Thursday, September 22, 2011

Object Thinking - Objects: a neurological basis

This post deals with how the brain perceives the world as objects.

A neurological perspective of how perception work, via studying perceptual disorders, is covered in chapter two of Neuropsychology: from theory to practice [1]. This is a review of that chapter.

Studying perceptual disorders tell us how we work by looking at damaged brains in people, or damaging brains in animals, and seeing how that affects what is perceived.

The chapter concentrates largely on visual perception, due to “the natural dominance of our visual sensory system”. It starts out by identifying two major pathways in the brain, the “what” pathway, which is responsible for identification of objects, and the “where” pathway, which is responsible for location position and motion. These were originally identified in monkeys in 1983 by Mishkin, Ungerleider and Macko. Milner and Goodale (1995) expanded on this model to explain that the “where” pathway is dedicated to the preparation of movement.

This demonstrates that humans understand the world as objects and actions. 

The chapter goes onto explain that these two pathways are linked, essentially the flow of data goes primary visual cortex → “what” pathway → “where” pathway → motor cortex. The system also gets feedback, via other pathways, from interactions with the environment to aid in learning. This of course means that we get better at performing actions the more we do them.

The next section of the chapter deals with sensation versus perception. It is not particularly relevant to this discussion. In short summary: sensation occurs before perception, and is not consciously recognised. In vision the sensation pathways are those that link the retina to the visual cortex. People with damage to these pathway will not notice that they don't see something, unless they are made aware of it appearing and disappearing from view.

Discussion of the hierarchy of the visual cortex follows on. This has quite a strong neurological focus, and describes a lot of the brain's structure in this area. The key point relevant here is that the brain is modular and parallel, which means that human thinking is modular and parallel, which is clearly analogous to separation of concerns. The parallelism is accomplished through pathways that allow feedback between modules. This could be thought of as message passing, although it might be a stretch to say it scales up to conscious thought.

Next the chapter discusses what certain disorders show us about visual perception. The two types of disorder covered are apperceptive agnosia – a condition that means the patient has a difficulty distinguishing between objects – and associative agnosia – in which the patient is unable to recognise objects or their functions.

Apperceptive agnosia, and its milder counter part; categorisation deficit, give strong evidence that the mind perceives the world as objects. People with these disorders cannot discern one object from another. This impedes problem solving, as the person with the condition does not know how to act on what they see. In fact, in the case of apperceptive agnosia, it can be equivalent to blindness, as those with the condition find it easier to navigate with their eyes shut.

Associative agnosia, prevents people from being able to recognise objects or their functions. This class of agnosia can affect any of the senses. The book focuses on vision.

People with associative agnosia can copy (e.g. by drawing) and match objects, but they cannot recognise. So it appears that primary perceptual processing is intact.

The current theory for what causes this agnosia is that the “what” pathway has become disconnected from the memory store for associative meaning. People with this condition can write something down, such as their name or address, but are completely unable to read it back. This is clear evidence that we use background knowledge to solve problems.

The chapter gives an example (p. 53) of a patient, with associative visual agnosia, who can only tell what a banana is after eating it, and even then only through logical deduction: “...and here I go right back to the stage where I say well if it's not a banana, we wouldn't have this fruit.”

The next section of the chapter discusses object and face recognition. The focus is on how this works at a neurological level, and the difference between face recognition and object recognition. The key point it makes is that the left hemisphere of the brain deals with parts of objects, and the right deals with objects as a whole. (Faces, are a special case, however, as they seem to be perceived as a whole, and not as parts, i.e. most of facial recognition is done in the right hemisphere.) The brain is set up to understand about composition.

The rest of the chapter focuses on describing top down (using past experience to influence perception) and bottom up (working from first principles) processing of visual information, and come to a conclusion about how the left and right hemispheres interact to give what we see meaning. Essentially they work together, the left hemisphere identifying objects and the meaning of objects, while the right analyses structural form, orientation and does holistic analysis of an object.

So, in conclusion, the chapter lays out clearly that human beings perceive the world as objects, even at a neurological level. This is our nature. Thus is makes sense when designing software to think of our problem space in terms of the objects in it. 

The next section will deal with why action is integral to how we think about the world, and can be found here: Object Thinking - Objects have actions.

[1] Neuropsychology: from theory to practice, David Andrewes (2001, Psychology Press)