The pretreatment weighting technique.

A common procedure that is frequently used in perceptual studies interested in group differences (e.g., young vs. aged) involves initially equating all subjects in a pretreatment condition by differentially weighting the subjects on a particular stimulus dimension. The experimental treatment of interest is then introduced to these equal-baseline groups. The present article questions the general validity of this procedure by focusing on the critical assumption that the pretreatment weighting and the experimental treatment are independent. A research example is included demonstrating that differentially weighting individuals in order to produce equal-baseline performance may, in fact, provide the basis for later performance differences under the experimental treatment.

Spare the rod and spoil the icon.

Short-term visual storage was investigated with a successive field paradigm, so that correct performance depended upon combining visual information from two targets that were never on simultaneously. In the first two experiments, the stimuli consisted of two slides, each containing a 10' red dot on a gray surround, and separated by an interstimulus interval (ISI) from 20 to 400 msec. Subjects had to determine if the dots were vertically or horizontally aligned. In Experiment 1, the stimuli had either no contrast for the rods or no luminance contrast for the cones or high contrast. At short ISIs the cone contrast determined performance, whereas at long ISIs the rod contrast determined performance. In fact, when the dots were invisible to the rods, the task was impossible for long ISIs. In Experiment 2, performance was compared for zero log rod contrast and for small departures from zero. Even a small departure from zero log rod contrast resulted in above-chance performance. In Experiment 3, the stimuli were luminous rectangles and the task was to decide whether or not a 4' spatial gap was present between the two successively presented rectangles. Wavelength, luminance, and ISI were varied under both photopic and scotopic adapting conditions. The result was that the rods performed the task for ISIs of 150 msec or longer under scotopic conditions and under the photopic conditions that we were able to test. The results of the experiments taken together are consistent with the hypothesis that the cone icon is short, whereas the rod icon is robust and long lasting.

The case for peripheral persistence: effects of target and background luminance on a partial-report task.

The effects of varying target and background luminances were examined on a partial-report task of iconic memory. Experiment 1 demonstrated that with a dim preexposure and postexposure field, varying target luminance has a marked effect on performance. Increased luminance results in significantly longer iconic storage. In Experiment 2 performance on the task was determined as the background luminance was systematically increased. It was found that not only is the duration of iconic storage reduced by increasing background luminance but changes in target luminance have progressively less of an effect as background luminance increases. These results are discussed in terms of the known response properties of cone and rod photoreceptors and their potential contribution to iconic memory. Finally, possible reasons for discrepancies with other iconic memory studies are detailed.

Receptor interactions and visual persistence.

Two experiments are described that provide evidence for an interaction between the cone and rod systems following the offset of a stimulus. Experiment 1 employs a visual persistence task in which the observer is required to integrate two brief flashes separated by a variable ISI. Threshold luminance as a function of ISI is determined for several conditions of target wavelength. Striking differences between short and long wavelength targets are found which indicate the possible exclusion of rod participation at the brief ISIs. Experiment 2 employs a forward-masking task to demonstrate that the stimulus conditions of the first experiment result in a transient reduction in rod system activity following stimulus offset while the cone system is still active. Results support this proposal. Implications for the locus of persistence within the visual system are discussed.

Visual persistence from brief letters and pictures.

The visual persistence from briefly presented letters and pictures was assessed by the popular probe-matching procedure over a range of background and target luminance levels and for several color conditions. It was determined that the fading visible persistence measured in this way increased with increasing target luminance and with decreasing background luminance. For small foveal presentations, photopically-matched targets of differing wavelength produced equivalent persistences; but for larger, parafoveal presentations, scotopically-matched targets of differing wavelength produced equivalent persistences. This was true for both letter and picture targets. Results were discussed in terms of an early sensory locus to such persistence effects. The strong consistency of these findings to some previous work and the apparent inconsistency with other work were treated in terms of different kinds of visual persistence effects assessed by different experimental methods.

Positive and negative afterimages from brief target gratings.

Threshold luminance levels for the production of negative afterimages from brief target gratings were determined as a function of background luminance and grating frequency. The obtained thresholds were extremely low--typically below the values that would maintain constant space-average luminance between target and background. The implications of these results for other studies that may have inadvertently produced negative afterimages with their stimulus conditions were noted. As a demonstration, visual persistence estimates from these gratings were determined under conditions that carefully excluded negative afterimages, and clear differences from previously published work were obtained.

The nature and basis of age-related changes in dynamic visual acuity.

