Lea Hyvärinen

Early visual deprivation alters modality of neuronal responses in area 19 of monkey cortex

Monkeys deprived on vision during the first year of life by lid suture appear functionally blind after the opening of the eyes, but move actively in familiar surroundings using somesthetic cues. Microelectrode recordings of multiple unit activity in the associative visual cortical area 19 of deprived monkeys indicated that 20% of the neuron groups studies responded only during active somatic exploration. In normal animals all neuron groups studied responses exclusively to visual stimuli, but in the deprived animals only 40% of them did. Visual deprivation alters the synaptic pathways to visual associative cortex enhancing the efficiency of those inputs that can mediate somatic information to this region.

Training-induced cortical representation of a hemianopic hemifield

Background: Patients with homonymous hemianopia often have some residual sensitivity for visual stimuli in their blind hemifield. Previous imaging studies suggest an important role for extrastriate cortical areas in such residual vision, but results of training to improve vision in patients with hemianopia are conflicting. Objective: To show that intensive training with flicker stimulation in the chronic stage of stroke can reorganise visual cortices of an adult patient. Methods: A 61-year-old patient with homonymous hemianopia was trained with flicker stimulation, starting 22 months after stroke. Changes in functioning during training were documented with magnetoencephalography, and the cortical organisation after training was examined with functional magnetic resonance imaging (fMRI). Results: Both imaging methods showed that, after training, visual information from both hemifields was processed mainly in the intact hemisphere. The fMRI mapping results showed the representations of both the blind and the normal hemifield in the same set of cortical areas in the intact hemisphere, more specifically in the visual motion-sensitive area V5, in a region around the superior temporal sulcus and in retinotopic visual areas V1 (primary visual cortex), V2, V3 and V3a. Conclusions: Intensive training of a blind hemifield can induce cortical reorganisation in an adult patient, and this case shows an ipsilateral representation of the trained visual hemifield in several cortical areas, including the primary visual cortex.

CONTRAST SENSITIVITY IN EVALUATION OF VISUAL IMPAIRMENT DUE TO DIABETES

Spatial contrast sensitivity of 19 diabetics with different degrees of visual impairment was studied. It was found that contrast sensitivity at intermediate and low spatial frequencies may decrease without corresponding loss of visual acuity. In advanced cases of diabetes the opposite may be true: contrast sensitivity was better than expected on the basis of visual acuity. Thus both visual acuity and contrast sensitivity measurements are useful in the evaluation of the nature of visual impairment due to diabetic eye disease.

Temporal sensitivity in a hemianopic visual field can be improved by long-term training using flicker stimulation

Background: Blindness of a visual half-field (hemianopia) is a common symptom after postchiasmatic cerebral lesions. Although hemianopia severely limits activities of daily life, current clinical practice comprises no training of visual functions in the blind hemifield. Objective: To find out whether flicker sensitivity in the blind hemifield can be improved with intensive training, and whether training with flicker stimulation can evoke changes in cortical responsiveness. Methods: Two men with homonymous hemianopia participated in the experiments. They trained with flicker stimuli at 30° or with flickering letters at 10° eccentricity twice a week for a year, and continued training with more peripheral stimuli thereafter. Neuromagnetic responses were registered at 1–2-month intervals, and the Goldmann perimetry was recorded before, during and after training. Results: Flicker sensitivity in the blind hemifield improved to the level of the intact hemifield within 30° eccentricity in one participant and 20° eccentricity in the other. Flickering letters were recognised equally at 10° eccentricity in the blind and intact hemifields. Improvement spread from the stimulated horizontal meridian to the whole hemianopic field within 30°. Before training, neuromagnetic recordings showed no signal above the noise level in the hemianopic side. During training, evoked fields emerged in both participants. No changes were found in the Goldmann perimetry. Discussion: Results show that sensitivity to flicker could be fully restored in the stimulated region, that improvement in sensitivity spreads to the surrounding neuronal networks, and that, during training, accompanying changes occurred in the neuromagnetic fields.

Parafoveal letter recognition at reduced contrast in normal aging and in patients with risk factors for AMD

BackgroundPatients with early age-related maculopathy (ARM) do not necessarily show obvious morphological signs or functional impairment. Many have good visual acuity, yet complain of decreased visual performance. The aim of this study was to investigate the aging effects on performance of parafoveal letter recognition at reduced contrast, and defects caused by early ARM and normal fellow eyes of patients with unilateral age-related macular degeneration (nfAMD).MethodsTesting of the central visual field (8° radius) was performed by the Macular Mapping Test (MMT) using recognition of letters in 40 parafoveal target locations at four contrast levels (5, 10, 25 and 100%). Effects of aging were investigated in 64 healthy subjects aged 23 to 76 years (CTRL). In addition, 39 eyes (minimum visual acuity of 0.63;20/30) from 39 patients with either no visible signs of ARM, while the fellow eye had advanced age-related macular degeneration (nfAMD; n = 12), or early signs of ARM (eARM; n = 27) were examined. Performance was expressed summarily as a “field score” (FS).ResultsPerformance in the MMT begins to decline linearly with age in normal subjects from the age of 50 and 54 years on, at 5% and 10% contrast respectively. The differentiation between patients and CTRLs was enhanced if FS at 5% was analyzed along with FS at 10% contrast. In 8/12 patients from group nfAMD and in 18/27 from group eARM, the FS was statistically significantly lower than in the CTRL group in at least one of the lower contrast levels.ConclusionUsing parafoveal test locations, a recognition task and diminished contrast increases the chance of early detection of functional defects due to eARM or nfAMD and can differentiate them from those due to aging alone.

