# Show Tag: motor

Select Other Tags

There is a theory called the premotor theory of visual attention' which posits that activity that can ultimately lead to a saccade can also facilitate processing of stimuli in those places the saccade will/would go to.

The SC is involved in tongue snapping in toads.

Satou et al. assume there is switch-like' behavior in toad tounge snapping and predator avoidance.

According to Satou et al., the optic tectum is where the decision to snap the tongue (at insects).

Tongue snapping can be evoked in toads by electrostimulation of neurons in their optic tectum.

The snapping-evoking area' in the toad optic tectum is in the lateral/vetrolateral part of the OT.

If sensory maps of uni-modal space are brought into register, then cues from different modalities can access shared maps of motor space.

Excitatory burst neurons (EBNs) in the paramedian pontine reticular formation (pprf) (pons) initiate saccades.

These neurons receive direct excitatory input from SC and inhibitory input from the nucleus raphe interpositus (RIP) (brainstem).

Wang et al. provide evidence that SC might during saccades turn of RIP inhibition through the central mesencephalic reticular formation (cMRF) while it drives EBNs.

Electrical stimulation of the SC can evoke motor behavior.

Electrical stimulation of the cat SC can evoke saccades.

Typically, these saccades go into that general direction in which natural stimuli would lead to activation in the area that was electrically stimulated.

The foveation hypothesis' states that the SC elicits saccades which foveate the stimuli activating it for further examination.

Reward mediated learning has been demonstrated in adaptation of orienting behavior.

Distributed Adaptive Control is a system that can learn sensory-motor contingencies

Verschure explains that, in his DAC system, the contextual layer overrules the adaptive layer as soon as it is able to predict perception well enough.

One version of DAC uses SOMs.

It has been found that stimulating supposed motor neurons in the SC facilitates visual processing in the part of visual cortex whose receptive field is the same as that of the SC stimulated neurons.

• either there are separate, competing fixation and motion systems,
• or there is competition between neurons in the SC coding for different motions.

According to Casteau and Vitu, a fixation population, if it exists, is probably located not in the SC but in the brainstem omnipause region.

According to Casteau and Vitu, a fixation population, if it exists, is probably located not in the SC but in the brainstem omnipause region.

Omnipause neurons (OPN) receive input from neurons across the SC, though more strongly from the rostral part.

Distractors close to a saccade target do not seem to affect saccade latency but change their landing sites.

Casteau and Vitu see the lack of a change in saccade latency due to distractors close to the saccade target as evidence against the lateral-inhibition theory of saccade generation.

Casteau and Vitu's results seem to show that it's not proximity between target and distractor but the ratio of their excentricities that saccade delay is dependent of.

The in-vitro study of the rat intermediate SC by Lee and Hall did not find evidence for the long-range inhibitory/short-range excitatory connection pattern theorized by proponents of the neural-field theory of SC fixation.

All sensory input signals are equal, a priori, as are all motor outputs.

The difference between inputs from different modalities and between different motor outputs is their sensory-motor contingencies.

Actually, motor outputs are not different from secondary sensory signals—efferent copies are exactly that: motor output used for sensory processing. (And raw sensory input can be used as motor signals, as Braitenberg has shown.)

O'Regan and Noë speak of the geometric laws that govern the relationship between moving the eyes and body and the change of an image in the retina.

The geometry of the changes—straight lines becoming curves on the retina when an object moves in front of the eyes—are not accessible to the visual system, initially, because nothing tells the brain about the spatial relations between photoreceptors in the retina.

O'Regan and Noë claim that the structure of the laws governing visual sensory-motor contingencies is different from the structure of other sensory-motor contingencies and that this difference gives rise to different phenomenology.

Kustov's and Robinson's results support the hypothesis that there is a strong connection of action and attention.

In order to work with spatial information from different sensory modalities and use it for motor control, coordinate transformation must happen at some point during information processing. Pouget and Sejnowski state that in many instances such transformations are non-linear. They argue that functions describing receptive fields and neural activation can be thought of and used as basis functions for the approximation of non-linear functions such as those occurring in sensory-motor coordinate transformation.

Deactivating or stimulating certain parts of the the deeper layers of the SC induces arousal, freezing and escape behavior as well as a raise in blood pressure, heart rate, and respiration.

Reactions can be as complex as running and jumping.

Whether or not the complex aversive behavior patterns evoked by deactivating or stimulating certain brain regions is a direct effect of SC activity or rather the result of actual fear which in turn may be due to that specific SC activity is unclear.

