The functional anatomy of inspection time: an event-related fMRI study

Twenty healthy young adults underwent functional magnetic resonance imaging (fMRI) of the brain while performing a visual inspection time task. Inspection time is a forced-choice, two-alternative visual backward-masking task in which the subject is briefly shown two parallel vertical lines of markedly different lengths and must decide which is longer. As stimulus duration decreases, performance declines to chance levels. Individual differences in inspection time correlate with higher cognitive functions. An event-related design was used. The hemodynamic (blood oxygenation level-dependent; BOLD) response was computed as both a function of the eight levels of stimulus duration, from 6 ms (where performance is almost at chance) to 150 ms (where performance is nearly perfect), and a function of the behavioral responses. Random effects analysis showed that the difficulty of the visual discrimination was related to bilateral activation in the inferior fronto-opercular cortex, superior/medial frontal gyrus, and anterior cingulate gyrus, and bilateral deactivation in the posterior cingulate gyrus and precuneus. Examination of the time courses of BOLD responses showed that activation was related specifically to the more difficult, briefer stimuli and that deactivation was found across most stimulus levels. Functional connectivity suggested the existence of two networks. One comprised the fronto-opercular area, intrasylvian area, medial frontal gyrus, and the anterior cingulate cortex (ACC), possibly associated with processing of visually degraded percepts. A posterior network of sensory-related and associative regions might subserve processing of a visual discrimination task that has high processing demands and combines several fundamental cognitive domains. fMRI can thus reveal information about the neural correlates of mental events which occur over very short durations.     

Left prefrontal cortex control of novel occurrences during recollection: a psychopharmacological study using scopolamine and event-related fMRI

Recollection and familiarity represent two processes involved in episodic memory retrieval. We investigated how scopolamine (an antagonist of acetylcholine muscarinic receptors) influenced brain activity during memory retrieval, using a paradigm that separated recollection and familiarity. Eighteen healthy volunteers were recruited in a randomized, placebo-controlled, double-blind design using event-related fMRI. Participants were required to perform a verbal recognition memory task within the scanner, either under placebo or scopolamine conditions. Depending on the subcondition, participants were required to make a simple recognition decision (old/new items) or base their decision on more specific information related to prior experience (target/non-target/new items). We show a drug modulation in left prefrontal and perirhinal cortex during recollection. Such an effect was specifically driven by novelty and showed an inverse correlation with accuracy performance. Additionally, we show a direct correlation between drug-related signal change in left prefrontal and perirhinal cortices. We discuss the findings in terms of acetylcholine mediation of the familiarity/novelty signal through perirhinal cortex and the control of the relative signal strength through prefrontal cortex.     

The role of dominant premotor cortex in language: a study using intraoperative functional mapping in awake patients

Although the role of the premotor cortex (PMC) was widely studied in motor function, very few data are currently available about the participation of this structure in language. We report a series of 25 right-handed patients harboring a low-grade glioma near or within the left dominant PMC, operated on under local anesthesia with intraoperative real-time sensorimotor and language mappings using electrical stimulations all along the resection. Language tasks consisted of counting and picture naming (preceded by the reading of a short sentence). Stimulations of the left PMC induced transient speech disturbances in all patients, with disruption of both counting and reading/naming during stimulation of the ventral PMC--due to elicitation of an anarthria--while generating an anomia during stimulation of the dorsal PMC. Moreover, corresponding subcortical pathways generated the same language disorders as at the cortical level when stimulated. Eloquent structures were systematically preserved, allowing the avoidance of definitive postoperative deficit. These findings suggest first that the left dominant PMC seems to play a major role in language and second that this structure could have a well-ordered functional organization, namely with the ventral PMC, which might be involved in planification of articulation, and the dorsal PMC, which might be involved in the naming network.