Biomedical subjects

E J Tehovnik

Publications and source records attributed to E J Tehovnik.

At least 19 recordsLinked to original sources

PubMed2006

Direct and indirect activation of cortical neurons by electrical microstimulation.

Electrical microstimulation has been used to elucidate cortical function. This review discusses neuronal excitability and effective current spread estimated by using three different methods: 1) single-cell recording, 2) behavioral methods, and 3) functional magnetic resonance imaging (fMRI). The excitability properties of the stimulated elements in neocortex obtained using these methods were found to be comparable. These properties suggested that microstimulation activates the most excitable elements in cortex, that is, by and large the fibers of the pyramidal cells. Effective current spread within neocortex was found to be greater when measured with fMRI compared with measures based on single-cell recording or behavioral methods. The spread of activity based on behavioral methods is in close agreement with the spread based on the direct activation of neurons (as opposed to those activated synaptically). We argue that the greater activation with imaging is attributed to transynaptic spread, which includes subthreshold activation of sites connected to the site of stimulation. The definition of effective current spread therefore depends on the neural event being measured.

Animals↗

PubMed2004

Phosphene induction and the generation of saccadic eye movements by striate cortex.

The purpose of this review is to critically examine phosphene induction and saccadic eye movement generation by electrical microstimulation of striate cortex (area V1) in humans and monkeys. The following issues are addressed: 1) Properties of electrical stimulation as they pertain to the activation of V1 elements; 2) the induction of phosphenes in sighted and blind human subjects elicited by electrical stimulation using various stimulation parameters and electrode types; 3) the induction of phosphenes with electrical microstimulation of V1 in monkeys; 4) the generation of saccadic eye movements with electrical microstimulation of V1 in monkeys; and 5) the tasks involved for the development of a cortical visual prosthesis for the blind. In this review it is concluded that electrical microstimulation of area V1 in trained monkeys can be used to accelerate the development of an effective prosthetic device for the blind.

Animals↗

PubMed2001

Look and see: how the brain moves your eyes about.

Two major cortical streams are involved in the generation of visually guided saccadic eye movements: the anterior and the posterior. The anterior stream from the frontal and medial eye fields has direct access to brainstem oculomotor centers. The posterior stream from the occipital cortices reaches brainstem oculomotor centers through the superior colliculus. The parietal cortex interconnects with both streams. Our findings suggest that the posterior stream plays an unique role in the execution of rapid, short-latency eye movements called 'express saccades'. Both the anterior and posterior streams play a role in the selection of targets to which saccades are to be generated, but do so in different ways. Areas V1, V2 and LIP contribute to decisions involved in where to look as well as where not to look. In addition, area LIP is involved in decisions about how long to maintain fixation prior to the execution of a saccade. Area V4 does not appear to be directly involved in eye-movement generation. In the anterior stream, the frontal eye fields, and to a lesser extent the medial eye fields, are involved in the correct execution of saccades subsequent to decisions made about where to look and where not to look.

Animals↗

PubMed2001

Eye movements modulate visual receptive fields of V4 neurons.

The receptive field, defined as the spatiotemporal selectivity of neurons to sensory stimuli, is central to our understanding of the neuronal mechanisms of perception. However, despite the fact that eye movements are critical during normal vision, the influence of eye movements on the structure of receptive fields has never been characterized. Here, we map the receptive fields of macaque area V4 neurons during saccadic eye movements and find that receptive fields are remarkably dynamic. Specifically, before the initiation of a saccadic eye movement, receptive fields shrink and shift towards the saccade target. These spatiotemporal dynamics may enhance information processing of relevant stimuli during the scanning of a visual scene, thereby assisting the selection of saccade targets and accelerating the analysis of the visual scene during free viewing.

Animals↗

PubMed2000

Effects of training on saccadic eye movements elicited electrically from the frontal cortex of monkeys.

It has been reported that training affects motor responses evoked electrically from the dorsomedial frontal cortex (DMFC). Once a monkey had been trained to generate visually-guided saccadic eye movements of a particular size and direction, electrical stimulation of the DMFC elicited saccades of only that size and direction [7]. The current study re-investigates this finding. Monkeys were trained to produce saccadic eye movements to a visual target. After training, electrical stimulation was delivered to the DMFC. It was found that stimulation of the DMFC always evoked saccadic eye movements of the same size and direction before and after training, such that training had little effect on the responses evoked electrically from the DMFC. Differences between the current results and previous results have disclosed new clues as to how the DMFC might be modulated by learning.

Animals↗

PubMed2000

Eye fields in the frontal lobes of primates.

