Temporal Variations in Visual Completion: A Reflection of Spatial Limits?

The completion of partly occluded objects appears instantaneous and effortless, but empirically takes measurable time. The current study investigates how amount of occlusion affects the time course and mechanisms of visual completion. Experiment 1 used a primed-matching paradigm to determine completion times for objects occluded by various amounts. Experiments 2 and 3 used a dot-localization paradigm to probe completed contour representations for a qualitative shift above some spatial limit. The results demonstrate that time to completion rises with amount of occlusion. Nonetheless, the visual system can complete highly occluded objects, even when the occlusion renders visible contours nonrelatable. Furthermore, prolonged completion times for highly occluded objects do not result from a breakdown of low-level interpolation processes: The same contour completion mechanism operates on objects occluded by different spatial extents. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Visual complexity in letter-by-letter reading: "pure" alexia is not pure

Standard accounts of pure alexia have favoured the view that this acquired disorder of reading arises from damage to a left posterior occipital cortex mechanism dedicated to the processing of alphanumeric symbols. We challenge these accounts in two experiments and demonstrate that patients with this reading deficit are also impaired at object identification. In the first experiment, we show that a single subject, EL, who shows all the hallmark features of pure alexia, is impaired at picture identification across a large set of stimuli. As the visual complexity of pictures increases, so EL's reaction time to identify the stimuli increases disproportionately relative to the control subjects. In the second experiment, we confirm these findings with a larger group of five pure alexic patients using a selected subset of high- and low-visual complexity pictures. These findings suggest that the deficit giving rise to pure alexia is not restricted to orthographic symbols per se but, rather, is a consequence of damage to a more general-purpose visual processing mechanism.     

Interactions between symmetry and elongation in determining reference frames for object perception.

Many theories of object recognition posit that objects are encoded with respect to a perceptual frame of reference. Such theories assume that factors such as symmetry and elongation are critical for the assignment of an object's primary axis, and consequently for the extraction of an object's reference frame. The present experiments directly examined the relative roles played by symmetry and elongation in the determination of an object's primary axis, and the extent to which symmetry and elongation interact with one another. A total of 55 Ss (aged 15–27 yrs) participated in the experiments. The authors found that observers use both symmetry and elongation in extracting an object's primary axis, that the extent to which each cue dominates depends on its relative salience, and that symmetry and elongation are processed interactively, rather than in encapsulated modules. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Models of stimulus uncertainty in motion perception.

Proposed a model to account for the loss in visibility of moving targets that occurs when an observer is uncertain about the target's direction of motion. In response to uncertainty about 2 possible directions of motion, the observer is assumed to use a mechanism whose peak sensitivity is to a direction midway between the 2 possible directions. Seven experiments (observers were 7 naive Ss and the 1st author), using both reaction time and forced-choice data, demonstrate the predictive advantages of this midway model over competing single-band and multiple-band models. The experiments reveal several new properties of human motion perception: (a) direction and velocity information have orthogonal representations in the visual system; (b) although motion sensitivity does not vary with direction, the precision with which small changes in direction can be recognized does, reflecting differential breadth of tuning for directionally selective mechanisms sensitive to various directions; and (c) motion-analyzing mechanisms are broadly tuned for direction as well as speed. (47 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Ocular fixation control as a function of age and exposure duration.

[Correction Notice: An erratum for this article was reported in Vol 3(1) of Psychology and Aging (see record 2008-10694-001). In the aforementioned article, the following corrections should be made: 1. The title of Table 1 should be changed to Mean Bivariate Areas (min-arc2) and Mean Horizontal and Vertical Standard Deviations (min-arc) of Fixations of Older and Younger Groups. 2. The equation on page 304 should have used the natural log rather than the log base 10. The corrected equation is provided in the erratum.] In previous work we reported that fixation stability did not deteriorate in older adults over relatively long viewing durations. In the present study we reanalyzed the data to examine potential aging effects on fixational control for viewing durations typically used in psychological experimentation. Monocular eye movements were recorded in 12 older and 12 younger observers using a dual Purkinje image technique, while observers fixated a stationary target. The two-dimensional scatter of eye positions was measured during nine viewing durations ranging from 100 ms to 12.8 s. Fixational control of the two groups was comparable at all of the viewing durations. Both younger and older observers were able to maintain fixation within an area several times smaller than the size of the fovea. Implications for aging studies that use briefly presented visual stimuli are discussed. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Self-reported visual problems of older drivers

We conducted a survey in order to gain insights into the reasons older persons decide to give up driving. Our survey focused on vision. We probed the relationship between visual deficiencies and driving status by asking older people about the problems they encountered while performing routine visual tasks. The results showed that older persons who had recently given up driving reported more visual problems than did their driving counterparts. These problems related to difficulties in dynamic vision, visual processing speed, visual search, light sensitivity, and near vision. The results suggested that older persons are sensitive to their own visual deficits and that this awareness influences decisions about driving.     

