The asymmetry of attention in visual processing

Chairs:  Rolf Verleger 

Dept. of neurology, University of Lübeck, Germany


Abstract:

Hardly noticeable in everyday life, our visual processing system consists of two separately functioning linked halves with asymmetrically distributed strengths. Here, this division of labor between brain hemispheres will be highlighted by focusing on right-hemisphere (RH) advantages in target identification presented in the left visual field. Methods include experimental psychology, neuropsychology, and fMRI and ERP measurements. In detail:

1) The second of two targets in rapid serial presentation is better identified in the left visual field. Verleger has studied how this RH advantage depends on the qualities of targets and distractors, and demonstrates that this advantage goes along with earlier ERP latencies in RH.

2) The division of labor between the two hemispheres is generally regarded as stable. Yet Finke shows how the RH advantage in identifying briefly presented stimuli depends on tonic alertness, and further corroborates this relationship by data from patients with hemi-neglect.

3) In a similar vein, O'Connell shows how the RH advantage in identifying briefly presented stimuli varies across time-on-task and is reflected by ERP and EEG measures.

4) Measuring parietal priming effects on motor-cortex activation by TMS impulses applied simultaneously at three sites, Koch reports inhibitory effects of the right parietal cortex on the left but not vice versa, and confirms these results by DTI.

5) The classic model of attentional control assumes that the left hemisphere covers the right visual field only whereas the RH covers both left and right visual fields. An alternative model is proposed by Hendler based on her fMRI and ERP measurements.

Altogether, these studies on the specific strengths of right-hemisphere visual processing may contribute to better understanding the pathologies of the right-hemisphere visual system, including disturbances of spatial sense and the hemi-neglect syndrome.

Talk 1:

Laterally presented targets in rapid visual series: Why left is better than right

Rolf Verleger¹, Kamila Śmigasiewicz¹ & Dariusz Asanowicz^2
1 University of Lübeck, Germany
² Jagellonian University Kraków, Poland

When two letter streams containing two targets (T1 and T2) are presented left and right, T2 is better identified in the left visual field. This remains true under strict fixation control and also holds true in right-to-left and top-to-bottom readers. By varying the type of targets and distractors, by comparing left & right to top & bottom streams, by degrading stimuli, by cueing T2 side, and by measuring ERPs, we studied whether this advantage is due to better processing in the right hemisphere or to overload of the left one.

Visual evoked potentials (VEPs) evoked by the ongoing stream of distractor stimuli peaked earlier at the right than at the left hemisphere at the onset of the stimulus series already, T2-evoked N2pc peaked earlier and T2-evoked P3 was larger with left than with right T2. Out of our many experimental variations of stimulus material, the most consistent interaction with the left-visual-field advantage was obtained by cueing T2 side in advance. Thus, the advantage probably arises due to better sensitivity of the right hemisphere to attentional cueing induced by the targets. The ERP results suggest that this might be due to both speed and capacity advantages of the right hemisphere.

Talk 2:

Dependence of hemispheric asymmetry on alertness in healthy subjects and in patients with focal brain lesions

Kathrin Finke¹, Ellen Matthias¹ &  Peter Bublak^2
1 Ludwig-Maximilians-University Munich, Germany
² Friedrich Schiller University Jena, Germany


Typically, in healthy subjects watching a multiple-object display, a slight processing advantage can be observed for left as compared to right visual field stimuli. This subtle visuospatial bias in attentional competition is assumed to result from a functional dominance of the right hemisphere for spatial attention. As evidenced by visual hemi-neglect patients, damage to this right-hemispheric spatial attention system gives rise to a clear rightward bias and visual extinction for left-sided stimuli. Recent evidence, however, indicates that this holds true only for the “default state” of arousal, i.e. in conditions of adequate intrinsic alertness in healthy subjects and of low-level intrinsic alertness in neglect patients.
In a series of studies we systematically investigated the consequences of experimental manipulations of arousal level on spatial and non-spatial components of visual attention in healthy subjects with different baseline arousal levels as well as in neglect patients. Our paradigms were based on Bundesen's (1990) theory of visual attention, which permits to extract parametric estimates of independent components of selective attention.
Our results indicate that the degree and the direction of spatial attentional imbalances are strongly influenced by arousal level: On the one hand, persons with a low general level of intrinsic alertness are specifically vulnerable to leftward extinction. On the other hand, enhancement of phasic alertness can optimize the spatial distribution of attentional weights even in neglect patients. We conclude that the brain system underlying spatial attention is decisively influenced by the alertness system and that efficient neglect treatment might build upon this relationship.

