Kinematics and eye-head coordination of gaze shifts evoked from different sites in the superior colliculus of the cat

Using superior colliculus principles of multisensory integration to reverse hemianopia

The diversity of our senses conveys many advantages; it enables them to compensate for one another when needed, and the information they provide about a common event can be integrated to facilitate its processing and, ultimately, adaptive responses. These cooperative interactions are produced by multisensory neurons. A well-studied model in this context is the multisensory neuron in the output layers of the superior colliculus (SC). These neurons integrate and amplify their cross-modal (e.g., visual-auditory) inputs, thereby enhancing the physiological salience of the initiating event and the probability that it will elicit SC-mediated detection, localization, and orientation behavior. Repeated experience with the same visual-auditory stimulus can also increase the neuron's sensitivity to these individual inputs. This observation raised the possibility that such plasticity could be engaged to restore visual responsiveness when compromised. For example, unilateral lesions of visual cortex compromise the visual responsiveness of neurons in the multisensory output layers of the ipsilesional SC and produces profound contralesional blindness (hemianopia). The possibility that multisensory plasticity could restore the visual responses of these neurons, and reverse blindness, was tested in the cat model of hemianopia. Hemianopic subjects were repeatedly presented with spatiotemporally congruent visual-auditory stimulus pairs in the blinded hemifield on a daily or weekly basis. After several weeks of this multisensory exposure paradigm, visual responsiveness was restored in SC neurons and behavioral responses were elicited by visual stimuli in the previously blind hemifield. The constraints on the effectiveness of this procedure proved to be the same as those constraining SC multisensory plasticity: whereas repetitions of a congruent visual-auditory stimulus was highly effective, neither exposure to its individual component stimuli, nor to these stimuli in non-congruent configurations was effective. The restored visual responsiveness proved to be robust, highly competitive with that in the intact hemifield, and sufficient to support visual discrimination.

Pulsed Stimuli Elicit More Robust Multisensory Enhancement than Expected

Neurons in the superior colliculus (SC) integrate cross-modal inputs to generate responses that are more robust than to either input alone, and are frequently greater than their sum (superadditive enhancement). Previously, the principles of a real-time multisensory transform were identified and used to accurately predict a neuron's responses to combinations of brief flashes and noise bursts. However, environmental stimuli frequently have more complex temporal structures that elicit very different response dynamics than previously examined. The present study tested whether such stimuli (i.e., pulsed) would be treated similarly by the multisensory transform. Pulsing visual and auditory stimuli elicited responses composed of higher discharge rates that had multiple peaks temporally aligned to the stimulus pulses. Combinations pulsed cues elicited multiple peaks of superadditive enhancement within the response window. Measured over the entire response, this resulted in larger enhancements than expected given enhancements elicited by non-pulsed (“sustained”) stimuli. However, as with sustained stimuli, the dynamics of multisensory responses to pulsed stimuli were highly related to the temporal dynamics of the unisensory inputs. This suggests that the specific characteristics of the multisensory transform are not determined by the external features of the cross-modal stimulus configuration; rather the temporal structure and alignment of the unisensory inputs is the dominant driving factor in the magnitudes of the multisensory product.

Tectal microcircuit generating visual selection commands on gaze-controlling neurons

The optic tectum (called superior colliculus in mammals) is critical for eye-head gaze shifts as we navigate in the terrain and need to adapt our movements to the visual scene. The neuronal mechanisms underlying the tectal contribution to stimulus selection and gaze reorientation remains, however, unclear at the microcircuit level. To analyze this complex--yet phylogenetically conserved--sensorimotor system, we developed a novel in vitro preparation in the lamprey that maintains the eye and midbrain intact and allows for whole-cell recordings from prelabeled tectal gaze-controlling cells in the deep layer, while visual stimuli are delivered. We found that receptive field activation of these cells provide monosynaptic retinal excitation followed by local GABAergic inhibition (feedforward). The entire remaining retina, on the other hand, elicits only inhibition (surround inhibition). If two stimuli are delivered simultaneously, one inside and one outside the receptive field, the former excitatory response is suppressed. When local inhibition is pharmacologically blocked, the suppression induced by competing stimuli is canceled. We suggest that this rivalry between visual areas across the tectal map is triggered through long-range inhibitory tectal connections. Selection commands conveyed via gaze-controlling neurons in the optic tectum are, thus, formed through synaptic integration of local retinotopic excitation and global tectal inhibition. We anticipate that this mechanism not only exists in lamprey but is also conserved throughout vertebrate evolution.

