Anna Oleksiak

The time course of hemispheric differences in categorical and coordinate spatial processing

Spatial relations between objects can be represented either categorically or coordinately. The metric, coordinate representation is associated with predominant right hemisphere activity, while the abstract, qualitative categorical representation is thought to be processed more in the left hemisphere [Kosslyn, S. M. (1987). Seeing and imagining in the cerebral hemispheres: A computational analysis. Psychological Review, 94, 148-175]. This hypothesized lateralization effect has been found in a number of studies, along with indications that specific task demands can be crucial for these outcomes. In the current experiment a new visual half field task was used which explores these hemispheric differences and their time course by means of a match-to-sample design. Within retention intervals that were brief (500 ms), intermediate (2000 ms), or long (5000 ms), the processing of categorical and coordinate representations was studied. In the 500 ms interval, the hemispheric effect suggested by Kosslyn (1987) was found, but in the longer intervals it was absent. This pattern of the lateralization effect is proposed to be caused by the differential effect the retention interval has on coordinate and categorical representations. Coordinate spatial relations appear susceptible to changes in retention interval and decay very quickly over time, congruent with previous findings about accurate location memory. The processing of categorical spatial relations showed less decay and only between 2000 ms and 5000 ms. Qualitative self reports suggest that the decay found for categorical relations might be caused by a switch from a visual to a more verbal memorization strategy.

Categorical and coordinate spatial relations in working memory: An fMRI study

Spatial relations within and between objects can be represented either coordinately or categorically. Coordinate representations concern metric and precise relations, and are strongly associated with right parietal cortex activity, while categorical representations relate to more qualitative, abstract relations, and have shown to have a, somewhat weaker, relationship with left parietal cortex activation [Trojano et al., 2002. Coordinate and categorical judgements in spatial imagery. An fMRI study. Neuropsychologia, 40, 1666-1674]. In the current study, a functional magnetic resonance imaging (fMRI) experiment enabled a closer examination of this proposed hemispheric lateralization within a working memory paradigm. A visual half field task in a match-to-sample format was conducted to examine these lateralization effects with a short (500 ms) and a long (2000 ms) interval between two stimuli, with either a categorical or a coordinate instruction. In the behavioural data, the hypothesized hemispheric specialization was found for the brief interval. The imaging data support the hemispheric lateralization as well. The proposed lateralization effect is present during spatial relation processing, but only within the superior parietal cortex and with certain temporal constraints. Additionally, categorical trials show a clear involvement of the left and right premotor and posterior parietal areas during the brief interval, while coordinate trials are related to higher activity in the left and right insula, during the long interval. We propose a refined view on lateralization of spatial relation processing, keeping in mind the temporal restrictions shown by this study.

The influence of biological motion perception on structure-from-motion interpretations at different speeds.

Nonrigid point-light representations of biological motion are ideal to test higher level influences on structure-from-motion (SFM) perception. Here, we investigated the influence of biological motion perception on 3D SFM interpretations at different speeds. We presented nonrigid biological motion and rigid structures rotating around the vertical axis. The familiarity of the stimuli was changed by presenting three walker types: normal, inverted, and phase scrambled. Subjects had to discriminate rotation in depth and rigidity. We found that at lower-than-natural gait speeds, subjects perceived nonrigid biological motion to be rotating in depth, especially when the walker type was less familiar. In contrast, the percept of rigidity was correct at all speeds. A second experiment, in which a constant fraction of the gait cycle was presented, confirmed the influence of speed and additionally showed that brief displays of a familiar form at a high speed facilitate biological motion interpretations. The more veridical percept of rotation toward higher speeds fits the idea of biological motion channels tuned to higher-more natural walking-speeds that overrule a default assumption to perceive trajectories in depth. We also speculate that the rotation-in-depth percept at lower speeds points toward the existence of low-speed-tuned object motion channels.

A review of lateralization of spatial functioning in nonhuman primates

The majority of research on functional cerebral lateralization in primates revolves around vocal abilities, addressing the evolutionary origin of the human language faculty and its predominance in the left hemisphere of the brain. Right hemisphere specialization in spatial cognition is commonly reported in humans. This functional asymmetry is especially evident in the context of the unilateral neglect, a deficit in attention to and awareness of one side of space, that more frequently occurs after right-side rather than left-side brain damage. Since most of the research efforts are concentrated on vocalization in primates, much less is known about the presence or absence of spatial functions lateralization. Obtaining this knowledge can provide insight into the evolutionary aspect of the functionally lateralized brain of Homo sapiens and deliver refinement and validation of the nonhuman primate unilateral neglect model. This paper reviews the literature on functional brain asymmetries in processing spatial information, limiting the search to nonhuman primates, and concludes there is no clear evidence that monkeys process spatial information with different efficiency in the two hemispheres. We suggest that lateralization of spatial cognition in humans represents a relatively new feature on the evolutionary time scale, possibly developed as a by-product of the left hemisphere intrusion of language competence. Further, we argue that the monkey model of hemispatial neglect requires reconsideration.

