Biomedical subjects

Kathy A Low

Publications and source records attributed to Kathy A Low.

8 recordsLinked to original sources

PubMed2006

Effects of measurement method, wavelength, and source-detector distance on the fast optical signal.

Fast optical signals can be used to study the time course of neuronal activity in localized cortical areas. The first report of such signals [Gratton, G., Corballis, P. M., Cho, E., Fabiani, M., Hood, D., 1995a. Shades of gray matter: Noninvasive optical images of human brain responses during visual stimulation. Psychophysiol, 32, 505-509.] was based on photon delay measures. Subsequently, other laboratories have also measured fast optical signals, but a debate still exists about how these signals are generated and optimally recorded. Here we report data from a visual stimulation paradigm in which different parameters (continuous: DC intensity; modulated: AC intensity and photon delay), wavelengths (shorter and longer than the hemoglobin isosbestic point), and source-detector distances (shorter and longer than 22.5 mm) were used to record fast signals. Results indicate that a localized fast signal (peak latency=80 ms) can be detected with both delay and AC intensity measures in visual cortex, but not with unmodulated DC measures. This is likely due to the fact that differential measures (delay and AC intensity) are less sensitive to superficial noise sources, which heavily influence DC intensity. The fast effect had similar sign at wavelengths shorter and longer than the hemoglobin isosbestic point, consistent with light scattering but not rapid deoxygenation accounts of this phenomenon. Finally, the fast signal was only measured at source-detector distances greater than 22.5 mm, consistent with the intracranial origin of the signal, and providing indications about the minimum distance for recording. These data address some of the open questions in the field and provide indications about the optimal recording methods for fast optical signals.

Adult↗

PubMed2006

Fast optical imaging of frontal cortex during active and passive oddball tasks.

This study used the high spatial and temporal resolution of the event-related optical signal (EROS) to investigate the timing of neuronal activity in frontal cortex during auditory target detection and passive oddball tasks. Activation in right middle frontal gyrus (MFG) peaked approximately 350 ms following rare target tones. This corresponded closely to the latency of the simultaneously recorded electrical P3 component. In addition, we found activation in left lateral MFG peaking at approximately 130 ms following tone onset for conditions that may have required response inhibition. These results correspond with activation patterns observed in similar fMRI studies, but provide temporal tags for the activated locations. These data may help bridge the gap between electrophysiological and hemodynamic measures of target detection and contribute to our understanding of the spatiotemporal dynamics of brain activity during target processing.

Acoustic Stimulation↗

PubMed2006

Reduced suppression or labile memory? Mechanisms of inefficient filtering of irrelevant information in older adults.

Cognitive aging theories emphasize the decrease in efficiency of inhibitory processes and attention control in normal aging, which, in turn, may result in reduction of working memory function. Accordingly, some of these age-related changes may be due to faster sensory memory decay or to inefficient filtering of irrelevant sensory information (sensory gating). Here, event-related brain potentials and the event-related optical signal were recorded in younger and older adults passively listening to tone trains. To determine whether age differentially affects decay of sensory memory templates over short intervals, trains were separated by delays of either 1 or 5 sec. To determine whether age affects the suppression of responses to unattended repeated stimuli, we evaluated the brain activity elicited by successive train stimuli. Some trains started with a shorter-duration stimulus (deviant trains). Results showed that both electrical and optical responses to tones were more persistent with repeated stimulation in older adults than in younger adults, whereas the effects of delay were similar in the two groups. A mismatch negativity (MMN) was elicited by the first stimulus in deviant trains. This MMN was larger for 1- than 5-sec delay, but did not differ across groups. These data suggest that age-related changes in sensory processing are likely due to inefficient filtering of repeated information, rather than to faster sensory memory decay. This inefficient filtering may be due to, or interact with, reduced attention control. Furthermore, it may increase the noise levels in the information processing system and thus contribute to problems with working memory and speed of processing.

Acoustic Stimulation↗

PubMed2005

Putting things into perspective: individual differences in working-memory span and the integration of information.

