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Contralateral visual field representation in area 17 of the cerebral cortex of the agouti: a comparison between the cortical magnification factor and retinal ganglion cell distribution.

The cortical representation of the contralateral visual field in area 17 of the agouti's brain was studied by multiunit recording. The borders of area 17 were determined by electrophysiological, cytoarchitectonic and myeloarchitectonic criteria. The results were plotted in flat, bidimensional representations of the cerebral cortex to minimize perspective distortions. The V1 map, a first order topological transformation of the visual field, shows a magnified representation of the horizontal meridian that corresponds to a retinal specialization, the visual streak. The visual field representation has asymmetries that are not directly related to the topography of the retinal ganglion cell density. Whereas the ganglion cell density shows a plateau along the visual streak, the areal cortical magnification factor is higher in the region that corresponds to the intersection of the horizontal and vertical meridians. This suggests functional specializations that are not obvious when one considers the distribution of the whole ganglion cell population but which might be related to the distribution of specific ganglion cell classes.

Head holder for lateral-eyed species in vision research.

The present report describes a head holder designed to be used for lateral-eyed species in vision research. The head-holder employs a prosthesis implanted on the skull and provides for adjustable movements in different planes, thus allowing precise positioning of the eye with respect to the visual space.

Head holder for lateral-eyed species in vision research

The present report describes a head holder designed to be used for lateral-eyed species in vision research. The head-holder employs a prosthesis implanted on the skull and provides for adjustable movements in different planes, thus allowing precise positioning of the eye with respect to the visual space

Retinal ganglion cell distribution in the cebus monkey: a comparison with the cortical magnification factors.

The distribution of ganglion cells was determined in whole-mounted Cebus monkey retinae. Ganglion cell density along the horizontal meridian was asymmetric, being 1.2-4.3 higher in the nasal retinal region when compared to temporal retina at the same eccentricities. The total number of ganglion cells varied from 1.34 to 1.4 million. Ganglion cell density peaked at 49,000/mm2 about 0.5 mm nasal to the fovea. Comparison between ganglion cell density and areal cortical magnification factors for V1 and V2 reveals that the relative representation of the fovea increases in the visual cortex. This effect seems to be a general feature of the visual system of primates.

The visual cortex of the agouti (Dasyprocta aguti): architectonic subdivisions.

1. We have studied the cytoarchitecture and myeloarchitecture of the agouti's cortical surface that can be activated by visual stimulation. Five architectonic subdivisions that correspond to distinctive visuotopic representations were characterized. 2. The largest portion of the visual cortex is occupied by area 17 which is situated lateral to the cingulate cortex, medial to area 18, posterior to the parietal cortex, and anterior to the agranular retrosplenial cortex. Additionally, four architectonic subdivisions in the extrastriate visual cortex were distinguished, i.e., from medial to lateral: area 18, area 19, anterior lateral area, and temporal posterior area. 3. Along the border of the extrastriate cortex a ring of nonvisual cortical fields was encountered encompassing parietal (somatic sensorial) cortex, temporal anterior and temporal intermediate (auditory) areas, a band of pre-rhinal cortex, and agranular retrosplenial cortex.

Distribution and size of ganglion cells in the retinae of large Amazon rodents.

The topographical distribution of density and soma size of the retinal ganglion cells were studied in three species of hystricomorph rodents. Flat-mounted retinae were stained by the Nissl method and the ganglion cells counted on a matrix covering the whole retinae. Soma size was determined for samples at different retinal regions. The agouti, a diurnal rodent, shows a well-developed visual streak, reaching a peak density of 6250 ganglion cells/mm2. The total number of ganglion cells ranged from 477,427-548,205 in eight retinae. The ganglion-cell-size histogram of the visual streak region exhibits a marked shift towards smaller values when compared to retinal periphery. Upper and lower regions differ in both cell density and cell size. The crepuscular capybara shows a less-developed visual streak with a peak ganglion cell density of 2250/mm2. The shift towards small-sized cells in the visual streak is less marked. Total ganglion cell population is 368,840. In the nocturnal paca, the upper half of the fundus oculi includes a tapetum lucidum. The retina of this species shows the least-developed visual streak of this group, with the lowest peak ganglion cell density reaching 925/mm2. The total ganglion cell number (230,804) is also smaller than in the two other species. Soma-size spectra of this species are characterized by the presence, in the lower hemi-retina, of very large perikarya comparable in size to the cat's alpha ganglion cells.

Electrophysiological determination of the refractive state of the eye of the opossum.

The refractive state of the eye of the South American opossum Didelphis marsupialis aurita was investigated with electrophysiological techniques. Using adult specimens, trapped from the wild, averaged cortical evoked responses were recorded from the region of projection of the central visual field. Stimuli consisted of a phase reversal of a square wave grating generated on a CRO screen, with luminance of 2.4 cd/m2 and contrast 0.84. The refractive state of the eye was altered by means of trial lenses and the amplitude of the cortical responses thus obtained compared to those obtained with no lens (control values). Refraction "tuning curves" were determined for each animal. The average refractive state was found to be -2.27 D indicating that this species when raised in its habitat shows, at low ambient luminosity, some degree of myopia. Determination of the Contrast Sensitivity Function indicate that induced ametropias lead to a reduction of the cut-off value of the spatial frequency and a loss of contrast sensitivity.

Contrast sensitivity function and visual acuity of the opossum.

