The pattern visual evoked cortical response in human ageing.

The N1, P1, N2 and P2 components of the pattern visual evoked response (PVER) have been recorded to the onset of presentation of a sinusoidal grating pattern of 3 and 8 cycles/deg in groups of young (15-34 years) and old (53-94 years) subjects. The negative components were taken to arise from striate cortex and the positive components from extrastriate cortex. For grating contrasts of 3-40%, the time-to-peak of the N1, P1 and N2 components, but not the P2 component, increased with age while the amplitude and rise time showed no consistent changes. Comparisons between 3 and 8 cycles/deg revealed a longer time-to-peak of N1 and P1 at 8 cycles/deg within the young group though not within the old group, i.e. an additional increment in the time-to-peak had occurred at 3 cycles/deg with ageing. This was also reflected for grating contrasts which were multiples of each individual's contrast threshold, irrespective of age. Now, the time-to-peak of N1, P1 and N2 was similar in young and old groups at 8 cycles/deg. At 3 cycles/deg, however, the time-to-peak was still longer in the old group, though the intervals between components were similar to those of the young group. This suggested the requirement for a further increment in contrast to make the time-to-peak similar, due to a selective loss of sensitivity in ageing within the low-spatial-frequency channels which are additionally sensitive to temporal modulation. Thus, it appeared that, by incrementing the stimulus contrast by the appropriate amount, the PVER of the old group could be made to resemble that of the young group. These results are consistent with the occurrence of neural changes during ageing in the retino-geniculate pathway prior to the visual cortex.