Activity in the SC drives the saccade burst generators which are oculomotor neurons in the reticular formation.

Tabareau et al. propose a scheme for a transformation from the topographic mapping in the SC to the temporal code of the saccadic burst generators.

According to their analysis, that code needs to be either linear or logarithmic.

Girard and Berthoz review saccade system models including models of the SC.

Except for two of the SC models, all focus on generation of saccades and do not consider sensory processing and in particular multisensory integration.

Activity in the deep SC has been described as different regions competing for access to motor resources.

Electrostimulation of putamen neurons can evoke body movement consistent with the map of somatosensory space in that brain region.

FEF stimulation elicits saccadic eye movements.

The motor map is not monotonic across the entire FEF, but sites that are close to each other have similar characteristic saccades.

Ijspert et al. show in an actual robot how the same spinal central pattern generators can produce swimming and walking behavior in a robotic model of a salamander.

Ijspert et al. use their robotic model of a salamander to test hypotheses about the neural networks that produce swimming and walking behaviors in salamanders.

It has long been known that stimulating the SC can elicit eye movements.

The size and direction of a saccade before which deep SC neurons show the greatest activity depends on where they are in the SC: Neurons in medial regions of the SC tend to prefer saccades going up, neurons in lateral regions of the SC tend to prefer saccades going down.

Long saccades are preceded by strong activity of rostral neurons, short saccades by activity of caudal neurons.

Deep SC neurons which have preferred saccades have these preferred saccades also in total darkness. They thus do not simply respond to the specific location of a visual stimulus.

Robinson reports two types of motor neurons in the deep SC: One type has strong activity just (~20 milliseconds) before the onset of a saccade. The other type has gradually increasing activity whose peak is, again, around 12-20 milliseconds before onset.

Currently, three types of saccade-related neurons are distinguished in the deep SC:

• Burst- and build-up neurons on the one hand,
• fixation neurons on the other.

Microstimulation of OT neurons in the barn owl can evoke pupil dilation.

Lesions of the tectospinal tract leads to deficits in motor responses, while lesions of brachium and parts of the tectothalamic system produce contralateral visual neglect.

Ablation of the SC leads to temporary blindness and deficits in visual following.

Brainstem premotor neurons producing the commands for eye movements are located in pons, medulla (horizontal movements), and the rostral midbrain (vertical movements).

The cerebellum is involved in saccade generation.

Certain Purkinje cells in the oculomotor vermis of the cerebellum have saccade-related activity: Helmchen and Büttner found neurons which displayed:

• a saccade-related burst (most of them),
• a saccade-related burst, followed by a pause,
• either a pause or a burst, depending on the direction of the saccade.

Unilateral deactivation of the caudal fastigial nucleus in the cerebellum leads to hypermetria of saccades to the ipsilateral and hypometria of saccades to the contralateral side.

Deactivation of the caudal fastigial nucleus in the cerebellum increases the variability of saccades.

There are neurons in the supplementary eye field which are related to

• eye movements,
• arm movements,
• ear movements,
• spatial attention.

There are projections from motor and premotor cortex to SC.

Visually active neurons in FEF do not project to SC. Motion-related neurons in FEF project to SC.

In the Simon task, subjects are required to respond to a stimulus with a response that is spatially congruent or incongruent to that stimulus: They have, for example, to press a button with the left hand in response to a stimulus which is presented either on the left or on the right. Congruent responses (stimulus on the left—respond by pressing a button with the left hand) is usually faster than an incongruent response.

It has been found that stimulating supposed motor neurons in the SC enhances responses of v4 neurons with the same receptive field as the SC neurons.

The SC is involved in generating gaze shifts and other orienting behaviors.

Eliasmith et al. model sensory-motor processing as task-dependent compression of sensory data and decompression of motor programs.

Muscle synergies are coordinated activations of groups of muscles.

There is the hypothesis that complex motions are comprised of combinations of simple muscle synergies, which would reduce the dimensionality of the control signal.

A low-dimensional representation of motion patterns in a high-dimensional space restricts the actual dimensionality of those motions.

I'm not so sure that a low-dimensional representation of motion patterns in a high-dimensional space necessarily restricts the actual dimensionality of those motions:

$\mathbb{Q}^3$ is bijective to $\mathbb{Q}$ (right?).

It is probably the case for natural behavior, though.

The concept of reduction of the dimensionality of motor space by using motor synergies has been used in robotics.

Cortical structures do not always control our overt behavior. Instead, sub-cortical areas sometimes override cortical tendencies.

