Two-photon excited autofluorescence imaging of freshly isolated frog retinas

The purpose of this study was to investigate cellular sources of autofluorescence signals in freshly isolated frog (Rana pipiens) retinas. Equipped with an ultrafast laser, a laser scanning two-photon excitation fluorescence microscope was employed for sub-cellular resolution examination of both sliced and flat-mounted retinas. Two-photon imaging of retinal slices revealed autofluorescence signals over multiple functional layers, including the photoreceptor layer (PRL), outer nuclear layer (ONL), outer plexiform layer (OPL), inner nuclear layer (INL), inner plexiform layer (IPL), and ganglion cell layer (GCL). Using flat-mounted retinas, depth-resolved imaging of individual retinal layers further confirmed multiple sources of autofluorescence signals. Cellular structures were clearly observed at the PRL, ONL, INL, and GCL. At the PRL, the autofluorescence was dominantly recorded from the intracellular compartment of the photoreceptors; while mixed intracellular and extracellular autofluorescence signals were observed at the ONL, INL, and GCL. High resolution autofluorescence imaging clearly revealed mosaic organization of rod and cone photoreceptors; and sub-cellular bright autofluorescence spots, which might relate to connecting cilium, was observed in the cone photoreceptors only. Moreover, single-cone and double-cone outer segments could be directly differentiated

Neurovascular coupling: In vivo optical techniques for functional brain imaging

10.1186/1475-925X-12-38BioMedical Engineering Online121-BEOI

Melanopsin Sensitivity in the Human Visual System

The human retina contains long [L]-wavelength, medium [M]-wavelength, and short [S]-wavelength cones, rods, and intrinsically photosensitive retinal ganglion cells expressing the blue-sensitive (λmax = ~480 nm) photopigment melanopsin. Previous animal studies have pointed to a role of melanopsin in advancing circadian phase, melatonin suppression, the pupillary light reflex (PLR), light avoidance, and brightness discrimination, often relying on genetic tools to study melanopsin in isolation in animal models. This work addresses the question of human melanopsin sensitivity and function in vivo using a spectrally tunable light source and the method of silent substitution, allowing for the selective stimulation of melanopsin in the human retina, in combination of pupillometry, psychophysics, and BOLD functional neuroimaging (fMRI). In three studies, we find (1) that the temporal transfer function of melanopsin in controlling the pupil in humans is low-pass, peaking at slow temporal frequencies (0.01 Hz), with a sharp drop off at higher frequencies (1-2 Hz); (2) that signals originating from S cones get combined in an antagonistic fashion with melanopsin signals and signals from L and M cones cones, demonstrating spectral opponency in the control of the human PLR; (3) that nominally cone-silent melanopsin-directed spectral modulations stimulate cones in the partial shadow of the retinal blood vessels (termed penumbral cones), leading to the entoptic percept of the subjective retinal vasculature; and (4) that there is no measurable signal due to melanopsin stimulation in human visual cortical areas (V1, V2/V3, MT, LOC; measured with BOLD fMRI) at temporal frequencies most relevant to spatial vision (0.5–64 Hz) while modulations directed at L+M, L–M and S photoreceptor combinations yield characteristic temporal transfer functions in these areas. This work advances to our understanding of the functional significance of melanopsin function in the human visual system, contributing to the study of human health in relation to light and color

Change blindness: eradication of gestalt strategies

Arrays of eight, texture-defined rectangles were used as stimuli in a one-shot change blindness (CB) task where there was a 50% chance that one rectangle would change orientation between two successive presentations separated by an interval. CB was eliminated by cueing the target rectangle in the first stimulus, reduced by cueing in the interval and unaffected by cueing in the second presentation. This supports the idea that a representation was formed that persisted through the interval before being 'overwritten' by the second presentation (Landman et al, 2003 Vision Research 43149–164]. Another possibility is that participants used some kind of grouping or Gestalt strategy. To test this we changed the spatial position of the rectangles in the second presentation by shifting them along imaginary spokes (by ±1 degree) emanating from the central fixation point. There was no significant difference seen in performance between this and the standard task [F(1,4)=2.565, p=0.185]. This may suggest two things: (i) Gestalt grouping is not used as a strategy in these tasks, and (ii) it gives further weight to the argument that objects may be stored and retrieved from a pre-attentional store during this task

