Cognitive Neuroscience, the biology of the mind, by M. Gazzaniga (fourth edition) – Summary chapter 6

Agnosias refer to failures of perception while the processes itself (e.g: the eye itself) is functioning normally. Visual agnosia refers to agnosia limited to the visual modality. There are four major concepts of object recognition: there is a difference between perceiving and recognizing (1), perception is of unified objects (2), perceptual abilities are flexible and robust (3) and the product of perception is interwoven with memory (4).

The parahippocampal area and posterior parietal cortex process information about places and scenes.

Output from the visual cortex is contained primarily in two major fibre bundles: fasciculi. The ventral (occipitotemporal) stream is the “what pathway” and the dorsal (occipitoparietal) stream is the “where pathway”. The what pathway is responsible for object perception and recognition and the where the pathway is responsible for spatial perception. There is also a what and where pathway for auditory stimuli. The pathways may carry similar information but they each support different aspects of cognition.

Patients with optic ataxia can recognize objects but cannot use visual information to guide their actions. Saccades are quick eye movements. In patients with optic ataxia, saccades may be directed inappropriately. The two pathways are not isolated from each other but communicate extensively.

Neurons in the parietal lobe have large, nonselective receptive fields that include cells representing both the fovea and the periphery. Neurons in the temporal lobe have large receptive fields that are more selective and always represent foveal information.

There are three major subtypes of agnosia:

1. Apperceptive agnosia
   This refers to a difficulty in recognizing objects from abnormal perspectives and is caused by a right-sided parietal lesion. Object recognition is limited when the perceptual input is limited. It can be seen as a problem with object constancy.
2. Integrative agnosia
   This refers to an inability to integrate features int parts or parts of an object into a coherent whole. Patients with this agnosia do not perceive objects holistically but rather as separate parts.
3. Associative agnosia
   This refers to a failure of visual object recognition that cannot be attributed to a problem of integrating parts to form a whole or to a perceptual limitation. People with this agnosia derive normal visual representations but cannot use this information to recognize things.

People with right-sided lesions cannot recognize objects. People with left-sided lesions cannot make the functional connection between two visual perceptions.

Facial stimuli activate the superior temporal sulcus and the inferotemporal gyrus. The FFA, the fusiform face area includes the fusiform gyrus and is the brain’s area for facial recognition. There are more face regions than this one. It is possible that different areas have different specialization for facial information. It is likely that the FFA is important for processing invariant facial properties and the superior temporal sulcus is important for processing more dynamic features. It is also possible that the FFA is used in fine perceptual discriminations among highly familiar stimuli, as people see a lot of faces every day.

Facial perception appears to use distinct physical processing systems. It is likely that the brain has different systems for object recognition and facial recognition. Prosopagnosia refers to difficulties in recognizing faces.

The parahippocampal place area (PPA) is an area used for making judgements about spatial properties or relations. The extrastriate body area (EBA) is important for recognizing parts of the body. The fusiform body area (FBA) is also important for recognizing body parts. Visual experience is not necessary for category-specific to develop within the organization of the ventral stream.