Differential effects of laminar stimulation of V1 cortex on target selection by macaque monkeys

We explored the effects of microstimulation on target selection by delivering stimulation at different depths within V1 (striate cortex) of the rhesus monkey (Macaca mulatta). Stimulation evoked saccadic eye movements that terminated in the receptive-field location of the activated neurons. The current thresholds for saccade evocation were highest (> or = 30 micro A) in the superficial layers and lowest (< or = 10 micro A) in the deep layers. To study target selection, one visual target was presented in the receptive-field location of the stimulated neurons and a second visual target was presented outside this location. Microstimulation delivered in concert with the appearance of the two targets decreased the probability that a monkey would select the target placed in the receptive-field location when the upper layers of V1 were stimulated, and it increased this probability when the lower layers were stimulated. We suggest that microstimulation of the upper layers of V1 disrupts visual signals from retina en route to higher cortical areas, whereas microstimulation of the lower layers activates V1 efferents that innervate the oculomotor system.     

Configuring intracortical microelectrode arrays and stimulus parameters to minimize neuron loss during prolonged intracortical electrical stimulation

BackgroundPrevious studies have shown that neurons of the cerebral cortex can be injured by implantation of, and stimulation with, implanted microelectrodes.ObjectivesObjective 1 was to determine parameters of microstimulation delivered through multisite intracortical microelectrode arrays that will activate neurons of the feline cerebral cortex without causing loss of neurons.Objective2 was to determine if the stimulus parameters that induced loss of cortical neurons differed for all cortical neurons vs. the subset of inhibitory neurons expressing parvalbumin.MethodsThe intracortical microstimulation was applied for 7 h/day for 20 days (140 h). Microelectrode site areas were 2000 and 4000 μm², Q was 2–8 nanocoulombs (nC) at 50 Hz, and QD was 50–400 μcoulombs/cm².ResultsNeuron loss due to stimulation was minimal at Q = 2 Ncp, but at 8 Ncp, 20%–50% of neurons within 250 μm of the stimulated microelectrodes were lost, compared to unstimulated microelectrodes. Loss was greatest in tissue facing electrode sites. Stimulation-induced loss was similar for neurons labeled for NeuN and for inhibitory neurons expressing parvalbumin. Correlation between neuron loss and QD was not significant.Electrodes in the medullary pyramidal tract recorded neuronal activity evoked by stimulation in the cerebral cortex. The pyramidal neurons were activated by intracortical stimulation of 2 nC/phase. 140 h of microstimulation at 2 nC/phase and 50 Hz induced minimal neuron loss.     

Using ocular dominance to infer the depth of the visual input layers of V1 in behaving macaque monkey

The goal of this study was to use the ocular dominance properties of multiple unit activity in area V1 of the visual cortex of the behaving rhesus monkey to infer the depth of the visual input layers. Multiple unit activity was examined with a recording electrode at different depths (in 100 micrometer increments) within V1 for responses to a visual stimulus presented to the dominant and non-dominant eye. The cortical depth at which there was a maximal difference in unit firing rate between the dominant and non-dominant eye was used to infer the depth of the visual input layers of V1. This depth was found to vary from 0.8 to 1.2 mm below the cortical surface. This range of depths overlaps with the approximate location of lamina IVc, which is the major recipient of visual fibres from the lateral geniculate nucleus.     

Hijacking cortical motor output with repetitive microstimulation

High-frequency repetitive microstimulation has been widely used as a method of investigating the properties of cortical motor output. Despite its widespread use, few studies have investigated how activity evoked by high-frequency stimulation may interact with the existing activity of cortical cells resulting from natural synaptic inputs. A reasonable assumption might be that the stimulus-evoked activity sums with the existing natural activity. However, another possibility is that the stimulus-evoked firing of cortical neurons might block and replace the natural activity. We refer to this latter possibility as "neural hijacking." Evidence from analysis of EMG activity evoked by repetitive microstimulation (200 Hz, 500 ms) of primary motor cortex in two rhesus monkeys during performance of a reach-to-grasp task strongly supports the neural hijacking hypothesis.     

