Essential neuronal pathways for reflex and conditioned response initiation in an intracerebellar stimulation paradigm and the impact of unconditioned stimulus preexposure on learning rate.

It has been demonstrated previously that pairing of tone CS and intracerebellar stimulation of lobule HVI white matter as the US produces conditioning that is robust and in many ways similar to that obtained with an airpuff US. The first study in this report addressed the effect of interpositus lesions on conditioned performance in rabbits trained with white matter stimulation as the US. It was found that interpositus lesions effectively eliminated the CR irrespective of the behavioral response measured. In addition, it was shown that the interpositus lesions also abolished the UR, providing strong evidence that the effects of the electrical stimulation were confined to the cerebellum and did not require the activation of brainstem structures. The second experiment examined performance on US-alone trials of varying durations. Response initiation within 100 ms of the US onset, regardless of US duration, indicated that reflex generation could not be due to rebound excitation of the interpositus following termination of Purkinje cell inhibition of that structure but instead likely reflects orthodromic activation of interpositus neurons via climbing fiber and/or mossy fiber collaterals. The impact of US preexposure on associative conditioning in this paradigm was also determined. Animals which received only 108 US-alone trials were massively impaired during subsequent training compared to rabbits that received fewer than 12 US-alone trials.

The nature of reinforcement in cerebellar learning.

In a now classic study, W. J. Brogden and W. H. Gantt (1942, American Journal of Physiology, 119, 277-278) demonstrated that movements (limbs, head, eyelid) elicited by direct electrical stimulation of certain regions of the cerebellum (particularly the ansiform lobe) could be trained to respond to neutral auditory or visual conditioned stimuli with appropriate pairing. In recent work we have replicated these results in detail and presented considerable evidence that the reinforcing or "teaching" pathway so activated for the learning of discrete movements is the inferior olive-climbing fiber projection system to the cerebellum. Very strong evidence now indicates that the memory traces for this "skilled response" learning are formed and stored in the cerebellum. The climbing fiber system and inhibitory pathway from the interpositus nucleus to the inferior olive appear to form a neural instantiation of the Resorla-Wagner formulation of classical conditioning and accounts for the "cognitive" phenomenon of blocking. It is concluded that reinforcement in this form of learning is not due simply to contiguity/contingency or to unconditioned stimulus aversiveness, per se, but rather to activation of a particular brain circuit, here the climbing fiber system, a circumstance that may apply to other forms of learning, with other reinforcement circuits, as well.

Classical conditioning with electrical stimulation of cerebellum as both conditioned and unconditioned stimulus.

Stimulating electrodes were implanted in rabbit cerebellum, providing an electrical conditioned stimulus (CS) activating cortical parallel fibers and thence Purkinje and other cells, and an electrical unconditioned stimulus (US) activating underlying white matter and eliciting unconditioned responses. Paired CS-US presentations led to the development of conditioned responses, which showed extinction following CS-alone trials and reacquisition with significant savings on reinstatement of paired trials. Increased local excitability as a result of paired training (but not following unpaired stimulus presentations) was observed in cerebellar cortex, as manifested in substantial decreases in CS threshold for response elicitation in all subjects. This preparation offers a model for the study of plastic neuronal interactions within cerebellar networks critically involved in associative learning.

Cerebellar stimulation as an unconditioned stimulus in classical conditioning.

Rabbits were implanted with chronic stimulating electrodes in white matter underlying lobule HVI of the cerebellar cortex. Stimulation elicited movements of the face or neck and, when paired with a tone conditioned stimulus (CS), produced learning comparable to that seen with peripheral unconditioned stimuli (USs). CS-alone trials produced extinction. Reinstatement of paired trials produced reacquisition with savings. Additional groups received either explicitly or randomly unpaired CS-US trials before paired conditioning. Low-frequency responding during these sessions indicated that the paired training results were associative and not due to pseudoconditioning or sensitization. Explicitly unpaired sessions retarded learning on subsequent paired trials compared with groups that received either randomly unpaired or no CS-US preexposure. These results are interpreted in terms of the role of the cerebellum and associated pathways in classical conditioning of motor responses.

