Vincent A. Nguyen

Increasing depth of binocular rivalry suppression along two visual pathways.

Binocular rivalry refers to the alternating perception that occurs when the two eyes are presented with incompatible stimuli: one monocular image is seen exclusively for several seconds before disappearing as the other image comes into view. The unseen stimulus is physically present but is not perceived because the sensory signals it elicits are suppressed. The neural site of this binocular rivalry suppression is a source of continuing controversy. We psychophysically tested human subjects, using test probes designed to selectively activate the visual system at a variety of processing stages. The results, which apply to both form and motion judgements, show that the sensitivity loss during suppression increases as the subjects task becomes more sophisticated. We conclude that binocular rivalry suppression is present at a number of stages along two visual cortical pathways, and that suppression deepens as the visual signal progresses along these pathways.

Effects of aging on vibration detection thresholds at various body regions

BackgroundThe ability to detect sinusoidal vibrations on the skin surface is dependent on the activation of two classes of receptors. The density of such receptors varies across the skin surface and is a factor in determining the sensory acuity of each skin area. However, the acuity of many sensory systems is known to deteriorate with advancing age. The aim of this study was to determine if vibrotactile sensibility of several skin surfaces deteriorated equally with advancing age.MethodsVibration detection thresholds for two frequencies of vibration (30 Hz and 200 Hz) were determined using a method of limits protocol, in two groups of healthy adults, one group aged 17 to 27 years and the other aged 55 to 90 years. Sinusoidal vibrations were generated by a computer and delivered to the skin surface via the probe (diameter = 2 mm) of a mechanical vibrator. Four skin sites (palmar surface of the tip of the middle finger, volar surface of the forearm, lateral aspect of the shoulder, cheek just caudal to the zygoma) were tested.ResultsThe fingertip was the most sensitive site for vibrotactile detection at both frequencies in a substantial majority of subjects. The older group of subjects showed significantly higher detection thresholds for both frequencies at all sites, except the fingertip, when compared to young subjects.ConclusionThe study confirms the deterioration of vibrotactile acuity at several skin sites previously reported in the literature. However, there appears to be no significant reduction in vibrotactile detection at the fingertips in older subjects. This may reflect the high receptor density of this area, or the functional importance of vibrotactile sensibility of the fingertips or some combination of both of these factors.

The depth and selectivity of suppression in binocular rivalry

Binocular rivalry occurs when the two eyes are presented with incompatible stimuli and the perceived image alternates between the two stimuli. The aim of this study was to find out whether the periodic perceptual loss of a monocular stimulus during binocular rivalry is mirrored by a comparable loss of contrast sensitivity. We presented brief test stimuli to one eye while its conditioning stimulus was dominant or suppressed. The test stimuli were varied widely across four stimulus domains—namely, the relative stimulation of medium- and long-wavelength-sensitive cones, duration, spatial frequency, and grating orientation. The result in each case was the same. Suppression depended slightly or not at all on the type of test stimulus, and contrast sensitivity during suppression was around 64% of that during dominance. The effect of suppression on sensitivity is therefore very weak, relative to its effect on the perceived image. Furthermore, suppression was largely independent of the similarity between the conditioning and the test stimuli, indicating that our results are better explained byeye suppression than bystimulus suppression. A model is presented to account for the small, monocular sensitivity loss during suppression: It assumes that test detection precedes conditioning stimulus perception in the visual pathway.

Detection of the depth order of defocused images

The sign of an accommodative response is provided by differences in chromatic aberration between under- and over-accommodated images. We asked whether these differences enable people to judge the depth order of two stimuli in the absence of other depth cues. Two vertical edges separated by an illuminated gap were presented at random relative distances. Exposure was brief, or prolonged with fixed or changing accommodation. The gap was illuminated with tungsten light or monochromatic light. Subjects could detect image blur with brief exposure for both types of light. But they could detect depth order only in tungsten light with long exposure, with or without changes in accommodation.

