Faces are not the only topic of specialization in the temporal cortex. We also see specialization for place, specifically pictures of indoor and outdoor scenes, which activate the parahippocampal place area (PPA) in the ventral stream (Goldstein & Brockmole, 2017). An individual with a visual agnosia in this area might be able to recognize the grocery store as a store, but might be unable to recognize the specific objects in the store, such as the displays, the food on the shelves, or other objects. So, it would seem that the spatial layout is intact, but the objects within the layout are not.
One other area of specialization in the region next to the primary visual cortex is the extrastriate body area (EBA), which is activated by the rest of the body parts other than faces (Goldstein & Brockmole, 2017). If visual agnosia occurs in the EBA, an individual might be able to recognize a face, but not a hand or leg. With this form of agnosia, you would be able to recognize your friend’s face in the store, and the shelves and produce, but not your friend’s hand or arm as a hand or arm
So, what about the environment and the types of stimuli encountered on a regular basis? Selective rearing occurs when an animal is raised in a particular environment with limited specific types of stimuli. Due to the limited selection, the neurons respond more to these stimuli. When this happens, the response potential for other stimuli is reduced. This is neural plasticity, or the shaping of neurons through perceptual experiences (Goldstein & Brockmole, 2017). As stimuli are limited, neural plasticity becomes more specialized to the stimuli that the animal has been exposed to. Blakemore and Cooper explored this ‘use it or lose it’ effect of neural plasticity by limiting the stimuli kittens were exposed to. Kittens were limited to viewing either horizontal or vertical stripes for the first five months of life. The kittens were then tested to see the effects of the selective rearing. Results indicated that cats raised in horizontal stimuli responded to horizontal but not vertical stimuli. The same occurred for cats raised in vertical stimuli (Goldstein & Brockmole, 2017).
Sensory Coding refers to how neurons represent different characteristics of the environment. When specialized neurons respond to specific stimuli, specificity coding has occurred. However, this idea is likely to be incorrect because the brain would need one different neuron to perceive every different object. Neurons usually respond to more than stimulus. It could be that there are a number of neurons that are involved in representing a stimulus. Another form of coding looks at the different neuronal firing patterns that can occur in the representation of a particular stimulus. Where population coding utilizes larger groups of neurons that can create a greater number of different patterns, sparse coding involves smaller groups of neurons. Sparse coding is in effect when a stimulus is represented by the firing of a smaller group of neurons (Goldstein & Brockmole, 2017).
Dorsal and Ventral Pathways
Neurons are also organized through streams, or pathways. The pathway leading from the striate cortex to the parietal lobe is called the dorsal pathway, and the pathway that leads to the parietal lobe is the ventral pathway. The ventral pathway identifies what a stimulus is, and the dorsal pathway identifies where a stimulus is located, and whether or not it is stationary. The dorsal and ventral pathways serve different functions, but they are connected and signals flow both up toward the parietal and temporal lobes and back. Some information is shared between them as what an object is and where it is interact. The dorsal pathway also seems to be linked to the actions of an object, including how an action is carried out (Goldstein & Brockmole, 2017).
Modularity and Agnosia
MODULARITY
VISUAL AGNOSIA
PARAHIPPOCAMPAL PLACE AREA (PPA)
EXTRASTRIATE BODY AREA (EBA)
While the pathways are connected, they do serve different functions. Based on this, different areas of the cortex respond to different stimuli. This is called modularity. The different specialized areas that process information are called modules. One area where specialization to specific stimuli is the face. Researchers have used fMRI brain imaging to identify areas of the brain where neurons respond best to faces when distinguished from other objects. The main area of activity occurs in the fusiform face area (FFA), located at the base of the brain below the IT cortex (Goldstein & Brockmole, 2017).
Distributed Representation
Now that we have looked at specific areas that specialize in faces, body parts, and environmental scenes, we need to understand that these specific areas do not exist in a vacuum. Other areas of the cortex, and the rest of the brain for that matter, are also involved in identification of these stimuli. This is called distributed representation, or activation in multiple different areas of the brain. This is important to know because while research continuously indicates areas of specialization, it also indicates that the activation is distributed to other areas of the brain at the same time (Goldstein & Brockmole, 2017).
So, why might this occur? Well for one, we discussed at the beginning of this lesson that processing does not occur in a straight line. The processing of the stimulus travels around to different areas of the brain. Additionally, a face is not just a face. Each face has different features and movements, all of which must be processed based on a multidimensional approach in different sections (Goldstein & Brockmole, 2017). Let’s relate it to our shopping trip. Think about making spaghetti and meatballs for dinner and shopping for the ingredients. Is it just spaghetti and meatballs? What is the sauce made up of? What about the meatballs?
Now, in addition to using the recipe, you have some understanding about what goes into spaghetti and meatballs stored in your memory. You remember perceptual experiences of cooking and eating spaghetti and meatballs previously. Next, we will look at how research has measured the relationship between memory and perception in the hippocampus, an area of the brain associated with memory formation and storage (Goldstein & Brockmole, 2017).
Perception and Memory
What would happen if you had your hippocampus removed on both sides of your brain? The following case study shows us. Henry Molaison (H.M.) had the hippocampus removed completely as doctors attempted to stop the epileptic seizures he was experiencing. The seizures were eliminated, but so was his ability to store experiences and form long-term memories. Other research showed that there is a connection between visual processes and the hippocampus that respond to our three areas again: faces, bodies and environment scenes (Goldstein & Brockmole, 2017). Where one neuron might respond to one face, another might respond to recognition of another known face. Thus, certain neurons would be responsible for certain categories or types of stimuli.
What has all of this taught us? Can we conclusively connect certain neurons to certain stimuli? Do we have a solid answer to the mind-body problem, or the question of how biological neural processes become our perceptual experience? Well, if we have seen anything with this lesson, it is the fact that each new discovery leads to more questions and potential exceptions to the explanations proposed. Lateral inhibition seems to make sense in some cases, but not all cases. Each person has their own individual experiences based on their own individual perspective and processing of information from a given stimulus.
If I cook a lot, my perception of that plate of spaghetti and meatballs might be a little more detailed as I note the spices mixed in the sauce. This goes along with the expertise hypothesis, which proposes that changes occur via the plasticity of experience that we looked at earlier in this lesson as individuals spend more time with a given stimulus (Goldstein & Brockmole, 2017). Of course, that does not mean that the expertise hypothesis would explain everything. Studies on faces and FFA neurons indicate that there is merit to this hypothesis as experts in a field indicate increased neuronal responses for what is known based on strong experience or expertise. Yet, some researchers argue that this has more to do with neural connections that are already there rather than strengthening and expanding new responses (Goldstein & Brockmole, 2017).