Misguided thinking on animals.
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Animal Welfare↗
Biomedical subjects
C Blakemore
Publications and source records attributed to C Blakemore.
Development of stimulus selectivity and functional organization in the suprasylvian visual cortex of the cat.
We have recorded from single neurons in the medial bank of the middle suprasylvian sulcus (PMLS) of anaesthetized and paralysed cats aged between nine days and eight weeks. Visual responses were assessed qualitatively, by using conventional projected stimuli, and quantitatively for drifting, high-contrast gratings of optimum spatial and temporal frequencies, but varying in orientation and direction of drift. At 9 days of age, some cells in the PMLS were spontaneously active but in three long penetrations only one visually responsive neuron was isolated. Between 9 and 15 days there was a rapid increase in the proportion of responsive units, which first appeared in small clusters in the lower layers (IV, V, VI). During the second and third postnatal weeks, spontaneous activity and the strength of visual responses increased to adult levels, and the proportion of cells showing rapid habituation to visual stimulation decreased. Even before two weeks of age, at least 85% of responsive cells in the PMLS were selective, by quantitative criteria, for image motion along one particular axis, and a majority of these were clearly direction-selective (responding to movement in one direction significantly more strongly than to that in the opposite). By the end of the third postnatal week the proportion of units with strong direction preference reached adult levels. The selective cells were initially more broadly 'tuned', on average, for the direction of motion of a grating (mean half-width in animals of 10-12 days was 32.6 degrees), but the sharpness of tuning improved to reach the adult level (ca. 23 degrees) during the third postnatal week. In animals younger than three weeks a slightly smaller proportion of cells than in adults (but always more than one third of all visually responsive cells) responded to stationary, contrast-modulated gratings. The majority of these cells showed clear selectivity for the orientation of a flashed grating. A few 'non-selective' cells were found in the youngest animals but by the end of the third postnatal week virtually all cells responsive to stationary gratings displayed orientation selectivity. There was always good agreement between the preferred orientations for stationary and drifting gratings. Even before two weeks of age, when responsive cells occurred only in small clusters, there was a clear tendency for neighbouring neurons to have similar or opposite preferred directions, just as in adult cats. By 2-3 weeks of age there were clear progressive shifts in stimulus preference along oblique or tangential tracks.(ABSTRACT TRUNCATED AT 400 WORDS)
Age Factors↗
Developmental plasticity in the visual and auditory representations in the mammalian superior colliculus.
Environmental factors play an important role in certain aspects of the development of sensory systems. But the way in which the maturation of different sensory modalities is coordinated is poorly understood. We have investigated this question neurophysiologically in the mammalian superior colliculus (SC), which contains topographically aligned maps of visual and auditory space. We report here that an essentially normal auditory map, in approximate register with the visual map, is found in the SC of adult ferrets reared with abnormal binaural localization cues. Also, if, early in life, one eye is deviated laterally, there is a compensatory shift in the auditory map, but early eye rotation totally disorders the auditory representation. These results imply that development of the auditory map is affected by visual activity or by information about eye position and that there is definite, but limited, capacity for the auditory map to reorganize so that it remains aligned with the visual map.
Acoustic Stimulation↗
The organization of corticocortical projections from area 17 to area 18 of the cat's visual cortex.
Retrogradely transported tracers were injected into area 18 of the visual cortex of the adult cat to study the organization of corticocortical projections from area 17 to area 18. All injections, whether very small or relatively large, and irrespective of their exact location in area 18, produced a discontinuous, clustered distribution of labelled cells, mainly in layers II, III and upper IV, in a topographically related region of area 17. The mean centre-centre distance between neighbouring patches was about 750 microns. We conclude that the overall population of cells projecting to area 18 is genuinely distributed in a patchy fashion and that they provide an efficient spatial sample of information from area 17. Comparison of the dimensions of each injection site and of the retrogradely labelled territory suggested that each region in area 18 receives a convergent input from a zone in area 17 whose visual field representation is about 0.8 M-1 deg larger in all directions (where M is the magnification factor in millimetres per degree at the termination site in area 18). Pairs of injection were made in area 18 by placing small volumes of two fluorescent tracers, fast blue and diamidino yellow, side-by-side in either a rostrocaudal or a mediolateral plane, with different distances between them. When the boundaries of the dense central cores of two injection sites were separated, at their closest points, by about 1.6 mm, the two corresponding distributions of labelled cells in area 17 were just non-overlapping, suggesting that each group of cells in area 17 sends a divergent projection in innervate a zone about 0.8 mm larger in all directions in area 18. More closely spaced injections led to overlap of the distributions of labelling by the two dyes, with shared clusters containing a mixture of labelled cells. The proportion of double-labelled cells in these shared clusters never exceeded 4.4% (but was 70% after sequential injection of the two dyes at a single point). We conclude that, although each cluster of cells sends a divergent projection to area 18, the majority of individual axons terminate more discretely, perhaps providing specific inter-connections between functionally corresponding 'columns' in the two areas.
