Neural dynamics of semantic categorization in semantic variant of primary progressive aphasia.

Semantic representations are processed along a posterior-to-anterior gradient reflecting a shift from perceptual (e.g., it has eight legs) to conceptual (e.g., venomous spiders are rare) information. One critical region is the anterior temporal lobe (ATL): patients with semantic variant primary progressive aphasia (svPPA), a clinical syndrome associated with ATL neurodegeneration, manifest a deep loss of semantic knowledge. We test the hypothesis that svPPA patients perform semantic tasks by over-recruiting areas implicated in perceptual processing. We compared MEG recordings of svPPA patients and healthy controls during a categorization task. While behavioral performance did not differ, svPPA patients showed indications of greater activation over bilateral occipital cortices and superior temporal gyrus, and inconsistent engagement of frontal regions. These findings suggest a pervasive reorganization of brain networks in response to ATL neurodegeneration: the loss of this critical hub leads to a dysregulated (semantic) control system, and defective semantic representations are seemingly compensated via enhanced perceptual processing.

Event-related brain potentials isolate the motor component in a tapping task.

Repetitive tapping is used to investigate temporal perception, memory, and reproduction. Intertap intervals and their variability, arise from cognitive and motor processes during the task. We used a measure of readiness potential onset to determine motor component latency during the timed interval. Subjects performed a paced, two-handed tapping task at four target intervals (1.5-2.75 s). Overall latency of production increased with increasing ISI, as did variability across target interval, conforming to a generalized Weber's law. In contrast, average motor latency was roughly 0.5 s across ISI. This constant motor latency may also indicate constant variability attributable to motor processing.

Toward a neurobiology of temporal cognition: advances and challenges.

A rich tradition of normative psychophysics has identified two ubiquitous properties of interval timing: the scalar property, a strong form of Weber's law, and ratio comparison mechanisms. Finding the neural substrate of these properties is a major challenge for neurobiology. Recently, advances have been made in our understanding of the brain structures important for timing, especially the basal ganglia and the cerebellum. Surgical intervention or diseases of the cerebellum generally result in increased variability in temporal processing, whereas both clock and memory effects are seen for neurotransmitter interventions, lesions and diseases of the basal ganglia. We propose that cerebellar dysfunction may induce deregulation of tonic thalamic tuning, which disrupts gating of the mnemonic temporal information generated in the basal ganglia through striato-thalamo-cortical loops.