Mark E. McCourt

Pseudoneglect: a review and meta-analysis of performance factors in line bisection tasks.

An exhaustive qualitative (vote-counting) review is conducted of the literature concerning visual and non-visual line bisection in neurologically normal subject populations. Although most of these studies report a leftward bisection error (i.e., pseudoneglect), considerable between-study variability and inconsistency characterize this literature. A meta-analysis of this same literature is performed in which the total quantitative data set, comprising 73 studies (or sub-studies) and 2191 subjects, is analyzed with respect to 26 performance factors. The meta-analytic results indicate a significant leftward bisection error in neurologically normal subjects, with an overall effect size of between -0.37 and -0.44 (depending on integration method), which is significantly modulated to varying degrees by a number of additional task or subject variables. For example, visual bisection tasks, midsagittal-pointing tasks and tactile bisection tasks all lead to leftward errors, while kinesthetic tasks result in rightward errors. Tachistoscopic forced-choice testing methods reveal much greater estimates of bisection error (effect size = -1.32) than do manual method-of-adjustment procedures (effect size= -0.40). Subject age significantly modulates line bisection performance such that older subjects err significantly rightward compared to younger subjects, and to veridical line midpoint. Male subjects make slightly larger leftward errors than do female subjects. Handedness has a small effect on bisection errors, with dextrals erring slightly further to the left than sinistral subjects. The hand used to perform manual bisection tasks modulated performance, where use of the left hand lead to greater leftward errors than those obtained using the right hand. One of the most significant factors modulating bisection error is the direction in which subjects initiate motor scanning (with either eye or hand), where a left-to-right scan pattern leads to large leftward errors while a right-to-left scan pattern leads to rightward errors.

Visuospatial attention in line bisection: stimulusmodulation of pseudoneglect

Neglect and pseudoneglect are asymmetries of spatial attention which are often assumed to possess a fundamental theoretical and neurological relationship to each other, although this assumption has never been directly tested and there is as yet no unifying quantitative theory. A total of 217 subjects participated in five experiments demonstrating that both the magnitude and direction of bisection errors in normal subjects (pseudoneglect) are modulated by stimulus factors that similarly influence the magnitude and direction of neglect. Stimulus positional uncertainty did not abolish pseudoneglect, indicating that bisection judgements are made within an object-centered frame of reference. Backward masking line stimuli had no influence on the magnitude of pseudoneglect, signifying that pseudoneglect is not a byproduct of covert directional scanning of the line stimulus in iconic or short-term visual memory. Finally, bisection errors are influenced by the direction of contrast gradients imposed on line stimuli, such that perceived line midpoint is drawn toward the lower-contrast line end. The magnitude and direction of pseudoneglect are modulated by stimulus factors that also influence the magnitude and direction of neglect. Both phenomena are succinctly described as biases in attention (i.e., neglect is a right-bias, whereas pseudoneglect is a left-bias). The two phenomena are modulated by stimulus factors as follows. Line length: there is an increased bias with increasing line length for both phenomena, and a cross-over to an reversed bias for short lines. Azimuthal line position: an increasing bias accompanies increasing leftward placement for both phenomena. Line aspect ratio: there is a decreasing bias with increasing line height for both phenomena. Line elevation: there is a decreasing bias with increasing elevation for neglect, and an increasing bias with increasing elevation for pseudoneglect. The only case in which a factors influence on the two phenomena is discrepant is for elevation, and this difference is explicable. Taken together these congruencies strongly support the notion that neglect and pseudoneglect are phenomena that are twin manifestations of parameter changes in a unitary set of underlying hemispheric attentional asymmetries.

