Spatial and temporal processing in a subject with cortical blindness following occipital surgery

Blindsight subjects are typically better at discriminating rapid, transient visual events than those with gradual on/off-sets. Surprisingly, the detailed investigation of temporal characteristics of mechanisms mediating blindsight is only reported in one subject (GY). It is of interest to establish whether these characteristics are similar to those in other cases of blindsight. Here, we report on a systematic study of spatio-temporal properties of mechanisms mediating blindsight in a subject VN. VN has a lower right quadranopia following surgical removal of the left occipital cortex above the calcarine sulcus, therefore, there are no remaining islands of intact visual cortex within this area. Similar to GY, the blindsight mechanisms in VN have narrowly tuned band-pass temporal characteristics with a peak sensitivity at 20 Hz and above chance performance at temporal frequencies greater than or equal to 10 and less than or equal to 33 Hz. The spatial channel in VN has low-pass characteristics with an upper cut-off less than or equal to 3.5 c/degrees. There is extensive spatial summation in the blindfield whereas no temporal summation was found in the time range tested (50-1600 ms). In agreement with our previous reports, pupillary responses can predict the existence of residual vision within the field defect and show similar spatial characteristics to those obtained psychophysically. The spatio-temporal characteristics of blindsight in VN reported here are similar to those reported in subjects with ischaemic lesions, suggesting that such visual capacities need not necessarily be attributed to spared areas of visual cortex. (C) 2003 Elsevier Science Ltd. All rights reserved.