Spatial contrast sensitivity changes after subcortical visual system lesions in the rat

Cerebral perfusion mapping during retrieval of spatial memory in rats

Studies of brain functional activation during spatial navigation using electrophysiology and immediate-early gene responses have typically targeted a limited number of brain regions. Our study provides the first whole brain analysis of cerebral activation during retrieval of spatial memory in the freely-moving rat. Rats (LEARNERS) were trained in the Barnes maze, an allocentric spatial navigation task, while CONTROLS received passive exposure. After 19 days, functional brain mapping was performed during recall by bolus intravenous injection of [¹⁴C]-iodoantipyrine using a novel subcutaneous minipump triggered by remote activation. Regional cerebral blood flow (rCBF)-related tissue radioactivity was analyzed by statistical parametric mapping from autoradiographic images of the three-dimensionally reconstructed brains. Functional connectivity was examined between regions of the spatial navigation circuit through interregional correlation analysis. Significant rCBF increases were noted in LEARNERS compared to CONTROLS broadly across the spatial navigation circuit, including the hippocampus (anterior dorsal CA1, posterior ventral CA1-3), subiculum, thalamus, striatum, medial septum, cerebral cortex, with decreases noted in the mammillary nucleus, amygdala and insula. LEARNERS showed a significantly greater positive correlation of rCBF of the ventral hippocampus with retrosplenial, lateral orbital, parietal and primary visual cortex, and a significantly more negative correlation with the mammillary nucleus, amygdala, posterior entorhinal cortex, and anterior thalamic nucleus. The complex sensory component of the spatial navigation task was underscored by broad activation across visual, somatosensory, olfactory, auditory and vestibular circuits which was enhanced in LEARNERS. Brain mapping facilitated by an implantable minipump represents a powerful tool for evaluation of mammalian behaviors dependent on locomotion.

Lateral masking effects on contrast sensitivity in rats

Changes in target visibility may be produced by additional stimulus elements at adjacent locations. Such contextual effects may reflect lateral interactions of stimulus representations in early cortical areas. It has been reported that the organization of orientation preference found in primates and cats visual cortex differs from that found in rodents, suggesting functional distinctions across species. In order to examine effects of lateral interactions at a perceptual level, contrast sensitivity in rats was measured for Gabor patches masked by two additional patches. Rats responded to target onset, and perceptual indices were based upon reaction time distributions across levels of luminance contrast. It was found that contrast sensitivity of targets without lateral masks corresponded to levels previously reported. For all measurements, the presence of sustained lateral masks systematically reduced sensitivity to targets, demonstrating interference by adjacent elements across levels of contrast. Effects of mask orientation or separation were not observed. These results may reflect reported non-systematic topography of orientation tuning across the cortex in rodents. Results suggest that intrinsic lateral connections in early processing areas play a minimal role in stimulus integration for rats.

Is blindsight an effect of scattered light, spared cortex, and near-threshold vision?

Blindsight is the term commonly used to describe visually guided behaviour elicited by a stimulus falling within the scotoma (blind area) caused by a lesion of the striate cortex. Such “vision” is normally held to be unconscious and to be mediated by subcortical pathways involving the superior colliculus. Blindsight is of considerable theoretical importance since it suggests that destriate man is more like destriate monkey than had been previously believed and also because it supports the classical notion of two visual systems. It is also of potential clinical importance, since it has been claimed recently that systematic practice in blindsight can lead to the recovery of normal visual function in patients with cortical lesions. From a review of the literature it is concluded that all of the phenomena of blindsight can be attributed either to light scatter into unimpaired parts of the visual field or to residual vision resulting from spared striate cortex. The possible contribution f other factors is also considered. It is concluded that blindsight studies have generally failed to control for such nonblindsight interpretations partly because of poor methodology and partly because of difficulties in defining the term “blindsight.”

Experiments were carried out to investigate the extent to which subjects can exhibit performance similar to blindsight when they are using scattered light as a cue. This was done both with hemianopic subjects (by manipulating the amount of scattered and direct light coming from a stimulus) and with normal subjects (by presenting targets within their blind spots). Good blindsight performance was observed when only scattered light was available as a cue to the subjects. It is therefore concluded that an adequate case for blindsight has not been made. It is probably impossible to demonstrate the existence of blindsight on purely behavioural grounds. What is required is the establishment of relationships between visual function and independent anatomical evidence.

The retinal projection to the pregeniculate nucleus in normal and destriate monkeys

Taking advantage of the segregation of different classes of ganglion cell fibres in the optic tract, we investigated the ganglion cell class that projects to the pregeniculate nucleus (PGN) in the normal macaque monkey (Macaca mulatta and Macaca fascicularis) and following long-standing removal of striate cortex in one hemisphere. Confining small pellets of horseradish peroxidase and biocytin into ventral or dorsal parts of the tract unilaterally, or placing several pellets throughout the tract, we labelled the retina retrogradely and the PGN anterogradely. Classification of ganglion cells according to soma size and dendritic morphology showed that implants in the dorsal part of the tract labelled predominantly, and perhaps exclusively, P beta cells, and produced dense anterograde label in the parvocellular lateral geniculate nucleus as well in the PGN. Labelling of the PGN was sparse or absent following implants into the ventral aspect of the tract, which labelled the magnocellular geniculate nucleus anterogradely and chiefly P alpha and P gamma cells retrogradely. The finding of a projection to the PGN from P beta cells has implications for the permanent selective sparing of a subpopulation of these cells after removal of striate cortex and their contribution to wavelength processing in blindsight.

