Lateralization of memory for the visual attributes of objects: evidence from the posterior cerebral artery amobarbital test

Apolipoprotein ε4 modifies obesity-related atrophy in the hippocampal formation of cognitively healthy adults

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Age-related inverted U-shaped curve of hippocampal myelin/neurite packing.

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Reduced hippocampal myelin/neurite packing and size/complexity in obesity.

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APOE modifies the effects of obesity on hippocampal size/complexity.

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Age-related slowing of spatial navigation but no risk effects on cognition.

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CA/DG predict episodic memory and subiculum predicts spatial navigation performance.

Characterizing age- and risk-related hippocampal vulnerabilities may inform about the neural underpinnings of cognitive decline. We studied the impact of three risk-factors, Apolipoprotein (APOE)-ε4, a family history of dementia, and central obesity, on the CA1, CA2/3, dentate gyrus and subiculum of 158 cognitively healthy adults (38-71 years). Subfields were labelled with the Automatic Segmentation of Hippocampal Subfields and FreeSurfer (version 6) protocols. Volumetric and microstructural measurements from quantitative magnetization transfer and Neurite Orientation Density and Dispersion Imaging were extracted for each subfield and reduced to three principal components capturing apparent myelin/neurite packing, size/complexity, and metabolism. Aging was associated with an inverse U-shaped curve on myelin/neurite packing and affected all subfields. Obesity led to reductions in myelin/neurite packing and size/complexity regardless of APOE and family history of dementia status. However, amongst individuals with a healthy Waist-Hip-Ratio, APOE ε4 carriers showed lower size/complexity than non-carriers. Segmentation protocol type did not affect this risk pattern. These findings reveal interactive effects between APOE and central obesity on the hippocampal formation of cognitively healthy adults.

Do spatial and recognition memories have a lateralized processing by the dorsal hippocampus CA3?

The present study evaluated the function of the right and left CA3 of the dorsal hippocampus (dHPC) in the processing of (i) recognition memory, (ii) recent and remote spatial memory, (iii) working memory and (iv) navigation strategy. Wistar rats were divided into four experimental groups: vehicle group (VG), animals received a bilateral injection of phosphate-saline buffer (PBS) in both right and left dorsal CA3; dHPC-R group, animals received an injection of ibotenic acid (IBO) in the right dorsal CA3; dHPC-L group, animals received an IBO injection in left dorsal CA3; and dHPC-Bi group, animals received bilateral injections of IBO in both dorsal CA3. Rats were submitted to a sequence of behavioral tests: Morris water maze (MWM), object recognition test (ORT), forced T-maze and MWM 30 days after the first exposure. The results showed no evidence of functional lateralization and the dorsal CA3 does not seem to be essential for learning and memory (recent and remote) processing and allocentric navigation analyzed in the MWM and T-maze, respectively. However, rats with right or bilateral lesions in the dorsal CA3 failed to recognize the familiar object in the ORT, suggesting a lateralized processing of recognition memory. That result is unprecedented and contributes to the knowledge about the compartmentalization of HPC functions.

Cerebral asymmetries in the level of attention required for word recognition

Words presented to the right visual field (RVF) are recognised more readily than those presented to the left visual field (LVF). This RVF advantage could reflect: (a) the direct connection between the RVF and left hemisphere, (b) an attentional bias directed towards the RVF, or (c) an attentional advantage, where the left hemisphere is able to recognise words using less attention than the right hemisphere. The attentional bias and advantage models were tested in 20 dextral adults during a divided visual field word-naming task. Spatial attention was manipulated with valid, invalid, or neutral central cues. Error and reaction time measures revealed a RVF advantage for word recognition. If the attentional bias model is correct, the RVF advantage should have been attenuated for valid and invalid cues compared to neutral cues. Instead of this, an interaction emerged whereby the cueing effect was stronger for words in the LVF than the RVF. This interaction has been reported previously in studies using peripheral spatial cues. The interaction suggests that the RVF requires less attention to process words than the LVF. This left hemisphere attentional advantage may reflect asymmetries between the hemispheres in their word processing styles.

The frontal lobe role in memory: a review of convergent evidence and implications for the Wada memory test

Functional imaging studies have implicated the frontal lobe in many of the memory processes often thought to be the domain of medial temporal structures. Results from fMRI studies of normal subjects have suggested that some components of memory formation, including those components tested during the Wada memory test, may involve frontal lobe regions. Specific behavioral disruptions during carotid amytal injections support a model for frontal lobe anesthesia in explaining results of the Wada memory test. Cortical stimulation data suggest that frontal lobe disruption is sufficient to cause memory disturbances. The convergence of evidence suggests frontal lobe memory may limit the predictive value of the Wada memory test in defining the risk of memory loss following temporal lobectomy.

An fMRI study of the functional neuroanatomy of picture encoding in younger and older adults

Age-related changes in the neural mechanisms of picture encoding were investigated using functional magnetic resonance imaging (fMRI). Seven younger and seven older adults were studied while they were encoding pairs of concrete-related, concrete-unrelated, and abstract pictures. Functional (T2*-weighted) and anatomical (T1-weighted) images of the brain were obtained using a 1.5 T MRI scanner. The results in the younger adults showed that the left dorsal prefrontal cortex (PFC) was activated during associative learning of the concrete-unrelated or abstract pictures. The results also suggest that both ventral and dorsal visual pathways are involved in the encoding of abstract pictures, and that the right superior parietal lobule likely mediates spatial information of the abstract pictures. The older adults showed significant activation in the left dorsal PFC under concrete-unrelated and abstract conditions. However, the older adults failed to activate either the left ventral and right dorsal PFC under the concrete-unrelated condition, or the parietal areas under abstract condition. A direct comparison between the two age groups demonstrates that the older adults had a reduced activation in the bilateral parieto-temporo-occipital areas under abstract condition, and in the right temporo-occipital area extending to the fusiform gyrus under the concrete-unrelated condition. Finally, age difference was found in correlation between memory performance and amplitude of signal change in the parahippocampal gyrus and fusiform gyrus under the concrete-unrelated and abstract conditions. These changes in neural response likely underlie the age-related memory decline in relation to pictorial information.