The Neural Correlates of Consciousness - An Update

This review examines recent advances in the study of brain correlates of consciousness. First, we briefly discuss some useful distinctions between consciousness and other brain functions. We then examine what has been learned by studying global changes in the level of consciousness, such as sleep, anesthesia, and seizures. Next we consider some of the most common paradigms used to study the neural correlates for specific conscious percepts and examine what recent findings say about the role of different brain regions in giving rise to consciousness for that percept. Then we discuss dynamic aspects of neural activity, such as sustained versus phasic activity, feedforward versus reentrant activity, and the role of neural synchronization. Finally, we briefly consider how a theoretical analysis of the fundamental properties of consciousness can usefully complement neurobiological studies

fMRI studies of amblyopia: Pediatric and adult perspectives

Functional magnetic resonance imaging (fMRI) is currently the technique of choice for mapping functional neuroanatomy in humans, and over the past 15 years there has been a dramatic growth in the number of studies that provide brain-behavior correlations in normal healthy adults. More recently, a few studies have begun to make such measures in healthy children. In addition, fMRI is increasingly being applied to study brain function in subjects with neurological disease. The overall aim of these studies was to apply fMRI methods to the study of amblyopia, the most prevalent developmental vision disorder. Amblyopia develops early in life, usually before 5 years old, and is most treatable during childhood. Our approach was to study both children and adults with either the strabismic or the anisometropic type of amblyopia. In our first experiment (Chapter 3), we applied fMRI techniques to map retinotopic visual organization in children. We conclude that cortical visual organization is measurable and highly mature in children aged 9 to 12 years. In our second experiment (Chapter 4), we applied similar techniques to adults with amblyopia. We conclude that visual field organization is abnormal in the brains of these adults. In our final experiment (Chapter 5), we applied these same techniques to children with amblyopia, and observed abnormalities similar to those seen in adults. These studies present a novel neurological characterization of amblyopia, and provide a basis for further studies of human visual development, in health and disease

Functional Magnetic Resonance Imaging

"Functional Magnetic Resonance Imaging - Advanced Neuroimaging Applications" is a concise book on applied methods of fMRI used in assessment of cognitive functions in brain and neuropsychological evaluation using motor-sensory activities, language, orthographic disabilities in children. The book will serve the purpose of applied neuropsychological evaluation methods in neuropsychological research projects, as well as relatively experienced psychologists and neuroscientists. Chapters are arranged in the order of basic concepts of fMRI and physiological basis of fMRI after event-related stimulus in first two chapters followed by new concepts of fMRI applied in constraint-induced movement therapy; reliability analysis; refractory SMA epilepsy; consciousness states; rule-guided behavioral analysis; orthographic frequency neighbor analysis for phonological activation; and quantitative multimodal spectroscopic fMRI to evaluate different neuropsychological states

Parafoveal Retinal Vascular Response to Pattern Visual Stimulation Assessed with OCT Angiography

We used optical coherence tomography (OCT) angiography with a high-speed swept-source OCT system to investigate retinal blood flow changes induced by visual stimulation with a reversing checkerboard pattern. The split-spectrum amplitude-decorrelation angiography (SSADA) algorithm was used to quantify blood flow as measured with parafoveal flow index (PFI), which is proportional to the density of blood vessels and the velocity of blood flow in the parafoveal region of the macula. PFI measurements were taken in 15 second intervals during a 4 minute period consisting of 1 minute of baseline, 2 minutes with an 8 Hz reversing checkerboard pattern stimulation, and 1 minute without stimulation. PFI measurements increased 6.1±4.7% (p = .001) during the first minute of stimulation, with the most significant increase in PFI occurring 30 seconds into stimulation (p<0.001). These results suggest that pattern stimulation induces a change to retinal blood flow that can be reliably measured with OCT angiography.National Institutes of Health (U.S.) (Grant R01 EY013516)National Institutes of Health (U.S.) (Grant Rosenbaum's P30EY010572)Research to Prevent Blindness, Inc. (United States) (Grant R01-Ey11289-26)United States. Air Force Office of Scientific Research (FA9550-10-1-0551

Endogenous and exogenous hemodynamic signals in primary visual cortex of alert non-human primates

The advent of neuroimaging techniques in particular the ones suitable for studies in alert humans has disseminated fast. Research in fields involving neuro-correlates of cognitive processes has flourished. Still the neural underpinnings of the neuroimaging signals remain to be fully characterized; this field is an active topic of research. In the context of behavior/cognition, the interpretation of neuroimaging signals is even more intricate