The ability of observers to resolve moving targets, or dynamic visual acuity (DVA), was determined for a group of young adults (M age = 19.6 years) and a group of older adults (M age = 67.6 years). Targets were presented at two luminance levels over a range of velocities (30, 60, 90, and 120 deg/s) and at 2 durations (200 and 600 ms). The younger subjects exhibited superior DVA under nearly all conditions, but this effect was essentially eliminated by the luminance adjustment. These results are interpreted in terms of decreased retinal illumination in the older subjects rather than in terms of age-related changes in the underlying eye movement systems. Practical and theoretical implications are discussed.

Foveal and parafoveal processing of asynchronous Stroop stimuli.

Two experiments are presented which used a form of the Stroop colour-word task to study the processing of target and distractor information under changing conditions of temporal and spatial proximity. Experiment 1 focused on the marked empirical discrepancies in previous research reported in the recent literature. A coloured target and a distractor (neutral, facilitative or interfering) that were separated by variable onset asynchronies were presented foveally to observers. The findings of this study indicate strong distractor effects on response latency at the intermediate stimulus onset asynchronies (SOAs) (around 0 ms) and a clear monotonic decrease in reaction time for colour naming at extreme positive and negative SOAs. Experiment 2 used this same task but presented targets parafoveally and distractors foveally. This manipulation resulted in the same general pattern of processing as found in Expt 1. In addition, a laterality effect was found when the target and distractor were incongruent; the left visual field/right hemisphere showed significantly greater interference as indicated by longer reaction times. These results are discussed in terms of current models of information processing of compound stimuli.

Paradoxical punishment as it relates to the battered woman syndrome.

The main goal of this article is to suggest some processes by which the behavioral interactions of an abusive couple can be developed and maintained. A summary of these processes as they have been adapted from the animal analog and human research follows. First, the pairing of abuse (punishment) with love and affection (reinforcement) could indicate that the battered woman responds to the reinforcement that follows an abusive incident. Second, the battered woman may seek to escape the fear of the tension-building phase but during the escape behavior she is punished (physical abuse), which fortifies and strengthens her initial fear. However, after the abusive incident, there is an absence of tension and fear (negative reinforcement) compounded by the positive reinforcement of the contrition phase. Thus, a cyclical pattern of behavior evolves and sustains itself. Possibly augmenting the vicious cycle is habituation or negative adaptation to the physical abuse, as well as the lack of opportunities to learn adaptive behavioral responses that may break the cycle of violence. Finally, the need for primary reinforcement (i.e., love and affection) may be so powerful that it somewhat mitigates the abuse endured to obtain it.

The retinal basis of iconic memory: Eriksen and Collins revisited.

The parameters of luminance, duration, wavelength, and interstimulus interval (ISI) were varied on the successive-field task first employed by Eriksen and Collins in 1967 and 1968. This task requires observers to combine visually the information presented in two brief successive presentations for successful trigram recognition. The results of the luminance and duration manipulations indicate that as stimulus energy is increased, iconic memory also increases. The wavelength and ISI manipulations reveal that performance at 20 msec ISI reflects cone (photopic) sensitivity but that performance at 250 msec ISI reflects rod (scotopic) sensitivity. These findings are interpreted in support of the retinal conception of iconic memory, with the cones providing a very transient icon and the rods providing the bulk of the persistence.

Complexity effects on reaction-time measures of visual persistence: evidence for peripheral and central contributions.

Two experiments are presented that attempt to determine the effect of operationally defined stimulus complexity on visual persistence. Two common measures of complexity are employed in both experiments. In Experiment 1, visual persistence for high-contrast, solid black figures increases with increasing target duration and with decreasing complexity. In Experiment 2, which uses outline figures of the same sizes and shapes as in the first experiment, visual persistence increases both with increasing target duration and with increasing complexity. It is proposed that the very different results from the two experiments may reflect the separate functioning of a visible, sensory persistence and a higher order, schematic persistence. The reaction-time (RT) measure of persistence that was employed may be sensitive to either of these persistences, depending upon stimulus and task conditions. Implications are suggested for (a) the importance of sensory processes in some manipulations of target complexity and (b) the possible role of peripheral and central factors in visual persistence.

Rod persistence on a partial-report task with scotopic and photopic backgrounds.

Two experiments were undertaken to demonstrate the participation of the rod system under certain stimulus conditions on the partial-report task of iconic memory. In Experiment 1, scotopically matched letter arrays of differing wavelength produced equivalent partial-report superiority but only if relatively large letters and a long cue delay were employed. In Experiment 2, variable photopic backgrounds were employed in contrast to the constant dark background used in the first experiment. For constant luminance targets, photopically matched red and blue-green backgrounds of 5 ftL (15.9 cd/m2) did not produce equivalent partial-report performance. Partial-report superiority was evident at much longer cue delays with the red background. This was attributed to the minimal stimulation of the rod system by this long wavelength background. However, if an intense adaptation field that saturated the rod system were interposed between stimulus trials, the photopically matched backgrounds then produced equivalent partial-report performance at all cue delays.

