Improved single-letter identification after whole-word training in pure alexia

Pure alexia is characterised by a very time-consuming letter-by-letter reading strategy due to an impaired identification or integration of single letters. So far, therapy interventions have addressed impaired letter identification using specific single-letter training approaches. In the present study, we report patient KA with pure alexia and letter-by-letter reading. Contrary to common approaches, we investigated whether a whole-word reading approach can be successful despite severe impairments at the level of single-letter identification. As a first step, auditory-visual verification tasks were used to familiarise the patient with the training items, which in a second step were read with limited exposure duration. After a four week intervention, KA's word reading performance improved significantly for trained and control items in terms of speed and accuracy. Although not specifically addressed during our training programme, even single-letter identification and text reading performance increased significantly. However, the patient's reading was still based on a letter-by-letter strategy.     

Exploitation of natural geometrical regularities facilitates target detection

We have evolved to operate within a dynamic visual world in which natural visual signals are not random but have various statistical regularities. Our rich experience of the probability structure of these regularities could influence visual computation. Considering that spatiotemporal regularity, co-linearity and co-circularity are common geometrical regularities in natural scenes, we explored how our visual system exploits these regularities to achieve accurate and efficient representations of the external world. By measuring human contrast detection performance of a briefly presented foveal target embedded in dynamic stimulus sequences (comprising six short bars appearing consecutively towards the fovea) imitating common regularity structures, we found that both contrast sensitivity and reaction time for target detection was facilitated by predictable spatiotemporal stimulus structure. Qualitatively consistent with natural image analysis that co-linearity is a stronger statistical feature than co-circularity, the facilitation in target detection was more evident for predictable stimulus sequences following a co-linear path than a co-circular path. Control experiments further showed that response bias and uncertainty reduction could not fully account for our observation. It seems that our visual system exploits geometrical natural regularities to facilitate the interpretation of incoming visual signals, such as constraining interpretation on the basis of contextual priors.     

The prevalence effect is determined by past experience, not future prospects

In a laboratory task similar to an X-ray baggage search at an airport, Wolfe, Horowitz, and Kenner (2005) reported a “prevalence effect” (i.e., a very high miss rate) when the presence of a target is very infrequent. The present study tested whether this prevalence effect is the result of a voluntary top-down control for future prospect or an implicit bottom-up priming from past experience. Experiments 1 and 2 showed that, regardless of instructions given on the likelihood of target presence, the magnitude of prevalence (i.e., the miss rate) was determined only by the actual prevalence of the target. In Experiments 3 and 4, target prevalence was indicated by background color on a trial-by-trial basis. Some blocks (i.e., constant blocks) were either comprised of all high-prevalence trials or all low-prevalence trials, whereas in other blocks (i.e., mixed blocks) high-prevalence and low-prevalence trials were randomly mixed. Target prevalence significantly affected the miss rate in the constant blocks, but had no effect in the mixed blocks. Overall, the prevalence effect is essentially the result of past experience and is not affected by future prospect.     

Functional imaging of gustatory perception and imagery: "top-down" processing of gustatory signals

By recalling gustatory memories, it is possible to generate vivid gustatory perceptions in the absence of gustatory inputs. This gustatory image influences our gustatory processing. However, the mechanism of the "top-down" modulation of gustatory perception in the human is still unclear. Our findings propose a new perspective on the neural basis of gustatory processing. Although gustatory imagery and gustatory perception shared common parts of neural substrates, there was an asymmetrical topography of activation in the insula: the left insula was predominantly activated by gustatory imagery tasks. In addition, the middle and superior frontal gyri were not activated by gustatory perception but they participated in the generation of gustatory hallucinations. These regions in the frontal cortex may mediate the "top-down" control of retrieving gustatory information from the storage of long-term memories.     

Atypical Categorical Perception in Autism: Autonomy of Discrimination?