Evoking highly focal percepts in the fingertips through targeted stimulation of sulcal regions of the brain for sensory restoration

BackgroundParalysis and neuropathy, affecting millions of people worldwide, can be accompanied by significant loss of somatosensation. With tactile sensation being central to achieving dexterous movement, brain-computer interface (BCI) researchers have used intracortical and cortical surface electrical stimulation to restore somatotopically-relevant sensation to the hand. However, these approaches are restricted to stimulating the gyral areas of the brain. Since representation of distal regions of the hand extends into the sulcal regions of human primary somatosensory cortex (S1), it has been challenging to evoke sensory percepts localized to the fingertips.Objective/hypothesisTargeted stimulation of sulcal regions of S1, using stereoelectroencephalography (SEEG) depth electrodes, can evoke focal sensory percepts in the fingertips.MethodsTwo participants with intractable epilepsy received cortical stimulation both at the gyri via high-density electrocorticography (HD-ECoG) grids and in the sulci via SEEG depth electrode leads. We characterized the evoked sensory percepts localized to the hand.ResultsWe show that highly focal percepts can be evoked in the fingertips of the hand through sulcal stimulation. fMRI, myelin content, and cortical thickness maps from the Human Connectome Project elucidated specific cortical areas and sub-regions within S1 that evoked these focal percepts. Within-participant comparisons showed that percepts evoked by sulcal stimulation via SEEG electrodes were significantly more focal (80% less area; p = 0.02) and localized to the fingertips more often, than by gyral stimulation via HD-ECoG electrodes. Finally, sulcal locations with consistent modulation of high-frequency neural activity during mechanical tactile stimulation of the fingertips showed the same somatotopic correspondence as cortical stimulation.ConclusionsOur findings indicate minimally invasive sulcal stimulation via SEEG electrodes could be a clinically viable approach to restoring sensation.     

Effects of tDCS on spontaneous spike activity in a healthy ambulatory rat model

BackgroundThe neurophysiological effects of transcranial direct current stimulation (tDCS) are typically described with respect to changes in cortical excitability, defined by using transcranial magnetic stimulation pulses to determine changes in motor evoked potentials. However, how individual cortical neurons change firing patterns under the influence of tDCS is largely unknown. While the relatively weak currents produced in the brain by tDCS may not be adequate to directly depolarize neuronal membranes, ongoing neuronal activity, combined with subthreshold changes in membrane polarization might be sufficient to alter the threshold for neural firing.ObjectivesThe purpose of this study was to determine the effects of tDCS on neurophysiological activity in motor cortex of freely moving, healthy rats.MethodsIn nine healthy, ambulatory rats, each studied under six different stimulation conditions varying in current intensity (maximum current density = 39.8 A/m² at 0.4 mA) and polarity (anodal or cathodal), neural activity was analyzed in response to 20 min of tDCS applied through bone screws insulated from the overlying scalp.ResultsAfter analysis of 480 multi-unit channels that satisfied a rigid set of neurophysiological criteria, we found no systematic effect of tDCS stimulation condition on firing rate or firing pattern. Restricting the analysis to the most responsive units, subtle, but statistically significant changes occurred only in the highest intensity anodal condition.ConclusionsThese results confirm that at current densities typically used in human or animal tDCS studies, observed effects of tDCS are likely to occur via mechanisms other than direct neuronal depolarization.     

