Right Hemisphere Systems Research Articles

Chapter 8 - Neural basis of speech perception

The functional neuroanatomy of speech processing has been difficult to characterize. One major impediment to progress has been the failure to consider task effects when mapping speech-related processing systems. We summarize a dual-stream model of speech processing that addresses this situation. In this model, a ventral stream processes speech signals for comprehension, and a dorsal stream maps acoustic speech signals to parietal and frontal-lobe articulatory networks. The model assumes that the ventral stream is largely bilaterally organized, although there are important computational differences between the left- and right-hemisphere systems, whereas the dorsal stream is strongly left-hemisphere-dominant.

Effects of Psychostimulants on Alertness and Spatial Bias in Healthy Participants

Converging evidence from neuropsychological and neuroimaging studies suggests that the ability to maintain an alert, ready-to-respond state is mediated by a network of right-hemisphere frontal and parietal cortical areas. This right lateralization may help to explain why visuospatial hemineglect, a cluster of deficits in detecting and responding to contralesional stimuli, is more common and persistent after right-hemisphere lesions. Indeed, it has been hypothesized that this asymmetry reflects a direct, functional link between alertness and spatial attention. In the present study, we investigated whether a pharmacologically induced increase in alertness would influence lateral bias in healthy people. Eighteen healthy participants were each given placebo or the psychostimulant drugs methylphenidate 40 mg or modafinil 400 mg on separate days and completed an hour-long version of the spatially sensitive landmark task. For those participants who demonstrated the expected alerting effect of modafinil, there was a significant Condition by Time interaction, consistent with the effects of the drug resisting time-on-task rightward drifts in spatial bias in the placebo condition. In contrast, no effect of methylphenidate on spatial bias was observed. These results suggest that spatial bias may be modulated by a psychostimulant-induced increase in alertness, supporting the hypothesis of a direct, functional link between right-hemisphere systems controlling alertness and visuospatial attention.

The cortical organization of speech processing

Despite decades of research, the functional neuroanatomy of speech processing has been difficult to characterize. A major impediment to progress may have been the failure to consider task effects when mapping speech-related processing systems. We outline a dual-stream model of speech processing that remedies this situation. In this model, a ventral stream processes speech signals for comprehension, and a dorsal stream maps acoustic speech signals to frontal lobe articulatory networks. The model assumes that the ventral stream is largely bilaterally organized--although there are important computational differences between the left- and right-hemisphere systems--and that the dorsal stream is strongly left-hemisphere dominant.

Peripheral and cerebral asymmetries in the rat.

Rats learn a novel foraging pattern better with their right-side whiskers than with their left-side whiskers. They also learn better with the left cerebral hemisphere than with the right hemisphere. Rotating an already learned maze relative to the external environment most strongly reduces right-whisker performance; starting an already learned maze at a different location most strongly reduces left-whisker performance. These results suggest that the right-periphery-left-hemisphere system accesses a map-like representation of the foraging problem, whereas the left-periphery-right-hemisphere system accesses a rote path. Thus, as in humans, functional asymmetries in rats can be elicited by both peripheral and cortical manipulation, and each hemisphere makes qualitatively distinct contributions to a complex natural behavior.

The previous visual field: Effects of lateralization and response accuracy on current performance

The accuracy of performance in identifying syllables flashed to the RVF (right visual field) or LVF (left visual field) was strongly affected by the lateralization of and response accuracy for the previous stimulus, presented 10–20 sec earlier. For both the LVF and RVF, performance was better on Pre-LVF trials (previous stimulus in LVF) than on Pre-RVF trials (previous stimulus in RVF). The Pre-LVF advantage was the same in subjects with a strong RVF advantage and those with no asymmetry. Their performance was better when a previous stimulus had received adequate attention for correct encoding (Pre-Correct) than when it had not (Pre-Wrong), for both fields on Pre-LVF trials and the LVF, but not RVF, on Pre-RVF trials. Relations are discussed in terms of the capacities and properties of the RH (right hemisphere) and LH (left hemisphere) cerebral arousal systems.