Convergent synaptic inputs from the caudal fastigial nucleus and the superior colliculus onto pontine and pontomedullary reticulospinal neurons

The caudal fastigial nucleus (FN) is known to be related to the control of eye movements and projects mainly to the contralateral reticular nuclei where excitatory and inhibitory burst neurons for saccades exist [the caudal portion of the nucleus reticularis pontis caudalis (NRPc), and the rostral portion of the nucleus reticularis gigantocellularis (NRG) respectively]. However, the exact reticular neurons targeted by caudal fastigioreticular cells remain unknown. We tried to determine the target reticular neurons of the caudal FN and superior colliculus (SC) by recording intracellular potentials from neurons in the NRPc and NRG of anesthetized cats. Neurons in the rostral NRG received bilateral, monosynaptic excitation from the caudal FNs, with contralateral predominance. They also received strong monosynaptic excitation from the rostral and caudal contralateral SC, and disynaptic excitation from the rostral ipsilateral SC. These reticular neurons with caudal fastigial monosynaptic excitation were not activated antidromically from the contralateral abducens nucleus, but most of them were reticulospinal neurons (RSNs) that were activated antidromically from the cervical cord. RSNs in the caudal NRPc received very weak monosynaptic excitation from only the contralateral caudal FN, and received either monosynaptic excitation only from the contralateral caudal SC, or monosynaptic and disynaptic excitation from the contralateral caudal and ipsilateral rostral SC, respectively. These results suggest that the caudal FN helps to control also head movements via RSNs targeted by the SC, and these RSNs with SC topographic input play different functional roles in head movements.

Motor functions of the superior colliculus

The mammalian superior colliculus (SC) and its nonmammalian homolog, the optic tectum, constitute a major node in processing sensory information, incorporating cognitive factors, and issuing motor commands. The resulting action-to orient toward or away from a stimulus-can be accomplished as an integrated movement across oculomotor, cephalomotor, and skeletomotor effectors. The SC also participates in preserving fixation during intersaccadic intervals. This review highlights the repertoire of movements attributed to SC function and analyzes the significance of results obtained from causality-based experiments (microstimulation and inactivation). The mechanisms potentially used to decode the population activity in the SC into an appropriate movement command are also discussed.

Sensitivity of the goldfish motion detection system revealed by incoherent random dot stimuli: comparison of behavioural and neuronal data

Background

Global motion detection is one of the most important abilities in the animal kingdom to navigate through a 3-dimensional environment. In the visual system of teleost fish direction-selective neurons in the pretectal area (APT) are most important for global motion detection. As in all other vertebrates these neurons are involved in the control of slow phase eye movements during gaze stabilization. In contrast to mammals cortical pathways that might influence motion detection abilities of the optokinetic system are missing in teleost fish.

Results

To test global motion detection in goldfish we first measured the coherence threshold of random dot patterns to elicit horizontal slow phase eye movements. In addition, the coherence threshold of the optomotor response was determined by the same random dot patterns. In a second approach the coherence threshold to elicit a direction selective response in neurons of the APT was assessed from a neurometric function. Behavioural thresholds and neuronal thresholds to elicit slow phase eye movements were very similar, and ranged between 10% and 20% coherence. In contrast to these low thresholds for the optokinetic reaction and APT neurons the optomotor response could only be elicited by random dot patterns with coherences above 40%.

Conclusion

Our findings suggest a high sensitivity for global motion in the goldfish optokinetic system. Comparison of neuronal and behavioural thresholds implies a nearly one-to-one transformation of visual neuron performance to the visuo-motor output. In addition, we assume that the optomotor response is not mediated by the optokinetic system, but instead by other motion detection systems with higher coherence thresholds.

Effect of reversible inactivation of superior colliculus on head movements

Because of limitations in the oculomotor range, many gaze shifts must be accomplished using coordinated movements of the eyes and head. Stimulation and recording data have implicated the primate superior colliculus (SC) in the control of these gaze shifts. The precise role of this structure in head movement control, however, is not known. The present study uses reversible inactivation to gain insight into the role of this structure in the control of head movements, including those that accompany gaze shifts and those that occur in the absence of a change in gaze. Forty-five lidocaine injections were made in two monkeys that had been trained on a series of behavioral tasks that dissociate movements of the eyes and head. Reversible inactivation resulted in clear impairments in the animals' ability to perform gaze shifts, manifested by increased reaction times, lower peak velocities, and increased durations. In contrast, comparable effects were not found for head movements (with or without gaze shifts) with the exception of a very small increase in reaction times of head movements associated with gaze shifts. Eye-head coordination was clearly affected by the injections with gaze onset occurring relatively later with respect to head onset. Following the injections, the head contributed slightly more to the gaze shift. These results suggest that head movements (with and without gaze shifts) can be controlled by pathways that do not involve SC.

Role of the primate superior colliculus in the control of head movements

One important behavioral role for head movements is to assist in the redirection of gaze. However, primates also frequently make head movements that do not involve changes in the line of sight. Virtually nothing is known about the neural basis of these head-only movements. In the present study, single-unit extracellular activity was recorded from the superior colliculus while monkeys performed behavioral tasks that permit the temporal dissociation of gaze shifts and head movements. We sought to determine whether superior colliculus contains neurons that modulate their activity in association with head movements in the absence of gaze shifts and whether classic gaze-related burst neurons also discharge for head-only movements. For 26% of the neurons in our sample, significant changes in average firing rate could be attributed to head-only movements. Most of these increased their firing rate immediately prior to the onset of a head movement and continued to discharge at elevated frequency until the offset of the movement. Others discharged at a tonic rate when the head was stable and decreased their activity, or paused, during head movements. For many putative head cells, average firing rate was found to be predictive of head displacement. Some neurons exhibited significant changes in activity associated with gaze, eye-only, and head-only movements, although none of the gaze-related burst neurons significantly modulated its activity in association with head-only movements. These results suggest the possibility that the superior colliculus plays a role in the control of head movements independent of gaze shifts.