Projections from cortical area SmII and claustrum to two functional subdivisions of SmI forepaw digit cortex of the raccoon

The retrograde HRP tracer method was used to study the projections from cortical area SmII and the claustrum to two electrophysiologically defined, functionally distinct subdivisions of the SmI forepaw cortex of the raccoon. Individual SmI cortical digit zones were found to receive ipsilateral projections from SmII and immediately adjoining cortical regions; the projections to the "heterogeneous" (hairy skin and claw) functional subdivision of a given digit zone were considerably more extensive than those to the glabrous skin functional subdivision of that zone. HRP-filled neurons within SmII were located primarily in layers VI and V, often formed clusters, and were distributed antero-posteriorly in a manner consistent with a loosely topographic representation of the digits. The SmI cortical digit zones received ipsilateral projections from approximately the middle 1/3 of the anterior-posterior extent of the insular claustrum; no clear difference was found in the projections to the two functional subdivisions of a given digit zone. Labeled neurons were typically scattered throughout much of the claustrum but were more numerous in its dorsal regions, tended to aggregate in clusters, and were distributed antero-posteriorly in an overlapping but roughly topographic fashion.     

Physiological changes in the somatosensory forepaw cerebral cortex of adult raccoons following lesions of a single cortical digit representation

The purpose of this study was to determine whether restricted lesions within primary somatosensory (SmI) cortex cause changes in the functional organization of cortical areas bordering on the site of injury. Focal ablations of cortical tissue were made in the representational area for digit 3 within the SmI forepaw cortex of adult raccoons. Electrophysiological mapping experiments done 15-17 weeks later showed that significant alterations had occurred in the response properties of clusters of neurons within those representational zones adjoining the lesion--the zones for digit 2, digit 4, and the palmar pads. These three cortical areas were modified by the appearance of new, usually weaker secondary inputs and changes in some properties of the normal primary inputs from the forepaw. (i) Many neurons responded to stimulation of previously ineffective skin regions; the new inputs often originated from digit 3 but frequently involved other digits or the pads as well. (ii) Neuronal receptive fields (RFs), mapped at a standard suprathreshold stimulus intensity, were larger than normal. (iii) Skin type and submodality sensitivity typically were less specific than normal; more neurons had RFs that included both glabrous and hairy skin or claws and displayed mixtures of responsiveness to skin touch, hair deflection, or claw touch. (iv) The representation of RF location, skin type, and submodality sensitivity was more variable as a function of horizontal and vertical distance through the cortex. In general, the physiological changes were found to degrade the somatotopic order and response specificity of the intact cortical areas adjoining the lesion.(ABSTRACT TRUNCATED AT 250 WORDS)     

The second somatic sensory area (smII) of opossum neocortex

Organization of the neocortical second somatic sensory area (SmII) of anesthetized Virginia opossums has been examined utilizing microelectrode recording techniques. SmII is situated between the first somatic sensory area (SmI) medially, and the rhinal fissure laterally. The head representation is located anteromedially within SmII, and the hindlimb representation posterolaterally, with the forelimb representation in between. Approximately 49% of SmII is devoted to representation of the head, 36% to forelimb representation, and 15% to trunk and hindlimb representation. All peripheral receptive fields (RF's) were either contralateral or bilateral. Approximately 63% of head RF's, 25% of forelimb RF's and 100% of hindlimb RF's were bilateral. For a given body locus, SmII RF's are larger than those for SmI. SmII is contained entirely within an area yielding evoked potentials responses to auditory click stimuli.     

Opossum somatic sensory cortex: a microelectrode mapping study.