A multiscale spatial filtering account of the White effect, simultaneous brightness contrast and grating induction

Blakeslee and McCourt ((1997) Vision Research, 37, 2849-2869) demonstrated that a multiscale array of two-dimensional difference-of-Gaussian (DOG) filters provided a simple but powerful model for explaining a number of seemingly complex features of grating induction (GI), while simultaneously encompassing salient features of brightness induction in simultaneous brightness contrast (SBC), brightness assimilation and Hermann Grid stimuli. The DOG model (and isotropic contrast models in general) cannot, however, account for another important group of brightness effects which includes the White effect (White (1979) Perception, 8, 413-416) and the demonstrations of Todorovic ((1997) Perception, 26, 379-395). This paper introduces an oriented DOG (ODOG) model which differs from the DOG model in that the filters are anisotropic and their outputs are pooled nonlinearly. The ODOG model qualitatively predicts the appearance of the test patches in the White effect, the Todorovic demonstration, GI and SBC, while quantitatively predicting the relative magnitudes of these brightness effects as measured psychophysically using brightness matching. The model also accounts for both the smooth transition in test patch brightness seen in the White effect (White & White (1985) Vision Research, 25, 1331-1335) when the relative phase of the test patch is varied relative to the inducing grating, and for the spatial variation of brightness across the test patch as measured using point-by-point brightness matching. Finally, the model predicts intensive aspects of brightness induction measured in a series of Todorovic stimuli as the arms of the test crosses are lengthened (Pessoa, Baratoff, Neumann & Todorokov (1998) Investigative Ophthalmology and Visual Science, Supplement, 39, S159), but fails in one condition. Although it is concluded that higher-level perceptual grouping factors may play a role in determining brightness in this instance, in general the psychophysical results and ODOG modeling argue strongly that the induced brightness phenomena of SBC, GI, the White effect and the Todorovic demonstration, primarily reflect early-stage cortical filtering operations in the visual system.

A spatial frequency dependent grating-induction effect

An inducing field containing a vertical sinewave luminance grating which surrounds a test field of similar space-average luminance induces within the homogeneous test field the appearance of a second sinewave grating of equal spatial frequency, but of opposite phase. The perceived contrast of the induced grating in the test field, as measured by a cancellation technique, can reach 90% that of the actual luminance contrast of the inducing grating. The perceived contrast of the induced grating decreases with increases in the spatial frequency of the inducing grating, and with increases in the dimension of the test field parallel to the orientation of the inducing grating. Square wave inducing gratings produce weaker induction effects than sinewave inducing gratings of the same spatial frequency and contrast. Additional observations indicate that the neural locus of this induction effect is cortical, lying at or beyond the level of spatial frequency selective channels.

A unified theory of brightness contrast and assimilation incorporating oriented multiscale spatial filtering and contrast normalization.

Brightness induction includes both contrast and assimilations effects. Brightness contrast occurs when the brightness of a test region shifts away from the brightness of adjacent regions. Brightness assimilation refers to the opposite situation in which the brightness of the test region shifts toward that of the surrounding regions. Interestingly, in the White effect [Perception 8 (1979) 413] the direction of the induced brightness change does not correlate with the amount of black or white border in contact with the gray test patch. This has led some investigators to reject spatial filtering explanations not only for the White effect but for brightness perception in general. Instead, these investigators have offered explanations based on a variety of junction analyses and/or perceptual organization schemes. Here, these approaches are challenged with a critical set of new psychophysical measurements that determined the magnitude of the White effect, the shifted White effect [Perception 10 (1981) 215] and the checkerboard illusion [R.L. DeValois, K.K. DeValois, Spatial Vision, Oxford University Press, NY, 1988] as a function of inducing pattern spatial frequency and test patch height. The oriented difference-of-Gaussians (ODOG) computational model of Blakeslee and McCourt [Vision Res. 39 (1999) 4361] parsimoniously accounts for the psychophysical data, and illustrates that mechanisms based on junction analysis or perceptual inference are not required to explain them. According to the ODOG model, brightness induction results from linear spatial filtering with an incomplete basis set (the finite array of spatial filters in the human visual system). In addition, orientation selectivity of the filters and contrast normalization across orientation channels are critical for explaining some brightness effects, such as the White effect.