Animals↗
Factors limiting the postnatal development of visual acuity in the monkey.
A major factor underlying the prolonged postnatal improvement of visual acuity in primates is an increase in the sampling limit imposed by the photoreceptor mosaic, which may be as much as 5-fold. Further factors operating peripheral to neurons of the lateral geniculate nucleus, which might include changes in neural connectivity, appear to contribute at most 1.5 octaves, between 3 weeks and 6 months of age. Neural factors at the geniculo-cortical synapse may add another half-octave between 10 weeks and 1 yr. Factors operating after the level of the striate cortex (changes in spatial degradation, attention, motivation, etc.) contribute up to another octave or more to the increase in behavioural acuity between birth and about 11 weeks of age. The foveal image is substantially undersampled in young animals and therefore aliasing could occur over a wide range of spatial frequencies.
Animals↗
The in vitro slice preparation for combined morphological and electrophysiological studies of rat visual cortex.
The morphological condition of slices of rat visual cortex, maintained in vitro in an interface-type recording chamber, was assessed. In addition, neurones in these slices were impaled with glass micropipettes for intracellular recording and horseradish peroxidase (HRP) injection. After fixation and embedding, slices were examined by light and electron microscopy. Slices sectioned orthogonally to the original plane of cutting showed a vertical zonation of tissue preservation. The upper zone contained dense and flattened neuronal somata, although the neuropil appeared normal. The central zone was well preserved, with the appearance of most somata, dendrites, axons and synapses comparing favourably with perfusion-fixed material. The lower zone contained many abnormal, vacuolated somata. The morphology of HRP-injected neurones was assessed by light microscopy. Dendrites could be visualised in great detail and spines were clearly visible. Local axon arbors were well represented. There was good correlation between electrophysiological and morphological criteria for the assessment of the condition of the slice. We conclude that, provided the extent of degeneration within the slice is monitored and appreciated, slices of visual cortex can provide both electrophysiological and morphological data of high quality.
Animals↗
Stimulus selectivity and its postnatal development in the cat's suprasylvian visual cortex.
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Animals↗
Development of spatial and temporal selectivity in the suprasylvian visual cortex of the cat.
We have studied the development of the spatial and temporal properties of neurons in the medial bank of the suprasylvian visual cortex (PMLS) in kittens aged between 9 d and 8 weeks. Quantitative measurements were made of the responses to drifting high-contrast gratings of optimum orientation and direction of motion, but varying in spatial and temporal frequency. The spatial resolution ("acuity") of cells increased rapidly and was fully mature (over 2 cycles/deg for the best cells) at 3 weeks of age. The optimum spatial frequency also tended to improve and reached adult values (around 0.5 cycles/deg for the best cells) at about the end of the third week. In younger kittens, the spatial resolution of neurons was not obviously correlated with the eccentricity of their receptive fields, but in older animals acuity was clearly elevated for receptive fields in the central visual field. The proportion of "low-pass" cells (showing no obvious attenuation of response for gratings of low spatial frequency) decreased with age and simultaneously there was a slight increase in the mean spatial bandwidth of "bandpass" cells. Responses to drifting sinusoidal gratings were generally dominated by an unmodulated elevation of discharge at all ages. In tests with stationary, contrast-modulated gratings presented at different spatial positions, cells in the youngest kittens behaved nonlinearly and showed mainly an unmodulated increase in discharge, whereas in older kittens, as in adult cats, most neurons responded to contrast-modulated gratings with a small, phase-dependent response at the temporal frequency of modulation and a larger component at twice the fundamental frequency. None of the cells recorded at any age had a true "null position." As in adult PMLS, the widths of receptive fields in kittens were, on average, about twice the size of the preferred spatial period (4 times the preferred bar width). At all ages, therefore, neurons in PMLS resembled striate complex cells with respect to the nonlinearity of their responses and the spatial structure of their receptive fields. The preferred temporal frequency and high-temporal-frequency cutoff also improved, on average, during the first 3 weeks of life, and the range of temporal frequencies over which cells responded continued to increase until at least 8 weeks. Although the low-spatial-frequency inhibition that creates spatial bandpass characteristics probably depends on cortical mechanisms, the postnatal development of both temporal and spatial resolution might well be limited by maturation at the level of the retina.