Motion perception: a modern view of Wertheimer's 1912 monograph

Max Wertheimer's 1912 monograph on apparent motion is a seminal contribution to the study of visual motion, but its actual contents are not widely known. This article attempts to clarify what the monograph did and did not contribute, emphasizing links between Wertheimer's principal findings and the results of subsequent investigations of motion perception, including currently active lines of research. The topics discussed include Wertheimer's experimental tests of explanations for apparent motion; his work with motion phenomena that lie between succession and optimum motion; his studies of the influence of attention on motion; explorations of various forms of hysteresis and motion transparency; and Wertheimer's work with a motion-blind patient.     

Blur and contrast as pictorial depth cues

OBJECTIVE: Infusion of GH secretagogues appears to be a novel endocrine approach to reverse the catabolic state of critical illness, through amplification of the endogenously blunted GH secretion associated with a substantial IGF-I rise. Here we report the dynamic characteristics of spontaneous nightly TSH and PRL secretion during prolonged critical illness, together with the concomitant effects exerted by the administration of GH-secretagogues, GH-releasing hormone (GHRH) and GH-releasing peptide-2 (GHRP-2) in particular, on night-time TSH and PRL secretion. PATIENTS AND DESIGN: Twenty-six critically ill adults (mean +/- SEM age: 63 +/- 2 years) were studied during two consecutive nights (2100-0600 h). According to a weighed randomization, they received 1 of 4 combinations of infusions, within a randomized, cross-over design for each combination: placebo (one night) and GHRH (the next night) (n = 4); placebo and GHRP-2 (n = 10); GHRH and GHRP-2 (n = 6); GHRP-2 and GHRH + GHRP-2 (n = 6). Peptide infusions (duration 21 hours) were started after a bolus of 1 microgram/kg at 0900 h and infused (1 microgram/kg/h) until 0600 h. MEASUREMENTS: Serum concentrations of TSH and PRL were determined by IRMA every 20 minutes and T4, T3 and rT3 by RIA at 2100 h and 0600 h in each study night. Hormone secretion was quantified using deconvolution analysis. RESULTS: During prolonged critical illness, mean night-time serum concentrations of TSH (1.25 +/- 0.42 mlU/l) and PRL (9.4 +/- 0.9 micrograms/l) were low-normal. However, the proportion of TSH and PRL that was released in a pulsatile fashion was low (32 +/- 6% and 16 +/- 2.6%) and no nocturnal TSH or PRL surges were observed. The serum levels of T3 (0.64 +/- 0.06 nmol/l) were low and were positively related to the number of TSH bursts (R2 = 0.32; P = 0.03) and to the log of pulsatile TSH production (R2 = 0.34; P = 0.03). GHRP-2 infusion further reduced the proportion of TSH released in a pulsatile fashion to half that during placebo infusion (P = 0.02), without altering mean TSH levels. GHRH infusion increased mean TSH levels and pulsatile TSH production, 2-fold compared to placebo (P = 0.03) and 3-fold compared to GHRP-2 (P = 0.008). The addition of GHRP-2 to GHRH infusion abolished the stimulatory effect of GHRH on pulsatile TSH secretion. GHRP-2 infusion induced a small increase in mean PRL levels (21%; P = 0.02) and basal PRL secretion rate (49%; P = 0.02) compared to placebo, as did GHRH and GHRH + GHRP-2. CONCLUSIONS: The characterization of the specific pattern of anterior pituitary function during prolonged critical illness is herewith extended to the dynamics of TSH and PRL secretion: mean serum levels are low-normal, no noctumal surge is observed and the pulsatile fractions of TSH and PRL release are reduced, as was shown previously for GH. Low circulating thyroid hormone levels appear positively correlated with the reduced pulsatile TSH secretion, suggesting that they have, at least in part, a neuroendocrine origin. Finally, the opposite effects of different GH-secretagogues on TSH secretion further delineate particular linkages between the somatotrophic and thyrotrophic axes during critical illness.     

Age-related changes in imitating sequences of observed movements.

Three experiments investigated the size and sources of age-related changes in visual imitation. In Experiment 1, young and older adults viewed sequences of quasi-random movements and then reproduced from memory what they had seen. As expected, older adults made more errors in imitation than their younger counterparts. However, older adults seemed to supplement their memory by exploiting an abstracted representation (gist) of a sequence. Experiments 2 and 3 apportioned the observed age-related changes in imitation performance among several possible causes. Experiment 2 showed that changes in precision of visual perception and motor control together accounted for only a small fraction of age-related changes in imitation quality; Experiment 3 showed that the bulk of the age-related changes arose from the older participants’ reduced ability to accommodate for increases in memory load, likely caused by diminished ability to encode or retain detailed information about movement sequences. Guided by these results, strategies are proposed for enhancing older adults’ imitation learning. (PsycINFO Database Record (c) 2010 APA, all rights reserved)