Blindsight - Research Paper Example

?Running head: Blindsight Blindsight [Institute’s Blindsight Introduction Occurrence of blindsight is rare and the case behind the problem being injury to the primary visual cortex. Primary visual cortex is in the brain and when it is damaged the person’s conscious visual awareness is affected (Weiskrantz, 1986). The result is that there is a reduction in the visual sensitivity that responds to luminance contrast; the case being more severe in case of high spatial and low temporal frequencies (Barbur, Harlow, & Weiskrantz, 1994). It is also a phenomenon that this sensitivity is not totally finished off in case of low spatial and high temporal frequencies and this is the reason that several reports have been made of residual visual capacities; in this is included detecting and discriminating any stimuli that may be present within the field defect. Such is what happens in forced-choice tests (Cowey, 2010). The phenomenon of blindsight does not just state that it is normal vision but in the absence of awareness. Besides the loss of primary visual cortex, there is another issue that has to be considered. Retrograde degeneration of relay neurons within the subsequent areas of the lateral geniculate nucleus and concomitant transneuronal degeneration of as much as 90% of the retinal ganglion cells (especially the P? ganglion cells) (Cowey, Stoerig, & Perry, 1989) is responsible for the extremely low contrast sensitivity for low temporal and high spatial frequencies, with subsequent damages to the capacity of discriminating form, reduced motion, and wavelength (Cowey, 2010) – these skills are usually dependent upon the parvocellular system (Schiller, Logothetis, & Charles, 1990). Literature Review The problem of blindsight in human beings has a certain exceptional property. This property states that there is a possibility of detecting and discriminating a stimulus even without there being any subjective awareness. That means, a person suffering from blindsight does still have certain visual abilities, and two of these capacities include detection and discrimination of movement (Weiskrantz, 1986). The patients suffering from blindsight are actually blind to conscious visual perception but they do have the ability of performing visual manual reaching works, which means that they can respond to visual information although they do not have any visual perception (Sanders, et al., 1974). According to certain studies cortically blind patients have the ability of discriminating the direction of motion of single spots (King, et al., 1996) and bars (Azzopardi & Cowey, 2001); they are better able to discriminate the faster moving things and this suggests that their sensitivity to high temporal frequencies is increased (Barbur, Harlow, & Weiskrantz, 1994). The cases of blindsight that have already been published had been caused due to lesions in the visual cortex. These patients still adhere to their feature of functional vision, for instance the ability to detect movement, to point correctly at light flashes in the absence of conscious visual perception, and to be able to guess if there is a stimulus in the visual blind field. The cortically blind patients do not hold the ability of discriminating the direction of the stimuli movement that does not change its location globally, for example, gratings and random dot kinematograms that depict transformation, comparative movement, and motion in depth (Azzopardi & Cowey, 2001). There is still a possibility of there being a difference in the direction discrimination and random kinematograms, gratings and so on, due to the fact that such stimuli concern themselves with various motion-processing methods that are reliable in relation to the variations that might be present between them regarding their local and global features. When this argument is considered it will be possible for there to be discrimination of direction in the cortically blind visual field on the grounds of a method which directly perceives movement information of the stimulus. Researchers have been able to prove blindsight in normal subjects that had been provoked due to transcranial magnetic stimulation (TMS) at the time of rapid motion, and they have also discussed the neural methods that are the cause behind such a behavior. More particularly, the authors have addressed the problem of whether there is any involvement of an efference copy in motion correction. There have been proposals of many methods of blindsight, and these include subcortical and alternative cortical networks. Experiments had been performed (Milner & Goodale, 1995) which suggest that separate processing in the ventral and dorsal cortical streams can be the cause behind blindsight. If the ventral stream is damaged the person would still be able to react to any visual stimulus; however, he will be able to slightly or not all consciously perceive the appearance of the visual object. In such a situation there can be a neural pathway bypassing the primary visual cortex and coming through the superior colliculus by means of the pulvinar to the parietal motor areas (Weiskrantz, 2002). Proof obtained from human beings suggests that the cause behind the correction of fast reaching movements is efference copy signals (Angel, 1976), and there has also been proof of such corrections preceding conscious awareness of motion at the time of performing those tasks (Castiello, Pulignan, & Jeannerod, 1991). For conducting further investigation in probable neural correlates underlying improvements of motion in the absence of conscious perception, Christensen, et al., (2008) adopted an experimental design that resembled with what Castiello et al (1991) had used. An important issue regarding the research of blindsight behavior is the degree to which the sufferers and/or even the healthy people are actually blind