Edward R. Gruberg
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Office: Department of Biology Phone: (215) 204-1920 |
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Our laboratory is currently working on three projects. Behavioral and physiological study of the perception of stationary objects by frogs. (Animations here) Within one second a human will lose sight of objects that are not moving with respect to the retina. That is normally not a problem even for stationary objects since our eyes are in almost perpetual motion with "micro-saccades" and other eye movements. We have discovered that frogs also have a keen ability to detect stationary objects, even holes in transparent covers (Saltzman et al., 2004). They can jump through such holes with great accuracy and can distinguish between holes they can fit through and holes they can not fit through. Yet they do not appear to generate micro-saccades or other eye movements. Further, frogs can detect such apertures in transparent covers even in the absence of the optic tectum, the principal target of the retina. We have found that the visual field of frogs is in motion relative the retina because they move their whole head. We are characterizing the nature of the movements of the head using digital video photography. As we characterize the head movements, we can reverse engineer visual stimuli which reflect the nature of the normal head movements that cause images to move relative to the retina. Since the tectum is not necessary to detect apertures we are looking for the neural substrate underlying the ability to detect stationary objects. With head-stabilized frogs we are using microelectrode recording in the brain to characterize non-tectal retinal targets that could be used to detect stationary objects and apertures.. Study of organization of nucleus isthmi using injections of solutions of three retrograde fluorescent labels into the frog optic tectum The optic tectum is the principal target of the retina. It mediates the frog's ability to correctly identify and appropriately respond to two important classes of moving objects: prey and looming objects. The tectum receives direct visuotopic retinal input only from the contralateral eye. Nucleus isthmi is a midbrain structure that combines information from the two eyes so that each tectum has access to information from both eyes. The nucleus isthmi also acts directly on retinotectal axons by modifying calcium uptake and hence controlling retinotectal synaptic transmission (Dudkin and Gruberg, 2003). The nucleus isthmi can be divided into an outer shell, the cortex, and an inner core, the medulla. Using three different fluorescent retrograde labels we have been studying the anatomical arrangement of medullary cells of nucleus isthmi that project to ipsilateral tectum and the medullary cells that project to contralateral tectum. In the region of overlap we find an admixture of cells that project ipsilaterally or contralaterally but not bilaterally. The use of identifiably different dyes and discrete tectal injections allows us determine the resolution of the visuotopic ipsilateral and contralateral isthmotectal projections.  The midbrain structure the optic tectum receives a large projection from the contralateral retina. Most of these retinotectal fibers use glutamate as their neurotransmitter to signal visual information to tectal cells. Curiously, the retinotectal axons express acetylcholine receptors on their surface. The midbrain structure nucleus isthmi receives input from the optic tectum and reciprocally projects back to the optic tectum to the same layers that receive retinotectal axons. Our laboratory earlier showed that isthmotectal fibers are cholinergic (Wallace, Ricciuti & Gruberg, 1990, Neuroscience). That is, they respond to stimulation by releasing acetylcholine. We hypothesized that cholinergic isthmotectal fibers should directly affect cholinoceptive retinotectal fibers. To test this we began by injecting a fluorescent calcium sensitive dye, calcium green-1 conjugated to a dextran, into the optic nerve. We found that the dye was transported to the tectum. The figure on the left shows a confocal montage image of a section of the tectum with the fluorescent dye seen as bright green. The dye remains sequestered in the optic nerve fibers and does not leak into surrounding tectal cells.
Physiological study of the role of nucleus isthmi in frog visual attention. It is noteworthy that a frog will respond to a ground-level moving prey stimulus at any position around the frog, i.e., it has a full 360o view. If several prey stimuli are presented simultaneously the frog does not get confused but rather, will choose a single prey and accurately respond to it. Frogs thus possess a form of selective visual attention. The nucleus isthmi has been implicated in the circuitry concerned with attention. We found (Winkowski and Gruberg, 2002 and earlier studies) that after unilateral lesion of nucleus isthmi frogs are inattentive to prey stimuli in the contralateral monocular field. Using extracellular microelectrode recording we have found that there is a visuotopic map of visual space in nucleus isthmi. We have begun recording single neuron visual responses to prey-like stimuli in the nucleus isthmi when one or several prey-like stimuli are presented to the frog. We are investigating whether there are significant differences in the response properties of these cells. We are testing whether nucleus isthmi can act as a "winner-take-all" module.
Recent publications: Winkowski, D. E. and E. R. Gruberg (2005) "Superimposed maps of the monocular visual fields in the caudolateral optic tectum in the frog, Rana pipiens." Vis Neurosci 22: 101-109. Saltzman, H., M. Zacharatos and E.R. Gruberg (2004) "Recognition of apertures in overhead transparent barriers by leopard frogs." Brain Behav Evol 64: 11-18. Dudkin, E. A. and E. R. Gruberg (2003). "Nucleus isthmi enhances calcium influx into optic nerve fiber terminals in Rana pipiens." Brain Res 969: 44-52. Winkowski, D. E. and E. R. Gruberg (2002). "The representation of the ipsilateral eye in nucleus isthmi of the leopard frog, Rana pipiens." Vis Neurosci 19: 669-79.
Model for tectal influence of right nucleus isthmi. Neurons are simple gating devices. |
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Calcium uptake from extracellular space to inside axon terminals is a prerequisite for release of neurotransmitter. Our calcium sensitive dye fluoresces more as the concentration of calcium increases. With an in vitro optic nerve/brain preparation using a sensitive photo diode we could measure any changes in fluorescence (i.e., intracellular calcium concentration) as a function of stimulation of nucleus isthmi alone, optic nerve alone or in combination (figure on right). We found that stimulation of nucleus isthmi alone has no effect on calcium uptake. However, if nucleus isthmi is stimulated along with the optic nerve the calcium uptake is twice as great as if the optic nerve were stimulated alone. Thus, we showed that nucleus isthmi can directly affect the retinal signal to the tectum. 