Effects of stimulus pulse rate on somatosensory adaptation in the human cortex

BackgroundIntracortical microstimulation (ICMS) of the somatosensory cortex can restore sensation to people with neurological diseases. However, many aspects of ICMS are poorly understood, including the effect of stimulation on percept intensity over time.ObjectiveHere, we evaluate how tactile percepts evoked by ICMS in the somatosensory cortex of a human participant adapt over time.MethodsWe delivered continuous and intermittent ICMS to the somatosensory cortex and assessed the reported intensity of tactile percepts over time in a human participant. Experiments were conducted over approximately one year and linear mixed effects models were used to assess significance.ResultsContinuous stimulation at high frequencies led to rapid decreases in intensity, while low frequency stimulation maintained percept intensity for longer periods. Burst-modulated stimulation extended the time before the intensity began to decrease, but all protocols ultimately resulted in complete sensation loss within 1 min. Intermittent stimulation paradigms with several seconds between stimulus trains evoked intermittent percepts and also led to decreases in intensity on many electrodes, but never resulted in extinction of the sensation after over 3 min of stimulation. Longer breaks between each pulse train resulted in some recovery in the intensity of the stimulus-evoked percepts. For several electrodes, intermittent stimulation had almost no effect on the perceived intensity.ConclusionsIntermittent ICMS paradigms were more effective at maintaining percepts. Given that transient neural activity dominates the response in somatosensory cortex during mechanical contact onsets and offsets, providing brief stimulation trains at these times may more closely represent natural cortical activity and have the additional benefit of prolonging the ability to evoke sensations over longer time periods.     

Involvement of rat posterior prelimbic and cingulate area 2 in vocalization control

Microstimulation mapping identified vocalization areas in primate anterior cingulate cortex. Rat anterior cingulate and medial prefrontal areas have also been intensely investigated, but we do not know, how these cortical areas contribute to vocalizations and no systematic mapping of stimulation‐evoked vocalizations has been performed. To address this question, we mapped microstimulation‐evoked (ultrasonic) vocalizations in rat cingulate and medial prefrontal cortex. The incidence of evoked vocalizations differed markedly between frontal cortical areas. Vocalizations were most often evoked in posterior prelimbic cortex and cingulate area 2, whereas vocalizations were rarely evoked in dorsal areas (vibrissa motor cortex, secondary motor cortex and cingulate area 1) and anterior areas (anterior prelimbic, medial‐/ventral‐orbital cortex). Vocalizations were observed at intermediate frequencies in ventro‐medial areas (infralimbic and dorsopeduncular cortex). Various complete, naturally occurring calls could be elicited. In prelimbic cortex superficial layer microstimulation evoked mainly fear calls with low efficacy, whereas deep layer microstimulation evoked mainly 50 kHz calls with high efficacy. Vocalization stimulation thresholds were substantial (70–500 μA, the maximum tested; on average ~400 μA) and latencies were long (median 175 ms). Posterior prelimbic cortex projected to numerous targets and innervated brainstem vocalization centers such as the intermediate reticular formation and the nucleus retroambiguus disynaptically via the periaqueductal gray. Anatomical position, stimulation effects and projection targets of posterior prelimbic cortex were similar to that of monkey anterior cingulate vocalization cortex. Our data suggest that posterior prelimbic cortex is more closely involved in control of vocalization initiation than in specifying acoustic details of vocalizations.     

A systematic exploration of parameters affecting evoked intracranial potentials in patients with epilepsy

BackgroundBrain activity is constrained by and evolves over a network of structural and functional connections. Corticocortical evoked potentials (CCEPs) have been used to measure this connectivity and to discern brain areas involved in both brain function and disease. However, how varying stimulation parameters influences the measured CCEP across brain areas has not been well characterized.ObjectiveTo better understand the factors that influence the amplitude of the CCEPs as well as evoked gamma-band power (70–150 Hz) resulting from single-pulse stimulation via cortical surface and depth electrodes.MethodsCCEPs from 4370 stimulation-response channel pairs were recorded across a range of stimulation parameters and brain regions in 11 patients undergoing long-term monitoring for epilepsy. A generalized mixed-effects model was used to model cortical response amplitudes from 5 to 100 ms post-stimulation.ResultsStimulation levels <5.5 mA generated variable CCEPs with low amplitude and reduced spatial spread. Stimulation at ≥5.5 mA yielded a reliable and maximal CCEP across stimulation-response pairs over all regions. These findings were similar when examining the evoked gamma-band power. The amplitude of both measures was inversely correlated with distance. CCEPs and evoked gamma power were largest when measured in the hippocampus compared with other areas. Larger CCEP size and evoked gamma power were measured within the seizure onset zone compared with outside this zone.ConclusionThese results will help guide future stimulation protocols directed at quantifying network connectivity across cognitive and disease states.     

