Chapter 7 - The striate cortex and hemianopia

Visual areas I and II of cerebral cortex of rabbit.

Perception Matches Selectivity in the Human Anterior Color Center

Changes in visual cortex excitability in blind subjects as demonstrated by transcranial magnetic stimulation. Gothe J, Brandt SA, Irlbacher K, Roricht S, Sabel BA, Meyer B-U. Brain 2002;125:479–490.

The connections of cingulate cortex with visual, motor, and parahippocampal cortices in the rabbit brain are evaluated by using a modified Brodmann cytoarchitectural scheme, electrophysiological mapping techniques, and the pathway tracers horseradish peroxidase (HRP) and tritiated amino acids. Rabbit cingulate cortex can be divided into areas 25, 24, and 29. Area 29 is of particular interest because area 29d has a lateral extension with a granular layer IV, area 29b has a caudal extension in which the connections differ from anterior area 29b, and there is a prominent area 29e. Cytoarchitectural delineation of the lateral border of area 29d with area 17 closely approximates the medial edge of the visual field representation in area 17 as determined electrophysiologically. The main interconnections between visual and cingulate cortices occur between cingulate areas 24b and 29d and visual areas 18 and medial parts of area 17. Projections between areas 29d and 18 are organized in a loose topographic fashion with rostral parts of each and caudal parts of each being reciprocally connected. Neurons mainly in superficial layer II-III of areas 17 and 18 project to layer I of area 29d, while the reciprocal projection originates from neurons in layer V of area 29d and project mainly to layer I of areas 17 and 18. The medial portion of motor area 8 projects to areas 18 and 29d and has a smaller projection to area 17. Postsubicular area 48 is reciprocally connected with area 29d, and it also projects to areas 29b and c. The subiculum projects to areas 29a and 29c but only to the anterior two-thirds of area 29b not the posterior one-third. Rostral area 29d receives the most extensive intrinsic cingulate projections including those from all major cytoarchitectural divisions. Interconnections between areas 29d and 29b appear to be topographically organized in the rostrocaudal plane. Area 29c projects more heavily to area 29b than vice versa. Finally area 29d projects mainly to area 24b in anterior cingulate cortex. In conclusion, rostral area 29d has extensive connections with visual areas 17 and 18, motor area 8, and all subdivisions of cingulate cortex. In light of these connections, it may play a pivotal role in associative functions of the rabbit cerebral cortex including visuomotor integration.

In a survey of methods for determining hemianopic visual field defects, a distinction is made between gross visual field screening and actual visual field examination with kinetic and automatic static perimetry. These methods may be arranged according to increasing the concentration required as well as to ability to cooperate, as follows: hand-movement stimulation of shifts of gaze, finger-counting with repetition by the patient, brightness and color comparison among the respective halves and quadrants of the visual field, then kinetic perimetry with the Goldmann perimeter, and, most exacting of all, automated perimetry. The advantages of kinetic perimetry in cases of incipient bitemporal visual field defects are discussed. For visual field examination with automated static perimeters, test programs are chosen which have their test-point pattern shifted with respect to the principal meridian. The diagnostic situations in which visual field testing may and must not be limited to 30 degrees are presented in tabular form. Guidelines and criteria for localizing lesions have been developed, i.e., the vertical and horizontal limits of the major axes which pass through the fixation point and the temporal crescent. Consideration of the course of the nerve fibers in the retina and of the fact that exact separation between superior and inferior may only be found temporal to the foveola with corresponding nasal defects enables horizontally limited visual field defects originating within the eye to be distinguished clearly from those originating in the visual cortex. Vertical limits due to semidecussation at the chiasma permit a differentiation between prechiasmic and postchiasmic lesions. In homonymous defects, consideration of further simple anatomic features of the visual pathway, namely the knee of Wilbrand and that portion of the optic radiation which extends anteriorly to the temporal lobes and which represents the inferior retinal halves with characteristic defects, also enables lesions in the visual pathway to be localized more accurately.

