User:Kelvin Dawson/sandbox

Behavioural and neuroimaging studies within psychology have provided rich data to undertake the task of disseminating affordances. Temporal information regarding affordances has been provided through the use of reaction times in behavioural studies and event-related potentials in electroencephalography (EEG). Functional magnetic resonance imaging (fMRI) techniques have allowed spatial information concerning affordances to be inferred. Disorders such as ataxia and visual agnosia have enhanced understanding of neural streams in the visual cortex associated with affordances.

Behavioural Studies
Studies investigating affordances arguably began with a seminal study by Warren (1984) who used a body-scale paradigm for the affordance of stairs for climbing. Ratios were calculated between the dimensions of the stair (riser height and length of treads) and biomechanics of the climber. The relation between environmental properties and climber (leg length), in terms of ratio, corresponded to the dynamics of the animal-environment system inherent in the affordance concept. However a shift away from body-scale studies to reaction time measurements in behavioural studies on affordance occurred, driven by the requirement to differentiate body scale and ability. The property of the climber in the stair climbing paradigm was not a leg length ratio but a function of ability; affordance here was re-understood in terms of a relation between stepping ability of climber and features of the environment (riser height). There is a differentiation between the subjective ability, the potential for action, relative to the actor, and the objective affordance properties of the object; what the object affords is relative to the subject-object interaction.

Most behavioural studies since have utilised reaction time as a measurement for prehension, the act of grasping, which has been aligned with the actual action afforded by an object. Familiar and novel objects have become the favoured paradigm for affordance studies. Tools in particular have become a unique focal point for affordance effects, not restricted to behavioural studies but utilised in neuroimaging too. Tools offer this distinct opportunity due to the representation for action associated with tools’ visual structure for affording action (grasping) and their specific functional identity (action plans relying on previous knowledge systems).

Tipper, Paul and Hayes, (2006) in an attempt to determine processes involved in the affordance effect, ascertained that the process is not completely automatic but dependant on the object’s properties. The action properties afforded by the object distinguish whether an affordance effect results; discrimination of shape was found to but colour had no effect. Information regarding shape is associated with action, grasping, whilst colour is irrelevant to action. It is the action-relevant features of the object which contribute to an affordance effect. Binary measurements used in traditional behavioural reaction tasks (the depressing of keyboard or stimulus box keys) can fail to capture subtleties in affordance tasks though. The use of a continuous force measure such as grip force is recommended which is sensitive to erroneous measurements of reaction. These measurements can reflect cognitive motor plans (the action afforded by the object) co-activated with incongruent task demands. This method provides direct evidence that motor plans, appropriate to the objects interaction, are activated by object affordances through visual processing; vision to action.

There is debate as to whether the affordance effect shown is an encoding of stimulus’ features relevant for action of appropriate motor plans, or a shift of attention to the stimulus’ orientation (location of graspable feature; a handle on the right of a pan would afford a faster response time from the right hand) which defines the response. This later attentional hypothesis proposes that the motor plans are automatically generated by an attentional modulation, a shift of directed visual attention to the spatial characteristics of the object (for instance the graspable feature provides a highly salient cue for attention). This orienting attentional process seems to account for the Simon effect, in which reaction times are faster when location of stimulus and response are congruent. Although attention is not excluded for the affordance effect the orienting attention found in the Simon effect is not necessary for affordance. Current thinking is that the two theories of affordance effects: motor affordance (encoding of pragmatic features of objects related to action) and spatial affordance (attention to location of graspable feature) are integrated for the visual processing of graspable objects.

Cognitive Neuroscience
Using techniques for studying affordances within the field of cognitive neuroscience, evidence has been trending away from the simplistic terminology of direct/indirect perception. fMRI and EEG have hinted at a complex interaction of cognitive pathways involved with affordances, and whilst not dismissing the Gibsonian concept of direct perception, have enabled a re-understanding of his theory. A perception-action theory that has proven to be influential in the field of affordances posited two visual systems (two-streams hypothesis) with different functional outputs: vision for perception through the ventral visual system and a vision for action through the dorsal visual system. Anatomically these neural pathways project to the inferotemporal cortex (ventral stream or ‘what’ pathway) and posterior parietal cortex (dorsal stream or ‘how’ pathway). Functional processes have been imparted for these streams such that the ventral stream perceptually processes recognition (characteristics of the object) and spatial relations whilst the dorsal system programs and controls the motor actions, the implementation of the skilled action with the object.

fMRI data using the unique characteristics of tools have supported the distinct functions of the two visual streams but have highlighted the importance of information integrated from both streams. The data have shown that planning and execution of tool-use skills happens in these separate areas but the activation of both networks with semantic tasks involving familiar tool-use strengthens the notion that affordance processes utilise both systems.

The ventral premotor cortex and supplementary motor cortex (BA 6) and posterior parietal cortex in particular the supramarginal gyrus (BA 40) in the inferior parietal lobule and BA 7 part of the superior parietal lobule are probable components for the neural dorsal stream for skilled object-related actions; whilst the posterior middle temporal gyrus and fusiform gyrus (BA 37) are implicated for the ventral stream for perception and recognition including accessing semantic information of objects.

There is a distinction made between planning, a conscious action in the ventral stream and programming, a direct unconscious action in the dorsal stream. This distinction has become apparent due to neuropsychological studies with patients with brain damage, in particular with DF who has visual agnosia with damage to her ventral stream but an intact dorsal stream. She is able to engage in specific visuo-motor actions but fails to consciously experience objects, her direct grasping action was intact but she made planning errors regarding which part of the object to grasp. She lacks the functional knowledge of the object (part of affordances). Double dissociations have been found which increase the robustness of findings regarding the functionality of the two streams. Patients with optic ataxia have damaged dorsal streams (posterior parietal cortex) such as patient IG. He is able to successfully identify objects (the inference is that this is due to his intact ventral stream) but has problems with actualising his behaviour, he cannot grasp those objects.

Evidence from event-related potentials suggest a hierarchal relationship in which the ventral system initiates action selection, planning based on information regarding object function, and directs the dorsal stream in a functional appropriate manner, the actualisation of an action response with the object. However the flexibility of the two visual systems can be detected in their interaction as an event-related potential study found that action affordance was coded faster than object recognition, a reversal of the hierarchal relationship seen above. Although the debate is still prevalent, both the dorsal stream and the ventral stream both play an important role in affordances of objects.