Horizontal propagation of visual activity in the synaptic integration field of area 17 neurons

The receptive field of a visual neuron is classically defined as the region of space (or retina) where a visual stimulus evokes a change in its firing activity. At the cortical level, a challenging issue concerns the roles of feedforward, local recurrent, intracortical, and cortico-cortical feedback connectivity in receptive field properties. Intracellular recordings in cat area 17 showed that the visually evoked synaptic integration field extends over a much larger area than that established on the basis of spike activity. Synaptic depolarizing responses to stimuli flashed at increasing distances from the center of the receptive field decreased in strength, whereas their onset latency increased. These findings suggest that subthreshold responses in the unresponsive region surrounding the classical discharge field result from the integration of visual activation waves spread by slowly conducting horizontal axons within primary visual cortex.     

A model for the neuronal implementation of selective visual attention based on temporal correlation among neurons

We propose a model for the neuronal implementation of selective visual attention based on temporal correlation among groups of neurons. Neurons in primary visual cortex respond to visual stimuli with a Poisson distributed spike train with an appropriate, stimulus-dependent mean firing rate. The spike trains of neurons whose receptive fields do not overlap with the "focus of attention" are distributed according to homogeneous (time-independent) Poisson process with no correlation between action potentials of different neurons. In contrast, spike trains of neurons with receptive fields within the focus of attention are distributed according to non-homogeneous (time-dependent) Poisson processes. Since the short-term average spike rates of all neurons with receptive fields in the focus of attention covary, correlations between these spike trains are introduced which are detected by inhibitory interneurons in V4. These cells, modeled as modified integrate-and-fire neurons, function as coincidence detectors and suppress the response of V4 cells associated with non-attended visual stimuli. The model reproduces quantitatively experimental data obtained in cortical area V4 of monkey by Moran and Desimone (1985).     

The influence of the visual cortex on the spatiotemporal response properties of lateral geniculate nucleus cells

Previous studies of the cortical input to the mammalian dorsal lateral geniculate nucleus (LGN) have identified a number of possible functions for the corticogeniculate pathway, including alteration of LGN spatial frequency selectivity and facilitation of both binocular interactions and orientation selectivity. These changes may be due to either a tonic or a phasic cortical facilitation or both. The temporal differences between each of these inputs suggests that their impact on LGN cell temporal tuning should be unique. To test this hypothesis, we reversibly blocked the visual cortex (VI) and measured the effects on several indices of the temporal properties of LGN cells, including peak frequency, bandwidth, and response phase. Macaque monkeys were anesthetized and paralyzed during single cell recording from the LGN while area VI was cryogenically deactivated. Single-cell responses were visually evoked with drifting, luminance-modulated, sine-wave gratings and discrete-Fourier analyzed. Cortical cooling produced statistically significant increases or decreases in response amplitude in 64% of cells recorded. In most cases, alterations in response amplitude occurred for stimuli that varied in spatial as well as temporal frequency. For those cells influenced by changes in stimulus temporal frequency, the majority showed changes over a broad range of frequencies. A minority of cells showed changes in either peak temporal tuning or temporal frequency bandwidth. Response phase angles for all temporal frequencies tested were unaffected by cortical cooling. Overall, these results suggest that the cortical input may alter the temporal response properties of LGN cells, perhaps by tonic, but not exclusively excitatory, corticofugal influences.     

Effects of cortical stimulation on red nucleus neurons in the guinea pig

The aim of the present work was to study the control that the cerebral cortex exerts on red nucleus (RN) neurons in the guinea pig. The experiments were carried out in anaesthetized animals. Electrical stimulation of localized cortical foci was performed by tungsten microelectrodes in frontal and parietal regions containing sensorimotor representations of the body. Single unit RN activity was extracellularly recorded through glass micropipettes, and the encountered RN neurons were recognized by searching their peripheral receptive field. Then, corticorubral influences were tested on RN neurons whose receptive field was located in the same body regions where motor responses were evoked by cortical stimulation. The stimulation with a single pulse evoked complex responses, typically consisting of long lasting inhibitions sometimes preceded by a weak facilitation and always followed by an excitatory rebound. The application of a second pulse modified this pattern, depending on the time interval between the two stimuli. In fact, the reduction of the interval below 300 ms enhanced the excitatory components whereas it shortened the inhibitory component; moreover, an "early" facilitation was evoked but only at intervals as short as 50-150 ms, or less. These results suggest that the corticorubral control may vary according to different levels of cortical activation, becoming more and more facilitatory as the cortical discharges increase from low frequency values (tonic activity) towards high frequency values (phasic activity).     