Organization of opossum somatic sensory cortex has been investigated utilizing closely spaced microelectrode penetrations (0.25-0.5 mm apart) and delicate mechanical stimulation of body surfaces including the facial vibrissae. Results may be summarized as follows: (1) the general organization of somatic sensory cortex, as originally defined by Lende ('63a) has been confirmed; (2) a double representation of the contralateral mystacial vibrissae and rhinarium, implicit in Lende's original data, was revealed in detail, the two representations being orderly, adjacent, mirror-images of each other; (3) units at a given cortical locus responded to deflection of between one and five mystacial vibrissae, about half responding to movement of a single vibrissa only; (4) about 40% of mystacial vibrissa units showed a directional specificity to the extent that they responded to deflections in only one or two cardinal directions; (5) units located in the medial vibrissa area showed a greater directional specificity than did units located in the lateral vibrissa area; (6) the surface area of rhinarial receptive fields was about ten times the area of first-order rhinarial unit receptive fields (B. Pubols et al., '73); (7) representation of the contralateral forelimb, especially the ventral surface of the forepaw, is extensive, orderly, and precise; (8) representation of the contralateral hindlimb, foot, and tail is minimal, and is confined to the midline convexity; (9) the presence of a small region of bilateral representation, lateral to the regions of contralateral representation, was confirmed. It is suggested that the region of contralateral postcranial representation plus the medial rhinarium and mystacial vibrissa areas are the homologue of SmI in placental mammals, and the region of bilateral representation is homologous to SmII of placental mammals, but that the lateral vibrissa and rhinarium areas are a specialization of somatic sensory cortex unique to the Virginia opossum.     

Somatotopic organization of the dorsal horn in the lumbosacral enlargement of the spinal cord in the neonatal cat

The somatotopic organization of the light touch receptive fields of single unidentified dorsal horn neurons in the lumbosacral spinal cord has been studied in the neonatal cat anesthetized with chloralose. Satisfactory recordings were obtained from single dorsal horn neurons in kittens aged 3-6 days. Reconstruction of recording tracks from pontamine blue dye spots and comparisons of the depths of recording sites with Nissl-stained sections of cord showed that most single-unit recordings were obtained from laminae III and IV of Rexed. In animals of all ages neurons were found which responded briskly to light cutaneous mechanical stimulation. Their receptive fields varied widely in size, being smallest on the distal digits and largest on proximal skin. Receptive field areas were similar in proportion to the size of the hindlimb to those seen in the equivalent region in the adult cat. Because of the shape of the dorsal horn and the relatively narrow dorsal columns in neonatal kittens it proved difficult to locate units with receptive fields on proximal skin. Nevertheless the main features of the somatotopic organization of the dorsal horn were similar to those in the adult cat. Thus the somatotopic map of the kitten showed a medial representation of glabrous skin that was bounded laterally by the representation of the hairy skin of the toes. Proximal skin was represented in the lateral parts of the dorsal horn, a region which was not easily accessible for microelectrode recording. The individual toes were represented in a rostral to caudal sequence such that toe 2 was represented rostrally and toe 5 caudally. Around the toe representation the medial surface of the foot was represented rostrally, the ventrolateral surface caudally, and the dorsal surface laterally. The results indicate that the mature organization of light touch receptive fields of dorsal horn neurons in the lumbosacral cord of the cat is already largely present at birth.     

Chronic peripheral nerve injuries alter the somatotopic organization of the cuneate nucleus in kittens

The effect of chronic peripheral nerve injuries on the somatotopic organization of the cuneate nucleus was examined in kittens, using electrophysiological techniques. In normal kittens, most cells in the dorsal part of the nucleus possessed small receptive fields on the ipsilateral front paw. Several weeks after paw denervation in young kittens, however, many cells in the corresponding dorsal part of the nucleus responded to tactile stimulation of the wrist, forearm, or trunk. Consistent with this change in receptive fields, the neurons in the dorsal part of the nucleus were more responsive to electrical stimulation of the medial cutaneous nerve, which innervates part of the forearm, in kittens after paw denervation than in control kittens. These somatotopic changes were not an artifact of severe atrophy of the dorsal part of the nucleus. This experiment confirms that after chronic peripheral nerve injuries, central somatosensory neurons can begin to respond to ascending afferent volleys originating from other undamaged peripheral axons, which were previously incapable of exciting the cells. Moreover, this change in functional connectivity is evident at the first synapse central to the injury site.     

Somatotopic organization of the second somatosensory cortical area after lesions of the primary somatosensory area in infant and adult cats

The somatotopic organization of the second somatosensory cortical area (SII) and receptive fields of multineuron responses to cutaneous stimulation were studied in cats 6–16 months after lesions of the forelimb representation in the primary somatosensory area (SI) at 4 days, 4 weeks of age or in adults. No change was detected in SII. The results contrast with findings of alterations in SII of macaque monkeys following similar ablations of SI.     

