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.

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.

Flicker sensitivity as a function of target area with and without temporal noise.

Flicker sensitivities (1-30 Hz) in foveal, photopic vision were measured as functions of stimulus area with and without strong external white temporal noise. Stimuli were circular, sinusoidally flickering sharp-edged spots of variable diameters (0.25-4 degrees ) but constant duration (2 s), surrounded by a uniform equiluminant field. The data was described with a model comprising (i) low-pass filtering in the retina (R), with a modulation transfer function (MTF) of a form derived from responses of cones; (ii) normalisation of the temporal luminance distribution by the average luminance; (iii) high-pass filtering by postreceptoral neural pathways (P), with an MTF proportional to temporal frequency; (iv) addition of internal white neural noise (N(i)); (v) integration over a spatial window; and (vi) detection by a suboptimal temporal matched filter of efficiency eta. In strong external noise, flicker sensitivity was independent of spot area. Without external noise, sensitivity increased with the square root of stimulus area (Piper's law) up to a critical area (A(c)), where it reaches a maximum level (S(max)). Both A(c) and eta were monotonic functions of temporal frequency (f), such that log A(c) increased and log eta decreased linearly with log f. Remarkably, the increase in spatial integration area and the decrease in efficiency were just balanced, so A(c)(f)eta(f) was invariant against f. Thus the bandpass characteristics of S(max)(f) directly reflected the composite effect of the distal filters R(f) and P(f). The temporal equivalent (N(it)) of internal neural noise (N(i)) decreased in inverse proportion to spot area up to A(c) and then stayed constant indicating that spatially homogeneous signals and noise are integrated over the same area.

Selective interference reveals dissociation between memory for location and colour.

The aim was to study whether there is indication of a dissociation in processing of visuospatial and colour information in working memory in humans. Experimental subjects performed visuospatial and colour n-back tasks with and without visuospatial and colour distractive stimuli presented in the middle of the delay period to specifically affect mnemonic processing of task-related information. In the high memory-load condition, the visuospatial, but not the colour, task was selectively disrupted by visuospatial but not colour distractors. When subvocal rehearsal of the memoranda in the colour task was prevented by articulatory suppression; colour task performance was also selectively disrupted by distractors qualitatively similar to the memoranda. The results support the suggestion that visual working memory for location is processed separate from that for colour.

The effects of temporal noise and retinal illuminance on foveal flicker sensitivity.

We measured foveal flicker sensitivity with and without external added temporal noise at various levels of retinal illuminance and described the data with our model of flicker sensitivity comprising: (i) low-pass filtering of the flickering signal plus external temporal and/or quantal noise by the modulation transfer function (MTF) of the retina (R): (ii) high-pass filtering in proportion to temporal frequency by the MTF of the postreceptoral neural pathways (P): (iii) addition of internal white neural noise; and (iv) detection by a temporal matched filter. Without temporal noise flicker sensitivity had a band-pass frequency-dependence at high and medium illuminances but changed towards a low-pass shape above 0.5 Hz at low luminances, in agreement with earlier studies. In strong external temporal noise, however, the flicker sensitivity function had a low-pass shape even at high and medium illuminances and flicker sensitivity was consistently lower with noise than without. At low luminances flicker sensitivity was similar with and without noise. An excellent fit of the model was obtained under the assumption that the only luminance-dependent changes were increases in the cut-off frequency (fc) and maximum contrast transfer of R with increasing luminance. The results imply the following: (i) performance is consistent with detection by a temporal matched filter, but not with a thresholding process based on signal amplitude; (ii) quantal fluctuations do not at any luminance level become a source of dominant noise present at the detector; (iii) the changes in the maximum contrast transfer reflect changes in retinal gain, which at low to moderate luminances implement less-than-Weber adaptation, with a 'square-root' law at the lowest levels; (iv) the changes of fc as function of mean luminance closely parallels time scale changes in cones, but the absolute values of fc are lower than expected from the kinetics of monkey cones at all luminances; (v) the constancy of the high-pass filtering function P indicates that surround antagonism does not weaken significantly with decreasing light level.

Flicker sensitivity as a function of spectral density of external white temporal noise.

Foveal flicker sensitivity at 0.5-30 Hz was measured as a function of the spectral density of external, white, purely temporal noise for a sharp-edged 2.5 deg circular spot (mean luminance 3.4 log phot td). Sensitivity at any given temporal frequency was constant at low powers of external noise, but then decreased in inverse proportion to the square root of noise spectral density. Without external noise, sensitivity as function of temporal frequency had the well-known band-pass characteristics peaking at about 10 Hz, as previously documented in a large number of studies. In the presence of strong external noise, however, sensitivity was a monotonically decreasing function of temporal frequency. Our data are well described (goodness of fit 90%) by a model comprising (i) low-pass filtering by retinal cones, (ii) high-pass filtering in the subsequent neural pathways, (iii) adding of the temporal equivalent of internal white spatiotemporal noise, and (iv) detection by a temporal matched filter, the efficiency of which decreases approximately as the power -0.58 of temporal frequency.

Squamous cell carcinoma of the conjunctiva. Failure to demonstrate HPV DNA by in situ hybridization and polymerase chain reaction.

