Increased motor excitability produced by alerting stimuli

The proposal that increased excitability of sensory and motor systems occurs in response to dishabituated sensory stimuli was supported by experiments with cats in which excitability was measured by the amplitude of the response evoked by an electrical test stimulus. Evoked EMG, evoked head movement and the postsynaptic visual cortex response were briefly augmented following such stimuli, with a latency less than 20 msec for auditory and less than 50 msec for visual stimuli. Evoked EMG responses were rare during habituation, but even faint stimuli (40 db (SPL) tone pips) were effective during conditioning procedures. The proposed organization of the brain was thought to have adaptive significance, helping the subject to cope with emergency situations by enhancing the processing of sensory and motor information.     

Paired-pulse behavior of visually evoked potentials recorded in human visual cortex using patterned paired-pulse stimulation

Paired-pulse stimulation techniques are used as common tools to investigate cortical excitability and cortical plastic changes. Similar to investigations in the somatosensory and motor system here we applied a new paired-pulse paradigm to study the paired-pulse behavior of visually evoked potentials (VEPs) in 25 healthy subjects. VEPs were recorded and the responses to the first and the second P100 peak were analyzed at different SOAs [stimulus onset asynchrony (SOA) = interstimulus interval (ISI) + pulse duration (13 ms)]. Two measures describe the paired pulse interaction: the “amplitude ratio”, the ratio of the second to the first amplitude, and the “latency shift”, the difference of the inter-peak interval between the P100 peaks and the respective SOA. To separate alterations in the amplitude of the second VEP response due to changes in paired-pulse inhibition from those originating from superposition of the two waveforms, particularly at short SOAs, we created a waveform template from recordings made at SOAs of 1 s, where interaction can be assumed to be negligible. Superposed traces of VEP recordings were then created by adding two templates at delays corresponding to the SOAs used. The original recordings were then digitally subtracted from the traces obtained by superposition. Analysis of the subtracted traces revealed evidence that at short SOAs the second VEP response is substantially suppressed, a finding comparable to the paired-pulse inhibition described for motor and somatosensory cortex following paired-pulse stimulation. However, paired-pulse inhibition seen in V1 varied considerably from subject to subject, both in respect to amplitude, and to time of maximal inhibition. We found paired-pulse inhibition ranging from 12 to 76% (mean 34%) at SOAs between 80 (shortest discriminable SOA) and 320 ms (mean 128 ms). At intermediate SOAs between 80 and 720 ms (mean 215 ms) the amplitude ratios were between 94 and 145% (mean 116%) indicative of slight paired-pulse facilitation. Comparable to recovery studies by means of paired-pulse median nerve stimulation in somatosensory cortex, at shorter SOAs we found a delayed second VEP response. Combined together, our findings suggest that VEPs are characterized by significant paired-pulse inhibition at short SOAs, a phenomenon reminiscent of findings reported in other modalities. Possible mechanisms and pharmacological properties of the described paired-pulse behavior in visual cortex remain to be explored.     

Operant Conditioning of Visual Evoked Potentials in Man

The operant conditioning technique was used to produce visual evoked response modifications in man. Our results show that: 1) 50% of our subjects are able to modify a definite component of their evoked potential. 2) These modifications still occur with irregular evoked potential intervals. 3) The criterion segment is the only part of the response which is modified; other components are not affected. 4) All components ranging from 90 to 385 msec can be modified; however, the second part of the response (after 200 msec) seems to be more easily modified.     

Interactions of the evoked responses to visual, somatic, and auditory stimuli in polysensory areas of the cat cortex

Recordings of evoked potentials in the middle suprasylvian gyrus were obtained to pairings tf visual, anditory, and somatic stimuli, and two response measures were compared. Amplitude recovery of the response to the second stimulus was delayed for pairings of like stimuli as compared to unlike pairings, an effect not altered by varying the intensity of the first stimulus.     

