Visual cortex controls retinal output in the rat

Retinal cross talk in the mammalian visual system

The existence and functional relevance of efferent optic nerve fibers in mammals have long been debated. While anatomical evidence for cortico-retinal and retino-retinal projections is substantial, physiological evidence is lacking, as efferent fibers are few in number and are severed in studies of excised retinal tissue. Here we show that interocular connections contribute to retinal bioelectrical activity in adult mammals. Full-field flash electroretinograms (ERGs) were recorded from one or both eyes of Brown-Norway rats under dark-adapted (n = 16) and light-adapted (n = 11) conditions. Flashes were confined to each eye by an opaque tube that blocked stray light. Monocular flashes evoked a small (5-15 μV) signal in the nonilluminated eye, which was named "crossed ERG" (xERG). The xERG began under dark-adapted conditions with a positive (xP1) wave that peaked at 70-90 ms and ended with slower negative (xN1) and positive (xP2) waves from 200 to 400 ms. xN1 was absent under light-adapted conditions. Injection of tetrodotoxin in either eye (n = 15) eliminated the xERG. Intraocular pressure elevation of the illuminated eye (n = 6) had the same effect. The treatments also altered the ERG b-wave in both eyes, and the alterations correlated with xERG disappearance. Optic nerve stimulation (n = 3) elicited a biphasic compound action potential in the nonstimulated nerve with 10- to 13-ms latency, implying that the xERG comes from slow-conducting (W type) fibers. Monocular dye application (n = 7) confirmed the presence of retino-retinal ganglion cells in adult rats. We conclude that mammalian eyes communicate directly with each other via a handful of optic nerve fibers. The cross talk alters retinal activity in rats, and perhaps other animals.

The centrifugal visual system of vertebrates: a comparative analysis of its functional anatomical organization

The present review is a detailed survey of our present knowledge of the centrifugal visual system (CVS) of vertebrates. Over the last 20 years, the use of experimental hodological and immunocytochemical techniques has led to a considerable augmentation of this knowledge. Contrary to long-held belief, the CVS is not a unique property of birds but a constant component of the central nervous system which appears to exist in all vertebrate groups. However, it does not form a single homogeneous entity but shows a high degree of variation from one group to the next. Thus, depending on the group in question, the somata of retinopetal neurons can be located in the septo-preoptic terminal nerve complex, the ventral or dorsal thalamus, the pretectum, the optic tectum, the mesencephalic tegmentum, the dorsal isthmus, the raphé, or other rhombencephalic areas. The centrifugal visual fibers are unmyelinated or myelinated, and their number varies by a factor of 1000 (10 or fewer in man, 10,000 or more in the chicken). They generally form divergent terminals in the retina and rarely convergent ones. Their retinal targets also vary, being primarily amacrine cells with various morphological and neurochemical properties, occasionally interplexiform cells and displaced retinal ganglion cells, and more rarely orthotopic ganglion cells and bipolar cells. The neurochemical signature of the centrifugal visual neurons also varies both between and within groups: thus, several neuroactive substances used by these neurons have been identified; GABA, glutamate, aspartate, acetylcholine, serotonin, dopamine, histamine, nitric oxide, GnRH, FMRF-amide-like peptides, Substance P, NPY and met-enkephalin. In some cases, the retinopetal neurons form part of a feedback loop, relaying information from a primary visual center back to the retina, while in other, cases they do not. The evolutionary significance of this variation remains to be elucidated, and, while many attempts have been made to explain the functional role of the CVS, opinions vary as to the manner in which retinal activity is modified by this system.

Photoreceptor plasticity in the albino rat retina following unilateral optic nerve section

Rhodopsin content of the retina increased when an albino rat was moved to a lower intensity cyclic light, and decreased when moved to a higher intensity. Unilateral optic nerve section was employed to test if intact optic nerves are necessary to maintain regulation of rhodopsin when albino rats are switched between two intensities, 3 lx and 600 lx (12 hr/12 hr, cyclic light). The ability to regulate rhodopsin content was altered but not lost in the eye which had the optic nerve transected. This alteration was in the direction of incomplete up-regulation when the intensity change was from high to low, and was in the direction of excessive down-regulation when the intensity change was from low to high. This indicates that efferent fibers from the brain to the retina may be involved in the regulation of rhodopsin content. Using histological techniques, cell number and rod outer segment (ROS) length were measured: surgery alone had no effect on ROS length or outer nuclear layer thickness and in all cases ROS lengths were inversely related to habitat illuminance.

The effect of unilateral optic nerve section on retinal light damage in rats

The effect of unilateral optic nerve section on the susceptibility of rat photoreceptors to damage by constant light was studied. Optic nerves of albino rats were cut intracranially by a ventral approach, so as not to interfere with structures in the orbit of the eye, with the brain or with the blood supply to the retina. Two separate experiments were performed on unilaterally optic nerve sectioned rats from two different sources. One group was purchased from a commercial supplier and the other group was born and raised in 3 lux cyclic light in our laboratory. For 1-4 weeks after surgery they were exposed to either 1000 lux for 24 hr or 80 lux for 48 hr. Light damage was quantified by measuring the outer nuclear layer area remaining in histological sections through the vertical meridian of the retina. It was found that retinas with optic nerves cut suffered substantially less damage from light than did those with intact optic nerves. Sham operated rats suffered the same amount of damage as did the optic nerve intact retinas of rats with one nerve cut. The extent of protection was greatest in the region previously shown to be most susceptible to damage. The protection afforded by optic nerve section could not be explained on the basis of behavior or rhodopsin photochemistry. The possible role of heat-shock proteins and a neuromodulator is discussed.

