Circling elicited from the anteromedial cortex and medial pons: refractory periods and summation

Visuomotor control in mice and primates

We conduct a comparative evaluation of the visual systems from the retina to the muscles of the mouse and the macaque monkey noting the differences and similarities between these two species. The topics covered include (1) visual-field overlap, (2) visual spatial resolution, (3) V1 cortical point-image [i.e., V1 tissue dedicated to analyzing a unit receptive field], (4) object versus motion encoding, (5) oculomotor range, (6) eye, head, and body movement coordination, and (7) neocortical and cerebellar function. We also discuss blindsight in rodents and primates which provides insights on how the neocortex mediates conscious vision in these species. This review is timely because the field of visuomotor neurophysiology is expanding beyond the macaque monkey to include the mouse; there is therefore a need for a comparative analysis between these two species on how the brain generates visuomotor responses.

The Visual Cortex in Context

In this article, we review the anatomical inputs and outputs to the mouse primary visual cortex, area V1. Our survey of data from the Allen Institute Mouse Connectivity project indicates that mouse V1 is highly interconnected with both cortical and subcortical brain areas. This pattern of innervation allows for computations that depend on the state of the animal and on behavioral goals, which contrasts with simple feedforward, hierarchical models of visual processing. Thus, to have an accurate description of the function of V1 during mouse behavior, its involvement with the rest of the brain circuitry has to be considered. Finally, it remains an open question whether the primary visual cortex of higher mammals displays the same degree of sensorimotor integration in the early visual system.

Behavioural state affects saccades elicited electrically from neocortex

Reviewed is how behavioural context influences saccadic eye movements elicited electrically from the neocortex of monkeys. Factors found to affect stimulation-evoked saccades include (1) motor state, i.e. whether stimulation is delivered during free-viewing, or during or after active fixation, or before an animal is about to execute a saccade to a target location, and (2) reward delivery, i.e. the characteristics and timing of reward, which can promote or inhibit the evocation of saccades. We conclude that anyone using electrical stimulation in neocortex to study sensory and cognitive processes must control for the possibility that stimulation of cortex is merely generating a saccade vector whose expression is being obscured by the behavioural paradigm in use. Areas of neocortex from which saccades can be evoked using low currents (<100 microA) are surprisingly widespread and include regions traditionally considered within the sensory domain (e.g. V1, V2, V4, and MT), in addition to visuomotor regions such as the lateral intraparietal area, the dorsomedial frontal cortex, the frontal eye fields, and the prefrontal cortex. This is especially true once the behavioural state of a stimulated animal is put under experimental control.

Effects of diazepam or haloperidol on convulsion and behavioral responses induced by bilateral electrical stimulation in the medial prefrontal cortex

1. Effects of diazepam (DZP) or haloperidol (HAL) on convulsions and behavioral responses (locomotion, circling, spying and head shaking) induced by bilateral electrical stimulation in the medial prefrontal cortex (mPFC) were examined. 2. Male Wistar rats were electrically stimulated (ten 30-sec trains, 60 Hz, 80-100 microA) bilaterally in the mPFC and their behavior was simultaneously observed in an open field in daily session. 3. DZP and HAL dose-response curves (0, 0.5, 1.25, 2.5 and 5 mg/kg, i.p., 30 min before electrical stimulation session) were determined after a baseline of behavioral responses was established. 4. DZP dose-dependently decreased head shaking and convulsions, had no effect in circling and spying behaviors, and increased locomotion except at the highest dose. HAL reduced locomotion, circling and spying behaviors in a dose-dependent manner, but did not affect convulsions or head shaking. 5. These results demonstrated that convulsion and behavioral responses induced by electrical activation of the mPFC were modified by DZP or HAL. Therefore, the mPFC is involved in the mediation of neural and/or behavioral activity that may be implicated in some central effects of psychoactive drugs.

