The superior colliculus : new approaches for studying sensorimotor integration
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Very young SC neurons are responsive only to unimodal inputs; it is not until many days later that some of them begin to respond to inputs from more than a single sensory modality, and even then they are not yet capable of integrating these inputs to produce the synthesized multisensory signals that characterize the normal adult. The most significant occurrence to precede this maturational change is the appearance of influences from association regions of the neocortex.
These influences appear abruptly on any given individual neuron, but because different neurons are targeted at different times during development, it takes many weeks before the mature complement of such neurons is achieved. It is likely that the maturational timing of the interplay between the cortex and SC determines not only the kinds of multisensory information that can be integrated in SC neurons, but the kinds of multisensory behaviors that SC neurons are able to mediate at different stages of development. Volume 5 , Issue 1.
The full text of this article hosted at iucr. If you do not receive an email within 10 minutes, your email address may not be registered, and you may need to create a new Wiley Online Library account. If the address matches an existing account you will receive an email with instructions to retrieve your username. Barry E. Search for more papers by this author. Mark T.
Terrence R. Tools Request permission Export citation Add to favorites Track citation. Share Give access Share full text access. Share full text access. Please review our Terms and Conditions of Use and check box below to share full-text version of article. However, how the information arising from the visual, auditory, and somatosensory systems maps is translated into collicular motor error signals for eye, head, or limb movements remains to be determined.
With the advent of new molecular genetic tools, we are poised to begin dissecting these circuits to define neurons within the colliculus that perform the computations leading to the conversion of sensory information into movement commands. McIlwain , was among the first to propose that the colliculus used a population code to produce saccades.
Thus, a weighted sum vector average of the activity of collicular neurons across the entire map was thought to determine the saccade vector. However, the data leading to this conclusion are largely from computational efforts or from experiments performed using only a single visual target, a condition not usually found naturally. These experiments point to the possibility that the colliculus operates using a winner-takes-all code, in which, ultimately, the population of neurons discharging at the greatest levels determines the saccade.
Thus, for single targets, the colliculus appears to use a population vector average scheme to determine the saccade vector, and for multiple possible targets, a winner-takes-all scheme. This two-model conundrum is evident in behavioral studies too.
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However, if the targets appear further apart or more time is provided, a saccade can be made to one or the other stimulus Ottes et al. Probabilistic population coding strategies offer a solution to this conundrum Ma et al. Recording from multiple neurons simultaneously from the colliculus while monkeys performed a simple task in which they chose one differently colored stimulus from an array of four stimuli revealed that the relative level of activity across both colliculi predicted whether monkeys would perform well or poorly, regardless of the saccade vector. This result indicates two things: First, the activity of neurons encoding saccades that are not made also contributes to the resulting saccadic eye movement, and second, the relative level of activity across the collicular map encodes the saccade choice, not just the saccade vector.
A further analysis comparing different models—vector averaging, winner-takes-all, and one using a Bayesian estimator, to understand how the colliculus encodes saccades—revealed that the Bayesian estimator provided the most accurate prediction of the choices made by the monkeys compared to the winner-takes-all and the vector average models.
A probabilistic approach is superior to winner-takes-all or population vector averaging in part because all the information contained in individual neuronal turning curves is used to determine the choice, and when all the information is combined across multiple neurons, the population activity encodes multiple options simultaneously.
The distribution of choices encoded by the population provides a conditional probability for each of the alternatives, eliminating the need for a switch between population coding schemes. Both the vector averaging code and the winner-takes-all code disregard information from distractor neurons.
Sensorimotor Integration and Spatially-Guided Behavior - Cynthia Moss
Furthermore, a Bayesian framework also allows prior information to influence saccade choices, an area yet to be explored. How Bayesian models are implemented in neuronal circuits remains an enigma in the field Ma et al. Schematic representation of activity across the movement map of the superior colliculus during a simple task in which one differently colored stimulus is chosen as a target for a saccade. The circles on the top are the possible targets red or green , and the maps below are heat maps in which warmer colors represent higher levels of neuronal activity.
One could think of these hills of activity as likelihoods; a linear sum across all activity will naturally lead to a probability distribution of all possible saccades. The peak would be the most likely saccade, whereas the width of population distribution provides an implicit representation of the certainty of the saccade choice. The physiological work described above reported another interesting observation.
The relative level of activity across the collicular map encodes saccadic eye movement choice. The relative activity scales linearly with performance accuracy. Discriminability correlates best with target choice and least with saccade parameters.
The experiments described above focused on the neurons in the motor layers of the colliculus with delay-period activity. In monkeys, there are at least two types of neurons found in the motor layers of the colliculus that have activity associated with the onset of saccadic eye movements.
These neurons are generally referred to as burst and as buildup, or prelude, neurons because of their characteristic response profiles. Both of these types of neuron project out of the colliculus Moschovakis et al.
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This arrangement leaves open the possibility that neurons with delay-period activity process information related to cognition, which is then transmitted to the burst neurons that are responsible for sending out the movement command. The superior colliculus of the rodent is an excellent model in which we can capitalize on novel molecular tools to unravel the neuronal circuits responsible for incorporating cognitive signals into movement commands.
As a first step toward this, rodent motor layer output neurons have been characterized as regular, burst, and late-spiking types, and they have multipolar morphology. Similar neurons are found in monkeys and have been classified as X and T neurons. X neurons have larger somata and larger, more complex dendritic fields, whereas T neurons have smaller somata and smaller, less complex dendritic fields.
