The use of chromatic information for motion segmentation: differences between psychophysical and eye-movement measures

Previous psychophysical studies have shown that chromatic (red/green) information can be used as a segmentation cue for motion integration. We investigated the mechanisms mediating this phenomenon by comparing chromatic effects (and, for comparison, luminance effects) on motion integration between two measures: (i) directional eye movements with the notion that these responses are mediated mainly by low-level motion mechanisms, and (ii) psychophysical reports, with the notion that subjects' reports should employ higher-level (attention-based) mechanisms if available. To quantify chromatic (and luminance) effects on motion integration, coherent motion thresholds were obtained for two conditions, one in which the signal and noise dots were the same 'red' or 'green' chromaticity (or the same 'bright' or 'dark' luminance), referred to as homogeneous, and the other in which the signal and noise dots were of different chromaticities (or luminances), referred to as heterogeneous. 'Benefit ratios' (theta(HOM)/theta(HET)) were then computed, with values significantly greater than 1.0 indicating that chromatic (or luminance) information serves as a segmentation cue for motion integration. The results revealed a high and significant chromatic benefit ratio when the measure was based on psychophysical report, but not when it was based on an eye-movement measure. By contrast, luminance benefit ratios were roughly the same (and significant) for both measures. For comparison to adults, eye-movement data were also obtained from 3-month-old infants. Infants showed marginally significant benefit ratios in the luminance, but not in the chromatic, condition. In total, these results suggest that the use of chromatic information as a segmentation cue for motion integration relies on higher-level mechanisms, whereas luminance information works mainly through low-level motion mechanisms.     

Dark-adaptation mechanisms of the long-wave foveal cones

The ordinary long-term rod and cone dark-adaptation curves have generally been assumed to follow a single exponential rate of recovery. However, in two previous papers on rod dark-adaptation (Stabell et al., 1986a, b), the recovery curve was found to consist of three different sections. The results of the present paper show the same type of recovery function with three different sections for the long-term dark-adaptation curve of the long-wave cone system. During the major, middle section log cone threshold, like log rod threshold, is linearly related to the logarithm of the concentration of bleached photopigment. Presupposing that the bleached cone photopigment acts as a ligand, the change in threshold level obtained during the middle section of the dark-adaptation curve is well described by the change in activity rate of an allosteric, postively cooperative enzyme built as a dimer.     

Awareness becomes necessary between adaptive pattern coding of open and closed curvatures

Visual pattern processing becomes increasingly complex along the ventral pathway, from the low-level coding of local orientation in the primary visual cortex to the high-level coding of face identity in temporal visual areas. Previous research using pattern aftereffects as a psychophysical tool to measure activation of adaptive feature coding has suggested that awareness is relatively unimportant for the coding of orientation, but awareness is crucial for the coding of face identity. We investigated where along the ventral visual pathway awareness becomes crucial for pattern coding. Monoptic masking, which interferes with neural spiking activity in low-level processing while preserving awareness of the adaptor, eliminated open-curvature aftereffects but preserved closed-curvature aftereffects. In contrast, dichoptic masking, which spares spiking activity in low-level processing while wiping out awareness, preserved open-curvature aftereffects but eliminated closed-curvature aftereffects. This double dissociation suggests that adaptive coding of open and closed curvatures straddles the divide between weakly and strongly awareness-dependent pattern coding.     

The magical number seven: Making a “bit” of “sense”

The maximum number of categories distinguishable in making an absolute judgment was estimated by Miller to be “seven plus or minus two,” corresponding to about 3 bits of information transmitted per stimulus. Later work extended this range to include at least 2 to 4 bits of information, which reached 16 categories. In contrast, the number of distinguishable differences between two stimuli is in the order of 100. Why is this so? It is shown here that an answer to these questions can be obtained by constructing anentropy function, Hs, which is a measure of the uncertainty of a subject (or a sensory receptor) as it perceives the magnitude of an applied stimulus. Using this function, it is demonstrated that the ubiquitous 3 bits of information per stimulus can be approximated from the expression \(\log _2 \sqrt {{{\tau _2 } \mathord{\left/ {\vphantom {{\tau _2 } {\tau _1 }}} \right. \kern-0em} {\tau _1 }}} \) , where τ₁ and τ₂ are known time constants. The same entropy function can be used to derive various other psychophysical laws, such as the Weber-Fechner law, Stevens’ law, and the Bunsen-Roscoe law.     