Transformation of peripheral inputs by the first-order lateral line brainstem nucleus

Extracellular recording techniques were used to record the responses of medial nucleus cells and posterior lateral line nerve fibers in mottled sculpin, Cottus bairdi, and goldfish, Carassius auratus, to a 50-Hz dipole source (vibrating sphere). Responses were characterized in terms of (1) receptive fields that relate responsiveness (spike rate and phase-locking) to the location of the source along the length of the fish, (2) input-output functions that relate responsiveness to vibration amplitude for a fixed source location, and (3) peri-stimulus time histograms that relate responsiveness to time during a sustained period of vibration. Relative to posterior lateral line nerve fibers, medial nucleus cells in both species were similar in showing (1) lower spontaneous and evoked rates of spike activity, (2) greater degrees of adaptation, (3) greater heterogeneity in all response characteristics, and (4) evidence for inhibitory/excitatory interactions. Whereas receptive fields of nerve fibers in both species faithfully reflect both pressure gradient amplitudes (with rate changes) and directions (with phase-angle changes) in the stimulus field, receptive fields of medial nucleus were more difficult to relate to the stimulus field, Some, but not all, receptive fields could be modeled with excitatory center/inhibitory surround and inhibitory center/excitatory surround organizations.     

A model for the early stages of motion processing based on spatial and temporal edge detection by X-cells

A model for the early stages of motion processing in the visual cortex is presented. The 'building block' for this model is the 'rebound response', which is the neuronal response evoked when a sufficient inhibitory stimulus is turned off. This response enables detection of temporal changes when the stimulus involves spatial changes. The model suggests that adjacent subunits in primary cortical cells have different weight functions for rebound responses, and thus a synergistic type of response is evoked in the preferred direction, which is predicted for both light and dark stimuli. Predictions of the model for different stimuli and receptive field structures are discussed. It appears to be more economical than previous motion models.     

Effects of saccades on the activity of neurons in the cat lateral geniculate nucleus

Effects of saccades on individual neurons in the cat lateral geniculate nucleus (LGN) were examined under two conditions: during spontaneous saccades in the dark and during stimulation by large, uniform flashes delivered at various times during and after rewarded saccades made to small visual targets. In the dark condition, a suppression of activity began 200-300 ms before saccade start, peaked approximately 100 ms before saccade start, and smoothly reversed to a facilitation of activity by saccade end. The facilitation peaked 70-130 ms after saccade end and decayed during the next several hundred milliseconds. The latency of the facilitation was related inversely to saccade velocity, reaching a minimum for saccades with peak velocity >70-80 degrees /s. Effects of saccades on visually evoked activity were remarkably similar: a facilitation began at saccade end and peaked 50-100 ms later. When matched for saccade velocity, the time courses and magnitudes of postsaccadic facilitation for activity in the dark and during visual stimulation were identical. The presaccadic suppression observed in the dark condition was similar for X and Y cells, whereas the postsaccadic facilitation was substantially stronger for X cells, both in the dark and for visually evoked responses. This saccade-related regulation of geniculate transmission appears to be independent of the conditions under which the saccade is evoked or the state of retinal input to the LGN. The change in activity from presaccadic suppression to postsaccadic facilitation amounted to an increase in gain of geniculate transmission of approximately 30%. This may promote rapid central registration of visual inputs by increasing the temporal contrast between activity evoked by an image near the end of a fixation and that evoked by the image immediately after a saccade.     

The influence of carbohydrate-feeding and insemination on oviposition of an Indiana strain of Aedes vexans (Diptera: culicidae)

In acute experiments on unanaesthetized cats immobilized with listenon, a study was made ofthe variability and interdependence of evoked responses to photic stimulation in the thalamo-cortical formations of the visual projection and the associative systems: the lateral geniculate body, pulvinar and the visual and parietal associative areas of the cerebral cortex. It has been shown that the variation coefficient of the amplitude from the maximum of the positive to the maximum of the negative oscillations in the parietal cortex statistically significantly exceeds the one in the lateral geniculate body and the projection cortex. Coefficients of paired correlation of evoked potentials' amplitudes to light more frequently reach significant levels for pairs consisting of one level formations, cortical or thalamic, than for pairs including formations of different levels.     

