Evidence For Coactivation Research Articles

Role of Active Phase in Fischer–Tropsch Synthesis: Experimental Evidence of CO Activation over Single-Phase Cobalt Catalysts

In order to understand the role of Co catalysts with different phases on Fischer–Tropsch synthesis, single-phase face-centered-cubic (fcc) and hexagonal-close-packed (hcp) Co were synthesized via a two-step approach, involving the formation of single-phase CoO materials followed by reduction in H2. The physicochemical properties of Co catalysts were thoroughly characterized by XRD, SEM, TEM, TPR, and H2 chemisorption. It was found that hcp-Co exhibits higher activity on hydrocarbon formation than fcc-Co in Fischer–Tropsch synthesis. For both catalysts, CO dissociation was suggested as the rate-determining step, on which hcp-Co presents ca. 40 kJ mol–1 lower activation energy than fcc-Co, in agreement with a reported computational study. As a result, hcp-Co is concluded to be the preferable phase for rational catalyst design.

Task‐specific tremor in violinists: Evidence of coactivation in the 3 to 8 Hz frequency range

Task-specific tremor in musicians severely impairs fine motor control. However, little is known about its pathophysiology. Here, we quantify electromyography (EMG) properties in primary bowing tremor-in particular, muscular coactivation-to determine whether primary bowing tremor affects a specific frequency range of coactivation. We quantitatively compared EMG properties of the wrist muscles between 4 professional violinists who had task-specific tremor and 4 age-matched healthy controls. We observed bowing tremor-specific muscular coactivation in the frequency range of 3 to 8 Hz only in the patients but not in the healthy controls. No muscular activity was observed at the resonance-frequency range. Our findings indicate an association between coactivation and bowing tremor at a specific frequency range (3-8 Hz). The absence of EMG activity and coactivation in the mechanical-reflex frequency of the wrist suggests that central mechanisms play a more dominant role than mechanical-reflex mechanisms in primary bowing tremor.

Age-related differences in interhemispheric visuomotor integration measured by the redundant target effect.

Presentation of bilateral redundant visual stimuli produces faster reaction times (RT) than presentation of a single unilateral stimulus; an effect known as the redundant target effect (RTE; Miller, 1982), and is a means of testing interhemispheric visuomotor integration (Ouimet, 2009). RTEs that exceed expectations, based on Miller's race model of inequality (RMI), are referred to as "enhanced RTEs" and imply neural coactivation. Paradoxically, enhanced RTEs are observed in cases of corpus callosum disruption. The Hemispheric Coactivation Hypothesis accounts for this paradox by positing that bihemispheric processing occurs to both unilateral and bilateral stimuli in the normal brain, but occurs only with bilateral stimuli in the disconnected brain. Neuroimaging has revealed decreases in the microstructural integrity of the corpus callosum with age (Ota et al., 2006), but research investigating the bilateral RTE in healthy older individuals is lacking. The present study investigated the bilateral RTE in healthy younger and healthy older adults using simple RT and choice RT tasks. Our prediction that older individuals would show significantly larger RTEs than younger individuals was found to be true for both tasks. Tests of the RMI produced little evidence for coactivation. The crossed-uncrossed difference, generally used as a means of testing visuomotor interhemispheric transfer, was also investigated, but no age effects were found. The observation of greater RTE in age is congruent with the Hemispheric Coactivation hypothesis (Miller, 2004) in which callosal disconnection is associated with increased RTE.

Perceptual Basis of Redundancy Gains in Visual Pop-out Search

The redundant-signals effect (RSE) refers to a speed-up of RT when the response is triggered by two, rather than just one, response-relevant target elements. Although there is agreement that in the visual modality RSEs observed with dimensionally redundant signals originating from the same location are generated by coactive processing architectures, there has been a debate as to the exact stage(s)--preattentive versus postselective--of processing at which coactivation arises. To determine the origin(s) of redundancy gains in visual pop-out search, the present study combined mental chronometry with electrophysiological markers that reflect purely preattentive perceptual (posterior-contralateral negativity [PCN]), preattentive and postselective perceptual plus response selection-related (stimulus-locked lateralized readiness potential [LRP]), or purely response production-related processes (response-locked LRP). As expected, there was an RSE on target detection RTs, with evidence for coactivation. At the electrophysiological level, this pattern was mirrored by an RSE in PCN latencies, whereas stimulus-locked LRP latencies showed no RSE over and above the PCN effect. Also, there was no RSE on the response-locked LRPs. This pattern demonstrates a major contribution of preattentive perceptual processing stages to the RSE in visual pop-out search, consistent with parallel-coactive coding of target signals in multiple visual dimensions [Müller, H. J., Heller, D., & Ziegler, J. Visual search for singleton feature targets within and across feature dimensions.