Performance consistency of normal observers in forced-choice tachistoscopic visual line bisection

Pseudoneglect (PN) refers to the leftward error exhibited by normal observers on line bisection tasks (Bowers and Heilman, Neuropsychologia, 18, (1980) 491-8). Although a thorough review of the literature has shown PN to be relatively robust (Jewell and McCourt, Neuropsychologia, 38, (2000) 93-110), controversy remains concerning the reliability of the phenomenon, with some studies reporting a relatively high incidence of normal subjects with rightward bisection errors. The present experiment assesses the consistency of bisection performance in normal young observers. Right-handed subjects (N=22) participated in a tachistoscopic forced-choice line bisection task. Each subject participated in 7-16 experimental sessions separated by at least 24 h (total bisection measurements=317). Individual bisection performance could thus be evaluated with respect to within-subject variability measures. An eyetracker recorded gaze position during the task in one session. A highly significant mean group bisection error of -0.26 degrees (P<0.001) was obtained (left negative), and individual subject means ranged from -0.55 degrees to +0.03 degrees. Of the 317 total bisection measurements, 9% (28) deviated rightward. Significant (P<0.05) mean leftward errors occurred in 91% (20/22) of subjects. Mean bisection error in two subjects was not significantly different from zero. No subject possessed a significant rightward error. Mean gaze deviation from screen (and line) center ranged from +/-0.9 degrees, and was positively correlated (P<0.05) with bisection error. It is concluded that forced-choice tachistoscopic line bisection measures are highly reliable; a mean correlation of +0.87 exists between mean error based on 15 trials and means estimated from a random sample of only two trials. The incidence of true rightward bisection error in the population of normal right-handed subjects is thus estimated to be less than 5%.

Asymmetries of visuospatial attention are modulated by viewing distance and visual field elevation: Pseudoneglect in peripersonal and extrapersonal space

Many factors influence the degree of leftward error (pseudoneglect) that typifies the line bisection performance of normal subjects. We find that viewing distance also exerts a modulating influence on spatial attention in normal subjects, as it appears to do in neglect syndrome. Using forced-choice tachistoscopic line bisection, 38 right-handed subjects (15 male, 23 female) bisected horizontal lines (13.7 degrees w x 0.24 degrees h) presented in the midsagittal plane as a function of line elevation (- 3.6 degrees, 0 degrees, and 3.6 degrees relative to horizontal midline) and viewing distance (45 and 90 cm). We find a significant main effect of viewing distance, F (1, 37) = 10.04, p = .003, where pseudoneglect is larger in peripersonal (45 cm) than in extrapersonal (90 cm) space. We replicate an effect of line elevation, F (2, 74) = 4.40, p = .016, where pseudoneglect is greatest in the superior visual field (McCourt and Jewell, 1999). The interaction was not significant, p > .05. Thus, we find evidence for independent spatiotopic (viewing distance) and retinotopic (line elevation) effects on line bisection performance in normal observers, suggesting that the allocation of visuospatial attention is modulated within multiple frameworks.

Similar mechanisms underlie simultaneous brightness contrast and grating induction.

The experiments explore whether the mechanism(s) underlying grating induction (GI) can also account for simultaneous brightness contrast (SBC). At each of three test field heights (1, 3 and 6 deg), point-by-point brightness matches were obtained from two subjects for test field widths of 32 deg (GI condition), 14, 12, 8, 6, 3 and 1 deg. The point-by-point brightness matches were quantitatively compared, using GI condition matches as a standard, to assess systematic alterations in the structure and average magnitude of brightness and darkness induction within the test fields as a function of changing test field height and width. In the wider test fields induction structure was present and was generally well-accounted for by the GI condition sinewave predictions. As test field width decreased the sinewave amplitude of the induced structure in the test field decreased (i.e., flattened), and eventually became negative (i.e., showed a reverse cusping) at the narrower test field widths. As expected, both subjects showed a decrease in overall levels of brightness and darkness induction with increasing test field height. For any particular test field height, however, relative brightness increased with decreasing test field width. This brightness increase began at larger test field widths as test field height increased. The results are parsimoniously accounted for by the output of a weighted, octave-interval array of seven difference-of-gaussian filters. This array of filters differs from those previously employed to model various aspects of spatial vision in that it includes filters tuned to much lower spatial frequencies. The two-dimensional output of this same array of filters also accounts for the GI demonstrations of Zaidi [(1989) Vision Research, 29, 691-697], Shapley and Reids [(1985) Proceedings of the National Academy of Sciences USA, 82, 5983-5986] contrast and assimilation demonstration, and the induced spots seen at the street intersections of the Hermann Grid. The physiological plausibility of the filter array explanation of brightness induction is discussed, along with a consideration of its relationship to other models of brightness perception.