An important function of working memory is the integration of incoming information into an appropriate model of the contextual situation. We hypothesized that individual variability in working-memory function (estimated using Engle's operation-span measure) may lead to differential reactivity to a changing context. We recorded event-related brain potentials (ERPs) and reaction-time measures to stimuli embedded in long stimulus series (two auditory discrimination tasks), and examined the participants' responses in relation to how the current stimuli fit with the context generated by the previous stimuli. In both tasks, participants with low working-memory span scores showed larger brain responses as a function of variations in the local stimulus sequence than participants with high span scores. These data suggest that the low working-memory span group is more affected by the local stimulus sequence than the high span group, possibly because they are more easily swayed by ongoing changes and are therefore less capable of maintaining their attention on the overall sequence.

Adolescent↗

PubMed2004

Sensory ERPs predict differences in working memory span and fluid intelligence.

The way our brain reacts to sensory stimulation may provide important clues about higher-level cognitive function and its operation. Here we show that short-latency (< 200 ms) sensory cortical responses elicited by visual and auditory stimuli differ dramatically between subjects with high and low working-memory span, as well as between subjects scoring high and low on a fluid intelligence test. Our findings also suggest that this link between sensory responses and complex cognitive tasks is modality specific (visual sensory measures correlate with visuo-spatial tasks whereas auditory sensory measures correlate with verbal tasks). We interpret these findings as indicating that people's effectiveness in controlling attention and gating sensory information is a critical determinant of individual differences in complex cognitive abilities.

Acoustic Stimulation↗

PubMed2004

The event-related optical signal to electrical stimulation of the median nerve.

The event-related optical signal (EROS) uses near-infrared light to study changes in neuronal optical properties in response to stimuli and endogenous events. EROS responses to electrical stimulation of the median nerve at 1, 5, and 8 Hz were collected from 80 channels in 7 subjects. Optical recording channels were spatially aligned by co-registering the digitized fiber locations with structural magnetic resonance images (MRI) for each subject separately. The co-registered data sets were then transformed into Talairach space to permit alignment across subjects. After alignment, data from channels underlying pixels of a surface projection were combined to produce maps of Z statistics. Waveforms associated with voxels within an a priori region of interest (ROI) over the hand area of primary somatosensory (SI) cortex were compared across the three stimulus frequencies. Reliable early increases in light propagation time (i.e., increased phase delay) were found in SI as early as 16-32 ms of poststimulus for all three frequency conditions, and both an increase in phase delay and a decrease in signal intensity were observed over SI at longer latencies. A split-half analysis of the 8 Hz condition demonstrated the replicability of the response. This represents the first direct comparison of intensity and delay measures of these components of the somatosensory response; further, it shows that these early cortical components are replicable across subjects and correspond well to individual subjects' anatomical landmarks for SI.

Adult↗

PubMed2004

Latent inhibition mediates N1 attenuation to repeating sounds.

Sound repetition typically reduces auditory N1 amplitudes, more so at higher rates. This has been attributed to refractoriness of N1 generators. However, evidence that N1 attenuation is delayed 300-400 ms after the first occurrence of a repeated sound suggests an alternative process, such as inhibition, that requires 300-400 ms to become fully operational. We examined the N1 to trains of fixed-interval (100, 200, 300, 400 ms) tones for evidence of effects predicted by models of refractoriness and of latent inhibition. Regardless of interval, latency of the eliciting tone from train onset determined N1 amplitudes during the first 400 ms of the train, which decreased in this window. The results show that N1 attenuation cannot be due simply to refractoriness, which would elicit the smallest N1 to the second tone. An inhibitory neural circuit can account for these and previous results, and may be important to auditory perceptual processing.

Acoustic Stimulation↗

PubMed2002

Response slowing in Parkinson's disease: a psychophysiological analysis of premotor and motor processes.

The mechanisms responsible for reaction time slowing in Parkinson's disease were investigated using movement-related potentials in a choice reaction time task. Parkinson's disease patients and control subjects were required to respond with the left or right hand to indicate whether a visual stimulus was relatively large or small. The difficulty of the size discrimination was manipulated, as was the complexity of the manual response (single key press versus sequence of three key presses). Behavioural responses of Parkinson's disease patients were slower than those of control subjects, especially when complex responses were required. Moreover, the timing of movement-related potentials indicated that motor processes clearly required extra time, relative to control subjects, for Parkinson's disease patients making complex responses. In addition, delayed onset of the movement-related potentials indicated that one or more premotor processes are also slowed in these patients.

Aged↗