Psychometrically matched tasks evaluating differential fMRI activation during form and motion processing.

Objective: Deficits in visual perception and working memory are commonly observed in neuropsychiatric disorders and have been investigated using functional MRI (fMRI). However, interpretation of differences in brain activation may be confounded with differences in task performance between groups. Differences in task difficulty across conditions may also pose interpretative issues in studies of visual processing in healthy subjects. Method: To address these concerns, the present study characterized brain activation in tasks that were psychometrically matched for difficulty; fMRI was used to assess brain activation in 10 healthy subjects during discrimination and working memory judgments for static and moving stimuli. For all task conditions, performance accuracy was matched at 70.7%. Results: Areas associated with V2 and V5 in the dorsal stream were activated during motion processing tasks and V4 in the ventral stream were activated during form processing tasks. Frontoparietal areas associated with working memory were also statistically significant during the working memory tasks. Conclusions: Application of psychophysical methods to equate task demands provides a practical method to equate performance levels across conditions in fMRI studies and to compare healthy and cognitively impaired groups at comparable levels of effort. These psychometrically matched tasks can be applied to patients with a variety of cognitive disorders to investigate dysfunction of multiple a priori defined brain regions. Measuring the changes in typical activation patterns in patients with these diseases can be useful for monitoring disease progression, evaluating new drug treatments, and possibly for developing methods for early diagnosis. (PsycINFO Database Record (c) 2011 APA, all rights reserved)     

Visual size processing in spatial neglect

Evidence from the use of the landmark task and from two size-matching tasks shows that many patients with left-sided neglect systematically under-perceive visual extent in leftward parts of space. This perceptual distortion of size serves to explain the occurrence of rightward line-bisection errors in most neglect patients. One possibility is that attentional biases of a chronic nature might underlie these perceptual changes seen in neglect patients. But contrary to this idea, we have found that attentional cueing in the landmark task causes changes in neglect patients exactly opposite to those seen in healthy subjects. The distortions of size perception seen in neglect could instead be caused by a high-level alteration of visual processing rather than by an attentional bias. In order to explore which visual stream of cortical processing might be compromised in such a disorder, we have begun to examine neglect patients on visuomotor as well as perceptual tasks. We have found clear evidence in one patient for intact grip scaling for object size in the neglected half of space, despite gross perceptual underestimations of the same objects. This result suggests that neglect can occur without major disruption of the dorsal stream, and may result instead from damage to a cortical system whose predominant visual input comes from the ventral stream.     

Capacity limitations in visual word processing.

The ability of subjects to process English words in a spatially parallel manner was examined in several redundant-target detection tasks. When redundant targets were identical in a given display, processing limitations were evident in a task that required subjects to make semantic categorizations of words. However, parallel processing of identical redundant target words was exhibited in a lexical decision task that required a structural analysis of letter strings, but not an analysis of word meaning. The difference in performance in the two tasks suggests that the capacity for semantic processing is limited. Analyses designed to examine whether the redundancy gain in Experiment 2 could be attributed to limited capacity processing in conjunction with positional preferences provided evidence against this possibility. In addition, these analyses suggested that the processing times for the redundant targets in Experiment 2 might be positively correlated. In the third and fourth experiments, the redundant-target displays contained two different words. Processing interference, in the form of a redundancy loss, was evident in the lexical decision task, but not in the semantic categorization task, confirming a difference in the mode of processing between the two tasks. The results provide evidence against the unlimited-capacity, parallel processing hypothesis of late selection theories of attention. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Visual selective attention and reading efficiency are related in children

We investigated the relationship between visual selective attention and linguistic performance. Subjects were classified in four categories according to their accuracy in a letter cancellation task involving selective attention. The task consisted in searching a target letter in a set of background letters and accuracy was measured as a function of set size. We found that children with the lowest performance in the cancellation task present a significantly slower reading rate and a higher number of reading visual errors than children with highest performance. Results also show that these groups of searchers present significant differences in a lexical search task whereas their performance did not differ in lexical decision and syllables control task. The relationship between letter search and reading, as well as the finding that poor readers-searchers perform poorly lexical search tasks also involving selective attention, suggest that the relationship between letter search and reading difficulty may reflect a deficit in a visual selective attention mechanisms which is involved in all these tasks. A deficit in visual attention can be linked to the problems that disabled readers present in the function of magnocellular stream which culminates in posterior parietal cortex, an area which plays an important role in guiding visual attention.     

