Extracallosal channels of interhemispheric transmission of visual information

Evoked potentials (EP) in response to light flashes were recorded in cats with different degree of optical tract disconnection. In intact and operated on animals, the latent period of the first component of these EP was the same as in the visual cortex. The different degree of disconnection of the classic and commissural optical tract brought about an increase in the amplitude of commissural potentials. The data obtained point to the existence of the effective callosal and extracallosal volleys of interhemispheric transmission of visual information, which are also likely to pay an essential part in compensatory processes of the optical system.     

Geniculate relay of interhemispherical transmission of visual information

Electrophysiological experiments were made on conscious cats with different degree of dissection of the classical and commissural optic tracts to the lateral geniculate body. The data obtained indicated that dissection of the direct retinal tracts to the lateral geniculate body (unilateral dissection of the optic tract) leads merely to the reduction in the amplitude of evoked potentials in response to light flashes. However, the latent period of the first component of the response in this structure appeared the same as in intact animals. Comparison of these data with those obtained in analogous experiments made on the visual cortex allows the conclusion that the geniculate body is the main subcortical relay of the visual commissural effects which are transmitted from the retina to the cortical regions of the brain.     

Interhemispheric transmission of visual information: behavioral and electrophysiologic aspects

Efficiency of callosal and extracallosal channels of interhemispheric transfer of visual information was studied by behavioural and electrophysiological methods on cats with sections of classical and commissural pathways of the visual system. The results showed a high ability of the operated animals to form conditioned defensive reflex to visual stimuli. The primary complexes of evoked potentials to light flashes in the projection cortex of both hemispheres were also preserved. The data show the significant role of the callosal and extracallosal pathways of the transmission of visual information across the midline.     

Pathways for transmission of visual and auditory information to the cat caudate nucleus

Visual and auditory projections to the cat caudate nucleus were investigated using the horseradish peroxidase retrograde axonal transport technique in conjunction with that of experimental degeneration of retinal axons. It was found that visual information may reach the caudate nucleus not just through well-known polysynaptic pathways from the cerebral cortex but also following oligosynaptic (transpulvinar, lateroposterior nucleus, suprageniculate nucleus, and nucleus limitans of the thalamus) as well as bisynaptic pathways (via the medial and lateral terminal nuclei of the accessory optical tract, pulvinar, pretectum, intermediary layer of the superior colliculus, and the supraoptic nucleus); some of these pathways were identified for the first time. Direct retinal inputs were found in the suprageniculate nucleus. Additional structures were discovered through which auditory information may reach the caudate nucleus, i.e., the dorsal nucleus of the parvocellular portion of the lateral geniculate body, the deep-lying superior colliculus, and the dorsal and ventral nuclei of the lateral lemniscus. The physiological significance of the pathways described for possible transmission of visual and auditory impulses is discussed and a new principle underlining the organization of sensory inputs into the caudate nucleus is put forward.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 19, No. 4, pp. 512–520, July–August, 1987.     

Bipolar cells use kainate and AMPA receptors to filter visual information into separate channels

Unlike cone photoreceptors, whose light responses have a uniform time course, retinal ganglion cells are tuned to respond to different temporal components in a changing visual scene. The signals in a mammalian cone flow to three to five morphologically distinct "OFF" bipolar cells at a sign-conserving, glutamatergic synapse. By recording simultaneously from pairs of synaptically connected cones and OFF bipolar cells, I now show that each morphological type of OFF bipolar cell receives its signal through a different AMPA or kainate receptor. The characteristic rate at which each receptor recovers from desensitization divides the cone signal into temporal components. Temporal processing begins at the first synapse in the visual system.     

Space-variant visual processing: spatially limited visual channels

A multichannel model incorporating visual inhomogeneity is presented in this paper. The parameters that describe inhomogeneity have been experimentally obtained both at threshold and in several suprathreshold conditions. At threshold, probability summation is taken into account in order to determine the spatial extent of visual channels from experimental data showing an asymptotic increase in sensitivity with increasing grating area. At suprathreshold contrast, the region where luminance variations at several scales are visible has also been found. The results support a spatially limited multichannel model of early visual processing and set out a basis for studying perceptual phenomena from the viewpoint of linear space-variant visual processing.     