Excitability changes of the visual cortex during discrimination behavior in the cat

Cats were conditioned to press a bar for food reward with repetitive stimulation of the lateral geniculate nucleus. The stimulation served both as a discriminative stimulus to generate the bar-press and to elicit a series of evoked responses in the visual cortex. With well-trained cats, we recorded evoked responses at the initiation of the bar-press. Each of the responses consisted of positive-negative diphasic waves of about 10 msec duration. The positive wave had small positive deflections on its rising phase. On repetitive stimulation, those waves were small at the beginning, then augmented and reached maximal amplitude. Bar-press occurred several tenths of a second after the maximum enhancement. Large augmentation of the response occurred when the first response was small and vice versa. We also found that single shock stimulation to the geniculate could serve as a discriminative stimulus for initiation of bar-pressing.     

Neurophysiological mechanisms of recovery from visual cortex damage in cats: Properties of lateral suprasylvian visual area neurons following behavioral recovery

Summary Damage to visual cortical areas 17, 18, and 19 in the cat produces severe and long-lasting deficits in performance of form and pattern discriminations. However, with extensive retraining the animals are able to recover their ability to discriminate form and pattern stimuli. Recent behavioral experiments from this laboratory have shown that a nearby region of cortex, the lateral suprasylvian visual area (LS area), plays an important role in this recovery (Wood et al., 1974; Baumann and Spear, 1977b). The present experiment investigated the underlying neurophysiological mechanisms of the recovery by recording from single neurons in the LS area of cats which had recovered from long-term visual cortex damage.Five adult cats received bilateral removal of areas 17, 18, and 19. They were then trained to criterion on two-choice brightness, form, and pattern discriminations. Recording from LS area neurons was carried out after the behavioral training, from 3 to 7 months after the visual cortex lesions. The properties of these neurons were compared to those of LS area neurons in normal cats (Spear and Baumann, 1975) and in cats with acute or short-term visual cortex damage and no behavioral recovery (Spear and Baumann, 1979). The results showed that all of the changes from normal which were produced by acute visual cortex damage were also present after the behavioral recovery. Moreover, all of the response properties of LS area neurons which remain after acute visual cortex damage were present in similar form after the behavioral recovery. There was no evidence for any functional reorganization in the LS area concomitant with its role in the behavioral recovery.These results suggest that functional reorganization plays little or no role in recovery from visual cortex damage in adult cats. Rather, the recovery of form and pattern discrimination ability appears to be based upon the functioning of residual neural processes in the LS area which remain after the visual cortex damage.     

Stimulus specificity of binocular cells in the cat's visual cortex: ocular dominance and the matching of left and right eyes

Summary Most cells in the striate cortex respond to visual stimulation through either eye. We have examined quantitatively the matching of response specificity for the two eyes. Our intention was to determine the degree to which this matching depends on ocular dominance. We used standard single cell recording techniques and studied responses to sinusoidal gratings of different spatial frequencies, orientations, and contrasts. For all tests, stimuli were randomly interleaved both with respect to the value of each parameter, and the eye which was stimulated. After estimating ocular dominance qualitatively and quantitatively, we measured: response modulation (to help identify whether a cell was simple or complex), orientation and spatial frequency tuning, and contrast response functions (to estimate contrast thresholds). Results show that: (1) Response modulation is well matched between the two eyes, but there is a slight tendency for the dominant eye to respond with less modulation. (2) Optimal orientation and spatial frequency and their respective tuning widths were similar for the two eyes. In general, tuning functions for the two eyes differed mainly in slope. However, in each case, there was a tendency for the dominant eye to have broader tuning widths. (3) In most cases, contrast response functions for the two eyes differed mainly in their slopes. Extrapolation to spontaneous levels suggests that estimated contrast thresholds are relatively independent of ocular dominance although, again, there was a tendency for the dominant eye to exhibit slightly lower estimated thresholds. These findings demonstrate that response characteristics between the two eyes are generally well matched regardless of relative response strength. There are, however, small but clear differences between the two eyes for all parameters we measured which are related to and demonstrate that ocular dominance influences the degree of matching between the two eyes.     

