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At least 1,279 records · Page 71Linked to original sources
PubMed1999

How good is the match between the plane of the text and the plane of focus during reading?

Because results from animal models demonstrate that retinal image defocus is a crucial factor in the visual control of eye growth, we have measured the precision of accommodation during reading at 1 m and at 30 cm distance. A newly developed photorefractor was used to sample both the refraction in the vertical meridian and direction of gaze at 25 Hz. Using these two parameters, a three-dimensional "refraction map" of the visual field was plotted. It showed the optic disc as an area with more myopic refractions and the course of refractions across a visual field of about +/- 25 deg. A special calibration scheme was employed to ensure that the precision of the refractions was 0.2 dpt or better (as estimated from the standard deviations of repeated measurements and the noise in the calibration curve). Twelve young adults (students from the lab) served as subjects. We found considerable inter-individual variability in the off-axis refractions but little variability among repeated measurements in the same subjects. Inter-individual variability reached a minimum in the foveal region. Both myopes wearing their spectacle corrections (n = 6) and emmetropes (n = 6) under-accommodated by about 0.3 D during reading at 30 cm distance but, at 1 m distance, only the emmetropes under-accommodated. Since both refraction groups under-accommodated similarly during reading at close distance, it remains unclear whether the small amount of defocus is critical for their future myopia development. Either accommodation errors differ at earlier times when myopia first appears (as suggested by the literature), or the subjects' eye growth was differently sensitive to defocus, or our simple protocol did not pick up existent differences in accommodation among the two groups.

Accommodation, Ocular↗
PubMed2004

The organization of orientation and spatial frequency in primary visual cortex.

Many studies have demonstrated that the primary visual cortex contains multiple functional maps of visual properties (e.g., ocular dominance, orientation preference, and spatial-frequency preference), but as yet no consistent picture has emerged as to how these maps are related to one another. Three divergent, prior optical-imaging studies of spatial frequency are reanalyzed and critiqued in this article. Evidence is presented that a nonstimulus-specific response biased the interpretation of results in previous studies. In addition to reexamining four prior cat experiments, we carried out one new experiment. Through the use of different methods and a careful removal of the nonspecific response, we are led in all instances to a unique view of cortical organization for spatial-frequency preference. In particular, we find little apparent evidence for a columnar organization for spatial frequency. The response recorded by each image pixel may be viewed as arising from an admixture of low- and high-spatial-frequency populations. For most pixels, the ratio of these populations is 1:1.

Animals↗
PubMed1995

Illusory contours activate specific regions in human visual cortex: evidence from functional magnetic resonance imaging.

The neural basis for perceptual grouping operations in the human visual system, including the processes which generate illusory contours, is fundamental to understanding human vision. We have employed functional magnetic resonance imaging to investigate these processes noninvasively. Images were acquired on a GE Signa 1.5T scanner equipped for echo planar imaging with an in-plane resolution of 1.5 x 1.5 mm and slice thicknesses of 3.0 or 5.0 mm. Visual stimuli included nonaligned inducers (pacmen) that created no perceptual contours, similar inducers at the corners of a Kanizsa square that created illusory contours, and a real square formed by continuous contours. Multiple contiguous axial slices were acquired during baseline, visual stimulation, and poststimulation periods. Activated regions were identified by a multistage statistical analysis of the activation for each volume element sampled and were compared across conditions. Specific brain regions were activated in extrastriate cortex when the illusory contours were perceived but not during conditions when the illusory contours were absent. These unique regions were found primarily in the right hemisphere for all four subjects and demonstrate that specific brain regions are activated during the kind of perceptual grouping operations involved in illusory contour perception.

Brain Mapping↗
PubMed2004

Prefrontal cortical activation associated with working memory in adults and preschool children: an event-related optical topography study.

