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Volume 3 Issue 4 (2007)

The role of spatial and temporal information in biological motion perception original article

pp. 419-428 | First published on 31 December 2007 | DOI:10.2478/v10053-008-0006-3

Joachim Lange, Markus Lappe

Abstract

Point-light biological motion stimuli provide spatio-temporal information about the structure of the human body in motion. Manipulation of the spatial structure of point-light stimuli reduces the ability of human observers to perceive biological motion. A recent study has reported that interference with the spatial structure of point light walkers also reduces the evoked event related potentials over the occipitotemporal cortex, but that interference with the temporal structure of the stimuli evoked event-related potentials similar to normal biological motion stimuli. We systematically investigated the influence of spatial and temporal manipulation on 2 common discrimination tasks and compared it with predictions of a neurocomputational model previously proposed. This model first analyzes the spatial structure of the stimulus independently of the temporal information to derive body posture and subsequently analyzes the temporal sequence of body postures to derive movement direction. Similar to the model predictions, the psychophysical results show that human observers need only intact spatial configuration of the stimulus to discriminate the facing direction of a point-light walker. In contrast, movement direction discrimination needs a fully intact spatio-temporal pattern of the stimulus. The activation levels in the model predict the observed event related potentials for the spatial and temporal manipulations.

Keywords: biological motion, model, task dependency, event-related potentials

Attentional demand influences strategies for encoding into visual working memory original article

pp. 429-448 | First published on 31 December 2007 | DOI:10.2478/v10053-008-0007-2

Jutta S. Mayer, Robert A. Bittner, David E. J. Linden, Danko Nikolić

Abstract

Visual selective attention and visual working memory (WM) share the same capacity-limited resources. We investigated whether and how participants can cope with a task in which these 2 mechanisms interfere. The task required participants to scan an array of 9 objects in order to select the target locations and to encode the items presented at these locations into WM (1 to 5 shapes). Determination of the target locations required either few attentional resources ("pop-out condition") or an attention-demanding serial search ("non pop-out condition"). Participants were able to achieve high memory performance in all stimulation conditions but, in the non pop-out conditions, this came at the cost of additional processing time. Both empirical evidence and subjective reports suggest that participants invested the additional time in memorizing the locations of all target objects prior to the encoding of their shapes into WM. Thus, they seemed to be unable to interleave the steps of search with those of encoding. We propose that the memory for target locations substitutes for perceptual pop-out and thus may be the key component that allows for flexible coping with the common processing limitations of visual WM and attention. The findings have implications for understanding how we cope with real-life situations in which the demands on visual attention and WM occur simultaneously.

Keywords: attention, working memory, interference, encoding strategies

Electrophysiological activation by masked primes: Independence of prime-related and target-related activities original article

pp. 449-465 | First published on 31 December 2007 | DOI:10.2478/v10053-008-0008-1

Werner Klotz, Manfred Heumann, Ulrich Ansorge, Odmar Neumann

Abstract

Visual stimuli that are made invisible by metacontrast masking (primes) have a marked influence on behavioral and psychophysiological measures such as reaction time (RT) and the lateralized readiness potential (LRP). 4 experiments are reported that shed light on the effects that masked primes have on the LRP. Participants had a go-nogo task in which the prime was associated with 1 of 2 responses even if the target required participants to refrain from responding. To analyze the electrophysiological responses, we computed the LRP and applied an averaging method separating the activation due to the prime and the target. The results demonstrated that (a) masked primes activate responses even in a nogo situation, (b) this prime-related activation is independent of masking, (c) and is also independent of whether prime and target require the same responses (congruent condition) or different responses (incongruent condition).

Keywords: metacontrast, EEG recording, LRP, Go/Nogo