Animals↗
The organization and post-natal development of area 18 of the cat's visual cortex.
1. We made extracellular recordings from 1176 single units in area 18 of adult cats and kittens aged between 7 days and 10 weeks, and from 137 single units in area 17 in kittens aged between 12 days and 10 weeks. 2. All cells examined in area 18 of adult cats were visually responsive, 84% being orientation selective, 9% orientation biased and 7% non-oriented. Orientation columns and ocular dominance columns were identified. There was an over-all bias towards horizontal and vertical in the distribution of preferred orientations in the rostral part of area 18, where we were recording. 3. In area 18 of 7-day-old, visually inexperienced kittens the majority of cells (60%) were visually unresponsive; the remainder were either non-oriented (25%) or orientation biased (15%), and no orientation-selective units were found. Nevertheless there seemed to be a rudimentary columnar system, even in the youngest animals, in that orientation-biased cells tended to occur in clusters with neighboring neurones having similar orientation preferences. In normal animals of 3 weeks and younger we found that the distribution of preferred orientations of a sample of neurones in area 18 with receptive fields scattered over much of the left lower quadrant of the visual hemifield was biased to oblique orientations. 4. As in adult cats, most cells in area 18 in young kittens were binocularly driven, and periodic alternation of dominance along oblique penetrations, characteristic of ocular dominance columns, was sometimes seen, even at the earliest ages. 5. Many of the developmental changes that we observed in area 18 occurred during the first 4 post-natal weeks. Orientation selectivity, orientation tuning, directionality and responsiveness of neurones matured rapidly over this period. The proportions of simple and complex cells were similar in kittens aged 4 weeks or more to those in adult cats, whereas prior to this most neurones that displayed an orientation preference appeared to be immature simple cells. 6. A laminar analysis revealed that very few units in the superficial layers (I, II and III) in area 18 are visually responsive in kittens during their first and second weeks, but orientation selectivity rapidly develops during the third week. By contrast, even in very young, visually inexperienced kittens, the majority of neurones found in deeper laminae (IV, V and VI) are visually responsive and a few of them already show an orientation preference; however, the subsequent appearance of larger proportions of orientation-selective cells in these lower layers is a more prolonged process than in upper laminae.(ABSTRACT TRUNCATED AT 400 WORDS)
Action Potentials↗
Effects of dark-rearing on the development of area 18 of the cat's visual cortex.
1. We recorded extracellularly from 420 single units in area 18 in visually inexperienced kittens aged 7 days and dark-reared kittens aged between 3 and 12 weeks and from 60 single units in area 17 in dark-reared kittens aged 5 and 11 weeks. 2. Visual deprivation generally depressed the maturation of area 18, although some features were affected more than others and certain developmental improvements still occurred. The percentage of visually responsive units in area 18 increased from 40% in 7-day-old kittens to about 75% in dark-reared animals 10-12 weeks. At each age a proportion of cells was orientation biased (between 15 and 45%) and these neurones appeared to be arranged in a crude columnar fashion. However, dark-rearing, from birth, prevented the development of a significant proportion of orientation-selective cells in area 18; no more than 5% of neurones were orientation-selective at any age. We found no major bias in the over-all distribution of preferred orientations of cells in area 18 in dark-reared kittens. 3. Simple cells, which are found in area 18 even in very young, visually inexperienced kittens, persisted after dark-rearing, although most retained immature properties. Relatively few complex cells were found in area 18 in visually deprived animals. 4. The majority of neurones in area 18 of dark-reared kittens were binocularly driven, many equally well by either eye; evidence for regional variation in ocular dominance (indicative of a columnar pattern) was found in these deprived animals. 5. A laminar analysis in area 18 showed that percentages of non-oriented and orientation-biased cells changed little, if at all, in lower laminae (IV, V, and VI) but increased substantially in upper layers (above layer IV) in the absence of visual stimulation, over the first 12 post-natal weeks. 6. A comparison of the effects of dark-rearing on areas 17 and 18 indicates that the normal development of visual responsiveness and specific receptive field properties is suppressed in both areas during the first 12 post-natal weeks. It is possible that area 17 has a greater degree of orientation selectivity than area 18 in young visually deprived kittens and this may reflect a difference in the type of afferent inputs.