Working memory for complex figures: An fMRI comparison of letter and fractal n-back tasks.

n-back letter and fractal tasks were administered to I I participants during functional magnetic resonance imaging to test process specificity theories of prefrontal cortex (PFC) function and assess task validity. Tasks were matched on accuracy, but fractal n-back responses were slower and more conservative. Maintenance (1-back minus O-back) activated inferior parietal and dorsolateral PFC, with additional activation in right ventrolateral PFC during letter n-back and left lingual gyrus during fractal n-back. Maintenance plus manipulation (2-back minus 0-back) activated inferior parietal, Broca's area, insula, and dorsolateral and ventral PFC, with greater right dorsolateral PFC activation for letter n-back. Manipulation only (2-back minus 1-back) produced additional and equivalent dorsolateral PFC and anterior cingulate activation in both tasks. Results support fractal n-back validity and indicate substantial overlap in working memory functions of dorsal and ventral PFC. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Cellular kinetic differences between Hodgkin''s and anaplastic large cell lymphomas: relation to the expression of p34cdc2 and cyclin B-1

Normal human subjects were tested for their ability to discriminate the orientation of a square plaque tilted in depth, using two different tasks: a grasping task and a perceptual matching task. Both tasks were given under separate monocular and binocular conditions. Accuracy of performance was measured by use of an opto-electronic motion analysis system, which computed the hand orientation (specifically, a line joining the tips of the thumb and index finger) as the hand either approached the target during grasping or was used to match the target. In all cases there was a very strong statistical coupling between hand orientation and target orientation, irrespective of viewing conditions. However, the matching data differed from the grasping data in showing a consistent curvature in the hand-target relationship, whereby the rate of change of hand orientation as a function of object orientation was smaller for oblique orientations than for those near the horizontal or vertical. The results are interpreted as reflecting the operation of two different mechanisms for analysing orientation in depth: a visuomotor system (assumed to be located primarily in the dorsal cortical visual stream) and a perceptual system (assumed to be located in the ventral stream). It may be that the requirements of visuomotor control dictate a primary need for absolute orientation coding, whereas those of perception dictate a need for more categorical coding.     

Emotion-attention interactions in recognition memory for distractor faces.

Effective filtering of distractor information has been shown to be dependent on perceptual load. Given the salience of emotional information and the presence of emotion-attention interactions, we wanted to explore the recognition memory for emotional distractors especially as a function of focused attention and distributed attention by manipulating load and the spatial spread of attention. We performed two experiments to study emotion-attention interactions by measuring recognition memory performance for distractor neutral and emotional faces. Participants performed a color discrimination task (low-load) or letter identification task (high-load) with a letter string display in Experiment 1 and a high-load letter identification task with letters presented in a circular array in Experiment 2. The stimuli were presented against a distractor face background. The recognition memory results show that happy faces were recognized better than sad faces under conditions of less focused or distributed attention. When attention is more spatially focused, sad faces were recognized better than happy faces. The study provides evidence for emotion-attention interactions in which specific emotional information like sad or happy is associated with focused or distributed attention respectively. Distractor processing with emotional information also has implications for theories of attention. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Coherent motion detection and letter position encoding

We identified 24 'good' and 24 'poor' coherent motion detectors from an unselected sample of young adults. The two groups were matched for reading ability, age and IQ. All subjects carried out two tasks in which optimal performance depended on accurate letter position encoding: a lexical decision task and a primed reaction time task. We found that accurate letter position encoding was predicted by performance in the motion detection task. Since coherent motion detection depends on input from the magnocellular pathway, these findings suggest that information carried by the magnocellular system may be required for encoding letter position. Furthermore, these results may have implications for reading disability which is said to be associated with magnocellular dysfunction.     

A spatiotemporal profile of visual system activation revealed by current source density analysis in the awake macaque

We investigated the spatiotemporal activation pattern, produced by one visual stimulus, across cerebral cortical regions in awake monkeys. Laminar profiles of postsynaptic potentials and action potentials were indexed with current source density (CSD) and multiunit activity profiles respectively. Locally, we found contrasting activation profiles in dorsal and ventral stream areas. The former, like V1 and V2, exhibit a 'feedforward' profile, with excitation beginning at the depth of Lamina 4, followed by activation of the extragranular laminae. The latter often displayed a multilaminar/columnar profile, with initial responses distributed across the laminae and reflecting modulation rather than excitation; CSD components were accompanied by either no changes or by suppression of action potentials. System-wide, response latencies indicated a large dorsal/ventral stream latency advantage, which generalizes across a wide range of methods. This predicts a specific temporal ordering of dorsal and ventral stream components of visual analysis, as well as specific patterns of dorsal-ventral stream interaction. Our findings support a hierarchical model of cortical organization that combines serial and parallel elements. Critical in such a model is the recognition that processing within a location typically entails multiple temporal components or 'waves' of activity, driven by input conveyed over heterogeneous pathways from the retina.     

