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 degrees or with flickering letters at 10 degrees 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 degrees eccentricity in one participant and 20 degrees eccentricity in the other. Flickering letters were recognised equally at 10 degrees eccentricity in the blind and intact hemifields. Improvement spread from the stimulated horizontal meridian to the whole hemianopic field within 30 degrees. 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.

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.

New standardised texts for assessing reading performance in four European languages.

AIMS: To develop standardised texts for assessing reading speed during repeated measurements and across languages for normal subjects and low vision patients. METHODS: 10 texts were designed by linguistic experts in English, Finnish, French, and German. The texts were at the level of a sixth grade reading material (reading ages 10-12 years) and were matched for length (830 (plus or minus 2) characters) and syntactic complexity, according to the syntactic prediction locality theory of Gibson. 100 normally sighted native speaking volunteers aged 18-35 years (25 per language) read each text aloud in randomised order. The newly designed text battery was then applied to test the reading performance of 100 normally sighted native speaking volunteers aged 60-85 years (25 per language). RESULTS: Reading speed was not significantly different with at least seven texts in all four languages. The maximum reading speed difference between texts, in the same language was 6.8% (Finnish). Average reading speeds (SD) in characters per minute are, for the young observer group: English 1234 (147), Finnish 1263 (142), French 1214 (152), German 1126 (105). The group of older readers showed statistically significant lower average reading speeds: English 951 (97), Finnish 1014 (179), French 1131 (160), German 934 (117). CONCLUSION: The authors have developed a set of standardised, homogeneous, and comparable texts in four European languages (English, Finnish, French, German). These texts will be a valuable tool for measuring reading speed in international studies in the field of reading and low vision research.

Topographic instability of spatial vision as a cause of dyslectic disorder: a case study.

A case of severe disorder of spatial perception with concomitant dyslexia is described. The patient, a 15-yr-old girl, has a systematic tendency to misperceive the spatial order of symbols or objects lying on the same horizontal level and within a few degrees of visual angle. The syndrome includes a spatial extinction phenomenon in which an object in her left visual field disturbs the perception of another object in the corresponding symmetry point of her right visual field. The order reversal symptom was abolished if the letters presented for her were shown for less than 15 msec. The disorder is probably due to a pathological interhemispheric interference in the processing of visual topographic information.

Resolution of gratings oriented along and across meridians in peripheral vision.

Grating resolution was measured at various locations of the visual field for four grating orientations. As an instance of the oblique effect, vertical and horizontal gratings produced the highest resolution values in the central area. At eccentricities larger than about 20 deg, the oblique effect was replaced by a meridional resolution effect, in which resolution was systematically best for meridionally oriented grating bars and worst for grating bars perpendicular to the visual-field meridians. The origin of the effect seems to be neural because it was not caused by peripheral refractive errors or optical distortion of the peripheral retinal image.

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.

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.

Persistent behavioural blindness after early visual deprivation and active visual rehabilitation: a case report.

Early long lasting binocular deprivation results in behavioural blindness in both man and experimental animals. However, few reported cases show that visual rehabilitation may improve visual behaviour. A 34-year-old man who had experienced 30 years of binocular deprivation due to bilateral cataracts received visual rehabilitation for one year. The rehabilitation included training in eye-hand co-ordination, recognition of objects, evaluation of distance and size, and mobility training. Despite signs of recovery of visual functions the patient never started to use vision in his normal life. The negative outcome of the rehabilitation is partly attributed to the patient's motivational problems and to the relatively short rehabilitation time. Visual rehabilitation may be successful when started immediately after the corrective operation on the eyes when the level of motivation is also high.

Considerations in evaluation and treatment of the child with low vision.

Habilitation of infants and children with visual impairments should be based on a thorough understanding of their visual capabilities and limitations. Comprehensive evaluation of visual function includes measurement of visual acuity, contrast sensitivity, visual field, color vision, adaptation, visual sphere, accommodation, and oculomotor functions. It is best completed through a team approach that uses therapists, special educators, and physicians. A complete evaluation also includes examination of the effect of other motor functions on the use of vision. Nearly all aspects of a child's vision can be measured in play situations with tests that are easy to use. However, further education of nonphysician team members in refractive errors, optics, and function of optical devices may be necessary to ensure an accurate and thorough evaluation.