N. Gangopadhyay, M. Madary, & F. Spicer
Oxford University Press (2010)
This book sets out to discuss a debate within the cognitive sciences as to the relationship of perception and action. One view is that this involves on a dual visual system driving the mainly separate functioning of perception and action. The alternative proposal is a system in which the sensory and motor systems are interactive with perception, and where consciousness is essentially viewed as a function of action and movement.
An initial proposal in this debate was that the perception of space and our position in it was driven by a different mechanism from the perception of objects. This postulated two separate pathways in the brain, sometimes referred to as the ‘what’ and ‘where stream’ or alternatively ‘vision for perception’ and ‘vision for action’. Goodale & Milner (1-3. 1992, 1995 & 2004) are proponents of this view. They argue that vision for perception is processed in the ventral stream that projects from the primary visual cortex to the inferior temporal cortex. Vision for action is processed by the dorsal stream that projects from the primary visual cortex to the posterior parietal cortex. Vision for perception serves to identify objects and events, whereas vision for action is involved with functions such as reaching and grasping.
The evidence for viewing these pathways as separate is based on the experience of patients who have deficits in one area of processing but not the other. It also relates to experimentally based claims that common optical illusions are related to perception, while actions such as grasping are immune from them.
Jacob and Jeannerod (4. 2003) propose that the same visual information from the external world undergoes two distinct types of processing within the brain. They also describe the dorsal stream as ‘pragmatic’ and the ventral stream as ‘semantic’ in the sense of giving meaning to objects.
Another researcher, Mohan Matthen, argues that perception provides an image that can be stored in and recalled from memory, while the dorsal stream is concentrated on guiding motion. These types of theories tend to allow goal-directed action to happen independently of consciousness. However, this last view looks to be more relevant to the type of trivial actions shown to be unconscious in Libet’s experiments than to more deliberative or emotionally driven activities.
Perception and action are seen as linking up only directly where the actions needed demand planning. This dual systems view is based on anatomically distinct dorsal and ventral pathways. Goodale and Milner have been principle exponent of this idea based on work with a patient (D.F.) with visual agnosia and also observation of other cases with optic ataxia. Agnosia is suggested to arise as a result of impairment of the ventral stream and ataxia as a result of impairment in the dorsal stream. Thus the patient D.F. could not report the existence of a slot in a disc, but she could pass her hand though the slot. Jacob and de Vignemont argue that experimental evidence suggests that D.F’s dorsal stream does not process a full range of visual features but only what is immediately relevant such as the width of a slot, rather than the shape of the object in which the shape is located.
These experiments have been criticised by the action-orientated theorists. Goodale and Milner concede that the experiments are not conclusive, but argue that they certainly point to the possibility of a separate streams system. They also assert that the deficit of optic ataxics in reaching applies to central as well as peripheral vision, contrary to claims that this deficit was just a feature of peripheral vision, and that perceptual deficits are not confined to egocentric processing. In addition (5. Aglioti et al, 1995) provides material indicating that visual illusions apply only to the ventral perceptual stream and not to the visual ability to grasp objects, although this is also challenged by the action-orientated theorists.
The alternative to these ‘two visual streams’ theories is the concept that perceptual awareness is something that results from an active or moving perceiver. These theories are referred to as action-orientated, and view action and perception as interdependent. Perceptual content is suggested to come from sensorimotor knowledge gained by exploring the environment. Physical movement is not necessary for the theory to apply, at least in some versions. It can be sufficient to understand what would happen if one were to move rather than actually moving. Other theories require actual movement to be involved and perception to subserve this movement.
There is a question as to whether there is enough experimental or observational support for the notion that conscious perception is the perceiver’s knowledge of the laws of motion. Experience suggests that perception contains many things that have little or nothing to do with motion or action. Maybe this is an illusion, but the burden of proving this does seem to be placed on the action-orientated theorists. Some of the arguments of these theorists can border on being rather convoluted, or come close to generating a problem with Ockham’s razor. It is suggested that D.F. experiences perception but cannot report it. The trouble with this is that there is no particular evidence for this assertion, and it appears to need some new element of neuroprocessing to explain how the reporting of a consciously experienced perception gets suppressed in this particular case.
The action-orientated theorists argue that consciousness cannot arise from the passive receipt of external signals. This sounds all right as far as it goes. Less convincing is the claim that the solution to the hard problem of consciousness arises from perception of the laws governing movement and interaction with the environment. In the rest of nature, such interactions get along very well without consciousness, and there is little in existing neuroscience that suggests consciousness would arise from signals related to movement and action in a way that it would not arise from other external signals, such as visual signals. Of course physical interaction can be perceived via the body, but in this case the body is just part of a signalling system, with the relevant brain state eventually arising in the somatosensory cortex. To suggest anything else in this respect would appear to be to advocate that the body could do something for consciousness that the brain can’t do. Some have suggested a reconciliation of the two positions with perception allowed to influence motor behaviour at the level of planning and general guidance. Thus Jacob and Jeannerod, supported by Jacob and de Vignemont, suggest that the ventral and dorsal represent two different types of processing that normally work closely in tandem, but reveal their different qualities when one processing system is impaired.
Discussion: Neural systems are anything but a simple one-to-one feed-forward process. The system is immensely complex with many sections of the brain capable of feeding into a single region and vice versa. Furthermore there are complex systems of feedback as between the cortex and the thalamus or the basal ganglia and both the limbic system and the frontal cortex. Moreover the last decade has seen a mainly welcome shift away from the brain in a vat approach of the last century’s computer based consciousness theory, towards ideas that take account of the interaction between brain and body, and body and environment. It seems highly likely that movement related and visceral-emotional related feedback from the body plays a part in neural processing, but it is quite another thing to try and make movement and action the sole drivers of consciousness. This approach poses a query as to why this and not the ventral processing is conscious, although in personal experience we do seem to be conscious of what passes through the ventral. Another problem with the action-oreintated approach is that it looks to entirely ignore a whole body of recent research into the way in the way in which reward is represented in the emotional areas of the brain, and from this acts to drive behaviour.