A specific deficit of dorsal stream function in Williams' syndrome

Williams' syndrome (WS) is a rare, genetically based disorder of cognitive development. Affected individuals show a severe deficit of spatial cognition but a relative sparing of language and face recognition. To examine the possible neural basis of the spatial deficit, we tested a group of WS children, aged 4-14 years, on two measures specific to dorsal cortical stream function: global motion coherence thresholds, in comparison with an analogous form-coherence test, and visuo-manual accuracy in posting a card through a slot, compared with matching the slot orientation. Deficits in these tasks provide the first evidence of specific involvement in WS of the dorsal stream, the cortical system believed to encode information about spatial relationships and the visual control of action.     

Feature-specific perceptual processing dissociates action from recognition.

Does visual processing differ for action and recognition? To address this question, the authors capitalized on research showing that color is preferred over binocular disparity in the ventral (recognition) stream, whereas disparity is preferred over color in the dorsal (action) stream. Participants searched for oblique targets among vertical distractors in displays defined only by color or disparity. Action-task participants stamped the target with a handheld block, whereas recognition-task participants lifted the block through a target-compatible gap. Analyses of reaction time and time-varying hand orientation showed that disparity and color displays were processed equally efficiently during action, but disparity was processed less efficiently than color during recognition. The results suggest that visual processing differs for action and recognition. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Divided attention and memory: Evidence of substantial interference effects at retrieval and encoding.

In 5 divided attention (DA) experiments, students (24 in each experiment) performed visual distracting tasks (e.g., recognition of words, word and digit monitoring) while either simultaneously encoding an auditory word list or engaging in oral free recall of the target word list. DA during retrieval, using either of the word-based distracting tasks, produced relatively larger interference effects than the digit-monitoring task. DA during encoding produced uniformly large interference effects, regardless of the type of distracting task. Results suggest that when attention is divided at retrieval, interference is created only when the memory and concurrent task compete for access to word-specific representational systems; no such specificity is necessary to create interference at encoding. During encoding, memory and concurrent tasks compete primarily for general resources, whereas during retrieval, they compete primarily for representational systems. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Attention constraints of semantic activation during visual word recognition.

The reduction of semantic priming following letter search of the prime suggests that semantic activation can be blocked if attention is allocated to the letter level during word processing. Is this true even for the very fast-acting component of semantic activation? To test this, the authors explored semantic priming of lexical decision at stimulus onset asynchronies (SOAs) of either 200 or 1000 ms. Following semantic prime processing, priming occurred at both SOAs. In contrast, no priming occurred at the long SOA following letter-level processing. Of greatest interest, at the short SOA there was priming following the less demanding consonant/vowel task but not following the more attention-demanding letter search task. Hence, semantic activation can occur even when attention is directed to the letter level, provided there are sufficient resources to support this activation. The authors conclude that the default setting during word recognition is for fast-acting activation of the semantic system. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Modulating irrelevant motion perception by varying attentional load in an unrelated task

Lavie's theory of attention proposes that the processing load in a relevant task determines the extent to which irrelevant distractors are processed. This theory was tested by asking participants in a study to perform linguistic tasks of low or high load while ignoring irrelevant visual motion in the periphery of the display. Although task and distractor were unrelated, both functional imaging of motion-related activity in cortical area V5 and psychophysical measures of the motion aftereffect showed reduced motion processing during high load in the linguistic task. These findings fulfill the prediction that perception of irrelevant distractors depends on the relevant processing load.     

Germ-line therapy for mitochondrial disease: some ethical objections

In an effort to examine the functional neuroanatomy of semantic memory, we studied the relative cerebral blood flow of eight healthy young subjects using 15O-water positron emission tomography (PET). Relative to a visual baseline control condition, each of four visual matching-to-sample tasks activated components of the ventral visual processing stream, including the inferior occipital and temporal cortices. Contrasting the task with the highest semantic component, a variation on the Pyramids and Palm Trees paradigm, with a size discrimination task resulted in focal activation in the anterior inferior temporal lobe, focused in the parahippocampal gyrus. There was additional activation in BA47 of the inferior frontal cortex. These data replicate and extend previously reported results using similar paradigms, and are consistent with cognitive neuropsychological models that stress the executive role of BA47 in semantic processing tasks.     