in the absence of perceptual experience of any kind of visual stimulus. It has been proved that providing patients with plain yes and no options and asking them if they are able to perceive a visual stimulus does not really result in a graded nature of visual perception. Thus, in this research a slightly different method was employed for measuring the perception awareness scale (Overgaard, et al., 2006) and in this method the participants had to rate on an ordinal scale how clearly they were able to see the movement in stimulus. The hypothesis made by Christensen, et al., (2008) for their research was that in case blindsight-like behavior is stimulated in normal subjects by means of TMS the subjects would still be able to perform corrections to the next light although they had not detected it. A comparison was made of the related situation with a control situation in which subjects were given TMS and carried out wrong corrections in the absence of the next light. In case this kind of a behavior is successfully achieved, the suggestion made was that the neural methods that support such a behavior do not need to be processed by means of the visual cortex but instead it might result in bypassing visual processing areas and thus have a direct affect on motor behavior. Another proposal was that such a link between sensory perception and motor control that perception arises just in case there is some kind of a simulation to interaction with the sensory stimuli (Cotterill, 2001); this is the theory of event coding (Hommel, et al., 2001). In addition, it was proposed that the lateral intraparietal region has some kind of an involvement in the binding of perception and response (Gottlieb, 2007). The method that has been set up is for investigating visual perception as well as motor control and thus it is very much suitable for testing if an interaction exists between sensory perception and motor control. The hypothesis put forward by Christensen, et al., (2008) was that there would be an interaction between perceptual clarity and motor performance, specifically that perceptual clarity would bring about an influence on correction reaction time (CRT) or the visuo-motor reaction time (RT). More so, in case of perceptual clarity increasing, motor performance will go down. The reason behind there existing such a relationship is that the networks that have the responsibility of perception and action are divided for either perception or action, therefore their performance is not exactly optimal for each of the two tasks together. Besides this, Christensen, et al., (2008) also tested if there was any influence on the performance of corrective movements by the movement time (MT) and if the corrections lead to longer MTs. The hypothesis the researchers put forward was that in case of an efference copy being involved with movement correction, the RT would be higher than the CRT. Previous reports by Sahraiea, et al., (2010) have suggested that a well-studied blindsight subject had the ability of performing at the same level in the absence and presence of conscious experience of a moving dot target, and the performance only depended on the stimulus properties (Weiskrantz, Barbur, & Sahraie, 1995). It was shown through a functional brain imaging study that there were contrasting patterns of brain activity under the two modes of processing (Sahraie, et al., 1997). Weiskrantz and his fellow workers (Weiskrantz, et al., 1974) comprehensively studied D.B. who was their first ever blindsight subject. Lately, it has been proven that in recognizing the first-order gratings in his blind and the sighted field or of the aged-matched physically fit subjects, he is more responsive to the former (Trevethan, Sahraie, & Weiskrantz, 2007). (The term first-order refers to gratings which are created by a sine-wave profile of shiny alterations of particular cycles in a degree in which the shine of the clear and dim bars is coordinated generally in the change occurred in the background). Another thing which he informs is the understanding of high-contrast first-order gratings. Thus, it was astounding to find out that he was not aware of the arrangement of adaptable gratings of high-contrast – also called second-order gratings – and yet he was still talented enough to carry out the earlier mentioned chance levels and in noticing their orientation and existence. (Second-order is gratings which are made by the presence of a sine-wave envelope and has quite dissimilar variations on a field which has granules or spots set around haphazardly). Since the period of the last 5-y, the same pattern of answers have been found. This gives an exclusive chance to plan the basics of D.B.’s visual attentiveness thoroughly. The effect this produces is a sequence of experimentation in which first-order and second-order gratings were calculated with methodical handling of spatial frequency and the other incentive properties to decide their consequences on detection and the given responsiveness. This result implies that although D.B. has the ability to recognize first-order and second-order stimuli across a series of spatial frequencies, his understanding of them significantly depends on the availability of low-frequency sine-wave shine data i.e. on first-order data. Synthesis From the results of Christensen, et al., (2008) it was suggested that the methods that account for the rapid visually guided corrective motions stand out of the range of the visual cortex and that the visual signals which are employed for the purpose of correcting motions are able to bypass visual cortex. Also, there is always a possiblity of there being subcortical routes for visually guided reaching which are able to bypass the cortical areas that have been influenced by TMS. It was shown already that there is possibility of inducing blindsight behavior in completely normal