Selective binocular vision loss in two subterranean caviomorph rodents: Spalacopus cyanus and Ctenomys talarum.

To what extent can the mammalian visual system be shaped by visual behavior? Here we analyze the shape of the visual fields, the densities and distribution of cells in the retinal ganglion-cell layer and the organization of the visual projections in two species of facultative non-strictly subterranean rodents, Spalacopus cyanus and Ctenomys talarum, aiming to compare these traits with those of phylogenetically closely related species possessing contrasting diurnal/nocturnal visual habits. S. cyanus shows a definite zone of frontal binocular overlap and a corresponding area centralis, but a highly reduced amount of ipsilateral retinal projections. The situation in C. talarum is more extreme as it lacks of a fronto-ventral area of binocular superposition, has no recognizable area centralis and shows no ipsilateral retinal projections except to the suprachiasmatic nucleus. In both species, the extension of the monocular visual field and of the dorsal region of binocular overlap as well as the whole set of contralateral visual projections, appear well-developed. We conclude that these subterranean rodents exhibit, paradoxically, diurnal instead of nocturnal visual specializations, but at the same time suffer a specific regression of the anatomical substrate for stereopsis. We discuss these findings in light of the visual ecology of subterranean lifestyles.

A simple method to microinject solid neural tracers into deep structures of the brain.

We have developed an instrument to perform microinjections of solid neural tracers into deep structures of the brain. The instrument consists of a thin hypodermic needle equipped with a movable internal rod, which is connected to a pressure chamber. When a pressure pulse is applied to the chamber, the rod moves forward and back inside the needle, pushing out a solid load previously packed inside the needle tip. By attaching a microelectrode to the instrument, it is also possible to have electrophysiological control of the injection placement. To test the instrument, we microinjected DiI and rhodamine crystals into selected structures of the visual system of pigeons. The results show small, well-defined injection sites, accurately located in the desired targets, together with well-developed anterogade and retrograde transport, selectively originated from the injection sites. This method extends the usage of solid tracers to most structures in the brain and may, in certain cases, be more advantageous than the conventional method of injecting tracer solutions.

Spike sorting based on discrete wavelet transform coefficients.

Using the novel mathematical technique known as wavelet analysis, a new method (WSC) is presented to sort spikes according to a decomposition of neural signals in the time-frequency space. The WSC method is implemented by a pyramidal algorithm that acts upon neural signals as a bank of quadrature mirror filters. This algorithm is clearly explained and an overview of the mathematical background of wavelet analysis is given. An artificial spike train, especially designed to test the specificity and sensibility of sorting procedures, was used to assess the performance of the WSC method as well as of methods based on principal component analysis (PCA) and reduced feature set (RFS). The WSC method outperformed the other two methods. Its superior performance was largely due to the fact that spike profiles that could not be separated by previous methods (because of the similarity of their temporal profile and the masking action of noise) were separable by the WSC method. The WSC method is particularly noise resistant, as it implicitly eliminates the irrelevant information contained in the noise frequency range. But the main advantage of the WSC method is its use of parameters that describe the joint time-frequency localization of spike features to build a fast and unspecialized pattern recognition procedure.

Spatiotemporal profile of synaptic activation produced by the electrical and visual stimulation of retinal inputs to the optic tectum: a current source density analysis in the pigeon (Columba livia).

The optic tectum of the pigeon is a highly organized, multilayered structure that receives a massive polystratified afference of at least five different populations of retinal ganglion cells and gives rise to various anatomically segregated efferent systems. The synaptic organization of retino-tectal circuitry is, at present, mostly unknown. To investigate the spatiotemporal profile of synaptic activation produced by differential (electrical and visual) stimulation of the retinal inputs, we performed a high-spatial-resolution current source density analysis in the optic tectum of the anaesthetized pigeon. Electrical stimuli consisted of brief pulses of different durations applied to the optic nerve head, while visual stimuli consisted of light flashes of different intensities. Electrical stimulation generated sinks confined to retinorecipient layers. The temporal structure, spatial location and thresholds of these sinks indicated that they are all due to primary tectal synapses of retinal fibers with different conduction velocities. Sinks evoked by the fastest retinal axons were more superficially located than sinks produced by slower retinal fibers. Visual stimulation, on the other hand, resulted in a more complex pattern of current sinks, with various sinks located in the retinorecipient layers and also well below. Visual stimulation induced action potentials at superficial as well as deep tectal levels. We conclude that electrical stimulation activates most of the populations of ganglion cells as well as their primary tectal synapses, but is unable to elicit a significant activation of secondary tectal synapses. Visual stimulation, on the contrary, activates just some of the incoming retinal populations, but in a way that produces noticeable secondary activation of intratectal circuits. Laminar segregation of retinally evoked tectal activity, as reported here, has also been found in other vertebrates. Similarities and differences with previous studies are discussed.