The ventral lateral geniculate nucleus and the intergeniculate leaflet: interrelated structures in the visual and circadian systems

The ventral lateral geniculate nucleus (vLGN) and the intergeniculate leaflet (IGL) are retinorecipient subcortical nuclei. This paper attempts a comprehensive summary of research on these thalamic areas, drawing on anatomical, electrophysiological, and behavioral studies. From the current perspective, the vLGN and IGL appear closely linked, in that they share many neurochemicals, projections, and physiological properties. Neurochemicals commonly reported in the vLGN and IGL are neuropeptide Y, GABA, enkephalin, and nitric oxide synthase (localized in cells) and serotonin, acetylcholine, histamine, dopamine and noradrenalin (localized in fibers). Afferent and efferent connections are also similar, with both areas commonly receiving input from the retina, locus coreuleus, and raphe, having reciprocal connections with superior colliculus, pretectum and hypothalamus, and also showing connections to zona incerta, accessory optic system, pons, the contralateral vLGN/IGL, and other thalamic nuclei. Physiological studies indicate species differences, with spectral-sensitive responses common in some species, and varying populations of motion-sensitive units or units linked to optokinetic stimulation. A high percentage of IGL neurons show light intensity-coding responses. Behavioral studies suggest that the vLGN and IGL play a major role in mediating non-photic phase shifts of circadian rhythms, largely via neuropeptide Y, but may also play a role in photic phase shifts and in photoperiodic responses. The vLGN and IGL may participate in two major functional systems, those controlling visuomotor responses and those controlling circadian rhythms. Future research should be directed toward further integration of these diverse findings.

The pretectum and visual discrimination learning in the hooded rat

In previous studies on the effects of pretectal lesions on visual discrimination performance, the lesions have either been small or their effects confounded with damage to the adjacent thalamus and the results have been inconsistent. This study compares the effects of large pretectal lesions (PRT) with lesions in lateral (TLP) and medial (MPT) posterior thalamus on simultaneous black versus white (BW), horizontal versus vertical (HV) discrimination acquisition and low frequency flicker detection. An unoperated control group treated regularly with a cycloplegic to produce mydriasis was also tested in the discrimination learning phase. PRT lesions and treatment with the cycloplegic produced mydriasis as well as an impairment on BW. Neither group was impaired on HV. Animals with MPT and TLP lesions were impaired on BW and HV and those with TLP lesions were also impaired on flicker detection. It is concluded that the pretectum is important for visual intensity discrimination learning but the effects of lesions are due to disturbances of pupillary control. The significance of this finding for the interpretation of deficits produced by lesions elsewhere in the thalamus is discussed.

Spatial contrast and flicker sensitivity following medial thalamic or visual cortex lesions in hooded rats

In rats, lesions of the medial thalamus that involve the intralaminar nuclei produce a number of visual impairments similar to those obtained with visual cortex ablation, suggesting that the former functionally disrupt visual cortex. This was tested by a direct comparison between the two lesions on spatial contrast and flicker sensitivity, using behavioural techniques. Both lesions depressed spatial contrast sensitivity, visual cortex damage more than thalamic lesions. Only medial thalamic lesions produced statistically significant changes in flicker sensitivity. This dissociation suggests that medial thalamic and visual cortex lesions are disrupting different mechanisms. Alternative explanations of the thalamic deficit are discussed.

Flicker sensitivity changes after subcortical visual system lesions in the rat

Sensitivity to sinusoidal flicker, as a function of flicker frequency, was measured behaviourally in hooded rats by reducing modulation depth in a two-choice flicker versus no-flicker discrimination until subjects could not perform at above 80% correct. Analogous methods were used to measure spatial contrast sensitivity. In both tasks the display area was 24 x 20 degrees. Bilateral lesions were made in one of 4 structures; superior colliculus (SC), pretectum (PRT), posterior thalamus (PT), or ventral lateral geniculate nucleus (LGv). A fifth group served as sham operated controls. On the basis of histology the LGv group was subdivided according to presence or absence of optic tract damage. PT, PRT and LGv lesions produced a statistically significant depression in flicker sensitivity, the impairment in the LGv sub-group with optic tract damage being significantly greater than that in the LGv group with optic tract sparing. In the latter, post-operative sensitivity correlated significantly with amount of surviving tissue in the thalamic radiations but not with surviving LGv itself. PRT and LGv lesions that involved the optic tract also significantly depressed spatial contract sensitivity. The implications of the finding, that PT and LGv lesions may depress flicker sensitivity without affecting spatial vision, for interpretation of the effects of comparable lesions on suprathreshold discrimination are discussed.