Action Potentials↗
Stimulus selectivity and functional organization in the lateral suprasylvian visual cortex of the cat.
1. We have examined the responses of cells in the postero-medial and postero-lateral lateral suprasylvian areas (p.m.l.s./p.l.l.s.) in the medial and lateral banks of the middle suprasylvian sulcus of the anaesthetized, paralysed cat. 2. Visual responses were assessed qualitatively (for projected spot and bar stimuli) and quantitatively (for drifting, high-contrast gratings of optimum spatial and temporal frequencies, but varying in orientation and direction of drift). There was excellent agreement between qualitative and quantitative estimates of preferred direction of motion. 3. Comparison of responses to motion in the preferred direction and the opposite direction confirmed that the percentage of units with strong directional preference is higher in this region of cortex than in other cortical and subcortical visual structures so far investigated. 4. Cells were comparatively well 'tuned' for the direction of motion of a grating: on average the half-width at half-amplitude for the variation in response around the principal preferred direction was 23.2 deg for p.m.l.s. and 25.3 deg for p.l.l.s., thus falling within the range found for complex cells in area 17. In this and other aspects of their direction selectivity, neurones in p.m.l.s. and p.l.l.s. were very similar to each other. 5. Quantitative analysis of the direction-response functions revealed them frequently to be more complex than previously described. For many cells there were subsidiary response peaks in addition to the main peak at the principal preferred direction. Often there was an accessory peak 180 deg from the optimum direction (i.e. directional preference was incomplete), but there was also a clear tendency for responses to be specifically elevated for directions orthogonal to the principal direction. 6. Tests with stationary, contrast-modulated gratings of the optimum spatial frequency, but differing in orientation, revealed that neurones responsive to such stimuli (though limited to about half the population) were orientation selective, their preferred orientations for stationary and drifting gratings being very similar. 7. The functional architecture of the lateral suprasylvian cortex was studied by means of single and multiple penetrations at different angles to the cortical surface: in most penetrations in p.m.l.s. the preferred directions of cells tended to shift progressively in small steps across the cortex, at a maximum rate of about 360 deg mm-1. However, there were occasional 180 deg differences between neighbouring recording sites or between cells recorded at different depths within a single radial column.(ABSTRACT TRUNCATED AT 400 WORDS)
Action Potentials↗
Spatial and temporal selectivity in the suprasylvian visual cortex of the cat.
We recorded from single units in the medial and lateral banks of the posterolateral suprasylvian visual cortex (PMLS/PLLS) of the cat. The responses to drifting high-contrast gratings of optimum orientation and direction of motion, but varying in spatial and temporal frequency, were examined quantitatively for a sample of cells, whose receptive fields covered a wide range of eccentricities. The optimum spatial frequencies (average about 0.2 cycles/deg) were low compared to the values reported for striate cortex but similar to those for area 18. The mean spatial bandwidth (about 2 octaves) was slightly broader than that of cells in other cortical visual areas. The cut-off spatial frequencies ("acuities") covered a wide range, from 0.05 to 2.1 cycles/deg, similar to those of cells in area 18. Responses to drifting sinusoidal gratings were usually dominated by an unmodulated elevation of discharge, although some modulation occurred at the temporal frequency of drift, especially at low spatial frequencies. Modulated responses were relatively stronger in PMLS than in PLLS. For those cells that responded to flashed stimuli, stationary, contrast-modulated gratings presented at different spatial positions typically evoked small responses at the fundamental frequency (dependent on spatial phase) and a larger component at the second harmonic of temporal frequency, with no overall "null-position." The optimum spatial frequency was usually higher than would be predicted by simple summation within the dimensions of the receptive field. Thus, neurons in PMLS and PLLS, like complex cells in areas 17 and 18, behave nonlinearly and their spatial selectivity is determined by "subunits" smaller than their receptive fields. The range of preferred temporal frequencies ranged from less than 2.5 Hz to more than 10 Hz. In their temporal selectivity neurons in PMLS resembled cells in area 17, with little attenuation at low temporal frequencies, whereas there was a tendency for cells in PLLS to prefer higher temporal frequencies, as is common in area 18.
Animals↗
Differences of visual field representation in the medial and lateral banks of the suprasylvian cortex (PMLS/PLLS) of the cat.
