Activation of cortical circuits during interictal spikes

Transient Cognitive Impairment in Epilepsy

Impairments of the dialog between excitation and inhibition (E/I) is commonly associated to neuropsychiatric disorders like autism, bipolar disorders and epilepsy. Moderate levels of hyperexcitability can lead to mild alterations of the EEG and are often associated with cognitive deficits even in the absence of overt seizures. Indeed, various testing paradigms have shown degraded performances in presence of acute or chronic non-ictal epileptiform activity. Evidences from both animal models and the clinics suggest that anomalous activity can cause cognitive deficits by transiently disrupting cortical processing, independently from the underlying etiology of the disease. Here, we will review our understanding of the influence of an abnormal EEG activity on brain computation in the context of the available clinical data and in genetic or pharmacological animal models.

Epileptiform activity in the mouse visual cortex interferes with cortical processing in connected areas

Epileptiform activity is associated with impairment of brain function even in absence of seizures, as demonstrated by failures in various testing paradigm in presence of hypersynchronous interictal spikes (ISs). Clinical evidence suggests that cognitive deficits might be directly caused by the anomalous activity rather than by its underlying etiology. Indeed, we seek to understand whether ISs interfere with neuronal processing in connected areas not directly participating in the hypersynchronous activity in an acute model of epilepsy. Here we cause focal ISs in the visual cortex of anesthetized mice and we determine that, even if ISs do not invade the opposite hemisphere, the local field potential is subtly disrupted with a modulation of firing probability imposed by the contralateral IS activity. Finally, we find that visual processing is altered depending on the temporal relationship between ISs and stimulus presentation. We conclude that focal ISs interact with normal cortical dynamics far from the epileptic focus, disrupting endogenous oscillatory rhythms and affecting information processing.

Changes in (14)C-labeled 2-deoxyglucose brain uptake from nickel-induced epileptic activity

Unilateral epidural applications of nickel solution to motor cortex were followed in about 1 h by contralateral forelimb myoclonus. In rats which displayed frequent myoclonal jerking during the 45-min 2-deoxyglucose (2-DG) uptake and clearing period, autoradiographic analysis showed that glucose utilization at the nickel implant site was greater in the supragranular and infragranular layers than in the granular layer (in normal cortex, activity is greatest in the granular layer), and was also greater in the substantia nigra and other subcortical centers. The same cortical and most of the subcortical changes in 2-DG uptake were also observed when metabolic activity was assessed 1 h after myoclonus had stopped, indicating that it may not have been the seizure activity itself that had altered metabolic activity, but some process engendered by the seizures - possibly a tissue response to excitotoxic damage. In fact, rats which displayed infrequent myoclonus showed negligible increases in cortical and subcortical uptake. These results do not support an earlier claim that increased glucose consumption is the metabolic signature of the interictal activity produced by seizure-inducing metals. Indeed, the findings raise the possibility that tissue damage is responsible for interictal hypermetabolism when it is observed in animal models of epilepsy.

Early magnetic field changes preceding the intracortical penicillin induced spikes

Events preceding interictal activity were studied using a combination of magnetoencephalography (MEG), electrocorticography (ECoG), and intracortical field potential recordings in rabbits. We measured MEG signals simultaneously with ECoG before and during interictal discharges induced by penicillin injected in the cortex (group 1: n = 12, medial cortex, regio retrosplenialis granularis; group 2: n = 4, cortical convexity, regio retrosplenialis agranularis; control group: n = 5); in group 3 (n = 12) a 16-channel depth electrode array was used to calculate the current source density in the cortical area exhibiting interictal epileptiform discharges. The modified Z-parameter as a lumped measure of magnetic field pattern changes and the global field power as a lumped measure of changes of field amplitude differences were calculated. In almost all recordings of both group 1 and 2, the Z-parameter of intra-individual MEG data became significantly larger than the control condition before the earliest change of the interictal spike recorded at the penicillin injection site (20-310 ms earlier, median: 91 ms, n = 151). The increase in Z-parameter in averaged MEG data of group 1 was significantly correlated with time as early as 790 ms before the spike (Pearson correlation coefficient, P < 0.05). After the start of the early increase of the Z-parameter, the global field power also began to increase before the ECoG spike. These results suggest a prespike field recruitment nearly 1 s before an interictal spike.

