Visual implicit memory in the left hemisphere: evidence from patients with callosotomies and right occipital lobe lesions

Authors:

• Andrew P. Yonelinas

• Neal E. A. Kroll

• Kathleen Baynes

• Ian G. Dobbins

• C.M. Frederick

• Robert T. Knight

• Michael S. Gazzaniga

Date: 2001

PubMed: 11476095

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Abstract:

Identification of visually presented objects and words is facilitated by implicit memory for past visual experiences with those items. Several behavioral and neuroimaging studies suggest that this form of memory is dependent on perceptual processes localized in the right occipital lobe. We tested this claim by examining implicit mem- ory in patients with extensive right occipital lobe lesions, using lexi- cal-decision, mirror-reading, picture-fragment, and word-fragment- completion tests, and found that these patients exhibited normal levels of priming. We also examined implicit memory in patients with com- plete callosotomies, using standard and divided-visual-field word- fragment-completion procedures, and found that the isolated left hemisphere exhibited normal priming effects. The results indicate that the right occipital lobe does not play a necessary role in visual im- plicit memory, and that the isolated left hemisphere can support nor- mal levels of visual priming in a variety of tasks.

Effects of emotional stimuli on event-related potentials and reaction times in orbitofrontal patients

Authors:

• Kaisa M. Hartikainen

• Keith H. Ogawa

• Maryam Soltani

• Robert T. Knight

Date: 2001

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Abstract:

We examined the effects of bilateral orbitofrontal damage on emotional processing and hemispheric attention capacity. Participants (four bilateral orbitofrontal patients and age- matched controls) discriminated between upright and inverted triangles (target). Targets were randomly presented in the left (LVF) or right visual hemifield (RVF, 150 ms). Emotional (pleasant or unpleasant; 150 ms) or neutral stimuli were presented centrally 350 ms prior to the target. Pleasant stimuli decreased whereas unpleasant stimuli increased RTs to LVF targets, with exaggerated effects in patients. Orbitofrontal damage differentially altered processing of emotional stimuli. In addition, orbitofrontal patients showed altered target ERPs, with evidence of increased activity over frontal sites for only LVF targets. The results suggest an interplay between orbitofrontal cortex and the right hemisphere.

Mechanisms of human attention: event-related potentials and oscillations

Authors:

• Christoph S. Herrmann

• Robert T. Knight

Date: 2001

PubMed: 11595268

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Abstract:

Electrophysiological and hemodynamical responses of the brain allow investigation of the neural origins of human attention. We review attention-related brain responses from auditory and visual tasks employing oddball and novelty paradigms. Dipole localization and intra- cranial recordings as well as functional magnetic resonance imaging reveal multiple areas involved in generating and modulating attentional brain responses. In addition, the in ̄uence of brain lesions of circumscribed areas of the human cortex onto attentional mechanisms are reviewed. While it is obvious that damaged brain tissue no longer functions properly, it has also been shown that functions of non-lesioned brain areas are impaired due to loss of modulatory in ̄uence of the lesioned area. Both early (P1 and N1) and late (P3) event-related potentials are modulated by excitatatory and inhibitory mechanisms.. Oscillatory EEG-correlates of attention in the alpha and gamma frequency range also show attentional modulation.