Permanent Functional Reorganization of Retinal Circuits Induced by Early Long-Term Visual Deprivation

Early sensory experience shapes the functional and anatomical connectivity of neuronal networks. Light deprivation alters synaptic transmission and modifies light response properties in the visual system, from retinal circuits to higher visual centers. These effects are more pronounced during a critical period in juvenile life and are mostly reversed by restoring normal light conditions. Here we show that complete light deprivation, from birth to periods beyond the critical period, permanently modifies the receptive field properties of retinal ganglion cells. Visual deprivation reduced both the strength of light responses in ganglion cells and their receptive field size. Light deprivation produced an imbalance in the ratio of inhibitory to excitatory inputs, with a shift toward larger inhibitory conductances. Ganglion cell receptive fields in visually deprived animals showed a spatial mismatch of inhibitory and excitatory inputs and inhibitory inputs were highly scattered over the receptive field. These results indicate that visual experience early in life is critical for the refinement of retinal circuits and for appropriate signaling of the spatiotemporal properties of visual stimuli, thus influencing the response properties of neurons in higher visual centers and their processing of visual information.

Inner retinal inhibition shapes the receptive field of retinal ganglion cells in primate

•  The receptive field of most retinal ganglion cells consists of an excitatory centre and an inhibitory surround. •  In retinal ganglion cells of non‐primates the receptive field surround is provided by lateral inhibition in both the outer and the inner retinal synaptic layers. •  We use whole cell recording methods to establish the spatial organisation of excitatory and inhibitory synaptic inputs onto ganglion cells in primate retina. •  We confirm centre–surround organisation in the excitatory inputs to ganglion cells, and show further that inhibitory inputs can also show centre–surround organisation. •  We show that lateral inhibition in the inner retina shapes the spatial profile of both excitatory and inhibitory synaptic inputs onto ganglion cells. •  Dynamic clamp experiments provide evidence that reduction of inner retinal inhibition reduces spatial tuning in ganglion cell output. •  These results show that lateral inhibition in the inner retina of primate shapes the analysis of spatial form and contrast.

Components of visual acuity loss in strabismus

Strabismus, the misalignment of the visual axis of one eye relative to that of the other eye, reduces visual acuity in the affected eye. Several processes contributing to that loss are: amblyopia, which results in a chronic acuity loss whether or not the fellow eye is viewing; strabismic deviation, which shifts the image of an acuity target onto more peripheral, and therefore less acute, retina when the fellow eye fixates; interocular suppression and binocular masking, which reduce visibility in the strabismic eye due to neural influences from the other eye. We measured the losses due to these processes in nine small-angle strabismic subjects. Amblyopia reduced acuity by a median of 34% relative to its value in subjects with normal binocular vision, and strabismic deviation produced a loss of 44%. Suppression and masking together reduced acuity by 20%, and therefore had substantially less effect than the other factors.

The value of clinical electrophysiology in the assessment of the eye and visual system in the era of advanced imaging

Electrophysiological techniques allow clinical investigations to include a ‘dissection’ of the visual system. Using suitable electrophysiological techniques, the ‘dissection’ allows function to be ascribed to the different photoreceptors (rod and cone photoreceptors), retinal layers, retinal location or the visual pathway up to the visual cortex. Combined with advances in genetics, retinal biochemistry, visual fields and ocular imaging, it is now possible to obtain a better understanding of diseases affecting the retina and visual pathways. This paper reviews core electrophysiological principles that can complement other examination techniques, including advanced ocular imaging, and help the interpretation of other clinical data and thus, refine and guide clinical diagnosis.

Controlling binocular rivalry.

Binocular rivalry is the alternating perception that occurs when the two eyes are presented with incompatible stimuli. We have developed a new method for controlling binocular rivalry and measuring its progress. One eye views a static grating while the fellow eye views a grating that smoothly and cyclically varies between two orientations, one the same as the static grating and the other orthogonal. Contrast sensitivity was tested monocularly a number of times during the stimulus cycle. When the eye viewing the static grating was tested, sensitivity varied between maximum and minimum values as the conditioning stimulus varied from binocularly compatible to incompatible. The interocular suppression thus demonstrated was limited to the eye viewing the static grating; variations in the fellow eyes sensitivity were due to interocular masking alone.

A2A: Benchmark Your Clinical Decision Support Search

Clinical Decision Support (CDS) systems aim to assist clinicians in their daily decision-making related to diagnosis, tests, and treatments of patients by providing relevant evidence from the scientific literature. This promise however is yet to be fulfilled, with search for relevant literature for a given patient condition still being an active research topic. The TREC CDS track was designed to address this research gap. We developed a platform to facilitate experimentation and hypothesis testing for information retrieval researchers working on this topic. It provides a large range of query and document processing techniques that are explored in the biomedical search domain.