Coupling of cortical and thalamic metabolism in experimentally induced visual and somatosensory focal epilepsy

Focal epileptic activity induces widespread metabolic disturbances beyond the area of the electroencephalographically detectable focus. In order to find out whether the metabolic coupling between the epileptic focus and other brain regions depends on the localization of the focus, two groups of rats with epileptic foci at different sites were investigated. In the first group acute epileptic activity was induced by application of penicillin to the secondary visual cortex (Oc2), and in the second group to the primary somatosensory cortex (Par1). Metabolism was analyzed using the [14C]deoxyglucose autoradiographic method. In both groups of animals, hypermetabolism in the area of the focus and in specific functionally coupled thalamic nuclei was observed. Focal epileptic activity in the secondary visual cortex induced significant hypometabolism in remote ipsilateral cortical areas. In rats with epileptic foci in the primary somatosensory cortex hypometabolism in extrafocal ipsilateral cortical areas was less prominent. These findings provide further support for the integral involvement of the thalamus in modulating metabolism in remote cortical brain regions during focal epileptic activity. The extent of metabolic alterations may depend on the site of the epileptic focus and the connectivity of the recruited thalamic nuclei.

Local cerebral glucose utilization in perfusion-fixed rat brains

Local cerebral glucose utilization (LCGU) was measured in 75 cortical areas and nuclei of adult, 3-4-month-old Wistar rats, using the [14C]2-deoxyglucose (2-DG) technique. Measurement of total brain radioactivity content was not significantly different in unfixed material compared to fixed brain tissue. Values of LCGU derived from fresh, unfixed material were compared with values obtained from rats fixed by perfusion 45 min after the [14C]2-DG bolus injection with phosphate-buffered 3.3% paraformaldehyde at room temperature. In the fixed material, the mean LCGU of all brain regions was significantly increased by about 25% compared with the unfixed specimen due to tissue shrinkage of 7.2% in the fixed brains. Shrinkage leading to a higher volume density of [14C]2-deoxyglucose-6-phosphate in brain tissue results in a higher grain density in the respective autoradiographs. The wash-out of blood-borne [14C]2-DG is negligible except for blood-rich structures like the pineal gland and the choroid plexus.

Regional hypometabolism in an acute model of focal epileptic activity in the rat

Focal epileptic activity can be expected to influence distant brain areas via far reaching connections. To investigate such interactions the effects of focal epileptic activity on the metabolism of the brain were investigated in the rat cortex. Focal epileptic activity was induced by the application of penicillin onto the motor cortex. The focus, and to a lesser extent homotopic contralateral brain areas, showed an increase in the regional cerebral metabolic rate of glucose (rCMRGlc) as measured by [14C]deoxyglucose autoradiography. This focal hypermetabolism was accompanied by widespread hypometabolism lateral to the focus. The decrease of rCMRGlc occurred in somatosensory cortical areas but not in the motor cortex behind or in front of the focus, the perirhinal cortex or the occipital cortex. It was associated with an increase in metabolic rate in the ventrolateral, ventroposteromedial, ventroposterolateral and, in particular, posterior nuclei of the thalamus. It is hypothesized that the widespread reduction of rCMRGlc in the somatosensory cortical areas is due to inhibition via thalamic nuclei caused by activity in the motor cortex.