A model of cortically induced synchronization in the lateral geniculate nucleus of the cat: a role for low-threshold calcium channels

Recently Sillito et al. (Nature 1994;369:479-82) discovered correlations in the spike trains of a relatively distant pair of cat lateral geniculate nucleus cells when simultaneously stimulated by a drifting grating; no such correlation occurs when the visual cortex is removed. In a further analysis of the data, we have found that short, high-frequency bursts contribute substantially to the synchronization and we hypothesize that the origin of the bursts is the low-threshold calcium spike. Guided by this hypothesis, our model of the corticogeniculate pathway and early visual system reproduces the experimental data in nearly every detail, as well as making predictions about cortical activity during the synchronizing process. We also discuss the possible behavioral relevance of correlations in the geniculo-cortical loop as well as other neural systems.     

Visual receptive-field properties of single neurons in cat's ventral lateral geniculate nucleus

1. Visual receptive-field characteristics were determined for 154 cells in the ventral lateral geniculate nucleus (VLG) of cats anesthetized with nitrous oxide. All cells were verified histologically to be within the VLG. Responses of 182 cells from laminae A and A1 of the dorsal lateral geniculate nucleus (DLG) were tested for comparison. 2. The VLG cells could be grouped into one of seven classes according to their responses to light stimulation. Twenty-seven percent of the cells had uniform receptive fields. They responded maximally to stationary stimuli flashed on or off anywhere within the receptive field and showed no evidence for antagonistic surround mechanisms. About 19.5% of the VLG cells had concentric receptive fields. They were similar to the uniform type, with the addition of a concentric inhibitory surround. Eight percent of the VLG cells had ambient receptive fields. These cells were characterized by an unusually regular maintained discharge which varied in rate in relation to the level of receptive-field illumination or of full-field ambient illumination. About 4% of the VLG cells were movement sensitive. They gave little or no response to stationary stimuli flashed on or off in the receptive field, and responded best to a contour moving across the receptive field in any direction. An additional 2.5% of the VLG cells were direction sensitive. Their response was dependent on the direction of stimulus movement through the receptive field. Sixteen percent of the VLG cells had indefinite receptive fields. They responded to whole-eye illumination or to localized visual-field stimulation; however, specific receptive-field properties could not be adequately defined. Approximately 23% of the VLG cells studied gave no convincing response to visual stimulation. 3. Responses of DLG cells agreed with those reported in previous studies. Almost all (97%) had concentric receptive fields, and a few (3%) had uniform receptive fields with no apparent antagonistic surround. None of the DLG cells had receptive fields like those in the other classes found for VLG cells. 4. The VLG cells tended to have large receptive fields; mean diameter was 10.6 degrees of visual arc. This was substantially larger than the diameter of receptive fields for DLG cells. In addition, VLG cells generally required larger stimuli than DLG cells to respond. There was no consistent relationship between receptive-field size and visual-field eccentricity for VLG cells, in contrast to the DLG. Most (57%) VLG cells were driven only by the contralateral eye, 30% were binocularly driven, and 13% were driven only by the ipsilateral eye. 5. A systematic visuotopic organization was present in the VLG. The lower visual field was represented anteriorly in the nucleus and the upper visual field posteriorly. The vertical meridian was represented along the dorsomedial border of the VLG where it abuts the DLG, and the temporal periphery was represented ventrolaterally. 6. Responses to electrical stimulation of the optic chiasm were studied for 55 VLG cells...     

Long-term observations of uterine contractions in nonpregnant dogs

Uterine contractile activity in nonpregnant conscious dogs was investigated based on 2- to 6-mo-long continuous recording by means of a chronically implanted force transducer. We found that nonpregnant uterine contractile activity could be classified into six major patterns: sporadic contractions, weak and strong tonic contractions, weak and strong phasic contractions, and phasic contraction bursts. The contractile patterns during proestrus and estrus were the most active, with strong phasic and tonic contractions and phasic contraction bursts. The phasic and tonic contractions were inhibited dose-dependently by a beta 2 adrenergic agonist, ritodrine, and reproduced by an alpha 2 adrenergic agonist, clonidine. In contrast, the cholinergic inhibitors atropine and hexamethonium did not affect the spontaneous occurrence of these contractions, although bethanechol evoked uterine contractions. Oxytocin and prostaglandin F2 alpha-induced contractions were phasic during estrus, whereas they showed tonic increases with phasic contractions during proestrus, diestrus, and anestrus, and these contractions did not resemble the spontaneous contractions. In conclusion, the nonpregnant uterus contracts continuously in harmony with the estrous cycle phases, and its contractile activity is enhanced by alpha adrenergic receptors and inhibited by beta 2 adrenergic receptors.     

