Can We Understand the Riddoch Phenomenon?

I would argue that the recent findings of Gallant and colleagues present a turning point in the field of neurophysiology. They have provided an intuitive way of understanding brain activity in the visual cortex. The essential findings of the study were that the researchers were able to reconstruct complex visual stimuli that had been presented to subjects on the basis of their brain activity alone. Thus they were able to ‘see’ the activity in the brain. I will not elaborate further on the details of the study which I have discussed previously (see this post). The nature of their investigations – looking specifically at the visual cortex lends itself to this type of visual analysis*. The video below demonstrates the outcome of their approach.

Video Reconstructions of Clips Presented to 3 Subjects. The average of the best-fit clips is on the left, while those on the right are the best fit clips. Each row represents a single subject.

Now that the methodology has been established it is possible for the researchers to turn to other phenomenon and start to ask questions which were solely the domain of cognitive psychologists, phenomenologists and philosophers and in so doing to integrate their findings with those of practitioners from these other disciplines. One interesting phenomenon is that of Cortical Blindness where a person may experience visual sensations but have no conscious awareness of these phenomenon. They can see in one sense and yet cannot see in another. Cortical blindness encompasses a variety of phenomenon and there are a corresponding variety of causes. One instance of Cortical Blindness is known as the ‘Riddoch phenomenon‘ described by Riddoch in 1917. A person exhibiting the Riddoch Phenomenon when presented with a stationary object in their field of vision and will be unable to identify this object. However when the subject is presented with a moving object they will be able to recognise the object’s movement. This is a rather simplistic description of the phenomenon and indeed there are various nuances. For instance one group have identified that stimuli of certain colours are more likely to produce this phenomenon than others. Whilst the original description by Riddoch focused on injury to the Occipital Cortex, other groups have identified the same phenomenon resulting from lesions elsewhere (see this freely available paper).

One remarkable figure in the field of Neurology who for some time was the editor of the Journal Brain was Sir Gordon Holmes. His pioneering work occurred in the first World War when he treated soldiers with head injuries. He closely followed Riddoch’s work and I was interested to learn of his views on the ‘Riddoch Phenomenon’. In his 1918 paper ‘Disturbances of Vision by Cerebral Lesions‘ Holmes writes that in those areas where the static stimuli cannot be appreciated but where the moving stimuli can be seen

But I have always found that the acuity of vision in these areas is considerably diminished; the patient may be uncertain of the stimulus, or he may describe it as indistinct, or as if seen through a mist; and on testing, it frequently happens that he responds only when the movement is abrupt and repeated but fails to do so when it is slow and gradual‘.

He further quotes from William James who writes about the extramacular parts of the retina thus

Its main function is that of a sentinel which when beams of light move over it, cries: ‘Who goes there?’ and calls the fovea to the spot

Holmes then moves to the point of his argument

I would like to say that the condition described by Riddoch should not be spoken of as a dissociation of the elements of visual sensation, since it is only a condition of visual hypoaesthesia in which the stronger and more adequate stimuli alone excite sensations‘.

Holmes left a remarkable legacy and his clinical acumen played a significant role in understanding of human visual perception. Thus we should pay close attention to his words above. Whilst the apparent dissociation of static and dynamic stimuli in the realms of perception seem perplexing and hint at sophisticated parcelling of function within the visual cortex, Holmes suggests instead that we consider instead a hierarchy of stimuli which are ever more likely to precipitate a perceptual response. What if we could transport Holmes through time to the laboratory of Gallant? What questions might he ask and what might he think of the technology? What might he expect to see in the reconstructed moving images.

Supposing subjects with Riddoch Phenomenon were to be investigated using the above methodology. They were presented with many hours of moving images whilst they were scanned. Whether there was any activity in the Visual Cortex would be the first telling finding. If Holmes were correct, it may be that activity in the Visual Cortex occurred only when the image stimuli were moving above a threshold velocity. Furthermore this would correspond to a conscious experience. Then similarly further moving images could be presented and reconstructed from recorded brain activity using the research paradigm. Where these findings to deviate from the predictions of Holmes, they would provide invaluable insights into the mechanisms behind this phenomenon. Suppose for instance that static images produced both activity within the visual cortex and that they could also be reconstructed from the brain activity. This would mean that these areas within the visual cortex are not necessary for conscious experience of visual stimuli. Answering questions of this sort would offer useful insights into an understanding of the qualia of conscious experience. These are interesting times for understanding such phenomenon.

References

Holmes G. Disturbances of the Vision by Cerebral Lesions. Brit J Opthal. 1918. 2. 353. Reproduced in Selected Papers of Sir Gordon Holmes. Compiled and Edited by F.M.R Walshe. Macmillan. 1956.

* Were the analysis to have involved the auditory cortex for instance, the researchers would have needed to reconstruct the auditory stimuli and we would be listening to the reconstructed sounds rather than the reconstructed moving images.

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2 thoughts on “Can We Understand the Riddoch Phenomenon?

  1. Pingback: Neuroanatomy Resources (Last Updated 7.4.12) « The Amazing World of Psychiatry: A Psychiatry Blog

  2. Chris

    Interesting. From a neuroanatomical point of view, it’s possible that Riddoch’s phenomenon arises from several extrastriatal pathways. The striate cortex (V1) is generally the first point of contact for the lateral geniculate nucleus, which relays all the retinal signals; however, there are at least 2 other pathways relaying retinal signals that bypass V1 and wind up in area V5, which is thought to be the motion processing part of our brain. One of them from the LGN straight to V5. As for the business with some colours precipitating lower thresholds for motion perception, it may be to do with specific tracts in the LGN, especially the koniocellular (blue/yellow signals) tract it would seem, having more output to V5. In most cases where Riddoch’s phenomenon was studied, relevant regions of V1 were shown to be inactive.

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