Organization of corticocortical connections in the parietal cortex of the rat

An analysis based on Nissl, anterograde degeneration, and sucinic dehydrogenase histochemical techniques reveals that there are two distinct regions of parietal cortex which are characterized by different cytoarchitectonic features and anatomical connections. The “granular” cortical zone possesses a well-defined fourth layer composed of small, densely-packed cells, receives dense projections from the ventral posterior nucleus of the thalamus, and is essentially free of callosal inputs. “Agranular” cortical areas which surround or lie embedded within the granular zone lack a well-defined fourth layer, receive sparse projection from the ventral posterior nucleus, but send and receive extensive callosal projections. These findings suggest that thalamic and callosal projections to the parietal cortex maintain a pattern of areal segregation. The granular cortical zone, which apparently corresponds to SmI, projects ipsilaterally to motor cortex, SmII, and adjacent agranular areas. The superficial layers of the granular cortex also project heavily upon the underlying layer V. This intracortical projection is not organized in discrete clusters within the “barrel field” cortex. This suggests that the specialized organization of thalamic afferents and granule cells within the “barrel field” is not maintained in the intracortical circuitry of this region.     

Topography,cytoarchitecture, and sulcal patterns in primary somatic sensory cortex (SmI) of the prosimian primate,Perodicticus potto

The topographic organization of the primary somatic sensory projection area (SmI) in relation to cytoarchitectural fields and sulcal patterns was examined in the prosimian primate Perodicticus potto. The area of cortex responding to low threshold (LT) cutaneous stimulation of the glabrous and hairy surfaces of the hand was determined by microelectrode mapping techniques, with standardized threshold stimuli for defining receptive fields. A single somatotopic projection of the two hand surfaces was found; the glabrous projection area is rostral to that of the hairy hand. Within both the glabrous and hairy areas, receptive fields on the distal digits are found anterior to those on the proximal hand. The glabrous hand projection area is coextensive with a dense granular area typical of koniocortex. The hairy hand area corresponds to a cytoarchitectural field which is less granular than the glabrous field. While koniocortex occupies the crown of the gyrus caudal to the coronally oriented sulcus, a large more rostral field, which contains both granule and large pyramidal cells, occupies the whole of the caudal bank of the sulcus. Force thresholds of many receptive fields (RFs) in Perodicticus were high both on the borders and within the LT area (perhaps because of the advanced age of these animals). However, the receptive field sizes for both the glabrous and hairy hand areas were of the same magnitude as those of Nycticebus (Carlson and FitzPatrick, '82). From the combined studies of three species of Lorisidae, Perodicticus, Galago (Carlson and Welt, '80), and Nycticebus (Carlson and FitzPatrick, '81), using similar mapping and stimulation techniques, both general and specific features of SmI hand area organization can be illustrated. A single projection of the glabrous and hairy hand is common to Perodicticus and Galago, but two glabrous projection areas are seen in Nycticebus. The projection area for the hand in Perodicticus is twice as large (relative to brain size) as in Galago and Perodicticus. The possible behavioral significance of increased differentiation of the hand area in Nycticebus and elaboration of the area in Perodicticus could be examined by study of hand use and tactile capacity in these same spcies.     

Interhemispheric neocortical connections of the corpus callosum in the normal mouse: a study based on anterograde and retrograde methods.

Interhemispheric neocortical connections are widely distributed through the corpus callosum in the mouse. Callosal connections are present in all cytoarchitectonic fields except field 25. The distal extemity representations of SmI, and MsI the representation of the mystacial vibrissae in SmI, and the more peripheral field representation of VI are relatively acallosal. Dense projections lie in the midline or truncal representations of SmI, MsI, SmII, at the vertical meridian representations bordering field 17, and medial to the AI representation. The radial distribution of terminals is bimodal in most cytoarchitectonic fields. It is unimodal in the supracallosal segment of field 29b and fields 49 and 27, trimodal in fields 13 and 35. The cells of origin of callosal fibers appear to have the same topographic pattern of distribution as the callosal terminals, observing the same steep and gradual density gradients. No cells giving rise to callosal axons are identified in the acallosal regions of fields 2 and 17. Further, superficial focal lesions in cortical areas which receive callosal connections give rise only to homotopic contralateral degeneration. Acallosal areas of 17 and 2 give rise to no callosal connections. The cells of origin of callosal connections are located at all laminar levels of the cortex and include pyramidal and polymorphic cells but not the granule cells of layer IV.     

