Antti Raninen

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.

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 Riccos area at each eccentricity, CFF became independent of visual field location.

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.

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 Riccos 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 Riccos area.

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 Riccos 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.

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.

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 Riccos 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.

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.

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 Riccos 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 Riccos 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 Riccos area.