation and patch size. Our results support the suggestion that the visual system can use multiple tags for location (Hess et al., Vis Res 1994;34:2439-2451; Badcock et al., Vis Res 1996;36:1467-1472). (+info)
Word ambiguity and the optimal viewing position in reading.
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The optimal viewing position phenomenon discovered by (O'Regan, J. K., Levy-Schoen, A., Pynte, J., Brugaillere, B. (1984). Convenient fixation location within isolated words of different length and structure. Journal of Experimental Psychology: Human Perception and performance. 10 (2), 250-257) is characterized by a minimization of gaze duration on a word and maximization of word recognition rates when the eye fixates a word near its center. Subsequent studies (Holmes, V. M., & O'Regan, J. K. (1987). Decomposing french words. In J. K. O'Regan, & A. Levy-Schoen, Eye movements: from physiology to cognition, North-Holland, Amsterdam, 459-466; O'Regan, J. K., & Levy-Schoen, A. (1987). Eye movement strategy and tactics in word recognition and reading. In M. Coltheart, Attention and performance XII: the psychology of reading, Erlbaum, Hillsdale N.J., 363-383) have shown that lexical structure can affect the location of the optimal viewing position. In this paper we show that the optimal viewing position is near to the position which minimizes word ambiguity arising from incomplete recognition of the letters in the word. This conclusion is supported by a statistical analysis based on inter-letter correlations in English and French word corpuses. (+info)
Computing feature motion without feature detectors: a model for terminator motion without end-stopped cells.
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Pointlike object features such as line-endings, have a privileged position in the computation of the veridical direction of object motion. Experiments confirm that the human visual system relies heavily on such features if they are present. It has been proposed that units such as end-stopped cells might be necessary for the computation of feature motion instead of the simple cells used in plaid motion models. Conventional plaid motion models have not been applied to feature motion. We present here a model, based on ordinary simple cells, using two parallel pathways (Fourier and non-Fourier) for the computation of the direction of two dimensional motion. Although similar in structure to popular models of plaid motion, our model includes a novel scheme for contrast normalisation and incorporates spatial pooling at the level of MT cells. The model predictions are consistent with psychophysical results for plaids. Furthermore, it computes directions within 5 degrees of the physical motion of line-endings. It is shown that the non-Fourier signal is necessary for the computation of veridical motion. (+info)
Understanding pedestrians' road crossing decisions: an application of the theory of planned behaviour.
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This paper reports a study applying the Theory of Planned Behaviour (TPB) to the prediction of pedestrians' road crossing intentions. Respondents (N = 210) completed questionnaires which included scenarios of three potentially dangerous road crossing behaviours, followed by measures of attitude, subjective norm, perceived behavioural control, self-identity and intention. The results indicated that the social psychological variables under consideration were able to explain between 39 and 52% of the variance in intentions to cross the road in the manner depicted in the scenarios. The perceived behavioural control component of the TPB emerged as the strongest predictor of pedestrians' intentions, suggesting that perceptions of control have an important role to play in road safety behaviour. The results are discussed in relation to the predictive utility of the TPB in this area and possible interventions to encourage safe road crossing behaviour. (+info)