Functional neuroimaging with SPECT in children with partial epilepsy

Single-photon emission computed tomography (SPECT) is being used increasingly in the investigation of children with intractable partial seizures. SPECT imaging of regional cerebral blood flow with 99mTc-hexamethylpropylene amine oxime, iodinated radiopharmaceuticals, and 133Xe typically reveals decreased cortical perfusion interictally and increased cortical perfusion ictally in the region of the epileptic focus. Studies in both adults and children indicate significantly greater sensitivity and specificity with ictal injection of radiopharmaceutical, with interictal SPECT not infrequently revealing nonlocalizing or falsely localizing information. Recent SPECT studies employing iodinated neuroreceptor ligands report altered receptor binding in the region of the epileptic focus, providing insight into the underlying neuropharmacology of partial epilepsy. SPECT has an established role in the presurgical localization of seizure foci in children with intractable partial seizures and may be a useful modality to study the functional anatomy and clinical semiology of partial seizures in childhood.

Dynamic changes of focal hypometabolism in relation to epileptic activity

The interictal hypometabolism in patients with focal epilepsy is usually regarded as stationary. In this study we investigated to which extent the hypometabolism may depend on the activity of the epileptic focus. In focal penicillin-induced epilepsy in rats the epileptic focus is hypermetabolic. This focus is accompanied by hypometabolism in widespread areas of adjacent cerebral cortex. The experiments revealed that these metabolic alterations are transient. Data from a patient experiencing a focal seizure during PET scanning gave similar results. They showed that the transition from interictal to ictal activity was accompanied by the development of hypermetabolic epileptic focus and the dynamic enlargement of the surrounding hypometabolism. Both, the experimental and clinical data provide evidence that the cerebral hypometabolism may vary in size depending on the activity of the epileptic focus. It is hypothesized that in human PET studies the large interictal hypometabolism may prevent the identification of hyperactive interictal epileptic foci due to the partial volume effects resulting from the limited spatial resolution of PET cameras.

Interictal and postictal focal hypermetabolism on positron emission tomography

Decreased glucose utilization in the epileptogenic zone is typically observed interictally on positron emission tomography (PET), whereas ictal PET studies reveal complex patterns of increased and decreased metabolism. PET findings of 7 children, ages 2 months to 16 years, are described and demonstrate small focal regions of hypermetabolism in the absence of clinical or electrographic seizure during the 2-deoxy-2[18F]fluoro-D-glucose (FDG) uptake period. Magnetic resonance imaging scans were nonlocalizing in 5 of 7 children. In 4 children, seizures had not occurred for at least several hours prior to PET. Electroencephalography during PET disclosed active spike-and-wave activity on the side of interictal hypermetabolism in these 4 children. The remaining 3 children had seizures within 15 min prior to FDG injection and were considered postictal; their PET images revealed focal hypermetabolism. Results indicated the need for electroencephalographic monitoring during functional neuroimaging studies of all epileptic patients. The biochemical basis of interictal hypermetabolism is probably related to increased energy consumption by an active epileptogenic focus, whereas postictal hypermetabolism is likely due to energy expenditure for the restoration of resting membrane potentials and chemical homeostasis following an epileptic event.

Magnetic resonance imaging and 31P spectroscopy of an interictal cortical spike focus in the rat

Magnetic resonance imaging (MRI) has proven to be an effective noninvasive technique for identifying lesions in patients with temporal lobe epilepsy. It has also been suggested that MRI may be sensitive to transient functional or metabolic changes in brain tissue. Increased brain electrical activity as monitored by electroencephalography causes changes in cerebral metabolism that may be responsible for focal or regional alterations in signal in the MRI of some patients. To test this hypotheses, experimental interictal cortical foci were produced in rats by topical application of penicillin to one hemisphere of the brain. In vivo MRI and phosphorous-31 (31P) spectroscopy of the focal and contralateral hemifield were performed in a 30-cm bore 1.89-T Bruker MSL system. 31P spectroscopy revealed no quantifiable differences in pH or in phosphocreatinine and ATP levels between the focal area and the contralateral hemisphere or between experimental and saline-treated control animals. There were also no differences in proton MRI. Similar areas of prolonged T2 were found near the cortex and in the deeper parenchyma in 55% of the experimental animals and 50% of the controls. These results suggest that the electrical activity from an interictal cortical spike focus is not severe enough to perturb cerebral metabolism sufficiently to be detectable by 31P spectroscopy or proton imaging techniques.