Commissural asymmetry in brachiopods

Consistent asymmetric folding of the commissure is a characteristic feature of a small but significant number of brachiopod species. The feature may be obligate or facultative and is almost entirely confined to rhynchonellids, most of which are Mesozoic. The detailed nature of the asymmetry is very variable, but does not extend to internal hard parts such as crura. Taken as a whole, asymmetric brachiopod species show no preference for any particular environment or geographic region, and in no circumstances seem to have been markedly more or less successful than symmetric species. We are thus led to suggest that asymmetry was a genetically based condition which cropped up periodically in brachiopod evolution, and which possibly was selected neither particularly for nor against.     

The transmission sense of information

Biologists rely heavily on the language of information, coding, and transmission that is commonplace in the field of information theory developed by Claude Shannon, but there is open debate about whether such language is anything more than facile metaphor. Philosophers of biology have argued that when biologists talk about information in genes and in evolution, they are not talking about the sort of information that Shannon’s theory addresses. First, philosophers have suggested that Shannon’s theory is only useful for developing a shallow notion of correlation, the so-called “causal sense” of information. Second, they typically argue that in genetics and evolutionary biology, information language is used in a “semantic sense,” whereas semantics are deliberately omitted from Shannon’s theory. Neither critique is well-founded. Here we propose an alternative to the causal and semantic senses of information: a transmission sense of information, in which an object X conveys information if the function of X is to reduce, by virtue of its sequence properties, uncertainty on the part of an agent who observes X. The transmission sense not only captures much of what biologists intend when they talk about information in genes, but also brings Shannon’s theory back to the fore. By taking the viewpoint of a communications engineer and focusing on the decision problem of how information is to be packaged for transport, this approach resolves several problems that have plagued the information concept in biology, and highlights a number of important features of the way that information is encoded, stored, and transmitted as genetic sequence.     

Pathways for the transmission of visual information in the protocerebrum of the drone fly Eristalis tenax]

Studies have been made on the structure of neuropiles and visual pathoways in the brain of the fly E. tenax L. (Diptera, Syrphidae). The retina is projected on laminar structures in the visual ganglia only; other protocerebrum neuropiles lack this projection. All the comissures connecting contralateral visual ganglia, consist of several hundreds of fibers, whereas the binocular zone of both eyes includes more than 4,000 ommatidia. Neither the visual ganglia, nor other protocerebrum neuropiles may serve as a substrate for topographic imposition of projections of the corresponding parts in both retines. The mechanism of binocular interaction in insects presumably differs from that in mammals (primates, carnovores).     

Measurement of visual channels by contrast adaptation.

Inspection of a high-contrast grating pattern affects our ability to detect patterns that are similar. This technique can be used to infer the underlying mechanisms of the visual system. By using this technique, measurements of the bandwidth of orientation channels are taken for different levels of adapting contrast and adapting duration. If the threshold elevation is plotted as the difference between the unadapted and adapted threshold in decibels, then the orientation bandwidth is invariant if taken at some fraction of the maximum elevation. This results from the fact that, as the orientation difference between the adapting and test patterns increases, the function relating threshold elevation to adapting contrast reduces in slope. These data contradict the often-used 'equivalent contrast transformation' (in which the fall off in the adaptation effect with respect to orientation is expressed in terms of an equivalent reduction in adapting contrast) as this would produce quite different bandwidths at different adapting contrasts. The data also address the issue of the neuronal mechanisms of adaptation.     

Visualization of ceramide channels by transmission electron microscopy

Functional studies have shown that the sphingolipid ceramide, self-assembles in phospholipid membranes to form large channels capable of allowing proteins to cross the membrane. Here these channels are visualized by negative stain transmission electron microscopy. The images contain features consistent with stain-filled pores having a roughly circular profile. There is no indication of tilt, and the results are consistent with the formation of right cylinders. The sizes of the pores range from 5 to 40nm in diameter with an asymmetric distribution indicating no apparent upper size limit. The size distribution matches well with the distribution of sizes calculated from electrophysiological measurements.