The Modulation Transfer Function (MTF) of the visual system of the opossum, D. marsupialis aurita, was determined using the amplitude of Visually Evoked Cortical Potentials (VECP) as response indicator. Stimuli consisted of a 180 degrees phase reversal of sinusoidally modulated gratings with an average luminance of 2.4 cd/m2. Contrast sensitivity was determined for various spatial frequencies and the MTF was calculated by the least square fit of an exponential function. The average acuity value obtained was 1.25 c/deg. The Fourier transform of the MTF was considered an approximation of the Line Spread Function of the visual system. The lowest value observed was 14 min of arc. The visual acuity observed in the mesopic range was not altered when stimulus intensity was raised to photopic levels.

AVG--A signal averager for biological data.

A paper presents a versatile computer program for averaging evoked responses. The program, called AVG is set up interactively on a DEC-PDP-12 computer, equipped with 12 kbytes memory. A protocol, recording the experimental conditions, as well as messages and prompters are displayed on the screen to inform the operator and indicate which action must be taken. AVG is written in assembly program LAP-6. The program has been useful for averaging cortical visual evoked responses.

A regional specialization in the opossum's retina: quantitative analysis of the ganglion cell layer.

The distribution of ganglion cells in the opossum's retina was determined from flat-mounted preparations stained with cresyl-violet. The retinal area is 109 mm2 (SD = 16 mm2). Maps of ganglion cell density were made from retinae of seven animals. In all maps iso-density lines were approximately concentric, showing a slight elongation towards the nasal region. Cell density varied from 400 cells/mm2 at the extreme periphery to 2,900 cells/mm2 in the region of highest count, the are centralis. The center of this region lies 1.85 mm (26.3 degree) temporal to the center of the optic nerve head. The average total number of ganglion cells is 77,384 (SD = 10,173). Based upon soma diameter histograms ganglion cells were classified into three groups, showing at area centralis peaks at 7 micrometer, 12 micrometer and 15 micrometer respectively. Cell soma diameter ranged from 6 micrometer to 21 micrometer, larger values being observed at the periphery.

Visual receptive fields of units in the pulvinar of cebus monkey.

Visually driven units, isolated in the ventrolateral group -- Pv1g (109) and in subnucleus Pmu (33) of the pulvinar of the cebus monkey, were studied in acute and chronic preparations under nitrous oxide N2O/O2 anesthesia during periods of EEG arousal. Taking into consideration the response properties to static or moving stimuli as well as the organization of the receptive fields, units isolated in the pulvinar were subdivided into 8 groups. Units displaying dynamic properties predominate over static ones. Static units were classified in 3 groups; of these, one showed uniform receptive fields; the remaining two groups, with non-uniform RFs, were further subdivided in terms of orientation selectivity. By testing for directional sensitivity, organization of the RFs and orientation selectivity, the dynamic units were divided in 5 groups. Among these there was a predominance of directional units, displaying uniform RFs and showing orientation selectivity. Although the receptive fields would extend into the ipsilateral hemifield (up to 10 degrees), their centers were always located in the contralateral visual hemifield. Binocularly driven units predominate in both static and dynamic categories.

Visuotopic organization of the cebus pulvinar: a double representation the contralateral hemifield.

The projection of the visual field in the pulvinar nucleus was studied in 17 Cebus monkeys using electrophysiological techniques. Visual space is represented in two regions of the pulvinar; (1) the ventrolateral group, Pvlg, comprising nuclei P delta, P delta, P gamma, P eta and P mu 1; and (2) P mu. In the first group, which corresponds to the pulvinar inferior and ventral part of the pulvinar lateralis, we observed a greater respresentation of the central part of the visual field. Approximately 58% of the volume of the ventrolateral group is concerned with the visual space within 10 degrees of the fovea. This portion of the visual field is represented at its lateral aspects, mainly close to the level of the caudal pole of the lateral geniculate nucleus (LGN). Projection of the vertical meridian runs along its lateral border while that of the horizontal one is found running from the dorsal third of the LGN's hilus to the medial border of the ventro-lateral group. The lower quadrant is represented at its dorsal portion while the upper quadrant is represented at the ventral one. In Pmu the representation is rotated 90 degrees clockwise around the rostrocaudal axis: the vertical meridian is found at the ventromedial border of this nucleus. Thus, the lower quadrant is represented at the later portion of Pmu and the upper at its medial portion. Both projections are restricted to the contralateral hemifield.

Single unit response types in the pulvinar of the Cebus monkey to multisensory stimulation.

Response to sensory stimulation was studied in 162 neurons in the pulvinar of Cebus monkeys in 4 acute and 5 chronic preparations. Two basic response patterns were observed: type I responses, similar to those obtained in primary relay centers, were only observed after visual stimulation. Type II responses were obtained after stimulation of more than one sensory modality. Characteristically these responses presented fatigue and habituation. Temporal relationship between stimulus and response was not as clear as in type I responses, afterdischarge frequently occurred. Taking these response types into consideration two groups of units were identified in the pulvinar. Units of group A (91 neurons) showed type I response to visual stimulation. For these units receptive fields similar to those found in other regions of visual projection could be defined. As a rule units of group A displayed type II responses to other sensory modalities. Units of group B (71) did not display type I responses; they always responded to visual, somatic, auditory and olfactory stimuli with type II responses. They could be activated by a single sensory modality (B, unimodal) or by more than one sensory modality (B, multimodal).