Cortical stimulation improves skilled forelimb use following a focal ischemic infarct in the rat

Abstract

Improving functional recovery following cerebral strokes in humans will likely involve augmenting brain plasticity. This study examined skilled forelimb behavior, neocortical evoked potentials, and movement thresholds to assess cortical electrical stimulation concurrent with rehabilitative forelimb usage following a focal ischemic insult. Adult rats were trained on a task that required skilled usage of both forelimbs. They then underwent an acute focal ischemic insult to the caudal forelimb area of sensorimotor cortex contralateral to their preferred forelimb. During the same procedure, they also received a stimulation electrode over the infarct area and two depth electrodes anterior to the lesion to record evoked potentials. One week following the surgery, rats received cortical stimulation during performance of the skilled task. Evoked potentials and movement thresholds were also determined. Functional assessment revealed that cortical stimulation resulted in superior performance compared to the no stimulation group, and this was initially due to a shift in forelimb preference. Cortical stimulation also resulted in enhanced evoked potentials and a reduction in the amount of current required to elicit a movement, in a stimulation frequency dependent manner. This study suggests that cortical stimulation, concurrent with rehabilitative training, results in better forelimb usage that may be due to augmented synaptic plasticity.     

Deep brain stimulation of the lateral cerebellar nucleus produces frequency-specific alterations in motor evoked potentials in the rat in vivo

The cerebral cortex is tightly and reciprocally linked to the cerebellum and the ascending dentato-thalalmo-cortical pathway influences widespread cortical regions. Using a rodent model of middle cerebral artery stroke, we showed previously that chronic, 20 Hz stimulation of the contralateral lateral cerebellar nucleus (LCN) improved motor recovery, while 50 Hz stimulation did not. Using motor evoked potentials (MEP) elicited by intracortical microstimulation, we now show the effect of LCN stimulation on motor cortex excitability as a function of pulse frequency in propofol-anesthetized rats. MEPs were recorded serially, at 15-s intervals, with cerebellar stimulation delivered in 10-min blocks at rates of 20, 30, 40, 50 or 100 Hz. Stimulation at 20, 30, 40 or 50 Hz enhanced the average MEP response across the block, with the maximal overall increase observed during 30 Hz stimulation. However, the effect varied as a function of both repeated trials within the block and LCN stimulation frequency, such that 40 Hz and 50 Hz stimulation showed a reduced effect over time. Stimulation at 100 Hz produced a transient increase in MEP amplitude in some animals; however the overall effect across the block was a trend towards reduced cortical excitability. These results suggest that direct stimulation of the LCN can yield frequency-dependent changes in cortical excitability and may provide a therapeutic approach to modulating cortical activity for the treatment of strokes or other focal cortical lesions, movement disorders and epilepsy.     

Non-ablative doses of focal ionizing radiation alters function of central neural circuits

BackgroundModulation of pathological neural circuit activity in the brain with a minimum of complications is an area of intense interest.ObjectiveThe goal of the study was to alter neurons' physiological states without apparent damage of cellular integrity using stereotactic radiosurgery (SRS).MethodsWe treated a 7.5 mm-diameter target on the visual cortex of Göttingen minipigs with doses of 40, 60, 80, and 100 Gy. Six months post-irradiation, the pigs were implanted with a 9 mm-wide, eight-shank multi-electrode probe, which spanned the radiation focus as well as the low-exposure neighboring areas.ResultsDoses of 40 Gy led to an increase of spontaneous firing rate, six months post-irradiation, while doses of 60 Gy and greater were associated with a decrease. Subjecting the animals to visual stimuli resulted in typical visual evoked potentials (VEP). At 40 Gy, a significant reduction of the P1 peak time, indicative of higher network excitability was observed. At 80 Gy, P1 peak time was not affected, while a minor reduction at 60 Gy was seen. No distance-dependent effects on spontaneous firing rate, or on VEP were observed. Post-mortem histology revealed no evidence of necrosis at doses below 60 Gy. In an in vitro assay comprising of iPS-derived human neuron-astrocyte co-cultures, we found a higher vulnerability of inhibitory neurons than excitatory neurons with respect to radiation, which might provide the cellular mechanism of the disinhibitory effect observed in vivo.ConclusionWe provide initial evidence for a rather circuit-wide, long-lasting disinhibitory effect of low sub-ablative doses of SRS.     