Prediction of presence of hippocampal sclerosis from intracarotid amobarbital procedure memory asymmetry scores and epilepsy on set age

Identification of the pathological status of the hippocampus prior to surgery is important since the absence of hippocampal sclerosis (HS) carries risks to memory function following anterior temporal lobectomy (ATL). We studied 62 patients undergoing ATL (31 L, 31 R) for intractable epilepsy of temporal lobe origin in whom no pathology was identified apart from HS. An intracarotid amobarbital procedure (IAP) was performed as part of the preoperative evaluation. All patients were left hemisphere dominant for language. IAP memory testing was according to the protocol of Loring. We examined IAP memory asymmetry scores at four levels of difference (<2, > or =2, > or =4, > or =6) as a function of the presence (HS+) or absence (HS-) of HS. A logistic regression analysis was performed with HS+ as the dependent variable, and age at onset of epilepsy, age at time of surgery, gender, side of surgery and significant IAP memory asymmetry as independent variables. At each level of memory asymmetry, onset age and memory asymmetry were the only predictors of HS+. Younger age at onset was associated with HS+. Curves were constructed showing probability of HS+ for age at onset for each level of asymmetry. These can be used to predict the likelihood of presence of HS based on age at onset of epilepsy and the IAP memory asymmetry score. It is concluded that IAP memory asymmetry scores reflect the functional and pathological status of the hippocampus, and greater asymmetry increases the probability of finding HS in the resected hippocampus.

Asymmetries in the effect of side of seizure onset on recognition memory following intracarotid amobarbital injection

PURPOSE

To assess interhemispheric differences in recognition memory for objects during the intracarotid amobarbital sodium procedure (IAP).

METHODS

The recognition memory for real objects of patients with either right (RTLE; n = 28) or left (LTLE; n = 22) temporal lobe epilepsy was assessed at baseline, and after left and right intracarotid amobarbital sodium injection.

RESULTS

There were no differences between groups on baseline performance. Performance following injection ipsilateral to the side of seizure focus was relatively lower for the LTLE as compared with the RTLE group, but this difference did not reach statistical significance. However, performance following injection contralateral to the side of seizure focus was significantly lower for the RTLE as compared with the LTLE group. Within-group differences in performance after ipsilateral as compared with contralateral injection were significant for the RTLE but not the LTLE group. The difference in interhemispheric asymmetry in IAP memory performance between RTLE and LTLE groups was reflected in decreased ability to classify LTLE patients as compared with RTLE patients about side of seizure onset, using a clinically applicable decision rule.

CONCLUSIONS

Recognition memory during the IAP for real objects, simultaneously named and presented visually during encoding, is mediated effectively by both the left and right hemisphere when there is no seizure focus present. However, memory appears to be more vulnerable to the presence of a seizure focus in the right as compared with the left hemisphere.

Effect of focus lateralization on memory assessment during the intracarotid amobarbital procedure

Despite the use of stimuli that can be processed by both hemispheres, a number of studies have reported lower memory scores after the left intracarotid amobarbital procedure (IAP) than after the right IAP. Because of that, failure after ipsilateral IAP is observed more often in patients with a left temporal seizure focus (LT) than in right temporal patients (RT), possibly needlessly excluding some LT patients from surgery. In order to overcome the deleterious effects of anesthetizing the dominant hemisphere, we designed an IAP protocol that did not promote verbal encoding of the stimuli. For this purpose, a large number of visual and tactile stimuli (colored pictures and real objects) were presented to be recognized later. The effect of seizure focus lateralization was examined in 82 temporal lobe epileptic patients who underwent IAP as part of their presurgical evaluation. As expected, for both RT and LT patients, long-term recognition of pictures presented under the effect of amobarbital was highly sensitive to the presence of a contralateral epileptic focus. However, contrary to what is generally reported, LT patients performed better than RT patients when their left (ipsilateral) hemisphere was anesthetized. In RT patients, although memory scores were lower after the left contralateral injection, the disparity in memory scores between the right and left injection was not as marked as in LT patients. These results are discussed in terms of the influence of type of processing required during the initial encoding on later recognition during IAP.

Lateralization of memory for music: evidence from the intracarotid sodium amobarbital procedure

The present study was conducted to determine whether material-specific memory for unfamiliar tonal patterns could be demonstrated for the right temporal lobe during the intracarotid sodium amytal procedure (IAP). Thirty-one patients with intractable complex partial seizures associated with either left temporal lobe epilepsy (LTLE) or right temporal lobe epilepsy (RTLE) underwent assessment of memory for tonal patterns during a baseline phase and right and left cerebral hemisphere anaesthesia during the IAP. Patients were presented unfamiliar tonal patterns which were later selected from a set of distracter patterns. Findings indicated that there was no difference between the LTLE and RTLE patients during injection of the right cerebral hemisphere suggesting no specific involvement of left mesial temporal structures. However, a significant effect was noted during left injection with the LTLE group performing significantly better than the RTLE group. This latter finding supports a specific role of right mesial temporal lobe structures in mediating memory for music.