Jaikishan Jayakumar, PhD
Research Officer National Health and Medical Research Council (NHMRC) Visual and Cognitive Neuroscience Laboratory Department of Optometry & Vision Sciences and Melbourne Brain Centre University of Melbourne Parkville, Vic 3010, Australia; E-mail: jja@unimelb.edu.au
Our perception of vision is largely as a result of the signals conveyed from the eye to the brain via the retino-thalamo-cortical pathway. Visual signals within this pathway originate from three cone photoreceptors (responsible for day vision) and one scotopic receptor, the rods. The cones are classified according to their spectral sensitivity peaks as Long(L), Medium(M) and Short(S) wavelength sensitive cones. Our chromatic perception is dependent upon how the brain processes variations in the activity among these photoreceptors. Chromatic signals are generally thought to be processed by two parallel streams, the red-green system by the parvocellular system1,2 and the blue-yellow system by the koniocellular system.3,4 A large body of work has already been documented to identify these streams within the retina and the main thalamic visual nucleus, the Lateral Geniculate Nucleus (LGN). However, our knowledge about the processing of the chromatic signal within the visual brain, particularly the pathways taken by the blue-yellow colour signals within the brain is at its infancy.
Majority of relay cells within the LGN project to the primary Visual cortex (V1) and hence is considered vital for our visual perception. However, in some clinical cases, even after extensive damage to V1, patients have some residual vision. This phenomenon is called “blind sight”.5,6 The patients usually exhibit little or no awareness of visual stimuli, however can perform tasks that seemingly needs vision such as navigation. Most common hypothesis that has been postulated explaining this phenomenon is that blindsight is mediated by inputs to middle temporal area or area MT that bypass V1.7,8 Area MT remains active even after complete ablation9 or reversible inactivation10 of V1, suggesting that there are alternative pathways that bypass V1 to reach MT. Experiments with inactivation of the superior colliculus showed that one such pathway exists via the colliculus.11 Whether there is any evidence that blindsight is made up of chromatic signals is still unresolved. The perception of colour is usually associated with the “what” pathway as evident from area V412 often considered to be the area that processes color information and for being the gateway to the ventral pathway. Area MT, an extrastriate visual area is considered to be part of the “ where” pathway and until recently was not considered to receive any colour signals but see Conway13 for a review of color signals in the dorsal and ventral pathways. However, more recent studies14,15 have shown that this area receives robust chromatic signal not only via V1 but also bypassing V1.15 Other studies16 have also shown that in subjects with blindsight found that within the hemianopic field, S-cone modulating stimuli were very effective in eliciting visual performance, and in fact in one patient presentation of narrow band blue stimuli (427 nm) led to excellent performance but not red stimuli (peaking at 630 nm). Thus, there is increasing evidence that S-cone signals reach area MT via a more direct route that bypasses V1 and providing an alternate explanation of blindsight in humans.
1. Kaplan E, Benardete E. The dynamics of primate retinal ganglion cells. Prog Brain Res. 2001; 134: 17-34.doi: 10.1016/s0079-6123(01)34003-7
2. Martin PR. Colour through the thalamus. Clin Exp Optom. 2004; 87: 249-257.doi: 10.1111/j.1444-0938.2004.tb05055.x
3. Roy S, Jayakumar J, Martin PR, et al. Segregation of short-wavelength-sensitive (S) cone signals in the macaque dorsal lateral geniculate nucleus. Eur J Neurosci. 2009; 30: 1517-1526. doi: 10.1111/j.1460-9568.2009.06939.x
4. Martin PR, White AJ, Goodchild AK, Wilder HD, Sefton AE. Evidence that blue-on cells are part of the third geniculocortical pathway in primates. Eur J Neurosci. 1997; 9: 1536-1541.doi: 10.1111/j.1460-9568.1997.tb01509.x
5. Weiskrantz L, Warrington EK, Sanders MD, Marshall J. Visual capacity in the hemianopic field following a restricted occipital ablation. Brain. 1974; 97: 709-728. doi: 10.1093/brain/97.1.709
6. Cowey A. The 30th Sir Frederick Bartlett lecture. Fact, artefact, and myth about blindsight. The Quarterly journal of experimental psychology. A Human experimental psychology. 2004; 57: 577-609.
7. Schmid MC, Mrowka SW, Turchi J, et al. Blindsight depends on the lateral geniculate nucleus. Nature. 2010; 466: 373-377. doi: 10.1038/nature09179
8. Weiskrantz L. Sources of blindsight. Science. 1993; 261: 494. doi: 10.1126/science.261.5120.494-a
9. Rodman HR, Gross CG, Albright TD. Afferent basis of visual response properties in area MT of the macaque. I. Effects of striate cortex removal. J Neurosci. 1989; 9: 2033-2050.doi: 10.1523/JNEUROSCI.09-06-02033.1989
10. Girard P, Salin PA, Bullier J. Response selectivity of neurons in area MT of the macaque monkey during reversible inactivation of area V1. J Neurophysiol. 1992; 67: 1437-1446.doi: 10.1152/jn.1992.67.6.1437
11. Rodman HR, Gross CG, Albright TD. Afferent basis of visual response properties in area MT of the macaque. II. Effects of superior colliculus removal. J Neurosci.1990; 10: 1154-1164.doi: 10.1523/JNEUROSCI.10-04-01154.1990
12. Roe AW, Chelazzi L, Connor CE, et al. Toward a unified theory of visual area v4. Neuron. 2012; 74: 12-29. doi: 10.1016/j.neuron.2012.03.011
13. Conway BR. Color signals through dorsal and ventral pathways. Vis Neurosci. 2014; 31: 197-209. doi: 10.1017/ S0952523813000382
14. Nassi JJ, Lyon DC, Callaway EM. The parvocellular LGN provides a robust disynaptic input to the visual motion area MT. Neuron. 2006; 50: 319-327. doi: 10.1016/j.neuron.2006.03.019
15. Jayakumar J, Roy S, Dreher B, Martin PR, Vidyasagar TR. Multiple pathways carry signals from short-wavelength-sensitive (‘blue’) cones to the middle temporal area of the macaque. J Physiol. 2013; 591: 339-352. doi: 10.1113/jphysiol.2012.241117
16. Alexander I, Cowey A. Edges, colour and awareness in blindsight. Consciousness and cognition. 2010; 19: 520-533. doi: 10.1016/j.concog.2010.01.008
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