Eye movements and the perceived location of phosphenes generated by intracranial primary visual cortex stimulation in the blind

BackgroundRestoring sight for the blind using electrical stimulation of the visual pathways is feasible but demands an understanding of the spatial mapping of the visual world at the site of targeted stimulation, whether in the retina, thalamus, or cortex. While a visual cortex stimulator can bypass the eye and create visual percepts, there is an inherent dissociation between this stimulation and eye movements. It is unknown whether and how robustly the brain maintains the oculomotor circuitry in patients with bare- or no-light perception.ObjectiveTo critically and quantitatively evaluate the effect of eye movements have on phosphene locations elicited by cortical stimulation that bypasses the eyes in order to restore sight in blind subjects.MethodsThe NeuroPace Responsive Neurostimulator (RNS) and the Orion visual cortical prosthesis devices were used to electrically stimulate the visual cortex of blind subjects with bare or no light perception. Eye positions were recorded synchronized with stimulation and the location of the percepts were measured using a handheld marker.ResultsThe locations of cortical stimulation-evoked percepts are shifted based on the eye position at the time of stimulation. Measured responses can be remapped based on measured eye positions to determine the retinotopic locations associated with the implanted electrodes, with remapped responses having variance limited by pointing error.ConclusionsEye movements dominate the perceived location of cortical stimulation-evoked phosphenes, even after years of blindness. By accounting for eye positions, we can mimic retinal mapping as in natural sight.     

Transcranial alternating current stimulation (tACS) modulates cortical excitability as assessed by TMS-induced phosphene thresholds

ObjectiveRecent developments in transcranial alternating current stimulation (tACS) provide a powerful approach to establish the functional roles of neuronal oscillatory activities in the human brain. Here, we investigated whether tACS can reach and modulate the excitability of the visual cortex in a frequency-dependent manner.MethodsWe measured the cortical excitability of the visual cortex using single pulse transcranial magnetic stimulation (TMS) while delivering tACS to the occipital region at different frequencies (5, 10, 20 and 40 Hz).ResultsWe found that tACS at 20 Hz decreased TMS–phosphene threshold (i.e., increased the excitability of the visual cortex) during the stimulation, whereas other frequencies did not affect TMS–phosphene thresholds.ConclusionsOur findings demonstrate direct interactions of tACS with the visual cortex in a frequency-dependent manner.SignificanceOur present work provides further demonstration of the potential of tACS as a method to selectively modulate the excitability of the visual cortex.     

Evaluation of extraocular electrodes for a retinal prosthesis using evoked potentials in cat visual cortex.

OBJECTIVE: To assess the efficacy of a device using extraocular electrodes as a retinal prosthesis by evaluating the responses evoked in the visual cortex to electrical stimulation. METHODS: In anaesthetised cats, a lateral orbital dissection and ipsilateral parietal craniotomy was performed. Two extraocular retinal prosthesis (ERP) disc electrodes were sutured to the sclera on the lateral and superior aspects of the globe. Retinal stimulation was performed with charge-balanced constant-current pulses. Potentials evoked in the visual cortex were measured with a ball electrode placed on the lateral gyrus after removal of the dura. RESULTS: Stable attachment of the ERP electrodes to the globe was achieved with scleral sutures. Visual cortex responses were recorded with the electrodes in bipolar and monopolar configurations. The evoked response consisted of an early component with a peak around 8 ms, and a late component with a peak after 50 ms. Thresholds for evoking a response occurred at current intensities as low as 500 microA. Through extrapolation from evoked response amplitude data, thresholds as low as 300 microA were calculated. Cathodal monopolar stimulation demonstrated lower thresholds than anodal stimulation for evoking cortical responses. CONCLUSIONS: The ERP electrodes can be easily attached to the globe and are effective in electrically stimulating the retina, evoking responses in the primary visual cortex. Threshold charge-density was within safe limits for neural stimulation.     