It is well known that lateral areas of the prefrontal cortex (LPFC) play a central role in working memory (a critical basis of various cognitive functions), but it remains unknown whether the LPFC of children of preschool age is responsible for working memory. To address this issue, we adopted a recently developed non-invasive imaging technique, optical topography (OT), which can potentially be applied to functional mapping in childhood. We firstly examined changes of activity in the LPFC using OT while adult subjects performed an item-recognition task, which requires working memory, under different memory-load conditions. We observed activation in the bilateral LPFC during performance of this task, the magnitude of which differed depending on memory-load. Then, we applied the same technique on 5- and 6-year-old children and observed the activation associated with working memory in the LPFC. Areas and properties of such activity were similar in adults and preschool children. Thus, for the first time, we demonstrate that the LPFC of preschoolers is active during working memory processes, indicating that in 5- and 6-year-old children, the LPFC has already developed processing of this important cognitive function.

Adult↗
PubMed2000

Increase of GAP-43 expression following kainic acid injection into whisker barrel cortex.

Plasticity after microinjection of kainic acid (KA) into the adult rat whisker barrel cortex was investigated with immunohistochemical staining of phosphorylated growth-associated protein (GAP)-43. After mapping the barrel cortex with the technique of intrinsic signal optical imaging, a small volume of KA was injected into one barrel. Rats were sacrificed at 2 days, 3 days, 1 week, and 6 weeks after lesioning. GAP-43 staining demonstrated intense immunoreactivity (IR) at the injected barrel which spread to the inter-barrel septa and the surrounding barrels. Elevated IR of GAP-43 was visible 2 days after KA injection, and increased gradually at least 6 weeks following the lesion. This model has the possibility of offering a simple and reliable tool for studying cortical plasticity.

Animals↗
PubMed1972

Comparison of receptive-field organization of the superior colliculus in Siamese and normal cats.

1. The superior colliculus has been studied in Siamese and normal cats by recording the responses of single tectal units to visual stimuli.2. The retinotopic organization of the superior colliculus has been compared in the two breeds. In the normal cat, the contralateral half-field is represented in the central and caudal part of the colliculus, and a vertical strip of the ipsilateral half-field, 15-20 degrees wide, is represented at the anterior tip. The Siamese cat superior colliculus receives an abnormally large projection from the ipsilateral half-field so that units with visual receptive fields which extend as far as 40 degrees into the ipsilateral half-field can be found. The area of the tectal surface devoted to the representation of the ipsilateral half-field is about twice as large in Siamese cats as in normal cats. The enhanced representation of the ipsilateral half-field in Siamese cats is reflected in a displacement of the vertical meridian and the area centralis on the tectal surface.3. The area centralis in the Siamese cat is located at about the same point on the tectal surface as would be occupied by a point in the visual field about 6-7 degrees contralateral to the area centralis in the normal cat. The smallest receptive fields in both breeds are located near the area centralis. The size of the receptive field for a tectal unit seems to be determined by the retinal location of the receptive field and not by the absolute position of the unit on the tectal surface.4. The receptive-field characteristics of tectal units show many similarities in the two breeds. The receptive fields of individual units consist of activating regions flanked by suppressive surrounds. Units respond well to stimuli of different shapes and orientation provided they are moving. The optimum stimulus for a given unit can be much smaller than the size of the activating region. About two thirds of the units studied in both breeds show directional selectivity. Most of the units studied in normal cats can be activated by stimulation of either eye, while in Siamese cats, 80% of the units studied can be driven only by the contralateral eye. A few monocularly driven units with two separated receptive fields have been observed in Siamese cats.5. In the left tectum of both breeds, units respond well to left-to-right stimulus movement. The reverse situation obtains in the right tectum. In Siamese cats, units located at the anterior tip of the tectum with their receptive fields located in the visual half-field ipsilateral to the tectum under study respond better to stimulus movement toward the area centralis than away from it. The preferred direction for a tectal unit seems to be determined by its tectal location rather than by the location of its receptive field in the retina.6. Visual cortex lesions in both breeds increase the responsiveness of tectal units to flashing spots and almost entirely remove the directional selectivity exhibited by tectal units, although units with asymmetric surrounds are still found. In normal cats, the lesions change the ocular dominance distribution, skewing it more strongly toward the contralateral eye. In Siamese cats, the ocular dominance distribution remains unchanged after a visual cortex lesion.7. The squint commonly exhibited by Siamese cats is regarded as a compensation for the anomalous retinotectal topography. It is suggested that, in the absence of an adaptive modification, the anomalous retinotectal projection would lead to mislocalization in Siamese cats just as it does in frogs and hamsters whose retinotectal projection has been experimentally altered. The convergent strabismus which Siamese cats commonly exhibit may be a cure for the abnormal retinal projections rather than a disease.