Electrical stimulation of neural tissue to evoke behavioral responses

This review yields numerous conclusions. (1) Both unit recording and behavioral studies find that current activates neurons (i.e., cell bodies and axons) directly according to the square of the distance between the electrode and the neuron, and that the excitability of neurons can vary between 100 and 4000 microA/mm2 using a 0.2-ms cathodal pulse duration. (2) Currents as low as 10 microA, which is considered within the range of currents typically used during micro-stimulation, activate from a few tenths to several thousands of cell bodies in the cat motor cortex directly depending on their excitability; this indicates that even low currents activate more than a few neurons. (3) Electrode tip size has no effect on the current density--or effect current spread--at far field, but tip size limits the current-density generated at near field. (4) To minimize neuronal damage, the electrode should be discharged after each pulse and the pulse duration should not exceed the chronaxie of the stimulated tissue. (5) The amount of current needed to evoke behavioral responses depends not only on the excitability of the stimulated substrate but also on the type of behavior being studied.

Motor cortex and pyramidal tract axons responsible for electrically evoked forelimb flexion: refractory periods and conduction velocities

Double-pulse methods are used here to measure the refractory periods and conduction velocities of the pyramidal tract axons which cause forelimb flexion in pentobarbital anesthetized rats. In the refractory period experiments, conditioning and test pulses were delivered to the motor cortex, the ipsilateral internal capsule, or the ipsilateral pyramid, and the maximum force exerted by the contralateral forelimb was measured at various conditioning-test intervals. The movements increased as conditioning-test interval increased from 0.5 to 1.0 ms in pyramid sites, from 0.6 to 1.5 in internal capsule sites, and from 0.6 to 2.0 ms in surface cortical sites, suggesting longer refractory periods for the substrates at more rostral sites. In cortical sites, as the conditioning-test interval increased from 4.0 to 20.0 ms, the movements decreased gradually to the single-pulse level, suggesting decreasing temporal summation at longer conditioning-test intervals. In the collision experiments, when conditioning pulses were delivered to one site and test pulses to a second site, the movements increased at conditioning-test intervals that were longer by 0.5-1.3 ms than the refractory periods in either site. This suggests that collisions occurred between orthodromic and antidromic action potentials in the pyramidal tract axons responsible for the limb movement. The collision-like increase was greater between internal capsule and pyramid than between cortex and pyramid, or between cortex and internal capsule. The estimated conduction times were 0.9-1.5 ms between cortex and pyramid, 0.4-0.8 ms between cortex and internal capsule, and 0.5-0.8 ms between internal capsule and pyramid. The range of conduction velocities, therefore, was quite narrow between all pairs (8.8-16.8 m/s). The largest pyramidal tract axons appear to be responsible for most of the force of forelimb flexion in pentobarbital anesthetized rats.

Behavioral responses induced by electrical stimulation of the caudate nucleus in freely moving cats

The caudate nucleus and adjacent structures of 26 freely moving cats were stimulated through multiwire electrodes chronically implanted. Two main effects here observed with trains of pulses of high frequency (100 Hz) and short duration (1 s): (1) contralateral head turning and (2) arrest reaction, which was associated with crouching and escape behavior. The responses follow a certain topographic distribution. Head turning was elicited with the lowest mean threshold in sites located in the internal two-thirds and caudal region of the caudate nucleus, while the arrest reaction was elicited from the ventromedial region of the caudate and adjacent nucleus accumbens. Stimulation of the corpus callosum and internal capsule produces postural instability, ventral flexion of the head and flexion of the contralateral limb. The extra-caudate responses were accompanied by contralateral head turning when the stimulated points were near of the caudate border. Experimental evidence suggested that striatal responses were not due to current spread to adjacent structures or to activation of corticofugal fibers. The head rotation was suppressed following interruption of the ipsilateral striatal outflow by electrolytic lesion of the globus pallidus and adjacent internal capsule. The chemical lesion of the substantia nigra and the ventral pallidum produced a significant increase in the stimulation threshold for head turning and arrest reaction, respectively. These results suggest a topographic arrangement of the responses evoked by electrical stimulation of the caudate nucleus in the cat, which are mediated by the substantia nigra pars reticulata and the ventral pallidum.

