So, I did some more psychology revision, and, seeing as perception is the most interesting (and I like to claim it as my speciality 😉 ), you can have a short, five-point post looking at the evidence for separate regions and abilities within the visual cortex.
How do we know that different regions of visual cortex are specialised for different aspects of vision? Illustrate your answer with reference to a range of visual areas. In short:
- Importance of specialisation. Double dissociation: identifying the different functioning between different areas of the visual cortex, eg. “If one can demonstrate that a lesion in brain structure A impairs function X but not Y, and further demonstrate that a lesion to brain structure B impairs function Y but spares function X, one can make more specific inferences about brain function and function localization.” (Wikipedia)
- Dorsal stream (occipital lobe to parietal lobe) versus ventral stream (occipital lobe to temporal lobe) – the where versus the what. Again, damage in one stream seems not to affect the functions of the other stream.
- Lesions – both TMS induced and of case study patients – such as the man who mistook his wife for a hat (he had visual agnosia), where he could not recognise objects, not for lack of seeing them; prosopagnosia: woman not recognising her doctor from a picture of his face; imaging and activation, eg. CAT (involves x-rays in a scanner; the resulting scan is different shades according to the different densities of areas of the brain: bone is white, Centrospinal Fluid is black, neurons grey. Wintermark’s developing children showed two times as many neurons in their brain – this is due to specialisation).
- Clinical failures of vision, malfunctions of systems, eg. visual agnosia – whilst the ability to recognise faces (deriving from the fusiform face area or gyrus, Brodmann 37) is intact, the ability to recognise objects is impaired or lost entirely. If caused by, for instance, direct/specific surgery, which has lacerated off the lateral occipital lobe area from any other parts of the brain, one would expect, if all aspects of vision stem from a single region, all vision to fail. Instead of complete blindness, only visual agnosia occurs, suggesting that 1) the single ability to recognise objects stems from there, and 2) the visual cortex operates different visual abilities from different parts of the visual cortex. Evidence for this has been found from fMRI scans, correlating the presence of electrical, Blood Oxygenation Level Dependency signals in the lateral occipital cortex of the ventral stream with the ability to recognise objects.
- Studies into the visual area action in general. Area Middle Temporal (MT or Visual Complex 5) in the dorsal stream has been studied heavily, despite it being a small area. It is very sensitive to motion vision, but not at all to colour vision. Why? A large portion of neuron cells here are tuned (ie. most brain sense selectively represent a particular type of info – and it can be broad or narrowed but sharp) to the speed and direction of moving stimuli. Damage here leads to Motion Blindness.
This is one way I’ve been chunking down my revision. I’m hoping its roughness is actually its power – after all, not all of our brains work in flowing sentences, and planning is the forefront of essay-writing. 🙂
Alexandrina Brant 