Metabolic anatomy of the focal epilepsy produced by cessation of chronic intracortical GABA infusion in the rat

Cessation of chronic (5 days), unilateral infusion of GABA into the somatomotor cortex of rats induces focal epileptic spikes which remain limited to the infused site and never evolve into generalized seizures. We have considered this finding as a new model of focal epilepsy and named it "GABA withdrawal syndrome". In the present study, we have measured local cerebral glucose utilization in order to map the cortical and subcortical regions involved in the GABA withdrawal syndrome. Local cerebral glucose utilization increased two- to three-fold in a 1-1.5 mm diameter area, involving all the cortical layers at the GABA-infusion site. This hypermetabolic area contained a central (1-2 mm diameter) hypometabolic zone showing neuronal depopulation in some animals. Except for the epileptic focus, the hemisphere ipsilateral to the infusion site was slightly hypometabolic. However, there was a large increase (three- to five-fold) in some ipsilateral thalamic nuclei (posterior oralis, ventralis postero-lateralis, centralis lateralis, ventralis lateralis and reticularis thalami nucleus). The local cerebral glucose utilization of the contralateral cortex and thalamus were unchanged. The present results confirm the focal nature of the epileptogenic syndrome produced by stopping chronic, intracortical GABA infusion. These results are markedly different from those described in the penicillin focal epilepsy model. Our data also show that specific ipsilateral thalamic relays may, by an as yet unknown mechanism, play a role in maintaining paroxysmal activity during the GABA withdrawal syndrome.

Lateralized effects of migraine and ANS seizures after closed head injury

Theoretical issues associated with memory, neurocognitive and noradrenergic mechanisms in posttraumatic migraine and dysautonomic complex-partial seizure disorders are reviewed, compared and discussed. Additionally, pretreatment Contingent Negative Variation (CNV) was recorded in a No-GO/GO reaction-time paradigm for 15 normal, and 18 posttraumatic migraine and seizure patients tested not more than three months postinjury. Normals demonstrated that CNV GO and NO-GO responses significantly differed. In both migraine and seizure patients GO and NO-GO trials did not differ significantly. In uncontrolled trials, it was noted that B-Blocker administration increased the difference between GO and NO-GO trials for both migraine and seizure patients over midline leads.

Comparisons between strychnine and penicillin epileptogenesis suggest that propagating epileptiform abnormalities require the potentiation of thalamocortical circuitry in neocortical layer 4

Simultaneous recordings from three laminae within the cat visual cortex following differential intralaminar injections of strychnine (i) confirmed that low strychnine concentrations (5 mM) induce interictal-like epileptiform abnormalities (late responses) only when injected into superficial layers 2 and 3, (ii) revealed that these abnormalities are generated locally within these layers, and (iii) showed that they remain local phenomena by not spreading vertically into other cortical layers. Higher strychnine concentrations (20 mM), however, (iv) obscured these laminar differences by increasing layer 4 sensitivity to this agent in addition to the maximally sensitive superficial layers, and further (v) revealed nonlocal, vertically propagating, interictal-like abnormalities (late responses) following layer 4 injections which are preceded by an increase in thalamocortically mediated activity within this layer (enhanced physiologic responses). When penicillin was used as the convulsant, propagated interictal-like responses (late responses) induced in any layer were always preceded by a thalamocortically mediated response from layer 4 (enhanced physiologic responses); a condition clearly unlike the 5 mM but similar to the 20 mM strychnine foci observed in this study. These results suggest that convulsant action upon the thalamocortical circuitry of layer 4 is essential for the development of propagating as opposed to local epileptiform activity. Further, these results may also help explain why some cortical seizure disorders remain localized (focal) whereas others secondarily generalize to distal brain sites (i.e., complex partial seizures of extratemporal origin).

