Doubts cast on the influential dual-stream theory of visual processing

Psychologists in Germany have challenged one of the most influential theories in neuropsychology – the dual stream model of visual processing proposed by Mel Goodale and David Milner. This model proposes that visual information entering the brain splits down two parallel paths: the dorsal path heads to the top and rear of the brain where the information is used for guiding actions; the ventral path reaches the temporal lobes where it is used for conscious perception and recognition. The model is hugely influential and will be familiar to all contemporary psychology graduates. The three seminal papers proposing and supporting the model have been cited over 930 times.

Much of the supporting evidence came from studies of the brain-damaged patient known in the literature as D.F. This woman's damage to her occipital and parietal lobes from carbon monoxide poisoning appeared to have left her with a rare form of "visual agnosia" – she was unable to recognise everyday objects but was perfectly able to grasp and use them. In other words, she appeared to have an impaired ventral stream but a preserved dorsal stream.

Marc Himmelbach and his team at Eberhard Karls University say that D.F. has become one of the most influential brain-damaged patients in neuropsychology, comparable to Paul Broca's aphasic patient Leborgne and Phineas Gage – the nineteenth century railway worker who survived an iron rod passing through his brain. However, as is the case with Leborgne and Gage, the German team believe that standards of testing have become more stringent since the seminal work with D.F. was published back in the 90s. In particular, conclusions were drawn about D.F. without comparing her performance and behaviour to age-matched controls.

For their paper, Himmelbach and his team have replicated the three main tests performed on D.F. with 20 female, age-matched healthy controls (mean age 36.5 years). These tests included indicating the size of various rectangular wooden blocks using the thumb and forefinger; actually reaching and picking up the blocks; indicating the orientation of a narrow slot in a disc; posting a card through that slot; and indicating the size and shape of odd-regular shapes and then actually picking up those shapes. Results from the original work with D.F. was compared against the results from these new healthy controls.

Himmelbach and his colleagues don't dispute that D.F.'s performance was far more impaired for recognition tasks compared with the reaching and grasping tasks. However, compared against their new control data, they say it's clear that D.F. was also severely impaired in her reaching and grasping performance, seemingly undermining the neat interpretation that she had a preserved dorsal stream. The German group also point to more recent tests of D.F. showing that she has obvious motor deficits when the task is more complicated – for example, she was unable to grasp a disc through three holes in its surface using her thumb, index and middle fingers.

Other evidence highlighted by Himmelbach and co concerns a more recently identified patient "J.S." who has a similar pattern of brain damage to D.F. and who is more impaired on recognition than motor tasks, but who nonetheless is clearly severely impaired on motor tasks compared with healthy controls. Based on a scan of J.S., the researchers also doubt that the pattern of brain damage suffered by D.F is as circumscribed as previously claimed. Finally, the researchers are critical of the lack of "kinematic data" from the original tests of D.F. – things like reaction times, peak velocity of movements and so forth. Such data, they say, would show whether her movements were really normal, or if she were, for example, taking longer than normal to compensate for her difficulties.

"In conclusion," the researchers said, "the behaviour and anatomy of D.F. on its own does not provide firm grounds for the perception vs. action interpretation of dorsal and ventral stream areas." They added that other sources of support for the dual stream model "do not provide unequivocal evidence in favour of or against [the model] without reference to D.F. and could also be integrated by alternative models that do not explicitly state an action-perception dissociation."

Further reading

Himmelbach, M., Boehme, R. and Karnath, H. (2012). 20 years later: A second look on DF's motor behaviour. Neuropsychologia, 50 (1), 139-144 DOI: 10.1016/j.neuropsychologia.2011.11.011