Mark Chappell

Dividing attention in the flash-lag illusion

A dual-task paradigm was used to examine the effect of withdrawing attentional and/or cognitive resources from the flash-lag judgment. The flash-lag illusion was larger, and performance in a detection task was generally poorer, under dual-task conditions than in single-task control conditions. These effects were particularly pronounced when decisions in the two tasks were required simultaneously, as compared to when they could be made sequentially. The results suggest that a time-consuming process is involved in the flash-lag decision, of such a nature that prolonging the process increases the magnitude of the illusion.

Planning following stroke: a relational complexity approach using the Tower of London

Planning on the 4-disk version of the Tower of London (TOL4) was examined in stroke patients and unimpaired controls. Overall TOL4 solution scores indicated impaired planning in the frontal stroke but not non-frontal stroke patients. Consistent with the claim that processing the relations between current states, intermediate states, and goal states is a key process in planning, the domain-general relational complexity metric was a good indicator of the experienced difficulty of TOL4 problems. The relational complexity metric shared variance with task-specific metrics of moves to solution and search depth. Frontal stroke patients showed impaired planning compared to controls on problems at all three complexity levels, but at only two of the three levels of moves to solution, search depth and goal ambiguity. Non-frontal stroke patients showed impaired planning only on the most difficult quaternary-relational and high search depth problems. An independent measure of relational processing (viz., Latin square task) predicted TOL4 solution scores after controlling for stroke status and location, and executive processing (Trail Making Test). The findings suggest that planning involves a domain-general capacity for relational processing that depends on the frontal brain regions.

Is there an auditory-visual 'flash-lag' effect?

A flash adjacent to the path of a moving object appears behind the moving object: the ‘flash‐lag effect’. We sought to test the flash‐lag effect with a ‘click’ instead of a flash: a white triangle horizontally traversed the screen at a constant 12°/s passing through a fixation cross in the presence of a quiet click. The subject judged whether the click occurred before or after the triangle passed through the cross. To be perceived as co‐instantaneous events, the click had to be presented 127 ms after the moving triangle reached the cross (a ‘click‐lead’ effect, providing falsification of predictive accounts of the flash‐lag effect), as opposed to a standard flash‐lag effect condition where a flashed triangle replaced the click and had to appear 60 ms before the moving triangle to appear aligned. With the auditory versus visual processing speed advantage considered, the neural time required to calculate a moving objects position is constant, independent of the modality of the flag.

Effect of motion smoothness on the flash-lag illusion

Two flash-lag experiments were performed in which the moving object was flashed in a succession of locations creating apparent motion and the inter-stimulus distance (ISD) between those locations was varied. In the first (n=10), the size of the flash-lag illusion was a declining non-linear function of the ISD and the largest reduction in its magnitude corresponded closely to the value where observers judged the continuity of optimal apparent motion to be lost. In the second (n=11) with large ISDs, we found the largest illusions when the flash initiated the movement, and no effect was observed when the flash terminated the movement. The data support motion position biasing or temporal integration accounts of the illusion with processing predominantly based on motion after the flash.

Relational processing following stroke

The research examined relational processing following stroke. Stroke patients (14 with frontal, 30 with non-frontal lesions) and 41 matched controls completed four relational processing tasks: sentence comprehension, Latin square matrix completion, modified Dimensional Change Card Sorting, and n-back. Each task included items at two or three levels of relational complexity. Relational processing was impaired in the stroke groups. This was due mainly to items at the intermediate ternary-relational level of complexity. Less complex binary-relational items and more complex quaternary-relational items (the latter are difficult for adults generally) were less sensitive to stroke status. Impairment was greater in frontal than non-frontal stroke patients. Positive inter-correlations among measures supported the domain-general nature of relational processing. Implications for assessment and intervention are discussed.

Mapping a field of suppression surrounding visual stimuli.

The brightness of a small incremental flash was found to be considerably suppressed in the vicinity of a moving visual stimulus (effect size, d, up to 6) and less so around a stationary stimulus. The pattern of suppression was mapped and extended 3.5 degrees away from a stationary stimulus and 10.5 degrees behind, and ahead of, a moving stimulus. A second experiment found that dark flashes appeared less dark in the presence of an inducing stimulus of either polarity. Combined results suggest that perceived contrast was being suppressed, in all cases by an inducing stimulus of lesser contrast, and in most cases by an inducing stimulus of lesser luminance. These findings were compared with a number of recent models of the perception of the position of moving visual stimuli. These assume that in the wake of such a stimulus, at certain retinal or cortical areas, there is a region of neural inhibition and that, preceding them, there is a (bow-wave-like) region of neural excitation. The current findings confirm the inhibitory, but not the excitatory, assumptions in these theories.

