Understanding anterograde amnesia: Disconnections and hidden lesions

Three emerging strands of evidence are helping to resolve the causes of the anterograde amnesia associated with damage to the diencephalon. First, new anatomical studies have refined our understanding of the links between diencephalic and temporal brain regions associated with amnesia. These studies direct attention to the limited numbers of routes linking the two regions. Second, neuropsychological studies of patients with colloid cysts confirm the importance of at least one of these routes, the fornix, for episodic memory. By combining these anatomical and neuropsychological data strong evidence emerges for the view that damage to hippocampal—mammillary body—anterior thalamic interactions is sufficient to induce amnesia. A third development is the possibility that the retrosplenial cortex provides an integrating link in this functional system. Furthermore, recent evidence indicates that the retrosplenial cortex may suffer “covert” pathology (i.e., it is functionally lesioned) following damage to the anterior thalamic nuclei or hippocampus. This shared indirect “lesion” effect on the retrosplenial cortex not only broadens our concept of the neural basis of amnesia but may also help to explain the many similarities between temporal lobe and diencephalic amnesia.     

Implicit response-irrelevant number information triggers the SNARC effect: Evidence using a neural overlap paradigm

There is evidence from the SNARC (spatial–numerical association of response codes) effect and NDE (numerical distance effect) that number activates spatial representations. Most of this evidence comes from tasks with explicit reference to number, whether through presentation of Arabic digits (SNARC) or through magnitude decisions to nonsymbolic representations (NDE). Here, we report four studies that use the neural overlap paradigm developed by Fias, Lauwereyns, and Lammertyn (2001) to examine whether the presentation of implicit and task-irrelevant numerosity information (nonsymbolic arrays and auditory numbers) is enough to activate a spatial representation of number. Participants were presented with either numerosity arrays (1–9 circles or triangles) to which they made colour (Experiment 1) or orientation (Experiment 2) judgements, or auditory numbers coupled with an on-screen stimulus to which they made a colour (Experiment 3) or orientation (Experiment 4) judgement. SNARC effects were observed only for the orientation tasks. Following the logic of Fias et al., we argue that this SNARC effect occurs as a result of overlap in parietal processing for number and orientation judgements irrespective of modality. Furthermore, we found stronger SNARC effects in the small number range (1–4) than in the larger number range (6–9) for both nonsymbolic displays and auditory numbers. These results suggest that quantity is extracted (and interferes with responses in the orientation task) but this is not exact for the entire number range. We discuss a number of alternative models and mechanisms of numerical processing that may account for such effects.     

Temporal integration of face view sequences and recognition of novel views

When faces are learned from rotating view sequences, novel views may be recognized by matching them with an integrated representation of the sequence or with individual views. An integrated-representation process should benefit from short view durations, and thus from the inclusion of views in a short temporal window, allowing the distribution of attention over the entire sequence. A view-matching process should benefit from long view durations, allowing the attention to focus on each view. In a sequential comparison task, we tested the recognition of learned and novel interpolated and extrapolated views after learning faces from rapid and slow sequences (240 ms or 960 ms for each view). We found a superiority of rapid over slow sequences, in favour of the integrated-representation hypothesis. In addition, the recognition pattern for the different viewpoints in the sequence depended on the absence or presence of extrapolated views, showing a bias of the distribution of attention.     

Organisation of executive functions: Hemispheric asymmetries

This address provides a review of evidence for a deconstruction of executive functions, the set of cognitive operations which allow goal-directed behaviour. The underlying working hypothesis is that some complementary and computationally diverse executive functions are dissociable not only functionally but also temporally and anatomically, along the left-right axis of prefrontal cortex and related neural networks. In particular, criterion setting—the capacity to flexibly set up and select task rules—is more left-lateralised; monitoring—the process of continuously evaluating the internal or external contingencies to optimise behaviour—is more right-lateralised; finally, superior medial prefrontal regions, including dorsal anterior cingulate cortex, play a role in energising weakly activated but relevant processes. Several lines of empirical evidence, including neuroimaging and neuropsychological findings, are presented to support this tripartite model of executive functions. Evidence which is difficult to explain with this model and some future directions are also discussed.