An uncrossed tectopontine pathway mediates ipsiversive circling

Stimulation of the superior colliculus (SC) of rodents, following knife cuts to the predorsal bundle decussation, evokes ipsiversive circling along with "cringing" or avoidance responses. A major uncut SC output is the uncrossed tectopontine pathway that projects heavily to the ventrolateral pons (VLP). Stimulation of this pathway in the VLP also evokes ipsiversive circling, but the circling is smoother, lacks the avoidance components, and begins with a shorter latency than SC circling. To determine whether continuous tectopontine axons mediate ipsiversive circling in both sites, the collision method of Shizgal et al. was used. Pairs of stimulating pulses were presented to the two sites, conditioning (C) pulses to one site and testing (T) pulses to the other site. Collision was evidenced when the frequencies required to evoke circling were higher at short conditioning-testing (C-T) intervals than at long C-T intervals. Between SC and VLP, collision varied from 25 to 64%. Refractory periods ranged from 0.4 to 1.0 ms in most VLP sites, and from 0.45 to roughly 3 ms in SC sites. Conduction velocities ranged from 1.2 to 19 m/s, but most were concentrated in two ranges, 1.2 to 2.7 m/s and 10 to 19 m/s. The contribution of the slower population was higher in electrode pairs where the percent collision was higher. Therefore, continuous axons from colliculus to ventrolateral pons mediate most of the ipsiversive circling produced by collicular stimulation. Slight asymmetries in the collision were observed between 3 pairs with high threshold colliculus electrodes, suggesting transsynaptic collisions across colliculus synapses transmitting from dorsal to ventral.

Electrically evoked turning: asymmetric and symmetric collision between anteromedial cortex and striatum

Electrical stimulation of the anteromedial cortex (AMC) or striatum of rats evoked contraversive eye, head and body movements. In these experiments we test which neurons and which pathways are responsible for the turning by delivering conditioning (C) pulses to one site and test (T) pulses to the second site, and measuring the frequency of pulse pairs required to evoke a full turn in 10 s. Decreases in the required frequency were usually found at C-T intervals from 0.6 to 1.0 ms, whether the C pulses were delivered to the AMC or to the striatum. This symmetric effect is attributed to collision in fast-conducting axons connecting cortex and striatum. Symmetric collision at C-T intervals of 2-4 ms was observed between cortex and 3 dorsal striatal sites, suggesting slower axons from cortex to these dorsal striatal sites. In several animals, asymmetric changes in required frequency also occurred. When the C pulses were presented via the striatal electrode, the recovery in required frequency occurred at C-T intervals of 1-4 ms, but when the C pulses were presented via the cortical electrode, recovery occurred at C-T intervals of 2-50 ms. This asymmetry is attributed to indirect (i.e., transynaptic) activation of corticostriatal or striatal output axons. These results suggest that in both cortex and striatum there are synapses, transmitting from rostral to caudal, which are important for electrically evoked turning. When C and T pulses were delivered to the same site, decreases in required frequency occurred at C-T intervals from 0.4 to 4 ms, attributable to recovery from refractoriness. In 3 striatal sites, however, large changes were also seen at C-T intervals from 6 to 50 ms. In all 3 sites, asymmetric collision occurred at these same intervals. The recovery at long C-T intervals could be due to transynaptic collision also, resulting from the simultaneous activation of presynaptic and postsynaptic axons by a single striatal electrode.

Efferent projections of the anteromedial cortex of the rat as described by Phaseolus vulgaris leucoagglutinin immunohistochemistry

Phaseolus vulgaris leucoagglutinin (PHA-L) immunohistochemistry was used to describe the corticofugal projections of the anteromedial cortex (AMC) of rats. PHA-L was injected iontophoretically into an area of the AMC which, when stimulated electrically, is known to induce contraversive head and body movements. It was found that the AMC innervates the midbrain via three separate pathways: a dorsal transthalamic pathway terminating in the pretectum, superior colliculus, and central grey area; and a ventral transthalamic pathway and a ventral capsular-peduncular pathway projecting to the central grey and mesencephalic and pontine reticular formation. The strongest terminations were found bilaterally in the mediodorsal thalamic nucleus and nucleus caudato-putamen. The functional significance of the pathways and terminations is discussed.

