Shifts in hemispheric advantage during familiarization with complex visual patterns

Habituation of attentional networks during emotion processing

Dysfunctional emotion processing is a key aspect of many neuropsychiatric disorders. This dysfunction may be due to an abnormal magnitude of neural substrate activation during emotion processing or due to an altered time course of the neural substrate response. To better understand the temporal characteristics of the neural substrate activation underlying implicit emotion processing, nine healthy female controls were repeatedly exposed to pictures of affective faces while performing a gender identification task in an fMRI. As the salience of the stimuli decreased with repeated exposure, brain areas implicated in a right hemispheric spatial attention network (including the posterior parietal cortex (BA 40) and the frontal eye fields (BA 6)) habituated while brain areas lateralized to the left hemisphere (including the angular gyrus (BA 39), posterior superior temporal gyrus (BA 39) and insula (BA 13)) sensitized. These results provide strong evidence that the time course of activation is a critical component when assessing the function of neural substrates underlying emotion processing (specifically whether habituation is altered) in neuro-psychiatric patients.

Left and right hemisphere contributions to physiognomic and verbal discrimination

The relative contributions of the right- and left-temporal lobes in rapid recognition of faces and letters were studied in patients with anterior right- or left-temporal lobe excisions and a matched control group. On the basis of findings in patients with unilateral and bilateral brain damage, it was hypothesized that left hemisphere damage would not change the reaction time of letters analyzed by the right hemisphere and that right hemisphere damage would not change the reaction time of faces analyzed by the left hemisphere. The hypothesis was supported for letters but not for faces. Patients in the right-temporal group, particularly those with large hippocampal removals, were slow to recognize faces in both visual fields. Two possible explanations for the findings with faces are explored: One holds that right hemisphere mechanisms are involved even when a face is presented to the left hemisphere for rapid recognition; the other holds that specialized encoding is carried out by the right hemisphere during learning, with the encoded template then being used by each hemisphere independently.

Regression of lateral asymmetry scores toward the mean

On most, if not all, laterality tasks, some subjects initially display a left sensory field advantage and others a right sensory field advantage. During the course of testing, the mean asymmetry score of those subjects who start with a left sensory field advantage decreases, whereas the mean asymmetry score of those subjects who start with a right sensory field advantage increases (asymmetry scores computed as L-R). Some previous studies (e.g., Ross-Kossak and Turkewitz, 1986) have suggested that these differential, temporal shifts in sensory field advantage reflect systematic changes in subjects' sensory field advantage (and associated hemispheric advantage) during familiarization and individual differences in those changes. Alternatively, the differential, temporal shifts may reflect unsystematic sources of changes because they are not distinguishable from statistical regression toward the mean. Consistent with this hypothesis, "time-reversed control" analyses of laterality data show that the differential trends do not reflect a factor associated with the specific direction of time passage as would be predicted in the case of increasing stimulus familiarization. Regression of asymmetry scores toward the mean, or its equivalent, imperfect reliability of asymmetry scores, may be significantly attributable to short-term fluctuations in subjects' hemispheric arousal asymmetries.

Selective increase in the right hemisphere transcranial Doppler velocity during a spatial task

Transcranial Doppler ultrasonography of the middle cerebral arteries was performed during two tasks: sorting of mah-jongg tiles and vibratory stimulation. These tasks selectively increase cerebral blood flow by positron emission tomography. The purpose of this study was to determine if analogous increases in cerebral blood flow velocity could be detected. We measured flow velocity during right hand manipulation followed by left hand manipulation, or vice versa, with resting studies in between. The average increase in the mean velocity, by paired t-test, was significant for the right middle cerebral artery with both left hand (p < .0005) and right hand (p < .005) tile sorting. For the left middle cerebral artery, there was an increase in the mean velocity with right hand (p < .005) but not for left hand sorting (p = .13). These findings support the importance of the right hemisphere in the performance of this type of spatial task. No significant flow velocity increase occurred during vibratory stimulation.

Gender, level of spatial ability, and lateralization of mental rotation

The present study indicates that some of the inconsistencies in studies of the lateralization of mental rotation may be a consequence of uncontrolled individual differences in the general level of spatial ability. In order to investigate the relation between spatial ability and the lateralization of mental rotation, 48 subjects (24 males and 24 females) were divided into three groups based on their performance on a standardized test of spatial ability. They then performed a lateralized two-dimensional mental rotation task. The results showed the typical mental rotation function in that angle of rotation and reaction time were linearly related. A significant spatial ability by visual field interaction indicated that subjects with low spatial ability had a left field advantage, whereas subjects with medium spatial ability showed no field advantage and subjects with high spatial ability showed a right field advantage. Gender also interacted with visual field, with males showing a left visual superiority and females an insignificant right visual field advantage. A significant three-way interaction of gender, spatial ability, and angle of rotation reflected the fact that low spatial males were more profoundly affected by rotation than the other groups. The results suggest that at least some of the inconsistent findings in studies of lateralization of mental rotation may be accounted for by differences in the level of spatial ability.

A clear left hemisphere advantage for visuo-spatially based verbal categorization

For a range of tachistoscopically presented stimuli that varied from modified complex Shepard and Metzler figures to simple lines, subjects had to judge whether or not the top or bottom of the stimuli contained the target feature. A marked and consistent right visual field (left hemisphere) advantage was observed for all such tasks. In addition, a task that required subjects to judge whether a solid black rectangle was in front of or behind a white rectangle also yielded a clear left hemisphere advantage. The results are explained in terms of a left hemisphere specialization for spatially based verbal categorizations in general support of Kosslyn's (Psychol. Rev. 94, 148-175, 1984) model.

Hemisphere priming through practice in a musical chords task

In a study of selective hemisphere activation, the performance of 30 subjects given either a "local" or "global" priming activity before and after a monoaurally presented chord analysis task was compared with an unprimed control group. The hypothesis was that ear advantage scores on the chords task would show increased left hemisphere involvement following local priming and increased right hemisphere involvement following global priming. The results failed to support this hypothesis. All subjects, however, regardless of priming condition, showed a very strong practice effect represented by a shift from a weak initial left ear (right hemisphere) advantage towards a significant right ear (left hemisphere) advantage (P less than 0.00006). The findings suggest that although the local/global priming activity did not lead to selective hemisphere activation, repeated exposure to the chords task resulted in increased use of analytic left hemisphere processing strategies as subjects became familiar with its processing requirements.