Visual Cortex - Introduction

Introduction

The primary visual cortex, V1, is the koniocortex (sensory type) located in and around the calcarine fissure in the occipital lobe. Each hemisphere's V1 receives information directly from its ipsilateral lateral geniculate nucleus.

Each V1 transmits information to two primary pathways, called the dorsal stream and the ventral stream:

• The dorsal stream begins with V1, goes through Visual area V2, then to the dorsomedial area and Visual area MT (also known as V5) and to the posterior parietal cortex. The dorsal stream, sometimes called the "Where Pathway" or "How Pathway", is associated with motion, representation of object locations, and control of the eyes and arms, especially when visual information is used to guide saccades or reaching.
• The ventral stream begins with V1, goes through visual area V2, then through visual area V4, and to the inferior temporal cortex. The ventral stream, sometimes called the "What Pathway", is associated with form recognition and object representation. It is also associated with storage of long-term memory.

The what vs. where account of the ventral/dorsal pathways was first described by Ungerleider and Mishkin. More recently, Goodale and Milner extended these ideas and suggested that the ventral stream is critical for visual perception whereas the dorsal stream mediates the visual control of skilled actions. Although these ideas are contentious among some vision scientists and psychologists, the proposed division of labor between vision-for-perception and vision-for-action is supported by a broad range of findings from neuropsychology, neurophysiology, and neuroimaging. Some of the most controversial evidence comes from studies of visuomotor behaviour in normal observers. For example, it has been shown that visual illusions such as the Ebbinghaus illusion distort judgements of a perceptual nature, but when the subject responds with an action, such as grasping, no distortion occurs. Other work suggests that both the action and perception systems are equally fooled by such illusions. More recent studies, however, provide strong support for the idea that skilled actions such as grasping are not affected by pictorial illusions. and suggest that the action/perception dissociation is a useful way to characterize the functional division of labor between the dorsal and ventral visual pathways in the cerebral cortex.

Neurons in the visual cortex fire action potentials when visual stimuli appear within their receptive field. By definition, the receptive field is the region within the entire visual field which elicits an action potential. But for any given neuron, it may respond best to a subset of stimuli within its receptive field. This property is called neuronal tuning. In the earlier visual areas, neurons have simpler tuning. For example, a neuron in V1 may fire to any vertical stimulus in its receptive field. In the higher visual areas, neurons have complex tuning. For example, in the inferior temporal cortex (IT), a neuron may only fire when a certain face appears in its receptive field.

The visual cortex receives its blood supply primarily from the calcarine branch of the posterior cerebral artery.