Two eye fields have been identified in the frontal lobes of primates: one is situated dorsomedially within the frontal cortex and will be referred to as the eye field within the dorsomedial frontal cortex (DMFC); the other resides dorsolaterally within the frontal cortex and is commonly referred to as the frontal eye field (FEF). This review documents the similarities and differences between these eye fields. Although the DMFC and FEF are both active during the execution of saccadic and smooth pursuit eye movements, the FEF is more dedicated to these functions. Lesions of DMFC minimally affect the production of most types of saccadic eye movements and have no effect on the execution of smooth pursuit eye movements. In contrast, lesions of the FEF produce deficits in generating saccades to briefly presented targets, in the production of saccades to two or more sequentially presented targets, in the selection of simultaneously presented targets, and in the execution of smooth pursuit eye movements. For the most part, these deficits are prevalent in both monkeys and humans. Single-unit recording experiments have shown that the DMFC contains neurons that mediate both limb and eye movements, whereas the FEF seems to be involved in the execution of eye movements only. Imaging experiments conducted on humans have corroborated these findings. A feature that distinguishes the DMFC from the FEF is that the DMFC contains a somatotopic map with eyes represented rostrally and hindlimbs represented caudally; the FEF has no such topography. Furthermore, experiments have revealed that the DMFC tends to contain a craniotopic (i.e., head-centered) code for the execution of saccadic eye movements, whereas the FEF contains a retinotopic (i.e., eye-centered) code for the elicitation of saccades. Imaging and unit recording data suggest that the DMFC is more involved in the learning of new tasks than is the FEF. Also with continued training on behavioural tasks the responsivity of the DMFC tends to drop. Accordingly, the DMFC is more involved in learning operations whereas the FEF is more specialized for the execution of saccadic and smooth pursuit eye movements.

Animals↗

PubMed1999

Reversible inactivation of macaque dorsomedial frontal cortex: effects on saccades and fixations.

Neural recording and electrical stimulation results suggest that the dorsomedial frontal cortex (DMFC) of macaque is involved in oculomotor behavior. We reversibly inactivated the DMFC using lidocaine and examined how saccadic eye movements and fixations were affected. The inactivation methods and monkeys were the same as those used in a previous study of the frontal eye field (FEF), another frontal oculomotor region. In the first stage of the present study, monkeys performed tasks that required the generation of single saccades and fixations. During 15 DMFC inactivations, we found only mild, infrequent deficits. This contrasts with our prior finding that FEF inactivation causes severe, reliable deficits in performance of these tasks. In the second stage of the study, we investigated whether DMFC inactivation affected behavior when a monkey was required to make more than one saccade and fixation. We used a double-step task: two targets were flashed in rapid succession and the monkey had to make two saccades to foveate the target locations. In each of five experiments, DMFC inactivation caused a moderate, significant deficit. Both ipsi- and contraversive saccades were disrupted. In two experiments, the first saccades were made to the wrong place and had increased latencies. In one experiment, first saccades were unaffected, but second saccades were made to the wrong place and had increased latencies. In the remaining two experiments, specific reasons for the deficit were not detected. Saline infusions into DMFC had no effect. Inactivation of FEF caused a larger double-step deficit than did inactivation of DMFC. The FEF inactivation impaired contraversive first or second saccades of the sequence. In conclusion, our results suggest that the DMFC makes an important contribution to generating sequential saccades and fixations but not single saccades and fixations. Compared with the FEF, the DMFC has a weaker, less directional, more task-dependent oculomotor influence.

Anesthetics, Local↗

PubMed1999

Behavioural conditions affecting saccadic eye movements elicited electrically from the frontal lobes of primates.

We assessed the effects of varying the time at which electrical stimulation was delivered to the dorsomedial frontal cortex (DMFC) and the frontal eye fields (FEF) relative to the onset of a visual target. Monkeys were required to fixate the visual target to obtain a drop of apple juice as reward. We found that the probability of eliciting saccades increased with increases in the delay of electrical stimulation relative to target onset. Also, the current threshold to evoke saccades decreased as electrical stimulation was delivered later following target onset. There were major differences in the magnitude of this effect with stimulation of the DMFC versus the FEF. The current threshold to evoke saccades from the DMFC was 16 times greater when electrical stimulation was delivered 200 ms after target onset as compared to when it was delayed 200 ms after target offset. In contrast, the current threshold to evoke saccades from the FEFs was only three times greater when stimulation was delivered under similar conditions. These results suggest that the FEF are more closely connected with the saccade generator for the execution of saccadic eye movements than is the DMFC, even though both regions have direct projections to brainstem oculomotor centres.

Animals↗

PubMed1998

Saccades induced electrically from the dorsomedial frontal cortex: evidence for a head-centered representation.