subjects and the result would be that those people would get the ability of performing rapid corrective reaching motions and not even be conscious of the presence of the visual signal which is responsible for such a behavior. Results showed that the older proposals were absolutely correct regarding the involvement of an efference copy with correction of motion. Moreover, it is apparent that it is a similar method without taking into account the clarity with which target switches are seen. In spite of all facts certain doubts are present regarding whether or not such capacities show sensitivity toward movement identifying the stimulus information (for instance, first order motion energy that is decided through spatiotemporal variations in luminance) (Adelson & Bergen, 1985), due to the fact that there is a confounding of transformational movement of spots and bars together with variations in the location, and discrimination can be made of this in the field defect in competition with movement (Poppel, Held, & Frost, 1973). From neuroimaging studies it has been deduced that motor areas like the intraparietal cortex and premotor cortex have some kind of an involvement in the subjective perceptual knowledge of visual objects (Christensen, et al., 2006). In case of perceptual clarity increasing, motor performance will go down, and this is shown in higher CRT or RT. We deduce that there is a possibility of performing determined aim directed movements although there is an absence of conscious visual perception. From older researches it was deduced that there is an involvement of an efference copy with movement correction, and if they were to compare the two, the CRT is lower than the RT (Angel, 1976). Conclusion According to our findings it may be suggested that preservation takes place of motion discrimination in the field defect of a patient having blindsight. Motion energy depends on motion energy detection but is independent of object or feature detection and this suggests that residual vision in blindsight has a great dependence upon the detection of transient variations in luminance. This fact is consistent with the older researches performed. There has been proof provided of blindsight even when all cortexes are stripped off unilaterally in cases of hemispherectomy. There is the likelihood of the second-order grating detection requiring cortical processing not just limited to the beginning of the collicular input. Still, there is always the possibility that type 1 second-order processing, which does not seem to be having enough low spatial frequency constituent, is performed completely in a subcortical manner. This remains a question and can be targeted by the future researches that would most probably be performed. References Adelson, E., & Bergen, J. (1985). Spatiotemporal energy models for the perception of motion. J Opt Soc Am A , 2, 284–299. Angel, R. (1976). Neurology , 26, 1164–1168. Azzopardi, P., & Cowey, A. (2001). Motion discrimination in cortically blind patients. Brain , 124, 30–46. Barbur, J., Harlow, A., & Weiskrantz, L. (1994). Spatial and temporal response properties of residual vision in a case of hemianopia. Philos Trans R Soc Lond B Biol Sci , 343, 157–166. Castiello, U., Pulignan, Y., & Jeannerod, M. (1991). Brain , 114, 2639–2655. Christensen, M. S., Kristiansen, L., Rowe, J. B., & Nielsen, J. B. (2008). Action-blindsight in healthy subjects after transcranial magnetic stimulation. Proceedings of the National Academy of Sciences , 105 (4), 1353-1357. Christensen, M., Ramsoy, T., Madsen, K., Lund, T., & Rowe, J. (2006). NeuroImage , 31, 1711–1725. Cotterill, R. (2001). Prog Neurobiol , 64, 1-33. Cowey, A. (2010). The blindsight saga. Exp Brain Res , 200, 3-24. Cowey, A., Stoerig, P., & Perry, V. (1989). Transneuronal retrograde degeneration of retinal ganglion cells after damage to striate cortex in macaque monkeys: Selective loss of P? cells. Neuroscience , 29, 65-80. Gottlieb, J. (2007). Neuron , 53, 9-16. Hommel, B., Musseler, J., Aschersleben, G., & Prinz, W. (2001). Behav Brain Sci , 24, 849–937. King, S., Azzopardi, P., Cowey, A., Oxbury, J., & Oxbury, S. (1996). The role of light scatter in the residual visual sensitivity of patients with complete cerebral hemispherectomy. Vis Neurosci , 13, 1-13. Milner, A., & Goodale, M. (1995). The Visual Brain in Action. Oxford: Oxford Univ Press. Overgaard, M., Rote, J., Mouridsen, K., & Ramsoy, T. (2006). Conscious Cog , 15, 700–708. Poppel, E., Held, R., & Frost, D. (1973). Letter: Residual visual function after brain wounds. Nature , 243, 295–296. Sahraie, A., Weiskrantz, L., Barbur, J. L., Simmons, A., Williams, S. C., & Brammer, M. J. (1997). Pattern of neuronal activity associated with conscious and unconscious processing of visual?signals. Proc Natl Acad Sci , 94 (17), 9406-9411. Sahraiea, A., Hibbard, P. B., Trevethana, C. T., Ritchiea, K. L., & Weiskrantzc, L. (2010). Consciousness of the first order in blindsight. PNAS , 107 (49), 21217-21222. Sanders, M., Warrington, E., Marshall, J., & Weiskrantz, L. (1974). Lancet , 303, 707–708. Schiller, P., Logothetis, N., & Charles, E. (1990). Role of the color-opponent and broadband channels in vision. Vis Neurosci , 5, 321-346. Trevethan, C., Sahraie, A., & Weiskrantz, L. (2007). Can blindsight be superior to 'sighted-sight'? Cognition , 103, 491–501. Weiskrantz, L. (1986). Blindsight. Oxford: Oxford Univ Press. Weiskrantz, L. (2002). Blindsight: A Case Study and Implications. Oxford: Oxford Univ Press. Weiskrantz, L., Barbur, J., & Sahraie, A. (1995). Parameters affecting conscious versus unconscious visual discrimination with damage to the visual cortex (V1). Proc Natl Acad Sci USA , 92, 6122–6126. Weiskrantz, L., Warrington, E., Sanders, M., & Marshall, J. (1974). Visual capacity in the hemianopic field following a restricted occipital ablation. Brain , 97, 709–728. Read More