A diminished top-down influence has been proposed in autism, to account for enhanced performance in low-level perceptual tasks. Applied to perceptual categorization, this hypothesis predicts a diminished influence of category on discrimination. In order to test this hypothesis, we compared categorical perception in 16 individuals with and 16 individuals without high-functioning autism. While participants with and without autism displayed a typical classification curve, there was no facilitation of discrimination near the category boundary in the autism group. The absence of influence of categorical knowledge on discrimination suggests an increased autonomy of low-level perceptual processes in autism, in the form of a reduced top-down influence from categories toward discrimination.     

Modulation of the spatial attention network by incentives in healthy aging and mild cognitive impairment

Impairments of spatial attention are common in Alzheimer's disease (AD), but may develop earlier in the course of the disease, a condition referred to as mild cognitive impairment (MCI). In a previous experiment, we showed that emotional content overcame the AD-related decline in selective attention to novel events [LaBar, K. S., Mesulam, M., Gitelman, D. R., & Weintraub, S. (2000). Emotional curiosity: Modulation of visuospatial attention by arousal is preserved in aging and early-stage Alzheimer's disease. Neuropsychologia, 38(13), 1734-1740]. The current experiment examined the influence of secondary reinforcers upon selective spatial attention in MCI and healthy aging (EC). Subjects performed a covert attention task while undergoing fMRI. They won money for fast responses and lost money for slow responses. In young subjects, this task had shown that the influence of incentive upon spatial attention is mediated by the posterior cingulate (PCC) and orbitofrontal cortices (OFC) [Small, D. M., Gitelman, D., Simmons, K., Bloise, S. M., Parrish, T., & Mesulam, M. M. (2005). Monetary incentives enhance processing in brain regions mediating top-down control of attention. Cerebral Cortex, 15(12), 1855-1865]. Both groups were able to use spatial cues to generate an anticipatory attentional shift towards the cued location. The prospect of winning (but not losing) money enhanced attentional shifts in EC subjects, an effect that was mediated by OFC activation. In contrast, only the prospect of losing money enhanced attentional shifts in MCI subjects, an effect that correlated with PCC activation. Behavioral effects of incentive upon spatial attention are only partially maintained in EC and MCI with corresponding modifications in the underlying neural circuitry. These results suggest a reorganization of the relationships between the limbic system and spatial attention network in healthy aging and MCI.     

“自上而下”法评定检验项目的测量不确定度

目的 对本核医学实验室电化学发光检验项目的测量不确定度进行评价.方法 依据CNAS技术报告“医学实验室一测量不确定度的评定与表达”,采用“自上而下”的方法,评定偏移和实验室内测量复现性两部分分量引入的不确定度并合成,计算相对扩展不确定度.结果 本核医学实验室电化学发光检验项目的相对扩展不确定度分别是AFP,8.56％;CEA,12.01％;t-PSA,11.87％;f-PSA,12.14％;FER,16.32％;CA125,8.53％;CA15-3,11.89％;CA19-9,9.40％(k=2).结论 核医学实验室对定量项目的测量不确定度进行评定很有必要.     

Scene context guides eye movements during visual search

How does scene context guide search behavior to likely target locations? We had observers search for scene-constrained and scene-unconstrained targets, and found that scene-constrained targets were detected faster and with fewer eye movements. Observers also directed more initial saccades to target-consistent scene regions and devoted more time to searching these regions. However, final checking fixations on target-inconsistent regions were common in target-absent trials, suggesting that scene context does not strictly confine search to likely target locations. We interpret these data as evidence for a rapid top-down biasing of search behavior by scene context to the target-consistent regions of a scene.     

Real-world visual search is dominated by top-down guidance

How do bottom-up and top-down guidance signals combine to guide search behavior? Observers searched for a target either with or without a preview (top-down manipulation) or a color singleton (bottom-up manipulation) among the display objects. With a preview, reaction times were faster and more initial eye movements were guided to the target; the singleton failed to attract initial saccades under these conditions. Only in the absence of a preview did subjects preferentially fixate the color singleton. We conclude that the search for realistic objects is guided primarily by top-down control. Implications for saliency map models of visual search are discussed.