Sub-cortical structures (like the SC) have bearing on cortical functionality.

In the Sprague effect, removing (or deactivating) one visual cortex eliminates visually induced orienting behavior to stimuli in the contralateral hemifield.

Lesioning (or deactivating) the contralateral SC restores the orienting behavior.

The heminanopia that follows unilateral removal of the cortex that mediates visual behavior cannot be explained simply in classical terms of interruption of the visual behavior cannot be explained simply in classical terms of interruption of the visual radiations that serve cortical function.
Explanation fo the deficit requires a broader point of view, namely, that visual attention and perception are mediated at both forebrain and midbrain levels, which interact in their control of visually guided behavior.''

(Sprague, 1966)

Patrick Winston states that predictive simulation is enabled by considerable reuse of perceptual and motor apparatus

Stuphorn et al. found neurons in the monkey SC whose activity was dependent on the retinotopic position of the target in a reaching task, but not to the actual path taken in reaching.

McHaffie et al. speculate that loops through various subcortical loops might solve the selection problem, ie. the gating of competing inputs to shared resources.

In the SC, this means that the basal ganglia decide which of the brain structures involved in gaze shifts access to the eye motor circuitry.

Zhao et al. propose a model which develops perception and behavior in parallel.

Their motivation is the embodiment idea stating that perception and behavior develop in behaving animals

Disparity-selective cells in visual cortical neurons have preferred disparities of only a few degrees whereas disparity in natural environments ranges over tens of degrees.

The possible explanation offered by Zhao et al. assumes that animals actively keep disparity within a small range, during development, and therefore only selectivity for small disparity develops.

Zhao et al. present a model of joint development of disparity selectivity and vergence control.

Zhao et al.'s model develops both disparity selection and vergence control in an effort to minimize reconstruction error.

It uses a form of sparse-coding to learn to approximate its input and a variation of the actor-critic learning algorithm called natural actor critic reinforcement learning algorithm (NACREL).

The teaching signal to the NACREL algorithm is the reconstruction error of the model after the action produced by it.

Vitay et al. note that while the brain can be seen as computing many things in parallel and mutually independent, they ultimately are affected by one another, as in walking and changing posture

SC is connected to motor plants via brainstem.

Lesions to SC and FEF individually do not eliminate saccades. Lesions to both do eliminate saccades.

Brainstem activation is very similar to actual muscle behavior.

Fujita models saccade suppression of endpoint variability by the cerebellum using their supervised ANN model for learning a continuous function of the integral of an input time series.

He assumes that the input activity originates from the SC and that the correction signal is supplied by sensory feedback.

In his model, Fujita abstracts away from the population coding present in the multi-sensory/motor layers of the SC.

The SC is multisensory: it reacts to visual, auditory, and somatosensory stimuli. It does not only initiate gaze shifts, but also other motor behaviour.

The deeper levels of SC are the targets of projections from cortex, auditory, somatosensory and motor systems in the brain.

The deeper layers of the SC project strongly to brainstem, spinal cord, especially to those regions involved in moving eyes, ears, head and limbs, and to sensory and motor areas of thalamus.

Neurons in the deep SC whose activity spikes before a saccade have preferred amplitudes and directions: Each of these neurons spikes strongest before a saccade with these properties and less strongly before different saccades.

Overt visual function occurs only starting 2-3 weeks postnatally in cats.

Less is known about the motor properties of SC neurons than about the sensory properties.

Electrical stimulation of the cat SC elicits eye and body movements long before auditory or visual stimuli could have that effect.

These movements already follow the topographic organization of the SC at least roughly.

Most multisensory SC neurons project to brainstem and spinal chord.

According to King, the principal function of the SC is initiating gaze shifts.

The SC also seems to be involved in reaching and other forelimb-related motor tasks and has been associated with complex vision-guided arm-gestures in humans.

People look where they point and point where they look.

Reasons why pointing and gazing are so closely connected may be

• that gaze guides pointing,
• that gazing and pointing use the same information,
• or that a common motor command guides both.

Open-loop control (in biological sensorimotor modeling) is a simplification because most motions are not ballistic.

Cost terms that are routinely minimized in sensorimotor control are

• Metabolic (muscular) energy consumption
• smoothness cost (time-derivative of acceleration)
• variance (of execution; usually assuming motor noise is control dependent)

Cost functions including multiple cost terms must be tuned by weighting, which is often arbitrarily done by the modeler.

Kalman filters are optimal estimators when dynamics and sensory measurements are linear and noise is Gaussian.