Temporal dynamics of chromatic tuning in macaque primary visual cortex

The ability to distinguish colour from intensity variations is a difficult computational problem for the visual system because each of the three cone photoreceptor types absorb all wavelengths of light, although their peak sensitivities are at relatively short (S cones), medium (M cones), or long (L cones) wavelengths. The first stage in colour processing is the comparison of the outputs of different cone types by spectrally opponent neurons in the retina and upstream in the lateral geniculate nucleus. Some neurons receive opponent inputs from L and M cones, whereas others receive input from S cones opposed by combined signals from L and M cones. Here we report how the outputs of the L/M- and S-opponent geniculate cell types are combined in time at the next stage of colour processing, in the macaque primary visual cortex (V1). Some V1 neurons respond to a single chromatic region, with either a short (68-95 ms) or a longer (96-135 ms) latency, whereas others respond to two chromatic regions with a difference in latency of 20-30 ms. Across all types, short latency responses are mostly evoked by L/M-opponent inputs whereas longer latency responses are evoked mostly by S-opponent inputs. Furthermore, neurons with late S-cone inputs exhibit dynamic changes in the sharpness of their chromatic tuning over time. We propose that the sparse, S-opponent signal in the lateral geniculate nucleus is amplified in area V1, possibly through recurrent excitatory networks. This results in a delayed, sluggish cortical S-cone signal which is then integrated with L/M-opponent signals to rotate the lateral geniculate nucleus chromatic axes.     

The receptive field of the primate P retinal ganglion cell, II: Nonlinear dynamics

The ganglion cells of the primate retina include two major anatomical and functional classes: P cells which project to the four parvocellular layers of the lateral geniculate nucleus (LGN), and M cells which project to the two magnocellular layers. The characteristics of the P-cell receptive field are central to understanding early form and color vision processing (Kaplan et al., 1990; Schiller & Logothetis, 1990). In this and in the following paper, P-cell dynamics are systematically analyzed in terms of linear and nonlinear response properties. Stimuli that favor either the center or the surround of the receptive field were produced on a CRT and modulated with a broadband signal composed of multiple m-sequences (Benardete et al., 1992b; Benardete & Victor, 1994). The first-order responses were calculated and analyzed in this paper (part I). The findings are: (1) The first-order responses of the center and surround depend linearly on contrast. (2) The dynamics of the center and surround are well described by a bandpass filter model. The most significant difference between center and surround dynamics is a delay of approximately 8 ms in the surround response. (3) In the LGN, these responses are attenuated and delayed by an additional 1-5 ms. (4) The spatial transfer function of the P cell in response to drifting sine gratings at three temporal frequencies was measured. This independent method confirmed the delay between the (first-order) responses of the center and surround. This delay accounts for the dependence of the spatial transfer function on the frequency of stimulation.     

A quantitative description of short-term plasticity at excitatory synapses in layer 2/3 of rat primary visual cortex

Cortical synapses exhibit several forms of short-term plasticity, but the contribution of this plasticity to visual response dynamics is unknown. In part, this is because the simple patterns of stimulation used to probe plasticity in vitro do not correspond to patterns of activity that occur in vivo. We have developed a method of quantitatively characterizing short-term plasticity at cortical synapses that permits prediction of responses to arbitrary patterns of stimulation. Synaptic responses were recorded intracellularly as EPSCs and extracellularly as local field potentials in layer 2/3 of rat primary visual cortical slices during stimulation of layer 4 with trains of electrical stimuli containing random mixtures of frequencies. Responses exhibited complex dynamics that were well described by a simple three-component model consisting of facilitation and two forms of depression, a stronger form that decayed exponentially with a time constant of several hundred milliseconds and a weaker, but more persistent, form that decayed with a time constant of several seconds. Parameters obtained from fits to one train were used to predict accurately responses to other random and constant frequency trains. Control experiments revealed that depression was not caused by a decrease in the effectiveness of extracellular stimulation or by a buildup of inhibition. Pharmacological manipulations of transmitter release and postsynaptic sensitivity suggested that both forms of depression are mediated presynaptically. These results indicate that firing evoked by visual stimuli is likely to cause significant depression at cortical synapses. Hence synaptic depression may be an important determinant of the temporal features of visual cortical responses.     