Talk 3:

Attentional load asymmetrically affects early electrophysiological indices of visual orienting

Redmond O'Connell¹, Edmund Lalor¹ & Mark Bellgrove^2
1 Trinity College Institute of Neuroscience, Dublin, Ireland
² University of Queensland, Australia

Recent behavioral studies suggest that asymmetries in visuospatial orienting are modulated by changes in the demand on nonspatial components of attention but the brain correlates of this modulation are unknown. Participants performed a dual-task involving monitoring for transient targets appearing at central and lateralised locations. Behavioural analysis indicated a subtle reaction time bias favouring targets appearing in the left visual field. Manipulation of the attentional load of the central task led to a disruption of the right hemisphere orienting response as indexed by ERP (N1) and source analysis. Further analysis revealed that participants’ left bias was abolished with increasing time-on-task and this was accompanied by a failure to suppress alpha activity during the inter-target interval. These data support the view that spatial asymmetries in visual orienting are modulated by nonspatial attention due to overlapping neural circuits within the right
hemisphere. Finally, we highlight some new experiments designed to isolate continuous electrophysiological markers of visuo-spatial bias.

Talk 4:

Asymmetry of parietal interhemispheric connections in humans

Giacomo Koch
Fondazione S Lucia IRCCS


Visuospatial abilities are preferentially mediated by the right hemisphere. Although this asymmetry of function is thought to be due to an unbalanced interaction between cerebral hemispheres, the underlying neurophysiological substrate is still largely unknown. Here, using a method of trifocal transcranial magnetic stimulation, we show that the right, but not left, human posterior parietal cortex exerts a strong inhibitory activity over the contralateral homologous area by a short-latency connection. We also clarify, using diffusion-tensor magnetic resonance imaging, that such an interaction is mediated by direct transcallosal projections located in the posterior corpus callosum. We argue that this anatomo-functional network may represent a possible neurophysiological basis for the ongoing functional asymmetry between parietal cortices, and that its damage could contribute to the clinical manifestations of neglect.

Talk 5:

Mind Your Left! It is the left visual field rather than the right hemisphere

Talma Hendler, Tali Siman Tov, Ilana Podlipsky, Hadas Okon Singer
Tel-Aviv Sourasky Medical Center and University of Tel-Aviv, Israel


A leftward bias is well known in humans and animals, and has been commonly related to the right (R) hemisphere dominance for spatial attention. Previous fMRI studies from our lab suggest that this bias is mediated by faster conduction from the R to left (L) parietal cortices than vice versa, supporting the importance of inter-hemispheric functional connectivity in the leftward bias. Furthermore, bilateral differential activations were demonstrated for fearful vs. neutral faces only if presented in the L hemifield. This was true not only in a core limbic region, the amygdala, but also in major visual-attention and orientation subcortical nodes, the pulvinar and superior colliculus, respectively. These results are consistent with the well- known leftward bias of danger-associated behaviors in animals. Recently, by using simultaneous EEG-fMRI and causal modeling, we found an association between faster neural conduction (ERPs) from R to L hemisphere and higher fMRI-activation in the left pulvinar, probably led by the R hemisphere. This finding suggests
the involvement of major sub-cortical attention nodes even in non-emotional leftward bias.
Together this multi-scale evidence highlights the relation between hemispheric dominance and asymmetric inter-hemispheric information transfer as the underlying mechanism of leftward bias in spatial attention. This insight may guide future efforts in alleviating attention deficits by focusing on brain-based diagnosis and improving delays in network connectivity. Additionally, the proposed neural model for asymmetry of visuospatial attention might provide important insights into the mechanisms underlying functional brain lateralization in general.