The amplitude and direction of saccadic eye movements evoked electrically from the dorsomedial frontal cortex (DMFC) of monkeys vary with starting eye position. This observation has been used to argue that the DMFC codes saccadic eye movements in head-centered coordinates. Whether the amplitude and direction of the evoked saccades are also affected by changes in head position has never been demonstrated. Such a result would argue against a head-centered representation, and instead would suggest a representation anchored to another body part. Tests were conducted on rhesus monkeys to determine whether changing the position of the head with respect to the trunk or changing the position of the head with respect to the gravitational axis alters saccadic parameters. The amplitude and direction of saccadic eye movements remained invariant to such manipulations. These findings confirm the claim that the DMFC encodes saccadic eye movements in head-centered coordinates.

Animals↗

PubMed1997

Excitability of neural elements within the rat corpus striatum.

The excitability of cholinergic, glutamatergic and dopaminergic elements within the rat neostriatum was studied in both in vivo and in vitro preparations. In vivo, the microdialysis technique was used to measure the release of striatal acetylcholine and dopamine under basal and electrically evoked conditions. For comparison, acetylcholine, dopamine and glutamate release was assayed in media obtained from superfused rat striatal slices. Electrical stimulation was used to derive the strength-duration functions and their chronaxies of stimulated elements containing the three neurotransmitter types. The chonaxies for experiments in vitro and in vivo were similar: the chronaxy values for elements containing acetylcholine were the shortest, the values for glutamate were intermediate, and the values for those containing dopamine were the longest. Based on the chronaxy estimates, it is proposed that the elements containing acetylcholine are the large cholinergic interneurons of striatum, and the elements containing glutamate and dopamine are the terminals of corticostriatal and nigrostriatal neurons, respectively. These results indicate that electrical stimulation of neural elements surrounding a microdialysis probe can be an additional tool to examine the factors that regulate neurotransmitter release. Likewise, investigators can activate specific striatal elements by using pulse durations that coincide with their chronaxies.

Acetylcholine↗

PubMed1997

Effective spread and timecourse of neural inactivation caused by lidocaine injection in monkey cerebral cortex.

We studied the effective spread of lidocaine to inactivate neural tissue in the frontal cortex of the rhesus monkey. Injections of 2% lidocaine at 4 microl/min were made while units were recorded 1 or 2 mm away. To inactivate units 1 mm away from the injection site 100% of the time, 7 microl of lidocaine had to be injected. To inactivate units 2 mm away from the injection site 100% of the time, 30 microl of lidocaine were required. Units were maximally inactivated around 8 min after the start of a lidocaine injection, and they gradually recovered, regaining most of their initial activity by around 30 min after the start of an injection. The volume of lidocaine required to inactivate neurons > 90% of the time could be estimated by the spherical volume equation, V = 4/3 pi (r)3. To prolong the inactivation, a slower infusion of lidocaine subsequent to an initial bolus was effective. Saline control injections had no effect. These results allow both a prediction of the timecourse of neural inactivation and an estimate of the spread of neural inactivation following injection of lidocaine into the monkey cerebral cortex.

Animals↗

PubMed1997

Reversible inactivation of macaque frontal eye field.

The macaque frontal eye field (FEF) is involved in the generation of saccadic eye movements and fixations. To better understand the role of the FEF, we reversibly inactivated a portion of it while a monkey made saccades and fixations in response to visual stimuli. Lidocaine was infused into a FEF and neural inactivation was monitored with a nearby microelectrode. We used two saccadic tasks. In the delay task, a target was presented and then extinguished, but the monkey was not allowed to make a saccade to its location until a cue to move was given. In the step task, the monkey was allowed to look at a target as soon as it appeared. During FEF inactivation, monkeys were severely impaired at making saccades to locations of extinguished contralateral targets in the delay task. They were similarly impaired at making saccades to locations of contralateral targets in the step task if the target was flashed for < or =100 ms, such that it was gone before the saccade was initiated. Deficits included increases in saccadic latency, increases in saccadic error, and increases in the frequency of trials in which a saccade was not made. We varied the initial fixation location and found that the impairment specifically affected contraversive saccades rather than affecting all saccades made into head-centered contralateral space. Monkeys were impaired only slightly at making saccades to contralateral targets in the step task if the target duration was 1000 ms, such that the target was present during the saccade: latency increased, but increases in saccadic error were mild and increases in the frequency of trials in which a saccade was not made were insignificant. During FEF inactivation there usually was a direct correlation between the latency and the error of saccades made in response to contralateral targets. In the delay task, FEF inactivation increased the frequency of making premature saccades to ipsilateral targets. FEF inactivation had inconsistent and mild effects on saccadic peak velocity. FEF inactivation caused impairments in the ability to fixate lights steadily in contralateral space. FEF inactivation always caused an ipsiversive deviation of the eyes in darkness. In summary, our results suggest that the FEF plays major roles in (1) generating contraversive saccades to locations of extinguished or flashed targets, (2) maintaining contralateral fixations, and (3) suppressing inappropriate ipsiversive saccades.

Anesthetics, Local↗

PubMed1997

Electrically evoked saccades from the dorsomedial frontal cortex and frontal eye fields: a parametric evaluation reveals differences between areas.

