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Gabriele Gratton

Publications and source records attributed to Gabriele Gratton.

At least 19 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

Lagged covariance structure models for studying functional connectivity in the brain.

Most cognitive processes are supported by large networks of brain regions. To describe the operation of these networks, it is critical to understand how individual areas are functionally connected. Here, we establish a statistical framework for studying effective and functional brain connectivity, using data obtained with a relatively new neuroimaging method, the event-related optical signal (EROS). The novelty of our approach is the use of timing information (in the form of lagged cross-correlations) in interpreting the connections between areas. Interpretation of lagged cross-correlations exploits the combination of spatial and temporal resolution provided by EROS. In this paper, we apply dynamic factor analysis as a method for testing various structural models on the lagged covariance matrices derived from the EROS data. We first illustrate the approach by testing a simple path model of neural activity propagation from area V1 to V3 in a visual stimulation task. We then build more complex structural equation models with latent variables, describing both within-hemisphere integrity, and interactions between the two hemispheres, to interpret data from a second task involving inter-hemispheric competition. The results demonstrate how the integrity of anatomical connections between the two hemispheres explains different patterns of cross-hemispheric interactions. This approach allows for fitting brain imaging data to complex models that capture dynamic cognitive processes as they rapidly evolve over time.

Adolescent↗
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↗
PubMed2006

Multiple levels of stimulus representation in visual working memory.

Object recognition presumably involves activation of multiple levels of representation. Here we use the encoding-related lateralization (ERL) method [Gratton, G. The contralateral organization of visual memory: A theoretical concept and a research tool. Psychophysiology, 35, 638-647, 1998] to describe the sequential activation of several of these levels. The ERL uses divided-field encoding to generate contralaterally biased representations in the brain. The presence and nature of these representations can be demonstrated by examining the event-related potentials (ERPs) elicited by centrally presented test probes for lateralized activity corresponding to the encoding side. We recorded ERPs during a memory-search task. Memory sets were composed of two or four uppercase letters displayed half to the left and half to the right of fixation. Probe stimuli were composed of one letter presented foveally in either upper- or lowercase. Letter case was manipulated to differentiate the time course of physical and symbolic levels of letter representation. Memory set size was manipulated to examine a relational level of letter representation. We found multiple ERLs in response to the probes: (1) An early (peak=170 msec) case-dependent (but set size independent) ERL, most evident at P7/P8, indexing the availability of a physical level of letter representation; (2) a later (200-400 msec) more diffusedly distributed ERL, independent of both letter case and set size, indexing a symbolic level of letter representation; (3) a long-latency (400-600 msec) ERL occurring at posterior sites, larger for the case match, Set Size 2 condition, indexing competition for neural representation across multiple letters. By assuming that these ERL activities track the progression of letter representation over time, we propose a model of letter processing in the context of visual working memory.

Adolescent↗
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↗
PubMed2005

Strategic control and medial frontal negativity: beyond errors and response conflict.

Errors in timed choice tasks typically produce an error-related negativity (ERN) in the event-related potential (ERP). The error specificity of the ERN has been challenged by studies showing a correct response negativity (CRN). Forty-five participants engaged in a flanker task in which both compatibility between flankers and target and the probability of compatible flankers were manipulated. Correct responses elicited a CRN, the amplitude of which increased with the degree of mismatch between the presence of conflict and conflict probability, even on low-conflict (compatible) trials. The fronto-central N2 component was larger on high-conflict (incompatible) correct response trials. However, in contrast to some recent accounts, this N2 was largest for highly probable stimuli. These findings suggest revision to models of the effects of conflict on response-related negativity to account for strategic adjustments made in preparation for the response.

Adult↗
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

Evidence of partial response activation in a memory-search task.

Studies using the lateralized readiness potential (LRP; an index of hand-specific motor preparation), suggest that the motor system can be activated on the basis of partial information, providing support for a parallel view of the human information processing system. However, data to the contrary have been obtained in memory-search paradigms, leading Heil et al. [Psychological Research 62 (1999) 289] to propose that hand-specific motor preparation cannot occur in parallel with memory scanning. To further investigate these findings we recorded the LRP in a fast-paced memory search task, in which the case of target letter and memory set size were manipulated and a choice-go/no-go procedure was used. The case of the test letter determined the response hand, whereas membership of the test stimulus in the memory set determined whether a response was required. We found different types of LRP effects as a function of time on task. Analyses based on the data from the first half of the experiment replicated those obtained by Heil et al. [Psychological Research 62 (1999) 289]. However, unlike Heil et al. [Psychological Research 62 (1999) 289], the second half of the data showed significant LRP activity on no-go trials and similar LRP onset latencies for go and no-go trials, as well as for different memory set size conditions in the stimulus-locked LRP waveforms. The response-related waveforms showed an earlier LRP onset in the set-size 4 than in the set-size 2 condition. These data suggest that motor preparation can occur simultaneously with memory search when participants are sufficiently practiced. Time-on-task and strategy differences between the two studies are discussed.

Adolescent↗
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↗
PubMed2003

Effects of alcohol consumption and alcohol susceptibility on cognition: a psychophysiological examination.