The effect of stimulus features on working memory of categorical and coordinate spatial relations in patients with unilateral brain damage

Spatial relations are typically divided into categorical and coordinate spatial relations. Categorical relations are abstract and show a left hemisphere (LH) advantage, whereas coordinate relations are metric and related to a right hemisphere (RH) advantage. In the current study a working memory task was used to asses categorical and coordinate performance with two different stimulus sets. In this task, participants had to compare two sequentially presented stimuli, consisting of a dot and a cross. The cross size used in the stimuli was either large or small; a direct manipulation of the amount of information provided to determine a category, or to assess a distance. Patients with damage in the LH or the RH and highly comparable controls were tested. In control participants, categorical processing is faster with the use of a large cross, i.e., more visual information about category boundaries. In contrast, coordinate performance was more accurate with a small cross, i.e., presenting less unnecessary visual information. LH patients showed a specific defect in processing categorical stimuli with a small cross and coordinate stimuli with a large cross. The RH patients were impaired in all conditions except for the categorical small cross condition. We conclude that a larger amount of information present in stimuli increases categorical processing performance and decreases coordinate processing performance, while opposite effects are found for less stimulus information.

Retinotopic mapping of categorical and coordinate spatial relation processing in early visual cortex

Spatial relations are commonly divided in two global classes. Categorical relations concern abstract relations which define areas of spatial equivalence, whereas coordinate relations are metric and concern exact distances. Categorical and coordinate relation processing are thought to rely on at least partially separate neurocognitive mechanisms, as reflected by differential lateralization patterns, in particular in the parietal cortex. In this study we address this textbook principle from a new angle. We studied retinotopic activation in early visual cortex, as a reflection of attentional distribution, in a spatial working memory task with either a categorical or a coordinate instruction. Participants were asked to memorize a dot position, with regard to a central cross, and to indicate whether a subsequent dot position matched the first dot position, either categorically (opposite quadrant of the cross) or coordinately (same distance to the centre of the cross). BOLD responses across the retinotopic maps of V1, V2, and V3 indicate that the spatial distribution of cortical activity was different for categorical and coordinate instructions throughout the retention interval; a more local focus was found during categorical processing, whereas focus was more global for coordinate processing. This effect was strongest for V3, approached significance in V2 and was absent in V1. Furthermore, during stimulus presentation the two instructions led to different levels of activation in V3 during stimulus encoding; a stronger increase in activity was found for categorical processing. Together this is the first demonstration that instructions for specific types of spatial relations may yield distinct attentional patterns which are already reflected in activity early in the visual cortex.

Temporal characteristics of working memory for spatial relations: an ERP study.

Spatial relations can be represented categorically, by means of abstract labels, or coordinately, in metric, absolute measures. These representations have been associated to the left and the right hemispheres respectively (Kosslyn, 1987). Recent studies have focused on the temporal dynamics of spatial relation processing, with working memory task designs. In this light, we examined the suggested lateralization effect in an ERP study incorporating a visual half field match-to-sample design, in which two sequentially presented stimuli were compared. By manipulating the length of the retention intervals between the two stimuli (500 ms, 2000 ms, and 5000 ms), spatial working memory effects were studied at three separate stages of working memory; encoding, memorization, and retrieval. The hypothesized interaction of instruction and visual field was found in the behavioural data, restricted to the 2000 ms retention interval. The EEG data indicate a strong overall right hemisphere effect, which is likely related to spatial working memory in general. Categorical and coordinate processing appears to already differentiate during the encoding stage in the P300 complex (300-500 ms after presentation of the first stimulus), where instruction interacts significantly with hemisphere in the parietal area. We found a clear right hemisphere advantage for coordinate processing and no lateralization for categorical processing. We argue that the outcome indicates qualitative rather than quantitative differences between categorical and coordinate processing.

Intermittent stimulus presentation stabilizes neuronal responses in macaque area MT

Repeated stimulation impacts neuronal responses. Here we show how response characteristics of sensory neurons in macaque visual cortex are influenced by the duration of the interruptions during intermittent stimulus presentation. Besides effects on response magnitude consistent with neuronal adaptation, the response variability was also systematically influenced. Spike rate variability in motion-sensitive area MT decreased when interruption durations were systematically increased from 250 to 2,000 ms. Activity fluctuations between subsequent trials and Fano factors over full response sequences were both lower with longer interruptions, while spike timing patterns became more regular. These variability changes partially depended on the response magnitude, but another significant effect that was uncorrelated with adaptation-induced changes in response magnitude was also present. Reduced response variability was furthermore accompanied by changes in spike-field coherence, pointing to the possibility that reduced spiking variability results from interactions in the local cortical network. While neuronal response stabilization may be a general effect of repeated sensory stimulation, we discuss its potential link with the phenomenon of perceptual stabilization of ambiguous stimuli as a result of interrupted presentation.

Temporal dynamics of decisions on spatial categories and distances do not differ

It has been proposed that spatial relations can be encoded in two different ways: categorically, where the relative position of objects can be described in prepositional terms (to the left/right, above/below, etc.) and coordinately, where a precise distance between the objects is assessed. Processing of categorical and coordinate spatial relations is believed to rely on the parvo- and magnocellular pathways or small and large receptive fields, respectively. We employed the response signal speed-accuracy trade-off procedure to obtain a description of temporal dynamics of information transfer for categorical and coordinate spatial decisions. In the two tasks the same procedure and stimuli were used, while the instructions called for different types of discrimination. We found no differences in information accrual speed between the tasks as would be expected from the parvo/magno cells or small/large receptive fields distinction. Theoretical consequences of these findings are discussed.