Biomedical subjects

Ceri T Trevethan

Publications and source records attributed to Ceri T Trevethan.

4 recordsLinked to original sources

PubMed2006

Increased sensitivity after repeated stimulation of residual spatial channels in blindsight.

Lesions of the occipital cortex result in areas of cortical blindness affecting the corresponding regions of the patient's visual field. The traditional view is that, aside from some spontaneous recovery in the first few months after the damage, when acute effects have subsided the areas of blindness are absolute and permanent. It has been found, however, that within such field defects some residual visual capacities may persist in the absence of acknowledged awareness by the subject (blindsight type 1) or impaired awareness (type 2). Neuronal pathways mediating blindsight have a specific and narrow spatial and temporal bandwidth. A group of cortically blind patients (n = 12) carried out a daily detection "training" task over a 3-month period, discriminating grating visual stimuli optimally configured for blindsight from homogeneous luminance-matched stimuli. No feedback was given during the training. Assessment of training was by psychophysical measurements carried out before and after training and included detection of a range of spatial frequencies (0.5-7 cycles per degree), contrast detection at 1 cycle per degree, clinical perimetry, and subjective estimates of visual field defect. The results show that repeated stimulation by appropriate visual stimuli can result in improvements in visual sensitivities in the very depths of the field defect.

Adult↗

PubMed2006

Can blindsight be superior to 'sighted-sight'?

DB, the first blindsight case to be tested extensively (Weiskrantz, 1986) has demonstrated the ability to detect and discriminate a range of visual stimuli presented within his perimetrically blind visual field defect. In a temporal two alternative forced choice (2AFC) detection experiment we have investigated the limits of DB's detection ability within his field defect. Blind field performance was compared to his sighted field performance and to an age-matched control group (n=6). DB reliably detected the presence of a small (2 degrees ), low contrast (7%), 4.6c/ degrees Gabor patch with the same space-averaged luminance as the background presented within his blind field but performed at chance levels at the same eccentricity (11.3 degrees ) within his sighted field. Investigation of detection as a function of stimulus contrast revealed DB's ability to detect the presence of an 8% contrast stimulus within his blind field, compared to 12% in his sighted field. No significant difference in detection performance between DB's sighted field and the performance of six age-matched control participants suggests poor sighted field performance does not account for the results. Monocular testing also rules out differences between the eyes as an explanation, suggesting that DB demonstrates superior detection for certain stimuli within his visual field defect compared to normal vision.

Choice Behavior↗

PubMed2003

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 20Hz and above chance performance at temporal frequencies >/=10 and </=33Hz. The spatial channel in VN has low-pass characteristics with an upper cut-off <3.5c/ degrees. There is extensive spatial summation in the blindfield whereas no temporal summation was found in the time range tested (50-1600ms). 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.

Adult↗

PubMed2003

Spatial channels of visual processing in cortical blindness.

Blindsight is the ability of some cortically blind patients to discriminate visual events presented within their field defect. We have examined a fundamental aspect of visual processing, namely the detection of spatial structures presented within the field defect of 10 cortically blind patients. The method outlined is based on the detection of high-contrast stimuli and is effective in flagging a 'window of detection' in the spatial frequency spectrum, should it exist. Here we report on the presence of a narrowly tuned psychophysical spatial channel optimally responding to frequencies less than 4 cycles/ degrees in eight out of 10 patients tested. The two patients who did not show any evidence of blindsight appear to have intact midbrain structures, but have lesions that extend from the occipital cortex to the thalamus. In addition, we have recorded subjective reports of awareness of the visual events in each trial. Detection scores of eight blindsight patients were subsequently subdivided based on the subjective reports of awareness. It appears that the psychophysical spatial channel-mediating responses in the absence of any awareness of the visual event have a narrower frequency response than those involved when the patients report some awareness of the visual event. The findings are discussed in relation to previous reports on the incidence of blindsight and performance on tasks involving spatial processing.

Adult↗