Both have axons that cross beneath the oculomotor nucleus and descend in the brainstem, but T neurons also have a commissural projection to the other colliculus. Both also display recurrent local collaterals Moschovakis et al. These recurrent collaterals form a local excitatory circuit that in rodents has been shown to modulate the robust bursting in neurons associated with the generation of orienting movements Isa et al.
Thus, local inhibitory circuits in both layers may modulate the bursting of motor layer neurons to drive a movement. This finding suggests that the delay-period activity of collicular neurons may not be a simple, passive reflection of synaptic inputs but an intrinsic property of the neurons themselves, providing cellular evidence for a collicular role in mechanisms that intervene between vision and action.
A careful look at the outputs of the superior colliculus reveals a surprising point: The terminations from ascending projections are much denser and more extensive than are the descending projections to the pons Figure 5. The ascending projections target regions of the thalamus, including the pulvinar, underscoring the idea that the superior colliculus plays an important role in cognitive aspects of brain function by influencing circuits upstream.
This enhancement of visual responses was interpreted as indicating an overt shift in the focus of attention to the target location. They presented monkeys with four patches of motion stimuli Figure 6. While fixating centrally, one of the patches changed its direction of motion.
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Monkeys reported this change by looking at a target placed elsewhere, dissociating the location of attention from the location of the required eye movement. One of the four locations was cued, indicating the location where the change would occur. Importantly, they also provided a foil patch that contained a motion direction change that differed from that occurring in the cued patch. When they inactivated a small region of the colliculus corresponding to the cued location, they found that monkeys ignored the cued motion change and reported the motion change occurring at the foil location.
Conversely, when the foil signal appeared in the inactivated region of the colliculus, the monkeys ignored this and reported the motion change from the cued location. This result was obtained whether monkeys reported the motion change with a saccade Figure 6b,c or a button press not shown , providing compelling evidence that the colliculus plays a critical role in determining the location of covert spatial attention. This suggests that the colliculus can modulate spatial attention through circuits operating independently of these cortical regions.
A role for the superior colliculus SC in attention. One patch then changed its motion direction, and monkeys reported this change by making a saccade to a choice target or pressing a button corresponding to the change in motion direction. The inset shows the trial condition.
Points falling along the dashed line indicate no change with activation. In this case, monkeys ignored it. Adapted with permission from Krauzlis et al. How can the superior colliculus influence attention, while bypassing cortical circuits? If the projections from the colliculus to thalamic nuclei, including the medial dorsal nucleus and the pulvinar that project in turn to the cerebral cortex, are not necessary for attention, what targets are involved?
Subcortical circuits through the basal ganglia have been proposed for visual selection and attention Krauzlis et al. The motor layers project to the intralaminar nuclei of the thalamus, including the centromedian nucleus and the parafascicular nucleus, and these project to the striatum, the key input nucleus of the basal ganglia. The striatum then projects to the substantia nigra pars reticulata, which projects directly back to the superior colliculus McHaffie et al.
It is possible that the activity in this subcortical loop could be responsible for changes in attention seen with inactivation Krauzlis et al. The results of Krauzlis and colleagues can be explained simply if the superior colliculus is responsible for only one component, the decision criterion. In this model, cortical activity is responsible for attentional mechanisms related to perception, and collicular activity is responsible for setting the decision criterion that determines how perceptual information is reported. Results from two recent experiments are consistent with this idea.
On some trials, monkeys adjusted their sensitivity to improve performance, and on other trials, they adjusted their criterion or response bias to improve performance. The investigators found that V4 activity correlated with changes in sensitivity and not changes in criterion. Because both processes are a part of attention, they reasoned that the experiments performed by Krauzlis and colleagues interfered with only one part of the attentional process.
Furthermore, performing the same experiment but substituting electrical stimulation of the superior colliculus mimicked the behavioral results Figure 7. Thus, neurons in the motor layers of the superior colliculus may not be shifting the focus of attention per se, but rather, their activity may signal the decision criterion and, therefore, how attention is expressed. Electrical stimulation of the colliculus changes decision criteria in predictable ways. Proportion of a Yes response is plotted as a function of decision difficulty coherence.
Panel a shows the results of priming with a visual stimulus, and panel b shows the results of priming with electrical stimulation of the motor layers of the colliculus. Blue shows the results with liberal priming, and orange shows the results with conservative priming. Black is baseline data with no priming or stimulation. This line of work extends the domain of the superior colliculus into decision making Redgrave et al.
In this work, the colliculus operates in partnership with corticobasal ganglia circuits, and the decision threshold is set by the weight of corticostriatal synapses.
Motor Functions of the Superior Colliculus
In this model, the colliculus plays a passive role by simply detecting threshold crossings and initiating the choice response, similar to the traditional role that the basal ganglia—superior colliculus circuit plays in saccade initiation Hikosaka et al. Considerable experimental focus is placed on the role of the monkey colliculus in cognitive processing, but far less experimental attention is placed on the circuits in the colliculus that can mediate these behaviors.
Recent experiments using coronal slices of the superior colliculus, in vitro, confirmed this direct link. Application of biccuculine, a GABA A receptor antagonist, enhanced these responses, resulting in robust bursts of action potentials, similar to those observed in vivo during saccades.