Corticofugal action on somatosensory response properties of rat nucleus gracilis cells

Single unit recordings were performed in the nucleus gracilis (Gr) of anesthetized rats to study the influences of the sensorimotor corticofugal projections on sensory responses of those cells. The effects of electrical stimulation of contralateral primary sensory cortex were studied in two conditions: when the receptive fields of the stimulated cortical area and the gracilis cells overlapped (matched) or when they were completely different (unmatched). Cortical stimulation at low intensities (<50 μA) evoked spike firing only in gracilis neurons with matched receptive fields. When the receptive fields were unmatched, the intensity of the stimulation had to be increased above 50 μA to elicit spike firing. To study the corticofugal actions on the responses of Gr neurons, the onset of peripheral stimulation was likened to a single cortical shock in the sensorimotor cortex. When receptive fields matched, cortical stimulation facilitated the cellular responses to the natural sensory stimulation of their RF in most of the Gr neurons (86%). In the unmatched receptive fields, cortical stimulation could either inhibit (66.7%), facilitate (20.8%) or did not modify (12.5%) the sensory response at all. Trains of cortical shocks during sensory stimulation demonstrated that the facilitatory and inhibitory effects on Gr neurons outlasted the period of stimulation by 30–60 s. Results indicate that the sensorimotor cortex exercises a very precise control of sensory transmission throughout the Gr nucleus and suggest that the corticofugal projection may play an important role in the plasticity of the sensorimotor system.     

Functional organization of tactile inputs from the hand in the cuneate nucleus and its relationship to organization in the somatosensory cortex.

Central processing of tactile inputs from the hand begins in the main cuneate nucleus and continues in the thalamus and area 3b cortex. Little is known about cuneate functional organization in primates or about how cuneate and area 3b organization are related. In this study, neurophysiologic approaches were used to evaluate how tactile inputs from the hand and adjacent body are organized in the cuneate nucleus of squirrel monkeys. Cuneate data on the organization of hand inputs were then compared with analogous area 3b data from our earlier cortical studies that used the same approaches. Evaluations of several cuneate properties, including (1) responsiveness to tactile stimulation, (2) incidences and sizes of receptive fields, (3) somatotopic progressions, (4) properties of representations, and (5) relationships between functional inputs and cytochrome oxidase staining, suggest that tactile afferents from the hand form consistently organized cuneate representations that, in turn, relate to the parcellated organization of cuneate structural substrates. Comparisons of cuneate and area 3b organization indicate that tactile processing from the brainstem to cortex involves a preservation of tactile responsiveness and somatotopic organization but, in addition, involves transformations that produce receptive field sharpening, suppression of hairy hand inputs, amplification and refinement of glabrous inputs, and relocations of representations. Ascending lemniscal substrates are characterized by cascading excitatory convergence/divergence that increments at successively higher levels between sensory afferents and area 3b. It is suggested that the observed preservations and transformations reflect this organization but, in addition, reflect mechanisms that cause counterbalancing sharpening and suppressions of hand inputs.     

Organization of the hand area in the primary somatic sensory cortex (SmI) of the prosimian primate,Nycticebus coucang

The topography of low-threshold (LT) cutaneous input from the hand to SmI cortex of the prosimian primate Nycticebus coucang was mapped in detail by means of threshold receptive field techniques. The area of cortex responding to LT stimulation of the hand can be divided into three distinct subdivisions on the basis of physiological as well as cytoarchitectonic criteria. The rostral subdivision in the hand area (G1) responds to stimulation of the glabrous surface of the digits and hand. Within this field cells located rostrally have receptive fields located on the glabrous digit tips, while caudally located cells respond to stimulation of the proximal glabrous palm. Receptive fields in G1 are small in size and have low thresholds. G1 is coextensive with an area of cortex having the cytoarchitectonic features of koniocortex. The caudal region of the LT hand area consists of two subdivisions. Laterally, cells in this caudal region (H) respond to stimulation of the dorsal hairy digits and hand. Receptive fields on the dorsal hairy digits are found rostrally in this subdivision, while receptive fields on the dorsal hand and wrist are encountered caudally. Thus, the hand is projected serially across G1 and H. Within H, thresholds are similar to those in G1, but receptive fields are much larger in H than in G1. A second glabrous hand area (G2), is found in the medial part of the caudal region of the LT hand area. In G2 receptive fields on the glabrous ulnar digits are encountered rostrally, while more caudally cells respond to stimulation of the radial digits. Few glabrous palm fields were found in G2. Receptive field sizes and thresholds in G2 are quite similar to those in G1. Subdivisions H and G2 have the cytoarchitectonic characteristics of parakoniocortex. Although H and G2 are not distinguishable from one another, they can be easily separated from the more rostral koniocortex which contains the first glabrous hand area (G1). Thus, the SmI hand area of Nycticebus is unique among those of the other prosimian, Galago, thus far mapped in that two separate somatotopic projection patterns of the glabrous hand are present within the LT area. In this respect, Nycticebus more closely resembles simian species in which multiple LT hand areas are present in SmI in all forms which have been mapped. This is not to suggest that G2 in Nycticebus is either homologous to, or a precursor of, those multiple cutaneous areas reported for Old and New World simian species. Rather we suggest that the second glabrous projection in Nycticebus, and that of simian species, may have been achieved by the displacement of the dorsal hand projection occupying the architectonically distinct caudal SmI field in earlier prosimian species.     

