ACCIÓN SOCIAL FRENTE A LOS PROYECTOS DE DESARROLLO

Many studies have reported that amblyopes show reduced pattem VEP amplitudes for high spatial frequency stimuli. Spekreijse, Khoe and Van der Tweel (1972) described a case study on a single strabismic amblyope. Using pattem reversal stimulation (5' and 20' checksizes, 3 degree display size) they found that, for a range of temporal frequencies (2Hz to 30Hz), the amblyopic eye VEP had a higher contrast threshold than did the fellow eye for all test frequencies. The amblyopic eye had a higher contrast threshold than the fellow eye for low frequency stimuli (2Hz to 17Hz), when presenting with 20' checks, but the same threshold as the fellow eye when presenting at high temporal frequency (20Hz to 30Hz). Pattem onset/offset stimulation with 20' checks was also studied. Both onset and offset VEPs were much reduced in amplitude from the amblyopic eye when compared to the fellow

eye. The onset/offset response of the amblyopic eye resembled that of the fellow eye elicited using an annulus (3-5 deg) stimulus field, thus suggesting that the VEP from the amblyopic eye may have a predominant contribution from the macular region. The amblyopic eye also had a reduced VEP amplitude as compared with the fellow eye for a large range of checksizes, from 2' to 100'. The fellow eye gave largest onset VEP amplitudes for 12' checks, while the amblyopic eye gave largest VEP amplitudes for 50' checks.

Sokol and Bloom (1973) compared monocular VEPs from non-amblyopic and amblyopic eyes in 15 subjects, using pattem reversal stimuli (15' checks, 18 x 18 degree display size, 12 reversals/second). All subjects had smaller VEPs from the amblyopic eye. Checksize effect was studied in one subject who showed a maximum VEP amplitude for 15' checks from the non-amblyopic, and a maximal amplitude for 60' checks from the amblyopic eye.

May les and Mulholland (1980) studied 62 strabismic patients aged 2 to 12 years. The stimuli were checksizes varying from 8' to 22', with a reversal frequency of 4Hz. For 8' checks there was a significant difference between normals and those with poor vision, however, for the 22' checks, there was no significant difference. It was found that amblyopic eyes did not produce a typical N80-P100-N145 reversal complex. The most common variation was the loss of the N145 component, postulated to be the outcome of the merging of the PlOO with a later positive component. The difference in latency delay for the eyes with good vision and those with worse than 6/24 vision was significant for 11' and 22' checksizes.

W anger and Nilsson (1978) compared ten amblyopic patients with ten normal subjects. M onocular and binocular pattem reversal stimulation, with 23'checks reversing at 1.4Hz was used. Seven of the ten patients showed a considerable amplitude asynunetry to

monocular stimulation or lack of normal increase of amplitude to binocular stimulation. Two patients displayed prolonged latency on stimulation of the amblyopic eye.

Sokol (1977) studied the pattem-reversai VHP in a 62 year old amblyopic adult. Acuities were 20/20 in the normal eye and 20/400 in the amblyopic eye. For a 12 degree field, the normal eye showed a maximal amplitude with 15' checks, and the amblyopic eye showed a maximal amplitude with 60' checks. There was a significantly larger signal at the 60' checks from the amblyopic eye compared with the normal eye. For a 6 degree field, the normal eye also peaked at 15'. Amplitudes for small checks (7.5' and 15') remained unchanged as field size decreased, but there was a drop in amplitude for the large checks. For the amblyopic eye, the maximal amplitude occurred with 30' checks. For a 3 degree field, both normal and amblyopic eyes showed a maximal amplitude for 15' checks, and there was no difference between the two eyes at other checksizes. This would seem to indicate that the increase of VEP amplitude in response to large checks in the amblyopic eye, (for both 12 and 6 degree fields), was the result of a greater contribution from the paramacular area in the amblyopic eye. To demonstrate this possibility, 12 and 6 degree fields, with the central 3 degree field blanked out were used, and it was found that the amblyopic eye made a greater contribution to the VEP response than did the normal eye. Also, to determine if regions outside the central 6 degree of the amblyopic eye showed any increase in their contribution for large check sizes, a 12 degree field with the a central 6 degree scotoma was presented to each eye and a large drop in the signals was found for the normal eye, especially for small checks. However, for the amblyopic eye, there was a drop in the signals obtained on stimulation with large checks. From these tests, Sokol concluded that regions outside the central 6 degree are essentially similar for the normal and amblyopic eyes, therefore, YEPs produced by the amblyopic eye receive a significant contribution from a retinal region that lies outside the central 3 degrees, but within the central 6 degrees.

Arden and Bernard (1979) studied 28 normal and 71 amblyopic children, aged 4 to 11 years. Of the amblyopic children, 56 had undergone occlusion therapy. It was found that in the amblyopic eyes, the latency of the VEP was increased and the amplitude was decreased, roughly in proportion to the loss of visual acuity. The VEP was found to be normal in the non-amblyopic eyes of the children who had never been treated by occlusion therapy. It was found that occlusion affected the fellow eyes of amblyopes, and increased the VEP latency, so that the response of the fellow eye was delayed beyond that o f the amblyopic eye. It was found that after the end of occlusion, the VEP usually returned to normal, however, in a subgroup of patients with prolonged occlusion (12 amblyopes with full time occlusion), the change had not completely reversed to normal when tested one year later. Sokol (1983) measured the P I00 latency of the pattern reversal VEP for small checks (15') in 68 normal and 32 amblyopic children. The amblyopic children showed longer P I 00 latencies in their amblyopic eye than in their fellow eye. Sokol suggested that there may be a selective loss of contrast-specific evoked potential mechanisms in amblyopia.

Animal studies have shown that amblyogenic factors predominantly lead to deficits in the parvocellular mechanisms and human VEP studies of amblyopia demonstrate attenuation of VEPs, mainly to macular-derived, small check (high spatial frequency) stimulation. This relatively selective deficit in amblyopic eyes can thus be used to investigate the macular/paramacular, and magno/parvocellular contributions to components of pattern VEPs. A comparison of pattem-onset, -reversal and -offset modes of stimulation, taking into account ipsilateral and contralateral half-field components appears not to have been previously investigated in amblyopes.

6.2 Methodology