Semantic memory deficit with a left thalamic infarct.

Previous studies of patients with thalamic lesions have reported consistent word finding difficulties without a proposed etiology. The authors describe a patient with a focal left thalamic infarct, pronounced word finding problems, and a relatively selective impairment in semantic memory for object recall from features. The thalamus is proposed to facilitate electrical activity between brain regions that encode object features, resulting in object recall. Thalamic dysfunction disrupting object recall is a plausible etiology for impaired word finding in some patients.

Distinct prefrontal cortex activity associated with item memory and source memory for visual shapes.

In contrast to item memory, which refers to recognition or recall of previously presented information, source memory refers to memory for the context of previously presented information. The relatively few functional MRI (fMRI) source memory studies conducted to date have provided evidence that item memory and source memory are associated with differential activity in right and left prefrontal cortex, respectively. To both confirm this distinction in prefrontal cortex and to determine whether other differences in the neural substrates associated with these cognitive functions exist, an event-related fMRI study was conducted. In this study, item memory and source memory encoding phases were identical; participants viewed a series of abstract visual shapes presented on the left or right side of the screen and were instructed to remember each shape and its spatial location. During the item memory retrieval phase, shapes from the encoding phase were intermixed with new shapes and participants made an old-new judgment. During the source memory retrieval phase, all shapes were from the encoding phase and participants made a left-right judgment. An event-related analysis of item memory and source memory revealed a right and left prefrontal cortex distinction. Moreover, only item memory was associated with activity in the medial temporal lobes. These results confirm and extend previous findings that item memory and source memory are associated with distinct neural substrates.

The effect of age on rate of functional recovery after intracarotid amobarbital injection.

PURPOSE: The duration of the drug effect during the intracarotid amobarbital procedure (IAP) is an important factor when considering the prioritization of behavioral testing during the IAP. Previous studies of thiopental, a barbiturate similar to sodium amobarbital, found that age and gender significantly affect the dose required to induce anesthesia, such that younger patients require higher dosage. METHODS: A total of 55 patients who underwent the IAP procedure at Johns Hopkins Hospital were included in the study (110 hemispheric injections). The patient group included 30 female and 25 male patients, ranging in age from 11 to 50 years. To determine if a higher dose of sodium amobarbital was needed for younger patients during the IAP, we analyzed the time to return to preinjection EEG baseline status and time to return to 5/5 strength as a function of patient age and gender. RESULTS: We found that younger patients (11-20 years old) returned to preinjection baseline EEG status and full (5/5) strength faster than older patients. No gender difference was found. CONCLUSIONS: The sodium amobarbital effect during IAP dissipates faster in young patients. Consideration of this difference in rate of recovery in 11-to 20-year-old patients has important clinical implications in terms of prioritizing behavioral testing.

Neural activation during an explicit categorization task: category- or feature-specific effects?

Previous lesion-deficit and functional imaging studies have suggested that there are distinct category-specific regions within the human brain. Using functional magnetic resonance imaging, we tested this claim with a task in which participants decided if two items, represented by words, were members of the same category. Signal changes detected for all pairs of items that were from the same category and for tool pairs were found in rostromedial BA 6, rostral cingulate, and at the BA 6/44 and BA 45/46 junctions. Activation was seen in BA 6 and the left BA 6/44 junction for fruits and vegetables pairs and only in rostromedial BA 6 for animal pairs. The common signal changes in the frontal lobes (BA 6/44) for the categories of fruits and vegetables and tools suggest that this region may be organized in a feature-specific, as opposed to a category-specific, manner.

Object activation from features in the semantic system.

The human brain is thought to elicit an object representation via co-activation of neural regions that encode various object features. The cortical regions and mechanisms involved in this process have never been elucidated for the semantic system. We used functional magnetic resonance imaging (fMRI) to evaluate regions activated during a task designed to elicit object activation within the semantic system (e.g., presenting the words "desert" and "humps" with the task to determine if they combine to form an object, in this case a "camel"). There were signal changes in the thalamus for word pairs that activated an object, but not for pairs that (a) failed to activate an object, (b) were simply semantically associated, or (c) were members of the same category. These results suggest that the thalamus has a critical role in coordinating the cortical activity required for activating an object concept in the semantic system.

Object activation in semantic memory from visual multimodal feature input.

The human brain's representation of objects has been proposed to exist as a network of coactivated neural regions present in multiple cognitive systems. However, it is not known if there is a region specific to the process of activating an integrated object representation in semantic memory from multimodal feature stimuli (e.g., picture-word). A previous study using word-word feature pairs as stimulus input showed that the left thalamus is integrally involved in object activation (Kraut, Kremen, Segal, et al., this issue). In the present study, participants were presented picture-word pairs that are features of objects, with the task being to decide if together they "activated" an object not explicitly presented (e.g., picture of a candle and the word "icing" activate the internal representation of a "cake"). For picture-word pairs that combine to elicit an object, signal change was detected in the ventral temporo-occipital regions, pre-SMA, left primary somatomotor cortex, both caudate nuclei, and the dorsal thalami bilaterally. These findings suggest that the left thalamus is engaged for either picture or word stimuli, but the right thalamus appears to be involved when picture stimuli are also presented with words in semantic object activation tasks. The somatomotor signal changes are likely secondary to activation of the semantic object representations from multimodal visual stimuli.