The "Poffenberger" and "Dimond" paradigms: interrelated approaches to the study of interhemispheric dynamics?

Modulation of Interhemispheric Inhibition between Primary Motor Cortices Induced by Manual Motor Imitation: A Transcranial Magnetic Stimulation Study

Imitation has been proven effective in motor development and neurorehabilitation. However, the relationship between imitation and interhemispheric inhibition (IHI) remains unclear. Transcranial magnetic stimulation (TMS) can be used to investigate IHI. In this study, the modification effects of IHI resulting from mirror neuron system (MNS) activation during different imitations are addressed. We measured IHI between homologous primary motor cortex (M1) by analyzing the ipsilateral silent period (iSP) evoked by single-pulse focal TMS during imitation and analyzed the respective IHI modulation during and after different patterns of imitation. Our main results showed that throughout anatomical imitation, significant time-course changes of iSP duration through the experiment were observed in both directions. iSP duration declined from the pre-imitation time point to the post-imitation time point and did not return to baseline after 30 min rest. We also observed significant iSP reduction from the right hemisphere to the left hemisphere during anatomical and specular imitation, compared with non-imitative movement. Our findings indicate that using anatomical imitation in action observation and execution therapy promotes functional recovery in neurorehabilitation by regulating IHI.

Callosal Sensitivity to Short-Range Stimulus Orientation and Long-Range Stimulus Context Orientation: Tachistoscopic Evidence

To study local-global relationships in interhemispheric interactions, tachistoscopically presented pairs of lines (1.15 degrees) were compared for their relative orientation by 48 neurotypical adults. Orientations of line stimuli (local aspect of the task) were vertical, horizontal, forward slash or backslash, as were those of the interstimulus axes. The latter created a global context that could influence line discrimination. Stimulus pairs were presented within a field (not requiring callosal participation for line orientation comparison) or one on each side of the visual field meridian (requiring callosal participation). The primary purpose of the design was to determine whether local or global violations of stimulus "homotopy" across the meridian would impose costs of interhemispheric integration. The rationale for this expectation is that the fiber projection of the corpus callosum is highly symmetric across the midsagittal plane (i.e., homotopic). The expected "callosal homotopy" effect was significantly upheld as a whole but broke down or became extravagant in certain specific conditions, with specific costs of interhemispheric integration varying from null to a highly significant 20-ms as a function of interactions of interstimulus and stimulus orientations. The corpus callosum seems to be particularly sensitive to local stimulus orientation in interaction with long-range stimulus context orientation.

Intra and inter hemispheric dynamics revealed by reaction time in the Dimond paradigm: a quantitative review of the literature

In stimulus matching tasks requiring discrimination of two unilaterally or bilaterally presented stimuli (Dimond paradigm), a well established intrahemispheric processing bottleneck model predicts that an increase in task difficulty as measured by reaction time should provide an advantage to bilateral stimulations. The purpose of the current investigation was to review the entire relevant literature on the Dimond paradigm and identify the experimental variables which reliably yield such effects. Forty nine experimental effects compatible with the "intrahemispheric processing bottleneck" model and 26 contrary effects were found. Manipulation of the complexity of the stimulus matching criterion significantly produced intrahemispheric bottleneck effects. This effect was also significantly greater when non-target stimuli required heavier processing. These two findings support the intrahemispheric bottleneck model: computationally complex tasks seem to overload a hemisphere׳s processing capacity, an effect seen in the unilateral presentation conditions. However, manipulating the similarity of target stimuli produced contrary effects. Contrary effects were also obtained more readily when two physical matching tasks were compared. These two latter effects may best be explained as low level visual-perceptual limitations of interhemispheric transfer or integration.

New variations of intrahemispheric and interhemispheric processing indexed by the dimond paradigm

We report 2 experiments designed to demonstrate that unilateral tachistoscopic stimulation would yield a response time (RT) advantage over bilateral stimulation in a simple experiment, whereas the opposite pattern would occur in a complex version of the same task, as predicted by the intrahemispheric resource limitation model of Banich and colleagues. Experiment 1 was a go/no-go task in which participants had to press a key when two shapes (circles or squares) were identical on the computer screen. A unilateral field advantage was obtained that was accentuated in several task conditions that yielded overall longer RTs, mostly in the bilateral condition. Experiment 2 was similar but required a more complex judgment: The go trials were to 2 stimuli identical on 1 dimension (shape or color) but not both or neither. The RTs were significantly and substantially longer than in Experiment 1 and exhibited a nonsignificant bilateral field advantage, which differed significantly from the unilateral field advantage obtained in Experiment 1. These results support the intrahemispheric resource limitation model of Banich and colleagues. However, several within-experiment effects are in direct opposition to this model and are best explained as limitations of commissural relay of perceptual information.

Homotopy and heterotopy and the bilateral field advantage in the Dimond paradigm

Pairs of homotopic and heterotopic visual bilateral stimuli and pairs of unilateral visual stimuli were presented to 12 normal right-handed university students requiring a key press if they were of the same form. As predicted from the known histology of the corpus callosum (massive preponderance of homotopic fibers), homotopic presentations yielded significantly faster reaction times than heterotopic stimulations. Bilateral pairs of stimuli were also advantaged in comparison with unilateral trials, replicating Sereno and Kosslyn [Sereno, A. B., & Kosslyn, S. M. (1991). Discrimination within and between hemifields: a new constraint on theories of attention. Neuropsychologia, 29, 659-675]. Moreover, certain attentional processes have never been investigated in the Dimond paradigm and this study provides evidence to the effect that discriminative reaction times to stimulus pairs are strongly influenced by their proximity to the fixation point. In similar previous experiments, the homotopy/heterotopy observation and the bilateral field advantage may have been distorted by that particular confound, as well as several others.

Efficiency of callosal transfer and hemispheric interaction

The corpus callosum (CC) is essential to hemispheric interaction, but it is unclear how individual callosal properties affect interaction between the cerebral hemispheres. A number of studies have demonstrated some relationship between morphology or structure of the CC and measures of hemispheric interaction. However, to the authors' knowledge, none of these studies has been able to show a clear, direct relationship between a behavioral measure of transfer speed through the CC, interhemispheric transfer time (IHTT), and a behavioral measure of hemispheric interaction. Eighty participants were tested over 6 sessions on 2 tasks assessing IHTT and hemispheric interaction. The Poffenberger paradigm was used to measure IHTT, and a divided split-visual-field letter-matching task was used to assess hemispheric interaction. A significant correlation that could not be accounted for by other factors such as functional lateralization, handedness, age, sex, or attention was found between these 2 measures. These results are discussed in relation to CC morphology and structure and functional and structural hemispheric lateralization.