Biomedical subjects

Katherine M Armstrong

Publications and source records attributed to Katherine M Armstrong.

5 recordsLinked to original sources

PubMed2006

Changes in visual receptive fields with microstimulation of frontal cortex.

The influence of attention on visual cortical neurons has been described in terms of its effect on the structure of receptive fields (RFs), where multiple stimuli compete to drive neural responses and ultimately behavior. We stimulated the frontal eye field (FEF) of passively fixating monkeys and produced changes in V4 responses similar to known effects of voluntary attention. Subthreshold FEF stimulation enhanced visual responses at particular locations within the RF and altered the interaction between pairs of RF stimuli to favor those aligned with the activated FEF site. Thus, we could influence which stimulus drove the responses of individual V4 neurons. These results suggest that spatial signals involved in saccade preparation are used to covertly select among multiple stimuli appearing within the RFs of visual cortical neurons.

Animals↗

PubMed2006

Visual and oculomotor selection: links, causes and implications for spatial attention.

Natural scenes contain far more information than can be processed simultaneously. Thus, our visually guided behavior depends crucially on the capacity to attend to relevant stimuli. Past studies have provided compelling evidence of functional overlap of the neural mechanisms that control spatial attention and saccadic eye movements. Recent neurophysiological work demonstrates that the neural circuits involved in the preparation of saccades also play a causal role in directing covert spatial attention. At the same time, other studies have identified separable neural populations that contribute uniquely to visual and oculomotor selection. Taken together, all of the recent work suggests how visual and oculomotor signals are integrated to simultaneously select the visual attributes of targets and the saccades needed to fixate them.

Attention↗

PubMed2003

Visuomotor origins of covert spatial attention.

Covert spatial attention produces biases in perceptual performance and neural processing of behaviorally relevant stimuli in the absence of overt orienting movements. The neural mechanism that gives rise to these effects is poorly understood. This paper surveys past evidence of a relationship between oculomotor control and visual spatial attention and more recent evidence of a causal link between the control of saccadic eye movements by frontal cortex and covert visual selection. Both suggest that the mechanism of covert spatial attention emerges as a consequence of the reciprocal interactions between neural circuits primarily involved in specifying the visual properties of potential targets and those involved in specifying the movements needed to fixate them.

Animals↗

PubMed2003

Selective gating of visual signals by microstimulation of frontal cortex.

Several decades of psychophysical and neurophysiological studies have established that visual signals are enhanced at the locus of attention. What remains a mystery is the mechanism that initiates biases in the strength of visual representations. Recent evidence argues that, during spatial attention, these biases reflect nascent saccadic eye movement commands. We examined the functional interaction of saccade preparation and visual coding by electrically stimulating sites within the frontal eye fields (FEF) and measuring its effect on the activity of neurons in extrastriate visual cortex. Here we show that visual responses in area V4 could be enhanced after brief stimulation of retinotopically corresponding sites within the FEF using currents below that needed to evoke saccades. The magnitude of the enhancement depended on the effectiveness of receptive field stimuli as well as on the presence of competing stimuli outside the receptive field. Stimulation of non-corresponding FEF representations could suppress V4 responses. The results suggest that the gain of visual signals is modified according to the strength of spatially corresponding eye movement commands.

Analysis of Variance↗

PubMed2003

Orlistat enhances warfarin effect.

OBJECTIVE: To report a case of increased international normalized ratio (INR) associated with the addition of orlistat to the drug regimen of a patient receiving warfarin therapy. CASE SUMMARY: A 66-year-old white man with a history of chronic atrial fibrillation (treated with a stable dose of warfarin), hypertension, and diet-controlled type 2 diabetes mellitus was started on orlistat for weight reduction. An increased INR was reported after the introduction of orlistat; there had been no other recent changes to medication or medical conditions. Warfarin was withheld and the dose reduced to allow INR control to be reestablished. According to the Naranjo probability scale, this reaction was probable. DISCUSSION: Control of the INR within therapeutic limits is always a challenge. Dietary intake of vitamin K, intercurrent illness, concomitant medication, herbal remedies, and other factors can interfere with warfarin dosing. Orlistat use may be associated with patient alteration of diet to compensate for adverse effects or other mechanisms by which orlistat alters warfarin control, including direct effect on absorption of vitamin K. CONCLUSIONS: The introduction of chronic dosing of orlistat may reduce the absorption of fat-soluble vitamins, including vitamin K, with the result that a lower dose of warfarin may be required. This may be due in part to change to a lower fat diet with decreased amounts of vitamin K. It may also be due to an effect on vitamin K absorption. Caution should be exercised when these 2 drugs are used concurrently.

Aged↗