Functional reorganization of adult raccoon somatosensory cerebral cortex following neonatal digit amputation

Amputation of a forepaw digit in raccoons 2–8 weeks of age produced dramatic changes in the functional organization of somatosensory cortex examined electrophysiologically 9–12 months later. The cortical region normally representing the digit that was amputated received widely overlapping input from the entire forepaw, with local disruption of somatotopic organization. Compared with normal animals, the receptive fields of cortical neurons in amputated animals were larger, often included both glabrous and hairy skin, sometimes involved discontinuous skin regions, and were much more variable in peripheral location as a function of recording distance across the cortex and of depth within the cortex.     

Horizontal Interactions in Cat Striate Cortex: III. Ectopic Receptive Fields and Transient Exuberance of Tangential Interactions

In this study the developmental changes of intracortical connectivity are related to changes of cortical receptor fields (RFs). The RFs of striate cortex neurons of 4- to 8.5-week-old kittens, reared under normal conditions (NR) or in a selective visual environment (SE), were analysed quantitatively and compared with adult cats. To unmask weak inputs from outside the conventional RF (CRF), cell excitability was raised by iontophoretic application of glutamate (GLU) and/or bicuculline methiodide (BIC) or by light stimulation of the CRF. Both the dominant discharge region (DDR) and the total RF (TRF) area were significantly larger in NR and SE kittens than in adult cats. Moreover, in kittens 18% of the cells had additional ectopic fields that were excitatory, had similar orientation preferences as the CRF, and ranged 4 degrees to 23 degrees from the centre of the CRF. In 74% of the cases the ectopic fields were direction-selective and 70% of them preferred stimuli moving toward the CRF. Ectopic fields occurred mainly in supragranular cells, were similarly frequent in simple and complex cells and slightly more frequent in SE (20.7%) than in NR (13.3%) kittens. In adult cats only one of 83 cells tested had an ectopic field. It is concluded that the age-dependent decrease in the RF size, the laminar distribution of cells having an ectopic RF, and the numerical reduction of these cells with age correlate well with the organization and postnatal pruning of tangential projections, suggesting that these contribute to the elaboration of specific response properties. Moreover, the authors infer from the early presence and from the selectivity of ectopic fields that the system of horizontal intrinsic connections mediates far-reaching, excitatory interactions between cortical neurons with similar functional properties and serves as a substrate for the processing of global aspects of visual patterns.     

Extrinsic visual and auditory cortical connections in the 4-day-old kitten

The major extrinsic projections to and from the visual and auditory cortical areas were examined in 4-day-old kittens using axonal transport of horseradish peroxidase (HRP) and/or tritiated proline. Six different afferent and seven different efferent systems were studied; all 13 were present by postnatal day 4 as revealed by either HRP, or autoradiography alone, or these two techniques combined. Topographical projections were found for the corticopetal pathways from the thalamus and claustrum and for the corticofugal pathways to the thalamus, claustrum, striatum, and tectum, as well as for the inter- and intrahemispheric pathways. No topographical relations were seen in projections to the cortex from the basal ganglia or the lower brainstem. The results of the present study indicate that most or all of the major extrinsic connections of the kitten's visual and auditory cortical areas are present neonatally, and that both the cells of origin and the axonal targets are arranged topographically much like those of adult cats. However, the origins of callosal projections from visual cortex are more widespread in newborn kittens than in adult cats. In addition, the laminar arrangements of the kitten's corticocortical connections differ from those of adult cats in a number of details. The results suggest that the sparing of some visual and auditory functions after neonatal lesions occurs despite the fact that the cortical areas removed have formed extrinsic connections.     