Animals↗
PubMed1974

Cat retinal ganglion cells: size and shape of receptive field centres.

1. Receptive field centres of 144 sustained and transient retinal ganglion cells were mapped in cats under light pentobarbitone anaesthesia.2. Sustained on-centre, sustained off-centre, transient on-centre and transient off-centre cells had different mean sizes of receptive field centre, with some overlap between their distributions.3. For each class of cell, central fields had the smallest field-centres; progressively larger field-centres were encountered more peripherally.4. All classes of ganglion cells tended to have slightly elliptical receptive field centres. Major axes of over half of all receptive fields were oriented within 20 degrees of horizontal. These trends were independent of pupil dimensions, or of receptive field eccentricity or position in the visual field. The results almost certainly reflect asymmetry in retinal wiring.5. Two cells of thirty-nine tested were sensitive to axis of motion; in both cases the preferred and major axis were horizontal. A further cell was orientation specific.

Action Potentials↗
PubMed2006

Indirect measurement of the lag time distribution of single cells of Listeria innocua in food.

The distribution of log counts at a given time during the exponential growth phase of Listeria innocua measured in food samples inoculated with one cell each was applied to estimate the distribution of the single-cell lag times. Three replicate experiments in broth showed that the distribution of the log counts is a linear mapping of the distribution of the detection times measured by optical density. The detection time distribution reflects the lag time distribution but is shifted in time. The log count distribution was applied to estimate the distributions of the lag times in a liquid dairy product and in liver paté after different heat treatments. Two batches of ca. 100 samples of the dairy product were inoculated and heated at 55 degrees C for 45 min or at 62 degrees C for 2 min, and an unheated batch was incubated at 4 degrees C. The final concentration of surviving bacteria was ca. 1 cell per sample. The unheated cells showed the shortest lag times with the smallest variance. The mean and the variance of the lag times of the surviving cells at 62 degrees C were greater than those of the cells treated at 55 degrees C. Three batches of paté samples were heated at 55 degrees C for 25 min, 62 degrees C for 81 s, or 65 degrees C for 20 s. A control batch was inoculated but not heated. All paté samples were incubated at 15 degrees C. The distribution of the lag times of the cells heated at 55 degrees C was not significantly different from that of the unheated cells. However, at the higher temperatures, 62 degrees C and 65 degrees C, the lag duration was longer and its variance greater.

Animals↗
PubMed2004

Early alcohol exposure induces persistent alteration of cortical columnar organization and reduced orientation selectivity in the visual cortex.

Fetal alcohol syndrome (FAS) is a major cause of learning and sensory deficits in children. The visual system in particular is markedly affected, with an elevated prevalence of poor visual perceptual skills. Developmental problems involving the neocortex are likely to make a major contribution to some of these abnormalities. Neuronal selectivity to stimulus orientation, a functional property thought to be crucial for normal vision, may be especially vulnerable to alcohol exposure because it starts developing even before eye opening. To address this issue, we examined the effects of early alcohol exposure on development of cortical neuron orientation selectivity and organization of cortical orientation columns. Ferrets were exposed to ethanol starting at postnatal day (P) 10, when the functional properties and connectivity of neocortical neurons start to develop. Alcohol exposure ended at P30, just before eye opening at P32. Following a prolonged alcohol-free period (15-35 days), long-term effects of early alcohol exposure on cortical orientation selectivity were examined at P48-P65, when orientation selectivity in normal ferret cortex has reached a mature state. Optical imaging of intrinsic signals revealed decreased contrast of orientation maps in alcohol- but not saline-treated animals. Moreover, single-unit recordings revealed that early alcohol treatment weakened neuronal orientation selectivity while preserving robust visual responses. These findings indicate that alcohol exposure during a brief period of development disrupts cortical processing of sensory information at a later age and suggest a neurobiological substrate for some types of sensory deficits in FAS.

