Lateral masking effects on contrast sensitivity in rats.

Changes in target visibility may be produced by additional stimulus elements at adjacent locations. Such contextual effects may reflect lateral interactions of stimulus representations in early cortical areas. It has been reported that the organization of orientation preference found in primates and cats visual cortex differs from that found in rodents, suggesting functional distinctions across species. In order to examine effects of lateral interactions at a perceptual level, contrast sensitivity in rats was measured for Gabor patches masked by two additional patches. Rats responded to target onset, and perceptual indices were based upon reaction time distributions across levels of luminance contrast. It was found that contrast sensitivity of targets without lateral masks corresponded to levels previously reported. For all measurements, the presence of sustained lateral masks systematically reduced sensitivity to targets, demonstrating interference by adjacent elements across levels of contrast. Effects of mask orientation or separation were not observed. These results may reflect reported non-systematic topography of orientation tuning across the cortex in rodents. Results suggest that intrinsic lateral connections in early processing areas play a minimal role in stimulus integration for rats.

Effects of pattern masks on the formation of perceptual grouping.

Mechanisms underlying perceptual grouping serve to bind stimulus components that are contained within grouped patterns. In order to examine the time course of grouping development, grids of spatially isolated dots were followed by pattern masks across a range of SOA. Subjects indicated the predominant perceived grouping of the dot patterns. Masks either spatially superimposed target elements (element mask), or superimposed elements as well as paths among elements (connection mask). Element masks thereby disrupted processing of target elements, while connection masks additionally disrupted representations in regions among elements. It was found that element masks disrupted grouping 12ms after target offset, after which masks had no effect. Connection masks disrupted grouping up to 47ms following target offset. Results suggest grouping mechanisms access the afferent signal for a brief period early in processing, after which binding formation proceeds for an addition 35ms. Shortening connection mask duration to 12ms enhanced performance during a brief temporal window within the interference period. For each set of conditions, target elements were visible during the time frame in which stimulus patterns could not be perceptually grouped. Full-field checkerboard masks degraded discrimination similarly as connection masks, although were more effective in disrupting discrimination with an SOA of 24 and 36ms. Degrading stimulus organization progressively extended the time scale for each masking effect. For the grouping of low-level stimulus features tested here, results support a model in which afferent signals are accessed early, followed by progressive binding among grouped elements. Effect of shortening connection masks may reflect incomplete disruption of target processing, or possibly re-entry of stimulus representations by feedback from higher processing areas.

Effects of contrast, spatial frequency, and stimulus duration on reaction time in rats.

Early visual processing in rats is mediated by several pre-cortical pathways as well as multiple retinal ganglion cell types that vary in response characteristics. Discrete processing is thereby optimized for select ranges of stimulus parameters. In order to explore variation in response characteristics at a perceptual level, visual detection in rats was measured across a range of contrasts, spatial frequencies, and durations. Rats responded to the onset of Gabor patches. Onset time occurred after a random delay, and reaction time (RT) frequency distribution served to index target visibility. It was found that lower spatial frequency produced shorter RTs, as well as increased RT equivalent of contrast gain. Brief stimulus presentation reduced target visibility, slowed RTs, and reduced contrast gain at higher spatial frequencies. However, brief stimuli shortened RTs at low contrasts and low spatial frequencies, suggesting transient stimuli are more efficiently processed under these conditions. Collectively, perceptual characteristics appear to reflect distinctions in neural responses at early stages of processing. The RT characteristics found here may thereby reflect the contribution of multiple channels, and suggest a progressive shift in relative involvement across parameter levels.

Priming effects on the perceived grouping of ambiguous dot patterns.

For ambiguous stimuli, complex dynamics guide processes of perceptual grouping. Previous studies have suggested two opposing effects on grouping that are produced by the preliminary stimulus state: one that enhances grouping towards the existing structure, and another that opposes this structure. To examine effects of the preliminary state on grouping directly, measurements were made of perceived grouping of dot patterns that followed a visual prime. Three stimuli were presented in sequence: prime, target, and mask. Targets were composed of an evenly spaced dot grid in which grouping was established by similarity in luminance. Subjects indicated the dominant perceived grouping. The prime either corresponded to or opposed the prevailing organization of the target. Contrary to the hypothesis, solid-line primes biased grouping away from the structure of the prime, even when the prevailing organization of dot patterns strongly favored the primes' structure. This effect occurred, although to a lesser extent, when primes did not occupy the same location of targets, but were presented in a marginal area surrounding the grid. Priming effects did not occur for primes constructed of dot patterns. Effects found here may be attributed to a forward masking effect by primes, which more effectively disrupts grouping of patterns matched to the prime. Effects may also be attributed to a type of pattern contrast, in which a grouped pattern dissimilar to primes gains salience. For the pattern contrast model, the partial activation of multiple grouped configurations is compared to the pattern of the solid-line primes.

Perceptual grouping across eccentricity.