Patterns of afferent projections to transitional zones in the somatic sensorimotor cerebral cortex of albino rats

The organization of somatosensory projections to the dysgranular areas of somatic sensory cortex was mapped in albino rats. Receptive fields that activate layer IV granule cells in these dysgranular zones were: cutaneous and deep (including muscle), roughly somatotopic, larger, and required stronger stimulation (tap) than the cutaneous light touch RFs of the adjacent granule cell zones.     

Functional reorganization in somatosensory cortical areas 3b and 1 of adult monkeys after median nerve repair: possible relationships to sensory recovery in humans

Previous studies have shown that the primary somatosensory cortex of adult mammals undergoes somatotopic reorganization in response to peripheral nerve transection. The present study assesses how cortical organization is affected when a transected nerve subsequently regenerates. The median nerve to one hand of adult owl monkeys was transected and repaired. Following nerve regeneration, the representations of the hand in cortical areas 3b and 1 were studied with neurophysiological mapping methods. The major results were as follows: Peripherally, median nerve transection, repair, and regeneration resulted in reinnervation of the median nerve skin territory. Centrally, both the initial loss and subsequent regeneration of median nerve inputs caused reorganizational changes in cortex. Reorganizational changes were specifically restricted to regions of the hand cortex where inputs from the median nerve were normally represented. The functional features of cortical regions that recovered tactile responsiveness from reinnervated skin regions were abnormal in several respects. Most notably, these regions contained recording sites with abnormally located or multiple cutaneous receptive fields, and contained major topographical changes, such as reestablishment of palmar pad or digit representations in small, discontinuous patches of cortex. Normal organizational features were reestablished to a more limited extent. These features included recovery of delimited, discrete receptive fields and reestablishment of topographic representations for localized skin areas. Different transformations in topographical organization were seen in areas 3b and 1 of the same monkey. These results suggest that nerve regeneration reestablishes the cortical capacity to process tactile information from reinnervated skin via a prolonged reorganizational process that appears dependent on peripheral and central factors. Cortical recovery mechanisms clearly appear to have limitations, since preinjury patterns of cortical organization are not widely recovered even almost 1 year after repair. We suggest possible relationships between cortical reorganizational changes in these primates, and postrepair sensory changes in humans.     

Plasticity in the organization of adult cerebral cortical maps: a computer simulation based on neuronal group selection

Recent experimental evidence from the somatosensory, auditory, and visual systems documents the existence of functional plasticity in topographic map organization in adult animals. This evidence suggests that an ongoing competitive organizing process controls the locations of map borders and the receptive field properties of neurons. A computer model based on the process of neuronal group selection has been constructed that accounts for reported results on map plasticity in somatosensory cortex. The simulations construct a network of locally connected excitatory and inhibitory cells that receives topographic projections from 2 receptor sheets corresponding to the glabrous and dorsal surfaces of the hand (a typical simulation involves approximately 1500 cells, 70,000 intrinsic and 100,000 extrinsic connections). Both intrinsic and extrinsic connections undergo activity-dependent modifications according to a synaptic rule based on heterosynaptic interactions. Repeated stimulation of the receptor sheet resulted in the formation of neuronal groups-local sets of strongly interconnected neurons in the network. Cells in most groups were found to have similar receptive fields: they were exclusively glabrous or dorsal despite equal numbers of anatomical connections from both surfaces. The sharpness of map borders was due to the sharpness of the underlying group structure; shifts in the locations of these borders resulted from competition between groups. Following perturbations of the input, the network underwent changes similar to those observed experimentally in monkey somatosensory cortex. Repeated local tapping on the receptor sheet resulted in a large increase in the magnification factor of the stimulated region. Transection of the connections from a glabrous region resulted in the organization of a new representation of corresponding dorsal region. The detailed simulations provide several insights into the mechanisms of such changes, as well as a series of predictions about cortical behavior for further experimental test.