To reverse or not to reverse: when is an ambiguous figure not ambiguous?

The role of bottom-up processes in our perception of reversible figures was examined. In Experiment 1 the overlapping squares figure and nonsense reversible figures were used. The effects of adapting subjects for differing durations to an unambiguous version of the figure before presentation of the traditional reversible figure were determined under conditions of varying precision of fixation. In Experiment 2 the research was expanded to other examples of reversible figures. In both experiments, results with two dependent measures (the subject's first percept and the number of reversals reported) were generally consistent with the interpretation of bottom-up processes underlying the adaptation effects. However, the crucial role of stimulus and procedural variables and the differential sensitivity of the two dependent measures was demonstrated.

The changing face of visual persistence.

In three experiments, the effect of target luminance on estimates of visual persistence was determined on a popular successive-field task. Even though the experiments differed from each other only in terms of minor changes in stimulus conditions, very different effects were obtained across the three experiments. The different findings were attributed to the often overlooked roles of masking and target degradation on tasks of persistence. Implications for the numerous contradictory findings in the persistence literature are discussed, and the current popularity of the successive-field task is questioned.

Task and size effects in the Oppel-Kundt and Irradiation illusions.

The present study examined the effects of type of task and stimulus size on two well-known illusions of perceived size: the Oppel-Kundt illusion and the Irradiation illusion. In Experiment 1, a forced-choice task, in which 87 male and female college observers chose the "longer" of two side-by-side stimuli, was used to determine the relative perceived size of stimuli of different length that were all white, all black, or white with various numbers of black stripes. In Experiment 2, an absolute judgment task, in which 73 male and female college observers reported their direct estimate of a single target's length, was used for the same determinations. For both kinds of perceptual judgment, the Oppel-Kundt illusion is strongly in evidence (p less than .001), but the magnitude of the illusion does not increase proportionally with the size of the stimuli. The Irradiation illusion is only found with the forced-choice judgments in Experiment 1. Implications for the generalizability of these illusions to nonlaboratory settings are noted.

Closure and acuity in pseudoisochromatic color tests.

While visual acuity was related to performance on several pseudoisochromatic color tests for 73 male college students, no evidence was found that perceptual closure also affected performance. No scores were correlated for the 69 undergraduate women.

Perceptual style and dual-task performance as a function of task difficulty and task emphasis.

Differences in dual-task performance by field-dependent and field-independent college students were investigated. The perceptual styles of the 102 subjects were determined by their performance on the Rod-and-Frame Test. Dual-task performance required the subjects to perform concurrently a central tracking task and a peripheral light-detection task. The effect of stress on dual-task performance was examined by employing three widely differing levels of tracking task difficulty. Three sets of instructions varied the relative importance of the two tasks within the dual-task situation. Both manipulation of task difficulty and instructional set had powerful effects on performance. However, performance differences among subjects with differing perceptual styles were found only on the central tracking task, with field-independent subjects consistently outperforming field-dependent ones. The results are discussed in terms of the theoretical basis for the perceptual style of field-dependence/field-independence as well as the potential value of this dimension in dual-task situations.

Effect of diets differing in fat content on chronic diarrhea in cats.

BACKGROUND: Fat-restricted diets have been advocated for dogs with diarrhea for many years. Recommendations for cats with diarrhea have varied between low-fat and high-fat diets, but there have been no published studies to support either recommendation. OBJECTIVES: The objective of this study was to compare the clinical responses of cats with chronic diarrhea to dietary management using either a high fat or a low fat, highly digestible diet. ANIMALS: Sixty pet cats with chronic diarrhea were recruited; 55 cats completed the study. METHODS: Randomized, double-blinded, controlled clinical trial. Upon completion of baseline measures, cats were fed 1 of 2 diets for 6 weeks, during which the owners recorded fecal scores daily using an illustrated fecal score chart ranging from 0 (very watery) to 100 (firm and dry). After 6 weeks, cats were reevaluated by the attending veterinarians. RESULTS: Fecal scores improved significantly, with 78.2% of cats improving by at least 25 points on the 100-point scale or having a final fecal score of at least 66. Over one third of the cats developed normal stools. There were no differences in clinical responses between the diets. Clinical improvement was noted within the 1st week, and maximized within 3 weeks. CONCLUSIONS AND CLINICAL IMPORTANCE: These results show that dietary management can be helpful in cats with chronic diarrhea, but dietary fat content does not appear to affect the outcome. Cats that do not respond within 3 weeks should be evaluated further.