We have studied the orderliness of representation of visual space in the medial and lateral banks of the middle suprasylvian sulcus. Penetrations were made either parallel to the sulcus, in one bank or the other, or vertical, thus crossing the sulcus between the postero-medial (PMLS) and posterolateral (PLLS) divisions of this area. In some cases we found clear evidence for topographical order in the representation of the visual field with a tendency (greater in PMLS than in PLLS) for the receptive fields of cells recorded deeper in the walls of the sulcus to lie closer to the area centralis, but along many penetrations the receptive fields were so large and so scattered that no retinotopic arrangement could be discerned. In PMLS the receptive fields of the majority of units we studied were centered below and close to the horizontal meridian, whereas in PLLS they were distributed over both the upper and lower visual fields with an over-representation of the upper field. Receptive fields were significantly larger in PLLS (mean field area = 442.2 deg2) than in PMLS (mean area = 154.4 deg2); there was also less clear correlation between receptive field size and eccentricity in PLLS (correlation coefficient = +0.25) than in PMLS (corr. coeff. = +0.72). Analysis of the distance between the receptive field centres of consecutively recorded units demonstrated that the mean scatter in both PMLS and PLLS amounts to about half the average receptive field diameter. In summary the topographical representation of visual space is less orderly in PLLS, and may involve a wider area of the visual field. These findings may relate to the segregated visual cortical and extrageniculate thalamic connections that the medial and lateral banks of the LS receive.
Animals↗
Organization of the visual pathways in the newborn kitten.
We have used retrograde and anterograde transport to examine the major visual pathways in newborn kittens. Retinal projections from both eyes to the dorsal lateral geniculate nucleus (dLGN) and superior colliculus (SC) are present and topographically organized. The dLGN projects topographically to areas 17 and 18 and receives reciprocal projections from cells in layer VI of areas 17, 18, 19 and suprasylvian cortex on the same side of the brain. Area 19 also has a sparse thalamic input but probably not from the dLGN. The laminar distribution of [3H]proline transported from dLGN to area 17 was quantified: label was spread through all layers, with a minimum at the border of layers I/II. Layer I was always labelled less heavily than IV. These results are critically compared with those based on other tracing techniques. Cells of layer V in areas 17, 18, 19 and the suprasylvian cortex project topographically to the superficial layers of the ipsilateral SC. Area 19 and the lateral suprasylvian cortex also send a crossed projection to restricted parts of the opposite SC. Thus these visual projections are not only present and topographically ordered on the day of birth, but, unlike certain highly exuberant interhemispheric and cortico-cortical projections, they are qualitatively remarkably mature, some days before the onset of visual activity. The major subcortical projections to and from the visual cortex appear to be constructed without the benefit of visual experience and much of the activity-dependent plasticity of cortical cells may well involve only local modulation of synaptic input.
Animals↗
Organization and post-natal development of the monkey's lateral geniculate nucleus.
We have studied the properties of neurones in the lateral geniculate nucleus (l.g.n.) of Old World monkeys, both in mature animals and throughout post-natal development. Cells were classified as X (linear) or Y (non-linear) on the basis of their responses to contrast-reversing achromatic gratings ('null position test'). In older animals virtually all parvocellular neurones and the majority of magnocellular units were X cells; only about 15% of magnocellular neurones displayed highly non-linear spatial summation, with no 'null position', typical of Y cells. X cells could not reliably be distinguished from Y cells, nor magnocellular from parvocellular, on the basis of their temporal patterns of discharge. Some Y cells responded transiently to contrast reversal of a grating far from the receptive field but X cells showed little or no such 'shift effect'. The spatial resolution of mature l.g.n. cells varied with the eccentricity of their receptive fields such that the best of them, at each point in the visual field, resolved drifting achromatic gratings about as well as a human observer. X cells in parvocellular and magnocellular layers had similar 'acuities', even in the central foveal representation, but Y cells generally had poorer resolution. Receptive fields in the temporal retina tended to have lower resolution than those at comparable eccentricities in the nasal retina. Even on the day of birth all cells we studied responded to visual stimulation and virtually all could be classified as X or Y. The laminar distribution of cell types and the general morphological appearance of the nucleus seemed very similar to those in the adult, but neurones in very young animals had low spontaneous activity, sluggish responses, and latencies to visual stimulation longer than any we saw in the adult. Until 3 weeks of age or so, many neurones suffered cumulative 'fatigue' when visually stimulated over several minutes. Visual latency was essentially mature by about 10 weeks. In the l.g.n. of the neonatal monkey there was little variation in neuronal 'acuity' with eccentricity: even in the foveal area the best cells could resolve only about 5 cycles/deg. Over the first year or more of life there is a gradual increase in responsiveness and about a 7-fold improvement in spatial resolution for foveal l.g.n. cells, correlating roughly with the behavioural maturation of visual acuity.