Functional mapping of limbic seizures originating in the hippocampus: a combined 2-deoxyglucose and electrophysiologic study

The pathways by which seizures spread from the hippocampus were studied both with multiple electroencephalographic recordings and 2-deoxyglucose autoradiography. The rapid kindling model described in the previous report was employed to compare mild versus severe limbic seizures. Seizures were accompanied by an increased glucose utilization in localized brain areas. The transition from mild to severe limbic seizures involved a greater spatial extent of paroxysmal electroencephalographic activity and metabolic signals. However, electrical recordings proved more sensitive in mapping seizures, as regions shown to be involved in mild or severe limbic seizures with electrical recordings did not necessarily show an increased glucose metabolism. Three types of circuits are important in dissemination of these seizures: interhippocampal connections, pathways leading out of the hippocampus to other limbic regions, and connections to certain extralimbic areas. The nucleus accumbens, amygdala, and substantia nigra emerge as important relay points in the spread of hippocampal-based seizures.

Laminar interactions during neocortical epileptogenesis

Interactions among laminar subpopulations of cat striate cortical neurons were assessed during the evolution of discrete and temporary epileptic foci, which were induced by selective microinjection of penicillin into different cortical layers. Field potentials and multiunit cellular discharges, evoked by selective visual field stimulation, were recorded simultaneously from 3 layers by multibarreled glass microelectrodes. Laminar response profiles at distinct stages of epileptogenesis were characterized for foci induced in superficial pyramidal, middle stellate, and deep pyramidal layers. Layer 4 was verified to be the most susceptible to epileptogenesis. Penicillin's action within this stellate layer appeared to be sufficient for epileptogenesis and was supportative of, if not necessary for, the development of foci originating in pyramidal cell layers. These findings could not be fully appreciated by monitoring only spontaneous interictal spike potentials. Of the two types of neuronal discharge routinely observed, early latency bursting was principally a characteristic of layer 4 stellate populations, whereas longer-latency bursts comparable to paroxysmal depolarization shifts were recorded equally well from both stellate and pyramidal layers. Epileptiform alterations in both field potential and unit responses were quickly evident in cortical laminae having known anatomic connections with the layer where the focus was induced: e.g. in layers 2-3 with layer 4 foci, in layers 5-6 with layers 2-3 foci, and in layer 4 with layers 5-6 foci. The spread of epileptogenesis was slower between laminae where pathways are purported to be less well developed, and appeared to be principally dependent upon the diffusion of penicillin.

Identification of penicillin diffusion at the onset of epileptogenesis in the cat striate cortex following differential laminar microinjection

Diffusion patterns of 14C-labeled penicillin, resulting from microinjections into different neocortical laminae, were compared with simultaneous electrophysiologic monitoring of epileptogenesis across these laminae. Injections into layer IV produced response alterations quickly in all recorded layers even with penicillin confined to this granular cell layer. Supra- and infragranular layer injections also initiated epileptic response abnormalities, though much more slowly and only after penicillin had diffused into layer IV. Injections where penicillin diffusion was shown to be limited to either of the non-granular layers did not induce epileptogenesis. These results indicate not only that layer IV is the most penicillin-sensitive neocortical layer, but that (1) it is sufficient for epileptogenesis when initially injected and, that due to this heightened sensitivity, (2) it serves as a natural trigger for epileptiform response development in other layers when they are injected first. Neurochemical and neuromorphologic correlates within striate cortex are discussed in an attempt to identify the reason for this granule cell layer sensitivity to penicillin-induced epileptogenesis.