Across the visual field, progressive differences exist in neural processing as well as perceptual abilities. Expansion of stimulus scale across eccentricity compensates for some basic visual capacities, but not for high-order functions. It was hypothesized that as with many higher-order functions, perceptual grouping ability should decline across eccentricity. To test this prediction, psychophysical measurements of grouping were made across eccentricity. Participants indicated the dominant grouping of dot grids in which grouping was based upon luminance, motion, orientation, or proximity. Across trials, the organization of stimuli was systematically decreased until perceived grouping became ambiguous. For all stimulus features, grouping ability remained relatively stable until 40°, beyond which thresholds significantly elevated. The pattern of change across eccentricity varied across stimulus feature, in which stimulus scale, dot size, or stimulus size interacted with eccentricity effects. These results demonstrate that perceptual grouping of such stimuli is not reliant upon foveal viewing, and suggest that selection of dominant grouping patterns from ambiguous displays operates similarly across much of the visual field.

Speed of perceptual grouping in acquired brain injury.

Evidence exists that damage to white matter connections may contribute to reduced speed of information processing in traumatic brain injury and stroke. Damage to such axonal projections suggests a particular vulnerability to functions requiring integration across cortical sites. To test this prediction, measurements were made of perceptual grouping, which requires integration of stimulus components. A group of traumatic brain injury and cerebral vascular accident patients and a group of age-matched healthy control subjects viewed arrays of dots and indicated the pattern into which stimuli were perceptually grouped. Psychophysical measurements were made of perceptual grouping as well as processing speed. The patient group showed elevated grouping thresholds as well as extended processing time. In addition, most patients showed progressive slowing of processing speed across levels of difficulty, suggesting reduced resources to accommodate increased demands on grouping. These results support the prediction that brain injury results in a particular vulnerability to functions requiring integration of information across the cortex, which may result from dysfunction of long-range axonal connection.

Effect of chronic kidney disease on the expression of thiamin and folic acid transporters.

BACKGROUND: Chronic kidney disease (CKD) is associated with significant cardiovascular, neurological and metabolic complications. Thiamin and folate are essential for growth, development and normal cellular function, and their uptake is mediated by regulated transport systems. While plasma folate and thiamin levels are generally normal in patients with CKD, they commonly exhibit features resembling vitamin deficiency states. Earlier studies have documented impaired intestinal absorption of several B vitamins in experimental CKD. In this study, we explored the effect of CKD on expression of folate and thiamin transporters in the key organs and tissues. METHODS: Sprague-Dawley rats were randomized to undergo 5/6 nephrectomy or sham operation and observed for 12 weeks. Plasma folate and thiamin concentrations and gene expression of folate (RFC, PCFT) and thiamin transporters (THTR-1 and THTR-2) were determined in the liver, brain, heart and intestinal tissues using real-time PCR. Hepatic protein abundance of these transporters was determined using western blot analysis. RESULTS: Plasma folate and thiamin levels were similar between the CKD and the control groups. However, expressions of both folate (RFC and PCFT) and thiamin (THTR-1, THTR-2) transporters were markedly reduced in the small intestine, heart, liver and brain of the CKD animals. Liver protein abundance of folate and thiamin transporters was significantly reduced in the CKD animals when compared with the sham-operated controls. Furthermore, we found a significant reduction in mitochondrial folate and thiamin transporters in the CKD animals. CONCLUSIONS: CKD results in marked down-regulation in the expression of folate and thiamin transporters in the intestine, heart, liver and brain. These events can lead to reduced intestinal absorption and impaired cellular homeostasis of these essential micronutrients despite their normal plasma levels.

Spectrum of tamoxifen associated endometrial pathology in breast cancer patients.

The objective of the study was to determine the incidence and type of endometrial abnormalities in long-term users of tamoxifen with breast cancer. All patients with a diagnosis of Oestrogen Receptor positive breast cancer on Tamoxifen therapy who had also undergone endometrial biopsy for abnormal bleeding or other symptoms were included. Among the 37 cases that had long-term follow up available, 21(57%) had evidence of endometrial pathology. There were seven cases of simple hyperplasia and thirteen of endometrial polyp. Only one case of endometrial carcinoma was seen. These findings support the association between prolonged tamoxifen therapy and endometrial pathology of possible neoplastic potential. Endometrial pathology is dependent on duration of exposure to Tamoxifen, therefore, close follow up of such patients is recommended.

Computer programming for generating visual stimuli.

Critical to vision research is the generation of visual displays with precise control over stimulus metrics. Generating stimuli often requires adapting commercial software or developing specialized software for specific research applications. In order to facilitate this process, we give here an overview that allows nonexpert users to generate and customize stimuli for vision research. We first give a review of relevant hardware and software considerations, to allow the selection of display hardware, operating system, programming language, and graphics packages most appropriate for specific research applications. We then describe the framework of a generic computer program that can be adapted for use with a broad range of experimental applications. Stimuli are generated in the context of trial events, allowing the display of text messages, the monitoring of subject responses and reaction times, and the inclusion of contingency algorithms. This approach allows direct control and management of computer-generated visual stimuli while utilizing the full capabilities of modern hardware and software systems. The flowchart and source code for the stimulus-generating program may be downloaded from www.psychonomic.org/archive.