Spatial context learning survives interference from working memory load.

The human visual system is constantly confronted with an overwhelming amount of information, only a subset of which can be processed in complete detail. Attention and implicit learning are two important mechanisms that optimize vision. This study addressed the relationship between these two mechanisms. Specifically we asked, Is implicit learning of spatial context affected by the amount of working memory load devoted to an irrelevant task? We tested observers in visual search tasks where search displays occasionally repeated. Observers became faster when searching repeated displays than unrepeated ones, showing contextual cuing. We found that the size of contextual cuing was unaffected by whether observers learned repeated displays under unitary attention or when their attention was divided using working memory manipulations. These results held when working memory was loaded by colors, dot patterns, individual dot locations, or multiple potential targets. We conclude that spatial context learning is robust to interference from manipulations that limit the availability of attention and working memory. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

How to study the kinetic depth effect experimentally.

G. Sperling et al (see record 1990-06589-001) proposed an objective 3D shape identification task with 2D artifactual cues removed and with full feedback (FB) to the subjects to measure kinetic depth effect (KDE) and to circumvent algorithmically equivalent KDE-alternative computations and artifactual non-KDE processing. (1) The 2D velocity flow-field was necessary and sufficient for true KDE. (2) Only the first-order (Fourier-based) perceptual motion system could solve our task because the second-order (rectifying) system could not simultaneously process more than two locations. (3) To ensure first-order motion processing, KDE tasks must require simultaneous processing at more than two locations. (4) Practice with FB is essential to measure ultimate capacity (aptitude) and, thereby, to enable comparisons with ideal observers. Experiments without FB measure ecological achievement—the ability of subjects to extrapolate their past experience to the current stimuli. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Anisotropies in visual motion perception: a fresh look

We measured motion-detection and motion-discrimination performance for different directions of motion, using stochastic motion sequences. Random-dot cinematograms containing 200 dots in a circular aperture were used as stimuli in a two-interval forced-choice procedure. In the motion-detection experiment, observers judged which of two intervals contained weak coherent motion, the other internal containing random motion only. In the direction-discrimination experiment, observers viewed a standard direction of motion followed by comparison motion in a slightly different direction. Observers indicated whether the comparison was clockwise or counterclockwise, relative to the standard. Twelve directions of motion were tested in the detection task and five standard directions (three cardinal directions and two oblique directions) in the discrimination task. Detection thresholds were invariant with direction of motion, but direction-discrimination thresholds were significantly higher for motion in oblique directions, even at low-coherence levels. Results from control conditions ruled out monitor artifacts and indicate that the oblique effect is relative to retinal coordinates. These results have broad implications for computational and physiological models of motion perception.     

Visual word recognition: A dissociation of lexical and semantic processing.

These experiments illustrate 2 new dissociations in word-recognition tasks. In one, relatedness facilitated lexical decision but impaired searching for a common letter in the same pairs of words (a cross-over interaction between relatedness and task). In the other dissociation, lexicality facilitated performance (words processed faster than nonwords) while relatedness impaired performance (related words processed slower than unrelated words) in the letter search task. Two classes of explanation are discussed. In the first, the perception of relatedness serves to focus attention to the word level, thereby making explicit letter level processing more difficult and/or increasing the number of competing lexical entries via priming. In the second, spreading inhibition makes related words more difficult to process than unrelated words. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Age-related differences in the detection of three-dimensional surfaces from optic flow.

The present study examined age-related differences in sensitivity to optic flow for the detection of 3-D surfaces. Observers were presented with optic flow displays simulating either a 3-D corrugated surface or a random velocity pattern and were asked to detect the 3-D corrugated surface display. Performance decrements were found for older (mean age?=?71) compared with younger (mean age?=?21) observers across variations in density and corrugation frequency. Older observers also showed performance decrements for a motion coherence task, but performance on this task was not significantly correlated with performance on the 3-D surface detection task. These results suggest that performance on 2-D motion tasks is not necessarily predictive of performance on complex 3-D motion tasks. In addition, the results suggest that different processes underlie the analysis of 2-D and 3-D motion. (PsycINFO Database Record (c) 2010 APA, all rights reserved)