Reliability and Representational Bandwidth in the Auditory Cortex

The concepts of the representation of visual field in primary visual cortex are based on studies of war wounds and correlations with brain imaging in small cohorts. Because of the difficulty of judging brain lesion extent and the small number of studied patients, there is lingering controversy as to whether the central 15° of visual field are mapped onto the posterior 25% of primary visual cortex or onto a larger area. To improve the delineation of MRI lesion extent, we have studied only patients with posterior cerebral artery (PCA) ischemic strokes. We accrued a cohort of 92 patients with PCA strokes from an electronic medical records search between 2009 and 2020 at a single tertiary care academic institution. Patients had reliable static perimetry demonstrating homonymous hemianopias and high-definition reviewable brain imaging. We divided the primary visual cortex on the MRI T1 sagittal sequence into 8 equal segments in right and left cerebral hemispheres and located lesions according to the segments they occupied. We correlated lesion locations with 3 visual field defects (VFDs): macular-sparing homonymous quadrantanopias, macular-splitting homonymous quadrantanopias, and homonymous paracentral scotomas. Among 25 cases with macular sparing, 13 had lesion-sparing confined to the posterior 25% of visual cortex. Among 6 cases with homonymous paracentral scotomas, 2 had lesions confined to the posterior 25% of visual cortex. Macular-splitting quadrantanopia did not occur in any patients with lesions confined to the posterior 25% of visual cortex, but did occur in 3 patients with lesions confined to the posterior 50% of visual cortex. These phenomena would not be expected if the central 15° of visual field were mapped onto a region extending beyond the posterior 25% of visual cortex. In patients with PCA strokes that involved the retrogeniculate visual pathway proximal to visual cortex, the visual cortex lesions were often less extensive than predicted by the VFDs, perhaps because of widespread damage to axons before they reached their destination in visual cortex. These results support the concept that the central 15° of the visual field are represented in the posterior 25% of visual cortex. Although this study contributes a larger cohort of patients with better-defined lesion borders than in past reports, its conclusions must be tempered by the variability of patient attention during visual field testing, the subjectivity in the interpretation of the defect patterns, and the difficulty in judging MRI lesion extent even on diffusion-weighted and precontrast T1 sagittal sequences.

NMDA receptor-dependent oscillatory signal outputs from the retrosplenial cortex triggered by a non-NMDA receptor-dependent signal input from the visual cortex

Specific functional asymmetries of the human visual cortex revealed by functional near-infrared spectroscopy

Two experiments were carried out with Mongolian gerbils to determine the roles of optic tectum and visual cortex in the mediation of visually guided head turns and locomotion elicited and controlled by discrete visual targets. In Experiment 1, the behavior of animals with either a sham operation, a bilateral lesion of optic tectum, or a bilateral ablation of areas 17, 18a, and 18b was recorded on videotape as they ran from the center of a circular arena toward a small visual target projected in different locations around the perimeter of the arena. The amplitude and direction of the head turns and the accuracy of their locomotor responses were reconstructed from a frame by frame analysis of the videotapes. Sham-operate gerbils made a series of head turns before running accurately and efficiently toward the target. The gerbils with lesions of areas 17, 18a, and 18b rarely made more than one head turn before running toward the perimeter of the arena. Although the single head turn they did make was often well-correlated with the position of the target in their visual field, the direction of their locomotor response was largely determined by the direction and amplitude of that head turn. As a consequence, these animals undershot the target more often than did the sham-operate animals, and even ran into the visual half field opposite the target if their head turn had also been made into that half field. Unlike the sham operates, these animals were unable to make further adjustments in their orientation toward the stimulus after their initial head turn. The head turns and locomotor behavior of the gerbils with lesions of optic tectum were even more disorganized and inaccurate than those of the posterior decorticates. Nevertheless, when the target was presented within 45 degrees from their visual midline, their head turns and locomotor responses showed a systematic relationship with the eccentricity of the target. Their behavior to stimuli outside this central wedge of their visual field was completely disorganized and showed no relationship to the location of the target. In Experiment 2, unilateral lesions of area 17 were performed in the gerbils that had already received bilateral tectal lesions to determine whether such lesions would affect the "residual" ability of these animals to orient toward stimuli located within the central portion of their visual field. During retesting, these animals were able to respond to targets only if they were located in the central portion of the field ipsilateral to the cortical lesion.(ABSTRACT TRUNCATED AT 400 WORDS)

The cholinergic system is a potent neuromodulatory system that plays critical roles in cortical plasticity, attention and learning. In this review, we propose that the cellular effects of acetylcholine (ACh) in the primary visual cortex during the processing of visual inputs might induce perceptual learning; i.e., long-term changes in visual perception. Specifically, the pairing of cholinergic activation with visual stimulation increases the signal-to-noise ratio, cue detection ability and long-term facilitation in the primary visual cortex. This cholinergic enhancement would increase the strength of thalamocortical afferents to facilitate the treatment of a novel stimulus while decreasing the cortico-cortical signaling to reduce recurrent or top-down modulation. This balance would be mediated by different cholinergic receptor subtypes that are located on both glutamatergic and GABAergic neurons of the different cortical layers. The mechanisms of cholinergic enhancement are closely linked to attentional processes, long-term potentiation (LTP) and modulation of the excitatory/inhibitory balance. Recently, it was found that boosting the cholinergic system during visual training robustly enhances sensory perception in a long-term manner. Our hypothesis is that repetitive pairing of cholinergic and sensory stimulation over a long period of time induces long-term changes in the processing of trained stimuli that might improve perceptual ability. Various non-invasive approaches to the activation of the cholinergic neurons have strong potential to improve visual perception.

