Procesamiento cortical de la visión: interacción de subsistemas parietales / Cortical processing of vision: parietal subsystems interaction

El procesamiento visual en el contexto de la neuropsicología exige la articulación de distintos niveles de análisis. Esto hace que el estudio de la visión se encuentre permeado por el avance en distintas disciplinas lo cual plantea como reto a la neuropsicología la construcción de un marco conceptual y de organización cerebral que permita la articulación de los vastos y distintos conocimientos que en la actualidad se tienen sobre el tema. Por tal motivo el presente artículo tiene como objetivo revisar los aportes de los principales modelos dicotómicos del procesamiento visual empleados en neuropsicología para detenernos con mayor detalle en la participación del Lóbulo Parietal Posterior y los retos que plantea a dichos modelos.

Visual processing requires, from a neuropsychological view, the integration of different analysis levels. This makes visual studies dependent on the advances produced by different scientific disciplines, leading into an increasing and comprehensive conceptual demand as well as more consistent brain functioning models. Thus, the aim of this article is to review the most outstanding dichotomic visual processing models used in neuropsychology, so as to painstakingly analyze the role of the Posterior Parietal Lobe and the risks taken by those different models.

Efficiency of information coding in various L/M retinal cone ratios

Previous evidence has shown that the number of L and M cones in retina varies significantly between subjects. However, it is not clear how the variation of L/M ratio changes the behavioral performance of the subject. A model of transformation of data from retina to visual cortex for evaluation of various L/M cones ratios is presented. While L/M cone ratios close to 1 brings the best performance for one of postreceptoral [magnocellular] channels, we showed that the performance in the second channel [parvocells] will improve when the ratio furthers away from 1. effects of different ratios of S were also explored

Changes in visual evoked potentials on acute induction to high altitude.

BACKGROUND & OBJECTIVES: Induction to high altitude leads to altered central nervous system (CNS) functions induced by hypobaric hypoxia. The sensory systems like visual and auditory systems are reported to be affected by hypoxia. The present study was undertaken to assess the effects of hypobaric hypoxia on visual evoked potentials (VEPs) at 3200 m and 4300 m. METHODS: The VEP of 30 human volunteers were recorded at sea level (SL) and then at high altitude (HA) of 3200 m (HA I) and 4300 m (HA II) in eastern Himalayas and on return to sea level (RSL). The absolute latencies and amplitude of positive and negative waves were recorded. RESULTS: High altitude of 3200 m did not alter the latency of major wave N1, P1 and N2 of the VEPs. At HA II (4300 m), there was a statistically significant increase (P < 0.01) in the latency of NI wave as compared to the SL value and HA I (3200 m) in both left and right eye. There was a slight increase in latenty of P1 wave in both left and right eye at HA II. INTERPRETATION & CONCLUSION: From these results it may be concluded that induction to HA causes increase in latency of N1 wave at 4300 m which is still within physiological limits. This increase in wave latency of N1 wave in both eyes may be due to synaptic delay and/or altered neuronal processing at HA.

Long distance communication in the human brain: timing constraints for inter-hemispheric synchrony and the origin of brain lateralization

Analysis of corpus callosum fiber composition reveals that inter-hemispheric transmission time may put constraints on the development of inter-hemispheric synchronic ensembles, especially in species with large brains like humans. In order to overcome this limitation, a subset of large-diameter callosal fibers are specialized for fast inter-hemispheric transmission, particularly in large-brained species. Nevertheless, the constraints on fast inter-hemispheric communication in large-brained species can somehow contribute to the development of ipsilateral, intrahemispheric networks, which might promote the development of brain lateralization.

Visual electrophysiology and objective visual function / 法医学杂志

This review summarizes objective assessment of visual function using visual electrophysiology. Objective assessment of visual acuity using pattern visual evoked potential (PVEP) and sweep pattern visual evoked potential (SPVEP), objective assessment of visual field using multifocal visual evoked potential (MVEP).

Visual evoked potentials in primary hypertension.

Functional integrity of sensory pathways in the brain has not been well documented in hypertension. It is suggested that vascular endothelial changes including hyalinisation during hypertension may lead to demyelination in the vulnerable areas of the brain. Since optic nerve is considered to be part of brain hence the present study was done to find out if visual pathways are involved in hypertension. Transient pattern reversal visual evoked potentials (VEPs) from 01, 02 scalp regions were recorded in 23 primary hypertensive patients and compared with 14 normotensive control. Of these, six patients showed delayed P1 latencies beyond 99% tolerance limit i.e. Mean + 3 SD of normal. The remaining 17 had latencies of all positive (P1-P3) and negative (N1-N3) waves comparable to those of the control group. Correlation Coefficient worked out, showed significant correlation between systolic BP and P1 latency in the control group only. No other parameter showed any correlation with P1 latency & amplitude in both the groups. These findings show that fluctuations of BP in normotensive subjects have correlation with P1 latency. This correlation ceased to exist in hypertensive patients and abnormality in P1 latency of VEP was detected in 26% cases.