Pedunculopontine tegmental nucleus-induced inhibition of muscle activity in the rat

The connections of the pedunculopontine tegmental nucleus (PPN) have led us to propose that this structure mediates striatally induced inhibition of muscle activity by directing basal ganglia output to an inhibitory reticulospinal system (nucleus reticularis gigantocellularis and ventralis, nrGi-V). We conducted experiments in order to examine the effects of electrical stimulation of the PPN on the activity of selected neck and shoulder muscles. PPN stimulation at low rates (0.1 Hz) elicited bilateral muscle excitation. As the rate of stimulation was increased (e.g. to 10 Hz), less excitation was observed. Anodal DC current inactivation of the nrGi-V during concurrent 10 Hz PPN stimulation resulted in an augmentation of muscle activity above the levels observed during 10 Hz PPN stimulation alone. PPN stimulation (10 Hz) also profoundly inhibited cortically-induced muscle activity. Further support for our proposal stems from increased baseline activity (0.1 Hz PPN-induced excitation) in animals with ibotenic acid lesions of the PPN as compared to normal animals. Apparently, destruction of the PPN releases the musculature from tonic and/or phasic inhibition. A model is discussed which attempts to account for both the rate-dependent changes in excitation and the inhibition of cortically induced muscle activity.

Head and body movements evoked electrically from the caudal superior colliculus of rats: pulse frequency effects

The effects of pulse frequency and current intensity on circling elicited from the caudal superior colliculus (SC) of rats were studied. The displacement of the head with respect to the body were measured for different levels of frequency (20, 29, and 50 Hz) and current (200 or 500 microA) at a pulse duration of 0.1 ms. The rate of circling increased monotonically with frequency and current. The rate at which the head was displaced laterally varied as a function of frequency. It is postulated that lateral head and body movements are affected by the firing frequency of SC output neurons.

Turning responses evoked by stimulation of visuomotor pathways

Lateral eye, head, and body movements are produced by electrical stimulation of many brain regions from frontal cortex to pons. A new collision method shows that at least 5 separate axon bundles mediate stimulation-elicited lateral head and body movements in rats. One bundle passes between the rostromedial tegmentum and medial pons, with conduction velocities of 0.8-18 m/s. A second bundle passes between the superior colliculus and contralateral medial pons, with conduction velocities of 1.7-13 m/s. A third bundle passes between the superior colliculus and ventrolateral pons, with conduction velocities of 1.3-20 m/s. A fourth bundle passes between the internal capsule and medial substantia nigra, with conduction velocities of 0.9-4.4 m/s. A fifth bundle passes between the anteromedial cortex and rostral striatum, with conduction velocities of 2.4-36 m/s. Collision effects have not been observed between the anteromedial cortex and the internal capsule, medial substantia nigra, superior colliculus, rostromedial tegmentum, or medial pons, which suggests that these sites are not connected by axons mediating turning. Possible synaptic linkages between the 5 bundles and possible transmitters are discussed.

Contraversive circling elicited from the internal capsule and substantia nigra: evidence for a continuous axon bundle mediating circling

Electrical stimulation of many brain sites (e.g., anteromedial cortex, internal capsule, substantia nigra, superior colliculus, rostro-medial tegmentum, and medial pons) evokes circling. The collision method of Shizgal et al. (J. Comp. Physiol. Psychol., 94 (1980) 227-237) was used to determine whether these sites are functionally connected for the production of circling in rats. If connectivity was evidenced, then refractory period and conduction velocity distributions were determined for axons passing through the connected stimulation sites. Collision of up to 90% was found between electrodes placed in internal capsule and substantia nigra, suggesting that these sites are connected by continuous axons that mediate circling. The refractory periods of these axons ranged from 0.5 to 4.5 ms, and the conduction velocities of these axons ranged from 0.9 to 4.4 ms. These velocities are similar to those of striatonigral axons. No collision was found between anteromedial cortex and any other sites tested, nor between pontine sites and internal capsule or substantia nigra.