Action Potentials↗
Effects of visual deprivation on the development of the monkey's lateral geniculate nucleus.
We have studied the physiological properties of cells in the deprived layers of the lateral geniculate nucleus (l.g.n.) in monkeys monocularly deprived from birth for up to 27 weeks, and compared them with results from the non-deprived layers in the same animals and in a series of normal animals. Despite the relative shrinkage of cell bodies in the deprived layers, units were easily isolated, were visually responsive and could readily be classified as linear (X) or non-linear (Y) by means of tests of spatial summation. The laminar distribution of cell types and the proportion of Y cells did not seem to be affected by deprivation. The patterns and latencies of discharge produced by contrast-reversing gratings did not differ grossly between deprived and non-deprived cells. The peak firing frequencies for drifting gratings were also similar. The degree of surround antagonism (though very variable from cell to cell) seemed unaffected by deprivation. Most surprising of all, there was little or no deficit in the spatial resolution of the receptive fields of deprived cells. Recordings were always taken ipsilateral to the deprived eye, and neural 'acuity' tended to be sligtly lower in the deprived laminae than the non-deprived. However, this nasal/temporal asymmetry in spatial resolution was not obviously more pronounced than in normal animals. Neural 'acuity' was not abnormally low in either contralateral or ipsilateral layers in the l.g.n. of an animal binocularly deprived from birth until a year of age. We have not examined chromatic properties or temporal characteristics adequately to say whether they are affected by deprivation. Paradoxically, although the post-natal maturation of visual acuity in normal monkeys seems to be mainly limited by peripheral factors, deprivation (which causes a profound defect of behavioural acuity) does not seem to interfere substantially with physiological development of the retina or the geniculate nucleus.
Action Potentials↗
Postnatal development of the ipsilateral retinocollicular projection and the effects of unilateral enucleation in the golden hamster.
Anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) was used to study the normal development of the ipsilateral retinocollicular projection in golden hamsters, and to examine the effect of enucleation of the other eye at birth. In neonatal animals there were retinal fibers and sparsely distributed granular labeling in the superficial layers of the ipsilateral superior colliculus over its entire areal extent. Differences in the uncrossed projections of normal and enucleated animals first became clear at day 5. In normal animals, retinal fibers withdrew from the superficial layers of the superior colliculus, and the projection became concentrated in the stratum opticum, where denser clumps of label in the rostral part of the superior colliculus were first seen at day 5. In enucleated animals, the retinal projection persisted in the most superficial layers, and the density of labeling was higher than in normals. The very sensitive WGA-HRP technique showed retinal fibers extending to the caudal pole of the superior colliculus at all ages: even in normal animals more than 2 weeks of age some fibers reached as far as the inferior colliculus. When the shrunken size of the superior colliculus in the enucleated animals was taken into account, the total areal distribution of the ipsilateral projection was similar in normal animals and enucleates. The major difference between the two groups was in the higher density of ipsilateral labeling, especially in the caudal part of the superior colliculus, and in its more superficial laminar distribution in the enucleated animals.
Aging↗
The postnatal development of the association projection from visual cortical area 17 to area 18 in the cat.
The postnatal development of the association projection from area 17 to area 18 was studied in normal and binocularly deprived kittens between 1 and 28 days of age, using retrograde transport of horseradish peroxidase conjugated with wheat germ agglutinin. The positions of injection sites in the visual cortex, defined in relation to the borders of visual areas 17, 18, and 19 located in Nissl- and cytochrome oxidase-stained sections, were confirmed by observing the patterns of labeling of cells in the lateral geniculate nucleus. The association projection is present and is arranged at least roughly topographically from birth onward; at all ages it arises from cells in both the superficial layers (II, III, and the upper part of IV) and the deep layers (V and VI). In older kittens (20 days or more), however, the origin of the pathway is principally from the upper layers, as in adult cats, whereas in younger animals the projection arises roughly equally from cells in superficial and deep laminae. Initially, the association neurons in area 17 are distributed uniformly along each lamina. Periodic clustering of labeled cells in the upper layers can just be discerned at 10 days, and this patchiness has reached its adult clarity by 20 days, at which stage the projection from the lower layers is greatly diminished. Binocular deprivation until the age of 28 days did not prevent these developmental changes in the projection. Various controls established that the patterns of labeling seen in this study were not due to direct spread of tracer into area 17, to uptake of tracers by fibers-of-passage, or to transcellular transport via the thalamus.
Animals↗