Two levels of functional heterogeneity of the neostriatum

Autoradiography following administration of 2-deoxy-[14C]glucose has shown that penicillin-induced epileptic foci in the cerebral cortex selectively coactivate associated neostriatal regions. The present material demonstrates that this coactivation may appear in patches in the rat neostriatum. Activity from a cortical area seems to spread first to islands of the associated neostriatal region and may either spread gradually to the surrounding 'matrix' or not involve the 'matrix' for tenths of minutes. The regionally restricted functional patchiness lends additional support to the notion that the neostriatum, like the neocortex, can be divided into regions which consist of modules.

Penicillin activity in brain tissue: a method for continuous measurement

A double-barrelled microelectrode is described which permits the continuous and simultaneous measurement of penicillin concentration and of the local bioelectric activity in nervous tissue. The electrode, which is based on a chloride ligand (Corning no. 477315), has a short response time and enables reproducible measurements of penicillin concentration even below 500 IU/ml.

Functional anatomy of limbic seizures: focal discharges from medial entorhinal cortex in rat

Focal seizure discharges were induced in the ventral aspect of the medial entorhinal cortex of awake, freely moving rats, either with cannula injections of penicillin or picrotoxin (0.02 microliters every 10-15 min) or by repetitive tetanic electrical stimulation. [14C]Deoxyglucose autoradiography (DG) was performed when animals were in a 'steady-state' with respect to electrographic discharges and/or behavioral changes. During simple interictal spikes behavior remained normal and DG labeling was increased only in the entorhinal focus and stratum moleculare of the ventral dentate gyrus. With complex spikes and short seizures animals exhibited staring, decreased responsiveness, and occasional wet dog shakes. DG labeling was increased in all layers of the dentate gyrus, Ammon's horn (ipsilateral greater than contralateral) and, to a lesser degree, in ipsilateral amygdala, and the accumbens-ventral pallidum area. During strong seizures, rearing and forelimb clonus occurred and metabolism was strongly activated bilaterally in the hippocampal formation, amygdala, accumbens, substantia nigra, and the anterior and periventricular thalamic nuclei. These studies indicate that the dentate gyrus initially restricts the entry of seizures from entorhinal cortex into the rest of hippocampus. As this is overcome there is rapid bilateral spread through the hippocampal formation with passive interruption of normal behavior. With prolonged seizure discharges there is further capture of amygdala and subcortical extrapyramidal and thalamic nuclei associated with behavioral convulsions.

Cellular mechanisms underlying the inhibitory surround of penicillin epileptogenic foci

Penicillin applied locally to the surface of in vivo hippocampus or neocortex is known to produce not only periodic interictal spikes within a focus, but also altered cellular activities around the focus. In the zone adjacent to a hippocampal focus, for example, Dichter and Spencer recorded hyperpolarization-depolarizing burst-hyperpolarization sequences, while further from the focus they recorded only hyperpolarizations (presumably IPSPs). We use here our previously developed model of interictal spike generation to show that these observations can be explained by the following two assumptions: (1) there is a gradient of effectiveness of synaptic inhibition (small within the focus and increasing with distance from the focus) caused by the gradient of penicillin concentration; and (2) the radius for recurrent inhibition is larger than the radius for recurrent excitation. Our model best fits the experimental data if we assume further that recurrent excitation extends from any one small cortical region to only some--not all--of its neighboring regions.

Focal cortical seizures cause distant thalamic lesions

Topical application of convulsants to the rat sensorimotor cortex in concentrations sufficient to cause repetitive focal motor seizures resulted in acute neuropathology (dark cell neuronal degeneration and spongiform neurophil changes) involving both the cortical seizure focus and certain thalamic nuclei within seizure pathways. Changes in the cortex were localized primarily in layer IV and those in the thalamus in nuclei having reciprocal connections with the cortical focus. The spongiform neuropil changes consisted of massively dilated presynaptic axon terminals in the cortex and postsynaptic dendrites in the thalamus. The dendritic and dark cell changes resemble the excitotoxic damage caused by glutamate and aspartate. Since these putative transmitters may be released locally from recurrent collaterals and remotely from corticothalamic axons, excessive release of glutamate or aspartate may account for the changes in both sites. The abnormal axons in sensory cortex appear to be terminals of thalamocortical neurons. Swelling of these axons may be caused by excessive anti- and orthodromic firing in the course of focal motor seizures.