Effects of redundant auditory stimuli on reaction time

Auditory redundancy gains were assessed in two experiments in which a simple reaction time task was used. In each trial, an auditory stimulus was presented to the left ear, to the right ear, or simultaneously to both ears. The physical difference between auditory stimuli presented to the two ears was systematically increased across experiments. No redundancy gains were observed when the stimuli were identical pure tones or pure tones of different frequencies (Experiment 1). A clear redundancy gain and evidence of coactivation were obtained, however, when one stimulus was a pure tone and the other was white noise (Experiment 2). Experiment 3 employed a two-alternative forced choice localization task and provided evidence that dichotically presented pure tones of different frequencies are apparently integrated into a single percept, whereas a pure tone and white noise are not fused. The results extend previous findings of redundancy gains and coactivation with visual and bimodal stimuli to the auditory modality. Furthermore, at least within this modality, the results indicate that redundancy gains do not emerge when redundant stimuli are integrated into a single percept.

Redundancy Gain and Coactivation in Bimodal Detection: Evidence for the Preservation of Coactive Processing in Older Adults

Previous investigations of adult age differences in the redundant signals effect suggest that both older and younger adults benefit from the presentation of redundant information. However, age deficits in divided attention may cause older adults to process redundant information in a different manner. In the present experiment, we tested between two competing explanations for the redundant signals effect: separate activation and coactivation. To investigate this issue, we used a bimodal detection task in which the auditory signal was a 1000-Hz tone and the visual signal was an asterisk. Both age groups showed significant violations of Miller's race model inequality, providing evidence for coactivation. These results suggest that, despite age-related deficits in divided attention, the ability to coactivate information from bimodal signals is spared with increased age.

Visuo-spatial neural response interactions in early cortical processing during a simple reaction time task: a high-density electrical mapping study

The timecourse and scalp topography of interactions between neural responses to stimuli in different visual quadrants, straddling either the vertical or horizontal meridian, were studied in 15 subjects. Visual evoked potentials (VEPs) were recorded from 64 electrodes during a simple reaction time (RT) task. VEPs to single stimuli displayed in different quadrants were summed (‘sum’) and compared to the VEP response from simultaneous stimulation of the same two quadrants (‘pair’). These responses would be equivalent if the neural responses to the single stimuli were independent. Divergence between the ‘pair’ and ‘sum’ VEPs indicates a neural response interaction. In each visual field, interactions occurred within 72–86 ms post-stimulus over parieto-occipital brain regions. Independent of visual quadrant, RTs were faster for stimulus pairs than single stimuli. This replicates the redundant target effect (RTE) observed for bilateral stimulus pairs and generalizes the RTE to unilateral stimulus pairs. Using Miller's ‘race’ model inequality (Miller J. Divided attention: evidence for coactivation with redundant signals, Cognitive Psychology 1982;14:247–79), we found that probability summation could fully account for the RTE in each visual field. Although measurements from voltage waveforms replicated the observation of earlier peak P1 latencies for the ‘pair’ versus ‘sum’ comparison (Miniussi C, Girelli M, Marzi CA. Neural site of the redundant target effect: electrophysiological evidence. Journal of Cognitive Neuroscience 1998;10:216–30), this did not hold with measurements taken from second derivative (scalp current density) waveforms. Since interaction effects for bilateral stimulus pairs occurred within 86 ms and require interhemispheric transfer, transcallosal volleys must arrive within 86 ms, which is earlier than previously calculated. Interaction effects for bilateral conditions were delayed by ≈10 ms versus unilateral conditions, consistent with current estimates of interhemispheric transmission time. Interaction effects place an upper limit on the time required for neuronal ensembles to combine inputs from different quadrants of visual space (≈72 ms for unilateral and ≈82 ms for bilateral conditions).

Dividing attention between color and shape: evidence of coactivation.

When attention is divided between spatially distinct objects, the time to detect a target decreases when two or more targets are present. This redundancy gain can be accounted for by an interactive race model (Mordkoff & Yantis, 1991) in which separate decisions are made about each object, but environmental contingencies among the objects can influence decision times. In the present study, we examined whether the model also accounts for performance when attention must be divided between stimulus attributes other than spatial location. Subjects made target-present responses when displays included a prespecified color, a prespecified letter, or both target features. The data violated the predictions of all separate-activations models, including the interactive race model. Two control experiments ruled out an alternative account based on task complexity. We conclude that coactivation occurs when target attributes from two separable dimensions are simultaneously present, but not when target attributes come from the same dimension. A modular hybrid of race and coactivation models is tentatively proposed.

Divided attention: Evidence for coactivation with redundant signals

Many models of divided attention assume that signals presented on different channels produce separate activations, any one of which can initiate a response. According to these models, detection responses are especially fast when signals are presented on two channels at the same time because the system can detect a signal in either of two ways. Such models predict a testable relation among reaction time distributions for conditions in which a single signal is presented as compared with a condition in which two signals are presented, and this prediction is tested in two tasks. A bimodal detection task required a simple speeded response to either a visual or an auditory signal. A letter search task required a choice response depending on whether or not a two-letter display included the signal letter “A.” Data from both tasks are inconsistent with the prediction. When two signals are presented, responses are faster than separate-activation models can explain. The results favor “coactivation” models, in which signals presented on different channels contribute to a common pool of activation that initiates a response.