In the old days, we conceived of two visual pathways known as the “what and where” streams. The “what” system operated along the junction of the occipital and temporal lobes, primarily involved in object recognition. The “where” system functioned along the junction of the occipital and parietal lobes, involved principally in the location of information.
But alas, this simple distinction proved to be too simple. The “what” pathway turned out to be a massive anatomical stream from primary visual cortex projecting into V4 and into the temporal lobe dealing with the analysis of shape, color, contour, discrimination and object recognition. As such, it processes “what” information, relying on memory circuity within the temporal lobe, sending its sensory neurons forward to the prefrontal cortex. This stream is also known as the ventral steam, and the vision-for-perception pathway.
The “where” pathway proved to be even more complicated, as heralded by the Amboy Dukes 50 years ago:
Leave your cares behind
Come with us and find
The pleasures of a journey to the center of the mind
Come along if you care
Come along if you dare
Take a ride to the land inside of your mind
Beyond the seas of thought
Beyond the realm of what
Across the streams of hopes and dreams where things are really not
Interconnected in parallel with the “what” or ventral pathway going from V1 to temporal lobe is the dorsal stream, also known as the “vision for action pathway”, blending “where” and “how” on its journey from V1 to the parietal lobe. The dorsal/parietal stream is involved in the guidance of actions and recognizing where objects are in space. The “where” information taps into depth and motion, allowing you to interact with the world. Do you want to grab or avoid objects in space? While the ventral stream is occupied with encyclopedic facts, the dorsal stream shuffles its visual field maps to detect, analyze and direct movement. Bear in mind the significant intermingling of the visual streams, as reviewed in this elegant article in Frontiers in Integrative Neuroscience.
A bit more information about the anatomical markers of the dorsal/parietal stream. The posterior parietal cortex (PPC) appears to be the hub for the perception and interpretation of spatial relationships, accurate body image, and the learning of tasks involving coordination of the body in space. The lateral intraparietal sulcus (LIP) contains neurons that produce enhanced activation when attention is directed via saccades toward the stimulus. The ventral intraparietal sulcus (VIP) is where visual and somatosensory information are integrated.
You may be wondering why I bring up the visual distinctions between who, what, when, where and how. It is prompted by a new book from New Jersey neuro-ophthalmologist Frederick Lepore.
In the course of telling his tale about Einstein’s brain, Dr. Lepore refers to the occipital lobe as one of the brain’s visual centers. I like that description. All of the brain’s lobes participate in vision, and multiple visual centers are located throughout the brain. Both developmental and acquired brain injuries and their clinical sequelae make that abundantly clear. Isn’t it time to replace the simplistic notion in high school and college textbooks equating vision with what occurs in the occipital lobe? It has far-reaching implications.