Functional MRI evidence for the decline of word retrieval and generation during normal aging.

This fMRI study aimed to explore the effect of normal aging on word retrieval and generation. The question addressed is whether lexical production decline is determined by a direct mechanism, which concerns the language operations or is rather indirectly induced by a decline of executive functions. Indeed, the main hypothesis was that normal aging does not induce loss of lexical knowledge, but there is only a general slowdown in retrieval mechanisms involved in lexical processing, due to possible decline of the executive functions. We used three tasks (verbal fluency, object naming, and semantic categorization). Two groups of participants were tested (Young, Y and Aged, A), without cognitive and psychiatric impairment and showing similar levels of vocabulary. Neuropsychological testing revealed that older participants had lower executive function scores, longer processing speeds, and tended to have lower verbal fluency scores. Additionally, older participants showed higher scores for verbal automatisms and overlearned information. In terms of behavioral data, older participants performed as accurate as younger adults, but they were significantly slower for the semantic categorization and were less fluent for verbal fluency task. Functional MRI analyses suggested that older adults did not simply activate fewer brain regions involved in word production, but they actually showed an atypical pattern of activation. Significant correlations between the BOLD (Blood Oxygen Level Dependent) signal of aging-related (A > Y) regions and cognitive scores suggested that this atypical pattern of the activation may reveal several compensatory mechanisms (a) to overcome the slowdown in retrieval, due to the decline of executive functions and processing speed and (b) to inhibit verbal automatic processes. The BOLD signal measured in some other aging-dependent regions did not correlate with the behavioral and neuropsychological scores, and the overactivation of these uncorrelated regions would simply reveal dedifferentiation that occurs with aging. Altogether, our results suggest that normal aging is associated with a more difficult access to lexico-semantic operations and representations by a slowdown in executive functions, without any conceptual loss.

Grasping an object comfortably: orientation information is held in memory.

It has been shown that memorized information can influence real-time visuomotor control. For instance, a previously seen object (prime) influences grasping movements toward a target object. In this study, we examined how general the priming effect is: does it depend on the orientation of the target object and the similarity between the prime and the target? To do so, we examined whether priming effects occured for different orientations of the prime and the target objects and for primes that were either identical to the target object or only half of the target object. We found that for orientations of the target object that did not require an awkward grasp, the orientation of the prime could influence the initiation time and the final grip orientation. The priming effects on initiation time were only found when the whole target object was presented as prime, but not when only half of the target object was presented. The results suggest that a memory effect on real-time control is constrained by end-state comfort and by the relevance of the prime for the grasping movement, which might mean that the interactions between the ventral and dorsal pathways are task specific.

Ideational action impairments in Alzheimer's disease.

We report data from a group of patients with mild Alzheimer's disease on a range of tasks requiring either stored semantic knowledge about objects (e.g., naming object use) or the execution of action to objects (e.g., miming and using objects). We found that the patients were impaired at miming in response to objects, even when they could describe the object's function. On the other hand, copying gestures was not impaired relative to naming gestures, indicating that an ideomotor deficit in action execution, per se, was unlikely to explain the impairments in object use. We suggest instead that the patients had an impairment in stored motor programmes for action, over and above their deficits in semantic knowledge. Despite this, the patients were better at using than at miming to objects, consistent with the view that proprioceptive input (when using objects) can directly constrain selection of the appropriate motor programme for action.

Postoperative speech disorder after medial frontal surgery: role of the supplementary motor area.

BACKGROUND: Patients undergoing surgical resection of medial frontal lesions may present transient postoperative speech disorders that remain largely unpredictable. OBJECTIVE: To relate the occurrence of this speech deficit to the specific surgical lesion of the supplementary motor area (SMA) involved during language tasks using fMRI. METHODS: Twelve patients were studied using a verbal fluency task before resection of a low-grade glioma of the medial frontal lobe and compared with six healthy subjects. Pre- and postoperative MR variables including the hemispheric dominance for language, the extent of SMA removal, and the volume of resection were compared to the clinical outcome. RESULTS: Following surgery, 6 of 12 patients presented speech disorders. The deficit was similar across patients, consisting of a global reduction in spontaneous speech, ranging from a complete mutism to a less severe speech reduction, which recovered within a few weeks or months. The occurrence of the deficit was related to the resection of the activation in the SMA of the dominant hemisphere for language (p < 0.01). Increased activation in the SMA of the healthy hemisphere on the preoperative fMRI was observed in patients with postoperative speech deficit. CONCLUSIONS: fMRI is able to identify the area at risk in the SMA, of which resection is related to the occurrence of characteristic transient postoperative speech disorders. Increased SMA activation in the healthy hemisphere suggested that a plastic change of SMA function occurred in these patients.

The real-object advantage in agnosia: Evidence for a role of surface and depth information in object recognition.

We examine the factors that lead to the identification advantage for real objects over line drawings in agnosia. In a single case study we show that identification is improved when shading cues can be used to guide the segmentation of objects into their parts. In addition we demonstrate that depth information, conveyed both by binocular disparity cues and by head movements, also facilitates object segmentation. The data indicate that information about depth and surface shading can contribute to object recognition in cases where edge-based object coding is impaired.