Verbal learning and memory following stroke

Abstract Objective: The research examined whether verbal learning and memory impairment previously observed 1 year after left hemisphere stroke endures over a longer period and whether stroke sufferers compensate for their impairments using working memory. Methodology: Twenty-one persons with left hemisphere lesions; 20 with right hemisphere lesions only and 41 matched controls completed the Hopkins Verbal Learning Test-Revised (HVLT-R), a working memory test (Letter-Number Sequencing, LNS) and the Boston Naming Test (BNT). Results: Persons with left hemisphere damage performed more poorly on HVLT-R than controls. They showed poorer immediate recall, delayed recall, recognition and learning, but intact retention, suggesting an encoding impairment. BNT and LNS scores predicted recall in this group. HVLT-R performance of persons with right hemisphere lesions only was comparable to controls. BNT (not LNS) predicted recall in these groups. Conclusions: Persons with left hemisphere damage relied more on working memory and recruited diverse left hemisphere regions to compensate for their impaired encoding. Implications: Tasks requiring verbal encoding and memory are effortful following left hemisphere stroke. This should be recognized and accommodated.

The flash-lag effect and equiluminance

An object briefly flashed adjacent to the path of another moving object appears to spatially lag the moving object in the direction of its motion: the ‘flash‐lag effect’. A simple differential lag model account of this effect suggests that it occurs because the moving object activates motion detectors in the faster magnocellular pathway, whereas the flashed object does not. This model was tested by reducing M‐pathway involvement using isoluminant stimuli. All four participants, who were university undergraduate students, were exposed to eight conditions, involving all possible combinations of moving and flashing objects coloured either white or green, shown against either a grey or a black background. Green objects were equiluminant with the grey background. The magnitude of the flash‐lag effect was found using the method of constant stimuli. No reliable support was found for the hypothesis that equiluminance of the moving object reduces the flash‐lag effect. Instead an interaction was found where there was an effect of equiluminance on the flash, but only when the moving object was not equiluminant. Such data is problematic for this and other simple differential lag models of the flash‐lag effect.

New twists for an old turning illusion

A Vernier-offset illusion induced by rotating lines, introduced by Matin et al (1976 Perception & Psychophysics 20 138–142) was re-examined using onset, offset, and reverse trajectories inspired by flash-lag illusion research, with both Vernier and alignment-with-vertical judgments being recorded. The pattern of illusions found was generally in agreement with a differential latency of stimulus ends account described by those authors, although certain variants of modern spatial projection theories, and a differential latency of attribute account, could also accommodate much of the data.

Testing Theories of Visual Position Perception by Manipulating Magnocellular Processing for Various Motion Trajectories

Spatial projection theories of visual localization posit that a moving stimulus’ perceived position is projected forwards in order to compensate for processing delays (Eagleman & Sejnowski, 2007; Nijhawan, 2008). Temporal integration theories (Krekelberg & Lappe, 2000) suggest that an averaging over positions occupied by the moving stimulus for a period of time is the dominant process underlying perception of position. Contrary to the predictions of these theories, reducing magnocellular (M) pathway processing by making stimuli equiluminant, and adding luminance noise (see Fig. 1A), had the opposite effect on localization judgments, assessed via the flash-lag illusion, when a smooth, continuous trajectory was used, compared to when the moving object suddenly appeared, or suddenly reversed direction (Fig. 1B, for interaction, F[2, 22] = 59.91, p = 1.3 × 10-9, eta_P^2 = .85). In addition, we have preliminary data indicating that simply reducing the contrast of the moving stimulus, without adding luminance noise, also yields a cross-over interaction similar to that shown in Fig. 1B. In order to explain the perception of the position of moving objects across all trajectories, our cross-over interaction result necessitates processes additional to those proposed by either the spatial projection or temporal integration theories. It also calls into question the utility of the onset and reversal trajectories for testing theories of localization for continuous trajectories.