Brief articleCan blindsight be superior to ‘sighted-sight’?☆
Introduction
Lesions of primary visual cortex can result in perimetrically blind areas in the corresponding region of visual field. Despite being clinically blind, some residual visual abilities termed ‘Blindsight’ may remain within these areas, which can be elicited through the use of specific testing conditions and stimulus parameters. Many aspects of visual processing in blindsight have been investigated, including; spatial vision (Weiskrantz et al., 1991, Barbur et al., 1994, Sahraie et al., 2002, Sahraie et al., 2003), processing of colour (Barbur et al., 1992, Stoerig and Cowey, 1992), emotional expression (de Gelder, Vroomen, Pourtois, & Weiskrantz, 1999) and semantic processing (Marcel, 1998). Investigation into spatial processing in areas of cortical blindness in a group of 10 patients revealed the presence of a narrowly tuned psychophysical spatial channel optimally responding to spatial frequencies below 4 c/° in 8 of the 10 cases tested (Sahraie et al., 2003). It appears that specific stimulus parameters are vital in determining performance and that in addition to the specific profile of spatial frequency sensitivity, relatively ‘salient’ stimuli (i.e. relatively large, high contrast, temporally modulated or moving) are often required for above chance detection performance. Indeed, dramatic differences in performance have been observed as a consequence of small alterations to stimulus parameters, which apparently make stimuli more salient (see Hess and Pointer, 1989, Weiskrantz et al., 1991, Sahraie et al., 2002, Sahraie et al., 2003).
DB, the first blindsight case to be tested extensively (Weiskrantz et al., 1974, Weiskrantz, 1986), has demonstrated the ability to detect and discriminate a range of visual stimuli presented within his perimetrically blind visual field defect (see Fig. 1). DB’s residual spatial processing channel peaks at the higher spatial frequency of 3 c/° (Weiskrantz, 2001) unlike other cases in which it is optimally tuned to 1 c/° (Sahraie et al., 2003). In a series of temporal 2AFC experiments we have investigated the limits of DB’s detection ability. Blind field performance was compared to his sighted field performance and to an age-matched control group (n = 6) and revealed apparently superior detection abilities for stimuli presented within cortically blind areas of visual field compared to his sighted field and age-matched controls.
Section snippets
Experiment 1
Initial investigations into DB’s ability to detect the presence of large (10°), high contrast (98%), temporally modulated (10 Hz) Gabor patches at a range of spatial frequencies between 0.5 and 7 c/° presented in a temporal 2AFC paradigm revealed that he was considerably more sensitive to these stimuli compared to a group of 10 cases previously tested (see Sahraie et al., 2003). Reducing the saliency of the stimuli through static presentation and a reduction in stimulus size and contrast did not
Experiment 2
In Experiment 1 an apparent dissociation between sighted and blind field detection of a specific stimulus was demonstrated in two separate experiments, one with separate blocked trials and one with interleaved stimulus presentation. In Experiment 2 in order to replicate and extend these findings DB was tested with the same stimuli (2° diameter, 4.6 c/°, static, Gabor patch) presented at a range of contrast levels within his blind and sighted fields (separate testing blocks for each field).
Experiment 3
In Experiments 1 and 2, DB demonstrated the ability to successfully detect the presence of certain stimuli presented within his cortically blind visual field defect which he was unable to detect when they were presented within his normal vision. These findings appear to represent a dissociation in detection ability between DB’s blind and sighted field, with his blind field outperforming his sighted field. One question this immediately raised was whether DB’s sighted field was actually
General discussion
Overall, the results reported here demonstrate that DB was able to detect the presence of certain stimuli when they were presented within his cortically blind field defect although he was unable to detect them successfully in an area of his normal vision. In addition, a group of six age-matched controls were also unable to successfully detect the presence of the target. To our knowledge this is the first clear demonstration of superior performance on a visual detection task within a cortically
Acknowledgements
Many thanks to DB for carrying out all the testing and to Mr. James Urquhart for technical support. This research was supported by a project grant from the Chief Scientist’s Office, Scottish Executive (CZB/4/40).
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This manuscript was accepted under the editorship of Jacques Mehler.