elephantcolor

What colour is an elephant
14 Oct 95

                                                 OUR knowledge of things in the world around us is
                                                 stored in networks of nerve cells all round our
                                                 brains. Researchers from the US National
                                                 Institute of Mental Health who have been looking
                                                 at where memories are stored now say that we
                                                 keep our knowledge about different properties of
                                                 objects, such as colour and movement, in
                                                 different places. And the knowledge about each
                                                 property is stored close to the part of the brain
                                                 that specialises in perceiving it.

                                                 People with brain damage occasionally suffer from
                                                 bizarre defects, such as being unable to
                                                 associate elephants with the colour grey while
                                                 having no difficulty perceiving and naming either
                                                 colours or elephants. Rather than having a single
                                                 file marked "elephant" in our mental filing
                                system, we seem to have a network of linked databases each
                                specialising in a particular kind of information. But when it comes to
                                identifying the brain areas responsible, brain damage can provide only
                                a rough guide: cases are rare, and the damaged areas are often large.

                                More detailed studies can be done by measuring blood flow in the
                                brain, using the scanning technique known as positron emission
                                tomography (PET). This allows researchers to identify peaks of
                                activity in different regions of normal people's brains while they carry
                                out particular mental tasks. Alex Martin and his colleagues at the
                                NIMH decided to use the technique to investigate knowledge about
                                colour and movement, because the areas of the brain that perceive
                                these attributes are already well known.

                                They showed people black and white line drawings of a number of
                                objects, and asked them to name either a colour or an action
                                associated with each one. For example, a picture of a pencil might
                                yield the responses "yellow" and "write".

                                Both tasks caused the brain to work harder in the prefrontal, parietal
                                and temporal lobes of the cortex, the outer layer of the brain that
                                deals with higher functions such as perception and language. Some
                                of this activity, however, was for subtasks such as word finding,
                                which come into play when naming both actions and colours. So
                                Martin and his colleagues computed the difference in brain activity
                                between the two tasks. Brain regions that responded equally to both
                                tasks yielded a difference of zero, while those that responded strongly
                                to one task and not the other showed a high value.

                                Their analysis revealed that the lower surfaces of each temporal lobe
                                contained an area that was most active when the subjects thought
                                about colour. This is close to a region other PET studies have
                                pinpointed as critical for colour perception. Similarly, an area near
                                the junction of the left temporal and occipital lobes was most active
                                when thinking up an action word - and this was immediately next to
                                the motion perception area (Science, vol 270, p 102).

                                "What's really important here is that knowledge is organised in a way
                                that's predictable from what we know about the organisation of
                                perception," says Martin. The results also support the idea that
                                perception of an object almost instantaneously makes available
                                everything we know about it. "We always see things in terms of the
                                meaning they convey," says Martin. "That's how we can identify them
                                so quickly."(see Diagram)

                                GEORGINA FERRY
                                From New Scientist magazine, vol 148 issue 1999, 14/10/1995, page
                                                            17