Real-time suppression and amplification of frequency-specific neural activity using stimulation evoked oscillations

BackgroundApproaches to predictably control neural oscillations are needed to understand their causal role in brain function in healthy or diseased states and to advance the development of neuromodulation therapies.ObjectiveWe present a closed-loop neural control and optimization framework to actively suppress or amplify low-frequency neural oscillations observed in local field potentials in real-time by using electrical stimulation. The rationale behind this control approach and our working hypothesis is that neural oscillatory activity evoked by electrical pulses can suppress or amplify spontaneous oscillations via destructive or constructive interference when the pulses are continuously delivered with appropriate amplitudes and at precise phases of the modulated oscillations in a closed-loop scheme.MethodsWe tested our hypothesis in two nonhuman primates that exhibited a robust increase in low-frequency (8–30 Hz) oscillatory power in the subthalamic nucleus (STN) following administration of the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). To test our neural control approach, we targeted 8–17 Hz oscillations and used electrode arrays and electrical stimulation waveforms similar to those used in humans chronically implanted with brain stimulation systems. Stimulation parameters that maximize the suppression or amplification of neural oscillations were predicted using mathematical models of the stimulation evoked oscillations.ResultsOur neural control and optimization approach was capable of actively and robustly suppressing or amplifying oscillations in the targeted frequency band (8–17 Hz) in real-time in the studied subjects.ConclusionsThe results from this study support our hypothesis and suggest that the proposed neural control framework allows one to characterize in controlled experiments the functional role of frequency-specific neural oscillations by using electrodes and stimulation waveforms currently being employed in humans.     

Multi-electrode stimulation evokes consistent spatial patterns of phosphenes and improves phosphene mapping in blind subjects

BackgroundVisual cortical prostheses (VCPs) have the potential to restore visual function to patients with acquired blindness. Successful implementation of VCPs requires the ability to reliably map the location of the phosphene produced by stimulation of each implanted electrode.ObjectiveTo evaluate the efficacy of different approaches to phosphene mapping and propose simple improvements to mapping strategy.MethodsWe stimulated electrodes implanted in the visual cortex of five blind and fifteen sighted patients. We tested two fixation strategies, unimanual fixation, where subjects placed a single index finger on a tactile fixation point and bimanual fixation, where subjects overlaid their right index finger over their left on the tactile point. In addition, we compared absolute mapping in which a single electrode was stimulated on each trial, and relative mapping with sequences containing stimulation of three to five phosphenes on each trial. Trial-to-trial variability present in relative mapping sequences was quantified.ResultsPhosphene mapping was less precise in blind subjects than in sighted subjects (2DRMS, 16 ± 2.9° vs. 1.9 ± 0.93°; t (18) = 18, p = <0.001). Within blind subjects, bimanual fixation resulted in more consistent phosphene localization than unimanual fixation (BS1: 4.0 ± 2.6° vs. 19 ± 4.7°, t (79) = 24, p < 0.001; BS2 4.1 ± 2.0° vs. 12 ± 2.7°, t (65) = 19, p < 0.001). Multi-point relative mapping had similar baseline precision to absolute mapping (BS1: 4.7 ± 2.6° vs. 3.9 ± 2.0°; BS2: 4.1 ± 2.0° vs. 3.2 ± 1.1°) but improved significantly when trial-to-trial translational variability was removed. Although multi-point mapping methods did reveal more of the functional organization expected in early visual cortex, subjects tended to artificially regularize the spacing between phosphenes. We attempt to address this issue by fitting a standard logarithmic map to relative multi-point sequences.ConclusionsRelative mapping methods, combined with bimanual fixation, resulted in the most precise estimates of phosphene organization. These techniques, combined with use of a standard logarithmic model of visual cortex, may provide a practical way to improve the implementation of a VCP.     