A multiscale spatial filtering account of the Wertheimer–Benary effect and the corrugated Mondrian

Blakeslee and McCourt [Blakeslee, B., & McCourt, M.E. (1997). Similar mechanisms underlie simultaneous brightness contrast and grating induction. Vision Research, 37, 2849-2869] demonstrated that a multiscale array of two-dimensional difference-of-Gaussian (DOG) filters provided a simple but powerful model for explaining a number of seemingly complex features of grating induction (GI), while simultaneously encompassing salient features of brightness induction in simultaneous brightness contrast (SBC), brightness assimilation and Hermann Grid stimuli. The DOG model (and isotropic contrast models in general) cannot, however, account for another important group of brightness effects including the White effect [White, M. (1997). A new effect of pattern on perceived lightness. Perception, 8, 413-416] and a variant of SBC [Todorovic, D. (1997). Lightness and junctions. Perception, 26, 379-395]. Blakeslee and McCourt [Blakeslee, B., McCourt, M.E. (1999). A multiscale spatial filtering account of the White effect, simultaneous brightness contrast and grating induction. Vision Research, 39, 4361-4377] developed a modified version of the model, an oriented (ODOG) model, which differed from the DOG model in that the filters were anisotropic and their outputs were pooled nonlinearly. Using this model, they were able to account for both groups of induction effects. The present paper examines two additional sets of brightness illusions that cannot be explained by isotropic contrast models. Psychophysical brightness matching is employed to quantitatively measure the size of the brightness effect for two Wertheimer-Benary stimuli [Benary, W. (1924). Beobachtungen zu einem experiment uber helligkeitskontrast. Psychologische Forschung, 5, 131-142; Todorovic, D. (1997). Lightness and junctions. Perception, 26, 379-395] and for low- and high-contrast versions of corrugated Mondrian stimuli [Adelson, E.H. (1993). Perceptual organization and the jugdement of brightness. Science, 262, 2042-2044; Todorovic, D. (1997). Lightness and junctions. Perception, 26, 379-395]. Brightness matches are obtained on both homogeneous and checkerboard matching backgrounds. The ODOG model qualitatively predicts the appearance of the test patches in the Wertheimer-Benary stimuli and corrugated Mondrian stimuli. In addition, it quantitatively predicts the relative magnitudes of the corrugated Mondrian effects in the various conditions. In general, the psychophysical results and ODOG modeling argue strongly that like SBC, GI, the White effect and Todorovics SBC demonstration, induced brightness in Wertheimer-Benary stimuli and in the corrugated Mondrian primarily reflects early-stage filtering operations in the visual system.

The effects of gender, menstrual phase and practice on the perceived location of the midsagittal plane

Women show menstrual phase-related cognitive changes that suggest altered hemispheric activation for a particular task, such that they demonstrate the greatest lateral performance differences on prototypical left hemisphere tasks during the luteal phase and on prototypical right hemisphere tasks during menstruation. Additionally, menstrual phase may alter total cerebral responsiveness, such that response times and performance accuracy for many tasks are best during the luteal phase and most impaired during the menstrual phase. We evaluated the effect of menstrual phase on spatial bisection (a perceptuospatial task) to help further understand hormonally-mediated changes in interhemispheric dynamics. Healthy young adult women and men blindly pointed to their midsagittal plane with either hand. Women were repeatedly tested according to menstrual phase, and men were tested at similar intervals. The mean pointing error in the luteal phase differed significantly from that of all other phases and did not differ significantly from those of men, who pointed significantly to the left across test sessions. These findings suggest that, in space bisection tasks, women are more likely to have asymmetric hemispheric activation during the luteal phase than during the menstrual phase. Thus, space bisection did not resemble other prototypical right hemisphere behaviors. The luteal phase may have nonspecifically activated both hemispheres on this task instead of suppressing right hemisphere function, and a slight functional asymmetry favoring the right hemisphere may have been promoted. In addition, intermanual pointing discrepancies in both subject groups decreased over repeated sessions. This suggests that, while practice alters an internal kinesthetic reference, it does not influence an imaginal extrapersonal spatial reference.