Distribution of AMPA-type glutamate receptor subunits in the chick visual system

Several glutamate receptor (GluR) subunits have been characterized during the past few years. In the present study, subunit-specific antisera were used to determine the distribution of the AMPA-type glutamate receptor subunits GluRl-4 in retinorecipient areas of the chick brain. Six white leghorn chicks (Gallus gallus, 7-15 days old, unknown sex) were deeply anesthetized and perfused with 4 per cent buffered paraformaldehyde and brain sections were stained using immunoperoxidase techniques. The AMPA-type glutamate receptor subunits GluRl, GluR2/3 and GluR4 were present in several retinorecipient areas, with varying degrees of colocalization. For example, perikarya in layers 2, 3, and 5 of the optic tectum contained GluRl, whereas GluR2/3 subunits appeared mainly in neurons of layer 13. The GluR4 subunit was only detected in a few cells of the tectal layer 13. GluRl and GluR2/3 were observed in neurons of the nucleus geniculatus lateralis ventralis, whereas GluR4 was only present in its neuropil. Somata in the accessory optic nucleus appeared to contain GluR2/3 and GluR4, whereas GluR1 was the dominant subunit in the neuropil of this nucleus. These results suggest that different subpopulations of visual neurons might express different combinations of AMPA-type GluR subunits, which in turn might generate different synaptic responses to glutamate derived from retinal gangliom cell axons.

Parallel pathways in the retina of old and new world primates

Old-world simians are all trichromats, but in most new-world primates there is a polymorphism; males are dichromats but most females are trichromats. In the old world simian, luminance and red-green chromatic channels defined by psychophysical experiments have as a basis parasol ganglion cells of the magnocellular (MC) pathway and midget ganglion cells of the parvocellular (PC) pathway respectively. Small bistratified ganglion cells provide a basis for a blue-yellow chromatic channel, which should probably be considered a separate entity. In both dichromatic and trichromatic new-world animals, the MC pathway and the small bistratified, blue-yellow system seem anatomically and physiologically similar to those in their old-world relatives. The midget ganglion cells of the parvocellular pathway in trichromats are anatomically and physiologically similar to the old-world pattern. In dichromatic animals, they are anatomically similar and physiologically resemble those of trichromatic animals, except for the lack of chromatic opponency. We conclude that these three systems may form a basic pattern for the visual pathway of primates. However, the results from dichromats indicate that the evolution of trichromacy may be found to be more complex than presently supposed.

Onset and offset inhibitions: effect of increase and decrease of cue luminance on manual reaction time to a visual target

Simple reaction time (RT) to a peripheral visual target is shortened when a non-informative cue is flashed at the target location 100-150 ms before target onset (early facilitation). With longer intervals, RT to targets appearing at cue hemifield is lengthened (inhibition of return). In the present study, we investigated these effects inverting the stimulus contrast in relation to background to see how these effects are related to the onset and/or to the offset of a cue darker or brighter than background. Ten subjects were asked not to respond to a non-informative cue (S1) appearing on a computer screen 6§ to the right or to the left of the center of a fixation cross (FP), but to respond, by pressing a microswitch, to a target (S2) occurring at 4§ from the FP in the same hemifield as S1 or in the opposite hemifield. There were two different types of sessions. In one, S1 and S2 were bright against a dark background and in the other, S1 and S2 were dark against a bright background. In each session there were two types of trials. In OFF trials, each trial began with the presentation of FP. Five hundred ms later, S1 appeared and remained on for 700 ms. S2 appeared 100 or 800 ms after the offset of S1. In ON trials, S1 onset occurred 1200 ms after the beginning of the trial and remained on until the end of trial. S2 appeared 100 or 800 ms after S1 onset. When S2 onset followed S1 onset by 100 ms, RT to S2 occuring in the same hemifield did not differ from RT when S1 and S2 were in opposite hemifields. In contrast, after an S1-S2 interval of 800 ms, S1 onset elicited an inhibition of its hemifield. This inhibition was similar to that observed 100 ms or 800 ms after S1 offset. The same results were obtained if the cue and target were brighter or darker than the background, showing that the ONSET and OFFSET inhibitions are related not to cue luminance increase and decrease, respectively, but to the appearance and the disappearance of a salient stimulus in a homogeneous background. Moreover, these results suggest that ON and OFF channels of the visual system have similar effects on the orienting of attention

Extraretinal modulation of accessory optic units in the pigeon

Electrophysiolgical methods were used to investigate the pretectal and telencephalic control of units within the nucleus of the basal optic root (nBOR) of the accessory optic system of pigeons. Electrical stimulation of the pretectal lentiform nucleus mainly produced excitatory effects on nBOR units with temporal-to-nasal directional preference (among 109 cells, 51% were excited, 23% were inhibited and 23% were not affected) and inhibitory effects on units with nasal-to-temporal preferences (among 88 cells, 43% were inhibited, 38% were not affected and 19% were excited). Electrical stimulation of the visual cortex) produced mainly inhibitory effects on units with downward preferences (among 123 units, 47% were inhibited, 24% were excited and 29% were not affected), and mixed effects on units with upward preferences (among 79 cells, 30% were excited, 36% were inhibited and 34% were not affected). Excitatory effects of LM stimulation had first-spike latente latencies ranging from 2-20 ms (mean ñ SEM, 5.5 ñ 0.7 ms), whereas latencies of W stimulation ranged from 6 to 30 ms (13.0 ñ 0.9 ms). These results provide additional information about the neural substrates of the optokinetic nystagmus