Electrophysiological correlates of local muscular fatigue effects upon human visual reaction time

Summary A study of the neurophysiological basis of reaction time change was undertaken as a means of exploring the physiological mechanisms of local muscular fatigue effects upon sensorimotor performance. The identification of electrophysiological indices of central and peripheral processes within the human electroencephalogram and electromyogram, enabled a fractionation of total reaction time into component latencies measuring sensory reception time, sensorimotor integration time, central motor outflow time and peripheral motor contraction time. Simple foot dorsiflexion reactions to visual stimuli were observed in 18 male college students. Foot responses under one condition were performed against a resistance which necessitated a moderate degree of muscular tension before movement could occur while a second condition required normal unresisted responses. Two intensities of serial isometric work resulting in the order of a 38% decrement in maximum voluntary contractile capability (MVC) were performed by each subject. While the rate and extent of MVC decrement varied with the inherent strength of the subject and the intensity of the exercise performed, unconditional changes were observed in the spatiotemporal dimensions of reaction time performance following exercise-induced fatigue. The quality of total reaction time was found to deteriorate, particularly when responses were resisted. Peripheral deficiencies, suggestive of a decreased rate of tension development, were evidenced by a marked elongation of resisted motor times, and less vigorous and extensive unresisted responses. Insofar as the energy of response electromyograms was also diminished, central mechanisms were implicated, possibly due to a shift in motor unit recruitment. Concomitant changes were also observed in central processing.This research project was undertaken in the Motor Integration Research Laboratory of the Department of Exercise Science at the University of Massachusetts     

Single neuron activity related to natural vestibular stimulation in the cat's visual cortex

Summary This study examines the possibility of a vestibular input to the visual cortex using chronically implanted cats subjected to horizontal sinusoïdal rotation in the dark. In areas 17 and 18 the activity of respectively 14% and 11% of units was modified by vestibular stimulation. Both non-specific and specific influences were observed. Specific influences (42% in area 17 and 33% in area 18) were similar to the types of responses recorded in the vestibular nuclei, and were encountered more frequently within the cortex subserving the peripheral visual field. Our results could provide a neurophysiological basis for some psychophysiological observations concerning visuo-vestibular interactions.Supported by I.N.S.E.R.M. (C.R.L. 79-53336)     

Parietal cortex neurons of the monkey related to the visual guidance of hand movement

Summary A class of neurons specifically related to hand movements was studied in the posterior parietal cortex while the monkeys manipulated different types of objects. We examined the neuronal activity during manipulation of objects by the hand in the light and in the dark. Fiftyfive neurons were active during manipulation in the dark and were classified as hand-movement-related neurons. Of these, 38/55 (69%) cells were also influenced by the visual stimulus. Most of the hand-movement-related neurons were selective in the type of objects manipulated. Moreover, some of these cells were selective in the axis of orientation of the object. These results suggest that the hand-movement-related neurons of the parietal cortex are concerned with the visual guidance of the hand movement, especially in matching the pattern of movement with the spatial characteristics of the object to be manipulated.     

Effect of body tilt on receptive field orientation of simple visual cortical neurons in unanesthetized cats

Summary The receptive field (RF) orientation of 53 simple visual cortical neurons was determined by recording the activity of single cells during presentation of stationary bars of light. An RF tuning curve was constructed for each cell by averaging the neural discharge resulting from the repeated presentation of a number of slit orientations. RF curves were then determined again, following a 45 ° roll tilt of the entire head and body, and subsequently after the return of the animal to the original horizontal position. RF tuning curves were typical of what others have found to characterize simple cells, and were highly replicable on the return to the starting position. In 73% of the cells studied, the RF orientation after tilt remained unaltered relative to the head axis (±15 °); in the remaining 27% of the cells RF orientations either under- or over-shot the retinal tilt by more than 15 °, and in some cases by as much as 45 °. These results support the hypothesis that the well documented vestibular inputs to visual cortex play a role in modifying the RF orientation selectivity of visual cortical neurons, and suggest that such information may be an important neurophysiological substrate underlying visual spatial constancy mechanisms.This research was supported by NIH grant NS12308, and grants to DLT and NMB from the University of Pittsburgh Medical Alumni Association     

Selection of visual targets activates prelunate cortical cells in trained rhesus monkey