Fechner’s paradox reflects a nonmonotone relation between binocular brightness and luminance

Twenty subjects judged the brightness of binocularly fused targets whose monocular luminances were varied independently. On each trial, the left eye was presented with one of two relatively high luminances and the right eye was presented with one of 15 luminances from the range in which Fechner’s paradox is effective. The objective of the experiment was to determine whether the psychophysical function over this range was nonmonotonic and Ll-shaped, as implied by several models of binocular brightness, or monotone increasing, but discontinuous at zero right-eye luminance. The functions associated with both left-eye intensities were found to be nonmonotone. Both minima were near the upper bound of the mesopic range.     

Size perception is less context-sensitive in males

Context sensitivity of size perception has previously been used to study individual differences related to the distinction between local, analytic, or field-independent and global, holistic, or field-dependent perceptual styles. For example, it has been used in several recent studies of autistic spectrum disorders, which may involve an excessive bias toward local processing. Autism is much more common in males, and there is evidence that this may be in part because males in general tend to be less context-sensitive than females, and thus are more affected by conditions that further reduce context sensitivity. There is also evidence that a bias to local processing is more common in professions that require attention to detail. Context sensitivity of size perception was therefore studied as a function of sex and academic discipline in sixty-four university staff and students by a simple, sensitive, and specific psychophysical measure based on the Ebbinghaus illusion. The results show that in this task males are on average less context-sensitive than females, that the overlap is large, and that subjects with very high or very low context sensitivity tend to have the sex and profession predicted by the above hypotheses.     

Perception of the form of stimulus increment as a method in assessment of the psychophysical relationship

Among numerous procedures for determination of the psychophysical relation, one approach has seldom been applied. Essential in this method is to present a set of stimuli whose intensity increases in fixed time following different forms. The objective stimulus increment, which the subjects perceive as linear growth directly, represents the inverse psychophysical relation. In this paper the method was tested in the fields of click frequency and sound pressure. This procedure was named "Perception of the Form of Stimulus Increment in Time". In comparison to other psychophysical approaches, this one has several advantages. The principal ones are the following: (1) In assessing the psychophysical relation it is not necessary to try to measure the perceptual magnitude; and (2) the psychophysical relation is directly determined by its dynamic pattern. In this paper modifications to the method are reported which facilitate the subject's task and avoid some differences in subjects' individual approaches to the task. The modified procedure leads to results satisfactory independent of the influence of the factors irrelevant to the psychophysical relationship.     

Nonhomogeneous resolution of images of natural scenes

The aim of this research is to model and simulate the loss of visual resolution as a function of retinal eccentricity in the perception of natural scenes. The model of visual resolution is based on a space-variant low-pass filter, having a variable convolution kernel according to retinal eccentricity. The parameters of the model are computed from psychophysical measures of visual acuity as a function of retinal eccentricity. The implementation of the model allowed us to generate images of scenes with nonhomogeneous space-variant resolution, simulating the filtering executed by the eye. These scenes are used to test and optimise the model by means of experiments in static vision (through tachistoscopic presentations) and in dynamic vision where the resolution of the scene is computed, in real-time, as a function of the location of gaze.     

Dark adaptation of the long-wave cones at different eccentricities

Using a Wright colorimeter the ordinary long-term, long-wave cone dark-adaptation curve was measured at 0, 2, 4, 7, 17, 25, 40 and 49 degrees nasally in the visual field. In opposition to previous findings, the results show that the dark-adaptation function of the long-wave cones changes markedly when the test field is moved outward from the rod-free fovea. It is suggested that the kinetics of the long-wave cone photopigment change with eccentricity. Also, at variance with previous findings, the present curves at all eccentricities may reasonably well be interpreted as consisting of three different sections; a first section where the threshold decreases rapidly, followed by a major, approximately linear section and a terminating section that converges asymptotically towards the final level of sensitivity. This finding suggests that the dark-adaptation process of the cone system, under the given experimental conditions, is based on three somewhat different processes.     

Temporal summation related to the nature of the proximal stimulus for the warmth sense

Magnitude estimations of the warmth aroused by radiant stimulation of the forehead showed that warmth obeys the psychophysical power law when the’ duration of the stimulus is relatively long (3 or 6 sec). When duration is short (0.25, 0.5. or 1 sec), however, warmth grows as a more complex function of irradiation. The family of psychophysical functions measured for the various durations can be used to generate the rules by which radiant intensity and duration trade to preserve constant warmth. These rules vary systematically from complete temporal summation (i.e., complete reciprocity) near threshold to less and less complete summation as warmth level increases. When, however, the stimulus is expressed as an equivalent change of temperature in the skin, there is no summation, only adaptation. It can be shown that temporal summation observed psychophysically must result from the heat-transfer properties of skin tissue.     