tACS phase-specifically biases brightness perception of flickering light

BackgroundVisual phenomena like brightness illusions impressively demonstrate the highly constructive nature of perception. In addition to physical illumination, the subjective experience of brightness is related to temporal neural dynamics in visual cortex.ObjectiveHere, we asked whether biasing the temporal pattern of neural excitability in visual cortex by transcranial alternating current stimulation (tACS) modulates brightness perception of concurrent rhythmic visual stimuli.MethodsParticipants performed a brightness discrimination task of two flickering lights, one of which was targeted by same-frequency electrical stimulation at varying phase shifts. tACS was applied with an occipital and a periorbital active control montage, based on simulations of electrical currents using finite element head models.ResultsExperimental results reveal that flicker brightness perception is modulated dependent on the phase shift between sensory and electrical stimulation, solely under occipital tACS. Phase-specific modulatory effects by tACS were dependent on flicker-evoked neural phase stability at the tACS-targeted frequency, recorded prior to electrical stimulation. Further, the optimal timing of tACS application leading to enhanced brightness perception was correlated with the neural phase delay of the cortical flicker response.ConclusionsOur results corroborate the role of temporally coordinated neural activity in visual cortex for brightness perception of rhythmic visual input in humans. Phase-specific behavioral modulations by tACS emphasize its efficacy to transfer perceptually relevant temporal information to the cortex. These findings provide an important step towards understanding the basis of visual perception and further confirm electrical stimulation as a tool for advancing controlled modulations of neural activity and related behavior.     

Differential distribution of barrel or visual cortex. Evoked responses along the rostro-caudal axis of the peri- and postrhinal cortices

The functional connections between the barrel cortex and visual cortex on the one hand and the perirhinal (PER) and postrhinal (POR) cortices on the other hand were investigated in the rat. Stimulation of the barrel cortex evoked field potentials throughout the longitudinal extent of both PER and POR. In contrast, visual cortex stimulation evoked responses only in the caudal portion of PER as well as in POR. Therefore, the information from the visual cortex on the way to the hippocampus is transferred preferentially by a relay in POR, whereas somatosensory information is transferred via both PER and POR. Moreover, stimulation of both cortical regions elicited firing of multiple units; however, unit activity was more commonly found in POR than in PER. We conclude that the transfer of somatosensory and visual information to the hippocampal formation is preferentially mediated by parallel channels through PER and POR respectively. Although the information transfer through these channels does overlap to some extent, each channel appears to have specific properties.     

Cortical microstimulation in auditory cortex of rat elicits best-frequency dependent behaviors

Electrical activation of the auditory cortex has been shown to elicit an auditory sensation; however, the perceptual effects of auditory cortical microstimulation delivered through penetrating microelectrodes have not been clearly elucidated. This study examines the relationship between electrical microstimulus location within the adult rat auditory cortex and the subsequent behavior induced. Four rats were trained on an auditory frequency discrimination task and their lever-pressing behavior in response to stimuli of intermediate auditory frequencies was quantified. Each trained rat was then implanted with a microwire array in the auditory cortex of the left hemisphere. Best frequencies (BFs) of each electrode in the array were determined by both local field potential and multi-unit spike-rate activity evoked by pure tone stimuli. A cross-dimensional psychophysical generalization paradigm was used to evaluate cortical microstimulation-induced behavior. Using the BFs of each electrode, the microstimulation-induced behavior was evaluated relative to the auditory-induced behavior. Microstimulation resulted in behavior that was dependent on the BFs of the electrodes used for stimulation. These results are consistent with recent reports indicating that electrophysiological recordings of neural responses to sensory stimuli may provide insight into the sensation generated by electrical stimulation of the same sensory neural tissue.