Summary Cells in monkey prelunate association cortex display an enhanced visual activity after the onset of a stimulus in the receptive field, when the stimulus is simultaneously selected as a target for a saccadic eye movement. In the present study we observed a separate activation which is independent of the passive visual on-response and occurs in a given cell when the animal saccades to a steady stimulus in its receptive field. The activation begins when the stimulus is selected for foveation before the eye actually moves, but is not necessarily predictive for an eye movement.This work was supported by the Sonderforschungsbereich Hirnforschung und Sinnesphysiologie (SFB 70/Tp B7)     

Source-density mapping of human visual receptive fields using scalp electrodes

Summary A triangular array of 20 electrodes spaced by 1.5 cm was used to record the distribution of current source-densities at 12 locations over the occipital scalp, in response to tachistoscopic presentation of a 1/2° × 1/2° scanning pattern element which explored a 2° × 2° area of the visual field. Single scalp locations had visual receptive fields of the order of 1°–2° in diameter, their shape varying somewhat according to the response in question. Source density analysis can resolve details of the AEP scalp map to better than 1 cm. Even with a 1/2° × 1/2° stimulus, averages of only a few tens of sweeps gave adequate signal-to-noise ratios.     

A current source density analysis of field potentials evoked in slices of visual cortex

Summary The method of one-dimensional current source density (CSD) analysis was applied to field potentials recorded from 350 m thick slices of the primary visual cortex of rats and cats. Field potentials were elicited by stimulation of the white matter and recorded along trajectories perpendicular to the cortical layers at spatial intervals of 25 to 50 m. The resulting CSD distributions resembled closely those recorded from the cat visual cortex in vivo. The responses with the shortest latency were distinct sinks in layers IV and VI probably reflecting monosynaptic EPSP's from specific thalamic afferents. From layer IV activity was relayed along three major routes: 1. to the supragranular layers via strong local connections to layer III and from there via both short and long range connections to layer II, 2. to targets within layer IV, and 3. to layer V. The source distributions suggest that the projections to layers III and II terminate on the proximal and distal segments, respectively, of apical dendrites of layer III pyramidal cells while the projection to layer V contacts the apical dendrites of layer VI pyramidal cells. These results indicate that all the excitatory pathways that are detectable with the CSD technique in the in vivo preparation remain intact in 350 m thick cortical slices. However, in the slice paired pulse stimulation did not lead to a depression of the response to the second stimulus while this is the case in vivo. This might be due to reduced inhibition in the slice which has been reported by several authors.     

Cortical responses to light in unanesthetized monkeys and their alteration by visual system lesions

Summary In unanesthetized rhesus monkeys, visual evoked responses were recorded and averaged from epidural electrodes over striate, prestriate, inferotemporal and frontal cortex before and after unilateral striate lesions, unilateral optic tract section and unilateral inferotemporal lesions. The early wavelets seen in the striate and prestriate responses were eliminated only by ipsilateral optic tract section or ipsilateral striate lesions. The slower components of the striate responses were drastically reduced by ipsilateral optic tract section. Contralateral optic tract section and striate ablation also reduced the amplitude of these components, but to a lesser extent. Whereas ipsilateral optic tract section markedly reduced the amplitude of the inferotemporal response, the most striking effect of ipsilateral striate ablation on this response was a reduction in its variability. The inferotemporal lesion had no effect on the striate responses. The results are related to the problems of visual input to cerebral cortex, the functions of the inferotemporal cortex and the effects of occipital damage in man.This research was begun in the Department of Psychology, Massachusetts Institute of Technology. It was supported by N.I.H. Grants MH-05673, MH-06723, MH-6418, HD-01907 and 1-K3-NB-31,816, N.S.F. Grants GB 4104 and GB 6999, N.A.S.A. Grant NsG-496 and grants to M.I.T. from the Hartford and Rockefeller Foundations. We thank Professor H.-L. xxTeuber for helpful discussions, Prof. W.J.H. xxNauta for performing the optic tract sections, Miss L. xxRolnitsky for computer programming, R.C. xxHull for experimental assistance and Mrs. C.E. xxThomson for preparation of the brain sections.     