Responsiveness of reorganized primary somatosensory (SI) cortex after local inactivation of normal SI cortex in chronic spinal cats

The cortical map of adult cats that sustained spinal cord transection at T12 when they were 2 weeks old is characterized by a clear duplication of the representation of the forelimb, rostral trunk, and neck. The novel representation is located in the cortical region that is, in nonoperated animals, normally devoted to the hindlimb representation. We have investigated the possibility that the reactivation of the deprived hindlimb cortex may be mediated by corticocortical projections from normal to reorganized cortex. The primary somatosensory (SI) cortex was initially mapped to determine the boundaries of the normal and reorganized cortical representations. Somatotopically corresponding regions in both normal and reorganized cortex representing the trunk, the web space, or the shoulder were more precisely mapped. Inactivation of normal cortex was achieved by the nanoinjection of a solution of lidocaine hydrochloride stained with Chicago sky blue. Two major findings are described. First, inactivation of a circumscribed region of normal cortex representing a given receptive field (RF) failed to reduce or inhibit the responsiveness of a somatotopically corresponding RF represented in reorganized cortex. Therefore, it is unlikely that intracortical connections between normal and reorganized cortex could account for the reorganizational processes observed in cats that sustained spinal cord transection at 2 weeks of age. Second, the chemical blockade of normal cortex provoked an increase of the responsiveness and of the size of the peripheral RFs represented in reorganized cortex. This finding suggests that there are corticocortical connections (possibly topographically organized) between normal and reorganized cortex, and that these connections are inhibitory.

Subcortical gating in the human visual system during spatial selective attention

The question of whether subcortical gating occurs as a function of spatial selective attention remains unsettled. This issue was investigated, using the paradigm of Eason et al. (1969) wherein subjects are instructed to attend to a specified location in a given visual field while attempting to ignore stimuli presented in the opposite field. Visual evoked responses falling within the 40-70 ms range were found to be significantly more negative when the location at which the evoking stimulus appeared was being attended to than when it was not. Also, later deflections (100-200 ms) were enhanced in amplitude and negatively biased. The very early effect provides further evidence for spatial attention-induced precortical gating. The later effects provide additional evidence for the amplitude enhancement of 'exogenous' components, along with the possible involvement of glial cell activity in the generation of slow wave negativity.

Centrifugal pathways to the retina: influence of the optic tectum

Two types of centrifugal pathways to the retina have been found in the vertebrates, according to the location of the cell bodies and presence or absence of connections with the optic tectum. One type is represented by the isthmo-optic nucleus (ION) of birds and, therefore, termed "ION-type" retinopetal system. The other type is termed "non-ION-type" retinopetal system. The ION-type retinopetal systems have been found in the cyclostomes, teleosts, reptiles, and birds. This review describes the anatomy and physiology of the ION-type retinopetal systems, mainly of birds and teleosts. On the basis of anatomical and physiological evidence cited in this review, the ION-type retinopetal systems can be regarded as the tectofugal pathways to the retina. The function of the ION-type retinopetal systems is discussed in detail, with special emphasis on their relation to the role of the tectum in mediating visuomotor behavior.

Optic nerve blockade influences the retinal responses to flash in rabbits

The presence of retinopetal fibers in mammals has been debated many times in the past two decades. Do rabbits have a retinopetal system? This question is addressed with the present investigations. In anesthetized and paralysed rabbits the b-wave and the oscillatory potentials (OP) are recorded at the cornea. The optic nerve is isolated retrobulbarly and is gently hooked to a curved injecting capillary. Through the latter lidocaine hydrochloride is pressure injected. This drug interrupts the neuronal flow travelling along the nerve. A steel electrode is positioned in the optic chiasm allowing us to monitor the evoked field potentials from the tested and untested eyes. The optic nerve blockade produces the following observations: (1) the amplitude of the b-wave is not significantly altered; and (2) the amplitudes of the long latency OP are significantly increased. The retinal capacity to respond to a second flash after the application of an initial light pulse was evaluated by varying the interval between the two flashes. After optic nerve blockade the recovery of the retinal responsiveness is considerably slower. Fourier analysis indicated that the highest power increases occurred around 200 Hz. It is difficult to escape the suggestion that rabbits possess retinopetal fibers.

Influences of cortico-pretectal fibers on responses of rat pretectal neurons

Responses of pretectal cells were studied following cortical excitation and inactivation in anesthetized rats. The visual cortex was excited electrically and by a topical application of strychnine while it was inactivated by a reversible cryoblockade. In a few experiments the superior colliculus was locally depressed by microinjections of lidocaine hydrochloride. Results showed a large spectrum of response latencies to electrical stimulations of the optic chiasm and visual cortex, and a significant correlation between responses to both sites of stimulation. It appears that pretectal cell responses strongly depend upon corticofugal impulses, as the disruption of the latter results in a profound decline of pretectal discharges. The suppressed-by-light cells were the most affected by a cryoblockade while the ON-tonic units were the least affected. The same types of cells were affected at the retinal level by cortical blockade.