Explicit or implicit representations of world dynamics are necessary for optimal controllers since they have to anticipate state changes before the arrival of the necessary sensor data.

An open-loop controller is just a special case of a closed-loop controller: one that does not have feedback.

The difference between open-loop and closed-loop controllers is greatest when during control there is time to acquire sensory data and act on it.

So the question for saccade control is: is there time and where does sensory data come from?

Are there representations, forward models of saccade controls in the SC?

According to control theory, optimal controllers "make no effort to correct deviations from the average behavior", which is called the 'minimal intervention' principle:

the reason is that the effort itself is expensive and introduces control-dependent noise.

'Minimal intervention' can be used to argue liberal economics.

Todorov argues that the minimal intervention principle accounts for large variances in execution dimensions which are not task relevant and states that PCA-related analysis of behavior therefore usually finds non-relevant dimensions.

Auditory signals gain relevance during saccades as visual perception is unreliable during saccades.

It would therefore be a good candidate for feedback if saccade control is closed-loop.

Tadpoles make eye movements which compensate for swimming movements independent of visual or vestibular input. Their rhythmic swimming motor commands are generated by spinal central pattern generators (CGPs). Efference copies of these motor commands appear to be what induces the eye movements.

The SC is also involved in eye, head, whole-body, ear, whisker and other body movements.

Map alignment in the SC is expensive, but it pays off because it allows for a single interface between sensory processing and motor output generation.

Some argue that the main task of cognition is generating the correct actions.

If the main task of cognition is generating the correct actions, then it is not important in itself to recover a perfect representation of the world from perception.

The idea that neural activity does not primarily represent the world but 'action pointers', as put by Engel et al., speaks to the deep SC which is both 'multi-modal' and 'motor'.

If there is a close connection between the state of the world and the required actions, then it is easy to confuse internal representations of the world with action pointers'.

The idea that the SC should learn to move the eyes such that it sees something interesting afterwards is in line with the idea that the brain should represent action pointers instead of actions.

One family of models for saccades and anti-saccades are the accumulator models'.

These models pose that activation of saccade and saccade suppression neurons race each other. The one first to reach a threshold wins.

Munoz and Everling assume that there are distinct populations of fixation and saccade neurons in the SC and FEF.

In a more recent paper, Casteau and Vitu state that there is some debate about that. However, they, too argue for distinct fixation neurons. On the other hand, they also state that fixation neurons probably are not located in the SC itself, which is in contrast of what Munoz and Everling write.

Omnipause neurons in the reticular formation tonically inhibit `the saccade-generation circuit'.

It seems unclear what is the original source of SC inhibition in preparation of anti-saccades. Munoz and Everling cite the supplementary eye fields (SEF), dorsolateral prefrontal cortex (DLPFC) as possible sources, and the substantia nigra pars reticulata (SNpr).

LIP may be where anti-saccade targets are decided upon.

Morén et al. present a spiking model of SC.

Trappenberg presents a competitive spiking neural network for generating motor output of the SC.

When reading, the standard deviation of the distribution of fixation targets within a word increases with the distance between the start and end of a saccade.

During reading, the further a saccade lands from the center of a word, the greater the probability of a re-fixation.

Pajak and Nuthmann found that saccade targets are typically at the center of objects. This effect is strongest for large objects.

The direction of a saccade is population-coded in the SC.

There exist two hypotheses for how saccade trajectory is population-coded in the SC:

• the sum of the contributions of all neurons
• the weighted average of contributions of all neurons

The difference is in whether or not the population response is normalized.

According to Lee et al., the vector summation hypothesis predicts that any deactivation of motor neurons should result in hypometric saccades because their contribution is missing.

According to the weighted average hypothesis, the error depends on where the saccade target is wrt. the preferred direction of the deactivated neurons.

Lee et al. found that de-activation of SC motor neurons did not always lead to hypometric saccades. Instead, saccades where generally too far from the preferred direction of the de-activated neurons. They counted this as supporting the vector averaging hypothesis.

The superior colliculus sends motor commands to cerebellum and reticular formation in the brainstem.

The cerebellum sends motor commands to the reticular formation

It is the reticular formation that initiates saccades.

Some models assume SC output encodes saccade amplitude and direction. In other models, each spike from a burst neuron encodes a motion segment, with length and direction depending on the position of the neuron and strength of connection to brainstem areas

Rucci et al. suggest that high saliency in the center of the visual field can act as a reward signal for pre-saccadic neural activation.