The projection of GABAergic neurons of the medial terminal accessory optic nucleus to the pretectum in the rat

Large numbers of neurons were retrogradely labeled in both the dorsal and ventral medial terminal nucleous (MTN) after fluoro-gold injections into the rat pretectal nucleus of the optic tract/dorsal terminal nucleus (NOT/DTN). Fluorescence immunocytochemistry for GABA in the same brains revealed GABA-positive neurons distributed mainly in the dorsal MTN. Approximately half of all the GABAergic neurons in the MTN were double-labeled. Therefore, GABAergic neurons comprise a significant component of the MTN-NOT-DTN projection which most likely inhibits the pretectal pathway mediating horizontal optokinetic nystagmus

Efferent projections of the pigeon intergeniculate leaflet containing neuropeptide Y

The present study was performed to map efferent projections of the pigeon intergeniculate leaflet (IGL) to other visual structures, with emphasis on the pathways containing neuropeptide Y (NPY). After injections of an anterograde tracer (Phaseolus vulgaris leucoagglutinin) into the IGL, labeled axons and presumptive terminals were seen in several retinorecipient and visually-related nuclei. All such areas contained immunoreactive fibers to antibodies against NPY. Electrolytic lesion of the IGL provoked a marked reduction in the number of NPY-labeled fibers in these visual structures. The data suggest that the IGL is the source of NPY-labeles axons which occur in many visual nuclei of the pigeon brain

Evidence for recrossing of the pathway from the retina to the ipsilateral nucleus of the optic tract

Single-unit recordings of the nucleous of the optic tract (NOT) under visual stimulation were performed in 5 opossums. Most of the units were directionally selective. Receptive fields for the contralateral eye lie mainly in the contralateral field while those for the ipsilateral eye were mainly in the ipsilateral field. As the nasal does not project ipsilaterally, recrossing must occur in the pathway from the retina to the ipsilateral NOT. Possible sites for this recrossing are discussed

Development of abnormal lamination and binocular segregation of retinal afferents onto the rat superior colliculus

Unilateral optic tract lesions made in newborn rats abnormal retinotectal pathways on the opposite side. The present in investigation was designed to study the development of the abnormal retinal projections in the superior colliculus using antomical tracing methods. The aberrant uncrossed retinotectal pathway develops within the first postnatal week. In spite of this, the retration of the crossed projection, wich indicates binocular segregation, is of late onset. This indicates that the induced segregation of retinal imputs is not dependent on regressive events such as ganglion cell death and terminal field retraction. These data and the results of lid-suture experiments are consistent with a role for spontaneous retinal activity in the regulation of the plasticity of retinal projections to the rat superior colliculus

Telencephalic and pretectal modulation of the directional selectivity of accessory optic neurons in the pigeon

Direction-selective units within the accessory optic system of the pigeon were shown to respond more strongly to motion along two main directions, downward-nasal and upward-temporal. Following ipsilateral telencephalic or pretectal lesions, these directions were modified in a systematic way. In the former, the principal response directions were downward-nasal and temporal and in the latter, downward-temporal and upward-temporal. These data indicate that the non-retinal afferents play an important role in the functional organization of the accessory optic system

A study of retinal projections in the ground squirrel, Spermophilus tridecemlineatus, using anterograde transport techniques

Luego de aplicar inyecciones intraoculares unilaterales de prolina radioactiva o de mezclas de fucosa y prolina, consistentemente se detectó radioactividad en el núcleo supraquismático, en los núcleos terminales medial, lateral y dorsal del sistema ópico accesorio, en los núcleos geniculado lateral dorsal y geniculado lateral ventral, en los núcleos pretectales anterior, olivar, posterior y núcleo del tracto óptico y en el colículo superior. En aquellas ardillas terrestres que recibieron una dósis grande de compuesto radioactivos y que sobrevieron durante períodos largos de tiempo después de las inyecciones, también se detectó radioactividad en el núcleo lateral posterior, el núcleo parabigémino y en una porción de la materia gris central del puente. Luego de que se aplicaran inyecciones intraoculares de peroxidasa de rábano, el marcador fue detectado en los lugares que convencionalmente se marcan, y en adición, en el núcleo lateral posterior. Por lo tanto, el marcador presente en todos los núcleo, parabigémino y la materia gris pontina, puede atribuirse a proyecciones directas de la retina. La mayor parte de las proyecciones fueron bilaterales y se podían observar algunas diferencias relacionadas con la lateralidad de las proyecciones. En el núcleo geniculado lateral dorsal se definieron tres láminas en base a las proyecciones de las retinas de ojo ipsilateral y contralateral; se definió una cuarta lámina en base a citoarquitectura. La proyección de la cabeza del nervio óptico se pudo observar en las láminas del núcleo geniculado latreral dorsal que recibían inervación contralateral y en el colículo superior contralateral al ojo inyectado