Binocular depth perception following early experience with interocular torsional disparity.

The relationship between the behavioral and physiological consequences of rearing with optically induced cyclotropia was assessed. Beginning at the age of 4 weeks, kittens wore goggles that rotated the visual field in opposite directions in each eye for several hours each day over a period of several weeks. The amounts of interocular rotation were 0 deg (control), 16 deg, and 32 deg. Subsequently, they were tested to determine their monocular and binocular depth thresholds and, in some cases, visual acuity. In several kittens recordings were also made from the visual cortex. Binocular performance of all kittens in the 0-deg condition and three out of six kittens in the 16-deg condition was comparable to, although slightly lower than, that of normally reared kittens. In contrast, none of the 32-deg kittens showed any evidence of the binocular superiority that would suggest the presence of stereopsis. Extracellular unit recordings from the visual cortex confirmed our earlier results with goggle-reared kittens. In 16-deg kittens, the distribution of the cells' preferred interocular disparities (IOD) in receptive-field orientation showed a compensating shift so that the mean matched the experienced rotational disparity. In the 32-deg kittens, binocularity was greatly disrupted and there was no compensatory shift in the IOD distribution. Two 32-deg kittens were afforded 3 years of subsequent normal visual experience. Both the behavioral and the physiological findings were unaffected by normal visual exposure in adulthood. Control measurements of acuity indicated that any deficits in depth perception were not due to reduced spatial-resolution abilities. The data indicate that the kitten visual system is able to maintain functional binocularity sufficient to subserve a moderate level of stereoacuity with interocular rotations of up to at least 16 deg.

Interocular torsional disparity and visual cortical development in the cat.

1. The present experiments were designed to assess the effects of relatively large optically induced interocular torsional disparities on the developing kitten visual cortex. Kittens were reared with restricted visual experience. Three groups viewed a normal visual environment through goggles fitted with small prisms that introduced torsional disparities between the left and right eyes' visual fields, equal but opposite in the two eyes. Kittens in the +32 degrees goggle rearing condition experienced a 16 degrees counterclockwise rotation of the left visual field and a 16 degrees clockwise rotation of the right visual field; in the -32 degrees goggle condition the rotations were clockwise in the left eye and counterclockwise in the right. In the control (0 degree) goggle condition, the prisms did not rotate the visual fields. Three additional groups viewed high-contrast square-wave gratings through Polaroid filters arranged to provide a constant 32 degrees of interocular orientation disparity. 2. Recordings were made from neurons in visual cortex around the border of areas 17 and 18 in all kittens. Development of cortical ocular dominance columns was severely disrupted in all the experimental (rotated) rearing conditions. Most cells were classified in the extreme ocular dominance categories 1, 2, 6, and 7. Development of the system of orientation columns was also affected: among the relatively few cells with oriented receptive fields in both eyes, the distributions of interocular disparities in preferred stimulus orientation were centered near 0 degree but showed significantly larger variances than in the control condition.(ABSTRACT TRUNCATED AT 250 WORDS)

Newer laboratory approaches for assessing visual dysfunction.

The crucial point that will be emphasized throughout this report is the potential utility of analyzing visual cortical receptive field (RF) properties of the single-cell level as a sensitive and reliable neurotoxicity screening tool. Numerous studies employing exposure of kittens to altered visual environments during the critical period have demonstrated that particular classes of RFs can be selectively affected while sparing others. There has been a rapid proliferation of new methods used to investigate such effects. An important current trend involves the development of multidisciplinary combinations of approaches. The various maneuvers reviewed here seem adaptable to studying neurotoxic insult of the sensitive properties of cortical visual neurons, particularly in the cat or monkey. Conceivably, a general disruption of cortical RF properties might be expected following toxic exposure since individual RF properties are generally not determined by completely independent mechanisms. In fact, some toxicants might produce a general degradation of RF properties akin to the electrophysiological results reported for long-term dark rearing or binocular deprivation.

Visual development: early experience with torsionally disparate images.