We examined the effects of reversible inactivation of a higher-order, pattern/form-processing, postero-temporal visual (PTV) cortex on the background activities and spike-responses of single neurons in the ipsilateral cytoarchitectonic area 19 (putative area V3) of anesthetized domestic cats. Very occasionally (2/28), silencing recurrent “feedback” signals from PTV, resulted in significant and reversible reduction in background activity of area 19 neurons. By contrast, in large proportions of area 19 neurons, PTV inactivation resulted in: (i) significant reversible changes in the peak magnitude of their responses to visual stimuli (35.5%; 10/28); (ii) substantial reversible changes in direction selectivity indices (DSIs; 43%; 12/28); and (iii) reversible, upward shifts in preferred stimulus velocities (37%; 7/19). Substantial (≥20°) shifts in preferred orientation and/or substantial (≥20°) changes in width of orientation-tuning curves of area 19 neurons were however less common (26.5%; 4/15). In a series of experiments conducted earlier, inactivation of PTV also induced upward shifts in the preferred velocities of the ipsilateral cytoarchitectonic area 17 (V1) neurons responding optimally at low velocities. These upward shifts in preferred velocities of areas 19 and 17 neurons were often accompanied by substantial increases in DSIs. Thus, in both the primary visual cortex and the “intermediate” visual cortex (area 19), feedback from PTV plays a modulatory role in relation to stimulus velocity preferences and/or direction selectivity, that is, the properties which are usually believed to be determined by the inputs from the dorsal thalamus and/or feedforward inputs from the primary visual cortices. The apparent specialization of area 19 for processing information about stationary/slowly moving visual stimuli is at least partially determined, by the feedback from the higher-order pattern-processing visual area. Overall, the recurrent signals from the higher-order, pattern/form-processing visual cortex appear to play an important role in determining the magnitude of spike-responses and some “motion-related” receptive field properties of a substantial proportion of neurons in the intermediate form-processing visual area—area 19.

Visual perception appears largely stable in time. However, psychophysical studies have revealed that theta waves (4–8 Hz) can modulate perception and have been linked to various cognitive states and functions. Neither the contribution of theta waves to cortical activity nor their impact during aberrant brain states have been resolved at high spatiotemporal scales. Here, using cortex-wide population voltage imaging in awake mice, we found that bouts of 5-Hz oscillations in the visual cortex are accompanied by similar oscillations in the retrosplenial cortex, occurring both spontaneously and evoked by visual stimulation. Injection of psychotropic 5-HT2AR agonist induced a significant increase in spontaneous 5-Hz oscillations, and also increased the power, occurrence probability and temporal persistence of visually evoked 5-Hz oscillations. This modulation of 5-Hz oscillations in both cortical areas indicates a strengthening of top-down control of perception, supporting an underlying mechanism of perceptual filling and visual hallucinations.

The cholinergic system is involved in cortical plasticity, attention, and learning. Within the visual cortex the cholinergic system seems to play a role in visual perception. The cholinergic neurons which project into the visual cortex are located in the basal forebrain. It has been shown that mice deficient for the low-affinity neurotrophin receptor p75NTR display increased numbers of cholinergic neurons in the basal forebrain and a denser cholinergic innervation of the hippocampus. This prompted us to analyze whether the cholinergic system is altered in adult p75NTR deficient mice. By analyzing the densities of cholinergic fibers within layer IV as well as within layer V of the visual cortex, we found that adult p75NTR deficient mice display increased cholinergic fiber densities. However, this increase was not accompanied by an increase in the density of local cholinergic neurons within the visual cortex. This indicates that the enhanced cholinergic innervation of the visual cortex is due to alteration of the cholinergic neurons located in the basal forebrain, projecting to the visual cortex. The increased cholinergic innervation of the visual cortex makes the p75NTR deficient mice an attractive model to study the necessity of the cholinergic system for the visual cortex.

The projection from striate and prestriate visual cortex to the pontine nuclei has been studied in the macaque monkey by means of anterograde tracer techniques in order to assess the contribution of anatomically and functionally distinct visual cortical areas to the cortico-ponto-cerebellar loop. No projection to the pons was found from central or paracentral visual-field representations of V1 (striate cortex) or prestriate visual areas V2, and V4. Small patches of terminal labeling occurred after injections of tracer into more peripheral parts of V1, V2 and V3, and into V3A. The terminal fields were located most dorsolaterally in the anterior to middle third of the pons and were quite restricted in their rostro-caudal extent. Injections of V5, however, yielded substantial terminal labeling, stretching longitudinally throughout almost the entire pons. This projection could be demonstrated to arise from parts of V5 receiving input from central visual-field representations of striate cortex, whereas parts of V4 receiving similarly central visual-field input had no detectable projection to the pons. Its distribution may overlap to a large extent with the termination of tecto-pontine fibers and with the termination of fibers from visual areas in the medial bank (area V6 or PO) and lateral bank (area LIP) of the intraparietal sulcus, as well as from frontal eye fields (FEF). It appears that the main information relayed to the cerebellum by the visual corticopontine projection is related to movement in the field of view.