Role of thalamic and cortical neurons in augmenting responses and self-sustained activity: dual intracellular recordings in vivo

Progressively increasing (augmenting) responses are elicited in thalamocortical systems by repetitive stimuli at approximately 10 Hz. Repeated pulse trains at this frequency lead to a form of short-term plasticity consisting of a persistent increase in depolarizing synaptic responses as well as a prolonged decrease in inhibitory responses. In this study, we have investigated the role of thalamocortical (TC) and neocortical neurons in the initiation of thalamically and cortically evoked augmenting responses. Dual intracellular recordings in anesthetized cats show that thalamically evoked augmenting responses of neocortical neurons stem from a secondary depolarization (mean onset latency of 11 msec) that develops in association with a diminution of the early EPSP. Two nonexclusive mechanisms may underlie the increased secondary depolarization during augmentation: the rebound spike bursts initiated in simultaneously recorded TC cells, which precede by approximately 3 msec the onset of augmenting responses in cortical neurons; and low-threshold responses, uncovered by hyperpolarization in cortical neurons, which may follow EPSPs triggered by TC volleys. Thalamic stimulation proved to be more efficient than cortical stimulation at producing augmenting responses. Stronger augmenting responses in neocortical neurons were found in deeply located (<0.8 mm, layers V-VI) regular-spiking and fast rhythmic-bursting neurons than in superficial neurons. Although cortical augmenting responses are preceded by rebound spike bursts in TC cells, the duration of the self-sustained postaugmenting oscillatory activity in cortical neurons exceeds that observed in TC neurons. These results emphasize the role of interconnected TC and cortical neurons in the production of augmenting responses leading to short-term plasticity processes.     

Sensory responsiveness of "broad-spike" neurons in the laterodorsal tegmental nucleus, locus coeruleus and dorsal raphe of awake rats: implications for cholinergic and monoaminergic neuron-specific responses

Although cholinergic neurons in the laterodorsal and pedunculopontine tegmental nuclei have been shown to have a pivotal role in neural mechanisms of paradoxical sleep, their function during wakefulness is less understood. To examine the latter, we have recorded from "broad-spike neurons", which were distinguished by their long spike duration, in the laterodorsal tegmental nucleus of undrugged, head-restrained rats, and examined their response properties to sensory stimuli such as light touch to the tail, air puff to the face, 2 kHz pure tone and flashes of light. Broad-spike neurons from the locus coeruleus and dorsal raphe nucleus were studied for comparison; these neurons have been demonstrated to be noradrenergic and serotonergic, respectively. The broad-spike neurons in the laterodorsal tegmental nucleus have also been suggested to be cholinergic. There were two kinds of responses: (1) a simple increase or decrease in firing, reflecting an elevated level of vigilance; and (2) a phasic response composed of a single spike or brief, high frequency burst, usually diminishing or disappearing upon repetition of the stimulus. When two or more types of stimuli were effective in a neuron, they evoked responses of the same quality. Most of the dorsal raphe neurons displayed only the simple increase of firing, whereas the locus coeruleus neurons gave a phasic response with rather weak attenuation upon repetition. Compared with these, the laterodorsal tegmental neurons were heterogeneous: about one-quarter showing only a simple change of firing (half increasing, half decreasing); and two-thirds displaying phasic responses. The latter response of many neurons attenuated strongly upon repetition. The laterodorsal tegmental neurons were classified into several groups according to their spontaneous firing behavior during sleep and wakefulness, but every neuron in a group did not show the same type of response. For example, some of the neurons which were most active during paradoxical sleep and essentially silent during wakefulness decreased or stopped firing upon sensory stimulation, while others in this group had strong phasic responses. These results suggest that putative cholinergic neurons in the laterodorsal tegmental nucleus have heterogenous properties not only with respect to their spontaneous activity during sleep and wakefulness but also with respect to their response to sensory stimulation. Some of these neurons may function to induce a global attentive state in response to a novel stimulus.     