CIRCULATION IN THE FUNDUS OF THE RABBIT EYE

The retinal vessels are clearly visible in their transparent chamber, and easy to study by ophthalmoscope; in albino animals the choroidal vessels are also observable. Nevertheless, difficulties are encountered when an attempt is made to examine their circulation. The usual microscopic study is rendered impossible by the distance from the anterior surface of the eye. Several operative methods which have been developed (Leopold 1951, Ashton and Cook 1954, Thurinsky 1957, Friedman, Smith and Kuwabara 1964) allow of study of the vessels in a very small area at large scale magnification, although these methods are not suitable for chronic experiments. Photographic studies of the retinal circulation have been essayed since the early 1930s (Lambert 1934, Puntenney 1939, Zwiauer and Bornshein 1949). An experimental study of fundus cinematography with fluorescein was first reported by Flocks, Miller and Chao in 1959. They studied the retinal circulation in 10 cats which had been anaesthetized with sodium pentobarbital, and recorded greater variation in the retinal circulation times than had been indicated by their earlier method (Chao and Flocks 1958) with tryptan blue. As the diffuse reflection of fluorescent light from the tapetum diminishes the contrast Letween vessels and the background, it is difficult to achieve good photographic quality in study of the cat by this means. Slightly different methods of fluorescence cineangiography have been described (Hart et al., 1963, Oberhoff, Evans and Delaney 1965) for study of the human retinal circulation, and for study of the retinal circulation in the anaesthetized pig (Ashton et al. 1966). Recently a new method of fluorescence cineangiography was developed in

Visual rehabilitation after long lasting early blindness.

A 23‐year‐old woman, blinded at an early age due to retinopathy of prematurity and cataract was succesfully operated. After the operation the patient was behaviourally blind. Visual rehabilitation was started 9 months later and lasted 1 1/2 years. During this time the visual behaviour of the patient improved. Quantitative tests were designed to depict the progress. The functional improvement of the patient is in agreement with the findings of animal studies; binocular deprivation during the critical period of development causes behavioural blindness, which is partially recoverable. The result of this study indicates that the visual rehabilitation facilitates the process of recovery. The progress in visual behaviour is likely to reflect an improvement of the function of the associative systems of the brain.

Dynamics of cortical activation in a hemianopic patient

Although residual vision in patients with cortical blindness is common, its brain mechanisms are poorly known. To study these mechanisms we measured neuromagnetic responses to visual stimuli in a patient with right posterior cerebral lesion and left visual field hemianopia. His vision had partially recovered with intensive training before our measurements. Compared with the processing in the healthy side, early occipital responses were attenuated for both passive viewing of checkerboard reversal patterns and a letter identification task. In both conditions there were prominent longer-latency responses at the right superior temporal cortex. We suggest that the activation in the superior temporal cortex can partially compensate for the failure to produce synchronized population responses at the early stages of visual cortical processing.

FLUORESCENCE CINEMATOGRAPHY OF THE OCULAR FUNDUS

Fluorescence photography of the ocular fundus has become a popular new method for studying the circulation and vascular changes of the eye. In the earlier works, pictures were taken once to three times a second (Novotny 1961, Dollery 1963, Oosterhuis 1965, Ferrer 1965). This method, however, does not visualize all phases of the circulation, so multiple injections of fluorescein are needed. To overcome this disadvantage, Hart (1963) developed the motion picture technique and found it very useful for most diagnostic and research studies. The Zeiss fundus camera has been used for fluorescein photography in many investigations. In our method this camera has been modified for fluorescein motion picture photography of the rabbit eye.

Contrast sensitivity in visually impaired children.

Spatial contrast sensitivity may decrease differently at different spatial frequencies when vision is impaired. Patients with central scotoma may have normal or decreased sensitivity at low spatial frequencies, their Snellen acuity often is lower than grating acuity estimated on the basis of contrast sensitivity values. In retinitis pigmentosa contrast sensitivity may decrease at the low spatial frequencies already in the early teens. Because the ability to discern low contrasts is important in daily living, spatial contrast sensitivity measurements should become a part of clinical evaluation of vision.