Environmental influences on the developing primary visual cortex of kittens were studied by exposing dark reared kittens to prism-induced interocular rotational disparities of 32 degrees, the visual input rotated equally and oppositely in the two eyes. The present report describes preliminary results obtained from two kittens that received this altered visual exposure during 1-6 hours each day from 4 until 12 weeks of age. Subsequent single-unit recordings from the striate cortex revealed three major changes in functional cortical visual physiology. First, there was a disruption in binocularity, with many more cells being monocularly driven in the rotated conditions compared to control conditions. Second, there was an increased variance in the distribution of cells' interocular differences in preferred stimulus orientation (interocular orientation disparity, or IOD) as compared to control conditions. Third, changes were noted in orientation tuning and in the distribution of orientation preferences: cells most selective for orientation tended to be in the extreme ocular dominance groups, and monocular cells were often the most highly selective; also, both binocular and monocular cells showed a tendency for preferred orientations for both eyes to fall near the horizontal or vertical (+/- 22.5 degrees). Thus, a large optically-induced orientation disparity between the two eyes' visual fields during the critical period can modify the characteristics of striate cortical neurons, particularly binocularity and IOD. In addition, these results indicate that an inherent cortical mechanism may ensure the encoding of horizontal and vertical orientation specificities for a subclass of primary visual cortical neurons.

Binocular differences in cortical receptive fields of kittens after rotationally disparate binocular experience.

Kittens were afforded visual experience only while wearing goggles fitted with prisms that rotated the inputs to the two eyes equally but in opposite directions about the visual axes (16 degrees for experimental subjects, 0 degrees for control subjects). Subsequently, receptive-field organization of the visual cortex was studied, special attention being given to the preferred orientation centered about the prism rotation experienced during early development. Thus, for moderate amounts of relative rotation, the development of interocular matching of orientation specificity in binocular cells of the visual cortex reflects the correspondence of early visual input between the two eyes.

Operant conditioning of single-unit response patterns in visual cortex.

Unit responses to photic stimuli were studied in cat visual cortex. After the baseline response pattern of a cell was determined, conditioning trials were given during which reinforcement was contingent upon increased firing during a selected segment of the poststimulus interval. Density of reinforcement increased substantially in about half the cells studied; significant increases in firing occurred within, but not outside, the criterion segment.

Effect of sensory stimulation on the uptake and incorporation of radioactive lysine into protein of mouse brain and liver.

Alterations in incorporation of tritiated lysine into protein of mouse brain and liver were observed following brief exposure to a variety of sensory stimuli. During a 15-min session, subjects were trained to perform a one-way active avoidance response or else were exposed to one of the stimulus components of the situation, including shocks, buzzers, lights, handling, and the apparatus alone. Twenty min after these behavioral treatments, tritiated lysine was injected subcutaneously, and its incorporation into total protein during a 10-min pulse was measured. Quiet mice, undisturbed until injection of the precursor, constituted the baseline group for biochemical comparisons. Most behavioral treatments increased the total amount of radioactivity in brain and liver. The treatments increased the incorporation of radioactivity into protein of both organs even more, thereby producing elevations of relative radioactivity (RR) of protein, a measure of the amount of radioactivity incorporated into protein relative to that in the acid-soluble pools. The RR increases following most of the behavioral experiences were approximately equal; however, exposure to lights or to the apparatus were less effective than the other treatments in eliciting these metabolic changes. The responses were greatly diminished in mice previously exposed to the treatments. Thus, the effectiveness of a stimulus in producing these metabolic alterations may depend upon its apparent magnitude and its novelty. The total radioactivity increases were larger in brain than in liver, while the RR increases were smaller in brain than in liver. Brain RR increases were of equal magnitude when the precursor was injected 5, 20, or 35 min after behavioral treatment, whereas the liver RR responses declined markedly over this period. Despite these differences, strong positive correlations between brain and liver across the various behavioral treatments existed. The RR changes occurred about equally in the cerebellum-brain stem, basal ganglia, hippocampus-septum, and ventral cortex, while the thalamus-hypothalamus and dorsal cortex showed smaller differences.