Visuo-oculomotor properties of cells in the superior colliculus of the alert cat

Visual responses and eye movement (EM) -related activities were studied in single units of the superior colliculus (SC) of alert cats. Spontaneous EMs were encouraged by training. Throughout the SC (i.e., in intermediate and deep layers as well as in superficial layers), units were found to respond well to visual stimuli. Strong and consistent responses could be elicited by very dim, low-contrast stationary stimuli. Visual responses varied from phasic to tonic; some units responded tonically to stationary stimuli in the center of the receptive field, and phasically to peripheral stimuli. Many cells responded more vigorously to moving than to stationary stimuli, but very few responded exclusively to stimulus movement. The vast majority of cells were directionally selective. A small number of units were sensitive to the absolute, as well as the retinal, position of visual stimuli. These cells were activated by visual stimuli which fell in the receptive field only if the cat's gaze was fixated on one half of the screen. It seems that these cells must receive information about both eye position and the retinal (receptive field) position of the stimulus. It is possible that they reflect coding of target location within a head (or body) frame of reference. EM-related units were of two types: (1) about 20% of the sample responded prior to spontaneous or visually-triggered EMs, and (2) another 10% (or more) responded with, but not before, EMs. Some cells in the second group discharge almost synchronously with EMs and, thus, cannot plausibly be said to respond to the movement of images across the retina. All cells in the first group were directionally selective. The percentage of EM-related cells in the deep layers of SC is lower in cat than in monkey. Possible reasons for such differences are discussed.     

Dynamics of membrane excitability determine interspike interval variability: a link between spike generation mechanisms and cortical spike train statistics

We propose a biophysical mechanism for the high interspike interval variability observed in cortical spike trains. The key lies in the nonlinear dynamics of cortical spike generation, which are consistent with type I membranes where saddle-node dynamics underlie excitability (Rinzel & Ermentrout, 1989). We present a canonical model for type I membranes, the theta-neuron. The theta-neuron is a phase model whose dynamics reflect salient features of type I membranes. This model generates spike trains with coefficient of variation (CV) above 0.6 when brought to firing by noisy inputs. This happens because the timing of spikes for a type I excitable cell is exquisitely sensitive to the amplitude of the suprathreshold stimulus pulses. A noisy input current, giving random amplitude "kicks" to the cell, evokes highly irregular firing across a wide range of firing rates; an intrinsically oscillating cell gives regular spike trains. We corroborate the results with simulations of the Morris-Lecar (M-L) neural model with random synaptic inputs: type I M-L yields high CVs. When this model is modified to have type II dynamics (periodicity arises via a Hopf bifurcation), however, it gives regular spike trains (CV below 0.3). Our results suggest that the high CV values such as those observed in cortical spike trains are an intrinsic characteristic of type I membranes driven to firing by "random" inputs. In contrast, neural oscillators or neurons exhibiting type II excitability should produce regular spike trains.     

Cortical Responses in Rats Predict Perceptual Sensitivities to Complex Sounds.

The common assumption that perceptual sensitivities are related to neural representations of sensory stimuli has seldom been directly demonstrated. The authors analyzed the similarity of spike trains evoked by complex sounds in the rat auditory cortex and related cortical responses to performance in an auditory task. Rats initially learned to identify 2 highly different periodic, frequency-modulated sounds and then were tested with increasingly similar sounds. Rats correctly classified most novel sounds; their accuracy was negatively correlated with acoustic similarity. Rats discriminated novel sounds with slower modulation more accurately than sounds with faster modulation. This asymmetry was consistent with similarities in cortical representations of the sounds, demonstrating that perceptual sensitivities to complex sounds can be predicted from the cortical responses they evoke. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Causes of initial visual loss following blunt ocular injuries: an experimental study

We have investigated in anaesthetised pigs the electrical responses of the retina to light after blunt injuries to the eye. Retinograms, cortical visual evoked responses and intraocular pressures were recorded before and after non-perforating mechanical trauma of sufficient severity to produce traumatic retinal opacification. After injury, applanation pressures rose up to 34 mm. Hg above normal, reaching peak values 4 to 12 minutes after trauma and returning to pre-trauma levels after about 45 minutes. These pressure changes were not responsible for the immediate abolition of the B wave of the retinogram or of the cortical visual evoked responses which occurred after trauma, but increases of intraocular pressure modified the recovery pattern of the visual evoked response.