Action Potentials↗
PubMed1992

Partition of function in the morphological subdivisions of the lateral geniculate nucleus of the tammar wallaby (Macropus eugenii).

Extra-cellular recordings from single cells in the dorsal lateral geniculate nucleus (dLGN) of the tammar wallaby, Macropus eugenii, were made to find out whether the stratification of the nucleus could be correlated with the receptive field properties of units. Retinofugal fibres terminate in the lateral geniculate nucleus of the wallaby in nine interleaved eye-specific layers. These may be grouped into a lateral alpha segment of six laminae and a medial beta segment of three, in which the cells are less densely packed. Ninety percent of the geniculate neurons recorded from in the alpha segment gave brisk responses to stimulation of their receptive fields. Cells with sluggish responses predominated in the beta segment, but there was also a sizable minority of cells with brisk responses that were indistinguishable from those recorded in the alpha segment. In contrast, other response properties were rarely differentiated in individual layers. Thus, in most layers, the numbers of cells with transient or sustained responses were not significantly different, and this was also true for cells with ON- or OFF- responses. For each of these response pairings, however, the numbers of one type (ON- and transient) predominated in every layer. The accumulation of this laminar distinction lead to significant differences in the alpha and beta segments and in the nucleus as a whole. We conclude that cells in the individual layers of the dLGN of the tammar wallaby show no evidence of having receptive field properties in common that might correlate with separate functional streams. There is a functional segregation of receptive field properties between the alpha and beta segments. The organization of these two segments resembles that of the A and the C layers of the dLGN in cats and, possibly, the magnocellular and koniocellular components of the dLGN in primates. These broad similarities in functional partition of the dLGN of different species suggests that this aspect of the organization of the nucleus is independent of lifestyle.

Animals↗
PubMed1976

Behavioral correlates of 'tectal compression' in goldfish. II. Visual acuity.

Visual acuity was measured in three normal goldfish and two goldfish in which the caudal optic tectum had been ablated bilaterally. Compression of the retinotectal map was assumed to have occurred in the two operated fish, which were tested more than 200 days after surgery. The results indicated that the visual acuity of the normal fish ranged from 0.94 to 0.95 degrees of visual angle. The visual acuity of the fish with presumed compression of their retinotectal maps ranged from 1.91 to 2.07 degrees. The results suggest that compression of the retinotectal map occurs at the expense of the spatial-resolution ability of the visual system.

Animals↗
PubMed1996

Genetic dissection of the retinotectal projection.

A systematic search for mutations affecting the retinotectal projection in zebrafish larvae was performed, as part of the large-scale Tubingen screen for homozygous diploid mutants in embryonic development. 2,746 inbred lines (F2 families) from males mutagenized with ethylnitroso urea were screened. In wild-type larvae, developing retinal axons travel along a stereotyped route to the contralateral optic tectum. Here, their terminals form a highly ordered retinotopic map. To detect deviations from this pattern, an axon tracing assay was developed that permits screening of large numbers of mutagenized fish. Two fluorescent tracer dyes (DiI and DiO) were injected at opposite poles of the eyes of day-5 aldehyde-fixed larvae. 12 hours later, retinal axons were labelled over their entire length, and could be observed through the intact skin. The assay procedure (aldehyde fixation, mounting, injection of dyes, microscopic analysis) took about 1 minute per fish. In total, 125,000 individual fish larvae were processed. During the screen, 114 mutations in approx. 35 genes were discovered. For the mutants subjected to complementation testing, the number of alleles per locus ranges from 1 to 15. The mutations affect distinct steps in the retinotectal pathway, from pathfinding between eye and tectum to map formation along the dorsal-ventral and the anterior-posterior axis of the tectum. Mutations that disturb axon pathfinding to the tectum for the most part do not disrupt retinotopic mapping, and vice versa. The majority of the mutants display associated defects in other tissues and die before day 10. These mutants provide new tools for studying the formation of neuronal maps. The results of this screen show that a large-scale genetic approach can be applied to relatively late and circumscribed developmental processes in the vertebrate brain.

Animals↗