Squamous cell carcinoma of the conjunctiva is a distinct rarity, often arising at the corneoscleral limbus and initially resembling pterygium or chronic keratoconjunctivitis. In this paper we report 4 patients with conjunctival squamous cell carcinoma/carcinoma in situ, which comprise all the cases found in the files of Kuopio University Hospital during 1959-1991. The clinical appearance, diagnosis and treatment of the lesions are described. All biopsies were studied for the presence of Human papillomavirus (HPV) DNA (recently demonstrated in conjunctival squamous cell papillomas, precancer lesions and carcinomas) by using in situ DNA hybridization (ISH) and polymerase chain reaction (PCR). Both techniques failed to demonstrate the DNA of any of the following HPV types: HPV 6, 11, 16 and 18 in any of the lesions. The results are discussed in the light of the recently proposed HPV etiology of these lesions.

Critical flicker frequency to red targets as a function of luminance and flux across the human visual field.

When the number of cells (cones at eccentricities 0-10 deg and ganglion cells above 10 deg) stimulated at various retinal locations was kept constant by enlarging the stimulus area with increasing eccentricity in the temporal visual field (M-scaling), CFF to red stimuli with dark surround increased as a single function of photopic luminous flux, collected by ganglion-cell receptive-field centres and calculated by multiplying Ricco's area with retinal illuminance at each eccentricity studied. The increase of CFF with the logarithm of photopic flux could be best explained by the Collins logarithmic law, the Kelly square-root law was almost equally good and the Ferry-Porter law was poorest. Adopting the general formulation of Corwin and Dunlap (Vision Research, 27, 2119-2123, 1987) the exponent of CFF is 0, 0.5, and 1 for the Collins, Kelly and Ferry-Porter laws, respectively. The exponent that best explained our results was found to be 0-0.3.

Cortical acuity and the luminous flux collected by retinal ganglion cells at various eccentricities in human rod and cone vision.

Using areally M-scaled, luminance-modulated orange-red and black-and-white gratings, we measured monocular resolution as a function of luminance at various eccentricities in the temporal visual field. In cone vision the increase of grating acuity with luminance became similar at all eccentricities when (1) acuity values were divided by the human cortical magnification factor to express grating resolution in cortical terms (c mm-1) and (2) retinal illuminance was multiplied by Ricco's area to express luminance in terms of photopic luminous flux. The same MF-scaling procedure also applied to scotopic acuity except that the amount of luminous flux collected by retinal ganglion-cell receptive-fields was in rod vision found to increase with eccentricity faster than photopic Ricco's area.

Critical flicker frequency as a function of stimulus area and luminance at various eccentricities in human cone vision: a revision of Granit-Harper and Ferry-Porter laws.

When the photopic luminous flux collected by ganglion cells was kept constant at all retinal locations by reducing average stimulus luminance in inverse proportion to photopic Ricco's area (F-scaling), critical flicker frequency to stimuli of 1.2-88 deg2 in area, presented at various eccentricities along the temporal meridian of the visual field, increased as a single logarithmic function of the number of retinal ganglion cells stimulated. Their number was calculated by multiplying stimulus area by the ganglion cell receptive field density of the human retina. When the number of ganglion cells stimulated was kept constant by enlarging the stimulus area in inverse proportion to the ganglion cell density (M-scaling), the logarithm of CFF to green, yellow, orange and red cone-targets increased as parallel linear functions of logarithmic flux, calculated by multiplying retinal illuminance by photopic Ricco's area.

Retinal ganglion-cell density and receptive-field size as determinants of photopic flicker sensitivity across the human visual field.

At 1, 10, and 50 Hz, photopic flicker sensitivity to a nonpatterned stimulus of constant area and luminance with a small equiluminous surround tended to decrease when eccentricity increased from 0 to 70 deg. The decrease was steeper for lower flicker frequencies. When the stimulus and surround were M scaled by magnifying them in inverse proportion to retinal ganglion-cell sampling density, flicker sensitivity tended to increase with eccentricity. The increase was steeper for higher flicker frequencies. When the stimulus and surround were F scaled by reducing their average luminance in inverse proportion to Ricco's area, flicker sensitivity again decreased with increasing eccentricity, but now the decrease was steeper for higher flicker frequencies. When the stimulus and surround were MF scaled, flicker sensitivity became independent of eccentricity at all flicker rates tested.

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.

Perimetry of critical flicker frequency in human rod and cone vision.

Photopic critical flicker frequency (CFF) to green and yellow-red targets became independent of visual field location when the decrease in the density of retinal ganglion cells and increase in their receptive-field size towards the retinal periphery were compensated for by increasing stimulus area in inverse proportion to the human cortical magnification factor squared (M-scaling) and by reducing stimulus luminance in inverse proportion to Ricco's area (F-scaling). In mesopic and scotopic vision CFF to green targets increased monotonically with eccentricity despite MF-scaling. Instead, CFF to MF-scaled yellow-red targets that predominantly stimulated cones was independent of eccentricity at all luminance levels tested.

Critical flicker frequency and M-scaling of stimulus size and retinal illuminance.

Using various stimulus areas and luminances we measured monocular critical flicker frequency (CFF) as a function of eccentricity in the temporal visual field. With constant stimulus area and luminance, CFF was not independent of visual field location. When stimulus area was scaled by the magnification factor of the human striate cortex to produce equal cortical stimulus areas from different retinal locations, CFF increased monotonically with increasing eccentricity. Hence, CFF cannot be made independent of visual field location by spatial M-scaling. However, when also retinal illuminance was M-scaled by reducing stimulus luminance in inverse proportion to Ricco's area at each eccentricity, CFF became independent of visual field location.