Quantifying private events: A functional measurement analysis of equisection

Functional measurement theory was applied to bisection, trisection, and quadrisection of grayness. Theoretically, these judgments should obey an averaging model. But the overt responses are not a valid measure of subjective magnitude (since they are made on the physical stimulus continuum), and so they cannot be used directly to test the model. However, scaling and model testing can both be accomplished simultaneously using functional measurement theory. If the subject is indeed averaging, then there exists a monotone transformation that makes the data additive; and this transformation can be computed with FUNPOT, Weiss’s (1973a) computer program which finds polynomial transformations that reduce selected effects. Further, determination of this transformation also reveals the psychophysical function, because it gives the relation between subjective magnitude and overt response. For bisection, the averaging model was successful; it was possible to find a monotone transformation that made the data additive. This psychophysical function differed somewhat in form from the Munsell scale. It gained cross-task validity from its agreement with a grayness scale obtained from rating data (Weiss, 1972). For trisection and quadrisection, the averaging model was not accepted; it was not possible consistently to find transformations which induced additivity.     

Binocular rivalry and visual awareness: the role of attention

When the right eye and the left eye view dissimilar scenes, the observer does not experience a stable superimposed percept of the images presented to the two eyes, but instead perceives an alternation between the images seen by each eye. A critical question confronting this robust and intriguing phenomenon of binocular rivalry is how the visual system selects the image to be perceived (dominant). The current main-stream literature emphasizes a bottom-up explanation in which the rivalry stimulus with the higher contour strength has the advantage, and becomes dominant in rivalry. Nevertheless, some workers in the past have favored an attention-selection explanation for binocular rivalry. We investigated the role of attention in binocular rivalry by employing novel psychophysical paradigms which capitalized on several established phenomena (e.g. the Cheshire Cat effect, attention cueing, pop-out effect). Our results revealed two major aspects of attention modulation in binocular rivalry. We found that a dominant image is less likely to be suppressed when voluntary attention is directed to it. This suggests the role of voluntary attention in retaining the dominant image in visual awareness. Second, a rivalry stimulus is more likely to become dominant if accompanied by a pop-out cue (in the same eye and proximity). Since a pop-out cue attracts involuntary attention to its location/eye, this result suggests that cue-mediated involuntary attention can promote the ability of a rivalry stimulus to reach visual awareness.     

Sensory and sensori-motor adaptations in strabismus: Their role in space perception

In comitant strabismus many perceptual adaptive processes take place, which involve perception of space. Suppression of the image of the deviated eye and anomalous retinal correspondence (ARC) are the two main antidiplopic mechanisms. ARC may be present without suppression in small-angle strabismus (up to 10 degrees), supporting an anomalous binocular cooperation in spite of the deviation. Both psychophysical and electrophysiological evidence for anomalous binocular vision in strabismus are provided. Sensori-motor adaptations in strabismus develop as well. They are represented by vergence eye movements which, although not identical to, have similar characteristics as normal fusional vergences. These anomalous fusional eye movements tend to return the eyes to their original deviation when elements are introduced to change the position of the eyes, e.g., prisms or surgery. In conjunction with ARC, these movements serve to maintain binocular visual perception despite the strabismus.     

Dark-adaptation of the human rod system: A new hypothesis

Following a long, full bleach, rod dark-adaptation curves from two normal trichromats were obtained with test fields of various size, exposure time and retinal eccentricity. The results show that there is a substantial region of threshold recovery with an approximately constant, linear slope of about 0.27 log per minute of dark-adaptation, which is independent of the test variables. It is suggested that the increase in sensitivity during this constant, linear slope is completely determined by changes in the concentration of bleached rhodopsin. The relationship between change of relative threshold (T) and fraction of bleached rhodopsin ( B ) is given by T=B ^3,7. This exponential law is well described by the displacement of the equilibrium between the active and inactive states of an allosteric enzyme built as a tetrarner.     

Two visual systems and two theories of perception: An attempt to reconcile the constructivist and ecological approaches

The two contrasting theoretical approaches to visual perception, the constructivist and the ecological, are briefly presented and illustrated through their analyses of space and size perception. Earlier calls for their reconciliation and unification are reviewed. Neurophysiological, neuropsychological, and psychophysical evidence for the existence of two quite distinct visual systems, the ventral and the dorsal, is presented. These two perceptual systems differ in their functions; the ventral system's central function is that of identification, while the dorsal system is mainly engaged in the visual control of motor behavior. The strong parallels between the ecological approach and the functioning of the dorsal system, and between the constructivist approach and the functioning of the ventral system are noted. It is also shown that the experimental paradigms used by the proponents of these two approaches match the functions of the respective visual systems. A dual-process approach to visual perception emerges from this analysis, with the ecological-dorsal process transpiring mainly without conscious awareness, while the constructivist-ventral process is normally conscious. Some implications of this dual-process approach to visual-perceptual phenomena are presented, with emphasis on space perception.     