Neural response to the second stimulus associated with poor speed discrimination performance in schizophrenia

Visual motion processing is compromised in schizophrenia (SZ), but it is uncertain what neural deviations account for their motion analysis abnormalities. Neural activations were measured with dense-array electroencephalography while 14 medicated SZ and 14 healthy persons performed a paired-stimuli forced choice speed discrimination task. SZ had (a) worse-at-speed discrimination, replicating previous findings, (b) normal early extrastriate neural activity (N1) to both motion stimuli, (c) reduced later extrastriate activity (P2) specifically to the second stimulus, and (d) following P2, an enhanced later N2 over parietal cortex. Stronger P2 and N2 responses were associated with better speed discrimination performance across groups. These findings indicate that the neural correlates of poor motion analysis in SZ may not be an early visual analysis abnormality but a problem with efficient use of speed information later in cognitive processing.     

Characterstics of operant control of centrally evoked potentials in rats

Rats were trained to increase and/or decrease the amplitude of one component of visual cortex evoked potentials. The potentials were elicited by shock of the optic chiasm. Operant control was demonstrated with both fixed and random interstimulus intervals. However, animals were better able to decrease than increase the evoked component amplitude. Significant amplitude changes were seen only in the prespecified evoked component. Potentials recorded from the contralateral visual cortex tended to show changes related to those at the trained site. These data were discussed in relation to previous research involving the operant control of peripheral evoked potential components, and in relation to arousal hypotheses utilized in other feedback studies.     

Current source density analysis: Methods and application to simultaneously recorded field potentials of the rabbit's visual cortex

This paper deals with the application of current source density (CSD) analysis to simultaneously recorded intracortical field potentials of the rabbit's visual cortex. Recordings were made with multielectrodes with either 8 contacts at distances of 300 m, or 16 contacts at distances of 150 m on one carrier needle. For synchronized activities, a spatial resolution of 150 m turned out to be sufficient to record all depth-varying details of the field potentials; for seizure potentials even a spacing of 300 m was adequate in most cases.For practical application, an appropriate spacing of the measuring points has to be chosen for a satisfactory estimation of the first and second derivatives of the field potentials. For this reason an interpolation procedure is applied to reduce the spacing from 300 m or 150 m electrode contact distances, respectively, and to obtain intermediate values at 75 m distances. With this spacing satisfactory estimations of the second derivative are obtained.Theoretically, CSD analysis has to be made three-dimensionally, but under certain conditions which are discussed, a one-dimensional analysis can be applied. An unknown quantity is _z, the vertical conductivity. It turned out that average values obtained from different experiments are not representative and that the vertical conductivity has to be measured in every experiment. This is caused by the great individual differences of the cortices even if the same stereotactic coordinates are chosen. Therefore, in every experiment relative conductivity measurements are performed. The influence of different conductivity values within the various layers and the influence of a conductivity gradient is discussed and demonstrated by examples.     

Visual evoked potential evidence for parallel processing of depth- and form-related information in human visual cortex

This paper describes the first of two complementary studies designed to identify and to investigate the properties and likely functional significance of independently generated components of scalp-recorded responses evoked by stationary patterns. These experiments compared the influence of various stimulus parameters, including site of stimulation, pattern form, nature of background field and several binocular and monocular depth cues on a single subject's visual evoked potentials. The results revealed the presence, inter alia, of two topographically distinct components with the following properties. The earlier component (C2), whose polarity depends on the stimulus location in the visual field, is: contour-specific; best evoked by discrete pattern elements, but not gratings, in the central few degrees of the visual field; insensitive to any depth cues. By contrast, the later (consistently) negative potential (LNP) is not dependent on the form of the stimulus and is larger for paracentrally (beyond 1.5°) than centrally located stimuli. It is also selectively enhanced by both monocular and binocular depth-cue stimuli, including the simulated forward movement of a pattern relative to a steady textured background; a stimulus which evokes no C2. The respective response properties of these scalp potentials suggest that there is parallel processing of depth- and contour-related features of stationary stimuli in anatomically separate regions of the human visual cortex.