Reversible mechanical fixation of eye position in awake head-restrained pigeons (Columba livia).

Here we describe a method to fix gaze positions and to significantly reduce saccadic oscillations in pigeons. The procedure consists of a mechanical immobilization of the eye through the use of an electromagnet that exerts a radial force upon a small metal rectangle glued to the dorsal part of the eye. The method can be used in avian visual neurophysiology in order to hold the eye immobilized for periods of time, long enough to map the properties of visual receptive fields and investigate the possible functions of saccadic oscillations.

Conduction velocity groups in the retino-tectal and retino-thalamic visual pathways of the pigeon (Columbia livia).

The anatomical characteristics of the avian visual system are well known. However, there are wide gaps in our knowledge with respect to the physiological characteristics of their visual system. For example, we lack both an operational identification of the different ganglion cell types present in the retinae of birds, and a description of their presumptive differential central projections. The results presented here address this latter point by classifying the conduction velocity groups of fibers present in the optic tract of the pigeon. We report the existence of at least 5 groups of axons in the optic tract of the pigeon, with conduction velocities of 22-18 m/s, 12-10 m/s, 8 m/s, 6 m/s and less than 2.5 m/s. All five groups project to the tectum but only the four fastest groups project to the dorsal thalamic complex. The homologies with the populations of retinal axons found in cats are discussed.

Saccade-related responses of centrifugal neurons projecting to the chicken retina.

Centrifugal projections to several sensory systems modulate the afferent activity during active behaviors. To see whether such modulation occurred in the visual system, we recorded the activity of isthmo-optic neurons in awake chickens during eye movements. We find that the discharge of all isthmo-optic neurons tends to stop during saccades, although every neuron does not pause for every saccade. The pause begins at approximately the same time as the saccade, and pause duration is correlated with saccade duration. Pausing during saccades occurs in both dark and light suggesting that it is motoric rather that visual in origin. In addition, we find that the spontaneous activity of isthmo-optic neurons increases in darkness. We discuss the significance of the saccadic modulation of isthmo-optic activity in terms of possible functions of the centrifugal projection in modulation of ganglion cell activity.

Why the cortical magnification factor in rhesus is isotropic.

We examine the arguments advanced recently [by Sakitt (Vision Res. 22, 417-421, 1982), and Tootell et al. (Science 218, 902-904, 1982)] leading to the conclusion that the retino-cortical magnification factor cannot be isotropic. The relevance of the issue is discussed through the use of complex function theory as the appropriate tool to express the geometry of interconnections between neural lamina. A general expression is obtained to compute the length of a curve under a conformal transformation. When this result is applied to circles in the visual field, it is shown that Sakitt's findings imply radial asymmetry of the retino-cortical map, and not local anisotropy, as claimed. In fact, by taking Schwartz's (Vision Res. 20, 645-670, 1980) proposal that the complex log map including a constant eccentricity term be used to represent the retino-cortical mapping, we show that both local isotropy and radial asymmetry are simultaneously valid. Furthermore, the values of cortical length predicted from this approach account exactly for the data shown by Sakitt. A quantitative prediction is offered also for the ratio between the magnification factor of vertical and horizontal meridians at different eccentricities, which is precisely the value reported by Tootell et al. We conclude that these results show that (1) conformal transformations are appropriate tools to describe the geometrical interconnections between neural lamina; (2) for the case of the retino-cortical mapping the appropriate conformal transform is an eccentric complex logarithm.