Saturation of light mixtures as a function of chromatic and achromatic excitation in the visual system

Summary The saturation of mixtures of white and chromatic lights may be measured by magnitude estimation as a function of the proportion of chromatic light in the mixture. When the intensity of the white light is kept constant, and the intensity of the chromatic light is varied, the psychophysical saturation curves change in a regular way: With higher chromatic intensity, they become less convex, and finally turn concave. Therefore it is possible to measure a whole class of stimulus-response curves representing the relation of chromatic light proportion to saturation. In this paper, a hyperbolic function is developed to describe this class of curves. The function is based upon the assumption that the data-processing system of human color vision evaluates the proportion of chromatic and achromatic excitation to produce subjective saturation. This assumption is in accordance with the known constancy of color perception under natural viewing conditions. The experimental data are approximated just as well, and in some cases better, by the hyperbolic function than by the commonly used power function.     

Distribution of ocular dominance and effect of image clarity

The purposes of this study were to determine if ocular dominance could be measured and defined in a quantitative and continuous manner, rather than dichotomously, and whether such a measure could be related to image sharpness, or acuity. Ocular dominance .w, as assessed on a dichoptic, but nonrivalry, task for 56 Ss, who were then assigned to groups according to their degree of dominance. Four positive spherical lenses (+0.75, +1.25, +1.75, +2.25) were used to induce differing amounts of refractive error in one eye. Primary results indicate that (1) dominance, as measured, is a continuous function which is normally distributed in the general population, (2) blurring the image in the dominant eye reduces the degree of dominance, and (3) bluring, the image to a nonpreferred eye increases the amount of dominance in the other eye.     

Evaluation of three sensitivity measures in magnitude estimation tasks

Taking into account the studies about the measure of sensitivity in magnitude estimation tasks, we analyze the three most common measures used in this topic: Pearson's product-moment correlation between the logarithm of the stimulus and the logarithm of the response (R), the exponent of Stevens' power function (K), and the measure "M" proposed by Garriga-Trillo. Using a sample of participants greater than usual in psychophysical studies (180 participants), we designed an experiment with two sets of stimuli with different stimulus ranges. In each of these sets, we used two kinds of stimuli (line segments and squares). Our conclusions were: (1) we rejected the use of K as a sensitivity measure because the results provided by this index were the opposite of those expected when we compared the two stimulus ranges. (2) We also rejected the use of M because this measure is a linear transformation of Kendall's coefficient of concordance. (3) Lastly, we suggest the mathematical transformation proposed by Fisher to achieve a normal distribution, and recommend this transformation as the best sensitivity measure.     

Phases of procedural learning and memory: characterisation with perceptual-motor sequence tasks

ABSTRACT

Procedural learning and memory has been conceptualised as consisting of cognitive and autonomous phases. Although the Serial Reaction Time Task (SRTT) is a popular task used to study procedural memory (PM), it has not been used to explore the different phases of PM. The present study employed a modified SRTT and investigated whether it can distinguish phases of PM. Our results revealed that performance at the beginning of typing a repeating sequence was marked by a steep learning curve, followed by gradual improvements and ending in high performance levels without further improvement. Steep performance increases characterise the effortful learning of the cognitive phase, gradual increases at higher performances characterise emerging automatisation of the associative phase, and sustained highest performance characterises autonomous procedures when PM has formed. Our study presents an easy-to-use measure, capable of distinguishing phases of PM, and which can be useful to assess PM during brain development.     

Errors of the standard in the horizontal-vertical illusion

Three experiments were conducted to test the claim that the magnitude of the horizontal-vertical illusion in an L figure varies as a function of which component is used as standard. No difference was found in Experiment 1, in which a staircase procedure was used to establish the PSE. However, by the use of a method of adjustment, the magnitude of illusion was affected by the component used as standard in Experiment 2. The results of Experiment 3, in which a staircase procedure was used with S in upright and recumbent body postures, confirmed those of Experiment 1. It was concluded that earlier differences associated with the standard were a methodological accompaniment of the psychophysical technique used.