Focal seizures disrupt protein synthesis in seizure pathways: an autoradiographic study using [1-14C]leucine

We have used a new autoradiographic technique developed by Smith et al.22,33 for visualizing rates of incorporation of [1-14C]leucine into protein in brain. Focal seizures caused by topical convulsants resulted in a marked decrease in autoradiographic density. This was primarily confined to the seizure focus, especially marked in pyramidal cell layers, and to subcortical seizure pathways. There were no distinct changes in cortico-cortical pathways beyond the seizure focus. Pure orthodromic pathways through basal ganglia showed an 18% inhibition of leucine incorporation in caudate nucleus and substantia nigra, pars compacta (P less than 0.05). By contrast, thalamic nuclei connected both ortho- and antidromically to the focus showed a 30-63% inhibition (P less than 0.01). The topographic pattern and intensity of the thalamic changes were related to the site, size and intensity of the seizure focus. As seizures became severe there was a more generalized depression of metabolism beyond seizure pathways, especially in the ipsilateral hemisphere. The results suggest that seizures block incorporation of leucine into protein either by an increase oxidation of the precursor, and/or an inhibition of protein synthesis per se. The effect is most severe in neurons undergoing epileptic burst discharge in the focus and in thalamic neuronal beds connected reciprocally with the focus.

Metabolic response to optic centers to visual stimuli in the albino rat: anatomical and physiological considerations

The functional organization of the visual system was studied in the albino rat. Metabolic differences were measured using the 14-C-2-deoxyglucose (DG) autoradiographic technique during visual stimulation of one entire retina in unrestrained animals. All optic centers responded to changes in light intensity but to different degrees. The greatest change occurred in the superior colliculus, less in the lateral geniculate, and considerably less in second-order sites such as layer IV of visual cortex. These optic centers responded in particular to on/off stimuli, but showed no incremental change during pattern reversal or movement of orientation stimuli. Both the superior colliculus and lateral geniculate increased their metabolic rate as the frequency of stimulation increased, but the magnitude was twice as great in the colliculus. The histological pattern of metabolic change in the visual system was not homogenous. In the superior colliculus glucose utilization increased only in stratum griseum superficiale and was greatest in visuotopic regions representing the peripheral portions of the visual field. Similarly, in the lateral geniculate, only the dorsal nucleus showed an increased response to greater stimulus frequencies. Second-order regions of the visual system showed changes in metabolism in response to visual stimulation, but no incremental response specific for type or frequency of stimuli. To label proteins of axoplasmic transport to study the terminal fields of retinal projections 14C-amino acids were used. This was done to study how the differences in the magnitude of the metabolic response among optic centers were related to the relative quantity of retinofugal projections to these centers. Fast and slow axoplasmic transport were studied using three separate amino acids. In each case over 64% of the radioactivity projecting contralateral from the eye was found in superior colliculus. considerably less isotope was found in dorsal lateral geniculate (11-17%), ventral lateral geniculate (3, 7-6.2%), pretectal nuclei (5-12%), and the accessory optic system (3-7%). The greatest concentration of radioactivity within each optic center was found in the visuotopic aspect subserving the superior visual field; particularly the medial aspects of the superior colliculus, olivary pretectal nucleus, and posterior pretectal nucleus, and the anterior portion of the nucleus of the optic tract. The representation of central vision in the colliculus was relatively pale, as was a zone within the middle of the contralateral dorsal lateral geniculate. The anatomical and physiological results of this study suggest that differences in deoxyglucose metabolism among optic centers are primarily related to the number of retinofugal endings and the kind of visual stimulation. Changes within any one center primarily reflect the density of retinal endings subserving the visual field.