Using electrical stimulation to evoke saccades from the dorsomedial frontal cortex (DMFC) and frontal eye fields (FEF) of rhesus monkeys, parametric tests were conducted to compare the excitability properties of these regions. Pulse frequency and pulse current, pulse frequency and train duration, and pulse current and pulse duration were varied to determine threshold functions for a 50% probability of evoking a saccade. Also a wide range of frequencies were tested to evoke saccades, while holding all other parameters constant. For frequencies beyond 150 Hz, the probability of evoking saccades decreased for the DMFC, whereas for the FEF this probability remained at 100%. To evoke saccades readily from the DMFC, train durations of greater than 200 ms were needed; for the FEF, durations of less than 100 ms were sufficient. Even though the chronaxies of neurons residing in the DMFC and FEF were similar (ranging from 0.1 to 0.24 ms) significantly higher currents were required to evoke saccades from the DMFC than FEF. Thus the stimulation parameters that are optimal for evoking saccades from the DMFC differ from those that are optimal for evoking saccades from the FEF. Although the excitability of neurons in the DMFC and FEF are similar (due to similar chronaxies), we suggest that the density of saccade-relevant neurons is higher in the FEF than in the DMFC.

Animals↗

PubMed1996

Compensatory saccades made to remembered targets following orbital displacement by electrically stimulating the dorsomedial frontal cortex or frontal eye fields of primates.

If the eye-position signal during visually-evoked saccades is dependent on the dorsomedial frontal cortex (DMFC), one would expect that saccades generated to briefly presented visual targets would be disrupted after displacement of the eyes via electrical stimulation of this cortical area. Compared are compensatory saccades evoked to brief targets following stimulation of the DMFC and frontal eye fields (FEF). Compensatory saccades produced to brief targets following perturbation via the DMFC were not affected. Accordingly, electrical stimulation of the DMFC does not disrupt the eye-position signal during the execution of visually-evoked saccades.

Animals↗

PubMed1996

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.

Animals↗

PubMed1995

Topographic distribution of fixation-related units in the dorsomedial frontal cortex of the rhesus monkey.

Most cells in the dorsomedial frontal cortex of the rhesus monkey had activity related to saccadic eye movement and/or visual fixation. This activity changed depending upon the position of a fixation target, which suggested coding for the target location in spatial coordinates. Further analysis of such activity revealed a topographical distribution of neurons: neurons in the rostral part of the area were more active with eyes to the contralateral position, while those in the caudal part were more active with eyes to the ipsilateral position; also, cells in the medial part of the area had higher activity with a downward fixation position, whereas those in the lateral part had higher activity with an upward fixation position. This distribution of units was in agreement with the map of termination zones of saccadic eye movements evoked by electrical stimulation of the same area. These observations provide evidence for the hypothesis that the dorsomedial frontal cortex is organized in spatial coordinates and is involved in specifying the position of visual fixation.

Animals↗

PubMed1995

The dorsomedial frontal cortex: eye and forelimb fields.

This review yields three conclusions: first, the eye field as described using unit recording and electrical stimulation on behaving monkeys trained to fixate visual targets is much larger than the 4 mm2 area originally described. Second, the eye field and forelimb field share a similar neural space within the dorsomedial frontal cortex (DMFC); thus the electrophysiogical studies that have been conducted on visually guided and sensory-triggered forelimb movements must be re-evaluated, since none of these studies controlled eye movement and eye position independently. Third, a topographic map representing eye position in orbit has been discovered in the DMFC; it is proposed that this topographic map records the order of positions of the eyes and forelimbs during the acquisition of visually guided movement sequences.

Animals↗

PubMed1994

Stimulation-evoked saccades from the dorsomedial frontal cortex of the rhesus monkey following lesions of the frontal eye fields and superior colliculus.

This study examined whether signals for the generation of eye movements from the dorsomedial frontal cortex (DMFC) reach brainstem oculomotor centers either through the frontal eye fields (FEF) or through the superior colliculi (SC). The DMFC was stimulated when the monkeys studied were intact and after either one FEF or one SC was ablated. Following lesions of either the FEF or SC, the topographic order of the DMFC was largely preserved. After either lesions, stimulation of anterior DMFC sites still evoked saccades that terminated in contralateral space, and stimulation of posterior DMFC sites still evoked saccades that terminated in central space. The probability of evoking saccades decreased and the latency to evoke saccades increased as fixation neared the termination zone (a restricted region within craniotopic space) both before and after either lesion. Ablation of the SC, but not of the FEF, eliminated the saccadic inhibition to visual targets which resulted when the DMFC was stimulated in the intact animal. The findings suggest that additional channels besides those coursing through the FEF and SC are utilized by the DMFC to access the saccade generator in the brainstem.

Animals↗