The present study sought to examine acute effects of alcohol on cognitive processing and performance within the context of two prominent theories of alcohol's effects; namely, that alcohol restricts the focus of attention (e.g. Steele and Josephs, 1990. Journal of Abnormal Psychology, 97, 196-205) and that alcohol impairs response inhibition (e.g. Fillmore and Vogel-Sprott, 1999. Experimental and Clinical Psychopharmacology, 7, 49-55; Fillmore and Vogel-Sprott, 2000. Journal of Studies on Alcohol, 61, 239-246). Forty-five participants were randomly assigned to receive either a placebo level of alcohol (0.04 g/kg), a moderate dose (0.40 g/kg), or a higher dose (0.80 g/kg). Brain electrical activity (ERPs) and behavioral responses (reaction time and accuracy) were measured while participants performed a modified flanker task, in which a target letter was flanked by response-compatible or response-incompatible letters. Analyses of behavioral data showed that alcohol increased response competition in accuracy but not response times, suggesting that alcohol influences response selection more than attentional processes per se. This finding is in-line with predictions derived from the response inhibition model. ERP latency data provided mixed support for both models. ERP amplitude data showed that the high dose of alcohol primarily influenced a mostly frontal negativity in the ERP, present on both correct and incorrect response trials. Differences in self-reported susceptibility to alcohol were most evident in the amplitude of the P3 component. Findings are discussed in terms of the differential effects of acute dose and susceptibility on information processing.

Adult↗
PubMed2003

Independent control of processing strategies for different locations in the visual field.

We investigated how the selection of information-processing strategies is organized. We contrasted three alternative ways in which strategies may be controlled: a centralized mechanism, a hemisphere-specific mechanism or a local mechanism. We built on the previous observation that the size of the noise-compatibility effect (flanker effect) is influenced by observers' expectancies for compatible or incompatible arrays. In three experiments, we varied the probabilities of compatible or incompatible noise in different locations of the visual field. We found that observers can adapt their processing strategies, as indicated by changes in the size of the noise-compatibility effect, independently in the left and right hemifields. However, processing of midline stimuli is determined by the expectancies existing in the lateral locations. These data support the notion of a hemisphere- or location-specific selection of processing strategies and suggest that the strategy selection process is not unitary.

Adult↗
PubMed2003

Effects of alcohol on person perception: a social cognitive neuroscience approach.

The acute effects of alcohol on cognitive processing of expectancy violations were investigated using event-related brain potentials and a cued recall task to index attentional and working memory processes associated with inconsistency resolution. As predicted, expectancy-violating behaviors elicited larger late positive potentials (LPP) and were recalled better than expectancy-consistent behaviors. These effects were moderated by alcohol and the valence of initial expectancies. For placebo group participants, positive targets performing negative behaviors elicited the largest LPP responses and were recalled best. For those in the alcohol groups, negative targets behaving positively elicited the largest LPP and recall responses. These findings suggest that alcohol does not globally impair working memory processes in person perception but instead changes the nature of valenced information processing. Findings are discussed in the context of alcohol's effects on working memory processes, reward sensitivity, and the prefrontal cortical structures thought to mediate them.

Adult↗
PubMed2003

Sound presentation rate is represented logarithmically in human cortex.

The encoding of temporal information is critical to auditory processing. Since the mismatch negativity component of the auditory event-related brain potential is thought to reflect properties of auditory sensory memory, we used it to examine the representation of acoustic time intervals in the human cortex. The mismatch negativity occurs in response to deviations in acoustic regularities, which are stored in sensory memory. We used 16 stimulus conditions, randomly presenting short trains of tones with fixed onset-to-onset intervals of 100, 200, 300 or 400 ms (all tones in the study were identical). The first four intervals between the tones established the acoustic regularity on each train (i.e. the 'standard'). The fifth tone in each train was preceded by an interval that varied randomly among the same four intervals. If this interval was different from the standard for that trial, it violated the acoustic regularity (i.e. it was a 'deviant'). The mismatch response to the fifth tone differed significantly among stimulus conditions and was proportional to the absolute value of the logarithm of the deviant/standard interval ratio. This indicates that short acoustic time intervals are represented with a ratio scale in the human cortex. When the fifth tone occurred 100 ms after the fourth, it elicited a somewhat different, although proportional response, supporting the hypothesis that a special integration mechanism may exist for very short time intervals.

Acoustic Stimulation↗
PubMed2003

Seeing right through you: applications of optical imaging to the study of the human brain.

A new set of techniques allows for the study of brain function by near-infrared light, exploiting two optical phenomena: Changes in light absorption are determined by changes in the concentration of substances like oxy- and deoxyhemoglobin, and changes in light scattering occur as a consequence of variations of properties of membranes and corpuscles in the neural tissue. Methods based on light absorption can be used to study hemodynamic changes in the brain, whereas those based on light scattering can be used to study neuronal activity and to provide anatomical information at a cellular and subcellular level. Three optical imaging approaches can be used to study living tissue: reflection, optical coherence tomography (OCT), and photon migration. These three approaches vary in their penetration (from less than a millimeter for reflection to up to 3-5 cm for photon migration) and spatial resolution (from a micron level for reflection and OCT to a millimeter and centimeter level for photon migration). This issue includes a collection of articles reviewing applications of these technologies to the study of brain and other bodily functions in humans.

Brain↗
PubMed2003

Optimum filtering for EROS measurements.

The event-related optical signal (EROS) is a relatively new technology that provides noninvasive data about the time course of neural activity in circumscribed cortical areas. However, much still remains to be learned about the physiology and physics underlying the observed signals. We examined the instrumental and physiological noise observed in the intensity modulation and phase-delay measurements produced by a frequency domain oxymeter in response to steady-state auditory stimuli. We present here data on the effects of different filters on the between-subjects response consistency for amplitude and phase measurements. The results of these analyses may help explain some apparently discrepant results previously reported from different laboratories (Steinbrink et al., 2000), and illustrate differences between data from different types of measurements.

Acoustic Stimulation↗